Ekaterina S Gulyaeva

Elena S Osipova

Ruqaya Buhaibeh

Yves Canac

Jean-Baptiste Sortais
email: jean-baptiste.sortais@lcc-toulouse.fr

Dmitry A Valyaev
email: dmitry.valyaev@lcc-toulouse.fr

Towards ligand simplification in manganese-catalyzed hydrogenation and hydrosilylation processes

Keywords: Manganese complexes, Bidentate ligands, Hydrogenation, Transfer Hydrogenation, Hydrogen borrowing, Hydrosilylation DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene, DFT, Density functional theory, Dipp, 2,6-diisopropylphenyl, KIE, kinetic isotope effect, LPO, dilauroyl peroxide (nC11H23COO)2, Mes, 2,4,6-trimethylphenyl, MP, N-methylpyrrolidine, NHC, N-heterocyclic carbene, Oct, 1-octyl, PDI, pyridine-diimine, PMHS, polymethylhydrosiloxane, Pent, 1-pentyl, Pr, 1-propyl, iPr, 2-propyl, Py, pyridyl, SET, single electron transfer, TEMPO, 2,2 trifluoromethylsulfonate, TMS, trimethylsilyl, TOF, turnover frequency, Tol, tolyl, TON, turnover number

come

Introduction

Homogeneous catalysis using organometallic manganese complexes have gained a tremendous progress during last decade [START_REF] Valyaev | Manganese organometallic compounds in homogeneous catalysis: past, present, and prospects[END_REF][START_REF] Carney | Recent advances of manganese catalysis for organic synthesis[END_REF][START_REF] Mukherjee | Homogeneous catalysis by cobalt and manganese pincer complexes[END_REF][START_REF] Hu | Inert C-H bond transformations enabled by organometallic manganese catalysis[END_REF][START_REF]Manganese catalysis in organic synthesis[END_REF] and continues to attract considerable attention from academic research community. In particular, since the seminal discovery of first efficient Mn-based (de)hydrogenation catalytic systems by Beller [START_REF] Elangovan | Selective catalytic hydrogenations of nitriles, ketones, and aldehydes by well-defined manganese pincer complexes[END_REF] and Milstein [START_REF] Mukherjee | Manganese-catalyzed environmentally benign dehydrogenative coupling of alcohols and amines to form aldimines and H2: a catalytic and mechanistic study[END_REF] in the beginning of 2016, this topic became one of the most rapidly emerging areas in catalysis using Earth-abundant transition metal complexes. Despite many impressive results obtained to date, the initial development of Mn(I)-based reduction catalysis was mainly performed using the transposition of chemical concepts previously applied for catalysts containing isoelectronic Fe(II) and Co(I) metal centers, namely the use of pincertype ligands with phosphine donor extremities [START_REF] Garbe | Homogeneous catalysis by manganese-based pincer complexes[END_REF][START_REF] Maji | Recent developments of manganese complexes for catalytic hydrogenation and dehydrogenation reactions[END_REF][START_REF] Filonenko | Catalytic (de)hydrogenation promoted by non-precious metals -Co, Fe and Mn: recent advances in an emerging field[END_REF][START_REF] Kallmeier | Manganese complexes for (de)hydrogenation catalysis: a comparison to cobalt and iron catalysts[END_REF][START_REF] Gorgas | Isoelectronic manganese and iron hydrogenation/dehydrogenation catalysts: similarities and divergences[END_REF][START_REF] Irrgang | 3d-metal catalyzed N-and C-alkylation reactions via borrowing hydrogen or hydrogen autotransfer[END_REF][START_REF] Widegren | Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation[END_REF][START_REF] Waiba | Manganese catalyzed acceptorless dehydrogenative coupling reactions[END_REF][START_REF] Rohit | Manganese-catalysed dehydrogenative coupling -an overview[END_REF][START_REF] Wang | Homogeneous manganese-catalyzed hydrogenation and dehydrogenation reactions[END_REF][START_REF] Sortais | Manganese-catalyzed hydrogenation and hydrogen transfer reactions[END_REF][START_REF] Weber | Manganese-catalyzed hydrogen-borrowing and dehydrogenative coupling reactions[END_REF][START_REF] Azouzi | Manganesenew prominent actor in transfer hydrogenation catalysis[END_REF][START_REF] Das | Advancements in multifunctional manganese complexes for catalytic hydrogen transfer reactions[END_REF][START_REF] Nad | Acceptorless dehydrogenative coupling reactions by manganese pincer complexes[END_REF] capable to activate inert chemical bonds via a metal-ligand cooperation [START_REF] Khusnutdinova | Metal-ligand cooperation[END_REF][START_REF] Alig | First-row transition metal (de)hydrogenation catalysis based on functional pincer ligands[END_REF][START_REF] Weber | The role of metal-ligand cooperation in manganese(I)-catalyzed hydrogenation/dehydrogenation reactions[END_REF][START_REF] Elsby | Strategies and mechanisms of metal-ligand cooperativity in first-row transition metal complex catalysts[END_REF] (A, Scheme 1). However, the analysis of recent literature provides growing evidence that tridentate ligand architectures are not mandatory for the design of efficient manganese (de)hydrogenation catalysts, as complexes bearing less elaborated bidentate ligands (B, Scheme 1) can be competitive or sometimes even superior in terms of catalytic activity and/or application scope. In addition, some of these successful Mn(I) catalytic systems contain only spectator donor ligands or even are based uniquely on metal carbonyl precursors, thus indicating the existence of unconventional reaction mechanisms undoubtedly arising from the specificity of organometallic manganese chemistry. Indeed, a similar trend can be also evidenced for Mn-catalyzed hydrosilylation-type processes: the evolution from the state-of-the-art Mn(II) catalysts developed by Trovitch [START_REF] Mukhopadhyay | A highly active manganese precatalyst for the hydrosilylation of ketones and esters[END_REF][START_REF] Mukhopadhyay | Mechanistic investigation of bis(imino)pyridine manganese catalyzed carbonyl and carboxylate hydrosilylation[END_REF][START_REF] Mukhopadhyay | Hydrosilylation of aldehydes and formates using a dimeric manganese precatalyst[END_REF] incorporating polydentate PDI-type scaffolds (C, Scheme 1) to the catalytic systems based on simpler bi-(D) and monodentate (E) donor ligands or even manganese carbonyl Mn2(CO)10. Scheme 1. Evolution of the ligand design in Mn-catalyzed hydrogenation and hydrosilylation.

While Mn-catalyzed hydrogenation-type processes have been extensively reviewed [START_REF] Garbe | Homogeneous catalysis by manganese-based pincer complexes[END_REF][START_REF] Maji | Recent developments of manganese complexes for catalytic hydrogenation and dehydrogenation reactions[END_REF][START_REF] Filonenko | Catalytic (de)hydrogenation promoted by non-precious metals -Co, Fe and Mn: recent advances in an emerging field[END_REF][START_REF] Kallmeier | Manganese complexes for (de)hydrogenation catalysis: a comparison to cobalt and iron catalysts[END_REF][START_REF] Gorgas | Isoelectronic manganese and iron hydrogenation/dehydrogenation catalysts: similarities and divergences[END_REF][START_REF] Irrgang | 3d-metal catalyzed N-and C-alkylation reactions via borrowing hydrogen or hydrogen autotransfer[END_REF][START_REF] Widegren | Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation[END_REF][START_REF] Waiba | Manganese catalyzed acceptorless dehydrogenative coupling reactions[END_REF][START_REF] Rohit | Manganese-catalysed dehydrogenative coupling -an overview[END_REF][START_REF] Wang | Homogeneous manganese-catalyzed hydrogenation and dehydrogenation reactions[END_REF][START_REF] Sortais | Manganese-catalyzed hydrogenation and hydrogen transfer reactions[END_REF][START_REF] Weber | Manganese-catalyzed hydrogen-borrowing and dehydrogenative coupling reactions[END_REF][START_REF] Azouzi | Manganesenew prominent actor in transfer hydrogenation catalysis[END_REF][START_REF] Das | Advancements in multifunctional manganese complexes for catalytic hydrogen transfer reactions[END_REF][START_REF] Nad | Acceptorless dehydrogenative coupling reactions by manganese pincer complexes[END_REF], these contributions have been essentially focused on the most popular pincer-type systems with a particular attention devoted to the comparison between manganese and their 3d metal congenersiron and cobalt [START_REF] Filonenko | Catalytic (de)hydrogenation promoted by non-precious metals -Co, Fe and Mn: recent advances in an emerging field[END_REF][START_REF] Kallmeier | Manganese complexes for (de)hydrogenation catalysis: a comparison to cobalt and iron catalysts[END_REF][START_REF] Gorgas | Isoelectronic manganese and iron hydrogenation/dehydrogenation catalysts: similarities and divergences[END_REF][START_REF] Irrgang | 3d-metal catalyzed N-and C-alkylation reactions via borrowing hydrogen or hydrogen autotransfer[END_REF]. In contrast, this article is focusing on a systematic overview of the catalytic properties of manganese complexes bearing bidentate donor ligands in (de)hydrogenation processes including the analysis of their relative performance depending on the nature of donor moieties (P-, N-and C-donors) and the comparison, when possible, with structurally related pincer scaffolds. The literature is covered until the end of September 2021. The application of Mn(I) complexes B in the catalysis of electro-and photochemical CO2 reduction [START_REF] Stanbury | Electro and photoreduction of CO2 driven by manganese-carbonyl molecular catalysts[END_REF][START_REF] Sinopoli | Manganese carbonyl complexes for CO2 reduction[END_REF][START_REF] Fernandez | Manganese complexes for electro-and photocatalytic transformations[END_REF] is out of scope of this contribution. As several excellent review articles on Mn-catalyzed hydrosilylation are available [START_REF] Trovitch | The emergence of manganese-based carbonyl hydrosilylation catalysts[END_REF][START_REF] Yang | Manganese-catalyzed hydrosilylation reactions[END_REF][START_REF] Darcel | Iron-, cobalt-, and manganese-catalyzed hydrosilylation of carbonyl compounds and carbon dioxide[END_REF][START_REF] Royo | Recent advances in catalytic hydrosilylation of carbonyl groups mediated by welldefined first-row late transition metals[END_REF][START_REF] Bhunia | Earth-abundant metal catalyzed hydrosilylative reduction of various functional groups[END_REF][START_REF] Nguyen | Manganese-catalyzed hydrosilylation and hydroboration reactions, Manganese-catalyzed hydrogen-borrowing and dehydrogenative coupling reactions[END_REF], in this account we aim to mainly show the recent evolution of ligand design in Mn-catalyzed hydrosilylative transformations of more challenging substrates (carboxylic acid derivatives, alkenes, alkynes).

Mechanistic aspects of these reactions and the influence of auxiliary ligands on the catalytic activity and/or chemoselectivity are presented in detail.

Manganese complexes bearing bidentate donor ligands in hydrogenation-type reactions

Hydrogenation of unsaturated C=O, C=N and C=C double bonds

Though the first precedents of catalytic hydrogenation using Mn(0) carbonyl complexes Mn2(CO)10 and Mn2(CO)8(PBu3)2 have been reported more than 40 years ago by the groups of Wender [START_REF] Weil | The Mn2(CO)10 catalyzed hydrogenation and hydroformylation of olefins[END_REF] and Cremer [START_REF] Fish | Homogeneous catalytic hydrogenation. 1. Regiospecific reductions of polynuclear aromatic and polynuclear heteroaromatic nitrogen compounds catalyzed by transition metal carbonyl hydrides[END_REF], these catalytic systems worked only under very harsh conditions (70-200 bar H2, 160-200 °C) with low efficiency (TON of 67 for 1-octene and of 20 for acridine). Later, it was demonstrated by Atwood and coll. [START_REF] Bogdan | Photocatalysis of hydrogenation and isomerization of alkenes by cis-HMn(CO)4PPh3[END_REF] that Mn(I) complex cis-(PPh3)(CO)4MnH under UV irradiation was able to hydrogenate 1-octene at room temperature and atmospheric hydrogen pressure, albeit the isomerization of the substrate to the mixture of 2-octenes was the major process. However, the first efficient hydrogenation reaction catalyzed by Mn(I) complexes bearing bidentate ligands was reported by Pidko and coll. in 2017. [START_REF] Van Putten | Non-pincer-type manganese complexes as efficient catalysts for the hydrogenation of esters[END_REF] Neutral [START_REF] Valyaev | Manganese organometallic compounds in homogeneous catalysis: past, present, and prospects[END_REF] and cationic (2a-b) complexes incorporating phosphine-amine ligands were shown to be active in ester hydrogenation (Scheme 2). Upon initial screening pre-catalyst 1 containing one Ph2PCH2CH2NH2 ligand was found to be ca. two times more active in the reduction of methyl benzoate than 2a incorporating two bidentate ligands (75 vs. 43% of substrate conversion), whereas the replacement of Ph substituents at the phosphorus atom by iPr ones resulted in further detrimental effect for the catalytic activity (13% conversion for 2b). Notably, catalytic system generated in situ from Mn(CO)5Br and one equivalent of PN ligand provided similar activity as for isolated complex 1. Under optimized conditions a variety of aliphatic and aromatic esters was successfully converted to the corresponding alcohols using only 0.2 mol% of catalyst (TON of 350-500), albeit in the presence of very high amount of base -75 mol% tBuOK. The catalytic system tolerates well terminal and internal alkene moieties, but the conjugated C=C bond in methyl cinnamate was fully reduced to form in fine Ph(CH2)3OH.

Scheme 2. Hydrogenation of esters catalyzed by Mn(I) PN complexes 1 and 2.

The reaction mechanism for model substrate MeCO2Me was studied in detail by DFT calculations [START_REF] Van Putten | Non-pincer-type manganese complexes as efficient catalysts for the hydrogenation of esters[END_REF][START_REF] Liu | Computational insights into the catalytic role of the base promoters in ester hydrogenation with homogeneous non-pincer-based Mn-P,N catalyst[END_REF] allowing in particular to clarify the critical importance of the strong excess of base for reaction efficiency especially in the case of esters containing small alkyl substituents. The key step of the catalytic cycle includes concerted hydride/proton transfer between the hydride complex 3 and ester to afford the intermediate 4, in which the hemiacetal moiety is hydrogen-bonded to deprotonated PN ligand (Scheme 3). The subsequent hydrogenative cleavage of C-O bond affords methanol and acetaldehyde complex 5, which is then easily reduced to 6 followed by the regeneration of the starting hydride complex 3 in the presence of dihydrogen. Importantly, the alternative amideassisted fragmentation of hemiacetal intermediate 4 leads to the formation of methoxide species 7 resistant to hydrogenolysis to form 3 and thus representing the main catalyst deactivation pathway. It was shown that high concentration of tBuOK, besides a transformation of 7 into more reactive tertbutoxide species 8, additionally increases the polarity of reaction medium making each step in the sequence 7→8→3 more thermodynamically favorable. Scheme 3. Simplified catalytic cycle for the hydrogenation of methyl acetate catalyzed by Mn(I) complex 1 based on DFT study (activation energy barriers are given in kcal/mol, in case of two-step process both values separated by comma are given).

Similar Mn-catalyzed ester hydrogenation protocol with an ample substrate scope was originally introduced by the group of Beller using complexes 9 and 10 exhibiting PNP pincer ligands (Scheme 4) [START_REF] Elangovan | Hydrogenation of esters to alcohols catalyzed by defined manganese pincer complexes[END_REF], which can be formally considered as an extension of PN-type ligand with a second CH2CH2PR2 arm. Both neutral and cationic pre-catalysts showed identical results assuming in situ formation of the same catalytically competent dicarbonyl hydride species (PNP)Mn(CO)2H. Importantly, in this case the use of small ethyl substituents at the phosphorous atoms was found to be crucial, as analogous manganese bromide complexes (PNP)Mn(CO)2Br with more common bulkier iPr and Cy groups showed negligible catalytic activity. Scheme 4. Hydrogenation of esters catalyzed by related Mn(I) PNP complexes 9 and 10.

The reaction proceeding under similar conditions as in the case of complex 1, demonstrates the same chemoselectivity vs. isolated/conjugated C=C bonds and according to DFT study goes via a conceptually close sequence of outer-sphere reduction of ester and aldehyde molecules. While the typical catalyst charge for PNP complex 10 is ten times higher compared to simpler PN congener 1, this fact is counterbalanced by a significantly lower amount of base required for efficient substrate conversion (10 vs. 75%). Though it is difficult to benchmark precisely the relative performance of PN and PNP Mn(I) catalysts due to some fluctuations in reaction conditions (50 vs. 30 bar H2 / 20 vs. 24 h of reaction time for 1 and 10, respectively), the reduction of methyl benzoate at 100 °C with the same charge of catalyst (2 mol%) and base (10 mol%) reveals a small preference for pincer-type system affording 97% GC yield of BnOH compared to 80% obtained in the case of its bidentate analogue.

The application of pyridinyl-phosphine manganese complexes 11a-b in ketone hydrogenation was reported in 2018 by Sortais and coll. (Scheme 5) [START_REF] Wei | Hydrogenation of carbonyl derivatives catalysed by manganese complexes bearing bidentate pyridinyl-phosphine ligands[END_REF]. As in the case of PN complexes 2a-b, precatalysts 11a containing Ph2P donor moiety was more active than its iPr2P analogue 11b (90 and 65% of acetophenone conversion using 1.0 mol% of catalysts at 80 °C). Aliphatic and aromatic ketones can be smoothly reduced at 50 °C using 0.5 mol% of complex 11a and 2 mol% of KHMDS or tBuOK as a base (TON of . Noteworthy, this catalytic system tolerates many functional groups including all types of halogens, CF3, ester, amide, terminal alkene as well as pyridine and unprotected aromatic amine. The hydrogenation of aromatic enones can be selectively performed at conjugated C=C bonds to form the corresponding saturated ketones isolated in excellent yields, which could be fully reduced at 80-100 °C using higher charge of catalyst and base (5 mol% 11a, 10 mol% tBuOK). The identified scope limitations include sterically bulky substrates as well as nitro, CN and thiophene moieties.

Scheme 5. Hydrogenation of ketones catalyzed by Mn(I) PN complexes 11.

Remarkably, the catalytic activity of cationic tricarbonyl Mn(I) complex 12 based on structurally related PN 3 P pincer-type ligand in ketone hydrogenation was far below the bidentate system 11a (Scheme 6) [START_REF] Bruneau-Voisine | Hydrogenation of ketones with a manganese PN 3 P pincer pre-catalyst[END_REF]. Indeed, in this case the utilization of 5.0 mol% of catalyst and heating at 130 °C was required to achieve reasonable substrate conversion. However, it was shown that the corresponding dicarbonyl neutral hydride derivative 13 was very efficient in hydrogenation of aldehydes at room temperature (Scheme 6) [START_REF] Glatz | Chemoselective hydrogenation of aldehydes under mild, base-free conditions: manganese outperforms rhenium[END_REF]. The selective reduction of aldehyde moiety readily proceeds using 0.05-0.1 mol% of catalyst 13 under base-free conditions and tolerates numerous functional groups including heterocycles and even conjugated C=C bond. The addition of 3 equivalents of DBU as base allowed to further decrease the amount of catalyst to 50 ppm providing TON up to 10400 for p-FC6H4CHO. Scheme 6. Hydrogenation of ketones and aldehydes catalyzed by Mn(I) PN 3 P complexes 12 and 13, respectively.

Mechanistic studies revealed the importance of both NH functions in complex 13 and ethanol as solvent for the catalytic cycle. Indeed, the corresponding NMe congener was completely inactive and metal alkoxide intermediate (PN 3 P)(CO)2MnOCH2C6H4F, which can be reversibly formed upon the reaction of 13 with excess of aldehyde in THF or benzene failed to react with dihydrogen. DFT calculations showed that reduction of PhCHO can proceed via two distinct reaction pathways (Scheme 7) characterized by similar energy barriers and implying the participation of ethanol as proton shuttle. Both of them include the direct hydride-ion transfer to aldehyde to form the common zwitterionic intermediate 14 containing C-H bound benzylate anion. This species can either directly react with dihydrogen to give the cationic η 2 -H2 complex 15 producing upon deprotonation with BnO - the starting hydride 13 and BnOH, or isomerize to the alkoxide complex 16. The hydrogenolysis of the latter proceeds via a bifunctional mechanism with a participation of acidic NH function to form the neutral η 2 -H2 complex 17 featuring a dearomatized pyridine moiety affording finally 13 upon cooperative H-H bond activation. Curiously, complex 13 being unable to hydrogenate ketones, esters and nitriles acts as efficient catalyst for hydrogenation of a priori less reactive CO2 occurring in THF/H2O at 100 °C in the presence of DBU as base and LiOTf as Lewis acid activator with TON up to 31600 [START_REF] Bertini | Carbon dioxide hydrogenation catalysed by well-defined Mn(I) PNP pincer hydride complexes[END_REF]. Scheme 7. Simplified mechanism for the hydrogenation of benzaldehyde catalyzed by Mn(I) PN 3 P complex 13 based on DFT calculations (activation energy barriers are given in kcal/mol).

The nature of central heterocyclic core in PNP pincer systems plays a crucial role for the catalytic performance, since the replacement of pyridine to more electron-rich para-substituted 2,4,6triazine resulted in PN 5 P complexes 18, which are very efficient for the hydrogenation of ketones at 80 °C (TON up to 950) using catalyst charge in a range of 0.1-1.0 mol% and ten-fold amount of tBuOK as a base (Scheme 8) [START_REF] Kallmeier | Highly active and selective manganese C=O bond hydrogenation catalysts: the importance of the multidentate ligand, the ancillary ligands, and the oxidation state[END_REF]. The electronic properties of the triazine substituent in para-position also strongly affects the catalyst productivity with NHC3H5 affording the best results. In contrast to bidentate PN catalyst 11a sterically bulky substrates and aromatic ketones bearing NO2 or CN groups could be efficiently reduced to the corresponding alcohols albeit with higher 1.0-2.0 mol% catalyst charge. Scheme 8. Hydrogenation of ketones catalyzed by Mn(I) PN 5 P complexes 18.

Pyridinyl-phosphine manganese complexes 11a-b were also shown to be viable catalysts for the hydrogenation of aldimines (Scheme 9) [START_REF] Wei | Manganese catalyzed reductive amination of aldehydes using hydrogen as a reductant[END_REF]. Complex bearing Ph2P moiety was again ca. 2 times more active than its iPr2P analogue affording 74 and 40% product yield in hydrogenation of model Ph(H)C=NPh under standard conditions using 2 mol% 11a and 11b, respectively. Noticeably, this catalytic system is water-tolerant, because the starting aldimines were obtained in situ by the condensation of aldehydes with 1.2 equivalents of the corresponding primary amines. This protocol showed a large substrate scope and target amine products were typically obtained in >90% yield. Other reducible functional groups (ketone, ketimine, ester, amide, alkene) remained completely intact and a variety of heterocyclic functions was well-tolerated. Importantly, the hydrogenation of aldimines derived from βor γ-aminoalcohols did not produce any side products from the competitive N-alkylation processes. The use of enantiomerically pure aminoalcohols and aminoesters in this Mncatalyzed reductive amination did not lead to the erosion of optical purity in the final products. Scheme 9. Hydrogenation of aldimines catalyzed by Mn(I) PN complexes 11.

While PN 3 P pincer analogue of complex 11b showed negligible activity in hydrogenation of Ph(H)C=NPh, PN 5 P complexes 18 exhibiting triazine core represent the most active to date Mn-based systems for C=N bond hydrogenation (Scheme 10) [START_REF] Freitag | Mechanistic studies of hydride transfer to imines from a highly active and chemoselective manganate catalyst[END_REF]. Contrary to ketone hydrogenation catalysis (Scheme 8) para-amino substituted species 18c-d were ca. 2 times less efficient than complexes bearing Me and especially Ph group supposing in the latter case the positive role of electronwithdrawing substituents. However, further increase of the accepting character of aryl using p-C6H4CF3 led to poor results. A variety of aldimines can be readily reduced using 0.4 mol% of 18b and ten-fold excess of tBuOK in only 4 hours at 50 °C. The presence of coordinating CN, pyridine, thiophene and SMe groups did not affect the reduction rate, while NO2 derivatives react more slowly (18 h) and require 0.6 mol% of catalyst to reach good conversion. Though, PN 5 P complex 18b was also active for ketone hydrogenation (Scheme 8), selective reduction of the C=N bond in aldimine was achieved in the presence of diarylketone moiety, thus illustrating a relevant example of selectivity control upon the change of solvent (toluene vs. THF) and reaction temperature (80 °C vs. 50 °C). Importantly, five examples of challenging ketimines incorporating CN, pyridine and pyrazine functions were efficiently reduced in 71-85% isolated yield using 1-3 mol% of 18b and 5-15 mol% of tBuOK under mild conditions (20 bar H2, 50 °C, 18 h). Scheme 10. Hydrogenation of imines catalyzed by Mn(I) PN 5 P complexes 18.

Interestingly, detailed mechanistic investigations revealed the dianionic hydride complex 19b having bis-deprotonated PN 5 P scaffold to be the key intermediate of this catalytic process (Scheme 11). Indeed, the reduction of C=N bond proceeds via the concerted outer-sphere hydride transfer to the substrate with concomitant protonation by tBuOH to form the target amine and mono-anionic 16electron species 20b regenerating the starting hydride 19b upon H2 activation in the presence of base. Scheme 11. Reaction mechanism for the hydrogenation of Ph(H)C=NPh catalyzed by Mn(I) PN 5 P complex 18b based on stoichiometric experiments and kinetic studies.

The bridge between phosphine and pyridine in PN complexes also directly influences catalytic activity (Scheme 12). Complex 21a containing CH2 unit instead of NH in 11a was less active in hydrogenation of acetophenone (Scheme 5) [START_REF] Wei | Hydrogenation of carbonyl derivatives catalysed by manganese complexes bearing bidentate pyridinyl-phosphine ligands[END_REF] and Ph(H)C=NPh (Scheme 9) [START_REF] Wei | Manganese catalyzed reductive amination of aldehydes using hydrogen as a reductant[END_REF] providing under standard conditions only 15% and 17% conversion, respectively. The replacement of pyridine moiety with quinoline in complex 21b did not affect the efficiency of ketone reduction, but almost inhibited the activity in the hydrogenation of aldimines. The same trend was observed in the tridentate series, where PNP complex (iPr2PCH2PyCH2PiPr2)Mn(CO)2H was by far less active in hydrogenation of aldehydes (Scheme 6) [START_REF] Glatz | Chemoselective hydrogenation of aldehydes under mild, base-free conditions: manganese outperforms rhenium[END_REF] than its PN 3 P analogue 13 (TON 21 vs. 2000 for p-FC6H4CHO under the same conditions). However, the group Khusnutdinova has recently shown that PN complex 21c featuring a CH2 bridge between two donor units was able to catalyze the hydrogenation of alkenes (Scheme 12) [START_REF] Rahaman | Hydrogenation of alkenes catalyzed by a non-pincer Mn complex[END_REF]. The catalytic system showed a moderate efficiency for styrenes and terminal aliphatic alkenes . The incorporation of electron-withdrawing groups or sterically bulky substituents at the C=C double bond led to further drop in yields to 23-34%. While 1,1-disubstituted styrenes can be reduced with reasonable efficiency (25-76%), internal alkenes were almost unreactive. Besides alkenes, this protocol was successfully applied to the hydrogenation of 1,5-cyclooctadiene and 1-pentadecyne selectively affording cyclooctene and pentadecane in 92% and 99% yield, respectively. Scheme 12. Hydrogenation of alkenes catalyzed by Mn(I) PN complex 21c.

Kirchner and co-workers have shown that alkene hydrogenation can be performed with much higher efficiency using σ-alkyl Mn(I) complexes 22 bearing aliphatic chelating diphosphines (Scheme 13) [START_REF] Weber | Rethinking basic conceptshydrogenation of alkenes catalyzed by bench-stable alkyl Mn(I) complexes[END_REF]. Though all complexes 22 were competent as catalysts for the hydrogenation of 1dodecene under base-free conditions providing full substrate conversion, only 22d was active at room temperature, whereas 22a and 22b-c required heating at 100 °C and 40 °C, respectively. Broad scope of mono-and 1,1-disubstituted alkenes was reduced at 25 °C in most cases in quantitative yield. In contrast to PN complex 21c, internal alkenes could be easily hydrogenated in excellent yields at slightly higher temperature (60 °C). This protocol was shown to tolerate tri-and tetra-substituted alkene fragments and unprotected alcohols. While esters and anhydride functions remained intact during hydrogenation process, α,β-unsaturated ketone (E)-PhCH=CHCOCH3 was reduced quantitatively at both C=C and C=O bonds. The reduction of pyridinyl alkene derivative was rather sluggish resulting in only 39% conversion, whereas the presence of nitrile or carboxylic groups totally inhibited the reaction.

Scheme 13. Hydrogenation of alkenes catalyzed by Mn(I) diphosphine complexes 22.

The reaction mechanism was investigated in detail by DFT calculations for the most active catalyst (Scheme 14) [START_REF] Weber | Rethinking basic conceptshydrogenation of alkenes catalyzed by bench-stable alkyl Mn(I) complexes[END_REF]. It was shown that the activation of starting σ-alkyl complex 22d proceeds as a sequence of migratory CO insertion across Mn-alkyl bond with concomitant H2 coordination, followed by hydrogenative cleavage of Mn-acyl bond in the resulting intermediate 23d to form 1butanal and 16-electron hydride complex 24d. This proposition was in full agreement with more difficult activation of complexes 22a vs. 22b-c due to the increase of the activation barrier for migratory insertion in Me>Et>Pr row and with the absence of catalytic activity for the corresponding tricarbonyl hydrides. Coordination of the substrate to the latter leads to the formation of η 2 -alkene complex 25d being probably the resting state of this catalytic cycle. The alkene insertion across Mn-H bond in 25d followed by H2 coordination affords alkyl-dihydrogen intermediate 27d liberating the propane via a formal Mn-C bond protonation and the binding of next substrate molecule to close the catalytic cycle. Scheme 14. Simplified activation scheme of pre-catalyst 22d and catalytic cycle for propene hydrogenation based on DFT study (activation energy barriers are given in kcal/mol, in case of twostep process both values separated by comma are given).

Complex 22a was also active in the hydrogenation of nitriles in the absence of base (Scheme 15) [START_REF] Weber | Old concepts, new application -additive-free hydrogenation of nitriles catalyzed by an air stable alkyl Mn(I) complex[END_REF]. Aliphatic nitriles and their aromatic analogues bearing electron-donating or electronwithdrawing substituents were readily reduced to the corresponding amines using typically 2 mol% of catalyst. The catalytic system tolerated also halogen substituents, NH2, CF3 and heterocyclic moieties and failed only for nitro derivatives. Scheme 15. Base-free hydrogenation of nitriles catalyzed by Mn(I) diphosphine complex 22a.

DFT calculations of reaction mechanism (Scheme 16) [START_REF] Weber | Old concepts, new application -additive-free hydrogenation of nitriles catalyzed by an air stable alkyl Mn(I) complex[END_REF] revealed the same activation mode for pre-catalyst 22a affording upon substrate coordination nitrile-hydride complex 28a. Intramolecular hydride transfer to the nitrile carbon atom in the latter leads to azavinylidene species 29a transforming in the presence of dihydrogen into the σ-iminyl derivative 30a and then by the protonation to the imine intermediate 31a. The reduction of coordinated imine in 31a proceeds via stepwise hydride transfer, hydrogenolysis of Mn-N bond in amide complex 32a and the coordination of next nitrile molecule. Scheme 16. Simplified catalytic cycle for PhCN hydrogenation by complex 22a based on DFT study (activation energy barriers are given in kcal/mol, in case of two-step process both values separated by comma are given).

It was also shown that diphosphine Mn(I) complex 22d, which can be activated under very mild conditions, was able to catalyze base-free ketone hydrogenation at room temperature using only 10 bar hydrogen pressure (Scheme 17) [START_REF] Weber | Manganese-catalyzed hydrogenation of ketones under mild and base-free conditions[END_REF]. Though the typical catalytic charge (3 mol%) remains quite elevated, this reaction represents a sole example of Mn-based ketone hydrogenation in the absence of base known to date. This catalytic protocol allowed to reduce a variety of aromatic and aliphatic ketones with a good tolerance to halogen, NH2 and heterocyclic moieties, but was found to be completely inefficient for nitro-and cyano-containing aromatic ketones. Interestingly, the chemoselectivity for hydrogenation of α,β-unsaturated aldehydes and ketones using complex 22d was strongly dependent on the reaction temperature. It was demonstrated that uniquely C=O bonds were reduced at 25 °C to give the corresponding allylic alcohols being opposite to the behavior of related PN system 11a (Scheme 5) [START_REF] Wei | Hydrogenation of carbonyl derivatives catalysed by manganese complexes bearing bidentate pyridinyl-phosphine ligands[END_REF]. In contrast, at 60 °C the conjugated C=C bonds seemed to be hydrogenated before C=O ones affording in fine fully saturated products. Scheme 17. Room temperature hydrogenation of ketones catalyzed by Mn(I) diphosphine complex 22d under base-free conditions.

The proposed catalytic cycle based on DFT calculations (Scheme 18) includes the initial Ocoordination of acetophenone to the unsaturated species 24d followed by inner-sphere reduction of C=O bond in the resulting complex 33d to form alkoxide intermediate 34d stabilized by a C-H agostic interaction. The subsequent coordination of H2 molecule to the latter, intramolecular proton transfer to form the intermediate 36d and the displacement of the final product by another acetophenone molecule ends up the catalytic cycle. Scheme 18. Simplified catalytic cycle for acetophenone hydrogenation by complex 22d based on DFT study (activation energy barriers are given in kcal/mol, in case of two-step process both values separated by comma are given).

The application of Mn(I) diphosphine complex 22d for CO2 hydrogenation to formate was recently reported by Kirchner, Gonsalvi and co-workers (Scheme 19) [START_REF] Kostera | Carbon dioxide hydrogenation to formate catalyzed by a bench-stable, non-pincer-type Mn(I) alkylcarbonyl complex[END_REF]. Under optimized conditions formate salt was obtained with TON values up to ca. 1000 after 24 h at 80 °C in the presence of DBU as base. The utilization of lithium triflate as Lewis acid promoter and extension of the reaction time to 48 h allowed to double the TON value leading to almost quantitative CO2 conversion.

Scheme 19. Hydrogenation of CO2 to formate catalyzed by Mn(I) diphosphine complex 22d.

The proposed catalytic cycle (Scheme 20) includes the inner-sphere hydride transfer in Ocoordinated CO2 complex 37d to afford κ 1 -O bound formate intermediate 38d stabilized by a C-H agostic interaction. The latter is in equilibrium with slightly more stable κ 2 -O form 39d representing a catalyst resting state. The catalytic cycle is completed by the coordination of dihydrogen to complex 38d, intramolecular proton transfer in the resulting η 2 -H2 intermediate 40d and base-assisted liberation of formic acid from hydride species 41d. Scheme 20. Simplified catalytic cycle for CO2 hydrogenation by complex 22d based on DFT study (activation energy barriers are given in kcal/mol).

The groups of Kirchner [START_REF] Weber | Hydrogenation of nitriles and ketones catalyzed by an airstable bisphosphine Mn(I) complex[END_REF] and García [START_REF] Garduño | Non-pincer Mn(I) organometallics for the selective catalytic hydrogenation of nitriles to primary amines[END_REF] have independently demonstrated that nitrile hydrogenation can be also efficiently catalyzed by other Mn(I) diphosphine derivatives 42 and 43 under similar conditions, but uniquely in the presence of base (Scheme 21). Both catalytic systems were able to reduce aliphatic, aromatic and heteroaromatic nitriles in good yields in the presence of other potentially reducible functional groups (ester, conjugated alkene, internal alkyne). It was shown that for complex 43 using secondary alcohols as solvents nitrile reduction can partially proceed as transfer hydrogenation (vide infra Scheme 62) and its relative contribution decreased with higher dihydrogen pressure and replacement of iPrOH with 2-BuOH. Despite higher catalyst loading complex 43 bearing diphosphine ligand with iPr groups seemed to be more active than its Prsubstituted analogue 42. As a representative example, the reduction of less reactive p-MeOC6H4CN using pre-catalyst 43 proceeded at lower temperature (90 °C vs. 100 °C), hydrogen pressure (7 vs. 50 bar), amount of base (3.3 vs. 5 equiv.) and revealed much higher TOF values (ca. 130 vs. 2.2 h -1 ). Scheme 21. Hydrogenation of nitriles catalyzed by Mn(I) diphosphine complexes 42 and 43.

While the formation of the corresponding hydride complexes fac-(R2PCH2CH2PR2)Mn(CO)3H was evidenced in both cases by NMR spectroscopy of the reaction mixtures [START_REF] Weber | Hydrogenation of nitriles and ketones catalyzed by an airstable bisphosphine Mn(I) complex[END_REF][START_REF] Garduño | Non-pincer Mn(I) organometallics for the selective catalytic hydrogenation of nitriles to primary amines[END_REF], it was demonstrated that at least one of them (R = iPr) was totally inactive under catalytic conditions and was proposed to be a catalyst deactivation product [START_REF] Garduño | Non-pincer Mn(I) organometallics for the selective catalytic hydrogenation of nitriles to primary amines[END_REF]. Unlike the case of σ-alkyl diphosphine complexes 22a-d (Schemes 16,[START_REF] Sortais | Manganese-catalyzed hydrogenation and hydrogen transfer reactions[END_REF][START_REF] Azouzi | Manganesenew prominent actor in transfer hydrogenation catalysis[END_REF], the exact mechanism of nitrile hydrogenation catalyzed by Mn(I) bromide or triflate derivatives 42 and 43 remained unclear to date. While Kirchner and coworkers proposed the existence of base-assisted outer-sphere hydride transfer from fac-(Pr2PCH2CH2PPr2)Mn(CO)3H to the substrate [START_REF] Weber | Hydrogenation of nitriles and ketones catalyzed by an airstable bisphosphine Mn(I) complex[END_REF], García and coll. postulated that catalyst activation included triflate ligand substitution in 43 with 2-BuOK, decarbonylation of the corresponding alkoxide derivative 44 and hydrogenative cleavage of the Mn-alkoxide bond with release of 2-butanol leading to the unsaturated hydride species 24d (Scheme 22) [START_REF] Garduño | Non-pincer Mn(I) organometallics for the selective catalytic hydrogenation of nitriles to primary amines[END_REF], which can then coordinate the molecule of nitrile and perform the catalytic hydrogenation similarly as depicted in Scheme 16.

Scheme 22.

Plausible route for the activation of Mn(I) diphosphine complex 43 in the presence of base.

Complex 42 was also active in ketone hydrogenation providing good yields for aromatic and cyclic aliphatic substrates (Scheme 23) [START_REF] Weber | Hydrogenation of nitriles and ketones catalyzed by an airstable bisphosphine Mn(I) complex[END_REF]. The comparison of the catalytic activity of diphosphine complex 42 with its phosphine-pyridine analogue 11a operating under very similar conditions (Scheme 5) revealed slightly higher performance of the PN-based system (TON 180 vs. 94 in hydrogenation of acetophenone using 11a and 42, respectively). Scheme 23. Hydrogenation of ketones catalyzed by Mn(I) diphosphine complex 42.

The association of phosphine and NHC donors in bidentate ligand was shown to boost significantly the catalytic activity in Mn-catalyzed ketone hydrogenation (Scheme 24) [START_REF] Buhaibeh | Phosphine-NHC manganese hydrogenation catalyst exhibiting a nonclassical metal-ligand cooperative H2 activation mode[END_REF]. Indeed, complex 45 afforded quantitative conversion of acetophenone using only 0.1 mol% catalyst loading at 60 °C in toluene and the amount of catalyst may be even reduced to 0.05 mol% at higher temperature (100 °C). Importantly, this reaction can be even better performed in tAmOH providing TON up to 6200 at 100 °C using 0.01 mol% of 45 and 1 mol% of tBuOK as base. A variety of aromatic and aliphatic ketones was efficiently reduced with this system using typically 0.1-0.5 mol% catalyst charge. In contrast to PN complex 11a sterically demanding substrates can be readily hydrogenated in 72-85% yield on 0.5 mol% amount of 45. Halogens, CF3 and pyridine groups were well tolerated, whereas the reduction of thiophene-and pyrrole-containing ketones was found to be rather sluggish.

Scheme 24. Hydrogenation of ketones catalyzed by Mn(I) phosphine-NHC complex 45.

Stoichiometric experiments revealed that complex 45 in the presence of a strong base transformed to 18-electron cyclometallated species 46 easily activating H2 even at low temperature to form hydride product 49 (Scheme 25). DFT calculations showed that this process proceeds via initial isomerization of complex 46 into non-classical NHC-phosphonium ylide intermediate 47, followed by H2 coordination and cooperative H-H bond activation in the resulting η 2 -H2 complex 48. It was found that hydride complex 49 was inactive in hydrogenation of acetophenone under base-free conditions even at 5 mol% charge, but can readily re-enter into the catalytic cycle in the presence of base.

Scheme 25. Simplified mechanism of cooperative H2 activation by NHC-phosphine complex 45 in the presence of KHMDS based on DFT calculations (activation energy barriers are given in kcal/mol).

With the aim to improve the catalytic performance of 45, the modification of this pre-catalyst by anchoring an additional phosphine or pyridine donor moiety to NHC core was then performed (Scheme 26) [START_REF] Buhaibeh | Cationic PCP and PCN NHC core pincer-type Mn(I) complexes: from synthesis to catalysis[END_REF]. However, the resulting cationic Mn(I) tricarbonyl pincer-type complexes 50-52 were much less active in acetophenone hydrogenation than parent bidentate derivative 45. The catalytic activity of these systems seemed to correlate with relative rigidity of the pincer ligand, the best results being obtained for PCN complex 52 exhibiting 5,6-membered chelating system. Since the corresponding neutral bidentate complexes fac-(Ph2PCH2NHC-L)Mn(CO)3Br displayed very similar results with cationic pincer derivatives 50-52, it can be reasonably concluded that the same catalytically competent species were formed. Better results obtained for complex 52 may be ascribed to the easier decoordination of the pyridine moiety in a less thermodynamically stable 6-membered metallacycle. Scheme 26. Hydrogenation of acetophenone catalyzed by Mn(I) pincer complexes 50-52 incorporating phosphine-NHC moiety.

Remarkable potential of NHC-phosphine containing ligands in Mn-catalyzed hydrogenation was further demonstrated by the group of Pidko (Scheme 27) [START_REF] Yang | Robust and efficient hydrogenation of carbonyl compounds catalysed by mixed donor Mn(I) pincer complexes[END_REF]. It was found that cationic complex 53 bearing NHC-amine-phosphine ligand was extremely efficient in the hydrogenation of carbonyl derivatives and aldimines providing excellent levels of conversions at 0.005-0.02 mol% (50-200 ppm) catalyst loading. Interestingly, the use of KBHEt3 as base provided much better results than conventional tBuOK, namely 3-7 fold rate acceleration and the absence of an induction period, which was rationalized in terms of more efficient pre-catalyst activation (vide infra). Under optimized conditions, acetophenone could be fully reduced after 9 h using only 5 ppm of catalyst charge in dioxane at 120 °C providing TON of ca. 200000. Detailed kinetic studies revealed that this catalytic system was on the first order on the pre-catalyst 53 and had a TOF value more than 40000 h -1 . A variety of arylketones was successfully hydrogenated using 50 ppm of catalyst including sterically hindered substrates. Interestingly, the reduction of dialkylketones and related compounds containing amino groups was possible at lower temperature (80 °C) in isopropanol. While potentially reducible ester group was tolerated under the latter conditions albeit with full transesterification, α,βunsaturated ketone such as benzylideneacetone was unexpectedly found to be totally unreactive. Aromatic and heteroaromatic aldehydes could be selectively hydrogenated in iPrOH without any undesired condensation processes. Alkyl formates were also amenable substrates for this catalytic system leading to full conversion in dioxane at 120 °C with only 0.02 mol% of catalyst charge. Finally, catalyst 53 remained for the moment the most productive for hydrogenation of aromatic aldimines (TON of 900 vs. 245 for Ph(H)C=NPh using 53 and 18b, respectively).

Scheme 27. Hydrogenation of aldehydes and ketones catalyzed by Mn(I) pincer complex 53.

Mechanistic experiments revealed the significant difference in the activation of complex 53 using tBuOK and KBHEt3. It was shown that neutral amide species 54 obtained in a quantitative manner upon deprotonation of the starting material with tBuOK (Scheme 28) reacted with dihydrogen very sluggishly to form pincer hydride species 55 even at 50 °C. In contrast the reaction of 53 with KBHEt3 instantaneously led to the formation of 54 and tricarbonyl hydride product 56 bearing bidentate NHC-amine ligand with pendant phosphine moiety. Noteworthy, NMR monitoring of the stoichiometric reaction between the mixture 55/56 and acetophenone showed higher reactivity of the bidentate complex. Though a slow decarbonylation of 56 into 55 was possible, the realization of two distinct catalytic cycles involving hydride intermediates with both bi-and tridentate ligand coordination modes cannot be excluded. Scheme 28. Activation of Mn(I) pincer complex 53 using tBuOK. Scheme 29. Activation of Mn(I) pincer complex 53 using KBHEt3.

The group of Hu has found that bidentate ligands incorporating carbon-based donors applicable in Mn-catalyzed hydrogenation are not restricted to NHCs (Scheme 30) [START_REF] Pan | A catalytically active [Mn]-hydrogenase incorporating a non-native metal cofactor[END_REF][START_REF] Pan | Biomimetic hydrogenation catalyzed by a manganese model of [Fe]hydrogenase[END_REF]. It was shown that Mn(I) tetracarbonyl complexes 57 and 58 bearing chelating pyridine-acyl and pyridine-carbamoyl ligands, respectively demonstrated modest activity in hydrogenation of aldehydes and ketones in the presence of N-methylpyrrolidine (MP) weak base. Despite complex 57 contains non-innocent 2hydroxypyridine fragment, its analogue 58 having a priori no cooperative functionalities was much more active in hydrogenation of ketones and imines providing ca. 2-, 7-and 9.5-fold increase of TON values for the reduction of acetophenone, Ph(H)C=NPh and Ph(Me)C=NPh, respectively. Though the activity of catalyst 58 was far below the best Mn-based catalytic systems, it was capable to catalyze efficiently the hydrogenation of aryl-substituted imidazolinium salts failed for Mn(I) complexes 1 (Scheme 2) and 9 (Scheme 4, R = iPr) bearing PN and PNP ligands, respectively. Scheme 30. Hydrogenation of aldehydes, imines and imidazolinium salts using Mn(I) complex 58.

Such C=N bond hydrogenation was then used as one of key steps for the asymmetric relay hydrogenation of benzoxazinones and benzoxazines using a combination of catalyst 58 with planarchiral ferrocene derivative 59 (Scheme 31) [START_REF] Pan | Biomimetic hydrogenation catalyzed by a manganese model of [Fe]hydrogenase[END_REF]. The reduction of the latter produced the saturated compound 60 capable to perform the enantioselective hydride transfer to these substrates in the presence of La(OTf)3 as Lewis acid. This protocol allowed to get the target products in good-toexcellent yields and useful level of enantioselectivity. Scheme 31. Relay hydrogenation of benzoxazinones and benzoxazines using the combination of Mn(I) complex 58 and planar-chiral ferrocene-based imine 59.

The association of two 2-hydroxypyridine units in the bidentate NN ligand was successfully used by Khusnutdinova, Nervi and co-workers for Mn-catalyzed CO2 hydrogenation to formate or diethylformamide (Scheme 32) [START_REF] Dubey | Bio-inspired Mn(I) complexes for the hydrogenation of CO2 to formate and formamide[END_REF]. Under optimized conditions up to 6250 TON for the formation of DBU formate salt was achieved with complex 61a in acetonitrile at 65 °C. The corresponding cationic complex [(NN)Mn(CO)3(MeCN)](OTf) displayed the same performance thus assuming a facile dissociation of the bromide ligand in 61a in highly polar medium. This Mn(I) NN system was ca. 3 times superior in terms of TON than PP complex 22d (Scheme 19) [START_REF] Kostera | Carbon dioxide hydrogenation to formate catalyzed by a bench-stable, non-pincer-type Mn(I) alkylcarbonyl complex[END_REF] under lower temperature and optimal H2/CO2 ratio (1:1 vs. 2:1 for 61a and 22d, respectively). The presence of hydroxyl groups in the proximity to the metal center in complex 61a was shown to be a critical factor since its parahydroxylated 61b or methylated 61c analogues revealed negligible catalytic activity (TON 18 and 160, respectively). Importantly, diethylformamide could be also prepared using this catalytic system with TON up to 588, but higher temperature and dihydrogen pressure were necessary in this case. Scheme 32. Hydrogenation of CO2 to formate and diethylformamide catalyzed by Mn(I) complex 61a bearing bidentate NN ligand.

The mechanism of this reaction was studied by DFT calculations by the groups of Pati [START_REF] Das | Mechanistic insights into catalytic CO2 hydrogenation using Mn(i)-complexes with pendant oxygen ligands[END_REF] and Yang [START_REF] Yan | Unexpected concerted two-proton transfer for amination of formic acid to formamide catalysed by Mn bipyridinol complexes[END_REF] (Scheme 33). It was proposed that pre-catalyst 61a in the presence of large excess of DBU could undergo single or double deprotonation to afford neutral 62a and anionic 63a intermediates, respectively each of them containing one dearomatized pyridine moiety. Both compounds in the presence of dihydrogen transform into the corresponding η 2 -H2 complexes 64a and 65a, which can undergo either a cooperative H-H bond activation assisted by the pendant aryloxide moiety or the H-H bond deprotonation with external molecule of DBU to afford hydride complexes 66a-68a differing in the overall molecule charge. The activation barriers for DBU-assisted routes are lower than for cooperative activation, especially for monoanionic complex 65a. Direct hydride transfer from complexes 66a-68a to CO2 afforded the intermediates 69a-71a exhibiting a formate anion hydrogenbonded to Mn(I) center regenerating the corresponding dihydrogen complexes by the subsequent release of the product. The energy barriers for CO2-to-formate conversion gradually decreased in 66a→67a→68a row being consistent with the increased hydricity of anionic species. While the lowest energy profile 63a→65a→68a→71a includes the double deprotonation of NN ligand, the formation of anionic complex 63a was not detected by IR spectroscopy in the reaction of 61a with six equivalents of DBU. Though this pathway can not be totally excluded, it is more probable that the catalytic cycle is mainly operating via monodeprotonated intermediate 62a presented in high concentration along the sequence 62a→64a→67a→70a being only marginally higher in energy than the previous one. Scheme 33. Different mechanistic pathways for CO2 hydrogenation to formate in acetonitrile catalyzed by Mn(I) complex 61a based on DFT calculations (activation energy barriers are given in kcal/mol).

It was also shown by Yang and coworkers that hydroxyl groups in complex 61a play a critical role in the transformation of CO2 into diethylformamide (Scheme 34) [START_REF] Yan | Unexpected concerted two-proton transfer for amination of formic acid to formamide catalysed by Mn bipyridinol complexes[END_REF]. The catalytic cycle in this case consists in the O-coordination of formic acid molecule obtained from CO2 as shown in the Scheme 33 to Mn(I) complex 62a followed by the nucleophilic attack of diethylamine to the resulting intermediate 72a via the transition state 73a, in which both oxygen atoms simultaneously participate in N-H bond deprotonation and O-H bond formation. The resulting intermediate 74a easily loses the target diethylformamide and water with regeneration of starting complex 62a. Importantly, the presence of two proton relay sites in 72a leads to a very low activation barrier (8.1 kcal/mol), whereas alternative mechanisms for Mn-catalyzed process without direct participation of NN ligand or direct reaction between HCOOH and HNEt2 are much more energy demanding (>40 kcal/mol). Scheme 34. Simplified mechanism for CO2 hydrogenation to formamide catalyzed by Mn(I) complex 61a (activation energy barriers are given in kcal/mol).

The relevance of related Mn(I) hydride complexes bearing aromatic nitrogen-based (75-81), [START_REF] Rawat | Catalytic hydrogenation of CO2 by manganese complexes: role of π-acceptor ligands[END_REF][START_REF] Rawat | Flexible proton-responsive ligand-based Mn(I) complexes for CO2 hydrogenation: a DFT study[END_REF] phosphine-NHC (49) [START_REF] Das | Unraveling the catalytically preferential pathway between the direct and indirect hydrogenation of CO2 to CH3OH using N-heterocyclic carbene-based Mn(I) catalysts: a theoretical approach[END_REF] and bis(NHC) (82) [START_REF] Jing | The potential of d 6 non-noble metal NHC catalysts for carbon dioxide hydrogenation: group and row effects[END_REF] ligands for CO2 hydrogenation was predicted by DFT calculations (Scheme 35), but to date none of them have been tested experimentally.

Interestingly, it was recently shown by Beller and co-workers that Mn(I) complexes bearing bidentate NN ligands were highly active in the reverse reactiondehydrogenation of formic acid to form CO2 and H2 (Scheme 36) [START_REF] Léval | Hydrogen production from formic acid catalyzed by a phosphine free manganese complex: investigation and mechanistic insights[END_REF][START_REF] Léval | Manganese(I) κ 2 -NN complex-catalyzed formic acid dehydrogenation[END_REF]. The catalytic system was initially optimized for aqueous triglyme medium (conditions A) [START_REF] Léval | Hydrogen production from formic acid catalyzed by a phosphine free manganese complex: investigation and mechanistic insights[END_REF] and it was found that the presence of at least one cyclic N=C-N fragment in ligand structure was crucial for good catalytic activity. The methylation of noncoordinated nitrogen atom of the imidazoline did not significantly alter the catalytic activity, as catalysts 83a and 83b displayed similar performance. Importantly, the activity of well-defined Mn(I) complex 83a was identical to the catalytic system prepared in situ from Mn(CO)5Br and pyridineimidazoline. Kinetic experiments revealed a strong decrease of the reaction rate catalyzed by 83a after ca. 30 min corresponding to ca. 8% of formic acid conversion, which was attributed to the increasing pH of the medium. Since low pH values due to excess of formic acid were also detrimental for catalysis, this problem was solved by continuous slow addition of HCOOH revealing a maximal TON value of 5763 after 45 h of reaction time corresponding to 67% of substrate conversion. Later on it was found that the replacement of KOH base by dimethyl(octyl)amine allowed to achieve better substrate conversion without slow addition of formic acid due to more efficient pH control (conditions B) [START_REF] Léval | Manganese(I) κ 2 -NN complex-catalyzed formic acid dehydrogenation[END_REF]. Under these conditions bis-imidazoline complex 85a was more active than previously used 83a providing TON of 3089 and 6738 after 3 h and 12 h, respectively, corresponding in the latter case to full conversion. The stability of this catalytic system was also demonstrated by a long-term experiment using the regular addition of new portions of HCOOH (12 cycles during 4 days) revealing an almost unchanged H2 evolution rate with a final TON value of 7883. Notably, CO content in the resulting CO2/H2 mixture under these conditions (39-60 ppm) was much lower than using catalyst 83a (757-832 ppm, conditions A), which is of critical importance for application in fuel cells.

The proposed mechanism of formic acid dehydrogenation (Scheme 37) included the initial formation of formate complex 87a, detected by NMR in the reaction mixture and characterized by Xray diffraction in the case of pre-catalyst 83a. The decarboxylation of the latter species led to the hydride 88a, further undergoing the protonation to η 2 -H2 intermediate 89a and dihydrogen loss closing the catalytic cycle. The observation of significant KIE for DCOOD/DCOOK using both H2O

(1.89) and D2O (2.08) was consistent with decarboxylation of complex 87a as a rate determining step. This mechanism clearly explained the critical role of pH in order to maintain in the reaction medium sufficient concentration of both formate anion required for the formation of 87a and free acid necessary for the efficient decomposition of the hydride 88a. Scheme 37. Mechanism for formic acid dehydrogenation catalyzed by Mn(I) complex 83a bearing pyridine-imidazoline ligand.

Very recently, Topf and coll. showed that catalytic system generated in situ from Mn(CO)5Br and 2-picolylamine in the presence of base was active for hydrogenation of ketones and alkenes bearing electron-withdrawing groups (Scheme 38) [START_REF] Vielhaber | Manganese-catalyzed homogeneous hydrogenation of ketones and conjugate reduction of α,β-unsaturated carboxylic acid derivatives: a chemoselective, robust, and phosphine-free in situ-protocol[END_REF]. The 1:1 metal/ligand ratio was found to be optimal providing 99% product yield upon hydrogenation of acetophenone under rather harsh conditions (3 mol% of catalyst, 3 mol% tBuOK, 50 bar H2, THF, 120 °C, 12 h), whereas two-fold excess of diamine ligand led to inferior results (25% alcohol yield). Interestingly, the catalytic activity of well-defined diamine complex (NN)Mn(CO)3Br at the same catalytic charge was much lower affording only 8% product yield. Though a variety of non-functionalized aromatic and aliphatic ketones was reduced in nearly quantitative yield using 3 mol% catalyst, this system remains significantly less active compared to the Mn(I) complexes bearing PN, PP and PC ligands (vide supra). While F, Cl, CF3 and CN groups were well tolerated the hydrogenation of bromo-and iodosubstituted aromatic ketones led to only 4-23% yields. The reduction of α,β-unsaturated esters proceeds selectively at the C=C bond to form the target products in good-to-excellent yields, albeit two-fold increase of catalyst charge was required in some cases. Scheme 38. Hydrogenation of ketones and α,β-unsaturated esters catalyzed by Mn(I) catalyst generated in situ from Mn(CO)5Br and equimolar amount of 2-picolylamine and tBuOK.

Interestingly, Beller and co-workers have demonstrated that hydrogenation of quinolines can be performed efficiently using simple Mn(CO)5Br precursor without any auxiliary ligands (Scheme 39) [START_REF] Papa | Development of a practical non-noble metal catalyst for hydrogenation of N-heteroarenes[END_REF]. A variety of diversely-substituted tetrahydroquinolines was obtained under mild conditions (typically 15 bar of H2, 45 °C, 18 h) using 2 mol% of pre-catalyst. Halogens, triflate, CF3, pyrazole or unprotected OH groups were readily tolerated and this protocol could be also extended to the reduction of other fused N-heterocycles, namely isoquinoline, quinoxaline, acridine and naphthyridine. Importantly, this catalytic system was particularly chemoselective for hydrogenation of quinoline ring leaving intact other reducible fragments including ketones, esters, nitriles and conjugated C=C bonds. Only for 3-acetoxy-and 3-cyanoquinolines small amounts of over-reduction products were detected affording however acceptable yields of target tetrahydroquinolines (47 and 65%, respectively). It is noteworthy that the use of 2.5 mol% of Mn(I) PNP complex (HN(CH2CH2PiPr2)2)Mn(CO)2Br for hydrogenation of quinoline provided only 9% yield even under more forcing conditions (50 bar H2, THF, 140 °C, 18 h) thus demonstrating remarkable advantage of using auxiliary ligand-free Mn-based catalytic system. Scheme 39. Hydrogenation of quinolines catalyzed by Mn(CO)5Br.

The catalytic cycle for quinoline hydrogenation based on detailed mechanistic studies is shown in Scheme 40. The key step of this transformation involved the hydrogenative cleavage of Mn-Br bond in Mn(CO)5Br to form the corresponding Mn(I) hydride 90 and HBr instantaneously trapped with basic substrate. The reduction of the resulting activated quinolinium salt was likely to proceed as 1,4-hydride addition with concomitant regeneration of starting Mn(CO)5Br to form 1,4dihydroquinoline. The latter undergoes the rearrangement into 3,4-dihydroquinoline followed by protonation and the reduction of activated C=N bond. It was also shown that Mn(CO)5Br partially disproportionated under reaction conditions to Mn2(CO)10 -an additional source of Mn(CO)5H by hydrogenative metal-metal bond cleavage and MnBr2, which could serve as Lewis acid additive for substrate activation. Scheme 40. Proposed catalytic cycle for hydrogenation of quinolines catalyzed by Mn(CO)5Br.

Shortly after the publication of Beller, the group of Wang independently reported the hydrogenation of quinolines and imines catalyzed by Mn(CO)5Br and Mn2(CO)10 (Scheme 41) [START_REF] Wang | Simple manganese carbonyl catalyzed hydrogenation of quinolines and imines[END_REF]. The use of 3 mol% Mn2(CO)10 also led to a full quinoline conversion, thus showing that acidic activation of the substrate with HBr (Scheme 40) was not mandatory under harsh conditions. Importantly, aromatic aldimines and ketimines could be hydrogenated with excellent yields (Scheme 41). Though catalyst charge remained quite elevated at the moment, this system clearly illustrates the potential of simple acidic Mn(I) carbonyl hydrides for the hydrogenation of C=N bonds in the presence of Brønsted or Lewis acid as activators.

Scheme 41. Hydrogenation of quinolines and imines catalyzed by Mn(CO)5Br and Mn2(CO)10.

Transfer hydrogenation processes

The first example of transfer hydrogenation catalyzed by Mn(I) complexes 91 bearing bidentate 2-picolylamine ligands was reported in 2017 by Sortais and coll. (Scheme 42) [START_REF] Bruneau-Voisine | Transfer hydrogenation of carbonyl derivatives catalyzed by an inexpensive phosphine-free manganese precatalyst[END_REF]. Acetophenone could be quantitatively reduced to the corresponding alcohol using isopropanol as a hydrogen donor only after 15 min of heating at 80 °C in the presence of 0.5 mol% of pre-catalyst 91a and two-fold excess of tBuOK as base. The amount of catalyst could be divided by ten still affording 58% conversion in 10 min and a complete consumption of the starting material after 2 h. A series of experiments at 0.01-0.1 mol% catalyst loading revealed the average TOF of 3600 h -1 and maximum TON of 2000 in this reaction. While complex 91b bearing N-methylated ligand containing NHMe moiety was only slightly less performant than 91a, the analogous derivatives 91c and 91d incorporating NMe2 and OH donor extremities, respectively showed negligible catalytic activity. Scheme 42. Transfer hydrogenation of ketones, aldehydes and imines catalyzed by Mn(I) complex 91a bearing 2-picolylamine ligand. This protocol showed a spectacular application scope and functional group tolerance. Both aromatic and aliphatic ketones were readily reduced including sterically bulky and highly electronrich substrates. Aldehydes could also serve as suitable substrates providing the primary alcohol products with low amounts (up to 5-7%) of aldol-like condensation products. Remarkably, the reaction can readily proceed even at 30 °C, albeit much more slowly (16 h), with similar or sometimes even better efficiency. Halogens, CF3, aromatic amines and heterocycles were well-tolerated and precatalyst 91a was exceptionally selective for activated C=O bonds, as other potentially reducible fragments such as ester, amide, CN, NO2, terminal alkyne and even conjugated C=C bond were almost not affected. The identified limitations of this catalytic system include 2-acetylpyridine or 2acetylfuran and 1,3-diketones, probably due to catalyst poisoning by strong chelating coordination. Finally, an equimolar mixture of Mn(CO)5Br and 2-picolylamine could be readily used instead of well-defined pre-catalyst 91a affording similar levels of conversion [START_REF] Wang | Practical (asymmetric) transfer hydrogenation of ketones catalyzed by manganese with (chiral) diamines ligands[END_REF].

The same Mn(I) complex 91a was also found to be active in transfer hydrogenation of aldimines under similar conditions (Scheme 42), though the use of higher catalyst charge (2 mol%) was required in this case [START_REF] Wei | Manganese-catalyzed transfer hydrogenation of aldimines[END_REF]. The reduction of aromatic aldimines with this catalytic system was rather efficient to give the amines in excellent yields, whereas the transformations for the substrates derived from aliphatic amines proceeded sluggishly (12-38% yield) and ketimines were found to be unreactive at all. Similar functional group tolerance as in the previous case [START_REF] Bruneau-Voisine | Transfer hydrogenation of carbonyl derivatives catalyzed by an inexpensive phosphine-free manganese precatalyst[END_REF] was observed with notable exceptions of NO2-containing substrate totally deactivating the catalyst and highly efficient reduction of 2-iminosubstituted nitrogen heterocycles.

The mechanism of the transfer hydrogenation of ketones using Mn(I) complex 91a (Scheme 43) was studied experimentally [START_REF] Bruneau-Voisine | Transfer hydrogenation of carbonyl derivatives catalyzed by an inexpensive phosphine-free manganese precatalyst[END_REF] and by DFT calculations [START_REF] Zhou | The origin of different driving forces between O-H/N-H functional groups in metal ligand cooperation: mechanistic insight into Mn(i) catalysed transfer hydrogenation[END_REF]. The deprotonation of 91a led to the formation of Mn(I) amide intermediate 92a, which can either rapidly form the corresponding alkoxide complex 94a upon reaction with isopropanol or dimerize to the binuclear species 93a, isolated in the absence of iPrOH and characterized by X-ray diffraction [START_REF] Bruneau-Voisine | Transfer hydrogenation of carbonyl derivatives catalyzed by an inexpensive phosphine-free manganese precatalyst[END_REF]. Importantly, welldefined dimer 93a was shown to be a competent catalyst under base-free conditions requiring however some induction period to dissociate into the mononuclear species. Alkoxide complex 94a underwent a β-hydride elimination to form acetone with an activation barrier of 13.1 kcal/mol resulting in the thermodynamically favorable formation of the hydride intermediate 95a, which regenerated the amido complex 92a upon the reaction with acetophenone followed by elimination of the product. According to DFT calculations low efficiency of the complex 91d bearing N,O donor extremities was explained by much greater stability of the alkoxide complex 94d making the formation of the key hydride intermediate 95d highly endothermic process. Scheme 43. Simplified mechanism of transfer hydrogenation of acetophenone catalyzed by Mn(I) complex 91a based on DFT calculations (activation energy barriers are given in kcal/mol).

Interestingly, the formal substitution of one hydrogen atom in NH2 moiety of Mn(I) complex 91a by a second 2-picolyl fragment was shown to produce a detrimental effect to the activity in transfer hydrogenation (Scheme 44) [START_REF] Perez | Molecularly defined manganese pincer complexes for selective transfer hydrogenation of ketones[END_REF]. Indeed, for cationic Mn(I) complex 97a bearing tridentate NNN ligand prolonged heating at 70 °C was required to achieve 96% conversion of acetophenone using 1 mol% of catalyst (TOF of 4 h -1 vs. ca. 3600 h -1 for less elaborated 91a). The substrate scope was quite large, but for many acyclic aliphatic or heteroaromatic ketones increased catalyst charge (5 mol%) was required to access good levels of conversion. In contrast to the bidentate series 91a-c, the presence of NH moiety in the coordination sphere of manganese was not mandatory for NNN ligands as complexes 97a and 97b showed virtually the same catalytic performance thus presuming the existence of another mechanism for the transfer hydrogenation of ketones. Scheme 44. Transfer hydrogenation of ketones and transfer dehydrogenation of secondary alcohols catalyzed by Mn(I) NNN complexes 97.

It is noteworthy, that both Mn(I) NNN complexes 97 were able to catalyze more efficiently the reverse reaction, namely the transfer dehydrogenation of secondary alcohols using acetone as hydrogen acceptor (Scheme 44) [START_REF] Budweg | Transfer-dehydrogenation of secondary alcohols catalyzed by manganese NNN-pincer complexes[END_REF]. The use of well-defined complexes 97 or their generation in situ from Mn(CO)5Br and the corresponding bis(picolyl)amines afforded the same results. Remarkably, in this transformation, NH-containing complex 97a was slightly less active than its analogue 97b bearing uniquely tertiary amine donors affording 78% and 97% yield of acetophenone after 30 min of reaction, respectively. Alkyl(aryl)-substituted alcohols were more easily dehydrogenated using this protocol (2 h of reaction using 0.5 mol% of 97b), whereas fully aliphatic derivatives required 10-fold increase of both catalyst charge and reaction time. The reaction tolerated halogens, primary and tertiary amines, esters and α,β-unsaturated C=C bonds providing in the latter case excellent yields of target products without any side processes.

In 2019, Clark and coll. have demonstrated that cationic complexes 98 incorporating chiral ferrocenyl phosphine arm in picolylamine scaffold could be successfully applied in asymmetric transfer hydrogenation of ketones (Scheme 45) [START_REF] Widegren | Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation[END_REF]. This system was capable to operate at relatively low catalyst loading at 50 °C with TOF values (1024-1360 h -1 at 50 °C) comparable to those of NN prototype 91a (ca. 3600 h -1 at 80 °C), but required much higher amount of base (2 vs. 25-50 equivalents for 91a and 98, respectively). Despite relatively modest enantiomeric induction levels were obtained in ketone reduction using Mn(I) complexes 98, these species were shown to be much more performant in hydrogenation processes using molecular dihydrogen [START_REF] Widegren | Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation[END_REF].

Importantly, PNN complexes 98 were shown to catalyze transfer hydrogenation of esters using ethanol as hydrogen donor (Scheme 45) [START_REF] Oates | Manganese-catalysed transfer hydrogenation of esters[END_REF]. The presence of the additional phosphine moiety in complexes 98 was crucial in this case as complex 91a showed very low activity (15% conversion and 2% yield of the target alcohol), which can be tentatively attributed to higher nucleophilic character of the corresponding PNN hydride intermediate facilitating the reaction with less active carbonyl substrate. The replacement of ethanol by methanol led to much lower conversion mostly affording trans-esterification product, whereas the use of secondary alcohol 2-butanol as hydrogen donor was completely inefficient. Under optimal conditions, a variety of alkyl esters (Me, Et, iPr) derived from aromatic or aliphatic acids was reduced with good efficiency. While this protocol tolerated halogens or heterocyclic moieties, the substrates bearing free OH, NH2 and CN groups were identified as limitation. The use of optically pure α-substituted and α,β-unsaturated esters led to chiral center racemization and fully reduced products, respectively. Scheme 45. Transfer hydrogenation of ketones and esters catalyzed by Mn(I) complexes 98 bearing PNN-type ligands.

The proposed reaction mechanism (Scheme 46) includes the deprotonation of the starting complex 98c into the corresponding amido species 99c, which undergoes a sequence of decarbonylation, EtOH activation and β-elimination processes to give the hydride intermediate 101c and acetaldehyde being in the equilibrium with the corresponding anionic acetal derivative. Nucleophilic attack of the hydride in 101c to the ester substrate affords the Mn(I) hemiacetal intermediate 102c forming by a similar sequence of transformations the corresponding aldehyde finally hydrogenated to the target primary alcohol products.

Very recently, the group of Maji demonstrated that the association of picolylamine motif with hemilabile O-or S-based arms in Mn(I) complexes 105-106 provided excellent results in external base-free transfer hydrogenation of nitriles using ammonia-borane as hydrogen donor under mild conditions (Scheme 47) [START_REF] Sarkar | Phosphine-free manganese catalyst enables selective transfer hydrogenation of nitriles to primary and secondary amines using ammonia-borane[END_REF]. The outcome of the reaction for aromatic nitriles ArCN depended drastically on the nature of solvent affording selectively primary amines ArCH2NH2 in hexane and secondary amines (ArCH2)2NH in isopropanol, the latter product arisen from the condensation between the transient imine intermediates ArCH=NH and ArCH2NH2 being facilitated in this polar solvent. Noticeably, the realization of the latter reaction in the presence of slight excess (1.5 equivalents) of the external aliphatic or aromatic amines RNH2 led to the exclusive formation of unsymmetrical amines ArCH2NHR. Aliphatic nitriles could be also efficiently reduced to primary amines in excellent yields. This protocol tolerated well the presence of halogen or ester groups, but ketone function was found to be reduced under these reaction conditions. Scheme 46. Proposed mechanism for transfer hydrogenation of esters catalyzed by Mn(I) complex 98c bearing PNN-type ligand.

Scheme 47. Chemo-divergent transfer hydrogenation of nitriles with ammonia-borane catalyzed by Mn(I) complexes 105-106 bearing hemilabile SNN-type ligands. Though all Mn(I) pre-catalysts 105-106 were active in both reaction media, their relative performance were rather different with a net preference for SNN-based ligands. Indeed, complexes 105b and 106b were almost equally active in transfer hydrogenation of PhCN in hexane being ca. 3 times more efficient in terms of product yield than O-based analogues 105a and 106a. For the synthesis of symmetrical secondary amines, the use of 105b exhibiting thiophene arm was found to be optimal (91% vs. 47-60% yield for other pre-catalysts), whereas pre-catalyst 106b provided remarkable selectivity for the transfer hydrogenation process in the presence of external primary amine.

The reaction mechanism (Scheme 48) was investigated in detail experimentally and by DFT calculations for complex 106b [START_REF] Sarkar | Phosphine-free manganese catalyst enables selective transfer hydrogenation of nitriles to primary and secondary amines using ammonia-borane[END_REF]. The reaction started by the deprotonation of the latter by traces of NH3 present in ammonia borane and decarbonylation to give 16-electron complex 107b bearing SNN pincer ligand. The latter can dehydrogenate H3N-BH3 to form the hydride species 110b and H2N=BH2 either via concerted outer-sphere process or via stepwise inner-sphere mechanism including cooperative activation of the N-H bond, decoordination of the sulphur moiety to form the intermediate 109b exhibiting agostic B-H interaction and β-hydride elimination. While the calculated activation energies for both pathways were very similar revealing maximal barriers of 20.0 and 21.5 kcal/mol for outer-and inner-sphere processes, KIE measurements clearly showed the implication of B-H and N-H bond activation processes in rate-determining step thus being consistent with concerted ammonia-borane dehydrogenation. Similarly, the hydrogenation of nitrile to imine can proceed by simultaneous hydride/proton transfer or as a sequence of the S-arm dissociation with concomitant binding of PhCN molecule to the metal, intramolecular hydride transfer in the resulting 111b with a re-coordination of S-donor and finally imine liberation upon proton transfer in 112b from NH moiety of the pincer ligand. While again, calculations data did not allow to distinguish between two mechanisms (17.5 vs. 19.0 kcal/mol maximal activation barriers for outer-and inner-sphere Scheme 48. Simplified mechanism for the transfer hydrogenation of benzonitrile with ammoniaborane catalyzed by Mn(I) complex 106b based on DFT study and KIE measurements (activation energy barriers are given in kcal/mol, n.r.transition state was not located). processes, respectively), catalytic tests in the presence of coordinating ligands (pyridines, PPh3) revealed a significant drop of activity. Taking into account that addition of PPh3 did not affect the step of amine-borane dehydrogenation in the absence of nitrile, these data showed that nitrile reduction proceeded at least partially via inner-sphere mechanism highlighting the vital importance of hemilabile S-donor group for the catalytic cycle. Finally, the chemo-divergent character of this transfer hydrogenation protocol can be rationalized in terms of relative difference in the reaction rates between the hydrogenation of imine intermediates ArCH=NH and their condensation with primary amines ArCH2NH2 depending on the solvent polarity and the nature of Mn(I) catalyst.

The simplicity of picolylamine ligand scaffold and the efficiency of the corresponding Mn(I) complex 91a in transfer hydrogenation inspired many research groups to develop similar catalytic systems by the replacement of the pyridine moiety by another donor fragment. In 2018 Leitner and coll. reported Mn(I) complexes 113 based on aminotriazole ligands active in transfer hydrogenation of ketones (Scheme 49) [START_REF] Martínez-Ferraté | Aminotriazole Mn(I) complexes as effective catalysts for transfer hydrogenation of ketones[END_REF]. Similarly, as it was observed for picolylamine derivatives 91 the presence of the NH moiety coordinated to the metal was essential for catalytic activity (complex 113c was almost inactive) and the alkylation of the NH2 group resulted in slightly lower efficiency (90% vs. 83% of acetophenone conversion using complexes 113a and 113b, respectively). While the catalytic system based on complex 113a readily reduced aromatic ketones, transfer hydrogenation of aliphatic substrates worked in less efficient manner affording only 47-76% conversion. In contrast to catalyst 91a the presence of nitro group in aromatic ketones induced a serious drop in the reaction efficiency (19-31% yield). Interestingly, for α,β-unsaturated ketones E-ArCH=CHCOMe bearing phenyl and 2-furfuryl groups different chemoselectivity of the C=C/C=O bonds reduction was observed giving fully saturated and allylic alcohols in 85 and 88% yield, respectively. The comparison between aminopicolyl and aminotriazole Mn(I) complexes 91a and 113a revealed significant decrease of catalytic activity in the latter case (TOF of 1.5 h -1 vs. 3600 h -1 for the reduction of acetophenone at 80 °C), which can be attributed not only to the pyridine-to-triazole change, but also to the increasing size of chelating system (five-vs. six-membered metallacycle, respectively).

Scheme 49. Transfer hydrogenation of ketones catalyzed by Mn(I) complexes 113 bearing aminotriazole ligands.

Later in 2019 the group of Kundu prepared a series of Mn(I) complexes 114 based on various amino-benzimidazole ligands having five-membered metallacycle similarly as in the prototype 91a (Scheme 50) [START_REF] Ganguli | Cooperative Mn(I)-complex catalyzed transfer hydrogenation of ketones and imines[END_REF]. Unlike picolylamine (Scheme 42) and amino-triazole (Scheme 49) systems, the mono-methylation of the NH2 moiety had a slightly positive effect to the catalytic performance being even more pronounced for phenyl-substituted ligand affording in the reduction of acetophenone 62%, 68% and 96% yields for 114a, 114b and 114c, respectively). Noteworthy, the N-H bond in the benzimidazole seemed to play a role in catalysis since the complex 114d was found to be ca. two times less active than 114c. As in the case of picolylamine-based NN and NNN ligands (Scheme 44), the introduction of additional nitrogen substituent into amino-benzimidazole scaffold in complex 115 almost killed any catalytic activity. Scheme 50. Transfer hydrogenation of ketones, aldehydes and aldimines catalyzed by Mn(I) complex 114c bearing amino-benzimidazole ligand.

Interestingly, significant TOF (ca. 200-250 h -1 ) and TON (450-500) values were achieved for 114c regardless some contribution of background Meerwein-Ponndorf-Verley reduction (25% conversion without Mn(I) catalyst under the same conditions). Under optimized conditions, a variety of ketones was successfully reduced at 90 °C using only 0.2 mol% of pre-catalyst 114c, but the presence of large excess of iPrONa as base (20 mol%, 100 equivalents) was essential. Aromatic and aliphatic aldehydes could also serve as good substrates for this reaction, the latter however being more impacted by aldol-type side reactions due to a higher amount of base present in solution. This catalytic system displayed similar functional group tolerance as picolylamine catalyst 91a allowing in particular a net preservation of the conjugated C=C bonds. Importantly, complex 114c was active in transfer hydrogenation of imines albeit in much higher catalyst charge (5 mol%) and under more forcing conditions. Unlike catalyst 91a (Scheme 42) aldimines derived from aliphatic amines and aromatic ketimines could be efficiently reduced with 114c being actually a sole Mn-catalyst capable to perform transfer hydrogenation of the latter substrates.

DFT calculations of transfer hydrogenation mechanism for the pre-catalyst 114c (Scheme 51) showed that both key steps of the process namely iPrOH dehydrogenation with amido species 116c and acetophenone hydrogenation with the hydride 118c proceeded via concerted outer-sphere pathway with low activation barriers of about 11.2 and 11.7 kcal/mol, respectively. In contrast, innersphere mechanism operating in the case of picolylamine complex 91a (Scheme 43) was found to be unlikely for amino-benzimidazole system due to much higher activation energies for β-hydride elimination from the alkoxide 117c and direct reaction of hydride 118c with acetophenone to form 119c (32.9 and 19.5 kcal/mol, respectively). These results may explain the beneficial role of the phenyl substituent in 114c in catalysis due to the increase of the N-H bond acidity facilitating a concerted proton/hydride transfer. Scheme 51. DFT modelling of two possible mechanisms for transfer hydrogenation of acetophenone catalyzed by Mn(I) complex 114c (activation energy barriers are given in kcal/mol).

The next important step towards NN ligand design in Mn(I)-catalyzed transfer hydrogenation was realized by Sortais and coll. by the extensive screening of various diamine ligands in combination with Mn(CO)5Br forming catalytically active species in situ upon heating to 80 °C (Scheme 52) [START_REF] Wang | Practical (asymmetric) transfer hydrogenation of ketones catalyzed by manganese with (chiral) diamines ligands[END_REF]. While the suppression or elongation of the alkyl spacer in picolylamine ligand led to poor results (0% and 7% of acetophenone conversion for 2-aminopyridine and PyCH2CH2NH2, respectively) thus confirming the importance of metallacycle size, it was found that simple aliphatic diamines could be valuable alternatives. Remarkably, the best results were obtained for ubiquitous ethylenediamine and after optimization of the reaction conditions complex 120a was shown to reduce a variety of aromatic ketones including sterically demanding representatives typically in a nearly quantitative yield (Scheme 52). The low price of this diamine ligand coupled with in situ catalyst generation and easy product isolation by the filtration through a short plug of silica makes this catalytic procedure an attractive alternative to classic stoichiometric ketone reduction with NaBH4 from both economical and practical (no hydrolysis/extraction work-up) points of view.

The extension of this methodology to commercially available chiral diamines allowed to detect two structural families based on 1,2-diphenyl-1,2-ethanediamine and 1,2-diaminocyclohexane motifs combining good catalytic activity with promising levels of enantiomeric induction (36% and 52% ee for acetophenone reduction, respectively). Interestingly, the methylation of NH2 moieties in both NN ligands considerably improved enantioselectivity levels providing the best results for complex 120b with 64% ee (Scheme 52). The evaluation of the application scope of asymmetric transfer hydrogenation with generated in situ complex 120b revealed that ortho-substituted acetophenones or their analogues with larger alkyl substituents afforded the target secondary alcohols with 68-90% optical purity, whereas ee values significantly drops to 30-46% for para-substituted acetophenones.

Scheme 52.

Transfer hydrogenation of ketones catalyzed by Mn(I) complexes 120 generated in situ from Mn(CO)5Br and equimolar amount of the corresponding aliphatic diamine.

Later Pidko and coll. evaluated the application of well-defined Mn(I) diamine complexes 121 (Scheme 53) in asymmetric transfer hydrogenation of acetophenone and studied in detail the mechanism of this reaction [START_REF] Van Putten | Mechanistic complexity of asymmetric transfer hydrogenation with simple Mn-diamine catalysts[END_REF]. It was found that isolated complexes 121a and 121b differing only in the configuration of N-methyl groups afforded 74% ee at 60 °C, which was slightly better than the value obtained using in situ generated system (69% ee). The complex 121c bearing bulkier isopropyl groups at the nitrogen atoms was almost inactive under these conditions. Scheme 53. Well-defined Mn(I) complexes 121 bearing chiral aliphatic diamines tested in asymmetric transfer hydrogenation of acetophenone.

DFT study revealed that transfer hydrogenation in this case proceeds through inner-sphere mechanism similarly to picolylamine system (Scheme 43) with the β-hydride elimination from the corresponding isopropoxide intermediate as the rate-determining step. The results of this theoretical investigation were consistent with kinetic measurements data and the observation of a strong primary KIE of 2.7 in the case of fully deuterated isopropanol.

Very recently, the group of Sun evaluated a series of well-defined Mn(I) complexes 122-123 bearing aminoacid-derived benzimidazole-amine ligands in asymmetric transfer hydrogenation of ketones (Scheme 54) [START_REF] Wang | Amino acid derived chiral aminobenzimidazole manganese catalysts for asymmetric transfer hydrogenation of ketones[END_REF]. Initial catalyst screening for the reduction of acetophenone (2 mol% cat., 4 mol% tBuOK, 80 °C, 20 min) revealed that in complexes 122a-d the increase of steric volume of the alkyl group R'' at the stereogenic carbon atom was beneficial for the enantioselectivity (42%, 65% and 66% ee for iPr, tBu and Ad, respectively) providing in the best cases similar values obtained in the case of in situ generated complex 120b (Scheme 52) [START_REF] Wang | Practical (asymmetric) transfer hydrogenation of ketones catalyzed by manganese with (chiral) diamines ligands[END_REF]. In contrast, the replacement of the methyl substituent at benzimidazole nitrogen atom by an ethyl one led to serious drop of ee (65% vs. 24% for 122b and 122c, respectively). While the introduction of Ph group at the adjacent position of the aromatic benzimidazole ring in NH2-containing complexes 122 totally inhibited the catalytic activity (complex 122e), this strategy allowed to improve significantly the efficiency of proline derived analogues 123 (48% vs. 82% ee for 123a and 123b, respectively). Importantly, transfer hydrogenation may be also performed at 40-50 °C for 24 h achieving excellent levels of substrate conversion and slightly better enantiomeric induction. Scheme 54. Transfer hydrogenation of ketones catalyzed by well-defined Mn(I) complexes 122 and 123 bearing chiral NN ligands.

Diversely substituted acetophenone derivatives were typically reduced at 50 °C using 2 mol% of complex 123b with excellent yield and ee in a range of 85-90%. Unlike the case of complex 120b the introduction of the substituents at the para-position of aromatic ring did not lead to the decrease of enantioselectivity. In contrast, complex 123b displayed slightly lower performance for furan-and thiophene derived ketones (51-53% yield, 69-71% ee) and was inefficient for the substrates bearing bulky aliphatic groups (8% yield and 77% ee for PhCOtBu). However, such substrates could be readily reduced using complex 122b even at 40 °C providing the target tBu-substituted secondary alcohols in 76-94% yield and 89-93% ee. As expected, the decrease of steric volume of alkyl groups led to lower levels of enantiomeric induction being the worse for long-chain and cyclic alkyls (69-71% ee).

A formal change of pyridine moiety in picolylamine system by phosphine was reported by the group of Lacy in 2019 (Scheme 55) [START_REF] Vigneswaran | β-Amino phosphine Mn catalysts for 1,4-transfer hydrogenation of chalcones and allylic alcohol isomerization[END_REF]. It was shown that complexes 124 bearing PN ligands easily available from optically pure amino-acids were competent catalyst for transfer hydrogenation of various ketones typically at 60 °C using 0.5 mol% catalyst loading albeit with negligibly low enantiomeric induction (11% ee max). The reaction afforded good-to-excellent product yields and well tolerated halogens, heterocycles, NO2 groups and failed completely only for phenol-containing substrates. Interestingly, this catalytic system allowed the selective reduction of C=C bond in chalcone derivatives to give the corresponding saturated ketones, which could be readily transformed to secondary alcohols using higher amount of catalyst and longer reaction time. The potential of phosphine-amine ligands for asymmetric transfer hydrogenation of ketones was recently demonstrated by Bastin, Sortais and coll. (Scheme 56) [START_REF] Azouzi | Asymmetric transfer hydrogenation of ketones promoted by manganese (I) pre-catalysts supported by bidentate aminophosphines[END_REF]. The key to success in this reaction seemed to be the use of the axially chiral binepine phosphorous scaffold in well-defined Mn(I) complex 125 allowing a relatively fast reaction rate at 30 °C for aromatic ketones and giving generally ca. 10-15% higher ee values than those observed for related diamine analogue 120b (Scheme 52). In contrast to NN-based system 120b, the deleterious effect of para-substituent of the aryl on the enantiomeric induction was less pronounced here providing in most cases ee values higher than 55%. Though aliphatic ketones could be also reduced using this synthetic protocol the enantioselectivity was very low (90% conversion and 18% ee for CyCOMe at 50 °C). Scheme 56. Asymmetric transfer hydrogenation of ketones catalyzed by well-defined Mn(I) complex 125 bearing chiral phosphine-amine ligand.

While the use of phosphine-amine ligands more donating than NN analogues in Mn(I)catalyzed transfer hydrogenation of ketones did not improve the overall catalyst performance, the situation changed dramatically for NHC-amine complexes 126 developed by Pidko and co-workers (Scheme 57) [START_REF] Van Putten | Efficient and practical transfer hydrogenation of ketones catalyzed by a simple bidentate Mn-NHC complex[END_REF]. While complex 126b without coordinated NH moiety was inactive as expected, its analogue 126a was highly efficient in acetophenone reduction with TOF up to 10800 h -1 . Importantly, the reaction efficiency in this case was strongly dependent on temperature especially for low catalyst loading. The analysis of kinetic data for acetophenone transfer hydrogenation catalyzed by 50 ppm of 126b at 60 and 70 °C revealed that despite faster initial reaction rate in the latter case TON value after 6 h (11100) was inferior to the one obtained at lower temperature (15100). This difference became even more spectacular for 25 ppm catalyst loading (3100 vs. 17300 TON for 70 and 60 °C, respectively) clearly showing faster catalyst deactivation at higher temperatures. Taking into account this information, the evaluation of substrate scope was performed at 40 °C using 0.05 mol% of catalyst affording in most cases more than 80% product yields for various aromatic and aliphatic ketones. The identified limitations included sterically bulky ketones such as MesCOMe or Cy2CO, phenol-and nitrogen heterocycles containing compounds. Aromatic substrates bearing NH2, CN and NO2 groups were reduced with lower 12-27% yield, which could be surely improved at higher catalyst loading and/or more elevated temperature. Scheme 57. Transfer hydrogenation of ketones catalyzed by Mn(I) complex 126a bearing NHCamine ligand.

Recently, Khusnutdinova and coll. demonstrated the viability of another strategy for the design of Mn(I)-based catalysts bearing bidentate NN ligands for transfer hydrogenation involving no amine/amide ligand cooperativity (Scheme 58) [START_REF] Dubey | Transfer hydrogenation of carbonyl groups, imines and N-heterocycles catalyzed by simple, bipyridine-based Mn I complexes[END_REF]. A series of bipyridine complexes 61 was initially tested in the reduction of acetophenone at 80 °C using 0.3 mol% catalyst charge in the presence of 1 mol % of tBuOK. As could be expected complex 61a bearing hydroxyl groups in the close proximity to metal center and successfully used in CO2 hydrogenation (Scheme 32) [START_REF] Dubey | Bio-inspired Mn(I) complexes for the hydrogenation of CO2 to formate and formamide[END_REF] was the most active in this transformation providing a quantitative conversion after 24 h. Surprisingly, its isomer 61b exhibiting distal hydroxyl groups and poorly active in CO2 hydrogenation afforded similar results being even slightly more productive after 1 h of reaction at 80 °C (88 vs. 93% yield for 61a and 61b, respectively). Moreover, in contrast to completely inactive fac-(bipy)Mn(CO)3Br, complexes 61c-e containing ortho-methoxy-substituted bipy ligands also led to full substrate conversion at 1 mol% catalyst charge in the presence of 3 mol% of tBuOK (background base-assisted reaction in the presence 5 mol% of tBuOK gave only 20% conversion under these conditions) providing the evidence that cooperative OH fragment was not strictly required for catalytic turnover. The TON values of ca. 215/175/95 obtained for 61c-e at lower catalyst loading (0.3 mol%) showed that complex 61c was the most active in the methylated series. The comparison of kinetic data for transfer hydrogenation of acetophenone at 50 °C revealed much higher performance of complex 61a displaying first-order behavior over 61b with more complicated kinetic pattern, whereas catalysts 61c-d were inactive under these conditions.

Both aromatic and aliphatic ketones were reduced in excellent yield using 0.3 mol% of catalyst 61a and the reaction tolerated some potentially coordinating functional groups (CN, SMe, furan). Curiously, the reduction of aldehydes was slower and slight increase of catalyst charge to 0.5 mol% was required to get good product yields. Importantly, this catalytic system provided highest TON known to date in transfer hydrogenation of aromatic aldimines (for example TON values observed for p-ClC6H4CH=NPh reduction were ca. 46, 17 and 170 for Mn(I) NN complexes 91a, 114c and 61a, respectively), though the synthetic scope was not developed. The potential of complex 61a for transfer hydrogenation of N-heterocycles was also highlighted. While the reduction of quinoline was relatively sluggish (45% yield after 72 h of reaction), the reaction for acridine, quinoxaline and 1,5napthhydrine afforded excellent isolated yields being comparable in term of TON with Mn(CO)5Br-catalyzed direct hydrogenation procedure developed by Beller (Scheme 39) [START_REF] Papa | Development of a practical non-noble metal catalyst for hydrogenation of N-heteroarenes[END_REF]. Noticeably, complex 61c bearing a methoxy group was almost unreactive for N-heterocycles showing the importance of free OH groups for the catalytic cycle. Scheme 58. Transfer hydrogenation of ketones, aldehydes, aldimines and N-heterocycles using Mn(I) complex 61a bearing bipyridine ligands.

Reaction mechanism of transfer hydrogenation of acetophenone and 1,5-napthhydrine with complex 61a based on DFT calculations and deuterium-labelling experiments is shown in the Scheme 59 [START_REF] Dubey | Transfer hydrogenation of carbonyl groups, imines and N-heterocycles catalyzed by simple, bipyridine-based Mn I complexes[END_REF]. It was demonstrated that the isopropanol dehydrogenation with monodeprotonated intermediate 62a proceeds as concerted outer-sphere process with activation barrier of only 9.0 kcal/mol to form the hydride complex 66a destabilized by 6.3 kcal/mol relative to 62a. The hydrogenation of acetophenone and N-heterocycles with 66a also proceeds in concerted manner regenerating the initial deprotonated species 62a. Importantly, in the case of 1,5-napthhydrine the second OH group is also directly involved into the first-rate determining C=C bond reduction step (transition state 127a) forming a hydrogen bond with the nitrogen atom of neighboring heterocyclic ring. This additional interaction allows to decrease the activation barrier (16.3 kcal/mol) compared to quinoline (24.3 kcal/mol) thus explaining the lower efficiency of the catalyst 61a for transfer hydrogenation of this substrate. Scheme 59. Simplified reaction mechanism for transfer hydrogenation of acetophenone and 1,5napthhydrine using Mn(I) complex 61a based on DFT calculations (activation energy barriers are given in kcal/mol).

In 2019, the group of Xia reported the application of structurally similar Mn(I) complexes 128 based on heteroaromatic NN systems containing a single OH group in the proximity to metal center in transfer hydrogenation of aldehydes and ketones (Scheme 60) [START_REF] Zhang | Manganese(I)-catalyzed transfer hydrogenation and acceptorless dehydrogenative condensation: promotional influence of the uncoordinated Nheterocycle[END_REF]. Though these catalytic systems were significantly less efficient than those based on complexes 61 and required strong excess of base, it was found that fusion of the second nitrogen ring in complex 128a was slightly beneficial for catalytic performance. Interestingly, complex 128d lacking cooperative OH moiety displayed the same activity as 128b in acetophenone reduction, which again may indicate the existence of an unconventional reaction mechanism. Scheme 60. Transfer hydrogenation of ketones and aldehydes catalyzed by Mn(I) complexes 128 bearing heteroaromatic NN ligand.

The unusual example of Mn-catalyzed transfer hydrogenation of carbonyl compounds using phenol as hydrogen donor under controlled potential electrolysis was recently reported by Siewert and coll. (Scheme 61) [START_REF] Fokin | Chemoselective electrochemical hydrogenation of ketones and aldehydes with a well-defined base-metal catalyst[END_REF]. It was found that binuclear complex 129a containing two Mn(I) units based on pyridine-imidazole ligand and proximal OH group was moderately active for the reduction of aliphatic aldehydes and ketones with Faradaic efficiency (FE) 60-93%. Scheme 61. Electrochemical hydrogenation of ketones and aldehydes using phenol as hydrogen donor catalyzed by Mn(I) complex 129a bearing bidentate pyridine-imidazole ligand.

The reaction tolerated isolated C=C bonds leading however to the decrease of both Faradaic and isolated product yields (for example 1-pentanal and 1-pent-4-enal were reduced with 99% (FE 93%) and 40% (FE 47%) yields, respectively). It was proposed that the reaction mechanism involves the reduction of (NN)(CO)3Mn-Br units in 129a into the corresponding anionic metallates (NN)(CO)3Mn -followed by their protonation with phenol to form hydride species (NN)(CO)3MnH capable to hydrogenate C=O bond. Interestingly, the presence of hydroxyl group and the second metal center was crucial for the catalysis since both complexes 129b and 130 were completely inefficient.

Though nitrogen donors present in most Mn-based catalysts for transfer hydrogenation known to date, other bidentate ligand architectures can be efficient as well. In particular, García and coworkers have shown that complexes 131 bearing chelating aliphatic diphosphines are active in transfer hydrogenation of nitriles using 2-butanol as hydrogen donor (Scheme 62) [START_REF] Garduño | Manganese-catalyzed transfer hydrogenation of nitriles with 2-butanol as the hydrogen source[END_REF]. Under these conditions, a mixture of target primary amine RCH2NH2 and the product of its condensation with 2butanone RCH2N=C(Me)Et were formed, the latter being the major component for aliphatic nitriles. The catalytic performance increased in 131a→131c→131b row order affording 58%, 94% and full conversion of benzonitrile after 3 h at 100 °C, respectively. While the reduction of electron-rich aromatic nitriles readily proceeded with 3 mol% of 131b, more electron-poor p-CF3C6H4CN and aliphatic substrates required higher amounts of catalyst (5-10 mol%) to achieve synthetically useful yields. It is important to note that despite the implication of the same alkoxide and hydride intermediates in the catalytic cycle (Scheme 22), transfer hydrogenation catalyzed by complex 131b was much slower than direct nitrile hydrogenation using closely related Mn(I) triflate complex fac-(iPr2PCH2CH2PiPr2)Mn(CO)3(OTf) 43 (Scheme 21) [START_REF] Garduño | Non-pincer Mn(I) organometallics for the selective catalytic hydrogenation of nitriles to primary amines[END_REF] (for example TOF of ca. 10 and 120 h -1 were observed for the reduction of PhCN using 3 mol% of 131b and 43 at 100 °C and 90 °C, respectively). Scheme 62. Transfer hydrogenation of nitriles catalyzed by Mn(I) complexes 131 bearing diphosphine ligands.

In 2018, Driess and coworkers shown that Mn(II) complexes bearing more exotic Nheterocyclic silylene ligands were able to perform transfer semi-hydrogenation of internal alkynes using ammonia-borane as hydrogen donor (Scheme 63) [START_REF] Zhou | Stereoselective transfer semi-hydrogenation of alkynes to E-olefins with N-heterocyclic silylene-manganese catalysts[END_REF]. While well-defined Mn(II) complexes 132a-b and 132e provided a quantitative conversion at 1 mol% catalyst loading, catalytic systems 132c-d generated in situ from MnCl2 and equimolar amount of the corresponding disilylene were much less efficient (41-48% conversion). Importantly, both bidentate coordination and the nature of the bridge between two silylene moieties directly affected the stereoselectivity of this transformation being the best for pre-catalyst 132a. This protocol was shown to be very efficient for diaryl-, arylalkyl and arylsilyl internal alkynes affording the target alkenes with typically more than 90% E-selectivity. The reaction tolerated halogens, thiophene, OMe and ester groups leading however to lower stereoselectivities for the two latter cases (60-82%). The reduction of aliphatic 6-dodecyne was more sluggish (67% conversion) and non-stereoselective, whereas disilylate alkyne Me3SiC≡CSiMe3 was unreactive at all. The mechanism of this reaction is not clear for the moment. Since the addition of TEMPO did not affect the reaction outcome the formation of free radicals seems to be unlikely. Taking into account the reduction properties of ammonia-borane and the possibility to create an efficient catalytic system from diaminopyridine-based disilylene and Mn(CO)5Br, the formation of Mn(I) complexes from 132a cannot be excluded. Such reduction mechanism was recently inferred from DFT calculations by Azofra, El-Sepelgy and coworkers for highly Z-selective transfer semi-hydrogenation of internal alkynes catalyzed with PNP pincer Mn(II) complex under similar conditions (1 mol% of (iPr2PCH2PyCH2PiPr2)MnCl2, 1 equiv. H3N•BH3, MeOH, 60 °C, 20 h) [START_REF] Brzozowska | Highly chemo-and stereoselective transfer semihydrogenation of alkynes catalyzed by a stable, welldefined manganese(II) complex[END_REF]. However, generated in situ structurally similar PNP complex 132f under the conditions of Driess was poorly active and displayed no stereoselectivity thus the oxidation state of catalytically competent manganese disilylene species remains the open question. Scheme 63. Transfer semi-hydrogenation of alkynes catalyzed by Mn(II) complexes 132 bearing Nheterocyclic silylene ligand.

Hydrogen-borrowing transformations

Since its discovery in 2016, various phosphine-based PNP [START_REF] Mukherjee | Manganese-catalyzed environmentally benign dehydrogenative coupling of alcohols and amines to form aldimines and H2: a catalytic and mechanistic study[END_REF][START_REF] Elangovan | Efficient and selective N-alkylation of amines with alcohols catalysed by manganese pincer complexes[END_REF], PN 3 P [98], PN 5 P [START_REF] Kallmeier | Manganese-catalyzed sustainable synthesis of pyrroles from alcohols and amino alcohols[END_REF] and PNN [START_REF] Espinosa-Jalapa | Synthesis of cyclic imides by acceptorless dehydrogenative coupling of diols and amines catalyzed by a manganese pincer complex[END_REF] pincers were shown to be dominant ligand structures in the area of Mn-catalyzed hydrogen-auto transfer reactions [9-13, 15-17, 19, 22]. The first evaluation of Mn(I) complexes bearing bidentate ligands in hydrogen-borrowing processes was mentioned in the report of Maji [START_REF] Barman | Phosphine-free NNN-manganese complex catalyzed αalkylation of ketones with primary alcohols and Friedländer quinoline synthesis[END_REF] dealing with α-alkylation of arylmethylketones with primary alcohols (Scheme 64), originally introduced by Beller and coll. [START_REF] Peña-López | Manganese-catalyzed hydrogen-autotransfer C-C bond formation: α-alkylation of ketones with primary alcohols[END_REF]. Initial catalyst screening (2.5 mol% cat., 20 mol% tBuOK, tAmOH, 120 °C, 24 h) for the alkylation of acetophenone with BnOH revealed that in situ generated picolylamine complex 91a was moderately active providing 31% GC product yield, whereas the corresponding tridentate analogue 97a was found to be slightly more performant (42% yield). Though the complex 133 based on 2-hydrazinylpyridine was low-efficient (10% yield), it was shown that incorporation of an additional pyridine moiety to the ligand resulted in highly active catalyst 134b affording 89% GC yield of the product, which can be further improved to almost quantitative yield at higher temperature (140 °C) using only 10 mol% of base. Importantly, the presence of NH moiety in this ligand was critical, because structurally close catalyst 134a containing CH2 linker was only moderately active (29% yield). Under optimized conditions a variety of aryl(methyl)ketones was alkylated with benzylic alcohols in good-to-excellent yields. Similarly as it was found in the case of Mn(I) PNP complexes [START_REF] Mastalir | Sustainable synthesis of quinolines and pyrimidines catalyzed by manganese PNP pincer complexes[END_REF], the application of 2-aminobenzyl alcohols smoothly afforded the corresponding quinolines in more than 70% yield. The use of aliphatic alcohols and ketones was also possible, but methanol cannot be activated with this catalytic system in contrast to those based on PNP [START_REF] Sklyaruk | Catalytic C1 alkylation with methanol and isotope-labeled methanol[END_REF] and PN 3 P [105] ligands. While the reaction was tolerant for the presence of halogens, CF3 and various heterocyclic fragments (pyridine, furan, thiophene) it failed completely for nitrile-and ester-containing substrates. Scheme 64. Alkylation of ketones with primary alcohols and Friedländer synthesis of quinolines catalyzed by generated in situ Mn(I) complexes bearing NN and NNN ligands.

The proposed reaction mechanism (Scheme 65) includes the deprotonation of in situ generated complex 134b to form the intermediate 135b with dearomatized pyridine fragment, which then reacts with alcohol to form the alkoxide species 136b. The latter undergoes the β-hydride elimination to form manganese hydride 137b and the aldehyde participating in base-induced condensation with ketone to afford enone, which can either be further hydrogenated with 137b to form the saturated ketone or cyclized with pendant amine function to provide quinoline. Scheme 65. Proposed mechanism for alkylation of acetophenone with primary alcohols catalyzed by generated in situ Mn(I) complex 134b bearing NNN ligand.

Later on the same group reported selective monoalkylation of arylacetonitriles with primary alcohols catalyzed by Mn(I) complexes 138 bearing similar hydrazine-containing NN ligands (Scheme 66) [START_REF] Jana | Manganese catalyzed α-alkylation of nitriles with primary alcohols[END_REF]. Well-defined Mn(I) catalysts 138a and 138c afforded similar results being marginally better for the latter one, whereas the N-methylated complex 138b displayed very low efficiency thus showing the importance of NH moiety for alcohol dehydrogenation step. This protocol showed a wide substrate scope and provided the target products in moderate-to-high yields for both aliphatic (besides methanol) and aromatic alcohols. The mechanistic studies revealed that the coordination of thiophene moiety is not directly implicated in the catalytic cycle and the key step of alcohol dehydrogenation proceeds via an outer-sphere process. Scheme 66. Alkylation of nitriles with primary alcohols catalyzed Mn(I) complexes 138 based on NN ligand bearing a thiophene arm.

In contrast to previous systems 138 the hemilabile properties of sulphur-containing arm was shown to be more important for the Mn(I)-catalyzed assembly of cycloalkane scaffolds by the double alkylation of aryl(methyl)ketones with 1,n-aliphatic diols (Scheme 67) [START_REF] Jana | A phosphine-free manganese catalyst enables stereoselective synthesis of (1+n)-membered cycloalkanes from methyl ketones and 1,n-diols[END_REF]. While all Mn(I) complexes bidentate picolylamine (91e, 105a) and picolylhydrazine (138a, 138c) ligands at 4 mol% charge were active in the annulation of C6Me5COMe with HO(CH2)5OH affording the final product in a similar 45-62% yield range, the use of the same amount of catalyst 105b led to a Scheme Stereoselective synthesis of aryl(cycloalkyl)ketones from 1,n-aliphatic diols catalyzed by Mn(I) complexes 105 bearing hemilabile NNS ligand. significant improvement (88% yield). This protocol showed an ample substrate scope and appeared to be the most efficient for the formation of six-membered rings (60-to-98% yield). The formation of cyclopentyl-and cycloheptyl-substituted ketones proceeded generally with 20-30% lower yields still being attractive from a synthetic point of view. Both ketone and diol partners involved in this process could possess various substitution patterns and contain halogen and methoxy groups. Importantly, the employment of 3-substituted 1,5-pentandiols led to the formation of trans-isomers of 1,4disubstituted cyclohexanes with 97-99% diastereoselectivity.

The reaction mechanism supported by isotope-labelling experiments and DFT calculations is shown on Scheme 68. The initial pre-catalyst 105b first transforms into the pincer-type species 107b upon a sequence of deprotonation and CO loss. Then the cooperative Mn-amido type activation of O-H bond in the diol takes place (N-methylated complex 105c was totally inactive) to form the alkoxide intermediate 139b. The β-hydride elimination in the latter leads to the formation of hydride complex 110b and aldehyde, which undergoes a base-assisted condensation with ketone to get the Scheme 68. Reaction scheme for the condensation of acetophenone with 1,5-pentanediol catalyzed by Mn(I) NNS complex 105b (up) and the simplified mechanism of the key diol dehydrogenation and enone hydrogenation steps based on DFT calculations (down, activation energy barriers are given in kcal/mol, n.r. transition states were not localized). corresponding enone product. The reaction of the latter with hydride 110b proceeds as a sequence of C=C bond coordination to form η 2 -alkene complex 141b, hydride insertion and Mn-alkyl bond cleavage in the resulting σ-alkyl species 142b to give the initial amide 107b and saturated ωhydroxyketone undergoing similar series of steps to produce the final aryl(cycloalkyl)ketone. Noticeably, the hemilability of a pendant thiophene arm was shown to be important for the realization of alcohol dehydrogenation and enone hydrogenation steps, which was confirmed by poisoning experiments in the presence of auxiliary L-type ligands such as PPh3 or pyridine.

It was also shown by Maji and coll. that Mn(I) NN complex 105b was efficient in α-alkylation of aromatic ketones with secondary alcohols (Scheme 69) [START_REF] Waiba | Manganese complex-catalysed α-alkylation of ketones with secondary alcohols enables the synthesis of β-branched carbonyl compounds[END_REF]. In this case the presence of both picolylamine core and hemilabile thiophene moiety was even more important for catalytic performance since the corresponding furan-(105a) and picolylhydrazine-containing (138a) analogues were much less efficient (85%, 25% and >10% product yield in the alkylation of C6Me5COMe with cyclohexanol using 2 mol% of 105b, 105a and 138a, respectively). This protocol was successfully applied to a variety of cyclic or acyclic aliphatic alcohols as well as to a series of 1-[hetero]arylethanol derivatives. Interestingly, very high cis-diastereoselectivity was observed for 4substituted cyclohexanols, which was rationalized as a result of equatorial attack of Mn(I) hydride to the conjugated enone at the final reaction step. The reaction tolerated various heterocyclic moieties and internal aromatic alkyne, albeit with lower yield in the latter case. The reaction mechanism is similar to that proposed for the alkylation of aromatic ketones with primary alcohols (Scheme 65) with alcohol dehydrogenation as rate-determining step (KIE kH/kD = 2.14 for alkylation of C6Me5COMe with Me(Ph)CDOH). The addition of 10-fold excess of coordinating additives was shown to significantly slow down the reaction between C6Me5COMe and Me(Ph)CHOH (45% and 11% yield for pyridine and 4-picoline, respectively vs. 85% under optimal conditions) thus demonstrating the importance of hemilabile coordination of thiophene fragment for this process. Scheme 69. Alkylation of aromatic ketones using secondary alcohols catalyzed by Mn(I) NNS complex 105b.

Very recently, the same research group reported the application of Mn(I) complexes with bidentate NN ligands in a formal anti-Markovnokov hydroamination of allylic alcohols in the presence of a weak base -K2CO3 (Scheme 70) [START_REF] Das | Manganese-catalyzed anti-Markovnikov hydroamination of allyl alcohols via hydrogen-borrowing catalysis[END_REF]. The presence of the picolylamine ligand scaffold was again found to be critical for the occurrence of this transformation since picolylhydrazine-containing complex was almost inactive. The complex 106b bearing thioether arm was more active than its thiophene-and methoxy-containing analogues (92% vs. 78% vs. 60% yield in the amination of allylic alcohol with N-methylaniline for 106b, 105b and 106a, respectively). The reaction can be applied to a wide variety of secondary aromatic or aliphatic amines with complete regioselectivity and typical product yield higher than 90%. Primary aromatic amines could be also employed but with slightly reduced efficiency (55-60% yields). This protocol well tolerated numerous functional groups including halogens, CF3, heterocycles, amide, alkene, secondary hydroxyl group and was successfully applied for the derivatization of several complex organic molecules. Scheme 70. Formal anti-Markovnikov hydroamination of allylic alcohols catalyzed by Mn(I) complexes bearing hemilabile NNS ligands.

The reaction mechanism (Scheme 71) includes the Mn-catalyzed dehydrogenation of allylic alcohol into the α,β-unsaturated carbonyl compound, which undergoes a condensation with amine to form the corresponding iminium species. The Michael addition of the second amine molecule to the latter, followed hydrolysis and the hydrogenation of resulting ketone intermediate leads to the formation of the final γ-amino-alcohol product. The formation of allyl-substituted iminium salt and β-aminoketone was confirmed by HRMS spectra of reaction mixtures. According to the results of deuterium-labelling experiments (KIE kH/kD = 3.43) inner-sphere alcohol dehydrogenation seems to be implicated into the rate-determining step of the process. The potential role of the S-based donor as hemilabile ligand was confirmed by the reduced reaction efficiency in the presence of exogenous phosphine donor ligands (37% and 16% yields in the presence of PPh3 and PCy3, respectively). Scheme 71. Reaction scheme for the amination of allylic alcohols with primary or secondary amines catalyzed by Mn(I) NNS complex 106b.

It was also shown by Maji that well-defined Mn(I) picolylamine complexes can catalyze monoalkylation of anilines with benzylic alcohols (Scheme 72) [START_REF] Das | Synthesis and characterization of N,N-chelate manganese complexes and applications in CN coupling reactions[END_REF]. The complex 106b having additional S-donor moiety was found again more active than its analogue 91e affording 91% and 40% yields for the reaction between aniline and BnOH in the presence of 40 mol% of tBuOK as base. Interestingly, N-methylated catalyst 106c was still able to perform some catalytic turnover (18% yield). This protocol showed only moderate efficiency being far below the best Mn(I) catalytic systems for N-amine alkylation with alcohols based on PNP [START_REF] Neumann | Improved and general manganese-catalyzed N-methylation of aromatic amines using methanol[END_REF] and PN 3 P [112] ligands. In addition, the use of stoichiometric amount of base was required in most cases to achieve reasonable product yield. The alkylation of aromatic primary ortho-diamines catalyzed by Mn(I) NN complex 128c exhibiting different type of cooperativity to form the corresponding substituted benzimidazoles under similar reaction conditions was reported by Chen, Xia and coll. (Scheme 72) [START_REF] Zhang | Manganese(I)-catalyzed transfer hydrogenation and acceptorless dehydrogenative condensation: promotional influence of the uncoordinated Nheterocycle[END_REF]. The reaction typically proceeded with 75-80% yield but required an excess of strong base thus limiting the application for the substrates bearing functional groups. Scheme 72. Alkylation of aromatic amines with benzylic alcohols catalyzed by Mn(I) complexes bearing bidentate NN and hemi-labile NNS ligands.

The use of different nitrogen heterocycles in the design of Mn(I) complexes with NN ligands was also evaluated. In particular, Chai and coll. reported that complexes 143 bearing pyridinepyrazole ligands were able to perform the dehydrogenation of benzylic alcohols in the presence of primary amines (Scheme 73) [START_REF] Chai | A highly selective manganese-catalyzed synthesis of imines under phosphine-free conditions[END_REF]. While both catalysts displayed similar conversion in the reaction between PhNH2 and BnOH, it was remarkable that complex 143b containing N-methylated pyrazole moiety was more efficient in the C=N bond hydrogenation (77:23 ratio between PhN=NBn and PhNHBn), whereas 143a provided 98% selectivity of imine formation. Negligible activity of 143a in hydrogenation reactions was exploited for the development of selective imine synthesis with ample substrate scope. Both aromatic and aliphatic amines incorporating halogens, CF3 and various heterocyclic fragments can be engaged in this process to form the target products in typically 80-90% isolated yields. Scheme 73. Synthesis of imines from primary alcohols catalyzed by Mn(I) complex 143a bearing pyridine-pyrazole ligand.

The influence of a pendant pyrazole group in Mn(I) complexes based on pyridinebenzimidazole ligands on the selectivity in β-alkylation of secondary alcohols with primary alcohols was studied by Yu and coll. (Scheme 74) [START_REF] Liu | Manganese-catalyzed β-alkylation of secondary alcohols with primary alcohols under phosphine-free conditions[END_REF]. While the use of 2.1 mol% of complex 144 afforded higher amount of target alcohol product in the reaction of Ph(Me)CHOH with BnOH at 110 °C (92% conversion, 78% selectivity) than its analogues 145a-b (84-86% conversion, 53-67% selectivity), the catalysts 145c combining the pyrazole-containing NN ligand and methoxide led to significantly improved results (96% conversion, 94% selectivity). A variety of substituted secondary alcohols can be obtained from both aromatic and aliphatic substrates requiring however the two-fold increase of catalyst charge and reaction temperature to 140 °C in the latter case. The efficient double alkylation of cyclopentanol with benzylic alcohols was also developed. The presence of halogens, CF3 and pyridine groups was well tolerated, but unexpectedly the tri-substituted C=C double bond in cholesterol was fully hydrogenated under reaction conditions. Scheme 74. Selective β-alkylation of secondary alcohols with primary alcohols catalyzed by Mn(I) complex 145c bearing pyridine-benzimidazole ligand.

Finally, it was demonstrated that even Mn(II) precursors in combination with bidentate nitrogen ligands could be applied to the α-alkylation of ketones (Scheme 75) [START_REF] Kabadwal | Mn(II)-catalysed alkylation of methylene ketones with alcohols: direct access to functionalised branched products[END_REF]. The application of catalytic system generated in situ from Mn(acac)2 and 1,10-phenanthroline to the reaction between propiophenone with BnOH led to the preferential formation of alkylated ketone accompanied by its hydrogenation product (80% and 20% GC yields, respectively). The utilization of other tertiary aromatic and aliphatic diamines under the same conditions led to lower conversions and significantly higher amounts of secondary alcohol by-products. The reaction was found to be the most efficient for benzylic alcohols to give branched ketone products in 70-80% typical yield and may be Scheme 75. Alkylation of ketones with primary alcohols catalyzed by the catalyst generated in situ from Mn(acac)2 and 1,10-phenanthroline. even used for one-pot double alkylation of acetophenones via the consecutive addition of two different alcohol components. The use of aliphatic and heterocycle-containing substrates was also possible, but the product yields dropped to 30-50%. Though isotope-labelling control experiments were in agreement with a realization on Mn-catalyzed alcohol dehydrogenation, the nature of the catalytic species and corresponding metal oxidation state are not clear for the moment.

In 2020, Wass and coll. evaluated the catalytic activity of Mn(I) complexes based on PN and PP bidentate ligands for synthesis of isobutanol from ethanol and methanol using Guerbet reaction (Scheme 76) [START_REF] King | Manganese diphosphine and phosphinoamine complexes are effective catalysts for the production of biofuel alcohols via the Guerbet reaction[END_REF], previously described by Liu [START_REF] Fu | Manganese-catalyzed upgrading of ethanol into 1-butanol[END_REF] and Jones [START_REF] Kulkarni | Catalytic upgrading of ethanol to n-butanol via manganese-mediated Guerbet reaction[END_REF] for Mn(I) PNP complexes. While complex 1 containing single phosphinoamine ligand showed low activity (TON 12), the introduction of the second PN ligand in 2a led to better results (TON 87). The inversed trend was observed for PP ligands, since pre-catalyst 148 bearing two chelating dppm ligands was less productive than 146a (TON 22 and 67, respectively). Though the replacement of dppm ligands in 146 with dppe afforded very similar results (149: TON 26), better performance was achieved for complex 147 exhibiting three coordinated and one pendant phosphine moiety (TON 113). Scheme 76. Selective synthesis of isobutanol by Guerbet reaction catalyzed by Mn(I) complexes 146 bearing dppm-type diphosphine ligand.

Finally, it was shown that the introduction of phenyl moiety into the dppm bridge in 146a was the optimal strategy since the activity of the resulting catalyst 146b was increased by ca. three times (TON 206) keeping the same level of selectivity (n-propanol and 2-butanol were systematically detected as by-products). These results are in accord with a recent observation of the strongly beneficial effect of dppm bridge substitution in complexes 146 for cooperative dihydrogen activation (Scheme 77) [START_REF] Kireev | Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H2 activation[END_REF]. Indeed, it was shown that diphosphinomethanide complex 150b obtained by the deprotonation of the bromide precursor 146b was capable to activate H2 under very mild conditions (1 atm H2, 25 °C, 5 min) to give the corresponding hydride 151b in 89% yield, whereas the reaction from its non-substituted analogue 150a required more drastic conditions (50 atm. H2, 50 °C, 16 h) and was less efficient (50% yield of 151a). Though the exact role of dppm bridge modification in the Guerbet reaction is not clear for the moment, it can be proposed that the presence of Ph substituent increases the acidity of the adjacent C-H bond thus facilitating hydrogenation/dehydrogenation steps involved in this process. Scheme 77. Simplified scheme of cooperative hydrogen activation with Mn(I) dppm complexes 146.

Later on Peng and coll. have shown that catalytic systems generated in situ from MnCl2 or Mn(CO)5Br with simple triphenylphosphine were able to perform the N-alkylation of anilines with primary alcohols (Scheme 78) [START_REF] Wei | N-alkylation of amines with alcohols catalyzed by manganese(II) chloride or bromopentacarbonylmanganese(I)[END_REF]. The combination of 10 mol% MnCl2 with two-fold excess of PPh3 was found to be optimal, whereas the use of more electron-donating triaryl-(Tol3P, Mes3P) and trialkylphosphines (PEt3, PCy3, PtBu3) provided a deleterious effect for the catalytic activity. A variety of anilines was alkylated in moderate-to-good yield using slight excess of benzylic alcohols and sur-stoichiometric amount of strong base. While, halogens and various heterocycles were tolerated in this protocol, the substrates bearing CN, ester, NO2 and OH functions afforded no desired products. In contrast to MnCl2/2PPh3 combination and Mn(I) complexes with NN ligands (Scheme 72), the catalytic system derived from Mn(CO)5Br was able to activate aliphatic primary alcohols (besides thus showing the preference of P-based ligands bidentate amines in type of processes. Scheme 78. Alkylation of aromatic amines with primary alcohols catalyzed by the combination of MnCl2 or Mn(CO)5Br with two equivalents of PPh3.

Similarly to hydrogenation and transfer hydrogenation processes, some remarkable results by using Mn(I) complexes bearing NHC-containing ligands were recently obtained. In 2019, the group of Ke reported highly efficient alkylation of anilines with alcohols catalyzed by bis(NHC) complexes 152 (Scheme 79) [START_REF] Huang | Room temperature N-heterocyclic carbene manganese catalyzed selective N-alkylation of anilines with alcohols[END_REF]. Complex 152a was found to be highly active under very mild conditions (100 mol% tBuOK, 25 °C, 24 h) leading to 85% product yield in a model reaction between PhNH2 and BnOH at 1.5 mol% catalyst loading, whereas its analogue 152b based on more bulky bis(NHC) ligand was less efficient (58% yield). Importantly, the presence of two NHC donor moieties was critical since the related complexes 153 bearing pyridine-NHC ligands were practically inactive under these conditions.

The unique catalytic properties of complex 152a constituted a highly efficient protocol for aniline alkylation using only stoichiometric amount of alcohol, which represents the first example of such process catalyzed by a 3d transition metal complex at room temperature. Both benzylic and aliphatic alcohols can be successfully employed to form the products in good-to-excellent yields. The reaction was compatible with numerous functional groups including halogens, CF3 and alkenes as well as coordinating SMe, thiophene and pyridine moieties requiring however for the latter the increase of catalyst charge to 3 mol%. High activity of catalyst 152a also allowed to use of methanol as alkylating agent at higher temperature (100 °C) to afford the target products typically in 70-90% yield keeping similar functional group tolerance. Stoichiometric experiments and DFT calculations revealed that both alcohol dehydrogenation and imine hydrogenation steps are likely to proceed via an outer-sphere mechanism with the last step being kinetically rate-determining. Scheme 79. Selective mono-alkylation of aromatic amines with alcohols catalyzed by Mn(I) complexes 152 bearing bis(NHC) ligand.

Shortly after the same group has shown the application of complex 152a for the α-alkylation of ketones and Friedländer's quinoline synthesis (Scheme 80) [START_REF] Lan | Nonbifunctional outer-sphere strategy achieved highly active α-alkylation of ketones with alcohols by N-heterocyclic carbene manganese (NHC-Mn)[END_REF]. Unlike the previous case, complexes 152a and 61f exhibiting NHC-pyridine and bipyridine ligands also displayed some catalytic activity in acetophenone alkylation with BnOH being ca. 2 times lower than bis(NHC) system (27-30% vs. 64% yield). Under optimized conditions the reaction possessed a wide substrate scope with functional group tolerance similar to that previously observed in the work of Maji (Scheme 64) [START_REF] Barman | Phosphine-free NNN-manganese complex catalyzed αalkylation of ketones with primary alcohols and Friedländer quinoline synthesis[END_REF]. Despite the catalyst charge for bis(NHC) complex 152a in this case was higher than for NNN pincer-type Scheme 80. Alkylation of ketones with primary alcohols and Friedländer synthesis of quinolines catalyzed by Mn(I) complex 152a bearing bis(NHC) ligand. complex 134b (4 vs. 2 mol%), the bidentate system operated at lower temperature (110 vs. 140 °C) in shorter reaction time (2 vs. 24 h) using weaker base (NaOH vs. tBuOK). Though it is difficult to benchmark precisely the activity of CC and NNN systems due to different reaction conditions, it can be noted that the former one is more efficient in terms of TOF (11.3 and 1.9 h -1 for 152a and 134b, respectively) in the representative reaction involving acetophenone and BnOH.

The comparison of bidentate and pincer-type Mn(I) complexes containing NHC donor units in β-alkylation of secondary alcohols was recently performed by Ke and coll. (Scheme 81) [START_REF] Lan | Sustainable and selective alkylation of deactivated secondary alcohols to ketones by non-bifunctional pincer N-heterocyclic carbene manganese[END_REF]. It was shown that complex 152a at 2 mol% charge was superior to bulkier CNC pincer-type derivatives 154b-c both in terms of activity (82% vs. 45-56% yield for acetophenone alkylation with BnOH) and selectivity towards the formation of ketone products (93% vs. 78-81%). However, pre-catalyst 154a bearing methyl groups at the nitrogen atoms was found to be much efficient resulting in 93% of the target product with more than 99% selectivity even at 1 mol% catalytic charge. A variety of aromatic ketones can be alkylated with benzylic alcohols in good-to-excellent yields using typically 1 mol% of 154a in the presence of cheap base NaOH. Aliphatic alcohols were employed as well with good efficiency, however five-fold amount of catalyst was required in that case. The presence of coordinating N-or S-groups in the substrates has a deleterious effect to the catalytic activity, but still 30-50% product yields could be achieved using 2 mol% catalytic charge. Scheme 81. Selective β-alkylation of secondary alcohols with primary alcohols catalyzed by Mn(I) complex 154a bearing NHC-pyridine-NHC ligand.

As in the previous case the performance of NHC-containing system was found to be better than for the related amine-based complex 145c described by Yu and coll. (Scheme 74) [START_REF] Liu | Manganese-catalyzed β-alkylation of secondary alcohols with primary alcohols under phosphine-free conditions[END_REF] revealing TOF 14.8 and 1.8 h -1 respectively in the alkylation of acetophenone with BnOH at the same temperature and base amount. The last three examples clearly highlight the great potential of Mn(I) NHC complexes for the future design of hydrogenation catalysts.

Evolution of the ligand design in Mn-based hydrosilylation catalysis

Hydrosilylation of activated carbonyl derivatives using Mn(I) and Mn(II) complexes bearing different types of donor ligands has already been well developed [START_REF] Trovitch | The emergence of manganese-based carbonyl hydrosilylation catalysts[END_REF][START_REF] Yang | Manganese-catalyzed hydrosilylation reactions[END_REF][START_REF] Darcel | Iron-, cobalt-, and manganese-catalyzed hydrosilylation of carbonyl compounds and carbon dioxide[END_REF][START_REF] Royo | Recent advances in catalytic hydrosilylation of carbonyl groups mediated by welldefined first-row late transition metals[END_REF][START_REF] Bhunia | Earth-abundant metal catalyzed hydrosilylative reduction of various functional groups[END_REF][START_REF] Nguyen | Manganese-catalyzed hydrosilylation and hydroboration reactions, Manganese-catalyzed hydrogen-borrowing and dehydrogenative coupling reactions[END_REF] providing in particular several catalytic systems working at room temperature, under solvent-free conditions with outstanding TOF numbers up to 297000 h -1 [START_REF] Mukhopadhyay | Hydrosilylation of aldehydes and formates using a dimeric manganese precatalyst[END_REF], 76800 h -1 [START_REF] Mukhopadhyay | A highly active manganese precatalyst for the hydrosilylation of ketones and esters[END_REF] and 19800 h -1 [START_REF] Mukhopadhyay | Mechanistic investigation of bis(imino)pyridine manganese catalyzed carbonyl and carboxylate hydrosilylation[END_REF] for aldehydes, ketones and alkylformates, respectively. In contrast, only few Mn-based catalytic systems for efficient and chemoselective reduction of carboxylic acid derivatives as well as for hydrosilylation of multiple C-C (alkenes, alkynes) and C-N (imines, nitriles) bonds are known in literature. A rapid progress in this area achieved during the last five years owing to the utilization of simpler manganese complexes is performed in this final section.

The first example of hydrosilylation of esters catalyzed by Mn(II) complexes bearing bidentate ligands was reported in 2017 by Sydora, Stradiotto, Turculet and coll. (Scheme 82) [START_REF] Kelly | A manganese pre-catalyst: mild reduction of amides, ketones, aldehydes, and esters[END_REF]. It was shown that well-defined complex 155 based on N-phosphinoamidinate ligand was capable to reduce methyl benzoates with phenylsilane at room temperature with more than 90% conversion after 4 h. Aliphatic methyl heptanoate was also reduced in almost quantitative manner under these conditions. While Cl and CF3 substituents in para-position of the aromatic ring were well tolerated the reaction failed for NO2 or pyridine containing esters as well as for lactones like phthalide. The catalytic activity of complex 155 was comparable with state-of-the-art Mn(II) pre-catalysts based on multidentate PDI ligands developed by Trovitch [START_REF] Mukhopadhyay | A highly active manganese precatalyst for the hydrosilylation of ketones and esters[END_REF]. Scheme 82. Hydrosilylative reduction of esters and tertiary amides catalyzed by Mn(II) complex 155 bearing N-phosphinoamidinate ligand.

The same catalyst 155 could be employed in the hydrosilylative reduction of tertiary amides with phenylsilane to form the corresponding amines (Scheme 82) [START_REF] Kelly | A manganese pre-catalyst: mild reduction of amides, ketones, aldehydes, and esters[END_REF]. Typically high conversions were observed for aromatic and aliphatic derivatives using only 2 mol% of catalyst at room temperature for 18-48 h, while in some cases increase of catalyst loading to 5 mol% and heating at 75 °C was required. Though halogens, OMe, and thiophene moieties were compatible with this protocol, the presence of CN or NO2 groups led to complex mixtures of products. The catalytic system was also not applicable to secondary and primary amide substrates. Interestingly, it was demonstrated that simple bis(amide) complex Mn(N(SiMe3)2) 156 could also catalyze amide reduction at 75 °C but with lower efficiency thus demonstrating the importance of bidentate PN ligand [START_REF] Kelly | A manganese pre-catalyst: mild reduction of amides, ketones, aldehydes, and esters[END_REF][START_REF] Macaulay | A comparative analysis of hydrosilative amide reduction catalyzed by first-row transition metal (Mn, Fe, Co, and Ni) N-phosphinoamidinate complexes[END_REF].

Later, Trovitch and coll. demonstrated that binuclear Mn(II) hydride complex 157 bearing bulky βketiminate ligands was efficient for alkene hydrosilylation (Scheme 83) [START_REF] Mukhopadhyay | A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight[END_REF]. Only phenylsilane was found to be suitable for this transformation, whereas less active secondary (Ph2SiH2) or tertiary (Ph3SiH, (EtO)3SiH) silanes gave no conversion. Though this reaction proceeded under rather harsh conditions (prolonged heating at 130 °C in benzene), it was found to afford excellent regioselectivity. Indeed, for the aliphatic terminal alkenes the exclusive formation of anti-Markovnikov addition productlinear silane was observed in good yield. Disubstituted geminal alkenes, cyclic alkenes and dienes were also reactive, but longer reaction time was typically required in these cases. This difference in relative reaction rate allowed in particular to perform fully selective hydrosilylation of terminal alkene in myrcene in 96% yield keeping intact geminal and trisubstituted moieties. In contrast the reaction of styrene derivatives under these conditions led to the preferential formation (89-100% selectivity) of branched silanes resulting from Markovnikov addition. Interestingly, the presence of coordinating PPh2 group in styrene substrate did not strongly affect the catalytic process and the corresponding product was isolated in 78% yield as a single isomer. The catalyst 157 was also successfully used to form silicones from 1,2,4-trivinylcyclohexane and PMHS under solvent-free conditions at 0.05 mol% catalyst charge. Scheme 83. Hydrosilylation of aliphatic and aromatic alkenes catalyzed by Mn(II) complexes 157 and 158 bearing β-ketiminate and PDI ligands, respectively.

The comparison of β-dekeniminate catalyst 157 with catalytic system based on a combination of the Mn(II) complex 158 bearing tridentate PDI ligand and tBuONa activator (Scheme 83) [START_REF] Carney | Manganese-catalyzed hydrofunctionalization of alkenes[END_REF] shows the superiority of the latter for the hydrosilylation of terminal aliphatic alkenes. In this case the reaction proceeds rapidly at room temperature with anti-Markovnokov selectivity and can be employed for less reactive tertiary silanes. However, styrene substrates afforded only ca. 30% yields of linear silanes under these conditions thus showing that the use of less active bidentate catalytic system can be complementary to selectively achieve the alternative isomer.

The reaction mechanism for alkene hydrosilylation catalyzed by Mn(II) complex 157 based on stoichiometric experiments is shown in Scheme 84 [START_REF] Mukhopadhyay | A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight[END_REF]. It was demonstrated that contrary to classic Chalk-Harrod mechanism implicating Si-H bond activation binuclear catalyst 157 undergoes dissociation and alkene insertion processes leading to σ-alkyl intermediates 159 and 160 (the structure of 160 derived from 4-tBu-styrene was confirmed by X-ray diffraction), which then form the final products and starting hydride complex 157 upon σ-bond metathesis with silane. While the formation of primary and thiophene groups. In contrast 3-cyanopyridine was reacted sluggishly to afford only 8% yield and PentCN was found to be unreactive at all. Scheme 86. Hydrosilylation of nitriles catalyzed by Mn(I) complex 162 bearing benzimidazolepyridine ligands.

The replacement of nitrogen donor moieties in Mn(I) complexes by phosphines [START_REF] Behera | Hydrosilylation of esters catalyzed by bisphosphine manganese(I) complex: selective transformation of esters to alcohols[END_REF] and NHCs [START_REF] Sousa | Bench-stable manganese NHC complexes for the selective reduction of esters to alcohols with silanes[END_REF] in the context of ester hydrosilylation (Scheme 87) was recently performed in the groups of Bagh and Royo, respectively. Both catalytic systems led to the selective formation of alcohols. In particular, complex 163 derived from Xantphos was capable to reduce a variety of esters in high yield under solvent free conditions using 1 mol% of catalyst and one equivalent of phenyl silane per ester moiety. Secondary (Ph2SiH2) or tertiary (Ph3SiH, Et3SiH) silanes could also be employed with similar efficiency, but longer reaction time was required to achieve a full conversion (12 h vs. 6 h for PhSiH3 for PhCO2Bn reduction). This process was equally efficient for the substrates containing aliphatic or aromatic groups at both carboxylic acid and alcohol components including lactones and even poly (1,6-hexamethyleneadipate). The list of functional group compatibility included halogens, furan, terminal or internal alkene and free hydroxyl in the α-position to the ester moiety. Scheme 87. Hydrosilylation of esters to alcohols catalyzed by Mn(I) complexes 163 and 152a bearing diphosphine and bis(NHC) ligands, respectively. NHC-containing Mn(I) complexes reported by Royo et al. were operating under similar conditions with bis(NHC) catalyst 152a being slightly more active than its pyridine-NHC congener 153a requiring 3 and 6 hours, respectively for the quantitative reduction of methyl benzoate at 1 mol% catalyst charge. Interestingly, complex 152a can be reused by five successive additions of ester and silane leading after 4 days to a TON of 485 for the reduction of PhCO2Me. Importantly, high activity of this system allowed to replace phenylsilane with 3 equivalents of cheaper PMHS resulting in comparable product yields. While simple aromatic and aliphatic esters could be readily reduced using complex 152a, the reaction of substrates containing NO2, NH2, pyridine, CN and conjugated alkene groups led to the formation of complex product mixtures.

Selective hydrosilylative transformation of esters to ethers catalyzed by Mn(I) σ-acyl complex 164b supported by PPh3 was reported in early work of Cutler et al. (Scheme 88) [START_REF] Mao | Catalytic hydrosilylation of organic esters using manganese carbonyl acetyl complexes[END_REF]. It was shown that the conversion of EtOAc to Et2O with 85% NMR yield could be achieved within 15 min using 3 mol% of catalyst 164b at room temperature in the presence of PhSiH3, whereas for less reactive silanes such as Ph2SiH2 or Ph2MeSiH the reaction stopped at the formation of the corresponding silylacetals isolated in 89-95% yield. Phosphine-free acyl complex 164a in combination with PhSiH3 worked much slower under these conditions requiring 1.5 h for full conversion. Curiously, even simple carbonyl derivatives such as Mn(CO)5Me and Mn(CO)5Br were catalytically active in the reduction of ethyl acetate to form Et2O in 85 and 55% NMR yield, respectively after 4 h. This protocol was found to be very efficient for the aliphatic esters besides very bulky and phenol-derived ones, in which the mixtures of ethers and silylacetales were formed. Scheme 88. Hydrosilylation of esters to ethers catalyzed by Mn(I) σ-acyl complex 164b bearing monodentate PPh3 ligand.

The potential of Mn(I) carbonyl complexes incorporating simple monodentate phosphines in hydrosilylation of alkynes was demonstrated by Wang and coll. (Scheme 89) [START_REF] Yang | Dichotomy of manganese catalysis via organometallic or radical mechanism: stereodivergent hydrosilylation of alkynes[END_REF]. While the hydrosilylation of diphenylacetylene with PhSiH3 at 150 °C catalyzed with 5 mol% of Mn(CO)5Br led to only 25% combined yield of the corresponding vinylsilanes with 1.3:1 E/Z ratio, the addition of PPh3 or AsPh3 greatly improved both yields (60-85%) and E-stereoselectivity (14:1 to 43:1). Curiously, the addition of PCy3, dppe or bipy-type ligands totally inhibited the catalytic activity. Both well-defined and generated in situ complexes 165 showed comparable product yields, but the Estereoselectivity was found to be slightly better in the presence of an excess of monodentate ligand. Finally, the catalytic system generated in situ from 5 mol% of Mn(CO)5Br and 20 mol% of AsPh3 revealed to be the most efficient in terms of yield and stereoselectivity was observed with a variety of internal alkynes and silanes. Diaryl alkynes could be stereoselectively hydrosilylated in high yield using primary, secondary and tertiary silanes. The reaction for arylalkyl-substituted internal alkynes proceeded with synthetically useful level of regioselectivity (1:5 to 1:13) favoring the addition of silyl fragment to the adjacent position of the alkyl group. Halogens, CF3 and thiophene functions were well tolerated and nitrile group remained completely untouched.

The reaction mechanism proposed based on extensive stoichiometric studies with phosphine complexes is presented in Scheme 90. The catalytic cycle starts with formation of σ-silyl intermediate 166a from the reaction of the bromide precursor 165a with diphenylsilane. The latter is transformed via thermal CO ligand substitution with diphenylacetylene to the η 2 -alkyne complex 167a, which undergoes the alkyne insertion across Mn-Si bond to form σ-vinyl derivative 168a. The coordination of silane molecule followed by σ-bond metathesis leads to the formation of the final product and the regeneration of silyl intermediate 166a.

Scheme 89. E-selective hydrosilylation of alkynes catalyzed by well-defined or generated in situ Mn(I) complexes 165. Scheme 90. Proposed reaction mechanism for E-selective hydrosilylation of diphenylacetylene with Ph2SiH2 catalyzed by Mn(I) complex 165a.

While singular low-efficient examples of hydrosilylation of terminal aliphatic alkenes using simple carbonyl complexes (CO)5MnSiPh3 [START_REF] Pratt | Hydrosilation catalysis via silylmanganese carbonyl complexes: thermal vs. photochemical activation[END_REF] and Mn2(CO) 10 [135] were known in literature, this reaction was generalized by Wang and coll. only in 2018 using Mn(CO)5Br (Scheme 91) [START_REF] Yang | Diverse fates of β-silyl radical under manganese catalysis: hydrosilylation and dehydrogenative silylation of alkenes[END_REF]. The optimization of the reaction between styrene and Me2BnSiH to avoid dehydrogenative alkene silylation (vide infra) revealed that heating in hexane in the presence of 5 mol% of catalyst and two equivalents of tertiary silane was required to provide quantitatively anti-Markovnikov addition product with excellent selectivity. It was also shown that both Mn(CO)5Br and Mn2(CO)10 complexes under UV irradiation at room temperature gave similar results but with slightly lower conversions. A vast variety of styrenes and tertiary silanes was readily employed at 60 °C using this catalytic system to form the target products in 80% average yield. The scope for terminal aliphatic alkenes was less extended and heating up to 150 °C was sometimes required. Importantly, the presence of halogen, CF3, C6F5, pyridine and even reducible ester groups was perfectly tolerated providing much larger scope than for Mn(II) catalysts 157 and 158 bearing β-keteniminate and PDI ligands (Scheme 83). Scheme 91. Hydrosilylation of alkenes catalyzed by Mn(CO)5Br.

Very recently, the group of Xie studied in detail the role of monodentate phosphine additives in anti-Markovnikov alkene hydrosilylation in the presence of an equimolar amount of silane catalyzed by hydride complexes 170 generated in situ from Mn2(CO)10, phosphine and silane (Scheme 92) [START_REF] Dong | Manganese-catalysed divergent silylation of alkenes[END_REF]. It was shown on the model reaction between TolCH=CH2 and Et3SiH that the reaction efficiency was improved with the increase of both steric volume of the phosphine ligand (10%, 35% and 55% for 170a-c, respectively) and their accepting properties (59%, 79% and 80% for 170d-f, respectively). Among all ligands JackiePhos-derived complex 170e was selected to evaluate the substrate scope being optimal in terms of yield and chemoselectivity vs. dehydrogenative silylation process. Despite the reaction yields were close to simpler of Wang, it is notable that ketone group was also tolerated in such process and hydrosilylation of 1,2,4-trivinylcyclohexane with PMHS led to silicone material in 76% yield. Scheme 92. Hydrosilylation of alkenes catalyzed by generated in situ Mn(I) hydride complexes 170 bearing monodentate phosphine ligands.

The proposed reaction mechanism for styrene hydrosilylation with simple Mn(I) carbonyl complexes (Scheme 93) includes the formation of 17-electron manganese radicals 171-172 upon thermal or photochemical cleavage of Mn-Mn or Mn-Br bonds. These species abstract hydrogen atom from silane to form the corresponding Mn(I) hydride complexes and the silyl radical. The latter undergoes the addition across C=C alkene bond to form benzyl radical reacting with hydride complexes to afford the final product and the starting 17-electron organometallic species.

The formation of minor amounts of vinylsilane byproducts in alkene hydrosilylation catalyzed by Mn(I) carbonyl complexes via the dehydrogenative silylation motivated the group of Wang to search the optimal conditions for the realization of this highly challenging transformation as a major reaction pathway. This goal was initially achieved for a series of styrenes using 10 mol% of Mn2(CO)10 at high temperature in the presence of sub-stoichiometric amount of particular tertiary silane (Scheme 94) [START_REF] Yang | Diverse fates of β-silyl radical under manganese catalysis: hydrosilylation and dehydrogenative silylation of alkenes[END_REF]. Scheme 93. Simplified mechanism of hydrosilylation of styrene with tertiary silanes catalyzed by Mn(CO)5Br and phosphine-based Mn(I) complexes 170.

Later, the group of Yang showed that this transformation can be generalized for allylarenes to form allylsilane products in moderate-to-good yield (Scheme 94) [START_REF] Wu | Manganese catalyzed dehydrogenative silylation of alkenes: direct access to allylsilanes[END_REF]. The key factor here allowing to control the selectivity for other tertiary silanes was the solvent-free conditions at more elevated temperature. Despite harsh conditions several functional groups such as CF3, halogens, NO2, CN, thioether, furan, thiophene were well-tolerated. Scheme 94. Dehydrogenative silylation of styrenes and allylarenes catalyzed by Mn2(CO)10.

The real breakthrough in this area was found later by Xie and coll., who showed that the application of generated in situ complex 173 bearing short-bite diphosphine ligand led to the development of extremely general and robust procedure for dehydrogenative silylation of diverse terminal alkenes (Scheme 95) [START_REF] Dong | Manganese-catalysed divergent silylation of alkenes[END_REF] outperforming all transition metal catalysts known to date.

2 .

 2 Manganese complexes bearing bidentate donor ligands in hydrogenation-type reactions 2.1. Hydrogenation of unsaturated C=O, C=N and C=C double bonds 2.2. Transfer hydrogenation processes 2.3. Hydrogen-borrowing transformations 3. Evolution of the ligand design in Mn-based hydrosilylation catalysis 4. Conclusions Declaration of Competing Interest

Scheme 35 .

 35 Scheme 35. Mn(I) complexes bearing bidentate ligands studied by DFT calculations as CO2 hydrogenation catalysts.

Scheme 55 .

 55 Scheme 55. Transfer hydrogenation of ketones and chalcones catalyzed by Mn(I) complexes 124 bearing phosphine-amine ligands.

Acknowledgements

Acknowledgements

We thank the Centre National de la Recherche Scientifique (CNRS), the Russian Science Foundation (grant no. 20-73-00311), the Université Toulouse III, Paul Sabatier, and the Institut Universitaire de France (IUF) for a general support. E. S. G. is grateful to French Embassy in Moscow for a joint PhD fellowship (Vernadski program). R. B. is grateful to the Embassy of Yemen in Paris and the program Pause for financial support.

Scheme 84. Simplified reaction mechanism for Markovnikov and anti-Markovnikov alkene hydrosilylation catalyzed by Mn(II) β-keteniminate complex 157. alkyl species 159 is expected for aliphatic alkenes, styrenes are typically prone to react at the benzylic position to form secondary alkyl intermediate of type 160 thus explaining the observed regioselectivity trend.

The evaluation of more stable carbonyl Mn(I) complexes bearing bidentate NN ligands in the hydrosilylation of esters and carboxylic acids was first reported by Werlé, Leitner and coll. (Scheme 85) [START_REF] Martínez-Ferraté | Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(I) complexes bearing triazole ligands[END_REF]. It was demonstrated that the hydrosilylation of ethyl benzoate catalyzed by imino-triazole complex 113c (2 mol% cat, 2 eq. PhSiH3, THF, 80 °C, 3 h) led to 53% of substrate conversion providing the ether BnOEt with 94% selectivity. Similar bidentate complex 113a bearing aminetriazole ligand was slightly more active under the same conditions (67% conversion) affording equal amount of ether and alcohol products after hydrolysis, whereas the use of its benzyl-substituted analogue 113b led to only 18% conversion with preferential formation of alcohol product. The activity of pincer-type analogue 161 obtained by a formal addition of phosphine arm to 113c was found to be superior leading to a full substrate conversion to form both products with a ratio dependent on the reaction temperature and the rate of silane addition. This reduction protocol was applicable for both benzoic and aliphatic ester derivatives to form generally alcohols as major products. Interestingly, the reduction of benzoic acid using complexes 113c and 161 afforded almost exclusively the benzylic alcohol with again higher activity of the PNN system compared to the NN one (50 vs. 28% conversion). Simple aliphatic and aromatic carboxylic acids were readily reduced in good-to-excellent yield, while in the case of m-nitrobenzoic acid 12% of the target product was only obtained.

Scheme 85. Hydrosilylation of esters and carboxylic acids catalyzed by Mn(I) complexes bearing triazole-amine ligands.

The hydrosilylation of nitriles to amines catalyzed by Mn(I) complexes based on bidentate diamine ligands was reported by the group of Kundu (Scheme 86) [START_REF] Ganguli | Benzimidazole fragment containing Mn-complex catalyzed hydrosilylation of ketones and nitriles[END_REF]. Complex 162 exhibiting the benzimidazole-pyridine scaffold was more efficient than its secondary amine analogue 114c in the reduction of PhCN both in terms of conversion (>99% vs. 80%) and product selectivity towards benzylamine (25% of imine BnN=C(H)Ph was formed in the latter case). The reduction of numerous aromatic and benzylic nitriles was successfully achieved with a tolerance to halogen, OMe, CF3, SMe Scheme 95. Highly versatile dehydrogenative silylation of alkenes catalyzed by Mn(I) hydride complex 173 in situ from Mn2(CO)10 and short-bite diphosphine ligand.

Remarkably, the reaction was extremely specific to this ligand providing only traces of desired product for structurally similar Ph2PNHPPh2 and dppm ligands. The substrate scope for both tertiary silanes and alkenes was extremely vast including not only regular aromatic and aliphatic olefins, but also conjugated dienes and even simplest C2-C4 alkenes. Functional group tolerance was remarkable as well since this protocol was fully compatible with OH, CN, ketone, ester, amine, numerous types of heterocycles and can be applied for the direct functionalization of several drugs and natural products.

The reaction mechanism based on DFT calculations is shown in Scheme 96. The hydride complex 173 formed in situ from Mn2(CO)10 and the corresponding diphosphine undergoes at high temperature an homolytic Mn-H bond cleavage to form dihydrogen and 17-electron radical 174, which is capable to abstract hydrogen atom from tertiary silane to regenerate again hydride species. The subsequent recombination of Mn-centered radical 174 and alkyl radical obtained from Et3Si • and alkene leads to the formation of σ-alkyl intermediate 175. The decarbonylation of the latter to the complex 176 stabilized by an agostic C-H interaction followed by β-hydride elimination afforded the final vinylsilane closing the catalytic cycle. It was found that the origin of chemoselectivity in this process resides in the lower energetic barrier for the reaction of alkyl radical with metallo-radical 174 (23.9 kcal/mol) than hydrogen abstraction from hydride 173 (27.9 kcal/mol) due to the bulky nature of diphosphine ligand. Scheme 96. Simplified mechanism of dehydrogenative silylation of TolCH=CH2 with Et3SiH catalyzed by Mn(I) complex 173 based on DFT calculations.

The application of Mn2(CO)10 under photochemical activation for the selective transformation of carboxylic acids to bis-silylacetals, which could be either isolated or hydrolyzed to the corresponding aldehydes was first reported by Sortais and coll. (Scheme 97) [START_REF] Zheng | Selective reduction of carboxylic acids to aldehydes through manganese catalysed hydrosilylation[END_REF]. The chemoselectivity of this transformation was perfectly controlled by the choice of appropriate reducing agent leading to aldehyde derivatives for tertiary silanes (Et3SiH, Ph2MeSiH, PhMe2SiH) and to alcohols with more reactive silanes (Ph2SiH2, Et2SiH2, TMDS). This protocol was very efficient for the reduction of aliphatic carboxylic acids besides sterically bulky ones such as 2-phenylpropionic acid. Benzoic acids reacted more sluggishly (30-40% yield), whereas stronger trifluoroacetic acid was totally unreactive. The reaction tolerated the presence of halogens, CF3, unprotected aromatic amine or indole and thiophene moieties and failed for nitro-substituted derivatives. While internal alkene moiety was fully tolerated, the terminal one in hept-6-enoic acid underwent partial hydrosilylation to form the 2:1 mixture between unsaturated and saturated products. The reduction of phenol-or conjugated C=C bond containing substrates was accompanied by OH group silylation and hydrogenation, respectively. Scheme 97. Chemoselective reduction of carboxylic acids and esters to silylacetals catalyzed by Mn2(CO)10 under photochemical activation.

Similar protocol was later applied by the same group for the selective transformation of esters to mixed silylalkylacetales (Scheme 97) [START_REF] Wei | Manganese and rhenium-catalyzed selective reduction of esters to aldehydes with hydrosilanes[END_REF]. A variety of esters derived was reduced with excellent selectivity using only slight excess of Et3SiH. The reaction was almost insensitive to the nature of alcohol fragment and to steric crowding comparing to the reduction of carboxylic acids providing 70-90% yields for benzoic or 2-phenylbutanoic acid derivatives. As in the previous case halogens, CF3, internal alkene and furan moieties were well tolerated.

The hydrosilylative reduction of carboxylic acids to alcohols using Mn(CO)5Br under thermal conditions was recently reported by Werlé and coll. (Scheme 98) [START_REF] Antico | Reduction of carboxylic acids to alcohols via manganese(I) catalyzed hydrosilylation[END_REF]. This process readily proceeded only with 2.5 equivalents of phenylsilane, whereas the use of less active Ph2SiH2 or PMHS afforded ca. 30% yield for the reduction of 3-cyclohexylpropanoic acid using 6 and 10-fold excess of silane, respectively. Both aliphatic and aromatic carboxylic acids were smoothly converted to the corresponding alcohols with the tolerance to halogen, CF3, OH, thiophene and indole groups. As for others Mn-based catalytic systems for the reduction of carboxylic acid derivatives the presence of nitro group resulted in complete inhibition of the process. The reaction for levulinic acid led to the formation of the corresponding diol due to concomitant reduction of ketone group. Interestingly, the group of Garcia showed that the same transformation under more forcing conditions (2.5 mol% Mn(CO)5Br, 100 °C, toluene, 24 h) afforded 2-methyltetrahydrofuran in good yield [START_REF] Roa | Mild reduction with silanes and reductive amination of levulinic acid using a simple manganese catalyst[END_REF]. Scheme 98. Hydrosilylative reduction of carboxylic acids to alcohols with phenylsilane catalyzed by Mn(CO)5Br.

The hydrosilylation of alkynes with PhMe2SiH catalyzed by Mn2(CO)10 in the presence of peroxide activator under thermal conditions was initially reported by the group of Wang (Scheme 99) [START_REF] Yang | Dichotomy of manganese catalysis via organometallic or radical mechanism: stereodivergent hydrosilylation of alkynes[END_REF]. In contrast to the results obtained with phosphine/arsine derivatives of Mn(CO)5Br (Scheme 89), under these conditions the reaction was found to be highly Z-selective. A series of vinylsilanes was obtained in good yield from diaryl-and alkylaryl-substituted acetylenes, being perfectly regioselective in the latter case for the formation of products, in which silyl group was attached to the alkyl end. Terminal alkynes such as 3-ethynylthiophene were also smoothly reacted to form the product in 78% yield.

Later, Zhang and coll. have shown that similar transformation can be performed at room temperature under UV irradiation and further expanded the application scope (Scheme 99) [START_REF] Liang | Visible-light-initiated manganesecatalyzed E-selective hydrosilylation and hydrogermylation of alkynes[END_REF]. In a complement to the results of Wang, it was demonstrated that aliphatic internal alkynes were also suitable substrates. This protocol was highly efficient for hydrosilylation of a broad scope of nonfunctionalized aromatic and aliphatic terminal alkynes using different tertiary silanes to form the corresponding vinylsilanes in excellent yields and 93-99% Z-selectivity. Reducible ketone and ester groups remained intact during the transformation and this method was successfully applied to the modification of complex natural products. Scheme 99. Z-selective hydrosilylation of alkynes catalyzed by Mn2(CO)10 under thermal and photochemical activations.

The reaction mechanism for both processes (Scheme 100) includes the homolytic cleavage of Mn-Mn bond in Mn2(CO)10 either in the presence of peroxide at high temperature [START_REF] Yang | Dichotomy of manganese catalysis via organometallic or radical mechanism: stereodivergent hydrosilylation of alkynes[END_REF] or via photochemical activation [START_REF] Liang | Visible-light-initiated manganesecatalyzed E-selective hydrosilylation and hydrogermylation of alkynes[END_REF]. The resulting 17-electron complex 171 abstracts a hydrogen atom from tertiary silane to form Mn(I) hydride 90 and silyl radical. The reaction of the latter with alkynes followed by hydrogen atom transfer from 90 to the resulting radical affords the final product with concomitant regeneration of starting odd-electron complex 171. The observed Z-selectivity in product formation is caused by the preferential trans-arrangement of alkene substituent and bulky silyl group in vinyl radicals to minimize steric constraints. Scheme 100. Proposed reaction mechanism for Z-selective hydrosilylation of alkynes catalyzed by Mn2(CO)10.

Conclusions

The results collected in this contribution demonstrate that manganese complexes bearing bidentate ligands are more efficient in transfer hydrogenation than their pincer-type analogues, show a comparable performance in hydrogenation processes and are generally less active for hydrogen borrowing reactions. In addition, there is also growing evidence of the importance of the hemilability phenomenon in tridentate Mn(I) catalysts being one of the key factors in the design of several systems with outstanding performance in (transfer) hydrogenation. Another important message arisen from the recent results in manganese catalysis clearly illustrates that cooperative ligands are not necessarily required for the design of efficient hydrogenation catalysts based on first row transition metals. Similar trend was evidenced in parallel in the area of Mn-catalyzed hydrosilylation showing that the evolution from highly sensitive Mn(II) catalysts bearing elaborated polydentate ligands to air-stable Mn(I) complexes containing simple mono-or donor ligands, often resulted in higher activity and/or chemoselectivity. Finally, in some reduction catalytic processes the utilization of trivial carbonyl derivatives such as Mn(CO)5Br or Mn2(CO)10 led to remarkable results.

One of the direct consequences for this rather counterintuitive aspect of manganese reduction catalysis, which can be shortly formulated as "simpler is sometimes better" could play an important role for the development of real sustainable 3d metal catalysis. Taking into account that the relative price of both metal and auxiliary ligands is equally important, manganese offers an unique opportunity to replace elaborated and expensive pincer ligands often unavoidable in Fe-or Co-based hydrogenation-type catalysts with ubiquitous phosphine ligands or bi-or tridentate scaffolds based on nitrogen-, carbon-or sulphur-based donor moieties. Reinvestigation of basic organometallic manganese chemistry in the context of search for new reaction pathways and unconventional catalyst activation modes will surely provide additional opportunities for the future development of greener homogeneous reduction catalysis attractive for industrial applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.