Christophe Fliedel
email: christophe.fliedel@lcc-toulouse.fr

Rinaldo Poli

sulfide and selenide bis(diphenylphosphino)amine (DPPA)-based ligands and their Nsubstituted/functionalized derivatives

Keywords: Aminophosphine -Phosphine oxide -Hybrid ligands -Chelating ligands -Transition metal complexes -Catalysis allyl, η 3 -CH2CHCH2, av, average, COD, 1,5-cyclooctadiene, Cp, η 5 -C5H5, Cp*, η 5 -C5Me5, DME, 1,2-dimethoxyethane XRD, X-ray diffraction

    Coordination chemistry of neutral mono-oxide, sulfide and selenide bis(diphenylphosphino)amine (DPPA)-based ligands and their N-substituted/functionalized derivatives

Introduction

Coordination/organometallic complexes hold a leading position in many areas of Chemistry, including for example homogeneous and heterogeneous catalysis, the development of high-tech materials or therapeutic molecules and photophysical chemistry. However, the development of suitable candidates for these applications is, in most cases, related to an appropriate design of the supporting ligand(s). In this context, short-bite ligands, such as bis(diphenylphosphino)amine (DPPA, Scheme 1) [START_REF] Appleby | [END_REF][2][3] or bis(diphenylphosphino)methane (DPPM), have attracted much attention for their ability to bring in close proximity two identical or different metal centers, allowing metal-metal interactions [4][5][6][7][8][9][10][11][12][13][14] or cooperative effects [15]. Derivation of these ligands by mono-or bis-oxidation do not only affect the bite-angle of the resulting chelating ligands, from 4-to 5-or 6-membered metallocycles, respectively.

The presence of different chemical functions, which might be a combination of hard and soft donors, in such heterotopic/hybrid ligands within the same molecule may generates hemilabile systems [16][17][18][19].

Consequently, the weak chelation may facilitate the generation of (re)active and/or coordinatively unsaturated species and lower the activation energy of metal-mediated transformations [20][21][22].

The (coordination) chemistry of DPPA and related ligands, such as DPPM or other aminophosphines (P-N bond containing molecules), was the subject of several reviews [START_REF] Appleby | [END_REF][2][3]23]. A certain interest in the mono-anionic DPPA-H -ligand (Scheme 1), resulting from the deprotonation of the N-H group of DPPA, and analog ligands (varying the P-substituents) could be evidenced by a couple of review articles [24][25][26]. A comprehensive review dedicated to the synthesis, coordination chemistry and applications of DPPA-type ligands, functionalized on the N atom by an additional chemical function (e.g. amine, (thio)ether, phosphine, Scheme 1) was recently provided by Fliedel, Ghisolfi and Braunstein [27].

We were interested in providing a survey of the coordination chemistry of neutral mono-(DPPA-E) and homo-and hetero-bis-oxide (P=O), sulfide (P=S) and selenide (P=Se) (DPPA-EE') bis(diphenylphosphino)amine-based ligands and their N-substituted/functionalized derivatives (Scheme 1, insets). The present review has been restricted to mono-oxidized systems (Scheme 1, solid inset), while a following report will be dedicated to recent work on bis-oxidized systems (Scheme 1, dashed inset) [28]. The influence of 1) the nature of the P=E donor, and 2) the substitution of the N-H group by a hydrocarbyl or heteroatom-containing group will be highlighted, based (especially) on NMR and structural data. The reactivity and applications of the resulting complexes is also discussed. However, coordination compounds in which the ligand(s) will be deprotonated (DPPA-E-H -in Scheme 1) are outside the scope of this review, which will focus on metal complexes supported by neutral DPPA-E ligands and their N-substituted derivatives. Our objective is to provide the researchers studying the coordination chemistry of such DPPA-E-type ligands a rapid and comprehensive access to the characteristic spectroscopic and structural data available for all complexes reported to date.

The coordination chemistry of bis-oxidized DPPA-EE' (E = O, S, Se) [26], and derivatives in which the PPh was replaced by other aryl, alkyl, or OR groups, has been overviewed in 1998 [29] and 2001 [30], but mostly concern metal complexes in which the ligands are deprotonated. As mentioned above, we will shortly provide an update on the coordination chemistry of neutral bis-oxidized DPPA-EE' (E = O, S, Se) ligands and their N-substituted derivatives, which will be complementary to the latter reviews [28]. The access to mixed phosphine-phosphine oxide ligands, their coordination chemistry and the catalytic applications of the resulting metal complexes, was thoroughly reviewed in the mid 2000's by Grushin [31]. However, DPPA-O and its derivatives were not discussed in that review. Scheme 1. Bis(diphenylphosphino)amine (DPPA) ligand and its derivatives. The solid inset highlight the ligands topic of this review, and the dashed inset mark the subject of our complement report [28].

DPPA mono-oxide (DPPA-O)

Woolins and co-workers reported the synthesis, isolation and characterization of the mixed P III /P V ligand DPPA-O, the mono-oxide derivative of DPPA, in the second half of the 90's (Scheme 2) [32,33].

DPPA-O was obtained by treatment of DPPA with H2O2 as oxidizing agent. The moderate yield obtained (ca. 49%) is due to the concomitant formation of the bis-oxide derivative (DPPA-O2, see section 6), even at low temperature (THF, -20°C). However, the lower solubility of the latter in THF allowed its separation. Ligand DPPA-O exhibits a characteristic 31 P NMR spectrum, containing two doublets centered at 28.1 (P III ) and 21.3 ppm (P V ) with a 2 JP,P=O coupling constant of 62 Hz (Table 1).

The integrity of the N-H moiety is shown by a characteristic ν(N-H) broad absorption band at 3031 cm - DPPA-O molecules are present in the solid-state, resulting in the formation of OPNHOPNH macrocycles (Figure 1). Characteristic structural data are summarized in Table 1. Scheme 2. Preparation of DPPA-O by oxidation of DPPA. The bis-oxide DPPA-O2 side product is removed by filtration (insoluble in THF) [32,33].

Figure 1. Solid-state structure of DPPA-O showing the H-bond between the P=O and N-H moieties [33].

Group 10 metal complexes

Group 10 metal complexes, including phosphine-stabilized species, are of great interest because of their multiple applications. Palladium complexes were extensively studied for their exceptional performances in catalysis, especially in cross-coupling reactions [34][35][36], and direct C-H functionalization/arylation [34,37,38], and continuing effort are devoted to develop new efficient catalysts for these transformations. Moreover, phosphine-containing palladium and nickel complexes found a niche in the field of olefin oligo/polymerization and co-polymerization with CO [27,[39][40][41]. Platinum complexes are well established in the photophysics and photochemistry areas for their unique luminescent properties [9,12,42]. More recently, platinum-catalyzed organic transformations, including by C-H functionalization, gained increasing interest [42][43][44].

Reaction between isolated DPPA-O and the dimeric [(allyl)Pd (μ-Cl)]2 precursor led to a cleavage of the chloride bridges and the formation of the mononuclear [(allyl)PdCl(DPPA-O)-P)] complex 1 (Scheme 3) [32]. In the latter, DPPA-O acts as a monodentate ligand, binding the metal center only through the phosphine ligand. As a result, the 31 P NMR resonance of the P III atom is strongly downfield shifted compared to the free ligand (Δδ = 32.8 ppm), while the P V resonance was only slightly affected (Table 1). For all Group 10 metal complexes of DPPA-O, both P III and P V resonances appear as doublets, which originate from an AX spin system, with JP,P=O couplings ranging from 3 to 37 Hz (Table 1).

Chelation of DPPA-O can be forced by a chloride abstraction reaction, using AgBF4 in CH2Cl2, affording the mononuclear cationic complex [(allyl)Pd(DPPA-O)-P,O)][BF4] (2, Scheme 3). The formation of a five-membered PdPNPO metallacycle could be confirmed in the solid-state by XRD analysis. Both P-N (av) and P=O bond lengths were not significantly affected by the coordination of the Pd center, while the PNP angle was found slightly more acute in 2 compared to free DPPA-O (Table 1). In contrast to 1, the 31 P NMR spectrum of 2 exhibits two doublets in the same (low field) region (Table 1). In the FT-IR spectra of complexes 1 and 2 a characteristic ν(N-H) absorption band at 3134 and 3180 cm -1 , respectively, confirms the absence of N-H deprotonation and the neutral nature of the ligand.

Similarly, the [(methallyl)Ni (DPPA-O)-P,O)][SbF6] (3) complex was prepared by a two-steps one-pot procedure that involves 1) a bridge cleavage reaction of [(methallyl)Ni (μ-Br)]2 with 2 eq. DPPA-O, followed by 2) a bromide abstraction using AgSbF6 (Scheme 3) [45]. In solution, the Ni II complex 3 probably adopts a (distorted) square-planar geometry, resulting in a diamagnetic species, which exhibits a 31 P NMR spectrum comparable to that of the closely related Pd II complex 2 (Table 1).

The N-H resonance could be observed, as a broad singlet at 5.74 ppm, in the 1 H NMR spectrum of 3.

The Ni II complex 3, along with other cationic Ni II complexes of mixed P/P=O chelating ligands, were evaluated in the catalytic oligomerization of ethylene [45]. All complexes are efficient catalyst precursors for the oligomerization of ethylene to linear, higher α-olefins. However, the moderate selectivity to α-olefins in the case of 3 (53%) highlights that the latter is also an efficient catalyst for the isomerization of α-olefins to internal olefins. The authors highlighted that the maximum chain length of produced olefins increased with decreasing the chelate ring size of the complex (5-to 7-membered metallocycles were studied). 9)] as deduced by 31 P NMR (Table 1). Successful halide exchange, from Cl to I, was performed by treatment of 4 (Pd) or 10 (Pt) with excess NaI, affording 6 (Pd) or 11 (Pt), respectively.

The 31 P NMR resonance of the PPh2 fragment of the DPPA-O ligand was strongly affected upon coordination to the Pd II centers in complexes 4-6 (17.6 < Δδ < 30.5 ppm), while the P=O resonance remained nearly unchanged, supporting non-coordination of the latter (Table 1). Moreover, the P III resonance of the cis-isomer of 4 was significantly more downfield shifted compared to those of the trans species [δ 58.3 (4-cis) vs. ≈ 47 (4-trans-6) ppm, Table 1].

The 31 P NMR spectra of the Pt II complexes exhibited, as expected, two doublets for the nonequivalent P III and P V centers. However, both are flanked by typical 1 JP,Pt and 3 JP=O,Pt satellites, which significantly vary as a function of the ligands arrangement (cis/trans, Table 1) [33]. Similarly to their palladium analogs, the 31 P NMR resonance of the PPh2 fragment in the platinum dihalide complexes (7-9) was downfield shifted compared to the free DPPA-O ligand (Δδ = 8-10 ppm), but without significant difference between cis and trans isomer (Table 1). The P=O moiety in 7-9 resonates in a similar range than the free DPPA-O ligand. The 31 P NMR spectra of the mixed halide/alkyl Pt II complexes (10)(11) are very similar to those of their dihalide analogs (7)(8)(9). However, the chemical shift of the coordinated PPh2 moiety are more strongly downfield shifted (ca. 20 ppm) and the JP,P=O coupling constants are weaker (Table 1).

The solid-state structures of complexes 7 (cis) and 10 (trans) could be established by XRD analyses, which confirmed a slightly distorted square planar geometry around the Pt II center [33]. The uncoordinated P=O bond lengths were slightly shorter than in the free DPPA-O ligand, while the P-N ones were nearly unaffected (Table 1). In contrast to the P,O-chelate complex 2 [PNP = 120.8(4)°], the PNP bond angles in 7 and 10 (131.5°), were found larger than in the free ligand [125.6(2)°]. Scheme 4. Group 10 metal complexes of DPPA-O [33].

As shown for the transformation of 1 to 2 (Scheme 3), the chelation of the DPPA-O ligand can be favored by ionization of the parent complexes, i.e. 4 (PdCl2) or 7 (PtCl2), using a silver salt (AgBF4) as chloride abstractor, to afford the dicationic bis-chelated cis-[M{(DPPA-O)-P,O}2][BF4]2 [M = Pd (12), Pt (13)] complexes (Scheme 5). The 31 1). In contrast, the neutral or cationic nature of the metal complexes does not affect much the 31 P NMR resonance of the PPh2 moiety (Δδ ≤ 2 ppm , Table 1). The cis arrangement of the DPPA-O ligands and the slightly distorted square planar coordination geometry around the metal center could be established by XRD in the case of the Pt II complex 13 (Table 1 and Figure 2). Both P-N and P=O bond lengths were found in the same range than in the free DPPA-O ligand, while the PNP angle imposed by the metal coordination was much more acute [118.1(7) vs. 125.6(2)°]. The other structural parameters involving the Pt II metal center were found closely related those of the cationic P,O-chelate Pd II complex 2 (Table 1).

The authors reported an alternative route to the formation of the dicationic bis-P,O-chelated complexes 12-13 [33]. This pathway involves in a first step the deprotonation of the DPPA-O N-H moiety by a strong base in 4 and 7, to afford the neutral bis-P,O-chelated complexes 12a-13a (not the topic of this review). The latter can then be selectively re-protonated with HBF4•OEt2 to form complexes 12-13 nearly quantitatively (Scheme 5). Cationic (12)(13) and neutral bis-P,O-chelated complexes (12a-13a) can readily be inter-converted through deprotonation/re-protonation reactions. The reactivity/acidity of the N-H fragment was also illustrated by the room temperature deprotonation of the transient cis-[Pt(CH3)2{(DPPA-O)-P}2] complex (14, Table 1), resulting from the reaction between [Pt(CH3)2(COD)] and DPPA-O (1:2 molar ratio), which afforded complex 13a and CH4 (2 eq.) [33]. Scheme 5. Formation of dicationic Pd II and Pt II complexes (12-13) of DPPA-O in P,O-chelate coordination mode [33]. [46]. The characteristic 31 P NMR data (of the DPPA-O ligand) of 15 were found very similar to its cis-[PtCl2{(DPPA-O)-P}2] (7) precursor (Table 1). Scheme 6. N-PPh2 bond cleavage of DPPA-O in 7 [46].

The formation of complex 15 resulted from a rather unusual/unexpected reactivity, and afforded several by-products, therefore the authors developed a more rational and general route to access Pt II complexes with mixed P-based ligands, through a bridge cleavage of the [PtCl(μ-Cl)(PMe2Ph)]2 dimeric precursor with 2 eq. of DPPA-O (Scheme 7) [46]. The latter reaction afforded exclusively the trans-[PtCl2{(DPPA-O)-P}(PMe2Ph)] complex (16-trans), which exhibited in its 31 P NMR spectrum a PPh2 resonance significantly downfield shifted compared to a related bis-DPPA-O complex, i.e. trans-[PtI2{(DPPA-O)-P}2] (9-trans), of ca. 7 ppm (Table 1), and accompanied by a typical and very large 2 JP,PMe2Ph coupling constant of 522 Hz. This difference is due to the stronger basicity of the trans PMe2Ph ligand, compared to the PPh2 moiety of DPPA-O. The authors evidenced that the presence of a catalytic amount of DPPA-O ligand cleanly and quantitatively isomerizes complex 16-trans into 16-cis, as evidenced by 31 P NMR by; 1) an upfield shift of the PPh2 signal in the same region than other cis-[PtCl2(P,P)] complexes (see e.g. 7, 8, 15, Table 1), and 2) a much lower value for the 2 JP,PMe2Ph coupling constant (18 Hz). In both complexes (16-trans and 16-cis), the dangling P=O moiety resonates in the same region as 7-11, 15 and free DPPA-O (Table 1). The structure of complex 16-cis, established in the solid-state by XRD analysis, confirmed a slightly distorted square-planar geometry around the Pt II center, with the most obtuse angle being PDPPA-O-Pt-PMe2Ph [97.62(6)°], probably due to steric factors.

The structural parameter (bond lengths and angles) involving the DPPA-O ligand are very close to those measured in the related cis-[PtCl2{(DPPA-O)-P}2] complex (7, Table 1).

Similarly to complexes 1 (Scheme 3), 4 or 7 (Scheme 5), the formation of the P,O-chelate complexes 17-trans and 17-cis from their respective chloride precursor (16-trans and 16-cis) was achieved by a chloride abstraction reaction, using a stoichiometric amount of AgBF4 (Scheme 7). The coordination of the P=O moiety was clearly evidenced by 31 P NMR spectroscopy, by a significant downfield shift of its resonance (Table 1). The retention of configuration trans vs. cis was nicely illustrated, by the values of the different P,P and P,Pt coupling constants, and in particular by comparing the 2 JP,PMe2Ph of 475 Hz and 13 Hz measured in the trans and cis isomers, respectively [46]. Scheme 7. Synthesis of neutral (16) and cationic (17) mixed P-based ligands platinum(II) complexes [46].

A series of C,N-cyclometallated Pd II chloride complexes with monodentate P-coordinated DPPA-O, of general formula [PdCl(C,N){(DPPA-O)-P}] (18-22, Scheme 8), resulted from the bond cleavage of [Pd(μ-Cl)(C,N)]2 with 2 eq. of DPPA-O [47,48]. The 31 P NMR spectra of complexes 18-22 exhibit the expected two doublets (AX spin system) for the two inequivalent P III and P V nuclei. While the resonance assigned to the coordinated PPh2 moiety is downfield shifted of ca. 38-45 ppm relative to the free ligand, the P=O group resonates at chemical shift close to that of free DPPA-O, indicating no interaction with the metal center (Table 1). Interestingly, when the 31 P NMR spectrum of complex 18 was recorded in MeOH (C6D6 insert), the resonance of the P=O moiety was significantly downfield shifted (from 21.1 to 50.5 ppm) along with a reduction of the 2 JP,P=O coupling constant (from 33 to 22 Hz, Table 1). Both observations led the authors to hypothesize that in a methanolic solution the P=O group interact with the metal center, probably via the decoordination of the chloride ligand and the formation of a cationic complex. Fliedel and Braunstein and co-workers more recently demonstrated such reactivity in the case of nickel(II) complexes of mono-sulfide N-functionalized DPPA-type ligands (see section 5.1).

Complexes 18 and 20 could readily be converted into their cationic P,O-chelate (23)(24) derivatives via chloride abstraction with AgBF4 (Scheme 8).

[Pd(C,N){(DPPA-O)-P,O}][BF4]
As it was observed for the "Pd(ally)" complex 2, the P,O-chelation was accompanied by a significant downfield shift of the P=O resonances (Table 1). The formation of the P,O-chelate and the mutual arrangement of the ligands, i.e. P trans to N and O trans to C, were confirmed by single-crystal XRD studies for complex 23 (Figure 3). The coordination angles around the metal are in the range 84.4(3)-96.3(2)°, highlighting a slightly distorted Pd II center. The P-Pd and O-Pd bond lengths are respectively 0.057 Å shorter and 0.048 Å longer than those measured in the "Pd(allyl)" analog complex 2 (Table 1). Scheme 8. Synthesis of neutral and cationic C,N-cyclometallated Pd II complexes of DPPA-O [47,48]. Table 1. 31 P NMR data and characteristic structural parameters of free DPPA-O and Group 10 metal (+2 oxidation state) complexes 1-25. a

    (P III ) b  (P V ) b 2 JP,P=O 1 JP,M 2 or 3 JP=O,M P=O P-N c P-M O-M PNP OMP Ref. DPPA-O 28

Group 9 metal complexes

Within the late transition metals, the development of well-defined rhodium and iridium complexes as (pre)catalysts for various organic transformations represents an attractive field of investigations.

Rhodium and iridium species are particularly performant catalysts for the (asymmetric) hydrogenation of unsaturated substrates, such as alkenes or ketones, using molecular hydrogen (H2) or by H-transfer (typically using an alcohol as H-source) [50][51][52]. Numerous contributions highlight also the use of Group 1 and Table 2).

The Ir I complex [IrCl(COD){(DPPA-O)-P}] (27) was prepared following the same procedure as its Rh I analog (26, Scheme 10) [48]. The 31 P NMR spectra of all Ir I complexes of DPPA-O exhibit two doublet for the non-equivalent P III and P V nuclei, accounting for a AB spin system, with 2 JP,P=O in the range 22-37 Hz (Table 2). As observed for all the metal complexes in which the DPPA-O ligand is only P-bonded, the resonance of the latter donor in 27 is downfield shifted compared to the free ligand, while the P=O resonance is found in the same region.

Similarly to the Group 10 metal complexes (see Section 2.1), the P,O-chelation of the DPPA-O ligand can be favored by halogen abstraction in the Rh I and Ir I complexes 26 and 27, using 1.5 eq. of AgBF4 (Scheme 10) [57]. The corresponding cationic complexes [M(COD){(DPPA-O)-P,O}][BF4] [M = Rh (28), Ir (29)] exhibited a significant downfield shift of both P III and P V resonances in their 31 2).

In the different Group 9 metal complexes of DPPA-O presented in this section, the neutral nature of the ligand could be assessed by the observation of a ν(N-H) absorption band in the 3163-3244 cm -1 region in their respective FT-IR spectra [48,57].

Scheme 11. Synthesis of Rh III and Ir III complexes of DPPA-O [57].

Group 8 metal complexes

Group 8 metal complexes are attractive species due to their wide range of catalytic applications, which embrace for example C-H bond functionalization, C-C coupling and (asymmetric) hydrogenation or silylation [53,[60][61][62][63][64]. Within the Group 8 metals, the development of well-defined iron catalysts is attracting increasing attention due to its high abundance, low cost, and relatively low toxicity [65][66][67].

Smith and Woolins and co-workers reported the synthesis of two series of Ru II , Os II The 31 P NMR signal of the PPh2 group of the DPPA-O ligand was subject to a significant downfield shift (ca. 32 ppm) upon coordination to Ru in 37 and 39 (Table 3). In contrast, the P=O resonance was not much affected in the latter, and was found in the same range than for the free ligand.

Similar observations were made for the Group 9 and 10 metal complexes supported by the DPPA-O ligand in a monodentate P-coordination mode (see Sections 2.1 and 2.2). Interestingly, the Os II complex 38 did not exhibit a similar behavior: while the P=O resonance remained in the expected region for the non-coordinated P=O moiety (ca. 20 ppm), the PPh2 resonance was slightly upfield shifted to a similar chemical shift (Table 3). Upon cationization and P,O-chelation, both PPh2 and P=O 31 P NMR resonances of complexes 40-42 were significantly downfield shifted compared to their neutral parent complexes (37)(38)(39), especially the P=O group which shifted from the "non-coordinated" region(ca. 20 ppm) to 51-66 ppm (Table 3). No deprotonation of the N-H moiety was observed, as assessed by the presence of the characteristic ν(N-H) absorption band in the 3132-3253 cm -1 region in the FT-IR spectra of complexes 37-42 [57,68]. The P=O, P-N and P-Os bond lengths are very close in the two compounds and in the same range than those measured in the Rh I complexes 26 and 28 (Table 3 vs. a Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b appears at doublet unless specified; c average value; d in C6D6/THF; e in CDCl3; f average value due to the presence of two molecules in the asymmetric unit; g not resolved.

N-substituted mono-oxide DPPA-type ligands

Woolins and co-workers reported the derivation of a chiral diphosphine (DPPA-type) ligand, based on the (S)-alanine methyl ester scaffold, by oxidation of one (or two) P atom(s) with H2O2 [70].

Unfortunately, the targeted mono-oxide N-substituted DPPA-type ligand 46 was found unstable in solution and transforms into the bis-oxide derivative, however its 31 The heteroditopic ligand DPPA-S can be readily prepared by reaction between DPPA and elemental sulfur (S8, Scheme 16), however a small amount (≈ 5%) of the bis-sulfide derivative (DPPA-S2) was

inevitably formed [71]. The 31 P NMR spectrum of DPPA-S exhibited the expected two doublets ( 2 JP,P=S = 86 Hz) for the non-equivalent P III and P V (=S) nuclei (AX spin system), centered at 27.6 and 60.9 ppm, respectively (Table 5). A characteristic absorption band at ν = 3165 cm -1 , undeniably attributed to the N-H group stretching mode, is very close to the frequency of same vibration in DPPA (ν = 3230 cm -1 ).

Scheme 16. Synthesis of DPPA-S [71]. and trans isomers) and 49 exhibited two triplets for the magnetically non-equivalent P III and P V nuclei of each bis-chelate complex, significantly downfield shifted compared to the free ligand (Table 5). A complex AA'BB' spin system, involving 2+3 JP,P=S = 2+3 JP',P'=S, 3+4 JP,P'=S, 4 J P=S,P'=S, 2 J P,P', could be evidenced and successfully simulated for the Ni II complex 49. Moreover, the solid-state molecular (centrosymmetric) structure of the latter, determined by XRD analysis, confirmed the formation of a tetra-coordinate dicationic Ni II center, bis-chelated by two DPPA-S ligands, trans to each other (Figure 7). Both P-Ni and S-Ni bond lengths are very close, and the P-Ni-S angle approaches 90°, which resulted in a very weak distortion from ideal square-planar geometry. The latter geometry resulted in the formation of a diamagnetic Ni II complex. The P=S bond length of 2.0281(15) Å is somewhat longer than the P=O bond in Group 10 metal complexes of DPPA-O (range: 1.472-1.512 Å, Table 1). The dicationic charge of the metal complex is balanced by two chlorides, which exhibit H-bonding interactions with the N-H atom. Characteristic ν(N-H) absorption bands at ca. 2700 cm -1 were recorded in the FT-IR spectra of complexes 48 and 49 [71,72].

Scheme 17. Synthesis of Group 10 metal complexes of DPPA-S [71,72]. (54, Scheme 18) was isolated. In contrast, the use of CH2Cl2 as solvent promoted, as for 51 and 52, the deprotonation of the DPPA-S ligand [74]. The 31 and 56, consequently to the "loss" of the N-group [75]. The identity of complexes 55 and 56 was established by NMR and XRD analysis, which confirmed the formation of piano-stool Rh III and Ir III complexes coordinated by one η 5 -Cp* ring, one DPPA-S ligand in a P,S-chelating mode, and one chlorine atom, while the cationic charge is balanced by one BF4 anion (Figure 8). The characteristic bond lengths and angles in both complexes 55 and 56 are very close, and in the same range than those measured in the Ni II complex 49, with the exception of the P-M and S-M bond lengths, as anticipated for a larger atomic radius for Rh and Ir vs. Ni (Table 5).

Scheme 18. Synthesis of Group 8 and 9 metal complexes of DPPA-S [73,74]. Note: complex 55 was not obtained via this route, see text and ref [75].

N-substituted mono-sulfide DPPA-type ligands

The mono-sulfide DPPA-type ligands 58-61, substituted on the N atom by pure hydrocarbyl groups [i.e.

Ph, (S)-CHMePh, i Pr and 1-Naphtyl, respectively], were easily accessible via oxidation by an equimolar amount of elemental sulfur of the parent diphosphines (Scheme 20) [77][78][79][80][81]. Interestingly, these mixed P/P=S ligands were isolated without contamination of the bis-sulfide derivatives after work-up.

Similarly to DPPA-S, the 31 P NMR spectra of ligands 58-61 exhibited two sharp doublets (AX spin system) in the 52.4-54.5 and 67.5-73.1 ppm regions for the non-equivalent P III and P V nuclei, respectively (Table 6). The solid-state molecular structure of ligand 59, determined by XRD analysis, revealed a nearly trigonal planar N atom linked asymmetrically to the P III [1.749( 4) Å] and P V [1.695( 4) Å] atoms. This asymmetry is generally also present, although less pronounced, in the coordination compounds of N-substituted/functionalized mono-oxidized DPPA-type ligands. (Scheme 20) [82]. While complexes 72 and 73 may exist as two isomers, their 31 P NMR spectra revealed the formation of only one isomer, which exhibited two doublets at ca. 98 and 69 ppm, as expected for the non-equivalent P III and P V nuclei, respectively (Table 6). The downfield shift of the PPh2 resonance Me in complexes 77 and 82 drives the co-existence of 6 and 3 isomers, respectively. In contrast to the latter complexes, when the allyl ligand is di-substituted with two bulkier Ph groups, only 2 and 1 isomers are observed by solution NMR for complexes 78 and 83, respectively. In Table 6 are only summarized the 31 P NMR data for the syn, trans-to P III Ph2 (75, 76, 80, 81) or syn/syn-allylic isomers (

which are observed for each complex, however the authors clearly detailed the 1 H, 13 C and 31 P NMR data for all (diastereo)isomers in ref [83]. The solid-state molecular structure of complex 74, established by XRD analysis, revealed the presence of only one diastereoisomer. The square-planar coordination geometry around the Pd II center is strongly distorted, due to the acute C-Pd-C' angle [67.5(5)°] imposed by the allyl ligand, while the P,S-chelate allows a P-Pd-S angle of 94.6(8)° closer to the expected 90° (Table 6). Apart from the P-Pd bond length of 2.285(2) Å, which is close to that measured in the DPPA-O analog of 74 [2: P-Pd = 2.283(2) Å, Table 1], the other characteristic structural parameters of complexes 2 and 74 differ significantly. The latter observation highlights how subtle changes in DPPAtype ligands, i.e. N-substitution and/or P=E oxidation, can affect the resulting coordination compounds and may influence their reactivity and/or catalytic activity. In this context, in-situ generated pre-catalysts based on ligands 59, 60 and their non-oxidized P,P parent ligands were evaluated in the allylic alkylation of (E)-1-phenyl-2-propenyl acetate with dimethyl malonate (conditions: 1 mol% Pd, 2.5 mol% ligand, 24 h, 25°C) [83]. All complexes exhibited similar activity, i.e. complete conversion under the studied conditions, and regio-selectivity, affording 95-97% of the linear product. The authors assumed that this high selectivity is related to steric factors, which favor the nucleophilic substitution to operate on the unsubstituted terminus of the substrate.

Scheme 20. Mono-sulfide DPPA-type ligands with pure hydrocarbyl N-substituents (58-61) and their

Pd II and Pt II complexes (62-83) [77][78][79][80][81][82][83]. Note: R 1

-R 2 -allyl = η 3 -1,3-R 1 -R 2 -CHCHCH, L2 = COD or (PhCN)2.
Increasing efforts have been made over the last decades to introduce various chemical functions as N-substituent in DPPA-type ligands [27], however the number of examples of mono-oxidized derivatives is rather limited. The presence of an additional donor on the N-substituent may result in the formation of polynuclear assemblies (see e.g. [4,7,[84][85][86]), but the latter can also act as a stabilizing ligand for reactive/unsaturated species during a catalytic cycle. From the adjunction of a suitable Nfunctionalized tail raised the possibility to anchor coordination compounds on solid matrices or on metal surfaces (see e.g. [87][88][89][90][91]).

The synthesis of the mono-sulfide N-functionalized DPPA-type ligands (PPh2)N{P(S)Ph2}(R) [R = o-(C6H4)OMe (84), p-(C6H4)SMe (85), (CH2)3SMe ( 89)] was achieved by reaction of the parent diphosphine ligands with elemental sulfur (Scheme 21) [72,92,93]. However, all mono-sulfide ligands were always contaminated by the bis-sulfide derivatives and could never be isolated in a pure form.

While Smith and co-workers used the isolated 84 (ca. 90% pure) for the coordination reactions [92],

Fliedel, Braunstein and co-workers generated in-situ the mono-sulfide ligands 85 and 89, using 1.5 eq. of sulfur, before addition of 1 eq. of metal precursor, and then removed the unreactive (towards the metals studied) bis-sulfide derivatives formed [93]. Ligands 84-85 and 89 exhibited, in their 31 P NMR spectra, the awaited two doublets (AX spin system) for the non-equivalent P III and P V nuclei at very similar chemical shifts (ca. 54 and 73 ppm, respectively). While the nature of the N-substituent did not affect (much) the chemical shift of the P atoms, the 2 JP,P coupling constant varies much more (58-61, 84-85, 89, Table 6). Substitution of the labile COD or (PhCN)2 (L2) ligands in [MCl2(L2)] (M = Pd, Pt) by 1 eq. of the chelating mixed P/P=S ligand led to the formation of the complexes [MCl2(84-85)-P,S}] (86-88, Scheme 21) [92,93]. Upon coordination to palladium, both P III and P V NMR resonances were downfield shifted, while the coordination to platinum led only to downfield shift of the P III resonance accompanied by the appearance of typical 1 JP,Pt and 2 JP=S,Pt satellites (Table 6). The slightly distorted square-planar coordination geometry around the d 8 metal ions was confirmed in the case of 87 and 88 by XRD analysis. The characteristic bond lengths and angles were found in the same ranges as for the

Pd II and Pt II complexes of N-substituted DPPA-type ligands (63-65, Table 6). Although a clear explanation for the different catalytic activities within this series of complexes could not be provided, several studies were conducted in the recent years to rationalize the (pre)catalyst/organoaluminum reagent interaction(s) and to identify potential active species in olefin polymerization catalysis [95].

Table 6. 31 Hz) and 76.0 ppm ( 2 JP=S,Rh = 3.0 Hz), respectively. The NMR data of complexes 96 and 97 followed similar trends (Table 7). The X-ray structure of complex 96 revealed a tetra-coordinated Rh I center in a slightly distorted square-planar geometry, with coordination angles ranging from 87.5(2) to 92.8(7)° [80]. The confirmed cis arrangement of the carbonyl ligand with respect to the phosphine donor contrasts with the structure of the Pd II complex 70, in which the neutral donor (N) is located trans to the P donor (see section 5.1 and Scheme 20). The characteristic bond lengths and angles in 96 (Table 7) are comparable to those measured in 73 (similar P,S, L, X coordination environment, Table 6). 7). Interestingly, while two geometric isomers, cis and trans, could be formed with the asymmetric ligand 59, a single isomer was formed for the Rh I complex 98. In the case of Ir I , although the initial reaction (CH2Cl2, RT, 2 h) afforded a single isomer of 99 [δ = 95.1 (P III ) and 72.9 (P V ) ppm], after five days in CDCl3 a second isomer was formed [δ = 82.0 (P III ) and 77.0 (P V ) ppm]. The authors could establish that the isomerization is favored by the addition of THF or of a catalytic amount of excess ligand, suggesting the formation of a five-coordinate intermediate, which then undergoes either 1) a pseudo-rotation to exchange the ligand positions or 2) a cleavage of one chlorido-bridge followed by rotation around the M-Cl-M axis before re-formation of the chloridobridge. The authors suggested that the back-donation of the bridging Cl to the coordinated P atom should be lower in the case of the cis isomer, because both P share the same Cl atom as trans ligand. As a consequence, the 31 P NMR resonance of the P III nucleus in the cis isomer should be at lower field, and the P,P coupling should be greater, compared to the trans isomer. It was concluded that the initial product formed was 99-cis, which also exhibited a higher asymmetry in the ν(Ir-Cl) region of the FT-IR spectrum, while the more stable complex is 99-trans. was downfield shifted compared to the free ligand (Δδ = 55.1 ppm, Table 8).

Scheme 24. Synthetic routes to the P,S-chelate Ru II complex 102 [74]. 8). The authors studied the thermodynamic parameters of the aforementioned ligand substitution reaction, establishing that the reaction with the non-oxidized PN(Ph)P ligand is more exothermic (ΔHreaction > by ca. 10 kcal.mol -1 ) than those involving its mono-oxidized PN(Ph)P=S (or Se) derivatives, both within the Cp and Cp* series. As a result, the P,S(or Se)-chelate complexes are less thermodynamically stable than their P,P-chelate analogs.

Furthermore, the authors observed that the ligand substitution reaction is less exothermic for the Cp*than the Cp-Ru system, rationalized by the higher electron-donating power of Cp* (vs. Cp), which renders the Ru II center more basic, hence less electrophilic, and does not favor the coordination of an additional incoming ligand with greater electron density.

Scheme 25. Piano-stool Ru II complexes (103-104) of the P,S-chelate ligand 58 [96]. . In this reaction, the Ru(0)3 core cluster formally undergoes an oxidative addition (of S), while the P V =S group is reduced (to P III ). However, since complexes 105-106 no longer contain any mixed P/P=S ligand, they will not be further discussed in the present review. It could be noticed that a similar reactivity was observed with a related P/P=S ligand in which the Ph substituents of the P atoms have been replaced by OPh groups [97]. Table 8. 31 P NMR data of free ligand 101 and metal complexes 102-104. a deprotonated derivative DPPA-Se-H [99]. The characteristic structural parameters of 111 are reported in Table 10, but no NMR nor FT-IR data were provided.

Valderrama and co-workers successfully applied the protocol reported for DPPA-S (see section 4) to the synthesis of piano-stool Ru II , Rh III and Ir III complexes of DPPA-Se [73,74] 5), with the P III and P V doublet resonances being respectively downfield and upfield shifted, compared to free DPPA-Se (Table 10).

The P V resonances of complexes 112-115 are flanked by 77 Se satellites and the P III resonance of the Rh complex 114 exhibit a typical 1 JP,Rh coupling, allowing the unambiguous assignment of each signal. 10) [73,74]. A lowering of the 2 JP,P and 1 JP,Se values was also observed.

Characteristic ν(N-H) absorption bands (3000-3300 cm -1 ), and/or N-H resonances (δ = 5.95-8.72 ppm)

were detected by FT-IR and 1 H NMR spectroscopy, respectively, confirming no deprotonation of the acidic N-H group of 112-117.

Conclusion

The present review highlights the versatility of the coordination chemistry of mono-oxidized DPPA,

incorporating

Scheme 3 .

 3 Scheme 3. Reactivity of DPPA-O towards the [(allyl)Pd (μ-Cl)]2 and [(methallyl)Ni (μ-Br)]2 precursors [32, 45].

Figure 2 .

 2 Figure 2. Solid-state structure of the complex cis-[Pt{(DPPA-O)-P,O}2][BF4]2 (13). The BF4 anions are omitted and only the N-H hydrogens are represented for clarity [33].

Figure 3 .

 3 Figure 3. Solid-state structure of the complex [Pd{(C12H12N)-C,N}{(DPPA-O)-P,O}][BF4] (23). The BF4 anion is omitted and only the N-H hydrogen is represented for clarity [47].

9

  as efficient tools for C-C, C-O and C-N bond formation via C-H bond activation/functionalization [53-55]. Two different Rh I complexes of DPPA-O, of general formula [RhCl(COD){(DPPA-O)-P}] (26) and [RhCl(CO){(DPPA-O)-P}2] (31, Scheme 10), were accessible via a bridge cleavage reaction (combined with CO substitution for 31) of [Rh(μ-Cl)(COD)]2 or [Rh(μ-Cl)(CO)2]2 with 2 or 4 eq.DPPA-O, respectively[48]. The 31 P NMR spectra of both Rh I complexes are representative of the expected AMX spin system ( 103 Rh, I = ½, 100% abundance), with the most shifted (to higher frequencies compared to free DPPA-O) resonances at ca. 61 ppm attributed to the PPh2 group, while the P=O moiety resonates at a similar chemical shift to that of the free ligand (Table2). The 1:2 reaction between [Rh(μ-Cl)(CO)2]2 and DPPA-O led to a yellow solid, the structure of which was tentatively assigned to [RhCl(CO){(DPPA-O)-P,O}] (32), based on31 P NMR data and on the previously reported cis-[RhCl(CO){(Ph2PCH2P(O)Ph2)-P,O}] complex[56]. The solid-state molecular structure of complex 26 was established by XRD analysis and revealed a slightly distorted square-planar Rh I center coordinated by the chelating COD ligand, one chlorine and one DPPA-O ligand, mono-ligated through the P III donor.While the P=O and P-N (av) bond lengths are similar to those of the free ligand and of most of DPPA-O metal complexes, the P-Rh bond length is in the upper range of DPPA-O metal complexes. The PNP angle, larger than in the free ligand, is comparable to those measured in the other metal complexes containing monodentate P-bonded DPPA-O (Table

Figure 4 .

 4 Figure 4. Solid-state structure of the complex [Rh(COD){(DPPA-O)-P,O}][BF4] (28). The BF4 anion is omitted and only the N-H hydrogen is represented for clarity [57].

  and Ru IV complexes of DPPA-O [57, 68]. The first group of neutral complexes [(L)MCl2{(DPPA-O)-P}] [L = p-Cym, M = Ru (37) and Os (38); L = η 3 : η 3 -C10H16, M = Ru (39)], in which the mixed P,P=O is coordinated in a monodentate mode by the P III donor, are readily obtained by bridge cleavage reaction of their corresponding chlorido-bridged dimeric precursors, [(L)MCl(μ-Cl)] [L = p-Cym, M = Ru or Os; L = η 3 : η 3 -C10H16, M = Ru], with 2 eq. DPPA-O ligand (Scheme 12). This first set of compounds can be ionized by chloride abstraction with AgBF4, affording their cationic [(L)MCl{(DPPA-O)-P,O}][BF4] [L = p-Cym, M = Ru (40) and Os (41); L = η 3 : η 3 -C10H16, M = Ru (42)] derivatives, in which the DPPA-O ligand is P,O-chelating (Scheme 12).

Scheme 12 .

 12 Scheme 12. Synthesis of Ru II , Os II and Ru IV metal complexes of DPPA-O[57,68].

Figure 5 .

 5 Figure 5. Solid-state structures of the [(p-Cym)OsCl2{(DPPA-O)-P}] (38, left) and [(p-Cym)OsCl{(DPPA-O)-P,O}][BF4] (41, right) complexes. The BF4 anion (in 41) is omitted and only the N-H hydrogen and ipso-C of the PPh rings are represented for clarity [68].

[ 87 .

 87 Scheme 15. Synthesis of the palladium(II) chloride complex (47) of ligand 46[70].

Figure 6 .

 6 Figure 6. Solid-state structure of the complex [PdCl2{(46)-P,O}] (47). The H atoms are omitted for clarity [70].

  Reaction between DPPA-S and Group 10 metal precursors of the [MCl2(L2)] (M = Ni, Pd, Pt; L2 = DME, COD) type led to different outcomes. The reaction with the Pd II precursor, in a 2:1 ligand/metal ratio, afforded a mixture of both cis and trans isomers (1:5 ratio) of the expected dicationic complex [Pd{(DPPA-S)-P,S}2][Cl]2 (48, Scheme 17). In contrast, ligand deprotonation occurred upon coordination with Pt II under similar reaction conditions, forming the neutral complex cis/trans-[Pt{(DPPA-S-H)-P,S}2] [71]. The equimolar reaction between DPPA-S and [NiCl2(DME)] afforded solely the complex trans-[Ni{(DPPA-S)-P,S}2][Cl]2 (49, Scheme 17) and unreacted Ni II precursor [72]. These results strongly contrast with those observed with DPPA-O (see section 2.1), which preferably formed [MCl2{(DPPA-O)-P}2] (M = Pd, Pt) complexes, and suggest a much greater preference for DPPA-S to form chelates than DPPA-O. The 31 P NMR spectra of complexes 48 (mixture of both cis

Figure 7 .

 7 Figure 7. Solid-state structure of the complex [Ni{(DPPA-S)-P,S}2][Cl]2 (49). Only the N-H hydrogen is represented for clarity [72].

Figure 8 .

 8 Figure 8. Solid-state structure of the complex [Cp*IrCl{(DPPA-S)-P,S}][BF4](56). The BF4 anion is omitted and only the N-H hydrogen is represented for clarity[75].

5. 1 .

 1 Group 10 metal complexes P,S-chelate palladium (62, 64, 66, 68) and platinum (63, 65, 67, 69) dichloride complexes of the type [MCl2{(58-61)-P,S}] were obtained by substitution of the labile ligand(s) in [MCl2(L2)] [M = Pd, Pt; L2 = COD, (PhCN)2] precursors with the mixed P/P=S ligands 58-61 (Scheme 20) [77, 79-81]. Similarly, the organo-palladium complexes [PdCl(Me){(59-60)-P,S}] (70-71) were synthesized from [PdCl(Me)(COD)] and the respective ligand (Scheme 20) [82]. Al-Masri and Moussa observed that the reaction between the mixed P/P=S ligand 61 and [NiCl2(PPh3)2] afforded the P,P-chelate complex [NiCl2{(Ph2P)N(1-Naphthyl)}-P,P], resulting from the desulfurization of the P,P=S DPPA-type ligand 61 and formation of Ph3P=S and the corresponding P,P ligand [81]. 31 P NMR spectroscopy revealed that upon coordination to the Pd II or Pt II centers, the chemical shifts of the PPh2 doublet are downfield shifted, of ca. 40-50 and 10-20 ppm, respectively, compared to the free ligands, while the P=S resonances are only slightly affected (Table6). In all of these d 8 metal complexes, the 2 JP,P was found in the same range. The assignment of both P III /P V resonances was facilitated, in the case of the Pt II complexes (63, 65, 67, 69), by the presence of typical 1 JP,Pt and 2 JP=S,Pt satellites due to the couplings of the phosphorus atoms with the 195 Pt center (spin = ½). For the organo-palladium complexes 70-71, the cis-position of the Me group with respect to the coordinated P atom was assessed by the observation of a characteristic3 JP,H coupling of 3.9 Hz by 1 H NMR. Single-crystal XRD analysis on complexes 63-65 confirmed the expected square-planar arrangement of the ligands; one mixed P,S-chelate and two chlorines, around the Pd II or Pt II metal center. The characteristic P-N, P=S and P-M bond lengths and PNP and SMP angles are very similar in the three complexes (Table6). The only significant structural differences with respect to the related complex [PdCl2{(46)-P,O}] (47, Figure6) are the P=E and (P=)E-Pd bond lengths (E = O, S) due to a much larger atomic radius for S compared to O. The two C,N-cyclometallated P,S-chelate Pd II complexes [Pd{(C6H3-R'-4)-N=N-(C6H4-R'-4)-C,N}{(60)-P,S}][PF6] [R' = H (72), OC6H13 (73)] were obtained via a chlorido-bridge cleavage reaction of [Pd(μ-Cl){(C6H3-R'-4)-N=N-(C6H4-R'-4)-C,N}]2 with 2 eq. of ligand 60 in the presence of NH4PF6

in 72 and 73 ,

 73 compared to the free ligand 60, is very similar to that observed for the palladium dichloride derivatives(62,64,66,68). The arrangement of the P,S-and C,N-chelate ligands could be unambiguously established by XRD studies for 73. The σ-bonded C center occupies the position trans with respect to the S, as observed in the Pd-Me complexes 70-71. Structurally, the presence of a σbonded aryl group on the Pd II center in 73 resulted in the lengthening of the trans Pd-S bond compared to the P,S-PdCl2 system 64 [2.386(1) vs. 2.274(2) Å, Table6].Reaction of ligands 59 and 60 with the chlorido-bridged dimers [(η3 -1,3-R 1 -R 2 -C3H3)Pd(μ-Cl)]2 (R 1 , R 2 =H, Me or Ph) in the presence of NH4PF6 led to the cationic η 3 -allyl Pd II complexes [(η 3 -1,3-R 1 -R 2 -C3H3)Pd(59-60)-P,S}][PF6] (74-83, Scheme 20)[83]. The 31 P NMR spectra of the resulting complexes 74-83 feature an AX spin system for the non-equivalent P III and P V nuclei, which resonate in the 86-97 and 74-81 ppm regions, respectively (Table6). In the case of the non-substituted allyl derivatives (R 1 = R 2 = H, 74 and 79), the allylic symmetry is broken by the mono-sulfide DPPA-type ligands itself, resulting in the observation of; 1) five distinct resonances by1 H NMR, and 2) a dynamic behavior for the H atoms trans to the S donor. The NMR spectra of the complexes 74-78 are more complicated, because of additional possibility of face-coordinated diastereomeric pairs due the chiral nature of ligand 59. Mono-substitution of the allyl group by one Me or Ph group led to additional possibilities of cis/trans arrangement of the latter substituent with respect to the S (or P) donor. The Me derivatives 75 and 80 exist respectively as mixtures of 6 and 4 isomers. Unlike 75 and 80, the Ph derivatives 76 and 81 exist as mixtures of 4 and 2 isomers, respectively, due to the absence of any antiisomer because of higher steric congestion of Ph vs. Me. Bis-substitution of the allyl ligand with two

Scheme 21 .

 21 Scheme 21. Synthesis of Pd II and Pt II complexes (86-88) of mono-sulfide N-functionalized DPPA-type ligands (84-85)[92,93]. Note: L2 = COD or (PhCN)2.

Figure 9 .

 9 Figure 9. Formula isomer Ni II complexes 93/93' of ligands 89 in equilibrium [72].

  Ligand substitution reaction in [M(μ-Cl)(COD)]2 (M = Rh, Ir) by 59 (2 eq.) led to the formation of the dinuclear chlorido-bridged complexes [M(μ-Cl){(59)-P,S}]2 [M = Rh (98), Ir(99), Scheme 23], in which ligand 59 acts as a P,S-chelate [79]. This result strongly contrasts with that observed with DPPA-O, which selectively formed complexes [MCl(COD){(DPPA-O)-P}] (26-27, Scheme 10), in which the mixed P,(P=)O ligand acts as a P-monodentate donor, highlighting the different reactivity of monosulfide vs. mono-oxide derivatives of DPPA. The 31 P NMR resonances of both P III and P V nuclei were significantly downfield shifted in complexes 98 and 99 compared to the free ligand 59, and a typical 1 JP,Rh of 158 Hz could be observed for 98 (Table

  The piano-stool complexes [Cp R RuCl{(58)-P,S}] [Cp R = Cp (103), Cp* (104)] were readily obtained by substitution of the labile (PPh3)2 or COD (L2) ligands in [(Cp R )RuCl(L2)] by an equimolar amount of ligand 58 (Scheme 25) [96]. Analysis of the 31 P NMR spectra of complexes 103 and 104 revealed a significant influence of the Cp or Cp* ligand on the P resonances of ligand 58. While in both cases the P III resonance was downfield shifted of ca. 70 ppm, the P V resonance was found upfield shifted in 103 (Cp ring, Δδ = 28.7 ppm) and nearly not shifted in 104, the latter being supported by the more electron donating Cp* ligand (Δδ = 1.5 ppm, Table

  The stoichiometric reaction between the N-substituted mono-sulfide DPPA-type ligands 59-60 and [Ru3(CO)12], in the presence of Me3NO, afforded the mono-sulfur capped triruthenium clusters [Ru3(μ-CO)(CO)7(μ3-S){(Ph2P)2NR)-P,P}] [R = CHMePh (105), i Pr (106), Scheme 26] [80]

Scheme 26 .

 26 Scheme 26. Reaction of mono-sulfide DPPA-type ligands 59-60 with [Ru3(CO)12] [80].

  . The formation of complexes [(L)MCl2{(DPPA-Se)-P}] (112-115) (L = C6Me6, p-Cym), incorporating one DPPA-Se ligand coordinated in a P-monodentate mode, could be achieved following two different routes. The first path involved; 1) the preliminary synthesis of complexes [(L)RuCl2{(DPPA)-P}] by bridge cleavage reaction of [(L)RuCl(μ-Cl)]2 by 2 eq. DPPA, followed by 2) the oxidation of the pendant PPh2 group with grey selenium (112-113, Scheme 28). The second pathway involved the use of isolated DPPA-Se, which readily reacted with 0.5 eq. of the dimeric precursors [Cp*MCl(μ-Cl)]2 (M = Rh, Ir) to form the bridged cleaved products (114-115, Scheme 28). The 31 P NMR spectra of complexes 112-115 are similar to those of their DPPA-S analogs (50-53, Table

  Scheme 28), in which DPPA-Se acts as a P,Se-chelate ligand. Similar reactivity was observed for the DPPA-S derivatives 50/54 (Ru) and 53/56 (Ir). The31 P NMR spectra of 116-117 exhibited comparable trends to those of 54, 56, with downfield shifts of both P III and P V resonances upon cationization and P,Se-chelation (Table10)[73,74]. A lowering of the 2 JP,P and 1 JP,Se values was also observed.

  one P III Ph2 and one Ph2P V =O, =S or =Se donor, and their N-substituted derivatives, with pure hydrocarbyl or including an additional chemical function. The presence of a hard O donor in DPPA-O favors a P-monodentate coordination mode with Group 9 and 10 metal complexes. In contrast, the presence of the softer sulfur or selenide donors in DPPA-S or DPPA-Se and their N-substituted derivatives led, under similar conditions, to the formation of mono-or bis-P,S or P,Se-chelate complexes. However, formation of P,O-chelate species with DPPA-O can be promoted by halide abstraction in neutral metal halide complexes. The asymmetry in the donor set in mono-oxidized DPPAtype (P,P=E, with E = O, S, Se) ligands, compared to their parent diphosphine (P,P) ligands, also triggered original reactivity. As a result, P,P=S-chelate nickel(II) dihalide complexes suffer a solventinduced isomerization via ligand-redistribution, while their P,P-chelate analogs are totally inert. Noteworthy, the reaction of DPPA with a Re=O precursor led to the oxidation of the ligand via transfer of the O atom to form a DPPA-O Re complex. In contrast, the reaction of mono-S or -Se DPPA-type ligands with Ru3(CO)12 led to the reduction of P V =S (or Se) moiety and transfer of the S (or Se) atom to form P,P-supported (μ3-S)-Ru3 or (μ4-S)2-Ru4 clusters. Metal complexes of mono-oxidized DPPA-type ligands found applications in catalytic oligomerization of ethylene, allylic alkylation or carbonylation of methanol.

  P NMR resonance of the P=O moiety of the DPPA-O ligand was significantly downfield shifted upon coordination in 12 and 13 compared to 4 and 7, from ca. 21 ppm to 68.1 and 62.8 ppm, respectively (Table

  Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b appears at doublet unless specified; c average value; d in C6D6/THF; e in CDCl3; f P III and P V resonances were not unambiguously attributed; g in CD2Cl2; h | 2 JP,P=O + 4 JP,P'=O|; i in MeOH/C6D6; j | 2 JP,P=O + 3+4 JP,P'=O|; k in addition to other uncharacterized species; l the NMR data reported concern the DPPA-O ligand;

	21	Pd	72.3 e	21.1 e	33									[47]
	22	Pd	66.4 e	21.7 e	35									[48]
	23	Pd	73.4 e	51.5 e	22			1.512(4) 1.674	2.226(2) 2.151(5) 117.9(3) 87.7(1)	[47, 48]
	24	Pd	.1 d 73.3 e	21.3 d 53.9 e	62 22									[32] [47]
	DPPA-O 25	Pd	27.8 e 44.5 o	25.2 e 65.8 o	59 23			1.508(2) 1.679 1.509(2) 1.676	125.6(2) 2.188(8) 2.228(2) 117.4(2) 86.93(6) [49] [33]
	1	Pd	60.9 e	22.6 e	37									[32]
	2 f	Pd	59.0 e	67.8 e	35			1.496(6) 1.654	2.283(2) 2.103(6) 120.8(4) 86.0(2)	[32]
	3	Ni	65.7 g	71.5 g	27									[45]
	4-cis	Pd	58.3 e	21.1 e	36 h									[33]
	4-trans	Pd	46.7 e	20.9 e	25 h									[33]
	5 (trans)	Pd	48.5 e	22.7 e	22 h									[33]
	6 (trans)	Pd	45.4 e	24.1 e	23 h									[33]
	7 (cis)	Pt	35.7 e	21.6 e	22 h	3955	114	1.481 c	1.685	2.245 c	-	131.5 c	-	[33]
	8 (cis)	Pt	37.6 e	22.1 e	24 h	3933	104							[33]
	9-cis	Pt	36.8 e	21.2 e	29 h	3821	108							[33]
	9-trans	Pt	35.4 e	22.8 e	21 h	2544	83							[33]
	10 (trans) Pt	56.0 e	22.6 e	18 h	3202	84	1.472 c	1.678	2.295 c	-	131.5 c	-	[33]
	11 (trans) Pt	55.6 e	23.9 e	10 h	3154	88							[33]
	12 (cis)	Pd	56.3 i	68.1 i	6 j									[33]
	13 (cis)	Pt	37.4 i	62.8 i	3 j	3924	88	1.502(9) 1.670	2.213(4) 2.108(1) 118.1(7) 87.3(3)	[33]
	14 (cis) k	Pt	58.7 e	22.7 e	23 h	2050								[33]
	15 (cis) l	Pt	36.4 e,m	22.8 e,m	26	3963	110							[46]
	16-trans l	Pt	42.7 e,m	21.6 e,m	34	2657	79							[46]
	16-cis l	Pt	36.1 e,m	22.2 e,m	26	3915	101	1.473(4) 1.673	2.243(1) -	134.2(3) -	[46]
	17-trans l	Pt	53.1 e,m	83.7 e,m	36	2480	44							[46]
	17-cis l	Pt	46.5 e,m	68.6 e,m	9	3383	132							[46]
	18 n	Pd	73.3 e	21.1 e	33									[48]
	19	Pd	71.7 e	21.0 e	33									[48]
	20	Pd	69.4 e	21.9 e	31									[47]

a m additional coupling constant(s) involving the monodentate (PPh2OMe or PMe2Ph) phosphine ligand are present; n δ(P III ) = 72.2 ppm, δ(P V ) = 50.5 ppm, 2 JP,P=O = 22 Hz in MeOH/C6D6; o in d6-DMSO, P III and P V resonances were not assigned.

Table 2 )

 2 coordination and cationization. Single crystals of complex 28, analyzed by XRD, revealed a distorted square-planar Rh I metal center, bis-chelated by one COD and one DPPA-O ligands (Figure4). The characteristic structural parameters of 28, i.e. P=O, P-N, P-Rh and O-Rh bond length and PNP and OMP angles are very close to those of the [Pd(allyl)(P,O)][BF4] complex 2 (Table1 and .Oxidative-addition reactions are the key step of several catalytic processes. For instance, the Rhcatalyzed methanol carbonylation initially involves the oxidative-addition of MeI to [RhI(CO)(P,P)] species[58]. Therefore, the study of reaction mechanisms, kinetics and the preparation of model intermediates have attracted much attention[59]. In this context, the reaction of the Ir I complex 29 with MeI afforded the Ir III complex [Ir(Me)I(COD){(DPPA-O)-P,O}][BF4] (30) (Scheme 10). The31 P NMR resonances of the P III and P V nuclei in 30 were found upfield shifted by ca. 8 and 37 ppm, respectively,

P NMR spectra of ca. 17 and 40 ppm, respectively (Table

2

). The 1 JP,Rh coupling constant remained nearly unchanged after P=O relative to its Ir I parent complex 29. A typical resonance for the Ir-CH3 group, which appeared as doublet due to a 3 JP,H coupling constant of 5 Hz, was observed by 1 H NMR spectroscopy. The authors proposed a cis-arrangement of the Me and I ligands in complex 30 as depicted in Scheme 10. Scheme 10. Synthesis of Rh I and Ir I complexes of DPPA-O and reactivity

[48,57]

.

Table 2 .

 2 31 P NMR data and characteristic structural parameters of free DPPA-O and Group 9 metal complexes 26-36. a

			 (P III ) b	 (P V ) b	2 JP,P=O	1 JP,M	2 or 3 JP=O,M	P=O	P-N c	P-M	O-M	PNP	OMP	Ref.
	DPPA-O		28.1(d) d	21.3(d) d	62									[32]
	DPPA-O		27.8(d) e	25.2(d) e	59			1.508(2) 1.679			125.6(2)		[33]
	26	Rh I	60.4 (dd) e	21.3 (dd) e	40	159	5	1.478(4) 1.687	2.289(1)		132.6(3)		[48]
	27	Ir I	49.2 (d) e	22.6 (d) e	35									[48]
	28	Rh I	75.6 (dd) e	61.5 (d) e	35	154		1.507(4) 1.674	2.272(2) 2.092(4) 119.0(3) 85.9(1)	[57]
	29	Ir I	69.6 (d) e	65.5 (d) e	31									[57]
	30	Ir III	61.7 (d) e	28.2 (d) e	22									[57]
	31	Rh I	62.4 (dd) e	20.8 (dd) e	26	127	3							[48]
	32	Rh I	86.6 (dd) f	54.7 (d) f	32	161								[48]
	33	Rh III	66.2 (dd) e	21.1 (dd) e	41	147	4							[57]
	34	Ir III	35.7 (d) e	21.1 (d) e	37									[57]
	35	Rh III	76.5 (dd) e	58.0 (d) e	25	147								[57]
	36	Ir III	65.9 (d) e	58.4 (d) e										[57]

a Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b multiplicity of the NMR signals given in parenthesis; c average value; d in C6D6/THF; e in CDCl3; f in DMSO/C6D6.

Table 2

 2

). The transition of monodentate Pto bidentate P,O-chelating mode of the DPPA-O ligand induces a lowering of the PNP angle value from ca. 130 to 120°, as observed in analogous Group 9 and 10 metal complexes (see Table

1

-Table

3

).

Table 3 .

 3 Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b appears at doublet unless specified; c average value; d in C6D6/THF; e in CDCl3; f 2 JP,P=O is not observed, resulting in singlet resonances for both P III and P V nuclei; g P III and P V resonances were not assigned by the authors; h average value due to the presence of two molecules in the asymmetric unit.

					2 JP,P=O P=O	P-N c	P-M	O-M	PNP	OMP Ref.
	DPPA-O		28.1 d	21.3 d	62				[32]
	DPPA-O		27.8 e	25.2 e	59	1.508(2) 1.679		125.6(2)	[33]
	37	Ru II 62.1 e	20.4 e	40				[57]
	38	Os II	20.6 e,f,g	20.3 e,f,g f	1.479 h	1.677 h 2.342 h	129.6 h	[68]
	39	Ru IV 58.5 e	19.6 e	44				[57]
	40	Ru II 82.1 e,g	65.0 e,g	29				[57]
	41	Os II	70.2 e	51.3 e	26	1.514 h	1.677 h 2.318 h 2.139 h 119.9 h	83.5 h [68]
	42	Ru IV 66.5 e	65.8 e	34				[57]

31

P NMR data and characteristic structural parameters of free DPPA-O and Group 8 metal complexes 37-42. a   (P III ) b  (P V ) b a

Table 4 .

 4 31 P NMR data and characteristic structural parameters of free DPPA-O and metal complexes

	43-45. a
			 (P III ) b  (P V ) b 2 JP,P=O P=O	P-N c	P-M	O-M	PNP	OMP	Ref.
	DPPA-O		28.1 d	21.3 d 62					[32]
	DPPA-O		27.8 e	25.2 e 59	1.508(2) 1.679			125.6(2)	[33]
	43	Re III			1.522(1) 1.665 2.468(4) 2.055(9) 119.3(6) 80.9(4) [69]
	44	Re IV			1.512 f	1.668 f 2.499 f	2.068 f	118.8 f	79.6 f	[69]
	45	Au I	53.9 e	26.4 e g					[48]

  The31 P NMR spectrum of complex 47 exhibited the expected AX spin system composed of two doublets centered at 78.6 and 61.7 ppm ( 2 JP,P=O 34 Hz), for the non-equivalent P III and P V nuclei, respectively. Although they are markedly different, the cationic Pd II complexes of the P,O-chelating DPPA-O ligand(23)(24) exhibit comparable NMR data (Table1). Furthermore, the solid-state structure of 47, established by XRD analysis, confirmed a distorted square-planar Pd II center, with coordination angles ranging from 87.8(2)

	to 93.81(11)°, coordinated by one P,O-chelating ligand 46 and two chlorine atoms (Figure 6). While the
	P-Pd [2.214(3) Å] and (P=)O-Pd [2.039(7) Å] bond lengths were slightly shorter, the P=O [1.524(7) Å]
	and P-N (av. 1.703 Å) bond lengths of complex 47 were slightly longer than those measured in the Pd II

P NMR fingerprint could be recorded [δ 45.8 (d, PPh2), 33.2 (d, P=O) ppm; 2 JP,

P=O 80 Hz]

. Interestingly, the mixed P III /P V ligand 46 could be generated in-situ and further reacted with 1/3-1/2 eq. (depending on the purity determined by

31 P NMR)

of the [PdCl2(COD)] precursor to form the [PdCl2{(46)-P,O}] complex 47 (Scheme 15). complexes of DPPA-O (Section 2.1, Table

1

). On the other hand, both PNP [115.5(5)°] and OPdP

Table 5 .

 5 The authors observed a large deshielding of the PPh2 resonance upon coordination of DPPA-S (Δδ = absorption bands centered at 3070 and 3330 cm -1 were observed in the FT-IR spectra of the Ru II complexes 50 and 54[74].

	ca. +45 ppm) for the neutral Ru II and Rh III complexes (50-52), while a smaller shift was recorded for the
	Ir III derivative (53), similarly to what was observed for analogous complexes of DPPA-O (see sections
	2.2 and 2.3, Table 2 and Table 3). In all complexes where the P=S moiety remained uncoordinated (50-
	53), its characteristic doublet resonance was observed at ca. 51 ppm. The formation of the cationic P,S-

P NMR data of complexes 50-56 are reported in chelate species 54-56 resulted in downfield shifts of both PPh2 and P=S resonances. The assignment of the P III vs. P V resonance was facilitated by the presence of 1 JP,Rh couplings in 52 and 55. Characteristic ν(N-H)

Table 5 .

 5 31 P NMR data and characteristic structural parameters of free DPPA-S and metal complexes 48-57 (Table divisions: free ligand, Group 10 metals, piano-stool Group 8-9 metal complexes, Group

	11 metal). a
			 (P III ) b	 (P V ) b	2 JP,P=S P=S	P-N c	P-M	S-M	PNP	SMP	Ref.
	DPPA-S		27.6(d) d	60.9(d) d	86						[71]
	48-trans Pd II	79.7(t) e	76.1(t) e	46 f						[71]
	48-cis	Pd II	78.2(t) e	68.8(t) e	20 f						[71]
	49	Ni II	79.2(t) g	82.8(t) g	64 h	2.028(2) 1.658 2.205(1) 2.178(1) 121.0(2) 92.0(4) [72]
	50	Ru II	68.0(d) e	50.5(d) e	46						[74]
	51	Ru II	61.6(d) e	50.5(d) e	45						[73]
	52	Rh III 65.4(dd) e,i 51.3(d) e	45						[73]
	53	Ir III	35.3(d) e	51.2(d) e	41						[73]
	54	Ru II	99.3(d) e	69.1(d) e	36						[74]
	55	Rh III 87.2(dd) e,i 69.0(d) e	39	1.997(4) 1.674 2.293(4) 2.404(3) 121.1(6) 88.3(1) [75]
	56	Ir III	60.9(d) e	69.0(d) e	32	2.000(8) 1.665 2.263(6) 2.407(5) 122.5(1) 88.6(2) [73, 75]
	57	Cu I	32(br) e,j	67.8(m) e,j j						[76]

a Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b multiplicity of the NMR signals given in parenthesis; c average value; d in d6-DMSO; e in CDCl3; f | 2 JP,P=S + 3 JP,P'=S|; g in CD2Cl2; h measured 2+3 JP,P, but the complexity of the AA'BB' spin system is described in the main text; i 1 JP,Rh = 148 and 139 Hz for 52 and 55, respectively; j P III and P V resonances were not attributed and JP,P coupling constants not given.

Table 6 )

 6 . A similar behavior was observed by changing the nature of the N-substituent (from ligand 89 to 85) or the halide ligands on the Ni II (from Cl to Br). The resulting complexes (92/92'

	and 94/94') are depicted in Scheme 22 and their spectroscopic and structural data are reported in Table
	6 [93]. All the [NiX2{(85, 89)-P,S}] complexes (92-94), unambiguously characterized in the solid-state
	by XRD, exhibit very similar structural characteristics. They only differ from the Pd II or Pt II analogs by
	the values of the P-M and (P=)S-M bond lengths (Δδ = ca. 0.06 and 0.15 Å, respectively) due to a larger
	atomic radius for the heavier metals (Table 6). In contrast, the related P,P-chelate complexes
	[NiCl2(P,P)] do not exhibit any dynamic behavior in solution.

92/92', 93/93' or 94/94', respectively

  P NMR data and characteristic structural parameters of free mono-sulfide DPPA-type ligands (58-61, 84-85, 89) and Group 10 metal (+2 oxidation state) complexes 62-83, 86-88 and 90-95 (Table divisions: free N-substituted ligands, corresponding metal complexes, free N-functionalized ligands, ; k in CD3OD.5.2. Group 9 metal complexesReaction between ligands 58-60 and 0.5 eq. of the dimeric precursor [Rh(μ-Cl)(CO)2]2 afforded the mononuclear complexes [RhCl(CO){(58-60)-P,S}] (95-97, Scheme 23)[77,80]. The cis arrangement of the carbonyl ligand with respect to the phosphine donor is evidenced by typical ν(CO) absorption bands in the 1978-1985 cm -1 region. The 31 P NMR spectrum of 95 exhibits two doublets of doubletsfor the P III and P V atoms, representative of a AMX spin system, at 122.8 ( 2 JP,P=S = 71.3 Hz, 1 JP,Rh = 158.7

	81	Pd	93.6 e	76.5 e							[83]
	82	Pd	94.6 e	76.4 e							[83]
	83 corresponding metal complexes). a Pd 91.7 e 78.0 e 84 53.7 e 73.5 e 104						[83] [92]
	85	    (P III ) b 55.3 e	 (P V ) b 2 JP,P=S JP,Pt 72.4 e 105	2 JP=S,Pt P=S	P-N c	P-M	S-M	PNP	SMP	Ref. [93]
	58 89		54.5 d 53.9 h	72.4 d 72.6 h	105						[77] [72]
	59 86	Pd	52.4 e 78.6 e	72.0 e 105.7 e	13			1.955(2) 1.722		124.3(2)	[75, 79] [92]
	60 87	Pt	54.4 f 73.4 e	67.5 f 73.4 e	63	3884	110	2.010(2) 1.712 2.203(1) 2.295(1) 116.2(2) 92.5(1)	[78] [92]
	61 88	Pd	52.7 e 80.1 e	73.1 e 106.7 e	105			2.004(1) 1.701 2.204(9) 2.309(1) 118.5(2) 90.5(3)	[81] [93]
	Pd Ni Pt Ni Pd Ni Pt Ni Pd 93' Ni 90 62 91 63 92 64 93 65 66 67 Pt 94 Ni	106.2 d 84.6 h 76.6 d 86.3 h 98.0 e 83.8 e,j 70.2 e 86.6 e,j 97.3 e 86.3 k 69.6 e 83.7 e,j	80.5 d 105.8 h 75.4 d 95.6 h 75.9 e 72.7 e 75.2 e 94.4 k 71.6 e	57 i 53 i 48 48 51 i 48		88 104 100	2.020(4) 1.698 2.198(3) 2.180(3) 114.7(6) 93.2(1) 2.003(3) 1.706 2.197(2) 2.306(2) 115.4(4) 90.5(8) 2.017(1) 1.685 2.187(8) 2.190(7) 117.3(1) 90.2(3) 2.011(3) 1.698 2.214(2) 2.274(2) 116.0(3) 91.2(7) 2.004(1) 1.705 2.138(1) 2.163(1) 118.0(2) 94.3(4) 2.016(4) 1.712 2.212(3) 2.260(3) 115.1(6) 92.1(1) 2.012(2) 1.692 2.139(2) 2.154(2) 118.5(2) 94.1(6) 2.017 c 1.689 2.213 c 2.185 c 115.6 c 91.7 c 2.013(1) 1.706 2.144(1) 2.158(1) 118.1(2) 94.8(3)	[77] [93] [77] [72] [79] [93] [79] [72] [80] [72] [80] [93]
	68	Pd	102.9 e	74.0 e	59						[81]
	69	Pt	72.5 e	70.1 e	55						[81]	3 (90), 67.5 (91),
	70 67.2 (94') Hz, 3+4 JP,P'=S, = 66.3 (90), 5.7 (91), 7.4 (94') Hz, 4 J P=S,P'=S = 76.4 (90), 66.1 (91), 66.5 (94') Hz, 2 J P,P' = 76.4 (90), 68.1 (91), 68.4 (94') Hz; j broad singlet due Pd 96.3 f 70.9 f 58 [82]
	71 to the fast equilibrium between Pd 98.6 f 70.5 f	58						[82]
	72	Pd	97.8 e	69.1 e	49						[82]
	73	Pd	97.9 e	69.1 e	52			1.995(2) 1.697 2.246(1) 2.386(1) 113.7(2) 91.2(1)	[82]
	74 g Pd	90.0 e	78.1 e	82			1.992(3) 1.713 2.285(2) 2.375(2) 110.2(4) 94.6(8)	[83]
	75 g Pd	90.4 e	76.5 e	60						[83]
	76 g Pd	87.7 e	74.1 e	71						[83]
	77 g Pd	89.2 e	75.2 e	82						[83]
	78 g Pd	92.5 e	76.5 e	76						[83]
	79	Pd	93.7 e	78.7 e	90						[83]
	80	Pd	93.6 e	76.5 e	73						[83]

a Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b appears as doublet unless specified; c average value; d in CDCl3 or CD2Cl2; e in CDCl3; f in CH2Cl2; g the 31 P NMR data of only one of the two diastereoisomers are given; h in CD2Cl2; i 2+3 JP,P=S = 2+3 JP',P'=S = 77.

Table 7 .

 7 31 P NMR data and characteristic structural parameters of Group 9 metal complexes 95-100. a Chemical shifts, coupling constants, bond lengths and angles given in ppm, Hz, Å and deg, respectively; b multiplicity of the NMR signals given in parenthesis; c average value; d in CDCl3 or CD2Cl2; e in CDCl3. The 31 P NMR spectrum of 102 exhibited the expected two doublets for the nonequivalent P III and P V nuclei, and only the resonance of the P atom directly coordinated to the Ru II center

		M	 (P III ) b	 (P V ) b	2 JP,P=S	1 JP,M 2 JP=S,M	P=S	P-N c	P-M	S-M	PNP	SMP	Ref.
	95	Rh I	122.8(dd) d	76.0(dd) d	71	159	3						[77]
	96	Rh I	112.3(dd) e	68.5(d) e	63	167		1.992(5) 1.72	2.208(5) 2.369(5) 110.1(8) 89.2(2)	[80]
	97	Rh I	114.9(dd) e	69.2(d) e	65	164							[80]
	98	Rh I	104.6(dd) e	70.9(d) e	66	158							[79]
	99-cis	Ir I	95.1(d) e	72.9(d) e	66								[79]
	99-trans Ir I	82.0(d) e	77.0(d) e	57								[79]
	100	Rh I	103.0(dd) e	69.8(d) e	66	156							[75]

a

in the FT-IR spectrum of DPPA-O. Moreover, H-bonds involving the N-H and P=O moieties of two

Acknowledgments

We gratefully acknowledge the Centre National de la Recherche Scientifique (CNRS) and the Institut Universitaire de France (IUF) for funding.

Scheme 23. Synthesis of Rh I and Ir I complexes (95)(96)(97)(98)(99) of mono-sulfide N-substituted DPPA-type ligands (58)(59)(60) [77,79,80].

The one-pot reaction between [Rh(μ-Cl)(COD)]2, ligand 59 and NaBF4 in a 1:2:2 molar ratio, afforded the mononuclear cationic complex [Rh(COD){(59)-P,S}][BF4] (100), as deduced by EA, multinuclear NMR and FT-IR spectroscopic techniques (Table 7) [75]. 102 Ru II 108.4 75.6 50 [74] 103 Ru II 125.9 46.0 46 [96] 104 Ru II 117.9 76.2 70 [96] a Spectra recorded in CDCl3, chemical shifts and coupling constants given in ppm and Hz, respectively; b appears as doublet; c not given. 98]. The 31 P NMR spectra of the complexes 105-108 exhibit all two sharp doublets for the nonequivalent P III and P V nuclei, as expected for an AX spin system, with a P,P=S coupling constant nearly similar to that of the free ligands (Table 6 and Table 9). While the P III resonance was always found at lower field compared to the free ligands (Δδ = 45-68 ppm), the chemical shift of the P V resonances remained similar for 105-106 and was only slightly upfield shifted by ca. 6 ppm for 107-108. The assignment of the P III and P V resonances was unambiguously established due to the presence of 185 W satellites in 106 and 108 (Table 9).

Scheme 27. Synthesis of Group 6 metal complexes of mono-sulfide N-substituted DPPA-type ligands 58 and 61 [77,98]. Note: L2 = COD or (Pip)2.

Table 9. 31 particular structural features, which might result from steric hindrance around the metal center in 130

and 131, could be noticed; 1) the two apical CO ligands are not linear and 2) the N-aryl group is nearly perfectly orthogonal to the PNP plane (Figure 11), which is not always the case with N-aryl DPPA-type ligands [84,87,101]. (Scheme 30) [79]. The 31 acetic acid) than its mono-selenide DPPM analog [102].

The chlorido-bridged structure of complexes [M(μ-Cl){(120)-P,Se}]2 136 (Rh) and 137 (Ir), along with the non-equivalent donor set in ligand 120 (P,P=Se), led to the coexistence of two isomers (1:1 approx. ratio) after five hours reaction time [79]. However, the ratio evolved with time and, after three days, a 4:1 ratio in favor of the isomers resonating at higher field and exhibiting the lower P,P and P,Rh coupling constants was reached. These observations clearly illustrate the non-innocent influence of the P=E (E = O, S, Se) group, because the Rh I complex 98 (P,S ligand 59, section 5.2) was only obtained as a single isomer, while its P,Se counterpart 136 exists in two forms. The 31 P NMR data for both isomers are summarized in Table 11. However, a conclusive assignment to the cis and trans forms could not be provided. The proposed mechanism for the isomerization is the same as for the


Thomas G Ribelli

S M Wahidur Rahaman

Krzysztof Matyjaszewski

Rinaldo Poli

Catalyzed Radical Termination in the Presence of Tellanyl Radicals

Keywords: Radical termination, Tellanyl radical, Catalysis, V-601, DFT calculations

The decomposition of the diazo initiator dimethyl 2,2'azobis(isobutyrate) (V-601), generating the Me2C • (CO2Me) radical, affords essentially the same fraction of disproportionation and combination in media with a large range of viscosity (C6D6, DMSO-d6 and PEG 200) in the 25-100°C range. This is in stark contrast to recent results by Yamago et al. on the same radical generated from Me2C(TeMe)(CO2Me) and on other X-TeR systems (X = polymer chain or unimer model; R = Me, Ph). The discrepancy is rationalized on the basis of an unprecedented RTe • -catalyzed radical disproportionation, with support from DFT calculations and photochemical V-601 decomposition in the presence of Te2Ph2.

The mechanism of radical termination is of fundamental importance in polymer chemistry. Bimolecular radical termination can occur via either disproportionation (Disp) or combination (Comb). Disp results in two chains, one with an unsaturated chain-end and one with a saturated one, while Comb gives a single chain following C-C coupling. [START_REF] Moad | The Chemistry of free Radical Polymerization[END_REF] The respective amounts of Disp and Comb highly depend on the nature of the radical species. For example, styrenes [2] and acrylonitrile [3] radicals undergo predominately Comb while methacrylates undergo both Disp and Comb. [2b, 4] The termination of acrylates has been of intense, ongoing debate with contrasting evidence in favor of either Comb or Disp. [2a, 5] Recently, Yamago et al. have put forth a few contributions towards the understanding of the radical termination mechanisms using a tellurium-mediated radical polymerization (TERP) system. [6] Various TERP macroinitiators, for instance poly(methyl methacrylate) (pMMA-TeR), poly(methyl acrylate) (pMA-TeR) and polystyrene (pSt-TeR) (R = Ph or Me) species with controlled molecular weight (MW) and narrow MW distribution, as well as corresponding low molar mass models (unimers), were synthesized and used as radical species precursors. The C-Te bonds were photolyzed, resulting in the pair of carbon-(R0•) and tellurium-based (•TeR) radicals which then underwent spontaneous termination processes. The •TeR radicals ultimately combine to form the RTe-TeR dimer, whereas the fate of the carbon-based radicals was investigated by NMR, SEC and MS to quantify the respective contributions of Disp and Comb, see Scheme 1 (A and B). Furthermore, the generation of radicals via photolysis allowed these termination reactions to be investigated under a wide variety of conditions by varying temperature and solvent.

Scheme 1. 1 (top): work using organotellurium initiators to generate carbonbased radicals from (A) TERP macroinitiators and (B) the unimer model of the pMMA-TeMe dormant species. [6] 2 (bottom): activation of the diazo compound V-601 to generate methacrylate radicals in the absence of tellurium compounds.

While the Disp/Comb ratio was as expected on the basis of previous literature reports for the pMMA • and pSt • radicals, [6a] it was surprisingly found that acrylate radicals resulting from a pMA-TePh macroinitiator terminate almost exclusively (99%) via Disp, [6b] in contrast with several other previous [5b, 7] and subsequent [8] reports. In the most recent contribution, [6c] it was shown that the product distribution depends on the reaction medium, a greater viscosity resulting in an increase of the Disp/Comb ratio. For example, 59% Disp was observed for the methacrylate unimer model compound Me2C(TeMe)CO2Me at relatively low viscosity (η = 1.1 mPas), whereas this amount increased significantly to 94% at η = 84 mPas at room temperature. Even more glaring was the use of the styrene unimer model complex CH3(TeMe)CHPh in which case 14% Disp was observed at low viscosity (η = 1.1 mPas) and increased drastically to 99% at η = 84 mPas at room temperature. In order to rationalize these surprising results, Yamago and coworkers have put forth an "advanced collision model", which rests on the principle that Comb is more viscosity-sensitive than Disp, hence a viscosity increase would result in a more significant retardation of the Comb rate constant (kc) than of the Disp constant (kd).

We have also been interested in the termination mechanism of acrylates, particularly in the case when this is catalyzed by a transition metal complex such as certain Cu-based ATRP catalysts. [9] Initial studies have given strong indications that the Cu-catalyzed radical termination (CRT) overwhelmingly leads to Disp-type products in the ATRP of acrylates. [9b] In addition, a more recent analysis of the termination products of acrylate radicals led to the conclusion that pMA radicals spontaneously terminate predominantly by Comb, in contrast to the recent report by Yamago et al. [6b] These results, which will be reported separately, [10] led us to hypothesize that the presence of RTe • radicals may promote Disp. In order to verify this hypothesis, it was necessary to first investigate the fate of radicals related to those of the investigation by Yamago et al., but obtained in the absence of RTe • , and then to rationalize any observed differences. We report here the use of the diazo initiator dimethyl 2,2'azobis(isobutyrate) (V-601) as a radical precursor to study radical termination. [11] This compound, by either thermal or photochemical activation, yields a radical identical to one of those studied by Yamago et al., see Scheme 1C. We show that the viscosity effects reported for the termination using the Me2C(TeMe)CO2Me initiator are not observed in the absence of tellurium. Furthermore, introduction of Te2Ph2 in the photochemical V-601 decomposition in C6D6 increases the Disp/Comb ratio, indicating the action of PhTe • as a Disp catalyst, which was further supported by DFT calculations We propose a simpler model, based on a viscosity effect on the radical diffusion away from the solvent cage, to rationalize the previously reported results.

The Me2C • (CO2Me) radicals, generated from V-601, were allowed to terminate under conditions as close as possible to those used by Yamago et al. for the Me2C(TeMe)CO2Me decomposition (for details, see the SI). The reactions were carried out in the same media, except for PEG 200 instead of PEG 400 because of better NMR spectral resolution (however, the viscosity range of these two solvents partially overlap), and in a wider temperature range (25-100°C). The % Disp and Comb obtained in all our experiments are collected in the Supporting Information (Table S1) and shown graphically in Figure 1 & S12. Examination of these results shows quite clearly that the temperature and the solvent nature have little effect, if any, on the relative proportions of Disp and Comb, thus questioning the validity of the previous proposed "advanced collision model". [6c] A possible way to rationalize the different outcome of the two decomposition experiments is suggested by the analysis of the termination reaction as a stepwise process: (i) formation of the Me2C • (CO2Me) radical and a second partner radical within a solvent cage, (ii) escape from the cage and (iii) termination by Disp and/or Comb. For V-601, both isobutyryl radicals needed for the termination events are formed within the same solvent cage, hence bimolecular termination may occur either within the solvent cage or after cage escape. For the tellurium-based system, on the other hand, cage escape is necessary for bimolecular radical termination between two isobutyryl radicals to occur. Thus, the observed difference in product distribution may result from the presence of side reactions between the Me2C • (CO2Me) and tellanyl radicals within the cage. [12] Scheme Proposed mechanism for C-based radical disproportionation catalyzed by RTe • . (i) Radical pair generation; (ii) solvent cage escape; (iii) bimolecular radical termination A possible side reaction is β-H abstraction [13] from the Cbased radical by RTe • , resulting in RTe-H and an alkene (R = Me, Ph), as shown in Scheme 2, step (a). This process has been previously shown for Cr [14] and Co [15] complexes and can also be envisaged for any β-H-containing alkyl chains, such as pMMA, pMA, pSt and their unimolecular models. The RTe-H intermediate may then transfer the H atom to a second radical, yielding the saturated Disp product with regeneration of RTe • as shown in Scheme 2 step (b). Note that the tellanyl radical produced in step (b) may dimerize but may also induce disproportionation of additional radicals that have already escaped from their solvent cages.

The feasibility of this catalytic disproportionation has been analyzed by DFT calculations on both steps (a) and (b) of the catalytic cycle proposed in Scheme 2, using the same MeTe • and Me2C • (CO2Me) radicals of the experimental investigation presented in Figure 1 (see the computational details in the SI). The DFT results are summarized in Figure 2A. All optimized geometries and energies are collected in the SI. The unrestricted optimization of the MeTe • + Me2C • (CO2Me) radical pair, carried out under the broken symmetry approach (total spin S = 0) yielded a loose van der Waals minimum (vdW1), the shortest contact being 4.68 Å between the Te atom and one Me H atom. This adduct is slightly stabilized (-0.8 kcal/mol) relative to the sum of the two separate radicals on the gas-phase electronic energy scale (ΔEgas), but destabilized on the Gibbs energy scale after corrections for dispersion and solvation in MeCN (ΔGD3,MeCN = (+3.5 kcal/mol). In this adduct, each partner maintains its full spin density (1.000 for MeCH • (CO2Me), of which 0.788 on the unsaturated C atom; -1.000 for MeTe • , all on the Te atom). Step a is exoergic by 23.0 kcal/mol from the separated MeTe • + Me2C • (CO2Me) reagents. We failed to locate the transition state for this step (TS1) but our attempts prove that this must be located very near the vdW1 minimum and that the barrier is nearly nonexistent (see details in SI). Likewise, step b starts from a very loose van der Waals minimum (vdW2, shortest contact of 5.5 Å), slightly stabilized on the ΔEgas scale (-0.7 kcal/mol) but destabilized on the ΔGD3,MeCN scale (+0.6 kcal/mol) relative to the sum of the two separate species. The transformation is again exoergic through a very low energy and relatively "early" transition state TS2 with the Te-H bond stretched to 1.738 Å (relative to 1.676 Å in vdW2), an H•••C distance of 1.852 Å, and a low imaginary frequency of 177i cm -1 . In TS2, the spin density is still mostly localized on the C atom (0.657), though a significant amount is already transferred to the Te atom (0.200), whereas the H atom being transferred has a very low spin density of -0.036.

The above computations show the likelihood of the mechanism proposed in Scheme 2 for the MMA model radical. Thus, the same process may also take place for the other organotellurium reagents, in particular for the controversial case of the pMA • radical. This was further suggested by an analogous investigation on the PhTe • /MeCH • (CO2Me) radical pair, giving similar results (Figure 2B and further details in the SI). At this point, a reasonable interpretation of the results published in the contribution by Yamago and coworkers [6c] rests on the hypothesis that the solvent viscosity affects the rate of step (ii) (solvent cage escape). A viscosity increase slows down the radical diffusion away from the cage, providing a bias in favour of the RTe • -catalyzed Disp. As the solvent viscosity is lowered and thus radical escape from the cage becomes more efficient, a greater contribution of Comb is observed for all radicals generated from organotellurium compounds, approaching the "true" uncatalyzed fraction of Disp to Comb. [6] The presence of "in-cage" radical reactions has previously been shown using alkoxy radicals. [12] It is also noteworthy that the PSt • radical, known to lead preferentially to Comb, [2] was also forced to dramatically shift preference to yield a Disp/Comb ratio of 97/3 when generated from PSt-TePh under very high viscosity conditions. [6c] We propose that this result can more easily be rationalized by the combination of a viscosity-dependent solvent cage escape and the PhTe • -catalyzed Disp process, than by the "advanced collision model" proposed by Yamago et al. [6c] In order to substantiate the proposed tellanyl radicalcatalyzed disproportionation, the V-601 photochemical decomposition at room temperature in C6D6 was carried out in the presence of a tenfold excess of Ph2Te2, known to generate PhTe • under irradiation (see details in the SI). In this case, the putative PhTe • -catalyzed disproportionation may only occur after the V-601-generated radical pair has escaped from the solvent cage. Indeed, the Disp/Comb ratio increased from 50/50 in the absence of PhTe • radicals to 68/32. As a final remark, the Disp/Comb fraction obtained from organotellurium initiators appears to be much more skewed in favour of Disp for pMA than for pSt and pMMA compared to generally accepted Disp/Comb ratios. This could be related to the trend of RTe-alkyl bonds bond strengths (i.e. the energy associated to the homolytic C-Te bond cleavage, step (i) in Scheme 2). Radicals forming a weaker bond presumably have a greater recombination barrier (Hammond principle) and may be able to escape faster from the cage and thus be less susceptible to undergo the catalyzed Disp process. The DFT calculations support this view (see Table 1). For acrylate radicals, the occurrence of transfer reactions such as backbiting must also be carefully considered. [8,10] In conclusion, the V-601 diazo initiator was used to thermally or photochemically generate isobutyryl radicals, which were allowed to undergo bimolecular termination. The resulting fractions of disproportionated (Disp) and combined (Comb) products were found to remain essentially unchanged over a wide range of temperatures and solvent viscosities, contrary to recent reports using organotellurium initiators. In-depth DFT studies demonstrated that a previously undiscovered RTe • catalysed radical disproportionation reaction is viable, providing a likely interpretation of the previously published experimental results. [6] The proposed mechanism occurs via a β-H abstraction from the carbon-based radical forming an unsaturated chain end and a RTe-H intermediate. The latter species can then transfer the hydride to a second radical forming the saturated species and completing the catalytic cycle. At higher solvent viscosities, the alkyl/tellanyl radical pair diffuses more slowly out of the solvent cage and thus more time is available for the catalysed disproportionation to occur.

Figure 1 .

 1 Figure 1. Fraction of disproportionation vs. (A) temperature and (B) viscosity for the isobutyryl radical in benzene (squares), DMSO (circles) or PEG 200/400 (triangles). Both the results obtained in this work from the V-601 diazo initiator (filled symbols) and those reported by Yamago et al. [6c] using the MMA-TeMe initiator (unfilled symbols) are shown for comparison. [V-601]0 = 10 mM (DMSO & C6D6) or 100 mM (PEG 200) heated or irradiated for 24 hours. Colored figures can be seen in Figure S12.

Figure 2 .

 2 Figure 2. Results of the DFT calculations for steps a and b of Scheme 2. (A) R = Me, R1 = Me and (B) R = Ph, R1 = H. The gas-phase electronic energies (ΔEgas) and the solvation-and dispersion-corrected free energies (ΔGD3,MeCN, bold characters in parentheses) are in kcal/mol.

Table 1 .

 1 Homolytic bond strength calculated by DFT for a variety of organotellurium systems.

	Alkyl radical	• TeR	ΔEgas, kcal/mol	ΔGD3,MeCN, kcal/mol

Acknowledgements

We thank the Centre National de la Recherche Scientifique (CNRS) for support through the PICS06782 grant and through the Laboratoire International Associé (LIA) "Laboratory of Coordination Chemistry for Controlled Radical Polymerization". Support from the NSF (CHE 1707490) is also gratefully acknowledged. This work was granted access to the HPC resources of CINES and IDRIS under the allocation (2006 through 2017)-086343 made by GENCI (Grand Equipement National de Calcul Intensif) and to the resources of the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP). We thank one of the referees for a useful suggestion.

Entry for the Table of Contents

COMMUNICATION

The bimolecular termination of (CH3)2C • CO2CH3 radicals generated from V-601 is essentially solvent and temperature independent, contrary to that of the same radicals generated from ( CH3

Catalyzed Radical Termination in the presence of Tellanyl Radicals

Solvent viscosity