Fule Wu

Christophe Deraedt

Yann Cornaton

Laurent Ruhlmann

Lydia Karmazin

Corinne Bailly

Nathalie Kyritsakas

Nolwenn Le Breton

Sylvie Choua

Jean-Pierre Djukic

Fate of Cobaltacycles in Cp*Co-mediated C-H bond Functionalization Catalysis: Cobaltacycles May Collapse upon Oxidation via Co(IV) species

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INTRODUCTION

The 5 -pentamethylcyclopentadienyl ligand [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] -or Cp* -in spite of an abundant literature 2 is often considered as a "robust", sterically and electronically stabilizing [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF][START_REF] Rottink | Ligand and metal effects on the enthalpies of protonation of Cp'M(PR3)(PR'3)X complexes (M = ruthenium or osmium)[END_REF] ligand in its use in catalysts. [START_REF] Park | Comparison of the Reactivities and Selectivities of Group 9 [Cp*MIII] Catalysts in C-H Functionalization Reactions[END_REF] If in specific cases the endo migration of a hydrido ligand to Cp* was reported in the case of a bis-pyridine chelate of Rh(III), 5 literature is rather silent on occurrences of explicit reactions of the Cp* ligand with {N,C} -heterochelating ligands in metallacycles. [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] This issue has been overlooked in most reports dealing with the use of Cp*Co-based catalysts in aromatic C-H bond functionalization employing redox co-catalysts where cobaltacycles are central intermediates. [START_REF] Park | Comparison of the Reactivities and Selectivities of Group 9 [Cp*MIII] Catalysts in C-H Functionalization Reactions[END_REF]7 One can find a rare report on the metal-templated collapse of a metallacycle of a CpNi (II) complex reported by Ustinyuk et al. [START_REF] Ustynyuk | Nickelocene Interaction with o-halogenated azobenzenes. 4-Phenyl-4H-cyclopenta[END_REF] where an azo-benzene-based nickelacycle undergoes interligand condensation and metal decoordination under oxidative conditions, affording a new organic extended aromatic product (1). It is probably because of the scarcity of similar reports that Cp* bound metallacycles of group 9 metals at the +I and +III formal oxidation states are frequently considered as structurally viable [START_REF] Hull | Highly Active and Robust Cp* Iridium Complexes for Catalytic Water Oxidation[END_REF] candidate catalysts. 7c,10 A report by Mayer et al. 11 outlined that the two-electron oxidation of Cp*Ir metallacycles by PhI(OAc) 2 only affects the Cp* ligand in the case of a methyl-Ir(III) complex, whilst other chloro and iodo-Ir(III) complexes are left untouched. In the majority of reports dealing with group 9 metallacycles bearing an heterochelating carbanionic ligand {N,C} -with Cp* bound metals, the latter ligand never interferes explicitly with ligands bound to the metal by other means but steric repulsion and synergetic "through bonds" electronic effects.

(2) 1 2 3 4 5

"H"

In a recent report [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] we disclosed without further reporting on its origin the crystal structure of the I 3 -salt of an unprecedented cationic Cp*-2-phenylpyridine condensation product, i.e [2a] + (2), serendipitously discovered whilst attempting to grow crystals of 1a. It was obvious that [2a] + resulted from the collapse of cobaltacycle 1a by the cyclocondensation of the Cp* and 2-phenylenepyridine (abbr. phpy) ligands with the capture of an exogenous H atom (2). We therefore undertook Interestingly, it has been recently proposed that transient Co(IV) species [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] may play a central role in oxidative C-H bond thiolation. [START_REF] Gensch | Cobalt-Catalyzed C-H Thiolation through Dehydrogenative Cross-Coupling Angew[END_REF] Indeed, in an effort to clarify the mechanism of the catalysis reported by Glorius et al., [START_REF] Gensch | Cobalt-Catalyzed C-H Thiolation through Dehydrogenative Cross-Coupling Angew[END_REF] Perez-Temprano et al. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] showed in a report that appeared while the present manuscript was being finalized that the treatment of a trifluoromethylthiolato complex of a 2-phenylpyridine Cp*bound cobaltacycle with excess Ag(I) salts may lead to the oxidation of the Co(III) centre into a Co(IV) complex purportedly responsible for the formation of the expected orthosubstituted product of C-S coupling (Scheme 1) in various yields (from ca. 99% to 0%) depending on the amount and nature of the oxidant. In this recent report [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] the fate of the remaining amounts of cobaltacycle was not addressed by the authors though.

In the present report we disclose our conclusions on the observed ligand cyclocondensation in cobaltacycles 15 of 2phenylpyridine derivatives and its direct cause, i.e the irreversible oxidation of Co(III) centre into the Co(IV) state, which sheds a new light on the possible limitations of Cp*Co(III) based aromatic C-H functionalization catalysts when placed under oxidative conditions. The recently introduced quantum theory of atom in molecule (QTAIM [START_REF] Bader | Atoms in Molecules: A Quantum Theory[END_REF] )-based g inter interaction score 17 and IBSI index [START_REF] Klein | New Way for Probing Bond Strength[END_REF] are used here to reveal the effect of the oxidation of Co(III) cobaltacycles on the attractive noncovalent Cp*-phpy interactions and their possible driving role in this unexpected cyclocondensative collapse of cobaltacycles.

RESULTS AND DISCUSSION

Synthetic outlook. In our original report that briefly described compound [2a][I 3 ] [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] (2) it was stated that it crystallized from a solution that originally contained 1a. The presence of I 3

-as counter anion suggested that the initial solution of 1a might have been contaminated by adventitious amounts of either I 2 or some I 3 -salts introduced or formed in the course of the synthesis of [Cp*CoI 2 ] 2 , [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] i.e the precursor of 1a. In the present study, suspecting an oxidoreductive process responsible for the formation of the condensation product [2a] + , 1a was hence reacted with two equiv. of I 2 in dichloromethane (abbr. DCM) for 16 h at 20°C. The 1 H NMR analysis of the reaction mixture revealed the full conversion of 1a into a mixture of various products among which a few could be identified readily from their specific signature: 2-(2'iodophenyl)pyridine, tris--iodo dimeric Cp*cobalt (III) cation {[Cp*Co] 2 (-I) 3 } + and compound [2a] + were formed in 40%, 66% and 33% yield according to [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR analysis (3). Compound {[Cp*Co] 2 (-I) 3 }[I 3 ] produces one signal for the Cp* ligand at  1.80 ppm in CDCl 3 . For [2a][I 3 ] the 1 H NMR spectrum depicts the expected magnetic inequivalence of the five methyl groups of the cyclopentenyl fragment that show up at 0.96 (d), 1.31 (s), 1.56 (s), 1.75 (s) and 2.33(s) ppm. Proton H 5 (2) appears at  2.49 ppm as a broad multiplet integrating 1d,e were synthesized by the lithium acetamidateassisted cyclocobaltation method. [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] Given the intricate mixture produced by the reaction of 1a with I 2 , as an alternative to the latter, we decided to probe the oxidizing property of the one-electron oxidant [Ph 3 C][BArF 24 ]. [START_REF] Connelly | Chemical Redox Agents for Organometallic Chemistry[END_REF] The tritylium [Ph 3 C] + , which undergoes a swift decomposition 20 in the gas phase, is in solution a wellknown stable multi-facetted Lewis acidic cation. 21 It may act as a hydride [START_REF] Horn | Towards a Comprehensive Hydride Donor Ability Scale[END_REF] and/or a halide [START_REF] Hayatifar | Transition metal-free hydrodefluorination of acid fluorides and organofluorines by Ph3GeH promoted by catalytic [Ph3C][B(C6F5)4[END_REF] abstraction agent but also as a one-electron oxidant 24 (Ph 3 C / Ph 3 C + couple [START_REF] Bank | Substituent effect on the electrochemical oxidation of trityl anions[END_REF] ) in the ground as well as in the excited state 26 and may be involved also in the hydride abstraction process as reported by Mandon, Astruc et al. [START_REF] Mandon | Electron-transfer pathway in the hydride abstraction by tritylium from exo-substituted cyclohexadiene iron(0) complexes: a key step in the synthesis of heterobifunctional cyclohexadiene from benzene[END_REF] Quite interestingly, to the best of our knowledge there has been no mention of any major drawbacks related to the oxidative properties of the tritylium as "activator", i.e as a halido ligand "remover", in transition metal-mediated homogeneous catalysis. For instance hydrosilylation catalysis otherwise ineffective with Cp*-bound chloridoiridacycles 28 can be triggered if a [Ph 3 C][BArF 24 ] salt is added to the reaction mixture seemingly acting as a chloride scavenger. The reaction between 1a and [Ph 3 C][BArF 24 ] (Scheme 2) carried out at 20°C in DCM for 16 h produced [2a][BArF 24 ] with an NMR yield of 72% (Table 1, entry 1). In fact 1 H NMR spectroscopy monitoring showed that the reaction reached 100% conversion after only 30 min. Also noteworthy, the same experiment carried out with the chloridoiridacycle analogue left the substrate unchanged. The same reaction carried out using 2 equiv. of AgPF 6 as oxidant instead of [Ph 3 C][BArF 24 ] was completed with 100% conversion of 1a into a mixture of [2a][PF 6 ] (70% NMR yield, not isolated) and other aromatic products. Despite the fact that Ag + was also efficient in this reaction, the significant content in side organic products and the overall higher heterogeneity of the reaction mixture led us to privilege [Ph 3 C][BArF 24 ] as a standard oxidant for further physical investigations. Compound [2a][BArF 24 ] was isolated pure by repeated recrystallization with a major loss of material due to the difficulty that arose in the separation of this salt from other polar Cocontaining residues (isolated yield: 29 %). The structure of the cation of [2a][BArF 24 ] obtained by X-ray diffraction analysis is in all aspects identical (Figure 2 Noteworthy, adding 1 equiv. of (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) to the reaction mixture resulted in a significant inhibition (Table 1, entries 2 and 3) of the reaction perceptible from an NMR yield decrease to 35% after 17 h, suggesting that open shell species and radicals essential to the formation of [2a] + are partly trapped by the rather bulky nitroxyl radical. Tritylium-triggered condensation reactions were repeated with other Cp*-bound cobaltacycles, i.e 1b-e, which were all reactive towards [Ph 3 C][BArF 24 ], although showing distinct outcomes depending on the presence (1b, 1c) [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] or absence (1d, 1e) of methyl hydrogens substractable by [Ph 3 C] + (Table 1). The data listed in Table 1 were all acquired under standard conditions consisting of a reaction time of 17 h at room temperature. ) a isolated yield. b 1 H NMR yield in the presence of 1,3,5trimethoxybenzene as an internal standard. c 1 equiv TEMPO/Co, aliquot taken at 17h of reaction time. d 4 equiv TEMPO/Co, aliquot taken after 17 h of reaction.

Only the substrates bearing -Br and -CF 3 substituents (Figure 1), i.e 1d and 1e (Table 1, entry 7 and 10), afforded cyclic pyridiniums, i.e [2d] + and [2e] + , the 1 H NMR signature of which could be recognized readily in the spectrum of the raw mixture from the specific pattern of methyl groups showing up in the  0.9-2.5 ppm range. The reaction with compounds 1b and 1c yielded untraceable mixtures of soluble and insoluble residues with no distinguishable main product. The structures of [2d,e] + were both established by X-ray diffraction analysis, which revealed features akin to those of [2a] + (Figure 2). Cobaltacycle 1f [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] reacted with [Ph 3 C][BArF 24 ] and fully converted to afford the product of hydrodemetallation, that is namely the N-protonated benzo [h]quinoline, [START_REF] Robertson | Crystal chemistry of tetraradial species. Part 9. The versatile BPh4anion, or how organoammonium H(N) atoms compete for hydrogen bonding Can[END_REF] the structure of which was ascertained by X-ray diffraction analysis (cf. Supporting Material). Quite intriguing is the outcome of the reactions of 1a, 1d and 1e with [Ph 3 C][BArF 24 ] when adventitious moisture and air contamination made its way (Table 1, entries 4, 8, 9, 11, 12) to a new solution in CDCl 3 prepared by dissolving the raw reaction residue obtained upon DCM evaporation after an overnight reaction: compounds [3d][BArF 24 ], and [3e][BArF 24 ] were readily isolated as they showed much lower solubility in CDCl 3 than their respective precursors (scheme 2). In contrast, [3a][BArF 24 ], which showed solubility in chloroform similar to that of [2a][BArF 24 ] could not be isolated pure. (Table 1). The structures of [3d] + and [3e] + were inferred from X-ray diffraction analysis (Figure 2) and indicate that they result from the hydroxylation of the same C 5 -H position. The interference of Co species [START_REF] Nesterov | Pronounced retention of stereoconfiguration upon sp 3 CH bonds hydroxylation of dimethylcyclohexanes and decahydronaphthalenes with m-CPBA oxidant and a Cophthalocyanine catalyst[END_REF] released by the cyclocondensation process with the unreacted tritylium cation capable to interact with O 2 and/or water to form possible tritylperoxo species 32 can be suspected in this hydroxylation. Particularly informative is the reaction of the chlorido analogue of 1a, i.e 1g, with [Ph 3 C][BArF 24 ], which afforded [2a][BArF 24 ] in 21 % yield (NMR yield), at room temperature after about 17 h of reaction in DCM (scheme 3, Table 1 entry 13). The case of the Cp-containing complex 1h (Figure 1) is particularly striking (Scheme 4). The reaction of 1h with [Ph 3 C][BArF 24 ] carried out in DCM at room temperature yielded a mixture of mostly untraceable products among which condensation product [2h] + (Figure 3) was isolated in ca. 12% yield and crystallized. Compound [2h] + differs from the previous condensation products by the cationic spiro motif resulting formally from the condensation of the phpy ligand to the same carbon atom at the Cp ligand combined with a prototropy. The incorporation of two 2-phenylenepyridine units in [2h] + obviously entails two distinct processes of intra and intermolecular nature that, for obvious structural reasons, cannot occur with the Cp* complex 1a.

Thermochemistry of the reaction of 1a with [Ph 3 C][BArF 24 ].

The thermochemistry of the reaction of 1a with [Ph 3 C][BArF 24 ] was addressed by isotherm titration calorimetry at +25.00°C in 1,2-dichloroethane (DCE), placing the cobalt complex in the 1 mL cell and the tritylium salt in the servo-controlled injection syringe. Quite interestingly the heat flow decreased steadily down to a marked inflexion point at a [Ph 3 C][BArF 24 ] /1a ratio n of ~1:1 (Figure 4). The enthalpy of reaction was determined from the raw values of heat flow corrected by subtraction of the residual heat peaks at n>1.5:1 as H= 13.7 ± 0.5 kcal/mol (from three independent titrations), which is the total exothermicity of all the chemical processes that take place under those conditions up to the heat inflexion point before athermicity, that is namely: the oxidation of 1a and the cyclocondensation reaction, dechelation by protonation, possible radical captures and recombination in which it is expected that the solvent or any H-atom donor species might be a player. A control reaction carried out under identical conditions was checked by 1 H NMR spectroscopy and showed that a yield of 40% in [2a] + could be achieved (Table 1, entry 5), suggesting that about 60% of the amount of tritylium cation might be diverted to an oxidative chemical pathway either disconnected from or instrumental to the formation of [2a] + .

Attempts to Trace the H-atom source down.

To trace the origin of the H atom incorporated in [2a,d,e] + two sets of experiments were carried out using 2 H-labeled molecules, expecting that a major isotopic effect could allow the identification of the main source of H atom in the reaction of 1a with [Ph 3 C][BArF 24 ] (Scheme 5). The first set consisted of two parallel reactions in anhydrous deuterated and nondeuterated solvents, namely CD 2 Cl 2 and CH 2 Cl 2 , under identical conditions. 1 H NMR analysis of the raw reaction mixtures indicated that the yields in [2a] + were the same in both cases with no incorporation of deuterium at the key position C 5 (Scheme 5). [2a][BArF 24 ] in 64% yield with no incorporation of deuterium at the C 5 position. The latter result suggests that the most probable source of H atom is either the BArF 24 anion, 1a or any of the products of its degradation such as 2-phenylpyridine, {

N H ,[BArF 24 ] [2a] + [Ph 3 C][BArF 24 ]. CD 2 Cl 2 N H ,[BArF 24 ] [2a] + [d 15 .Ph 3 C][BArF 24 ] CD 2 Cl 2 CF 3 SO 3 H CH 2 Cl 2 1a N H D 2 O CD 2 Cl 2 N H ,[
[Cp*Co] 2 (-I) 3 }[BArF 24 ].
To eventually rule out [BArF 24 ] -anion as a potential source of H atom, the reaction of 1a with [Ph 3 C][PF 6 ] 33 was undertaken, which led to the formation of [2a] + in yields similar to those obtained with the tritylium BArF 24 salt. Whether the H atom is captured directly from 1a by an oxidized form of 1a or from an organic source such as Ph 3 CH, was not elucidated experimentally. Triphenylmethane could indeed arise from an hydride abstraction from the Cp* ligand of 1a by [Ph 3 C] + if one refers to the work of Tilley et al. [START_REF] Straus | Trityl tetraphenylborate as a reagent in organometallic chemistry[END_REF] that was originally carried out with [Ph 3 C][Ph 4 B]. However, the presence of Ph 3 CH was not overwhelming in all analysed reaction media. Furthermore, like shown below in the theoretical investigation, Ph 3 CH is a very unlikely source of H radical for thermochemical reasons. It is well established that the trityl radical recombines to give a polyaromatic species, i.e the so-called the Gomberg-Jacobson-Lankamp-Nauta-MacLean asymmetric quinoid-like structure [START_REF] Mcbride | The Hexaphenylethane Riddle[END_REF] (cf. Scheme 6) that is more favorable than Ph 3 C-CPh 3 , which displays a weak but reactive C-C bond. However, the absence of deuterium incorporation into [2a] + in the experiment carried out with [d 15 .Ph 3 C][BArF 24 ] 33 seemingly rules out this latter scenario too. Worthy of note, the treatment of 1a with 1 equiv. of CF 3 SO 3 H in CH 2 Cl 2 (Scheme 5) at room temperature swiftly led to the hydro-de-cobaltation of the phpy ligand with the formation of N-protio,2-phenylpyridinium, suggesting that any formation of H + in the reaction of 1a with [Ph 3 C] + should result in a significant hydrodemetallation of the phpy ligand, which is not the case. Treatment with 1 equiv. of a weaker acid, e.g. pure AcOH, under identical conditions resulted in the hydrodemetallation of 1a and the formation of 2-phenylpyridine, in lower yields though: 14% after 5 h, 34% after 24 h. Also noteworthy is the effect on the reaction of 1a with [Ph 3 C][BArF 24 ] in the presence of 5 eq of H 2 O or D 2 O that was probed under atmospheres of either argon or air (Scheme 5): disregarding the atmosphere, with D 2 O 1 H NMR analysis of the reaction mixture revealed no incorporation of deuterium at the C 5 position of [2a][BArF 24 ]. The formation of the latter in the presence of water occurs with a lower yield when the reaction was run under argon (~10%) as compared to the one ran under air (~80 %)(cf. Supporting Material). This result suggests that the combination of water contamination with oxidizing conditions (1 electron oxidant + air) favours the formation of [2a] + . Noteworthy, [Cp*Co] 2 (-I) 3 + , i.e the main side product of the collapse of 1a, fully decomposes in the reaction carried out in the presence of water under air (cf. Supporting Material). The body of evidence gathered here points at 1a or some degraded from of it as a probable source of the H atom that incorporates into [2a] + . A contribution of the solvent cannot be completely excluded neither at this stage as a major kinetic barrier to deuterium incorporation such as a transfer of H by quantum tunneling 35 (not addressed here) may well lead to the consumption of the residual protio-dichloromethane in the experiments ran in this deuterated solvent.

EPR and electrochemical investigations.

The mixture produced by the reaction of a 40 mM solution of 1a and [Ph 3 C][BArF 24 ] in DCM was analysed by EPR spectroscopy at low temperature in the frozen state. The EPR spectrum (Figure 5) recorded at 4 K after 5 minutes of reaction showed the superposition of the signatures of two species. One intense peak is centered at g2.003 (H pp =1.25 mT) and supports the presence of an organic radical 36 in frozen solution. The second signal of lower intensity displays a distorted axial pattern with broad resonances and effective g values at g 5.2 and g 2.02 similar to that already reported by Wang et al. [START_REF] Wang | Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex[END_REF] These main peaks point to high-spin Co(IV) species [START_REF] Wang | Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex[END_REF][START_REF] Zanders | A Rare Low-Spin CoIV Bis(β-silyldiamide) with High Thermal Stability: Steric Enforcement of a Doublet Configuration Angew[END_REF] (S= 3/2 or S= 5/2) as a result of the oxidation of 1a and in agreement with their disappearance at higher temperature in frozen solution, i.e from 30 K. [START_REF] Zanders | A Rare Low-Spin CoIV Bis(β-silyldiamide) with High Thermal Stability: Steric Enforcement of a Doublet Configuration Angew[END_REF] After 1 hour of reaction of the molten mixture, both signals disappeared when the sample was frozen back to 4 K and analysed. The electrochemical behaviour of the compounds involved in the cyclocondensation of 1a was studied by cyclic voltammetry under argon with solutions in the moderately volatile anhydrous 1,2-dichloroethane using [N(nBu) 4 ][PF 6 ] (0.1 mol L -1 ) as conducting electrolyte (Figure 6, Figures S13-S21 in the Supporting Material and Table 2). The cyclic voltammogram of {[Cp*Co] 2 (-I) 3 + }[I 3-] (Figure 6 black curve) shows three successive oxidation processes at -0.02 V, 0.36 V and 1.25 V vs. Ag/AgCl. The last oxidation at 1.25 V is irreversible and may correspond to the oxidation of the cobalt centre, while the two first oxidations are similar to the redox behaviour of tetraethylammonium iodide (abbr. TEAI) (Supporting Material, Figure S15). It corresponds to the redox properties of the triodide I 3 -counter-anion. In fact, at the potential applied at the start of the sweep I 3 -is first reduced to I -, and redox behaviour similar to that of TEAI is observed. Indeed, cyclic voltammogram of 0.4 mmol.L -1 solution of TEAI recorded in 0.1 mol L -1 [N(nBu) 4 ][PF 6 ]/DCE at a platinum electrode and scan rate of 0.1 V s -1 (Figure 6, blue curve and Figure S10) exhibited also two anodic oxidative peaks at E pa1 (a') = -0.08 V vs. Ag/AgCl and at E pa2 (c')= 0.40 V vs. Ag/AgCl coupled with an cathodic peaks at E pc1 (b') = -0.28 V vs. Ag/AgCl and E pc2 (d') = 0.35 V. The presence of two coupled peaks is well documented in the literature. It indicates the presence of two chemical processes following the two steps of the quasi-reversible electron transfer (E 1 C 1 E 2 C 2 mechanism). [START_REF] Dane | The iodine/iodide redox couple at a platinum electrode Electrochim[END_REF] According to the literature, 40 the peak a' (Figure 6a) is attributed to the E 1 C 1 mechanism: first the oxidation of two iodides to I 2 (electrochemical step E 1 , two electron oxidation wave) followed by the chemical reaction C 1 with I -giving triodide I 3 -(chemical step C 1 ). The peak b' (Figure 6a) is attributed to the reduction of I 3 -to regenerate I -. The second anodic peak (c', Figure 6a) yielded from I 3 -is a one electron oxidation wave (electrochemical step E 2 ) attributed to the oxidation of I 3 obtained after the E 1 C 1 process to form I 2 , while peak d' (Figure 6a) is attributed to the reduction of I 2 to I 3 -. For 1a, two irreversible oxidation peaks were detected at 0.52 and 1.11 V vs. Ag/AgCl (Figure 6a, red curve). The first oxidation corresponds to the oxidation of Co(III) to Co(IV). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] Note that the intensity of the peak current of the second irreversible oxidation at 1.11 V is a little less than half of the intensity of the first oxidation. The last reduction at -1.37 V (peak c) may be due to the irreversible reduction of the pyridinium group of [2a] + . Indeed, the cyclic voltammogram of [2a][BArF 24 ] shows similar irreversible reduction at -1.39 V (Figure 6a, green curve). This irreversible reduction is attributed to the formation of a reactive pyridyl radical which may undergo homocoupling through C-C bond formation. [START_REF] Huo | Photocurrent Generation from Visible Light Irradiation of Covalent Polyoxometalate-Porphyrin Copolymer Electrochim[END_REF] In the case of the cathodic scanning before the anodic scanning, we did not detect the additional signals obtained only after the irreversible oxidation of 1a (Figure 6b). However, one small reoxidation peak at around -0.5 V was detected and is probably arising from the species generated during the reduction of the solvent when reaching the cathodic limit or from the reduction of 1a (ill-defined peak (**) detected in Figure 6b). Finally, compounds 1d, 1e and the new "CpCo" complex 1h give a redox behaviour similar to that of 1a (Table 2 and Supporting Material) showing that the potential of oxidation of the Co(III) is little affected by the nature of substituent R (R = H, Br or CF 3 ) or by substitution of Cp* by Cp in cobaltacycles, like reported for other metallacycles. 42 However, in the case of 1g the redox behaviour is dissimilar with 1a: replacing the iodo by a chlorido ligand leads to only one irreversible oxidation and two irreversible reduction at 0.80 V, -1.21 V and -1.76 V respectively.

DFT evaluation of the mechanism of formation of [2a] + . NCI plots and IBSI suggest a predisposition to cyclocondensation.

A preliminary comparative study between the reactive cobalt complex 1a and the iodoiridium analogue Ir-1a was carried out to address the propensity of 1a to undergo the condensation of Cp* with phpy under oxidative conditions while its Ir analogue supposedly does not undergo cyclocondensation. Note that according to Mayer et al. 11 Ir-1a is reportedly insensitive to PhI(OAc) 2 . The computed difference in the Gibbs free energy of oxidation for the Co(III) and Ir(III) complexes is negligible and cannot explain the difference of behaviour (ΔΔG=0.1 kcal/mol). Our attention turned to the possible existence of attractive noncovalent interactions that could predispose 1a to its collapse by cyclocondensation.

Recent reports [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF][START_REF] Cornaton | A noncovalent interaction insight onto the concerted metallation deprotonation mechanism[END_REF] have indeed stressed the assisting effect of local attractive NCI interactions in the reactant complexes of base assisted aromatic C-H bond activation by Pd(II), Ni(II), Co(III) and Ir(III) centres. NCI plots are particularly informative of the interactions in reactant complexes that may prefigure new bonds appearing once the transition state is passed. [START_REF] Yepes | Real-Space Approach to the Reaction Force: Understanding the Origin of Synchronicity/Nonsynchronicity in Multibond Chemical Reactions[END_REF] NCI plots 43 were therefore scrutinized for peculiar features related to the uncommon cyclocondensation reported here. The NCI plots 43 depicted in Figure 7 display the isosurfaces of attractive domains (red coloured isosurfaces) and repulsive or non-bonding ones (blue coloured isosurfaces) in various singlet and doublet state geometries of complexes relevant to this report, including the one reported by Perez-Temprano. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] The circular attractive domain isosurface located underneath the Cp*ligand in all the cases depicted in Figure 7 is frequently observed in  5 -cyclopentadienyl and  6 -arene-metal complexes. 51 One uncommon feature however is when this ring-like isosurface is no more circular like in Ir-1a but displays attractive outgrowths, like in 1a: these outgrowths are symptomatic of a peculiar attractive relationship between one carbon atom at the cyclopentadienyl moiety and an atom of a group of atoms at a different vicinal ligand. This is the case for the attractive domains between carbon C 4 of the Cp* and carbon C 2' of the phpy ligand that bind during the cyclocondensation and which are present in singlet state 1a but not in singlet state Ir-1a. This difference in attractive NCI is even more pronounced when the Co(III) centre is oxidized to Co(IV) to give [1a'] + (Figure 7) . To get a more quantitative insight onto the difference in interaction strength between the Co and Ir complexes 1a and Ir-1a, at the +III and +IV formal oxidation states and for [Cp*Co(SCF 3 )(phpy)] and 1g at the +III and +IV formal oxidation state, the integrated independent Gradient Model (IGM)-based δg inter descriptor 17 (also called Δg inter interaction score) and the recently developed Intrinsic Bond Strength Index [START_REF] Klein | New Way for Probing Bond Strength[END_REF] (IBSI) were computed to quantify the interaction between phpy and each of the two other ligands. The IGM-δg inter descriptor can be seen as a measure of the electron sharing caused by the electron density interference between two user-defined fragments. It assesses the mutual density penetration by the deviation of the true electron density gradient from a non-interacting reference, i.e the independent gradient model bearing the same electron density as the real system. [START_REF] Wang | Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex[END_REF][START_REF] Zanders | A Rare Low-Spin CoIV Bis(β-silyldiamide) with High Thermal Stability: Steric Enforcement of a Doublet Configuration Angew[END_REF] Integrating this descriptor gives rise to the Δg inter interaction score that measures the interaction between the two user-defined fragments. IBSI is another new efficient quantitative index allowing to intra-molecularly probe the "strength" of a given covalent bond or weaker interaction also based on the so-called Independent Gradient Model, [START_REF] Klein | New Way for Probing Bond Strength[END_REF] which was validated by tests on 677 bonds in 235 molecular systems. Like many QTAIM [START_REF] Bader | Atoms in Molecules: A Quantum Theory[END_REF] -based analytical methods the Δg inter score and IBSI are exempt of issues related to basis set superposition error. Both Δg inter score and IBSI have previously been successfully applied in mechanistic studies for the rationalization of the discrimination between competing reaction pathways [START_REF] Khartabil | Mechanistic insights into Smiles rearrangement. Focus on - stacking interactions along the radical cascade Org[END_REF] and are particularly useful in assessing reactional predispositions in so-called reactant complexes. Like shown below, oxidized cobaltacycles can be considered as typical reactant complexes at a mechanistic crossroad that are capable of either evolving by reductive elimination by the coupling of ligand X with the phpy, which corresponds to observation reported by Perez-Temprano et al. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] for the thiolated complex [Cp*Co(SCF 3 )(phpy)] or by cyclocondensation of phpy with the Cp* ligand (this report). In the cases addressed here (cf. Table 3), Δg inter evidences a stronger interaction between the Cp* and phpy ligands in the iodo-Co case than in the iodo-Ir one, in the +III as well as in the +IV formal oxidation state. Oxidation increases the interaction between Cp* and phpy in the Co case, while the increase is almost null in the Ir one. Conversely, the interaction between phpy and the iodo ligand decreases with the oxidation on both cases and, while it is stronger in the Co case than in the Ir one at the +III formal oxidation state, it is the opposite at the +IV formal oxidation state. IBSI evidences a stronger interaction between C 3 of the Cp* moiety and C 2' of the phpy ligand, as well as between C 4 of the Cp* and N of the phpy, in 1a than in its Ir analogue, while the interaction between the four aforementioned atoms and the metal center is stronger in the Ir analogue. The same observation stands when the complex is oxidized. Moreover, in the Co case oxidation increases the strength of the interaction between the Cp* and the phpy, while decreasing the strength of the interaction between the ligands and the metal center. This analysis suggests that the condensation of the phpy ligand with the Cp* moiety could be somewhat encoded in the favorable IBSI and NCI signatures observed in 1a and its oxidized Co(IV) form [1a'] + . The general qualitative behaviour of the Δg inter interaction score and of the IBSI remains the same when replacing the iodo ligand by a chloro one in the Co complex (Table 3). The main difference lies in the Δg inter between phpy and the halido ligand which is lower in the chloro analogue than in the iodo at the +III formal oxidation state, while it is higher at the +IV formal oxidation state. When comparing 1a to [Cp*Co(SCF 3 )(phpy)], [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF][START_REF] Gensch | Cobalt-Catalyzed C-H Thiolation through Dehydrogenative Cross-Coupling Angew[END_REF] the general quantitative behaviour of the Δg inter interaction score between Cp* and phpy, and of the IBSI remain the same (Table 3). However, the Δg inter interaction score between phpy and the X ligand is higher when X= SCF 3 than when X= I : while it decreases in the latter case, it increases in the former, consistently with the favoured C-S bond formation by reductive elimination. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] More important is the fact the the g inter for the Cp*-phpy interaction in almost identical in 1a/[1a'] + and [Cp*Co(SCF 3 )(phpy)]/[Cp*Co(SCF 3 )(phpy)] + whereas in the latter case no metallacycle collapse has been reported. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] The DFT exploration of the formation of [2a] + . Several scenarios were considered for the mechanism of formation of [2a] + . In this study, only intramolecular mechanisms of condensation were considered, keeping in mind that from the experimental investigations mainly two questions remain unanswered, that are: a) where does the hydrogen H 5 originate from? b) what is the fate of the Co after the release of [2a] + ? We considered that the formulation of a plausible reaction energy profile should be based on the following crucial three steps starting from 1a: i) attack of the carbanionic carbon of the phpy ligand, on carbon C 4 at Cp* ii) attack of the nitrogen of the phpy on carbon C 3 at Cp* and iii) capture of hydrogen H 5 (likely as a H•). ≠ The oxidation of 1a. Like suggested by EPR spectroscopy, cyclic voltammetry and experimental evidence of the ready oxidation of 1a, the first step of the condensation reaction conspicuously involves the oxidation of singlet-state 1a into Co(IV) complex [1a'] + which was considered therein in its doublet-state (by about 20 kcal/mol more stable than the quartet 3/2 spin state) in a way similar to the treatment operated by Pérez-Temprano et al. [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] for [Cp*Co(SCF 3 )(phpy)]. The computed Gibbs free energies of the single electron oxidation of 1a by Ph 3 C + and by I 2 amount to + 9 kcal/mol and + 19 kcal/mol respectively. The Gibbs enthalpy value for Ph 3 C + is a rather low endergonic value for an equilibrated oxidation given that the irreversible consumption of [1a'] + draws this oxidation step to completion by virtue of the mass action law. An additional single electron oxidation of [1a'] + into singlet or triplet-state [1a"] + was eventually discarded, for its Gibbs free energy exceeded + 40 kcal/mol. Note that the spin density of [1a'] + is essentially located at the Co center. Out of these possible paths, the attack on C 4 of Cp* by C 2' (phpy) displays the lowest activation barrier (ΔG= + 7 kcal/mol, while ΔG=+ 48 kcal/mol for the concerted attack and ΔG= + 42 kcal/mol for the attack of N). Each of the three paths from the singlet state agostic [ag-Cp*-H] + complex display an activation barrier higher than + 30 kcal/mol. Therefore, all of these considered transition states are higher in energy than the one corresponding to the coupling of C has not yet been settled. From this investigation it appears that consideration for spin states at Co is important. Also important is the scrutiny of NCI plot patterns, which suggests that the studied cobaltacycles particularly in their Co(IV) state are somewhat prepared to cyclocondensate owing to marked attractive NCI domains that seemingly predispose the interaction of the {N,C} -ligand with Cp*. The relevance of this unprecedented process of collapse of the Co metallacycle particularly raises the fundamental question of the viability of such organometallic intermediates in C-H bond functionalization catalysis where "redox" co-catalysts are commonly used on a trial-and-error basis. [START_REF] Gensch | Cobalt-Catalyzed C-H Thiolation through Dehydrogenative Cross-Coupling Angew[END_REF][START_REF] Liu | Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed Direct Trifluoromethylthiolation of Arenes via C-H Activation[END_REF] If Pérez-Temprano et al. evidenced the role of Co(IV) species in C-H bond sulfurization [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] they did not mention the herein described irreversible collapse of the metallacycle. We speculate that in the context of complex catalytic mixtures containing reagents and co-catalysts endowed with oxidoreductive mediation roles, where similar metallacycles are claimed to be key intermediates, 7b,7c,56 the occurrence of such an unwanted cyclocondensation pathway depends on the nature of ligand X (Scheme 8), like suggested by the IBSI and g inter scores.

Particularly crucial in such catalytic reactions are the kinetics of the reductive elimination step that releases the pivotal "Cp*Co" moiety in the medium alongside the main organic product: a disfavoured reductive-elimination may result in low conversions due to a more favourable "autolysis" of the catalyst by the herein described cyclocondensation (Scheme 8).

Electrochemical investigation suggests that the dechelation of the Co centre to give the dimeric {[Cp*Co] 2 (-I) 3 } + competes with the cyclocondensation process, which might be a crucial information in understanding all the underlying issues of the actual mechanism of catalytic C-H bond functionalization and in improving the performance of catalysts.

Scheme 8. Which pathway prevails in the catalysis ?

The collapse of the metallacycle or the reductive elimination of the targeted product ? How does the nature of ligand "X" determine the outcome ?

In the context of the catalysis, the cyclocondensative collapse of cobaltacycles represents an irreversible destructive deactivation pathway, which further confirms that establishing the whereabouts of the Cp (*) Co(III)-based catalysts after catalysis is an imperious need. The combination of QTAIM-based IBSI, IGM and NCI-plot might prove particularly informative in this endeavour like shown above. From the view point of metal-mediated synthesis, the oxidation-induced cyclocondensation of Cp*Co(III) metallacycles reported herein is a new example of oxidation-triggered Co(IV)-centred "metaltemplated" 57 synthesis of valuable three-dimensional alkaloids of the Berberine 58 class, the hydroxylation reaction of which deserves further investigations.

EXPERIMENTAL PART

General information. All experiments were conducted under a dry argon atmosphere using standard Schlenk lines and dry glovebox techniques. The following compounds were purchased from Sigma Aldrich: 2-phenylpyridine (abbr. phpyH, 98%), tritylium chloride (97%). The following compounds were purchased from Alfa Aesar: 1,2,3,4,5pentamethylcyclopentadiene (abbr. Cp*, 94%), and sodium tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (abbr. Na-BArF 24 , 97%). The following compounds were purchased from TCL chemicals: 2-(4-bromophenyl)pyridine (abbr. Br-phpyH, 98%), 2- [4-(trifluoromethyl)phenyl]pyridine (abbr. CF 3 -phpyH, 98%), 2-(p-tolyl)pyridine (98%). The cobalt(III) complexes [Co(η 5 -C 5 Me 5 )I 2 ] 2 and LiNHAc were prepared as reported in the literature. [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF][START_REF] Sun | Air-Stable Carbonyl(pentamethylcyclopentadienyl)cobalt Diiodide Complex as a Precursor for Cationic (Pentamethylcyclopentadienyl)cobalt(III) Catalysis: Application for Directed C-2 Selective C-H Amidation of Indoles[END_REF] All NMR yields were calculated using 1,3,5-trimethoxybenzene as internal 1 H NMR standard. All solvents were distilled over sodium or CaH 2 under argon before use. 1,2-Dichloroethane used in cyclic voltammetry experiments was distilled over CaH 2 under argon. Deuterated solvents were dried over sodium or CaH 2 , filtered over activated neutral alumina, and stored under argon before use. 1 H (300, 400, 500, and 600 MHz) and13 C (75 and126 MHz) NMR spectra were measured on Bruker DPX 300 and 400, Avance I 500, and Avance III 600 spectrometers. Chemical shifts (expressed in parts per million) were referenced against solvent peaks. Mass spectra were run on a MicroTOF Bruker Daltonics spectrometer, using a TOF-ESI coupling analysis system. Elemental analyses were achieved with Thermo Scientific FLASH 2000 CHNS/O analyzers.

X-ray diffraction analysis. The crystals were placed in oil, and a single crystal was selected, mounted on a glass fibre and placed in a low-temperature N 2 stream. X-ray diffraction data collection was carried out on a Bruker PHOTON III DUO CPAD diffractometer equipped with an Oxford Cryosystem liquid N 2 device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 37 mm. The cell parameters were determined (APEX3 60 software) from reflections taken from 1 set of 180 frames at 1s exposure. The structures were solved using the program SHELXT-2014. [START_REF] Sheldrick | SHELXT -Integrated space-group and crystal-structure determination[END_REF] The refinement and all further calculations were carried out using SHELXL-2014 or SHELXL-2018. [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] For compound [3e][BArF 24 ], the hydrogen atom of the OH group was located from Fourier difference. The other H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F 2 . A semi-empirical absorption correction was applied using SADABS in APEX3.

Cyclic Voltammetry. All glassware was dried for 24 hours at 90°C prior to use. Voltammetric data were recorded using an Autolab PGSTAT30 potentiostat (Eco Chemie, Holland) driven by the GPSE software with a standard three-electrode system flushed with a gentle stream of argon. The electrolyte consisted of a solution of 0.1 mol L -1 of anhydrous tetrabutylammonium hexafluorophosphate ([NBu 4 ][PF 6 ]) in dry distilled 1,2-C 2 H 4 Cl 2 . The reference electrode was the Ag/AgCl electrode, which was electrically connected to the solution by a junction bridge filled with 5 mL of anhydrous CH 3 CN + 0.1 M [NBu 4 ][PF 6 ]. The working electrode was a platinum disk electrode (d = 3 mm in diameter). All potentials are given vs. Ag/AgCl used as external reference and are uncorrected from ohmic drop. EPR spectroscopy. EPR spectra were recorded on a continuous-wave X-band EMX-plus equipped with TE102 rectangular cavity (ER 4102ST, Bruker) and an ESR900 continuous flow cryostat controlled with an Oxford ITC503S. EPR spectra were recorded with the following parameters: microwave power of 1 mW, a modulation frequency and amplitude of respectively 100 kHz and 0.4 mT, a conversion time of 300 ms and a time constant of 20.48 ms. A 2 nd order polynomial baseline was subtracted to the spectra recorded between 0 and 450 mT. The simulation was done using Easyspin 63 toolbox working under MatLab (MathWorks).

ITC experiments. Isothermal titration calorimetry (ITC)

experiments were carried out on a Waters-SAS nano-ITC device (TA Instruments®) equipped with two stainless steel Hastelloy cells of 1 mL volume each) and housed in an argonfilled glovebox. Auto equilibration of the ITC was performed before every experiment to reach an acceptable baseline. The solutions of reactants were prepared in the same glovebox by dissolving a mass of substrate placed in a volumetric flask in pure, freshly distilled and degassed 1,2-dichloroethane. The ITC experiments were performed by sequential injection at 25.00⁰ C with a stirring rate of 300 rpm. The solution of tritylium salt was placed in the injection syringe and the solution of compound 1a in the measure cell. Three thermograms were acquired by sequential injection of 2-3 L with a delay between two injections not exceeding 900 s, and the raw enthalpy of the reaction was inferred by summing up the heat flow released from which was deduced the residual heat flow after the thermal inflexion point.

Computational details DFT-D computations. Geometry optimization of the reactants, intermediates, transition states, and products were performed using the SCM-ADF2019.01 [START_REF] Te Velde | Chemistry with ADF[END_REF] package, at the Density Functional Theory (DFT) level. The Perdew-Burke-Ernzerhof (PBE) [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] functional augmented with Grimme's dispersion corrections with the electronegativity equilibrium model (DFT-D4(EEQ)) [START_REF] Caldeweyher | A generally applicable atomic-charge dependent London dispersion correction[END_REF] was used in all geometry optimizations. All geometry computations were carried out using scalar relativistic effects with ad hoc all-electron (abbr. ae) single polarization functions triple-ζ Slater-type basis set (TZP) 64 . Solvation was treated by the Conductor-like Screening Model (COSMO) 65 procedure assuming CH 2 Cl 2 (ε= 8.9, r= 2.94 Å) as a solvent. Geometry optimization by energy gradient minimization were carried out in all cases with integration grid accuracy "Normal". [START_REF] Franchini | The Becke fuzzy cells integration scheme in the Amsterdam Density Functional program suite[END_REF] All transition states were found using the Nudged Elastic Band (NEB) [START_REF] Henkelmann | A climbing image nudged elastic band method for finding saddle points and minimum energy paths[END_REF] procedure and submitted to the Intrinsic Reaction Coordinate (IRC) 68 procedure to verify the connection to their reactive complexes and products. Vibrational modes were analytically computed to verify that the optimized geometries were related to energy minima or to transition states. For the monoatomic systems that were considered (H -and I -), as the enthalpy and entropy corrections to the energy are usually computed from the vibrational energies, these contributions were neglected and the Gibbs free energy was approximated as the bond energy. IBSI computations. Intrinsic Bond Strength Index (IBSI) of the key atom pairs were computed using the IGMPLOT program 17 using gaussian-type wavefunctions obtained from single-point calculations with the PBE functional 46 augmented with Grimme's correction to the dispersion with a Becke-Johnson damping function 69 (PBE-D3(BJ)), scalar relativistic effects with the balanced Karlsruhe 2 nd generation default triple-ζ valence plus polarization (def2-TZVP) basis set [START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] and Conductor-like Polarizable Continuum Model (CPCM) [START_REF] Barone | Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model[END_REF] simulating dichloromethane (ε= 8.9, r= 2.94 Å) as a solvent on the previously optimized geometry using the ORCA program system [START_REF] Neese | The ORCA program system[END_REF] ). [Ph 3 C][BArF 24 ] was prepared according to a literature procedure with minor modification. [START_REF] Bahr | Trityl Tetrakis(3,5-Bis(Trifluoromethyl)Phenyl)Borate -a New Hydride Abstraction Reagent[END_REF] Under argon atmosphere, a mixture NaBArF 24 (886.2 mg, 1 mmol) and tritylium chloride (278.1 mg, 1 mmol) was stirred overnight in ~20 mL of CH 2 Cl 2 at room temperature in a sealed Schlenk vessel. The reaction mixture was then filtrated through Celite, the filtrate was evaporated under reduced pressure. The resulting solid was first washed with n-pentane and then purified by recrystallization with CH 2 Cl 2 /n-pentane to afford bright yellow microcrystals (973 mg, 88% yield). [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (300 MHz, CDCl 3 ) δ 8.15 (ddt, J = 8.8, 7.3, 1.3 Hz, 3H), 7.80 -7.73 (m, 6H), 7.68 (p, J = 2.1 Hz, 8H), 7. 59 -7.54 (m, 6H), 7.48 (s, 4H).

Preparation of the d 15 -trityl chloride . . d 15 -Tritylium chloride was prepared according to a literature procedure with minor modification [START_REF] O'brien | Solid-State Molecular Rotors with Perdeuterated Stators: Mechanistic Insights from Biphenylene Rotational Dynamics in Ordered and Disordered Crystal Forms[END_REF] . Under argon atmosphere, a mixture of d 6benzene (5.0 mL, 0.05 mmol) and carbon tetrachloride (1.0 mL, 0.01 mmol) was dissolved in 20 mL of 1,1,2,2tetrachloroethane in a sealed Schlenk vessel, and cooled down to 0 °C. Aluminium trichloride (1.6 g, 0.01 mmol) was then added slowly during stirring. After 15 min of stirring, the solution was refluxed for 90 min and then cooled down to room temperature. The solution was treated with 1 M HCl, and the organic phase was extracted with DCM and dried over Mg 2 SO 4 . After removal of the solvent under vacuum, the crude solid was purified by column chromatography (hexanes/ethyl acetate = 9:1) to afford d 15 -tritylium chloride (1.2 g, 37%).

Synthesis of CpCo(CO)I 2 . CpCo(CO) 2 (1 mL, 7.5 mmol) and 20 mL of distilled Et 2 O were introduced in a Schlenk tube under argon atmosphere. I 2 (2 g, 7.9 mmol) dissolved in distilled Et 2 O was added to the Schlenk tube with a cannula. The reaction was stirred overnight at room temperature. The solvent was removed under reduced pressure and the residual solid washed 4 times with distilled pentane. 1 H NMR analysis in CDCl 3 revealed the disappearance of the signal of the starting compound at 5.06 ppm that was replaced by a new Cp singlet at 5.68 ppm. Analytical data were in accord with those reported in the literature. 75 Synthesis of [CpCoI 2 ] 2 . The synthesis of the [CpCoI 2 ] 2 was carried out by applying the methodology reported for [Cp*CoI 2 ] 2 . [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] In a round bottom flask, 1 g of [CpCo(CO)I 2 ] were introduced with 50 mL of distilled heptane. The solution was stirred and refluxed during 2 days. After reaction, the solid was filtrated and washed 3 times with distilled pentane. [CpCoI 2 ] was insoluble in most solvents, but slightly soluble in coordinating solvents such as CH 3 CN and THF giving rise to quite air and light sensitive solutions. All analytical data were in accord with data published in literature. 75b,76 General Procedure for the Synthesis of Iodocobaltacycles. 1a~e were prepared according to our published procedure. [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] A mixture of [Cp*CoI 2 ] 2 (1 equiv), LiNHAc (6 equiv.), and 2arylpyridine ligand (2 equiv.) were stirred in ∼10-15 mL of CH 2 Cl 2 at 50 °C for 48 h in a sealed Schlenk vessel. The reaction mixture was filtrated over Celite and the resulting filtrate was stripped of solvent. The resulting residue was then purified by column chromatography (DCM/n-pentane= 4/1) and recrystallization with CH 2 Cl 2 /n-pentane and/or washing with n-pentane. (d, J = 7.7 Hz, 1H), 7.00 (td, J = 5.9, 2.7 Hz, 1H), 6.84 (ddd, J = 7.8, 1.6, 0.7 Hz, 1H), 2.46 (d, J = 0.7 Hz, 3H), 1.48 (s, 15H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 181. 16, 167.16, 154.92, 143.21, 143.00, 139.08, 136.54, 124.07, 122.96, 120.76, 118.10, 93.36, 22.13, 10.39 [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (500 MHz, CDCl 3 ): δ 9.16 (ddd, J = 5.7, 1.5, 0.8 Hz, 1H), 8.34 (d, J = 1.9 Hz, 1H), 7.63 (ddd, J = 8.0, 7.1, 1.5 Hz, 1H), 7.59 (ddd, J = 8.2, 1.8, 0.8 Hz, 1H), 7.40 (d, J = 8.2 Hz, 1H), 7.17 (dd, J = 8.2, 1.9 Hz, 1H), 7.08 (ddd, J = 7.3, 5.7, 1.7 Hz, 1H), 1.50 (s, 15H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 184. 10, 166.10, 154.92, 144.21, 143.88, 136.78, 125.80, 124.07, 123.94, 121.52, 118.54, 93.58, 10.26 IN: C,48.64; H,4.08; N,2.58. Found: C, 48.55; H, 4.04; N, 2.64. [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (500 MHz, CDCl 3 ): δ 9. 25 -9.20 (m, 1H), 8.50 (dd, J = 1.8, 0.9 Hz, 1H), 7. 72 -7.66 (m, 2H), 7.63 (dd, J = 8.1, 0.9 Hz, 1H), 7.29 -7.26 (m, 1H), 7.14 (ddd, J = 6.6, 5.7, 2.2 Hz, 1H), 1.50 (s, 15H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 181. 93, 165.71, 155.20, 148.65, 138.28, 137.04, 130.04, 122.49, 122.31, 119.76, 119.35, 93.86, 10.36. 19

Synthesis of chloro(2-phenylpyridine- 2 -C,N)(pentamethylcyclopentadienyl)cobalt(III) (1g).

A mixture of 1a (95 mg, 0.2 mmol) and dry tetrabutylammonium chloride (100 mg, 0.35 mmol) was stirred overnight in ∼10-15 mL of acetonitrile at room temperature in a sealed Schlenk vessel. After reaction, washed and filtrated with H 2 O, the resulting residue was purified by recrystallization with CH 2 Cl 2 /n-pentane and/or washing with n-pentane, to afford the pure compound the analytical data of which matched published data 77 (60 mg, 79 %). [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (400 MHz, CDCl 3 ): δ 9.24 (dt, J = 5.6, 1.2 Hz, 1H), 8.28 (dd, J = 7.7, 1.1 Hz, 1H), 7. 70 -7.63 (m, 2H), 7.53 (dd, J = 7.6, 1.5 Hz, 1H), 7.32 (td, J = 7.4, 1.5 Hz, 1H), 7.15 -7.07 (m, 2H), 1.29 (s, 15H).

Synthesis

of iodo[2-phenylpyridine-κ 2 -C,N](cyclopentadienyl)cobalt(III) (1h). The synthesis of 1h was performed similarly to the one of 1a published recently. [START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] [CpCoI 2 ] 2 (40 mg, 0.053 mmol), 2-phenylpridine (15.1 µL, 0.106 mmol) and LiNHCOCH 3 (20 mg, 0.307 mmol) were introduced in a Schlenk tube and 5 mL of distilled CH 2 Cl 2 were added. The reaction was stirred two days at 50°C. The reaction was filtrated and the resulting filtrate was stripped of solvent under reduced pressure. The resulting solid residue was washed 3 times with distilled pentane and further purified by chromatography. A gradient of eluent starting from 100% of pentane to 100% of CH 2 Cl 2 to elute the desired purple compound. The eluate containing the latter was stripped of solvent under reduced pressure to afford 1h in 40% yield (17 mg). Anal. calcd. for C 16 H 13 CoIN•1/2 H 2 O: C 46.40, H 3.41, N 3.38. Found: C 46.26, H 3.24, N 3.29. [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (500 MHz, CDCl 3 ): δ 9.69 -9.63 (dd, 1H), 8.48 (dd, J = 7.7, 1.1 Hz, 1H), 7. 70 -7.60 (m, 3H), 7.31 (td, J = 7.4, 1.5 Hz, 1H), 7.14 -7.02 (2dd, 2H), 5.15 (s, 5H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 169. 83, 167.67, 157.17, 146.86, 144.25, 137.09, 129.44, 124.25, 123.28, 121.62, 119.03, 86.99. HRMS (ESI, m/z) calcd. for C 16 H 13 CoN + : 278.0380. Found: 278.0374.

General Procedure for the oxidative cyclocondensation of 1a,d,e. A mixture of iodocobaltacycles (1 equiv) and [Ph 3 C][BArF 24 ] (1 equiv) was stirred overnight in ∼10-15 mL of CH 2 Cl 2 at room temperature in a sealed Schlenk vessel. The reaction mixture was filtrated through Celite and the resulting filtrate was stripped of solvent under reduced pressure to afford a solid residue that was purified by column chromatography. Recrystallization of the eluted compound from a CH 2 Cl 2 /n-pentane mixture and further trituration of the residue with n-pentane afforded the product as a white powder.

[2a][BArF 24 ]. 1a (48 mg, 0.1 mmol) and [Ph 3 C][BArF 24 ] (111 mg, 0.1 mmol) in 10 mL of CH 2 Cl 2 , isolated yield: 29% (33 mg). Anal. calcd for C 53 H 36 BF 24 N: C 55. 18, H 3.15, N 1.21. Found: C 55.21, H 3.34, N 1.21. [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (500 MHz, CDCl 3 ): δ 8. 30 (dd, J = 6.5, 1.5 Hz, 1H), 8.06 (dd, J = 8.4, 1.6 Hz, 1H), 7.97 (ddd, J = 8.5, 7.3, 1.4 Hz, 1H), 7.87 (dd, J = 8.0, 1.3 Hz, 1H), 7.73 (td, J = 7.7, 1.3 Hz, 1H), 7.70 (dt, J = 5.2, 2.3 Hz, 8H), 7.59 -7.51 (m, 2H), 7.50 (s, 4H), 7.45 (ddd, J = 7.8, 6.6, 1.6 Hz, 1H), 2.42 -2.36 (m, 1H), 2.09 (dq, J = 2.7, 1.3 Hz, 3H), 1.70 (p, J = 1.3 Hz, 3H), 1.40 (s, 3H), 1.29 (s, 3H), 0.96 (d, J = 7.2 Hz, 3H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 162. 43, 162.04, 161.64, 161.25, 150.29, 148.75, 144.

Notes

≠ The first discarded mechanisms consisted in the spontaneous evolution of singlet state 1a wherein the Cp* first binds the phpy ligand before catching a hydrogen atom in a final step. This mechanism could indeed proceed in three different modes (cf. Scheme 2 and Figure S123 of the Supporting Material): a) the Cp* and the phpy condense concertedly, ii) the Cp* ligand is sequentially first attacked by the N center and then the carbanionic C atom of the phpy ligand, or iii) the reverse, i.e the Cp* is first attacked by the carbanionic carbon and secondly by the N center of the phpy ligand. For each of these three scenarios applied to 1a, the first activation barrier exceeds + 35 kcal/mol (ΔG= + 63 kcal/mol for the concerted binding, ΔG= + 62 kcal/mol for the binding on N and ΔG= + 37 kcal/mol for the binding on C). The abstraction of iodide from 1a by Ph 3 C + was also considered as the first step of the mechanism, but was eventually discarded due to its unfavorable thermochemistry (ΔG= + 22 kcal/mol).

The one electron oxidation of Cp*Co(III) metallacycles of 2-phenylpyridine derivatives by iodine, a silver (I) salt or a tritylium cation leads to the irreversible cyclocondensation of the organic ligands to afford cationic alkaloids. Under optimal conditions in either CH 2 Cl 2 , ClCH 2 CH 2 Cl or PhCl, the reaction produces the cationic aromatic alkaloids in up to 88% yield; it proceeds via a reactive Co(IV) intermediate and the capture of a H atom.

  for 1 proton. Additional electrospray-mass spectroscopy (abbr. ES-MS) analysis of the raw reaction mixture confirmed the presence of the latter three compounds alongside a strong signal of a [Cp*CoI] + moiety at m/z 321, stemming from the dimer {[Cp*Co] 2 (-I) 3 } + showing up with lower intensity at m/z 769. Worthy to note, compound {[Cp*Co] 2 (-I) 3 }[I 3 ] was successfully isolated and subsequently recrystallized to afford stable dark green crystals suitable for X-ray diffraction analysis. The structure of {[Cp*Co] 2 (-I) 3 }[I 3 ] shows three symmetrically bridging iodo ligands between the two Co(III) centres that are distant from each other by 3.177(5) Å (Figure 1). This first test reaction clearly revealed that at least two pathways are involved in the observed oxidative decomposition of 1a: 1) the reductive elimination of 2-(2'-iodophenyl)pyridine and 2) Cp*-phpy cyclocondensation, both triggered by the formal oxidation of the Co(III) centre to either the open shell Co(IV) or the diamagnetic Co(V) state. The major question remaining to be answered being that of the origin of the "H 5 " atom in [2a] + (2).

Figure 1 .

 1 Figure 1. ORTEP-type drawings of the structures of 1a, 1d, 1e, {[Cp*Co] 2 (-I) 3 }[I 3 ], and 1h, at 50% probability with partial atom numbering. Molecules of solvent were omitted for clarity.1d,e were synthesized by the lithium acetamidateassisted cyclocobaltation method.[START_REF] Wu | Making Base-Assisted C-H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation[END_REF] Given the intricate mixture produced by the reaction of 1a with I 2 , as an alternative to the latter, we decided to probe the oxidizing property of the one-electron oxidant [Ph 3 C][BArF 24 ].[START_REF] Connelly | Chemical Redox Agents for Organometallic Chemistry[END_REF] The tritylium [Ph 3 C] + , which undergoes a

  Scheme 2. The Co-templated oxidative cyclocondensation of 1a, 1d and 1e.

  ) to that already published 6 for [2a][I 3 ]. Alongside this organic salt, amounts of Ph 3 C-I and {[Cp*Co] 2 (-I) 3 } + were also identified by ES-MS analysis. More interesting was the formation of small amounts of N-protio,2-phenylpyridinium indicating that even with [Ph 3 C][BArF 24 ] the hydrodechelation of the phpy ligand jointly with the release of {[Cp*Co] 2 (-I) 3 }[BArF 24 ] is a competing process.

  Scheme 3. Synthesis of 1g and its reaction with [Ph 3 C][BArF 24 ].

Figure 2 .

 2 Figure 2. ORTEP-type drawings of the structures of [2a] + , [2d] + , [2e] + , [3d] + , [3e] + , at 50% probability with partial atom numbering. The [BArF 24 ] anion and lattice solvent were omitted for clarity.

  Scheme 4. Synthesis of 1h and its conversion to [2h][BArF 24 ] upon treatment with [Ph 3 C][BArF 24 ].

Figure 3 .

 3 Figure 3. ORTEP-type drawing of the structure of [2h] + at 50% probability with partial atom numbering. The [BArF 24 ] - anion and solvent were omitted for clarity.

Figure 4 .

 4 Figure 4. Thermographic trace of the isotherm calorimetric titration of a solution of 1a (0.94 mM) by sequential addition of 2 L of a solution of [Ph3C][BArF24] (19.6 mM) in DCE at 25°C in an argon-filled glovebox. Individual heat flow peaks correspond to the response of the cell containing 1a after the addition of portions of a solution of [Ph3C][BArF24]. The inflexion point considered at the 29 th peak (at ca. 55000 s) corresponds to a n=1:1 stoichiometry.

Figure 5

 5 Figure 5 X-band EPR spectrum recorded at 4.3 K after 5 min of reaction between 40 mM solutions of 1a and [Ph 3 C][BArF 24 ] in DCM . The spectrum was recorded at a frequency of 9.345 GHz.

Figure 6 a 3 +

 63 Figure 6 a) Cyclic voltammograms of {[Cp*Co] 2 (-I) 3 + }[I 3 -], TEAI, 1a and [2a][BArF 24 ] (0.4 mmol L -1 ) on Pt electrode in a DCE solution in the presence of [N(nBu) 4 ][PF 6 ] (0.1 mol L -1 ). v = 100 mV s -1 . b) Cyclic voltammograms of 1a running first anodic or cathodic sweep. **: Ill-defined peak which may be due to the reduction of 1a.

  It may corresponds to the oxidation of the intermediate of the reaction giving [2a] + (cf. Supporting Material) or may be related to the oxidation of the released {[Cp*Co(III)] 2 (-I) 3 } + (cf. Supporting Material). The irreversibility of the oxidation of 1a is fully in agreement with the reactivity of 1a with [Ph 3 C][BArF 24 ] leading to the formation of [2a] + , "CoI" and {[Cp*Co] 2 (-I) 3 }. During the reverse sweep after the oxidation 1a, three new irreversible peaks are detected at -0.25 V (peak a), -0.57 V (peak b) and -1.37 V (peak c). The three signals should correspond to the detection of the compounds formed during the irreversible oxidation. The first irreversible cathodic peak at -0.25 V has a redox potential similar to that of TEAI and may correspond to the reduction of I 3 -to I -indicating probably the release of I -during the oxidation of 1a. The second signal at -0.57 V (peak b) can be attributed to the reduction of the {[Cp*Co] 2 (-I) 3 } species which gives similar redox potential reduction wave at -0.51 V. The potential is a little different, which can be explained by the nature of the counter-anion (PF 6 -vs. I 3 -) for {[Cp*Co] 2 (-I 3 }[I 3 ].

  Potentials (in V) vs. Ag/AgCl were obtained from cyclic voltammetry in DCE with 0.1 mol L -1 [N(nBu)4][PF6]. Scan rate = 100 mV s -1 . Working electrode: Pt, d= 3 mm in diameter. The given half-wave potentials are equal to E1/2 = (Epa+ Epc)/2. Under bracket: △Ep = |Epa-Epc|. irr : Irreversible peak potential. a Slow process. Italic (blue color): couple I3 -/I -or I2/I3 -. b see (9).

Figure 7 .

 7 Figure 7. NCI plots 43 from ADFview2019 for optimized geometries of a) singlet ground state 1a, b) its singlet ground state Ir analogue Ir-1a, c) doublet ground state [1a'] + , d) doublet ground state [Ir-1a'] + e) doublet ground state [Cp*Co(SCF 3 )(phpy)] + and f) doublet ground state [1g'] + (SCM-ADF2019 44 ZORA 45 -PBE 46 -D4(EEQ) 47 / all electron TZP / COSMO 48 (CH 2 Cl 2 ) level). Color code for atoms: dark blue N, gray C, white H, yellow S, light green F, orange Ir; purple I; indigo Co. NCIs are materialized by reduced density gradient isosurfaces (cut-off value s= 0.5 a.u.,=0.02 a.u.) coloured according to the sign of the signed density i.e. in red for attractive NCI domains and in blue for repulsive (or nonbonding) NCI domains. Specific NCI attraction domains between the Cp* and the phpy ligand are circled in green (cf. Supporting Material for larger drawings).

  The doublet state Co(IV) complex [1a'] + not only exhibits a larger attractive domain between C 4 of the Cp* and C 2' of the phpy ligand, but an attractive domain between C 3 of the Cp* and N of phpy also appears. None of these attractions appear for the Ir(IV) analogue. It is also worthy to note the slight rapprochement between Cp* and phpy induced by oxidation in both casesCo(SCF 3 )(phpy)] + 13 and [1g'] + , i.e the oxidized form of 1g, (Figure7e,f respectively) all show a significant Cp*-phpy C-C attractive domain while the N-C 3 attractive domain develops weakly only for [1g'] + and is absent in Co(IV) [Cp*Co(SCF 3 )(phpy)] + . Worthy to note, in the case of [Cp*Co(SCF 3 )(phpy)] + the phpy-related C 2' -S interaction is strongly attractive whereas in [1a] + the C 2' -I interaction bears a repulsive/non-bonding character inwards C 2' .

  Scheme 6. The so-called Gomberg-Jacobson-Lankamp-Nauta-MacLean asymmetric quinoid-like structure as a source of H.

Figure 8 .

 8 Figure 8. Proposed computed Gibbs free energy profiles of the formation of [2a] + . The three possible pathways are materialized by solid coloured lines (green, blue, pink). a) Exergonic intrinsic Gibbs energy profile considering a fictitious H. b) Endergonic intrinsic Gibbs energy profile considering Ph 3 CH as the source of H. c) Endergonic intrinsic Gibbs energy profile considering Ph 3 C-C 6 H 5 C(Ph 2 ) as the source of H. The so-called Gomberg-Jacobson-Lankamp-Nauta-MacLean asymmetric quinoid-like structure Ph 3 C-C 6 H 5 =C(Ph 2 ) arising from the reaction of two units of Ph 3 C (G = + 9 kcal/mol)was also considered as a potential source of H as shown in Scheme 6. This hypothesis proved to be unfavourable as it would result in a surge in the overall reaction Gibbs enthalpy of G= + 46 kcal/mol compared to the reference reaction Gibbs enthalpy with H (Figure8c). The same observation could be made when considering the Cp* ligand from external 1a or [1a'] + to be the source of H• (ΔΔG= +82 kcal/mol and +68 kcal/mol, respectively). The doublet-state Co(IV) species [1a'] + was therefore considered as pivotal in the mechanism of formation of [2a] + . Three pathways appeared to be realistic with respect to the crucial H atom capture step (Scheme 7): 1) the capture of a formal H• by [1a'] + occurs before the condensation reaction (pathway 1), 2) the capture of a formal H• after the migration of phpy's C to the Cp* ligand and before the migration of the N centre (pathway 2) and 3) the condensation of the Cp* with the phpy ligand occurs prior to the capture of a formal H• (pathway 3). Pathway 1, the early capture of H• and spin state. The formation of a hydrido-cobalt(V) species by capture of a H• was considered to account for the exclusive endo positioning of H 5 . The oxidation of singlet state 1a into doublet state [1a'] + followed by the capture of H• to yield a putative cati-

Figure 9 .

 9 Figure 9. Singlet and triplet state C-H…Co agostic transients' geometries computed at the (COSMO)-ZORA-PBE-D4(EEQ)/all electron TZP level with selected interatomic distances. Color code for atoms: gray C, blue N, indigo Co, white H, purple I.

  as barrier-less, as the shift of the Co from a N position to a  2 position on the aryl fragment combined with the search for planarity of the phpy moiety leading to a proximity between C 3 of the former Cp* and the N, favours the formation of the C-N bond. Intermediate[pre-2a] + is more likely to stay in the triplet state, as its singlet equivalent lies 7 kcal/mol above. Pathway 2 and 3, the early C-C coupling. Also considered was the attack on the Cp* by the phpy ligand in [1a'] + before the capture of the hydrogen atom occurs later in the process. Again three options were considered: the prior attack on C 4 at Cp* by C 2' at the phpy ligand shows the lowest activation barrier (ΔG= + 19 kcal/mol, while ΔG= + 48 kcal/mol for the concerted attack and ΔG= + 42 for the prior attack by N). In these conditions, the formation of the C-C bond in intermediate [IntC'] + from [1a'] + is endergonic with a ΔG of + 12 kcal/mol. Pathway 2, the capture of H. The capture of H by intermediate [IntC'] + was considered by attempting the optimization of a hydrido-Co(III) species, which eventually ended up in the migration of H to C 5 , thus yielding the triplet-state intermediate [IntCH] + which undergoes the N-C coupling as mentioned for pathway 1. Pathway 3, the N-C coupling followed by the H capture. From [IntC'] + , the formation of the C-N bond is also endergonic (ΔG= + 20 kcal/mol) and barrierless. Tentative optimization of a hydrido-Co(III) species accounting for the capture of a H eventually ended up in the migration of H to C 5 , yielding [pre-2a] + . The fate of "CoI". The question of the fate of the "CoI" moiety when[2a] + is released has been addressed heuristically. Calculation reveals that the release of "naked" CoI as such from triplet-state [pre-2a] + is highly unfavourable whatever the spin multiplicity of CoI may eventually be (ΔG= + 81 kcal/mol for the singlet, ΔG= + 48.8 kcal/mol for the triplet, and ΔG= + 69 kcal/mol for the quintet). It was therefore inferred that the "CoI" moiety might undergo the oxidativeaddition of a solvent molecule. Insertion of the Co(I) of CoI in the C-Cl bond of a solvent molecule after the release of [2a] + was found highly stabilizing (ΔG= -48 kcal/mol). The addition of a solvent molecule before the release of[2a] + was also considered. The insertion of the Co in the C-Cl bond is endergonic (ΔG= + 4 kcal/mol) in the triplet state (lowest spin state). From there, the release of[2a] + , generating a [Co(Cl)(I)(CH 2 Cl)] complex, becomes exergonic (ΔG= -8 kcal/mol). In summary, all three proposed pathways start with the oxidation of 1a into doublet [1a'] + and subsequently diverge: -pathway 1: H is first captured by the Co(IV) radical yielding the triplet-state [1a-H] + , then C 2' and N of the phpy sequentially attack the Cp*, -pathway 2: C 2' of the phpy first attacks C 4 at the Cp*, then H is captured by the Co(IV) radical yielding the triplet state intermediate [IntCH] + before N-C bond formation, -pathway 3: the C-C and N-C couplings occur stepwise in the Co(IV) species before H is captured. Discrimination between these three pathways, for which Figure8amaterializes the intrinsic energy profile considering a fictitious H source, depends on the nature of the latter, which remains still to be identified. Figure8b-cshows how two different H sources, namely Ph 3 CH and Ph 3 C-C 6 H 5 C(Ph 2 ), may affect the overall energy reaction profile changing the energy precedence of pathways 1, 2 and 3 in the overall reaction network by increasing or lowering activation barriers at different stages.CONCLUSIONIn this study the sensitivity of Co(III) complexes to oxidation has been established by experimental methods and its relationship to the formation of[2a,d,e] + demonstrated. The intramolecular cyclocondensation of the {N,C} -heterochelating ligand with the Cp* moiety is a rather fast process induced by the formation of a Co(IV) transient. DFT calculations suggest a pathway where C-C and C-N bond forming steps may occur before the capture of a H atom, the origin of which

  F NMR (282 MHz, CDCl 3 ): δ -62.1 ppm. HRMS (ESI, m/z) calcd. for C 22 H 22 CoF 3 N + : 416.1036, found:416.1035.

  57, The Centre National de la Recherche Scientifique The University of Strasbourg GENCI-IDRIS Grant 2020-A0080811408 HPC Center of the University of Strasbourg Grant g2020a78c.

  29

	R'				1b, 1c	untraceable products
	N	Co	Cp* I	[Ph 3 C][BArF 24 ] DCM or DCE room temp.	1a, 1d, 1e	N	R= H, [2a] R= Br, [2d] R= CF 3 , [2e]
	R					H
	R= H, R'=H, 1a R= NMe 2, R'= tBu, 1b		R	, [BArF 24 ]
	R= Me, R'= H, 1c
	R= Br, R'= H, 1d
	R= CF 3 , R'= H, 1e
						 H 2 O/air
						N		R= H, [3a]
								R= Br, [3d]
							OH	R= CF 3 , [3e]
								, [BArF 24 ]
						R

Table 1 .

 1 Reaction of Co(III) cobaltacycles with [Ph 3 C][BArF 24 ] in various halogenated solvents and conditions (atmosphere: dry argon or ambient air).

	Entry Complex R,	Solvent	Product(s) (yield)
			R'	(atm.)
	1	1a	H,H DCM (Ar)	[2a][BArF 24 ] (29% a , 72% b )
	2	1a	H,H DCM (Ar) + TEMPO c	[2a][BArF 24 ] (35% b )
	3	1a	H,H DCM (Ar) + TEMPO d	[2a][BArF 24 ] (17% b )
	4	1a	H,H DCM CDCl 3 (am-and	[2a][BArF 24 ] (44% b ),
				bient air)	[3a][BArF 24 ] (44% b )
	5	1a	H,H DCE (Ar)	[2a][BArF 24 ] (40% b )
	6	1a	H,H d 5 .PhCl (Ar) [2a][BArF 24 ] (51% b )
	7	1d	Br, H	DCM (Ar)	[2d][BArF 24 ] (57% b )
	8	1d	Br, H	DCM (ambi-ent air)	[2d][BArF 24 ] (37% a , 84% b )
	9	1d	Br, H	DCM (Ar) and CDCl 3	[3d][BArF 24 ] (21% a )
				(air)
	10	1e	CF 3 , H	DCM (Ar)	[2e][BArF 24 ] (63% b )
	11	1e	CF 3 , H	DCM (ambi-ent air)	[2e][BArF 24 ] (35% a , 88% b )
	12	1e	CF 3 , H	DCM (Ar) and CDCl 3	[3e][BArF 24 ] (17% a )
				(air)
	13	1g	H, H DCM (Ar)	[2a][BArF 24 ]
					(21%

no deuterium incorporation no deuterium incorporation Scheme 5. The use of CD 2 Cl 2 and/or [d 15 .Ph 3 C][BArF 24 ] does not result in the incorporation of 2 H at the C 5 posi- tion in [2a][BArF 24 ]. Treatment with triflic acid results in hydrodemetallation. Treatment with D 2 O promotes the formation of [2a][BArF 24 ] under an atmosphere of air without incorporation of 2 H.

					~70%
			argon atmosphere
	[2a] +	BArF 24 ]	air or argon	argon atmosphere
	80% under air 10% under argon		atmosphere	argon atmosphere	64%
	no deuterium incorporation
			quantitative
	A [d 15 .Ph 3 C][BArF 24 ] 33 afforded second experiment,	ran	in	CD 2 Cl 2 ,	using

Table 2 . Electrochemical data for 1a, 1d, 1e, 1h, 1g, [[Cp*Co] 2 (-I) 3 + ][I 3 -], 2a, and TEAI.

 2

	Compounds	E 1/2
	1a	1.11 irr 0.52 irr
	1d	1.19 irr 0.59 irr	-1.60
	1e	1.23 irr 0.61 irr
	1h b	1.11 irr 0.49 irr
	1g		0.80 irr	-1.21 irr	-
					1.76 irr
	{[Cp*Co] 2 (-I) 3 }[I 3 ]	1.25 irr 0.36 (168)	-0.02 a (293)	-0.51 (120)	-1.76 irr
	[2a][ BArF 24 ]			-1.39 irr
	TEAI	0.40	-0.08 a
		(116)	(400)
	Fc		0.16
			(72)

Table 3 . Interatomic "Intrinsic Bond Strength Indexes" (IBSI) 18 and the integrated difference between the user-defined fragment-based density gradients with and without interference IGM 17 g inter computed for 1a and [1a'] + , Ir-1a [Ir-1a] + , 1g, [1g'] + , [Cp*Co(SCF 3 )(phpy)] and [Cp*Co(SCF 3 )(phpy)] + .

 3 Noncovalent interactions (NCIs) are evidenced 18 by 0 < IBSI < 0.15, transition-metal coordination bond by 0.15 < IBSI < 0.60, and covalent bonding by 0.15 < IBSI < 4.0 values.

						Me
					Me	Me
					Me	C 3 C 4	Me
						M	X
					N	C 2 '
		1a	Ir-1a	[1a'] +	[Ir-1a'] +	1g		[1g'] +	[Cp*Co(SCF3)(phpy)]	[Cp*Co(SCF3)(phpy)] +
	Atom-pair						IBSI (au)
	interactions
	C4-C2'	0.047	0.038	0.052	0.020	0.044		0.053	0.038	0.048
	C3-N	0.026	0.012	0.048	0.009	0.026		0.027	0.021	0.029
	C4-Co/Ir	0.253	0.274	0.232	0.248	0.244		0.203	0.242	0.192
	C3-Co/Ir	0.259	0.301	0.213	0.247	0.250		0.223	0.235	0.204
	C2'-Co/Ir	0.479	0.488	0.469	0.498	0.497		0.460	0.485	0.459
	N-Co/Ir	0.385	0.402	0.376	0.410	0.385		0.378	0.392	0.391
	C2'-X	0.026 a	0.024 a	0.024 a	0.020 a	0.030 b	0.027 b	0.046 c	0.032 c
	interacting					g inter (au)
	fragments
	Cp*-phpy	0.865	0.630	0.935	0.633	0.857		0.920	0.859	0.917
	phpy-X	0.292 a	0.343 a	0.220 a	0.216 a	0.288 b	0.270 b	0.458 d	0.513 d

a X= I. b X= Cl. c X= S. d X= SCF3

trifluoromethyl)phenyl]borate ([Ph 3 C][BArF 24 ]

  version 4.1.1.

	Preparation	of	the	Tritylium	tetrakis[3,5-
	bis(

  . HRMS (ESI, m/z) calcd. for C 22 H 25 CoN + : 362.1319, found: 362.1318. Cl 2 afforded a yield of 71% (393 mg). Anal. calcd for C 21 H 22 BrCoIN: C,45.52, H 4.00, N 2.53. Found: C 45.33, H 4.05, N 2.62.

	Iodo[2-(4-bromophenyl)pyridine-κ 2 -
	C,N](pentamethylcyclopentadienyl)cobalt(III)	(1d).
	[Cp*CoI 2 ] 2 (448 mg, 0.5 mmol), LiNHAc (195 mg, 3 mmol),
	and 2-(4-bromophenyl)pyridine (235 mg, 1 mmol) in 20 mL of
	CH 2

Iodo[2-(4-(trifluoromethyl)phenyl)pyridine-κ 2 - C,N](pentamethylcyclopentadienyl)cobalt(III) (1e).

  . HRMS (ESI, m/z) calcd. for C 21 H 22 BrCoN + : 426.0268. Found: 426.0262. [Cp*CoI 2 ] 2 (448 mg, 0.5 mmol), LiNHAc (195 mg, 3 mmol), and 2-(4-(trifluoromethyl)phenyl)pyridine (224 mg, 1 mmol) in 10 mL of CH 2 Cl 2 afforded 1e in 62% yield (335 mg). Anal. calcd for C 22 H 22 CoF 3

ACKNOWLEDGMENT

The CNRS and the University of Strasbourg are acknowledged for their financial support. F.W. thanks the China Scholarship Council for financial support. Y.C. would like to acknowledge the GENCI-IDRIS for providing access to national HPC resources and to the High Performance Computing Center of the University of Strasbourg for providing access to computing resources. We gratefully thank Prof. Dr Eric Hénon (University of Reims) for edifying discussions on IGM and IBSI. .

ABBREVIATIONS

DCM, dichloromethane; DCE, 1,2-dichloroethane; ITC, isothermal titration calorimetry; DFT, density functional theory; QTAIM, quantum theory of atoms in molecule; Fc, ferrocene.

Funding Sources

CDCl 3 ):  -62.4 ppm. HRMS (ESI, m/z) calcd. for C 22 H 23 F 3 N + : 358.1777. Found: 358.18.

[3d][BArF 24 ]. 3a (16 mg, 0.03 mmol) and [Ph 3 C][BArF 24 ] (30 mg, 0.03 mmol) in 10 mL CH 2 Cl 2 , isolated yield: 21% (8 mg). Anal. calcd for C 53 H 35 BBrF 24 NO: C 50.99, H 2.83, N 1.12. Found: C 50.69, H 2.76, N 1.18. [START_REF] Jutzi | The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry Comments[END_REF] H NMR (500 MHz, CDCl 3 ): δ 8. 46 (dd, J = 6.6, 1.4 Hz, 1H), 8.28 (ddd, J = 8.7, 7.4, 1.4 Hz, 1H), 8.18 (dd, J = 8.3, 1.6 Hz, 1H), 7.91 (d, J = 8.5 Hz, 1H), 7.85 -7.77 (m, 2H), 7.73 (d, J = 6.4 Hz, 1H), 7.73 -7.70 (m, 8H), 7.56 (s, 4H), 2.21 (q, J = 1.3 Hz, 3H), 1.80 (q, J = 1.3 Hz, 3H), 1.51 (s, 3H), 1.40 (s, 3H), 1.32 (s, 3H), 1.06 (s, 1H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 162. 90, 162.50, 162.11, 161.71, 149.63, 148.79, 145.21, 141.00, 138.82, 135.36, 134.91, 133.33, 132.47, 130.34, 129.54, 129.30, 128.55, 128.39, 126.57, 126.23, 125.74, 124.24, 124.06, 118.03, 86.18, 82.57, 55.64, 22.27, 21.38, 21.01, 14.22, 11.54. HRMS (ESI, m/z) 53 (dd, J = 6.5, 1.4 Hz, 1H), 8.33 (ddd, J = 8.7, 7.4, 1.4 Hz, 1H), 8.26 (dd, J = 8.4, 1.6 Hz, 1H), 8.18 (d, J = 8.2 Hz, 1H), 7.91 (d, J = 8.2 Hz, 2H), 7.79 (ddd, J = 7.9, 6.5, 1.6 Hz, 1H), 7.72 (p, J = 2.3 Hz, 8H), 7.55 (s, 4H), 2.21 (q, J = 1.4 Hz, 3H), 1.81 -1.75 (m, 3H), 1.51 (s, 3H), 1.44 (s, 3H), 1.32 (s, 3H), 1.11 (s, 1H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CD 2 Cl 2 ): δ 162. 69, 162.30, 161.90, 161.50, 149.61, 147.87, 145.42, 141.17, 137.81, 135.14, 134.69, 129.74, 129.59, 129.36, 129.34, 129.32, 129.29, 129.11, 129.09, 129.07, 129.04, 128.18, 127.71, 127.20, 126.58, 126.55, 126.01, 125.82, 125.79, 124.68, 123.85, 121.68, 117.88, 117.85, 117.81, 85.96, 82.60, 55.52, 21.99, 21.23, 20.79, 13.99, 11.34. 19 47 -7.38 (m, 3H), 7.26 -7.17 (m, 1H), 6.83 (dd, J = 5.4, 2.0 Hz, 1H), 5.75 (dd, J = 5.4, 2.7 Hz, 1H), 5.30 (s, 1H), 5.08 (d, J = 7.5 Hz, 1H), 3.06 (dd, J = 15.9, 8.0 Hz, 1H), 2.80 (dd, J = 15.9, 6.7 Hz, 1H). [START_REF] López-Resano | Redefining the Mechanistic Scenario of Carbon-Sulfur Nucleophilic Coupling via High-Valent Cp*CoIV Species Angew[END_REF] C NMR (126 MHz, CDCl 3 ): δ 162. 41, 162.02, 161.62, 161.23, 149.12, 148.74, 146.05, 145.93, 140.15, 139.67, 137.33, 135.62, 134.87, 131.30, 130.89, 130.38, 130.01, 129.70, 129.26, 128.99, 128.17, 127.83, 126.34, 125.66, 125.26, 124.45, 123.80, 123.49, 123.29, 122.53, 120.25, 117.68, 91.44, 85.17, 83.21, 82.47, 53.59, 49.67, 48.10

AUTHOR INFORMATION

Corresponding Author * J.-P. Djukic, Email: djukic@unistra.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interests.