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Cobaltacycles May Collapse Upon Oxidation V On the Fate of Cobaltacycles in Cp*Co-mediated C-H bond Functionalization Catalysis: Cobaltacycles May Collapse upon Oxidation via Co(IV) species Fule Wu, Christophe Deraedt, Yann Cornaton, Laurent Ruhlmann, Lydia Karmazin, Corinne Bailly, Nathalie Kyritsakas, Nolwenn Le Breton, Sylvie Choua, Jean-Pierre Djukic To cite this version: Fule Wu, Christophe Deraedt, Yann Cornaton, Laurent Ruhlmann, Lydia Karmazin, et al.. On the Fate of Cobaltacycles in Cp*Co-mediated C-H bond Functionalization Catalysis: Cobaltacycles May Collapse upon Oxidation via Co(IV) species. 2021. hal-03292255 HAL Id: hal-03292255 https://hal.archives-ouvertes.fr/hal-03292255 Preprint submitted on 20 Jul 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. On the Fate of Cobaltacycles in Cp*Co-mediated C-H bond Func- tionalization Catalysis: Cobaltacycles May Collapse upon Oxidation via Co(IV) species. Fule Wu, a Christophe Deraedt, a Yann Cornaton,a Laurent Ruhlmann, b Lydia Karmazin, c Corinne Bailly,c Nathalie Kyritsakas,c Nolwenn Le Breton,d Sylvie Chouad and Jean-Pierre Djukica,* a Laboratoire de Chimie et Systémique Organométalliques, Institut de Chimie de Strasbourg (UMR 7177) CNRS/Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France b Laboratoire d’Electrochimie et Chimie Physique du Corps Solide, Institut de Chimie de Strasbourg (UMR 7177) CNRS/Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France c Service de Radiocristallographie Fédération de Chimie Le Bel – FR2010 BP 296R8, 1 rue Blaise Pascal, F-67008 Stras- bourg Cedex, France d Laboratoire Propriétés Optiques et Magnétiques des Architectures Moléculaires Institut de Chimie de Strasbourg (UMR 7177) CNRS/Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France ABSTRACT: Recent reports have identified Cp*Co-based complexes as powerful catalysts for aromatic C-H bond activation under oxidative conditions. However, little is known about the speciation of Cp*Co species during catalysis. We now show that key intermediates, Cp*Co(III) metallacycles derived from 2-phenylpyridine (phpy-H), react swiftly in solution with one-electron oxidants to irreversibly collapse by a cyclocondensation of the organic ligands to afford cationic alkaloids in yields of >70 %. Low temperature EPR analysis of a mixture of cobaltacycle with the tritylium cation reveals the signatures of trityl and Co(IV)-centred radicals. Electrochemical analyses show that the oxidation of these cobaltacycles is irreversible and gives rise to several products in various amounts, among which the most salient ones are a cationic alkaloid resulting from the cyclocondensation of the phpy and + Cp* ligands, and the dimeric cation {[Cp*Co]2(-I)3} . DFT investigations of relevant noncovalent interactions using QTAIM- based NCI plots and Intrinsic Bond Strength Index suggest a ligand-dependent predisposition by “NCI-coding” for the Co(IV)- templated cyclocondensation, the computed reaction network energy profile for which supports the key roles of a short lived Co(IV) metallacycle and of a range of triplet state organocobalt intermediates. INTRODUCTION iodo-Ir(III) complexes are left untouched. In the majority of 5 1 reports dealing with group 9 metallacycles bearing an hetero- The -pentamethylcyclopentadienyl ligand - or Cp* - in spite - of an abundant literature2 is often considered as a "robust", chelating carbanionic ligand {N,C} with Cp* bound metals, sterically and electronically stabilizing1,3 ligand in its use in the latter ligand never interferes explicitly with ligands bound catalysts.4 If in specific cases the endo migration of a hydrido to the metal by other means but steric repulsion and synergetic ligand to Cp* was reported in the case of a bis-pyridine che- “through bonds” electronic effects. late of Rh(III),5 literature is rather silent on occurrences of PhCO3H - or explicit reactions of the Cp* ligand with {N,C} heteroche- NiCp Ar-Cl 6 N N lating ligands in metallacycles. This issue has been over- N N (1) o-xylene, reflux looked in most reports dealing with the use of Cp*Co-based Ustinyuk et al. 1969 & 1970 catalysts in aromatic C-H bond functionalization employing redox co-catalysts where cobaltacycles are central intermedi- Cp* oxidation ? 4,7 N N 2 ates. One can find a rare report on the metal-templated "H" 1 (2) Co 3 collapse of a metallacycle of a CpNi (II) complex reported by room temp. 5 Wu et al. 2020 I 4 Ustinyuk et al.8 where an azo-benzene-based nickelacycle H , I3 undergoes interligand condensation and metal decoordination 1a rac-[2a]+ under oxidative conditions, affording a new organic extended In a recent report6 we disclosed without further reporting on its - aromatic product (1). It is probably because of the scarcity of origin the crystal structure of the I3 salt of an unprecedented similar reports that Cp* bound metallacycles of group 9 metals cationic Cp*-2-phenylpyridine condensation product, i.e [2a]+ at the +I and +III formal oxidation states are frequently con- (2), serendipitously discovered whilst attempting to grow 9 7c,10 sidered as structurally viable candidate catalysts. A report crystals of 1a. It was obvious that [2a]+ resulted from the 11 by Mayer et al. outlined that the two-electron oxidation of collapse of cobaltacycle 1a by the cyclocondensation of the Cp*Ir metallacycles by PhI(OAc)2 only affects the Cp* ligand Cp* and 2-phenylenepyridine (abbr. phpy) ligands with the in the case of a methyl-Ir(III) complex, whilst other chloro and capture of an exogenous H atom (2). We therefore undertook the investigation of a series of Co(III) metallacycles derived identified readily from their specific signature: 2-(2’- from 2-phenylpyridine (abbr phpy-H), because such a demetal- iodophenyl)pyridine, tris--iodo dimeric Cp*cobalt (III) cation + + lative cyclocondensation reaction, if relevant in its extent, {[Cp*Co]2(-I)3} and compound [2a] were formed in 40%, 4 could limit further developments of Cp*Co(III) catalysts at a 66% and 33% yield according to 1H NMR analysis (3). Com- larger scale where recycling of metal catalysts might be cru- pound {[Cp*Co]2(-I)3}[I3] produces one signal for the Cp* cial for environmental reasons.12 1 ligand at 1.80 ppm in CDCl3. For [2a][I3] the H NMR spec- " " trum depicts the expected magnetic inequivalence of the five N Cp* -1e N Cp* N methyl groups of the cyclopentenyl fragment that show up at Co Co S 0.96 (d), 1.31 (s), 1.56 (s), 1.75 (s) and 2.33(s) ppm. Proton SCF3 (IV) SCF3 CF3 H5 (2) appears at 2.49 ppm as a broad multiplet integrating for 1 proton. Additional electrospray-mass spectroscopy Perez-Temprano et al. 2021 (abbr. ES-MS) analysis of the raw reaction mixture confirmed Scheme 1. The oxidation of the singlet state Co(III)- the presence of the latter three compounds alongside a strong thiolato complex leading to C-S bond formation. 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 Interestingly, it has been recently proposed that transient m/z 769. Worthy to note, compound {[Cp*Co]2(-I)3}[I3] was 13 Co(IV) species may play a central role in oxidative C-H successfully isolated and subsequently recrystallized to afford 14 bond thiolation. Indeed, in an effort to clarify the mecha- stable dark green crystals suitable for X-ray diffraction analy- 14 nism of the catalysis reported by Glorius et al., Perez- sis. The structure of {[Cp*Co] (-I) }[I ] shows three sym- 13 2 3 3 Temprano et al. showed in a report that appeared while the metrically bridging iodo ligands between the two Co(III) cen- present manuscript was being finalized that the treatment of a tres that are distant from each other by 3.177(5) Å (Figure 1). trifluoromethylthiolato complex of a 2-phenylpyridine Cp*- This first test reaction clearly revealed that at least two path- bound cobaltacycle with excess Ag(I) salts may lead to the ways are involved in the observed oxidative decomposition of oxidation of the Co(III) centre into a Co(IV) complex purport- 1a: 1) the reductive elimination of 2-(2’-iodophenyl)pyridine edly responsible for the formation of the expected ortho- and 2) Cp*-phpy cyclocondensation, both triggered by the substituted product of C-S coupling (Scheme 1) in various formal oxidation of the Co(III) centre to either the open shell yields (from ca. 99% to 0%) depending on the amount and Co(IV) or the diamagnetic Co(V) state. The major question nature of the oxidant. In this recent report13 the fate of the remaining to be answered being that of the origin of the “H ” remaining amounts of cobaltacycle was not addressed by the 5 atom in [2a]+ (2). authors though. In the present report we disclose our conclusions on the ob- N I 15 2 eq I2 1a [2a][I ] and and *CpCo CoCp* (3) served ligand cyclocondensation in cobaltacycles of 2- 3 I I DCM, room temp. I , [I3] phenylpyridine derivatives and its direct cause, i.e the irre- overnight versible oxidation of Co(III) centre into the Co(IV) state, 33% (NMR) 40% (NMR) 66% (NMR) which sheds a new light on the possible limitations of Cp*Co(III) based aromatic C-H functionalization catalysts when placed under oxidative conditions.
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