A Well-Defined Isocyano Analogue of HCo(CO)4. 3: Hydride Migration to Olefins, H-Atom Transfer and Reactivity toward Protic Sources

The tetraisocyano cobalt hydride complex HCo(CNArMes2)(4) (Ar-Mes2 = 2,6-(2,4,6-Me3C6H2)(2)C6H3) has previously been shown to serve as a well-defined structural and spectroscopic analogue to the classical carbonyl hydride HCo(CO)(4). The latter has been established as a precatalyst for olefin hydroformylation (i.e., the oxo reaction) and, in certain instances, as an H-atom transfer reagent for radical hydrogenation reactions. In this report, a detailed account of the reactivity of HCo(CNArMes2)(4) toward olefin substrates is reported to provide additional points of comparison with HCo(CO)(4). Treatment of HCo(CNArMes2)(4) with a range of monoene substrates leads rapidly to eta(6)-arene iminoacyl complexes of the formulation Co(eta(6)-(Mes)-kappa C-1-C(R)=NArMes2)(CNArMes2) and is accompanied by the loss of two isocyanide ligands. In each case, the incoming olefin is both reduced and incorporated into the alpha-carbon position of the newly formed iminoacyl unit. Mechanistic considerations for this transformation points to the proclivity of HCo(CNArMes2)(4) to undergo hydride migration to the CNArMes2 ligand. However, experiments with diene substrates demonstrate that alpha-migration is reversible and competes with beta-hydride elimination from putative Co alkyl intermediates. In addition to its reactivity with olefins, the ability of HCo(CNArMes2)(4) to engage in both hydrogen-atom transfer (HAT) and protolytic dihydrogen evolution processes was also probed. Upon reaction with the nitroxide radical TEMPO, TEMPOH was generated along with the eta O-2,N-nitroxide complex (eta O-2,N-TEMPO)Co(CNArMes2)(2). The latter likely arises from efficient trapping of the d(9) metalloradical Co(CNArMes2)(4) by TEMPO after HAT from HCo(CNArMes2)(4). This reaction outcome places an upper limit on the bond dissociation energy (BDE) of the Co-H unit in HCo(CNArMes2)(4) of ca. 69 kcal/mol, which is in line with that measured for related cobalt hydrides, including HCo(CO)(4). Upon treatment of HCo(CNArMes2)(4) with triflic acid (HOTf; [OTf](-) = [F3CSO3](-)), substoichiometric quantities of H-2 are evolved. However, the major product of this reaction is the aminocarbene salt [Co(eta(6)-(Mes)-kappa C-1-C(H)N(CH3)Ar-Mes2)(CNArMes2)]OTf, signifying that HCo(CNArMes2)(4) possesses dual sites of Bronsted basicity and lacks utility as a potential catalyst for the hydrogen evolution reaction (HER).

Automated summary

The reactivity of HCo(CNArMes2)(4) towards olefin substrates involves formation of new complexes and loss of isocyanide ligands, indicating a propensity for hydride migration. Additionally, the complex can engage in hydrogen-atom transfer and protolytic dihydrogen evolution reactions.