A Well-Defined Isocyano Analogue of HCo(CO)₄. 1: Synthesis, Decomposition, and Catalytic 1,1-Hydrogenation of Isocyanides

Reported here is the synthesis and characterization of the tetrakis­(m-terphenyl isocyanide)cobalt hydride HCo­(CNAr^Mes2)₄ (1; Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃). Monohydride 1 serves as a well-defined isocyano analogue of the tetracarbonyl hydride HCo­(CO)₄. While tetrakis-phosphine analogues of HCo­(CO)₄ have been reported previously, these compounds have failed to exhibit a reactivity profile that can be compared and contrasted with HCo­(CO)₄ in a systematic fashion. Herein, HCo­(CNAr^Mes2)₄ (1) is shown to be a readily accessed and reactive complex that allows for this comparison. For example, HCo­(CNAr^Mes2)₄ (1) is found to decompose smoothly to the κ¹-C-iminoformyl complex Co­(η⁶-(Mes)-κ¹C-C­(H)­NAr^Mes2)­(CNAr^Mes2) (2). Kinetic analysis of this decomposition and that of the d₁-isotopomer DCo­(CNAr^Mes2)₄ (1-d₁) revealed a unimolecular process characterized by a large primary k_H/k_D isotope effect (3.2(6)) and no dependence on the presence of free CNAr^Mes2. These data point to rate-limiting hydride α-migration and formation of the κ¹-C-iminoformyl species [Co­(κ¹-C-C­(H)NAr^Mes2)­(CNAr^Mes2)₃] as a critical intermediate. Indeed, ligand substitution reactions of HCo­(CNAr^Mes2)₄ (1), as well as ¹³C-labeling experiments of the decomposition product 2, demonstrate that hydride α-migration is the dominant mechanistic feature of this system. Most notably, this behavior is in contrast with that of HCo­(CO)₄, for which it has been established that CO ligand dissociation is the initial mechanistic feature. Additional support for the critical role of hydride α-migration in HCo­(CNAr^Mes2)₄ (1) was obtained by the development of catalytic CNAr^Mes2 1,1-hydrogenation to form a stable and isolable methylenimine.