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(CNArMes2)(4) (1; Ar-Mes2 = 2,6-(2,4,6-Me3C6H2)(2)C6H3). 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(CNArMes2)(4) (1) is shown to be a readily accessed and reactive complex that allows for this comparison. For example, HCo(CNArMes2)(4) (1) is found to decompose smoothly to the kappa(1)-C-iminoformyl complex Co(eta(6)-(Mes)-kappa C-2-C(H)NArMes2)(CNArMes2) (2). Kinetic analysis of this decomposition and that of the d(1)-isotopomer DCo(CNArMes2)(4) (1-d(1)) 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 CNArMes2. These data point to rate limiting hydride alpha-migration and formation of the kappa(1)-C-iminoformyl species [Co(kappa(1)-C-C(H)=-NArMes2)(cNAr(Mes2))(3)] as a critical intermediate. Indeed, ligand substitution reactions of HCo(CNArMes2)(4) (1), as well as C-13-labeling experiments of the decomposition product 2, demonstrate that hydride alpha-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 alpha-migration in HCo(CNArMes2)(4) (1) was obtained by the development of catalytic CNArMes2 1,1-hydrogenation to form a stable and isolable methylenimine.