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

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journal contribution
posted on 2016-06-01, 20:48 authored by Alex E. Carpenter, Arnold L. Rheingold, Joshua S. Figueroa
Reported here is the synthesis and characterization of the tetrakis­(m-terphenyl isocyanide)cobalt hydride HCo­(CNArMes2)4 (1; ArMes2 = 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(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 κ1-C-iminoformyl complex Co­(η6-(Mes)-κ1C-C­(H)­NArMes2)­(CNArMes2) (2). Kinetic analysis of this decomposition and that of the d1-isotopomer DCo­(CNArMes2)4 (1-d1) revealed a unimolecular process characterized by a large primary kH/kD isotope effect (3.2(6)) and no dependence on the presence of free CNArMes2. These data point to rate-limiting hydride α-migration and formation of the κ1-C-iminoformyl species [Co­(κ1-C-C­(H)NArMes2)­(CNArMes2)3] as a critical intermediate. Indeed, ligand substitution reactions of HCo­(CNArMes2)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(CNArMes2)4 (1) was obtained by the development of catalytic CNArMes2 1,1-hydrogenation to form a stable and isolable methylenimine.

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