Associative Ligand Exchange and Substrate Activation Reactions by a Zero-Valent Cobalt Tetraisocyanide Complex

Abstract

Tetraisocyanide complex Co(CNArMes2)4 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) serves as a crystallographically characterized, thermally stable isocyano analogue of the reactive binary carbonyl Co(CO)4. The enhanced stability of Co(CNArMes2)4 allows for a systematic investigation of its reactivity profile in an operationally convenient manner, which is not possible for transient Co(CO)4. Zero-valent Co(CNArMes2)4 undergoes ligand exchange reactions with a variety of 2e– donor, L-type ligands, including triphenylphosphine, tert-butylethylene, and phenylacetylene. Kinetic studies of ligand substitution of Co(CNArMes2)4 with phenylacetylene using UV–vis spectroscopy indicates an associative reaction mechanism. This is consistent with the mechanisms proposed for other isolable 17e– transition-metal carbonyls but contrasts with studies proposing zero-valent CoL4 complexes to react via dissociative reaction pathways. Zero-valent Co(CNArMes2)4 also reacts with elemental phosphorus and sulfur and with diphenyl disulfide to form products indicative of multielectron transformations. Under the conditions surveyed, Co(CNArMes2)4 did not display 1e– atom- or group-abstraction chemistry. The combined results suggest that this zero-valent, S = 1/2 cobalt complex reacts with substrates via associative ligand substitution and inner-sphere multielectron reduction, which is a mode of action that is likely general for this class of Co complexes.