Reactivity of carbon monoxide towards high valent molybdenum complexes

Reaction of carbon monoxide (3 atm) with Mo(O)₂(mes)₂ (mes = MESITYL = C₆H₂Me₃-2,4,6) in pyridine at room temperature affords dimesityl ketone 1 (5%) and 1,1′-bis(mesitoyl)-1,1′,4,4′-tetrahydro-4,4′-bipyridine 2 (25%), the molecular structure of which has been determined by an X-ray diffraction study. The formation of 2 represents the first example of N-acylation of a pyridine by CO mediated by a transition metal complex. It has been shown that the relative ratio of 1 and 2 is dependent on the reaction conditions and that the presence of pyridine is essential for the formation of 1. Reactions of CO with the isoelectronic complex Mo(N^tBu)₂mes₂ contrast sharply with Mo(O)₂mes₂ and, under the same conditions, no dihydropyridine is formed, the sole identified products being 1 (60%) and mesityltertiobutyl amide HN(^tBu)COmes (40%). In light of the experimental results and of extended Hückel calculations on dioxo and diimido d⁰ molybdenum complexes, a tentative mechanism for the formation of 2 is proposed which involves oxycarbene-like intermediates in the case of the more electrophilic dioxo molybdenum species.