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(t)Bu)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((t)Bu)COmes (40%). In light of the experimental results and of extended Huckel 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.