Mass Spectrometric Study of the Conversion of Rhenium Diolates to Metallaoxetanes and Carbenes. Coordination Number, Polar, and Steric Effects

A combination of mass spectrometric gas-phase ion−molecule reactions and density functional theory electronic structure calculations indicates that, at least in high-valent rhenium oxo complexes, a diolate complex can dissociate preferentially to form a carbene complex, which most likely proceeds via a metallaoxetane intermediate. The experimental study finds effects on the efficiency of carbene production due to coordination number, polar substituents, and steric hindrance. Both qualitative theory and DFT calculations confirm the central role played by coordination number and furthermore define the structural conditions under which the reaction sequence, diolate → metallaoxetane → carbene complex, should become favorable. The results are relevant, not only to oxidation chemistry but also for the initiation mechanism in some olefin metathesis catalysts.