Mechanistic Aspects of a Surface Organovanadium(III) Catalyst for Hydrocarbon Hydrogenation and Dehydrogenation

Understanding the mechanisms of action for base metal catalysis of transformations typically associated with precious metals is essential for the design of new technologies for a sustainable energy economy. Isolated transition metal and post-transition metal catalysts on oxides such as silica are generally proposed to effect hydrogenation and dehydrogenation by a mechanism featuring either σ-bond metathesis or heterolytic bond cleavage as the key bond activation step. In this work an organovanadium(III) complex on silica, which is a precatalyst for both olefin hydrogenation and alkane dehydrogenation, is interrogated by a series of reaction kinetics and isotopic labeling studies in order to shed light on the operant mechanism for hydrogenation. The kinetic dependencies of the reaction components are potentially consistent with both the σ-bond metathesis and the heterolytic bond activation mechanisms, however, a key deuterium incorporation experiment definitively excludes the simple σ-bond metathesis mechanism. Alternatively a two electron redox cycle, rarely invoked for homologous catalyst systems, is also consistent with experimental observations. Evidence supporting the formation of a persistent vanadium(III) hydride upon hydrogen treatment of the as prepared material is also presented.