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 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.