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revised on 10.06.2019 and posted on 12.06.2019by David Kaphan, Magali Ferrandon, Ryan R. Langeslay, Gokhan Celik, Evan Wegener, Cong Liu, Jens Niklas, Oleg Poluektov, Massimiliano Delferro
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.