Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a “Bulky Hydrogen Atom” Surrogate: Analogies to PCET

Recent computational
studies suggest that the phosphate support in the commercial vanadium
phosphate oxide (VPO) catalyst may play a critical role in initiating
butane C–H bond activation through a mechanism termed reduction-coupled
oxo activation (ROA) similar to proton-coupled electron transfer (PCET);
however, no experimental evidence exists to support this mechanism.
Herein, we present molecular model compounds, (Ph2N)3VN–P­(O)­Ar2 (Ar = C6F5 (2a), Ph (2b)), which are reactive
to both weak H atom donors and a Me3Si• (a “bulky hydrogen atom” surrogate) donor, 1,4-bis­(trimethylsilyl)­pyrazine.
While the former reaction led to product decomposition, the latter
resulted in the isolation of the reduced, silylated complexes (Ph2N)3V–NP­(OSiMe3)­Ar2 (3a/b). Detailed analyses of possible reaction
pathways, involving the isolation and full characterization of potential
stepwise square-scheme intermediates, as well as the determination
of minimum experimentally and computationally derived thermochemical
values, are described. We find that stepwise electron transfer (ET)
+ silylium transfer (ST) or concerted EST mechanisms are most likely.
This study provides the first experimental evidence supporting a ROA
mechanism and may inform future studies in homogeneous or heterogeneous
C–H activation chemistry, as well as open up a possible new
avenue for main group/transition metal cooperative redox reactivity.