Investigating the C–H Bond Activation Mechanism at Phosphorus(V) Oxides using Vanadium Phosphorus Oxide (VPO) Model Complexes

Alkanes, or saturated hydrocarbons, have long been zealously exploited for their energy content through combustion processes; however, practical processes for converting them directly to higher value commodity chemicals remain limited. This stems from the inherent inertness of alkanes and the subsequent difficulty in activating their strong and localized C–C and C–H bonds. The introduction of functionality into unactivated C–H bonds incites many practical advantages – from offering new methodologies for fine chemical synthesis, to far-reaching implications of replacing our current alkane petrochemical feedstocks. For the past several decades, there has been tremendous work in utilizing transition-metal complexes to activate inert C–H ¬bonds and install functionality under mild conditions. While profitable practical applications remain a long-term goal, our mechanistic understanding of these transition-metal mediated transformations has advanced tremendously, and has provided a framework to recognize new strategies for useful C–H bond activation. For example, recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst – used for the partial oxidation of butane to maleic anhydride – 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, there has been a general lack of experimental evidence to support this mechanism. Herein, we present a wide library of molecular model compounds to examine the validity of the proposed ROA mechanism, which incites C–H bond activation through a main-group/transition metal cooperative mechanism. We report the synthesis, characterization, and reactivity of a series of mono/multi-metallic vanadium phosphate complexes, including the first experimental evidence supporting the proposed ROA mechanism using 1,4-(bistrimethylsilyl)-pyrazine as a “bulky hydrogen” surrogate. Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential step-wise intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values are described. Additionally, ongoing work has shown that careful electronic tuning of these vanadium phosphate complexes can enable enhanced reactivity towards weak C–H bonds