The uptake of protons and hydrogen atoms into reducible metal oxides (RMOs) are two processes with implications in catalysis and energy saving applications; however, outside computational modeling, it is difficult to glean atomically precise information into the physicochemical implications of insertion of these species. In this work, we study proton-coupled electron transfer onto the surface of polyoxovanadate-alkoxide (POV-alkoxide) clusters. Being discreet molecular species that are soluble in organic solvents we are able to interrogate transformations on the cluster surface with atomic precision using conventional means such as <sup>1</sup>H NMR spectroscopy. We find that exposure of H-atom donors gives rise to the formation of an O-atom vacancy on the surface of the assembly. Kinetic analysis reveals that H-atom transfer from organic reagents follows a concerted proton-electron transfer mechanism, directly reducing the vanadyl unit to an oxygen vacancy by the sequential transfer of two H-atoms. The loss of surface oxygen atoms upon exposure to protons and hydrogen has been cited in extended metal oxides in the past, this analogous activity on molecular model complexes enables us to understand relevant material design principles with a high degree of atomic precision. We find that the oxidation state distribution of the component vanadium ions plays a significant role in H-atom abstraction as a consequence of the relevant thermochemical parameters (reduction potential, basicity) of each cluster charge state, and that a difference of one core electron can significantly affect hydrogen affinity of the cluster. In addition, by saturating bridging oxygen atoms with organic groups, we demonstrate the ability to obtain regioselectivity in the hydrogen uptake process to obtain the oxygen deficient assembly without competition from bridging oxide sites. This work provides atomistic cues for the development of heterogenous catalysts, as well as a methodology for studying how synthetic modifications (heterometal dopants, surface bridging ligands) alter hydrogen-atom affinity at metal oxides.

Oxygen-atom defect formation in Polyoxovanadate clusters via proton-coupled electron transfer

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