The striking influence of oxophilicity differences in heterometallic Mo-Mn oxide cluster reactions with water

Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of Mn_xMoO_y ^- (x = 1, 2; y = 3, 4) clusters toward H₂O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H₂ production, specifically for Mn_xMoO₃ ^- (x = 1, 2) clusters. Unlike previously studied Mo_xO_y ^- clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoO_y ^- and Mn₂MoO_y ^- preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the Mn_xMoO_y (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster-water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.