Nitrate and Perchlorate Reduction by a Dinuclear Mo(V) Complex

Nitrate (NO3–) and perchlorate (ClO4–) are persistent groundwater contaminants due to their high stability and solubility. Microorganisms reduce these anions using molybdenum-containing enzymes such as nitrate reductases and perchlorate reductases. Reported here is a bioinspired dinuclear Mo(V) catalyst, [Mo2O3(LBr)2(THF)2] (2), where LBr = 5-Bromo-2-hydroxybenzaldehyde thiosemicarbazone, and its reactivity with nitrate and perchlorate. Compound 2 was previously speculated to be an inactive byproduct formed when its monomeric Mo(VI) analog, [MoVIO2(LBr)(MeOH)] (1), catalyzes an oxygen atom transfer (OAT) between dimethyl sulfoxide (DMSO) and PPh3. This work reports the synthesis, spectroscopic and crystallographic characterizations, and catalytic reactivity of 2. Contrary to earlier expectations, 2 catalyzes OAT from DMSO to PPh3, and also reduces nitrate and perchlorate, making it one of the few homogeneous molybdenum catalysts known to do so. With nitrate, 2 performs two OAT steps per NO3– to generate 1 and N2O, likely via nitrite and nitroxy intermediates. With perchlorate, 2 catalyzes four OAT events per ClO4–, yielding Cl– and 1. The effect of Sc3+ as a cocatalyst was also investigated; it significantly enhances the rates for perchlorate reduction but has minimal impact on nitrate reduction.