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

Abstract

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(L^Br)₂(THF)₂] (2), where L^Br = 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, [Mo^VIO₂(L^Br)(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.