Selectivity of thiolate ligand and preference of substrate in Model Reactions of dissimilatory nitrate reductase

Abstract

Complexes analogous to the active site of dissimilatory nitrate reductase from Desulfovibrio desulfuricans are synthesized. The hexacoordinated complexes [PPh₄][Mo^IV(PPh₃)(SR)(mnt)₂] (R = -CH₂CH₃ (1), -CH₂Ph (2)) released PPh₃ in solution to generate the active model cofactor, {Mo^IV(SR)(mnt)₂}^1-, ready with a site for nitrate binding. Kinetics for nitrate reduction by the complexes 1 and 2 followed Michaelis-Menten saturation kinetics with a faster rate in the case of 1 (V_Max = 3.2 × 10^-2 s^-1, KM = 2.3 × 10^-4 M) than that reported earlier (V_Max = 4.2 × 10^-3 s^-1, KM = 4.3 × 10^-4 M) (Majumdar, A., Pal, K., and Sarkar, S. J. Am. Chem. Soc.2006, 128, 4196-4197). The oxidized molybdenum species may be reduced back by PPh3 to the starting complex, and a catalytic cycle involving [Bu4N][NO₃] and PPh₃ as the oxidizing and reducing substrates, respectively, is established with the complexes 1 and 2. Isostructural complexes, [Et₄N][Mo^IV(PPh₃)(X)(mnt)₂] (X = -Br (3), -I (4)) did not show any reductive activity toward nitrate. The selectivity of the thiolate ligand for the functional activity and the cessation of such activity in isostructural halo complexes demonstrate the necessity of thiolate coordination. Electrochemical data of all these complexes correlate the ability of the thiolated species for such oxotransfer activity. Compounds 1 and 2 are capable of reducing substrates like TMANO or DMSO, but after the initial 15-20% conversion, the product trimethylamine or dimethylsulfide formed interacts with the active parent complexes 1 and 2 thereby slowing down further oxo-transfer reaction similar to feedback type reactions. From the functional nitrate reduction, the molybdenum species finally reacts with the nitrite formed leading to nitrosylation similar to the NO evolution reaction by periplasmic nitrate reductase from Pseudomonas dentrificans. All these complexes (1- 4) are characterized structurally by X-ray, elemental analysis, electrochemistry, electronic, FT-IR, mass and ³¹P NMR spectroscopic measurements.