Catalytic Reduction of Hydrazine to Ammonia with MoFe3S4−Polycarboxylate Clusters. Possible Relevance Regarding the Function of the Molybdenum-Coordinated Homocitrate in Nitrogenase

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Abstract
The catalytic function of the previously synthesized and characterized [(L)MoFe3S4Cl3]2-,3- clusters (L = tetrachlorocatecholate, citrate, citramalate, methyliminodiacetate, nitrilotriacetate, thiodiglycolate) and of the [MoFe3S4Cl3(thiolactate)]24- and [(MoFe3S4Cl4)2(μ-oxalate)]4- clusters in the reduction of N2H4 to NH3 is reported. In the catalytic reduction, which is carried out at ambient temperature and pressure, cobaltocene and 2,6-lutidinium chloride are supplied externally as electron and proton sources, respectively. In experiments where the N2H4 to the [(L)MoFe3S4Cl3]n- catalyst ratio is 100:1, and over a period of 30 min, the reduction proceeds to 92% completion for L = citrate, 66% completion for L = citramalate, and 34% completion for L = tetrachlorocatecholate. The [Fe4S4Cl4]2- cluster is totally inactive and gives only background ammonia measurements. Inhibition studies with PEt3 and CO as inhibitors show a dramatic decrease in the catalytic efficiency. These results are consistent with results obtained previously in our laboratory and strongly suggest that N2H4 activation and reduction occur at the Mo site of the [(L)MoFe3S4Cl3]2-, 3- clusters. A possible pathway for the N2H4 reduction on a single metal site (Mo) and a possible role for the carboxylate ligand are proposed. The possibility that the Mo-bound polycarboxylate ligand acts as a proton delivery “shuttle” during hydrazine reduction is considered.