The catalytic function of the previously synthesized and characterized [(L)MoFe(3)S(4)Cl(3)](2)(-)(,3)(-) clusters (L = tetrachlorocatecholate, citrate, citramalate, methyliminodiacetate, nitrilotriacetate, thiodiglycolate) and of the [MoFe(3)S(4)Cl(3)(thiolactate)](2)(4)(-) and [(MoFe(3)S(4)Cl(4))(2)(&mgr;-oxalate)](4)(-) clusters in the reduction of N(2)H(4) to NH(3) 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 N(2)H(4) to the [(L)MoFe(3)S(4)Cl(3)](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 [Fe(4)S(4)Cl(4)](2)(-) cluster is totally inactive and gives only background ammonia measurements. Inhibition studies with PEt(3) 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 N(2)H(4) activation and reduction occur at the Mo site of the [(L)MoFe(3)S(4)Cl(3)](2)(-)(, 3)(-) clusters. A possible pathway for the N(2)H(4) 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.
The catalytic reduction of cis-dimethyldiazene by the
(Et4N)2[(Cl4-cat)(CH3CN)MoFe3S4Cl3]
cluster (Cl4-cat = tetrachlorocatecholate) is reported. Unlike the reduction
of cis-dimethyldiazene by the Fe/Mo/S center of
nitrogenase, which yields methylamine, ammonia, and methane (the latter
from the reduction of the C−N bond), the
reduction of cis-dimethyldiazene by the synthetic cluster
yields exclusively methylamine. In separate
experiments,
it was shown that the C−N bond of methylamine is not reduced by the
[MoFe3S4]3+ core, perhaps
accounting for
the differences observed between the biological and abiological
systems. 1,2-Dimethylhydrazine, a possible partially
reduced intermediate in the reduction of
cis-dimethyldiazene, was also shown to be reduced to
methylamine. Interaction
of methylamine with the Mo atom of the cubane was confirmed through the
synthesis and structural characterization
of
(Et4N)2[(Cl4-cat)(CH3NH2)MoFe3S4Cl3].
Phosphine inhibition studies strongly suggest that the Mo atom of
the
[MoFe3S4]3+ core, which has a
Mo coordination environment very similar to that in nitrogenase, is
responsible for
the binding and activation of cis-dimethyldiazene. The
reduction of a NN bond exclusively at the heterometal
site
of a nitrogenase-relevant synthetic compound may have implications
regarding the function of the nitrogenase Fe/Mo/S center, particularly in the latter stages of dinitrogen
reduction.
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