Ligand metathesis as rational strategy for the synthesis of cubane-type heteroleptic iron-sulfur clusters relevant to the FeMo cofactor.

Molybdenum-dependent nitrogenases catalyze the transformation of dinitrogen into ammonia under ambient conditions. The active site (FeMo cofactor) is the structurally and electronically complex weak-field metal cluster [MoFeSC] built of FeS and MoFeSC portions connected by three sulfur bridges and containing an interstitial carbon atom centered in an Fe trigonal prism. Chemical synthesis of this cluster is a major challenge in biomimetic inorganic chemistry. One synthetic approach of core ligand metathesis has been developed based on the design and synthesis of unprecedented incomplete ([(Tp*)WFeSQ]) and complete ([(Tp*)WFeSQ]) cubane-type clusters containing bridging halide (Q = halide). These clusters are achieved by template-assisted assembly in the presence of sodium benzophenone ketyl reductant; products are controlled by reaction stoichiometry. Incomplete cubane clusters are subject to a variety of metathesis reactions resulting in substitution of a -bridging ligand with other bridges such as N, MeO, and EtS Reactions of complete cubanes with MeSiN and S undergo a redox metathesis process and lead to core ligand displacement and formation of [(Tp*)WFeS(-Q)Cl] (Q = MeSiN, S). This work affords entry to a wide variety of heteroleptic clusters derivable from incomplete and complete cubanes; examples are provided. Among these is the cluster [(Tp*)WFeS(-NSiMe)Cl], one of the very few instances of a synthetic Fe-S cluster containing a light atom (C, N, O) in the core, which constitutes a close mimic of the [MoFeSC] fragment in FeMo cofactor. Superposition of them and comparison of metric information disclose a clear structural relationship [Tp* = tris(3,5-dimethyl-1-pyrazolyl)hydroborate(1-)].