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

Abstract

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 [MoFe₇S₉C] built of Fe₄S₃ and MoFe₃S₃C portions connected by three sulfur bridges and containing an interstitial carbon atom centered in an Fe₆ 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*)WFe₂S₃Q₃]^-) and complete ([(Tp*)WFe₃S₃Q₄]^2-) 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 <i>μ</i>₂-bridging ligand with other bridges such as N₃^-, MeO^-, and EtS^- Reactions of complete cubanes with Me₃SiN₃ and S₈ undergo a redox metathesis process and lead to core ligand displacement and formation of [(Tp*)WFe₃S₃(<i>μ</i>₃-Q)Cl₃]^- (Q = Me₃SiN^2-, S^2-). 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*)WFe₃S₃(<i>μ</i>₃-NSiMe₃)Cl₃]^-, 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 [MoFe₃S₃C] 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-)].