Stabilization of Fully Reduced Iron−Sulfur Clusters by Carbene Ligation: The [FenSn]0 Oxidation Levels (n = 4, 8)

Abstract

The all-ferrous [Fe₄S₄]⁰ state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe₄S₄(CN)₄]⁴⁻ (Scott, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A.2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Prⁱ₂NHCMe₂) has been found to stabilize the fully reduced clusters [Fe₈S₈(Prⁱ₂NHCMe₂)₆] (4) and [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (5), which are prepared by cluster assembly or phosphine substitution of Fe_nS_n (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe₇S₆(PEt₃)₅Cl₂] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the latter two are the first iron−sulfur clusters with Fe−C σ bonds. Cluster 4 possesses the [Fe₈(μ₃-S)₆(μ₄-S)₂] edge-bridged double cubane structure and 5 the cubane-type [Fe₄(μ₃-S)₄] stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and ⁵⁷Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.