[2Fe-2S] Clusters Supported by N-Heterocyclic Carbene Ligands

Abstract

The di-ferrous [2Fe-2S]0 state is the least understood oxidation level of [2Fe-2S] rhomb that is the smallest module of iron–sulfur clusters in enzymes. Reported synthetic models of the [2Fe-2S]0 state utilize bulky anionic ligands to achieve their stabilization, and their highly reducing nature renders detailed structural and spectroscopic studies difficult. Herein, we report the stabilization of the di-ferrous [2Fe-2S]0 state by using N-heterocyclic carbene (NHC) as supporting ligands. The charge-neutral cluster [Fe2(μ-S)2(ICy)4] (1, ICy = 1,3-bis-cyclohexyl-imidazol-2-ylidene) is synthesized from the reaction of the iron(0) precursor [(ICy)2Fe(η2-CH2=CHSiMe3)] with SPPh3. The attenuated reducing power of 1 as compared to those supported by anionic ligands allows its isolation in pure form. Further spectroscopic and theoretical studies established its S = 0 ground state resulting from anti-ferromagnetic coupling of two high-spin ferrous sites with an exchange-coupling constant J = -208 cm-1. The NHC ligand is also capable of stabilizing the mix-valent complex [Fe2(μ-S)2(ICy)4][BPh4] (2), which is synthesized from the reaction of 1 with [Cp2Fe][BPh4] and identified as a Robin-Day Class II complex with an S = ½ ground state.