[2Fe–2S] clusters supported by N-heterocyclic carbene ligands†

The di-ferrous [2Fe–2S]⁰ state is the least understood oxidation level of the [2Fe–2S] unit, which 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) moieties 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-CH₂CHSiMe₃)] with SPPh₃. The attenuated reducing power of 1 as compared to those clusters 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 mixed-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.