N-Heterocyclic Carbene Ligands in Nonsymmetric Diiron Models of Hydrogenase Active Sites

Reaction of [Fe2{μ-S(CH2)3S}(CO)6] (1) at room temperature with the N-heterocyclic carbenes IMe-(CH2)2-L (IMe = 1-methylimidazol-2-ylidene, L = NMe2, SMe) afforded the pentacarbonyl carbene
derivatives [Fe2{μ-S(CH2)3S}(CO)5{IMe-(CH2)2-NMe2}] (2a) and [Fe2{μ-S(CH2)3S}(CO)5{IMe-(CH2)2-SMe}] (2b). Reaction of 1 with IMe-CH2-IMe at room temperature provided the dimer [{Fe2(μ-S(CH2)3S)(CO)5}2{μ-(IMe-CH2-IMe)}] (3) together with the chelated bis-NHC complex [Fe2{μ-S(CH2)3S}(CO)4{IMe-CH2-IMe}] (4a) as the major product. The analogous reaction of 1 with IMe-(CH2)2-IMe yielded the chelated
bis-NHC complex [Fe2(μ-S(CH2)3S)(CO)4{IMe-(CH2)2-IMe}] (4b). Addition of HBF4 to compound 4a
afforded the stable bridging hydride complexes [Fe2(μ-H){μ-S(CH2)3S}(CO)4{IMe-CH2-IMe}](BF4) (5a,b)
with NHC ligands in a basal/basal and basal/apical mode of coordination in 5a,b, respectively. The
molecular structures of 2a, 3, 4a,b, and 5a were confirmed by X-ray diffraction studies. Low-temperature
NMR studies on the protonation of 4a showed spectroscopic evidence for the formation of a very unstable
terminal hydride and a bridging hydride species with a NHC ligand having a non classical mode of
coordination via a C-4(5) bond. Cyclic voltammetry revealed that 4a is a catalyst for proton reduction.