N-heterocyclic carbene ligands in nonsymmetric diiron models of hydrogenase active sites

Reaction of [Fe-2{mu-S(CH2)(3)S}(CO)(6)] (1) at room temperature with the N-heterocyclic carbenes I-Me-(CH2)(2)-L (I-Me = 1-methylimidazol-2-ylidene, L = NMe2, SMe) afforded the pentacarbonyl carbene derivatives [Fe-2{mu-S(CH2)(3)S}(CO)(5){I-Me-(CH2)(2)-NMe2}] (2a) and [Fe-2{mu-S(CH2)(3)S}(CO)(5){I-Me-(CH2)(2)-SMe}] (2b). Reaction of 1 with I-Me-CH2-I-Me at room temperature provided the dimer [{Fe-2(mu-S(CH2)(3)S)(CO)(5)}(2){mu-(I-Me-CH2-I-Me)}] (3) together with the chelated bis-NHC complex [Fe-2{mu-S(CH2)(3)S}(CO)(4){I-Me-CH2-I-Me}] (4a) as the major product. The analogous reaction of 1 with I-Me-(CH2)(2)-I-Me yielded the chelated bis-NHC complex [Fe-2(mu-S(CH2)(3)S)(CO)(4){I-Me-(CH2)(2)-I-Me}] (4b). Addition of HBF4 to compound 4a afforded the stable bridging hydride complexes [Fe-2(mu-H){mu-S(CH2)(3)S}(CO)(4){I-Me-CH2-I-Me}](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.