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

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