The effect of bridgehead steric bulk on the ground state and intramolecular exchange processes of (μ-SCH2CR2CH2S)[Fe(CO)3][Fe(CO)2L] complexes

The flexibility of the coordination sphere in the diiron organometallic is likely an important design component in nature's electrocatalyst for proton reduction or H-2 Oxidation, Le, the active site of [FeFe]hydrogenase. A series of complexes, (mu-SCH2CRR'CH2S)[Fe(CO)(3)][Fe(CO)(2)L] with steric bulk incorporated into the mu-S-to-S linker was synthesized and the compounds were analyzed by infrared spectroscopy and cyclic voltammetry [(R/R' = Me/Me, Et/Et, Bu/Et), (L = CO, PPh3, IMes (1.3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), and IMe (1,3-dimethylimidazole-2-ylidene))]. While added steric bulk at the bridgehead carbon of the mu-SCH2CR2CH2S produced little change in the ground state structures (X-ray diffraction) and electronic character for the (mu-SRS)[Fe(CO)(3)](2) complexes, monosubstitution of a CO with L produced distortions consistent with steric interference of the mu-SRS with nearby ligands as compared to the similar (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)L] (pdt = S(CH2)(3)S). Variable temperature NMR studies have shown that the activation barrier for CO site exchange on the sterically bulky complexes decreases in a manner predicted by theory [J.W. Tye, M.B. Hall, M.Y. Darensbourg, Inorg. Chem. 45 (2006) 1552].