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

Abstract

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, i.e, the active site of [FeFe]hydrogenase. A series of complexes, (μ-SCH 2 CRR′CH 2 S)[Fe(CO) 3 ][Fe(CO) 2 L] with steric bulk incorporated into the μ-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, PPh 3 , 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 μ-SCH 2 CR 2 CH 2 S produced little change in the ground state structures (X-ray diffraction) and electronic character for the (μ-SRS)[Fe(CO) 3 ] 2 complexes, monosubstitution of a CO with L produced distortions consistent with steric interference of the μ-SRS with nearby ligands as compared to the similar (μ-pdt)[Fe(CO) 3 ][Fe(CO) 2 L] (pdt = S(CH 2 ) 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].