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 H2 oxidation, i.e, the active site of [FeFe]hydrogenase. A series of complexes, (m-SCH2CRR0CH2S)[Fe(CO)3][Fe(CO)2L] with steric bulk incorporated into the m-S-to-S linker was synthesized and the compounds were analyzed by infrared spectroscopy and cyclic voltammetry [(R/R0 ¼ Me/Me, Et/Et, Bu/Et), (L ¼ 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 m-SCH2CR2CH2S produced little change in the ground state structures (X-ray diffraction) and electronic character for the (m-SRS)[Fe(CO)3]2 complexes, monosubstitution of a CO with L produced distortions consistent with steric interference of the m-SRS with nearby ligands as compared to the similar (m-pdt)[Fe(CO)3][Fe(CO)2L] (pdt ¼ S(CH2)3S). 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 [