[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar (µ-SCH2)2ER2 Linkers (E = Ge and Sn).

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E = Ge or Sn) adopt an unusual conformation with nearly coplanar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to steric interaction of the Me groups with the axial carbonyls of the Fe2(CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties due to the long C-Ge and C-Sn bond lengths. Gas phase photoelectron spectroscopy of these complexes shows a shift of ionizations with substantial sulfur orbital character to lower energies and, as supported by the computations, an increase in sulfur character in the predominantly metal-metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism in the absence of acid, and catalysis of proton reduction in the presence of acid. Two cyclic cluster dimers featuring the sulfur atoms of two Fe2S2(CO)6 cores bridged by CH2SnR2CH2, R = Me, Ph, linkers were also obtained and characterized.