[FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar μ-(SCH₂ )₂ ER₂ 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 co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the 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 owing to the long C-Ge and C-Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character 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 (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe₂ S₂ (CO)₆ cores bridged by CH₂ SnR₂ CH₂ , R=Me, Ph, linkers were also obtained and characterized.