Synthesis and characterization of [FeFe]-hydrogenase models with bridging moieties containing (S, Se) and (S, Te)

[FeFe]-hydrogenase-active-site models containing larger chalcogens such as Se or Te have exhibited greater electron richness at the metal centers and smaller gas-phase ionization energies and reorganization energies relative to molecules containing S atoms. Diiron complexes related to the much-studied molecule [Fe₂(μ-SC₃H₆S)(CO)₆] (1) have been prepared with one S atom replaced either by one Se atom to give [Fe₂(μ-SC₃H₆Se)(CO)₆] (2) or by one Te atom to give [Fe₂(μ-SC₃H₆Te)(CO) ₆] (3). The molecules have been characterized by use of mass spectrometry and ¹³C{¹H} NMR, ⁷⁷Se{ ¹H} NMR, IR, and photoelectron spectroscopic techniques along with structure determination with single-crystal X-ray diffraction, electrochemical measurements, and DFT calculations. He I photoelectron spectra and DFT computations of 2 and 3 show a lowering of ionization energies relative to those of the all-sulfur complex 1, indicating increased electron richness at the metal centers that favors electrocatalytic reduction of protons from weak acids to produce H₂. However, chalcogen substitution from S to Se or Te also causes an increase in the Fe-Fe bond length, which disfavors the formation of a carbonyl-bridged "rotated" structure, as also shown by the photoelectron spectra and computations. This "rotated" structure is believed to be important in the mechanism of H₂ production. As a consequence of the competing influences of increased electron richness at the metals with less favorable "rotated" structures, the catalytic efficiency of the Se and Te molecules 2 and 3 is found to be comparable to that of molecule 1.