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 [Fe-2(mu-SC3H6S)(CO)(6)] (1) have been prepared with one S atom replaced either by one Se atom to give [Fe-2(mu-SC3H6Se)(CO)(6)] (2) or by one Te atom to give [Fe-2(mu-SC3H6Te)(CO)(6)] (3). The molecules have been characterized by use of mass spectrometry and C-13{H-1} NMR, Se-77{H-1)}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 H-2. 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 H-2 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.