Preparation and characterization of homologous diiron dithiolato, diselenato, and ditellurato complexes: [FeFe]-hydrogenase models

In order to elucidate the influence of the bridging chalcogen atoms in hydrogenase model complexes, diiron dithiolato, diselenolato, and ditellurolato complexes have been prepared and characterized. Treatment of Fe ₃(CO)₁₂ with 3,3-bis(thiocyanatomethyl)oxetane (1) or a mixture of 2-oxa-6,7-dithiaspiro[3.4]octane (2a) and 2-oxa-6,7,8-trithiaspiro[3. 5]nonane (2b) in toluene at reflux afforded the model compound Fe ₂(μ-S₂C₅H₈O)(CO)₆ (3). The analogous diselenolato and ditellurolato complexes, Fe₂(μ- Se₂C₅H₈O)(CO)₆ (4) and Fe ₂(μ-Te₂C₅H₈O)(CO)₆ (5), were obtained from the reaction of Fe₃(CO)₁₂ with 2-oxa-6,7-diselenaspiro[3.4]octane (6) and 2-oxa-6,7-ditelluraspiro[3.4]octane (7), respectively. Compounds 3-5 were characterized by spectroscopic techniques (NMR, IR, photoelectron spectroscopy), mass spectrometry, single-crystal X-ray analysis, and computational modeling. The electrochemical properties for the new compounds have been studied to assess their ability to catalyze electrochemical reduction of protons to give dihydrogen, and the catalytic rate is found to decrease on going from the sulfur to selenium to tellurium compounds. In the series 3-5 the reorganization energy on going to the corresponding cation decreased from 3 to 4 to 5. Spectroscopic and computational analysis suggests that the increasing size of the chalcogen atoms from S to Se to Te increases the Fe-Fe distance and decreases the ability of the complex to form the structure with a rotated Fe(CO)₃ group that has a bridging carbonyl ligand and a vacant coordination site for protonation. This effect is mirrored on reduction of 3-5 in that the rotated structure with a bridging carbonyl, which creates a vacant coordination site for protonation, is disfavored on going from the S to Se to Te complexes.