Comparison of S and Se dichalcogenolato [FeFe]-hydrogenase models with central S and Se atoms in the bridgehead chain

In order to study the influence of sulfur and selenium atoms incorporated into the structure of complexes that model the active site of [FeFe]-hydrogenases, a series of diiron dithiolato and diselenolato complexes of the form (μ-ECH ₂XCH ₂E-μ)Fe ₂(CO) ₆ have been prepared and characterized, where the diiron bridging atoms E are S or Se, and the linker bridgehead X is CH ₂, S, or Se. The electron energies have been compared by gas-phase photoelectron spectroscopy, and the oxidation, and reduction behaviors, as well as the ability to reduce protons from acetic acid to form H ₂, have been compared by cyclic voltammetry. Density functional theory computations agree well with the structures and electron energies of these molecules, and shed additional light on the oxidation and reduction properties. The computations indicate that the HOMO of each molecule where the bridgehead X is S or Se contains substantial chalcogen 'lone pair' orbital character. The presence of the bridgehead chalcogen lone pairs favors the Fe(CO) ₃ 'rotated' structures for both the cations and dianions of these complexes, but in different ways. In the cations one Fe(CO) ₃ rotates to put one carbonyl ligand in a semibridging position, and the bridgehead chalcogen lone pair electrons donate to the vacant coordination site created on the iron to stabilize the positive charge. In the dianions one Fe(CO) ₃ rotates to put one carbonyl ligand in a fully bridging position, and one bridging chalcogen atom breaks its bond with an iron atom, pulling the bridgehead chalcogen lone pair away from the iron to minimize the electron-electron repulsions.