Vanadium, niobium and tantalum complexes with terminal sulfur radical ligands

Sulfur radicals terminally bound to the metal center can be considered as the one-electron reduction products of complexes with terminal sulfido ligands which serve as the reactive sites in enzymes and precursors. However, there is limited information regarding this kind of metal stabilized sulfur radical, which contrasts the more commonly known metal stabilized thiyl radical. In this work, we report the preparation of vanadium, niobium and tantalum radical complexes in the form of M(O)(S)F₂ from the reactions of laser-ablated metal atoms and SOF₂ in cryogenic matrixes. Combined with the results from infrared spectroscopy and density functional theory calculations, the sulfur ligand in M(O)(S)F₂ is characterized to be a terminally bound radical with the unpaired electron located on the sulfur 3p orbital. Besides this radical complex, calculations also predict the existence of MF₂(η²-SO) with a side-on SO ligand, but this less stable isomer is not observed as a result of high exothermicity along with its formation from metal atoms and SOF₂ that is large enough to overcome the energy barrier towards the occurrence of M(O)(S)F₂.