Carborane Stabilized "19-Electron" Molybdenum Metalloradical

Paramagnetic metal complexes gained a lot of attention due to their participation in a number of important chemical reactions. In most cases, these complexes are dominated by 17-e metalloradicals that are associatively activated with highly reactive paramagnetic 19-e species. Molybdenum paramagnetic complexes are among the most investigated ones. While some examples of persistent 17-e Mo-centered radicals have been reported, in contrast, 19-e Mo-centered radicals are illusive species and as such could rarely be detected. In this work, the photodissociation of the [Cp(CO)3Mo]2 dimer (1) in the presence of phosphines was revisited. As a result, the first persistent, formally 19-e Mo radical with significant electron density on the Mo center (22%), Cp(CO)3Mo·PPh2(o-C2B10H11) (5b), was generated and characterized by EPR spectroscopy and MS as well as studied by DFT calculations. The stabilization of 5b was likely achieved due to a unique electron-withdrawing effect of the o-carboranyl substituent at the phosphorus center.