Reactivity and Structure of a Bis-phenolate Niobium NHC Complex.

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand ( ) with the general formula . We achieved this by deprotonation of the corresponding azolium salt and subsequent reaction with . The pyridine ligand in can be removed by the addition of B(CF) as a strong Lewis acid leading to the formation of the pyridine-free complex . In contrast to similar vanadium(V) complexes, complex was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula (E = O (), S (), NH (), and PH ()). Furthermore, complex can be converted to alkyl complex () with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center () in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to gives the pyridine-free aryl complex , which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes and are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, , KC, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including H-, C{H}-, and H-N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.