Dioxo-, Oxo-imido-, and Bis-imido-Molybdenum(VI) Complexes with a Bis-phenolate-NHC Ligand

We report the facile synthesis of five rare high-valent Mo(VI)-NHC complexes with a dianionic OCO-pincer benzimidazolylidene ligand (L-1) with the general formula (LMoO2)-Mo-1 (1), (LMo)-Mo-1(O)((NBu)-Bu-t)(THF) (2), (LMo)-Mo-1((NBu)-Bu-t)((NHBu)-Bu-t)Cl (3), (LMo)-Mo-1((NBu)-Bu-t)(2) (4), and L-1 Mo(NMes)(2) (5). These complexes represent a complete series of high-valent molybdenum(VI) complexes, obtained by consecutive exchange of the oxo for imido ligands from the dioxo complex (1) to the bis-imido complexes (4 and 5). The complexes can be synthesized in high yields following a simple deprotonation protocol, using either triethylamine or lithium diisopropylamide (LDA) as a base. All complexes have been characterized by various techniques, including H-1, C-13{H-1}, and N-15 NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis. Except for the dioxo complex 1, all complexes adopt monomeric structures in the solid state. Compounds 1, 4, and 5 are rare examples of five-coordinated molybdenum(VI) complexes in the solid state, while 2 is six-coordinated, having a weakly bound THF ligand in the axial position. The dioxo complex 1 was found to be stable under air and moisture in the solid state as well as in solution for several days. Furthermore, preliminary reactivity studies and an approximate determination of the pK(a) value of the imido ligands in 4 are presented.