A new route to mixed oxo/arylimido complexes of molybdenum(VI) with a tris(pyrazolyl)borate co-ligand: syntheses, spectroscopic properties and ligand-centred redox activity

Reaction of the oxo-molybdenum(v) precursor [Mo(Tp(Me,Me))(O)Cl-2] [Tp(Me,Me) = hydrotris(3,5-dimethylpyrazol-1-yl)borate] with arylamines RNH2, in the presence of Et3N and air, afforded the oxo(imido)molybdenum(VI) complexes [Mo(O)(Tp(Me,Me))Cl(=NR)] 1 (R = 4-tolyl), 2 (R = C6H4NMe2-4); this is a new and simple route to these rare compounds. Use of 1,4-diaminobenzene afforded the mononuclear complex 3 (R = C6H4NH2-4) in addition to the dinuclear complex [{Cl(O)(Tp(Me,Me))Mo}(2) (NC6H4N)] 4. Reaction of various aminoferrocene derivatives Fc-X-NH2 with [Mo(Tp(Me,Me))(O)Cl-2] afforded the dinuclear ferrocenyl-molybdenum complexes [Mo(O)(Tp(Me,Me))Cl(=NXFc)] 5 (X = nothing), 6 (X = 1,4-C6H4), 7 (X = C6H4CH=CHC6H4 with all para substitution) and 8 (X = C6H4N=NC6H4 with all para substitution). Reaction of [Mo(Tp(Me,Me))(O)Cl-2] with mixed amine-phenol ligands HO-X-NH2 afforded the mixed-valence complexes [{Cl(O)(Tp(Me,Me))Mo}(2)(=NXO)] 9 (X = 1,4-C6H4) and 10 (X = 1,5-naphthalenediyl), and the mononuclear oxo-Mo(V) complex [Mo(O)(Tp(Me,Me))Cl(OC6H4NH2)] 9a with an unreacted amino terminus was also isolated. The complexes were characterised by H-1 NMR and IR spectroscopy, FAB mass spectrometry and UV/VIS spectroscopy. Whereas the simple oxo-imido-Mo(VI) cores of 1 and 5-10 undergo completely irreversible oxidations, complexes 2 and 3 undergo two reversible ligand-centred oxidations, one of the pendant amino group to give a radical cation, and the second of the imido fragment resulting in a quinonoidal structure {Mo=N=C6H4=NR2}(2+) (R = Me, H) for the arylimido ligand. Similarly in complex 4 the bridging fragment Mo=NC6H4N=Mo undergoes two reversible oxidations to give the quinonoidal dication {Mo=N=C6H4=N=Mo}(2+) via the semiquinone radical intermediate. Spectroelectrochemical studies on 2 and 4 confirmed the nature of these reversible oxidations as ligand centred. Complexes 5 to 8 undergo a reversible oxidation of the ferrocenyl fragment, and complexes 9, 9a and 10 display the reversible Mo(V)/Mo(VI) and Mo(V)/Mo(IV) redox couples expected for their oxo-Mo(V) fragments.