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

Abstract

Reaction of the oxo-molybdenum(V) precursor [Mo(Tp^Me,Me)(O)Cl₂] [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)Cl₂] afforded the dinuclear ferrocenyl–molybdenum complexes [Mo(O)(Tp^Me,Me)Cl(NXFc)] 5 (X = nothing), 6 (X = 1,4-C₆H₄), 7 (X = C₆H₄CHCHC₆H₄ with all para substitution) and 8 (X = C₆H₄NNC₆H₄ with all para substitution). Reaction of [Mo(Tp^Me,Me)(O)Cl₂] 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 ¹H 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 {MoNC₆H₄NR₂}²⁺ (R = Me, H) for the arylimido ligand. Similarly in complex 4 the bridging fragment MoNC₆H₄NMo undergoes two reversible oxidations to give the quinonoidal dication {MoNC₆H₄NMo}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.