Redox and UV/VIS/NIR spectroscopic properties of tris(pyrazolyl)borato-oxo-molybdenum(v) complexes with naphtholate and related co-ligands

A series of complexes has been prepared in which {(Tp*)Mo-V(O)Cl}(+) termin i [Tp* = tris(3,5-dimethylpyrazolyl) hydroborate] have been coordinated wit h ligands containing naphtholate binding sites. Mononuclear complexes, in w hich the sixth ligand is e.g. 1-naphtholate or 2-naphtholate, have been pre pared; in addition some dinuclear complexes were also prepared based on bri dging ligands containing either two naphtholate termini, or one naphtholate terminus and one phenolate terminus. The complexes have been studied by el ectrochemistry and UV/VIS/NIR spectroelectrochemistry to evaluate how the n aphtholate donors affect the properties of the complexes compared to their known phenolate-based analogues. It was found that mononuclear complexes wi th phenolate, 1-naphtholate and 2-naphtholate ligands give significantly di fferent electronic spectra in their oxidised [Mo(VI)] forms, with the chara cteristic phenolate --> Mo(VI) LMCT transition being at much lower energy f or the naphtholate complexes than the phenolate, an observation which is of significance for development of electrochromic near-IR dyes. In addition, the twist induced in some of the bridging ligands by the bulky naphtholate termini results in substantially decreased metal-metal electronic interacti ons compared to the equivalent phenolate-based complexes.