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

Abstract

A series of complexes has been prepared in which {(Tp*)Mo^V(O)Cl}⁺ termini [Tp*  = tris(3,5-dimethylpyrazolyl)hydroborate] have been coordinated with ligands containing naphtholate binding sites. Mononuclear complexes, in which the sixth ligand is e.g. 1-naphtholate or 2-naphtholate, have been prepared; in addition some dinuclear complexes were also prepared based on bridging ligands containing either two naphtholate termini, or one naphtholate terminus and one phenolate terminus. The complexes have been studied by electrochemistry and UV/VIS/NIR spectroelectrochemistry to evaluate how the naphtholate donors affect the properties of the complexes compared to their known phenolate-based analogues. It was found that mononuclear complexes with phenolate, 1-naphtholate and 2-naphtholate ligands give significantly different electronic spectra in their oxidised [Mo(VI)] forms, with the characteristic phenolate → Mo(VI) LMCT transition being at much lower energy for 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 interactions compared to the equivalent phenolate-based complexes.