Titanium, hafnium, and tantalum complexes of a potentially triphenolate phosphine ligand that is unexpectedly prone to O-protonation

Abstract

The exploratory coordination chemistry of titanium, hafnium, and tantalum complexes employing the potentially tetradentate triphenolate phosphine ligand [(2-O-3,5-tBu2C6H2)3P]3−·([O3P]3−) is described. The reactions of H3[O3P] with TiCl4(THF)2 or Ti(OEt)4, regardless of the stoichiometry of these starting materials, in toluene at 25°C generate the homoleptic, red crystalline Ti[O2P(OH)]2 (1), where [O2P(OH)]2− is a dianionic, facially tridentate biphenolate phosphine ligand tethering a P-bound phenol pendant. Metathetical reactions of in situ prepared Li3[O3P] with HfCl4(THF)x (x=0, 2) or TaCl5 in diethyl ether afford unexpectedly 7-coordinate colorless Hf[O2P(OH)]2(OH2) (2) or yellow Ta[O2P(OH)]2Cl (3), respectively. Comproportionation reactions of 1 with TiCl4(THF)2 or 3 with TaCl5 in toluene produce 6-coordinate dark red Ti[O2P(OH)]Cl2(THF) (4) or yellow Ta[O2P(OH)]Cl3 (5), respectively. The solution structures of these metal complexes were all characterized by multinuclear NMR spectroscopy and the solid-state structures of 1–4 determined by single-crystal X-ray diffraction analysis. The propensity of [O3P]3− to act preferentially in this study as a bi- instead of triphenolate phosphine chelate along with its unexpected inclination to O-protonation is discussed.