Novel Ti–O–Ti bonding species constructed in a metal-oxide cluster

Abstract

The preparation and structural characterization of a novel Ti–O–Ti bonding complex constructed in the mono-lacunary α-Keggin polyoxometalate (POM), are described. The water-soluble, crystalline complex with a formula of K₅H₂[[{Ti(OH)(ox)}₂(μ-O)](α-PW₁₁O₃₉)]·13H₂O 1 was prepared in 30.2% (0.60 g scale) yield in a 1 : 3 molar-ratio reaction of the tri-lacunary species of α-Keggin POM, Na₉[A-PW₉O₃₄]·19H₂O, with the titanium(IV) source, K₂TiO(ox)₂·2H₂O (H₂ox = oxalic acid), in HCl-acidic solution (pH 0.08), and characterized by complete elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solution (³¹P, ¹⁸³W, ¹H and ¹³C) NMR spectroscopy and X-ray crystallography. The complex was also obtained in 47.6% (0.81 g scale) yield in a 1 : 2 molar-ratio reaction of the mono-lacunary Keggin POM, K₇[PW₁₁O₃₉]·10H₂O, with the anionic titanium(IV) complex under acidic conditions. The molecular structure of [[{Ti(OH)(ox)}₂(μ-O)](α-PW₁₁O₃₉)]⁷⁻1a, was successfully determined. This POM in the solid state is composed of one host (mono-lacunary site) and two guests (two octahedral Ti groups), in contrast to most titanium(IV)-substituted POMs consisting of one host and one guest. On the other hand, the ³¹P NMR measurements revealed that in aqueous solution this POM was present under a dissociation equilibrium which depends upon both temperature and pH.