Novel Ti−O−Ti Bonding Species Constructed in a Metal−Oxide Cluster:  Reaction Products of Bis(oxalato)oxotitanate(IV) with the Dimeric, 1,2-Dititanium(IV)-Substituted Keggin Polyoxotungstate

The preparation and structural characterization of a novel Ti−O−Ti bonding complex constructed in a dilacunary α-Keggin polyoxometalate (POM), [[{Ti(ox)(H₂O)}₄(μ-O)₃](α-PW₁₀O₃₇)]^7- (H₂ox = oxalic acid) (1a), are described. The water-soluble, crystalline complex with a formula of K₆H[1a]·0.5KCl·10H₂O (1p) was prepared as the bulk sample in 28.0% (0.51 g scale) yield in a 1:4 molar-ratio reaction of the dititanium(IV)-substituted, dimeric form of an α-Keggin POM, K₁₀[(α-1,2-PW₁₀Ti₂O₃₉)₂]·18H₂O, with the titanium(IV) source K₂[TiO(ox)₂]·2H₂O in HCl-acidic solution (pH 0.08). Prior to formation of 1p, the KCl-free crystalline compound (1c) obtained was characterized with X-ray crystallography. The compound 1p was unequivocally characterized with complete elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, and solution (³¹P, ¹⁸³W, and ¹³C) NMR spectroscopy. The molecular structure of 1a was determined. The POM 1a in the solid state was composed of the four octahedral Ti groups (four guests), i.e., the two Ti−O−Ti groups linked with the μ-O atom, incorporated to the two adjacent, octahedral vacant sites (two hosts) in the dilacunary Keggin POM. The formation of 1a, as well as the recently found POM [{Ti(ox)(H₂O)}₂(μ-O)](α-PW₁₁O₃₉)]^5- (2a), was strongly dependent on the reaction with [TiO(ox)₂]^2-, i.e., the anionic titanium(IV) complex as the titanium(IV) source. The POM 1a is contrasted to most titanium(IV)-substituted POMs consisting of a combination of a monolacunary site (one host) and an octahedral Ti group (one guest) and also contrasted to 2a as a combination of a monolacunary site (one host) and two octahedral Ti groups or a Ti−O−Ti group (two guests).