Counteranion-Stabilized Titanium(IV) Isopolyoxocationic Clusters Isolated from Water

A novel titanium(IV) oxo cluster comprised solely of Ti, O, and H atoms, [Ti₆(O_μ)₈(O_tH₂)₂₀]⁸⁺ (Ti₆) was synthesized in high yield via controlled hydrolysis and condensation of TiX₄ (X = Cl, Br) in the presence of TBAX (TBA = tetrabutylammonium; X = Cl, Br) from water, while reactions of TiI₄ and TBAI yielded [Ti₈O₁₂(OH₂)₂₄]⁸⁺ (Ti₈). The structures and compositions of the clusters were determined by single-crystal X-ray crystallography, powder X-ray diffraction, inductively coupled plasma atomic emission spectrometry, and energy-dispersive spectrometry. Ti₆ is comprised of six-coordinated titanium(IV) atoms bridged with μ₂-O atoms, structurally similar to a typical Lindqvist polyoxometalate. On the basis of a structural comparison of Ti₆ and Ti₈, density functional theory calculations, and spectroscopic analysis, it is evident that both clusters are stabilized by halide counteranions via the formation of hydrogen bonds. This study not only presents the second example of a titanium(IV) isopolyoxocationic cluster isolated from water but also suggests that counteranions are generally important for synthesizing molecular fragments of titanium oxides.