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

A novel titanium­(IV) oxo cluster
comprised solely of Ti, O, and H atoms, [Ti6(Oμ)8(OtH2)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.