Unprecedented Oxo-Titanium Citrate Complex Precipitated from Aqueous Citrate Solutions, Exhibiting a Novel Bilayered Ti₈O₁₀ Structural Core

Aqueous titanium citrate solutions were prepared from the reaction of citric acid with titanium 2-propoxide in a range of molar ratios. Solutions containing two or fewer citrates per titanium resulted in the slow crystallization of an insoluble titanium oxo-citrate complex. Single-crystal X-ray analysis identified the species as Ti8O10(citrate)4(H2O)12·14H2O·3HOPri, crystallized in the tetragonal space group I41/a, with a = 30.775(7) Å, c = 14.528(7) Å, V = 13 759(8) Å3, and Z = 8. The trianionic citrate ligands supply both carboxylate and alkoxide coordination and stabilize the structure using simultaneous chelating and bridging modes of attachment. The compound is a neutral species, exhibiting titanium in three contrasting environments. Laser Raman microscopy and 13C CPMAS solid-state NMR data were consistent with those of the X-ray crystal structure. When exposed to air, the crystals rapidly lost water and became a powder. The dehydrated powder was noncrystalline to X-rays and insoluble, but 13C NMR results demonstrated retention of the carboxylate linkages.