Titanium phosphonate oxo-alkoxide "clusters": solution stability and facile hydrolytic transformation into nano titania.

Titanium (oxo-) alkoxide phosphonate complexes were synthesized using different titanium precursors and -butylphosphonic acid (BPA) as molecular models for interaction between phosphonates and titania surfaces and to investigate the solution stability of these species. Reflux of titanium(iv) ethoxide or titanium(iv)(diisopropoxide)bis(2,4-pentadionate) with -butylphosphonic acid in toluene-ethanol mixture or acetone yielded seven titanium alkoxide phosphonate complexes; [Ti(μ-O)(μ-O)(μ-HOEt)(μ-OEt)(μ-OEt)(μ-BPA)(μ-HBPA)(μ-BPA)(μ-HBPA)]·3EtOH, 1, [TiO(μ-OEt)(μ-OEt)(μ-BPA)], 2, [Ti(μ-O)(μ-OEt)(μ-HOEt)(μ-PBA)(μ-HPBA)]·4EtOH, 3, [Ti(μ-O)(μ-OEt)(μ-HOEt)(μ-PBA)(μ-HPBA)]·2EtOH, 4, [Ti(μ-O)(μ-O)(μ-OEt)(μ-OEt)(μ-BPA)(μ-HBPA)], 5, [Ti(μ-OPr)(acac)(μ-BPA)], 6 and [Ti(μ-O)(μ-O)(μ-OEt)(μ-OEt)(μ-HOEt)(μ-BPA)], 7. The binding mode of BPA to the titanium oxo-core were either double or triple bridging or a combination of the two. No monodentate or chelating coordination was observed. P NMR spectrometry of dissolved single crystals indicates that 1 and 5 retain their solid-state structures in solution, the latter even on moderate heating, while 6 and 7 dissolved into several other forms. The complexes were found to be sensitive towards hydrolysis, proceeding in a topotactic fashion with densification of the material into plates and lamellae resulting finally in "core-shell" nanoparticles with a crystalline core (anatase) and an amorphous outer shell upon contact with water at room temperature as observed by HRTEM and AFM analyses. P NMR data supported degradation after addition of water to solutions of the complexes. Hydrolysis under different conditions affords complex oxide structures of different morphologies.