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 tert -butylphosphonic acid ( t 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 tert -butylphosphonic acid in toluene-ethanol mixture or acetone yielded seven titanium alkoxide phosphonate complexes; [Ti 5 (μ 3 -O)(μ 2 -O)(μ-HOEt) 2 (μ-OEt) 3 (μ 2 -OEt)(μ 3 - t BPA) 3 (μ 3 -H t BPA)(μ 2 - t BPA) 2 (μ 2 -H t BPA)]·3EtOH, 1, [Ti 4 O(μ-OEt) 5 (μ 2 -OEt) 7 (μ 3 - t BPA)], 2, [Ti 4 (μ 2 -O) 2 (μ-OEt) 2 (μ-HOEt) 2 (μ 2 - t PBA) 2 (μ 2 -H t PBA) 6 ]·4EtOH, 3, [Ti 4 (μ 2 -O) 2 (μ-OEt) 2 (μ-HOEt) 2 (μ 2 - t PBA) 2 (μ 2 -H t PBA) 6 ]·2EtOH, 4, [Ti 6 (μ 2 -O)(μ 3 -O) 2 (μ 2 -OEt) 5 (μ-OEt) 6 (μ 3 - t BPA) 3 (μ 3 -H t BPA)], 5, [Ti 4 (μ- i OPr) 4 (acac) 4 (μ 2 - t BPA) 4 ], 6 and [Ti 5 (μ 4 -O)(μ 2 -O) 3 (μ 2 -OEt) 4 (μ-OEt) 6 (μ-HOEt)(μ 3 - t BPA)] 2 , 7. The binding mode of t 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. 31 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. 31 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.