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

摘要

Titanium (oxo-) alkoxide phosphonate complexes were synthesized using different titanium precursors and tert-butylphosphonic acid (tBPA) 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(mu(3)-O)(mu(2)-O)(mu-HOEt)(2)(mu-OEt)(3)(mu(2)-OEt)(mu(3)-tBPA)(3)(mu(3)-HtBPA)(mu(2)-tBPA)(2)(mu(2)-HtBPA)]center dot 3EtOH, 1, [Ti4O(mu-OEt)(5)(mu(2)-OEt)(7)(mu(3)-tBPA)], 2, [Ti-4(mu(2)-O)(2)(mu-OEt)(2)(mu-HOEt)(2)(mu(2)-tPBA)(2)(mu(2)-HtPBA)(6)]center dot 4EtOH, 3, [Ti-4(mu(2)-O)(2)(mu-OEt)(2)(mu-HOEt)(2)(mu(2)-tPBA)(2)(mu(2)-HtPBA)(6)]center dot 2EtOH, 4, [Ti-6(mu(2)-O)(mu(3)-O)(2)(mu(2)-OEt)(5)(mu-OEt)(6)(mu(3)-tBPA)(3)(mu(3)-HtBPA)], 5, [Ti-4(mu-(OPr)-O-i)(4)(acac)(4)(mu(2)-tBPA)(4)], 6 and [Ti-5(mu(4)-O)(mu(2)-O)(3)(mu(2)-OEt)(4)(mu-OEt)(6)(mu-HOEt)(mu(3)-tBPA)](2), 7. The binding mode of tBPA 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-31 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-31 NMR data supported degradation after addition of water to solutions of the complexes. Hydrolysis under different conditions affords complex oxide structures of different morphologies.