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

Abstract

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; [Ti₅(μ₃-O)(μ₂-O)(μ-HOEt)₂(μ-OEt)₃(μ₂-OEt)(μ₃-tBPA)₃(μ₃-HtBPA)(μ₂-tBPA)₂(μ₂-HtBPA)]·3EtOH, 1, [Ti₄O(μ-OEt)₅(μ₂-OEt)₇(μ₃-tBPA)], 2, [Ti₄(μ₂-O)₂(μ-OEt)₂(μ-HOEt)₂(μ₂-tPBA)₂(μ₂-HtPBA)₆]·4EtOH, 3, [Ti₄(μ₂-O)₂(μ-OEt)₂(μ-HOEt)₂(μ₂-tPBA)₂(μ₂-HtPBA)₆]·2EtOH, 4, [Ti₆(μ₂-O)(μ₃-O)₂(μ₂-OEt)₅(μ-OEt)₆(μ₃-tBPA)₃(μ₃-HtBPA)], 5, [Ti₄(μ-ⁱOPr)₄(acac)₄(μ₂-tBPA)₄], 6 and [Ti₅(μ₄-O)(μ₂-O)₃(μ₂-OEt)₄(μ-OEt)₆(μ-HOEt)(μ₃-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. ³¹P 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. ³¹P NMR data supported degradation after addition of water to solutions of the complexes. Hydrolysis under different conditions affords complex oxide structures of different morphologies.