Synthesis and spectroscopic characterization of tris(tert-butoxy)siloxy titanium and hafnium complexes: Molecular precursor to [M/Si/O (M = Ti, Hf)] materials

The reaction of one equivalent of metallocene dichloride Cp₂MCl₂ (M = Ti, Hf) with two molar equivalents of sodium salt of tri(tert-butoxy)siloxane, (^tBuO)₃SiONa, in toluene yields Cp₂Ti[OSi(O^tBu)₃]₂ (1) and Cp₂Hf[OSi(O^tBu)₃]₂ (2). These compounds were characterized by analytical and spectroscopic techniques (C/H/N analysis, FT-IR, NMR, EI-MS, and thermal analysis). An independent thermolysis of 1 and 2 at low temperature resulted in the formation of the corresponding metal silicates MO₂·2 SiO₂ [M = Ti (3) and Hf (4)] as evidenced by infrared spectroscopy, powder X-ray diffraction analysis (PXRD), and scanning electron microscopy (SEM). These thermally stable amorphous metal silicate materials (3 and 4), albeit exhibiting moderate surface areas, are found to be microporous by N₂ adsorption studies. UV-vis-NIR spectroscopy shows low energy band gap E_g = 2.63 eV for 3 and 2.99 eV for 4 due to the silicon oxide mixed with the corresponding transition metal oxides. The photocatalytic activity of titanium silicate material (3) has been explored in terms of degradation of methylene blue.