Thermolytic conversion of a bis(alkoxy)tris(siloxy)tantalum(V) single-source molecular precursor to catalytic tantala-silica materials

The new complex (iPrO)2Ta[OSi(O tBu)3]3 (1) was prepared via silanolysis of Ta(O iPr)5 with (tBuO)3SiOH and is a useful structural and spectroscopic (NMR, FTIR) model of Ta(V) on silica. The complex was also used to prepare tantalum-containing silica materials, via the thermolytic molecular precursor method (yielding Ta2O5 ⋅ 6SiO2 and Ta2O5 ⋅ 18SiO2) or by grafting 1 onto mesoporous SBA-15 silica (yielding a surface-supported tantala species, TaSBA-15). The solution phase thermolysis of 1 in nonpolar media afforded homogeneous, high-surface-area (ca. 450 m2 g−1) xerogels (Ta2O5 ⋅ 6SiO2) that are amorphous up to approximately 1100 °C. A more silica-rich tantala-silica material (Ta2O5 ⋅ 18SiO2) was prepared via a solution-phase co-thermolytic route with 1 and HOSi(O tBu)3, to yield a material with a Si/Ta ratio of 9/1. It was demonstrated that tantala-silica materials are active as catalysts for cyclohexene oxidation.