New Vanadium Tris(tert-butoxy)siloxy Complexes and Their Thermolytic Conversions to Vanadia−Silica Materials

The V(IV) alkoxysiloxy complexes (^tBuO)₃VOSi(O^tBu)₃ (1) and (^tBuO)₂V[OSi(O^tBu)₃]₂ (2) were synthesized via the silanolysis of V(O^tBu)₄ with 1 and 2 equiv of HOSi(O^tBu)₃, respectively. Complexes 1 and 2 are efficient single-source molecular precursors to homogeneous V/Si/O materials via the thermolytic molecular precursor method. The thermal transformations of these complexes occurred at low temperatures (≤160 °C), via the elimination of isobutene as the major carbon-containing product. Thermolyses of 1 in the solid state and in solution (forming a xerogel in the latter case) yielded V/Si/O materials (VOSi1_ss and VOSi1_xg, respectively) with low surface areas (30−40 m² g^-1). After calcination at 400 °C in O₂, these materials exhibited powder X-ray diffraction (PXRD) patterns consistent with the presence of V₂O₅. Similar thermolyses of 2 yielded V/Si/O materials (VOSi2_ss and VOSi2_xg) that exhibited higher surface areas (up to 170 and 70 m² g^-1, respectively) and V₂O₅ crystallite formation after calcination at 400 °C. Comparisons of VOSi1_xg and VOSi2_xg with a previously reported V/Si/O xerogel (VOSi3_xg), generated from the V(V) precursor OV[OSi(O^tBu)₃]₃, revealed interesting differences. Crystalline V₂O₅ was first observed in VOSi3_xg after calcination at only 300 °C in O₂. Transmission electron microscopy (TEM) and PXRD were used to determine that the average size of the V₂O₅ crystallites in the V/Si/O xerogels, after calcination at a given temperature, increased with increasing silicon content of the precursor and was highest for the V(V) tris(siloxide). The precursors containing vanadium in the lower oxidation state (IV) appear to initially provide more homogeneous V/Si/O materials. Higher Si content for the precursor leads to a greater surface area for the resultant material, but also to earlier phase separation during the calcination process.