SOMC grafting of vanadium oxytriisopropoxide (VO(O i Pr) 3 ) on dehydroxylated silica; analysis of surface complexes and thermal restructuring mechanism.

Vanadium oxytriisopropoxide (VO(O i Pr) 3 ), 1, was grafted on highly dehydroxylated silica (SiO 2-700 : aerosil silica treated at 700 °C under high vacuum) to generate compound 2 following the concepts and methodology of surface organometallic chemistry (SOMC). The resulting compound was analyzed by elemental analysis, FT-IR, 1 H, 13 C and 51 V solid state (SS) NMR, Raman and EPR spectroscopies. The grafting reaction of 1 to generate 2 was found to lead to the formation of a monopodal surface complex [([triple bond, length as m-dash]Si-O-)V(O)(O i Pr) 2 ], 2m, as well as bipodal [([triple bond, length as m-dash]Si-O-) 2 V(O)(O i Pr)], 2b, formed along with ([triple bond, length as m-dash]Si-O- i Pr) moieties as an effect of the classical rearrangement of 2m with strained siloxane bridges. Upon controlled thermal treatment at 200 °C under high vacuum, 2m and 2b were found to mainly rearrange to tetrahedral VO 4 moieties [([triple bond, length as m-dash]Si-O-) 3 V(O)] (3) with formation of propylene whereas the ([triple bond, length as m-dash]Si-O- i Pr) groups were preserved. The mechanism of the thermal rearrangement of the isopropoxide groups was investigated by a DFT approach revealing the occurrence of a concerted γ-H-transfer and olefin elimination mechanism.