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.