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

Abstract

Vanadium oxytriisopropoxide (VO(OⁱPr)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, ¹H, ¹³C and ⁵¹V 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 [(Si–O–)V(O)(OⁱPr)₂], 2m, as well as bipodal [(Si–O–)₂V(O)(OⁱPr)], 2b, formed along with (Si–O–ⁱPr) 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 VO₄ moieties [(Si–O–)₃V(O)] (3) with formation of propylene whereas the (Si–O–ⁱPr) 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.