On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes

Abstract

¹⁷O-labelled tungsten siloxide complexes [WOCl₂(OSitBu₃)₂] (1-Cl) and [WOMe₂(OSitBu₃)₂] (1-Me) were prepared and characterized by ¹⁷O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking ¹⁷O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO_2-700 afforded material 2, with surface species [(SiO)WOMe₂(OSitBu₃)] as shown by elemental analysis, IR and ¹H and ¹³C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient ¹⁷O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.