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

O-labelled tungsten siloxide complexes [WOCl(OSiBu)] (1-Cl) and [WOMe(OSiBu)] (1-Me) were prepared and characterized by O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO afforded material 2, with surface species [(SiO)WOMe(OSiBu)] as shown by elemental analysis, IR and H and C 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 O 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.