Characterization of reactive sites in supported catalysts by 51V/15N rotational echo double resonance NMR spectroscopy: formation of phenylimido groups at surface-bound oxovanadium sites.

Silica-supported oxovanadium groups were reacted in a gas−solid reaction with aniline at 175 °C. The reaction was clean as monitored in situ by UV−vis spectroscopy and resulted in the elimination of water as the principal product of the reaction and the disappearance of the terminal VO stretch in the Raman spectrum. 15N MAS solid-state NMR spectroscopy showed only a single nitrogen-containing species on the surface. Proton-dephased 15N NMR showed only weak attenuation of its intensity, indicating that there are no protons directly bonded to the nitrogen. The formation of a vanadium−imido covalent bond was characterized by 51V/15N rotational echo double resonance NMR spectroscopy where the quadrupolar 51V nucleus was monitored and the spin-1/2 15N nucleus was dephased.