Vibrational Spectroscopy of C-H Activation Intermediates and Dehydrogenation Products of Sequential Reactions of Niobium Cation with Methane.

Vibrational spectra of NbCH ( = 1-4) and dehydrogenation products NbCH and NbCH produced by the reaction of Nb with methane are measured using photofragment spectroscopy in the C-H and H-H stretching regions (2500-3600 cm). Entrance channel complexes Nb(CH) are observed with red-shifted vibrations of proximate C-H stretches. Calculations at the B3LYP/aug-cc-pVTZ level of theory assist in identifying the structures of the ions. The hydrogen coordination moves from η toward η as more methanes ligate. Exit channel complexes (H)NbCH(CH)(Ar) and (H)NbCH(CH) are also observed, identified based on their characteristic agostic carbene C-H stretch (∼2530 cm) and H-H stretch (∼3500 cm). Although dehydrogenation of methane by Nb is endothermic, the interaction with multiple methane molecules opens exothermic H loss pathways. The measured spectrum of the two H loss product NbCH consists of a single peak, confirming it to be Nb(CH). This is consistent with the calculations, which predict a preference for species with Nb-CH bonds.