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

Vibrational spectra of NbCnH4n+ (n = 1–4) and dehydrogenation products NbC3H10+ and NbC4H12+ 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–1). Entrance channel complexes Nb+(CH4)n 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 η3 toward η2 as more methanes ligate. Exit channel complexes (H2)NbCH2+(CH4)2(Ar) and (H2)NbCH2+(CH4)3 are also observed, identified based on their characteristic agostic carbene C–H stretch (∼2530 cm–1) and H–H stretch (∼3500 cm–1). Although dehydrogenation of methane by Nb+ is endothermic, the interaction with multiple methane molecules opens exothermic H2 loss pathways. The measured spectrum of the two H2 loss product NbC4H12+ consists of a single peak, confirming it to be Nb(CH3)4+. This is consistent with the calculations, which predict a preference for species with Nb–CH3 bonds.