Neon-matrix spectroscopic and theoretical studies of the reactivity of titanium dimer with diatomic ligands: comparison of reactions with nitrogen and carbon monoxide

Abstract

The reactivity of diatomic titanium with molecular carbon monoxide has been investigated in solid neon at very low temperature. In contrast to the spontaneous reaction observed between Ti₂ and N₂, our results show that the formation of dititanium oxycarbide (OTi₂C) from the condensation of effusive beams of Ti and CO in neon matrices involves several intermediate steps including one metastable intermediate. In the absence of electronic excitation, only formation of a Ti₂(CO) complex occurs spontaneously during the reaction at 9 K of ground state Ti₂ and CO, as reported in solid argon by Xu, Jiang and Tsumori (Angew. Chem. Int. Ed., 2005, 44, 4338). However, during deposition or following electronic excitation, this species rearranges into a new species: the more stable, OTi₂C oxycarbide form. Several low-lying excited states of OTi₂C are also observed between 0.77 and 0.89 eV above the ground state, leading to a complex sequence of interacting vibronic transitions, merging into a broad continuum above 1 eV. Observations of Ti₂¹²C¹⁶O, Ti₂¹³C¹⁶O and Ti₂¹²C¹⁸O and natural titanium isotopic data enable the identification of four fundamental vibrations in the ground electronic state and two others in the first two excited states. Quantum chemical calculations predict an open-shell ¹A_g ground state with Ti–C and Ti–O distances close to 184 pm, and 91° for the TiCTi and TiOTi bond angles, and give fundamental frequencies in good agreement with observation. The reaction paths of the Ti₂ + N₂→ Ti₂N₂ and Ti₂ + CO → Ti₂(CO) → OTi₂C have been investigated and a reaction scheme is proposed accounting for the similarities in nature and properties of the final products, as well as explaining the observation of a coordination complex with Ti₂ only in the case of the carbonyl ligand.