Infrared Spectroscopy of Gold Carbene Cation (AuCH2+): Covalent or Dative Bonding?

The present work explores the structure of the gold carbene cation, AuCH2+, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M+ = Ta+, W+, Os+, Ir+, and Pt+) that react with methane by dehydrogenation to form MCH2+ species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC2H4O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]+ product photofragmented by loss of H2 as induced by radiation provided by the free-electron laser for intracavity experiments in the 300-1800 cm-1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC+, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH2+ (1A1), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au+ (1S, 5d10) atom binds to methylene by donation of a pair of electrons from CH2(1A1) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC+ appearance spectrum from larger complexes are also considered, and H2CAu+(c-C2H4O) seems likely to contribute one band observed.