Interaction of Ruthenium(II) with Terminal Alkynes: Benchmarking DFT Methods with Spectroscopic Data

Helium tagging infrared photodissociation (IRPD) spectroscopy for the characterization of organometallic complexes is presented. The IRPD spectrum of the [RuCp­(PPh3)­(PhCCH)]+ complex reveals that more than 80% of the detected ions correspond to a structure with π-coordinated phenylacetylene, and the rest are complexes with the alkyne probably isomerized to its vinylidene form. The detected CC and C–H stretches of the terminal alkyne reflect the degree of activation of the triple bond. They are used to benchmark the popular DFT functionals used in theoretical studies of ruthenium catalysis. It is shown that there are only small differences between the methods. GGA methods (e.g., BP86 or PBEPBE) and (hybrid) meta GGA functionals (e.g., M06) provide slightly better descriptions of this system than hybrid DFT methods such as B3LYP. A notable exception is M06-2X, which significantly underestimates the activation of the CC bond by the coordination to the ruthenium complex.