Energetics and Structures of Adducts of JohnPhos(Au+), PPh3(Au+), and IPr(Au+) with Organic Substrates: A Mass Spectrometry and DFT Study

We report a mass spectrometry study of the interaction between three representative Au(I) catalysts containing the ligands L = JohnPhos (2-biphenylyl[bis(2-methyl-2-propanyl)]phosphine), TPP (PPh3, triphenylphosphine), the N-heterocyclic carbene IPr (carbene form of 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-imidazole), and 27 organic molecules S (substrates) bearing organic functionalities often present in reactants (alkenes, alkynes, allenes, enol ethers, aldehydes, ketones, and epoxides). An experimental scale of gas-phase relative binding energy between three L(Au+) ions and the organic substrates was established by energy-resolved experiments of the 81 cationic two-coordinate gold adducts L(Au+)S using a quadrupole ion trap mass spectrometer. The experimental scale is expressed in units of normalized collision energy for 50% dissociation (NCE50) of the precursor ion. In parallel, the gas-phase bond dissociation energetics and the structure of adducts were probed by DFT calculations. The experimental affinity order of each substrate for the three cationic gold complexes L(Au+), IPr(Au+) > TPP(Au+) > JohnPhos(Au+) is well reproduced by the calculated bond dissociation energies ΔE. At the computational level used in this study, the agreement between the calculated ΔE and the experimental NCE50 values is limited to series of substrates with the same functionality, and reasonable correlations NCE50 vs ΔE are observed within series. The DFT-optimized structures are discussed and compared with available X-ray crystal structures. Although no general trend can be observed between bond lengths or their changes upon coordination with the L(Au+) cations and dissociation energies, a significant correlation between the Au-O distance (S = O-bases) and ΔE is observed.