Complexation of gas-phase metal ions with furan: Experimental and quantum chemical binding energies

Binding of metal ions to furan in the gas phase to form M+(furan) complexes was studied by radiative association kinetics (RAK) in the Fourier transform ion cyclotron resonance (FT-ICR) spectrometer, and by density functional theory (DFT) calculations of the structures and binding energies. Three main-group metals and the first-row transition metal series were included. Binding energies were compared with previous studies of cation-pi binding to the aromatic pi face involving benzene and pyrrole ligands. It was found that furan is a somewhat weaker pi-binding ligand than benzene for all of the metal ions (by an amount of the order of 5 kcal mol(-1)), in contrast to pyrrole, which is stronger than benzene in most cases. This was rationalized as a largely electrostatic effect. Furan presents the competing possibility of metal-ion binding to the oxygen, but it was found that the oxygen binding site has lower affinity than the at site for all metal ions. The preference for the at site is of the order of 6 kcal mol(-1) for metal ions without active d electrons, and of the order of 20 kcal mol(-1) for the transition metal ions with partially filled d-orbital shells. A quantitative model was developed and applied to separate out the electrostatic contributions to ion/ligand interaction from other types of interactions.