Bond energies of copper ion-ligand L complexes CuL2+ determined in the gas phase by ion-ligand exchange equilibria measurements

The free energy changes, Delta G(2), for the reaction CuL2+ = Cu+ + 2L were obtained in the gas phase for some 23 different ligands L. These results were based on the determination of ligand exchange equilibria of the type CuA(2)(+) 2B = CuB2+ 2A. The equilibria were observed in a gas-phase ion-molecule reaction chamber sampled with a mass spectrometer. The entropy changes Delta S-2 degrees were determined by evaluation of the entropies S degrees, of the reactants from vibrational frequencies and moments of inertia obtained with HF/3-21G* basis sets. Combining Delta G(2) degrees and Delta S-2 degrees one obtained also the Delta H-2 degrees values. The range of Delta H-2 degrees values extended from 72 kcal/mol for the most weakly bonded ligand ethyl chloride to 135 kcal/mol for the most strongly bonding one, 1-methylimidazole. The results provide a partial confirmation of the Hard and Soft Acids and Bases (HSAB) principle. Comparison with available data for M+ = Li+ and Ag+ show that soft bases such as Me2S bond relatively more strongly to soft acids such as Cu+ and Ag+. However the actual bond energies Delta H-2 degrees are affected also by other interactions such as electrostatic contributions due to ion-ligand dipole attractive forces. Several of the ligands used correspond quite closely to the functional groups present on peptide residues. Therefore, a partial scale of peptide residue bonding to Cu+ can be established. Histidine is found to be the most strongly bonding residue.