Relative binding energies of gas-phase pyridyl ligand/metal complexes by energy-variable collisionally activated dissociation in a quadrupole ion trap

The relative binding energies of a series of pyridyl ligand/metal complexes of the type [(ML2)-L-I](+) and [(ML3)-L-II](2+) are investigated by using energy-variable collisionally activated dissociation in a quadrupole ion tr ap mass spectrometer. The pyridyl ligands include 1,10-phenanthroline and v arious alkylated analogues, 2,2'-bipyridine, 4,4'-dimethyl-2,2 -bipyridine, and 2,2':6',2 " -terpyridine, and the metal ions include cobalt, nickel, c opper, zinc, cadmium, calcium, magnesium, lithium, sodium, potassium, rubid ium, and cesium. The effect of the ionic size and electronic nature Of the metal ion and the polarizability and degree of preorganization of the pyrid yl ligands on the threshold activation voltages, and thus the relative bind ing energies of the complexes, are evaluated. Correlations are found betwee n the binding constants of [(ML3)-L-II](2+) Complexes in aqueous solution a nd the threshold activation voltages of the analogous gas-phase complexes d etermined by collisionally activated dissociation.