Evaluation of proton-binding capabilities of polyether and pyridyl ligands

The competitive proton-binding abilities of a series of pyridyl and polyether ligands were examined by application of ligand-exchange and collisionally activated dissociation (CAD) methods. Many of the ligands of interest are multidentate, thus giving them enhanced capabilities for coordinating the proton, and they also may undergo a substantial loss of entropy, predominantly in rotational and vibrational modes of freedom, when binding a proton because the electrostatic interactions between donor atoms and the proton create a degree of organization of the ligand. The presence of dominant mixed-dimer complexes formed during the ligand-exchange reactions provides key evidence that multiple hydrogen-bond formation is operative for one or both ligands. Although CAD provides some insight into the factors which influence proton-binding strengths of the ligands in dimers, several general issues emerge when applying the CAD (i.e. the kinetic method) to estimate proton affinities of multidentate ligands. Owing to the severe impact of entropic effects upon dissociation of the dimers involving multidentate ligands, the ratio of product ions does not reflect the order of proton affinities of the ligands involved in the dimer. The CAD experiments give the order of gas-phase basicities at a higher temperature in which the entropy term is much more significant. This effect is especially significant when one of the ligands is floppy and multidentate and the other is rigid or monodentate. © 1998 John Wiley & Sons, Ltd.