Tandem mass spectrometry of Cu(II) complexes: the effects of ligand donor group on dissociation.

A quadrupole ion trap mass spectrometer was used to study the dissociation patterns of Cu(II) complexes with various linear podand ligands. Cu(II) complexes having different combinations of nitrogen-, oxygen- and sulfur-containing terminal functionality attached to a diethylenetriamine (DIEN) framework were ionized by electrospray and collision-induced dissociation (CID) was used to generate product ions. Regardless of the particular functional groups present, the complexes undergo predominantly heterolytic cleavages of carbon-carbon bonds along the DIEN backbone with Cu remaining coordinated to one of the two terminal functional groups. Upon dissociation, Cu's preference to remain coordinated to a particular functional group follows the trend thioether > amine > imidazole > pyridine > ether. A simple evaluation of this trend based upon metal-functional group binding affinity appears not to be adequate for fully explaining these observations. The tendency of Cu(II) to be reduced upon dissociation helps explain the observed trend, as does the flexibility of the functional group, which affects its ability to orient its dipole effectively toward the metal.