Mass spectrometric studies on small open-chain piperazine-containing ligands and their transition metal complexes

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the complexes formed between open-chain piperazine-containing ligands and transition metal salts (Cobalt, Copper, Zinc, and Cadmium as chlorides, nitrates, and acetates). Only single-charged complexes were observed, formed of one ligand (L) and mainly one metal ion (M). Since the net charge of the complexes was one, a counterion (X) was attached to some of the complexes, with formation of [L + M + X]+ complexes, and a proton was lost from others, as in [L - H + M]+ complexes. In most cases the composition of the complexes was more dependent on the ligand than the metal salt. Collision-induced dissociation measurements showed that complexes with related composition often differed in structure, or that interactions between the ligand and the metal ion were not alike. The metal ion influenced considerably the fragmentation pathways of the ligands, so that the fragmentation products could be used to deduce the binding sites of the metal. The variations observed in fragmentation behavior of complexes possessing the same ligand but different metal ions can mostly be explained by the ionic radius and electronic configuration of the metal ion. The results indicated a preference of the piperazine ring of the coordinated ligand for the boat conformation.