Complexation of nickel(II) by ethylenediamine investigated by means of electrospray ionization mass spectrometry

Electrospray ionization (ESI) mass spectrometry is used to investigate the complexation of Ni(II) by ethylenediamine (en) in water/methanol solution of NiCl2, both in the presence and absence of N,N-dimethylformamide (DMF). The fragmentations of the resulting Ni(II) complexes are studied using collision-induced dissociation (CID) including labeling experiments. The solvated ions formed upon ESI, in the absence of DMF, were: [Ni(en)n]2+ with n=2-5, [Ni(en)nCl]+ with n=1-3, and the chloro-bridged binuclear ions [Ni2(en)nCl3]+ with n=2-4. The dications [Ni(en)n]2+ show neutral ligand loss, proton transfer between the ligands, and electron transfer from the ligand to Ni(II). CID experiments and comparative studies including related ligands suggest that the multiply solvated dications [Ni(en)n]2+ with n>3 bear a hexacoordinate [Ni(en)3]2+ core with the additional ligands bound via hydrogen bonds. The monocations [Ni(en)nCl]+ with n=2 and 3 undergo evaporation of ethylenediamine and elimination of HCl. The bisligated monocation [Ni(en)Cl]+ undergoes competing eliminations of HCl and NH3 at low collision energies, and Ni---Cl bond homolysis at elevated collision energies. The binuclear ions [Ni2(en)nCl3]+ show a loss of neutral NiCl2 leading to the formation of [Ni(en)2Cl]+. The ions formed upon ESI from solutions which contain DMF as a co-ligand are: [Ni(en)3(DMF)n]2+ with n=0-4, [Ni(en)2(DMF)n]2+ with n=0-2, the monocation [Ni(en)2(DMF)Cl]+ and the binuclear clusters [Ni2(en)nCl3]+ with n=2-4. The mixed solvated Ni(II) ions preferentially eliminate neutral DMF, indicating a favorable coordination of Ni(II) by ethylenediamine.