Electrospray ionization mass spectrometry of metal complexes. Gas phase formation of a binuclear copper(II)–5-Br-PADAP complex Original Research Article

The complexes formed from copper(II) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP or HL) in aqueous methanol solution was studied by electrospray ionization mass spectrometry. The solution of a 1:1 complex of Cu(II) with 5-Br-PADAP showed five peaks assignable to a binuclear complex [Cu2L2(AcO)]+ and mononuclear complexes [CuL]+, [CuL(H2O)]+, [CuL(AcOH)]+ and [CuL(HL)]+ (AcO=acetate). Collision activated dissociation revealed the relative order of bonding strengths; Cu–L>Cu–HL>CuL–AcOH>CuL–H2O. The peak intensities of the binuclear complex showed second-order dependency on those of the mono complex. As for the solution of Ni(II)–5-Br-PADAP, no binuclear complex was observed in the mass spectra. Thus, it was suggested that [Cu2L2(AcO)]+ was formed by the fast gas phase reaction: 2[CuL]++AcO−⇄[Cu2L2(AcO)]+.