Complexes of tetradentate β-keto enamine schiff bases: an electrospray mass spectrometric study

Abstract

Electrospray mass spectrometry (ESMS) has been used to investigate several types of derivatives of four tetradentate Schiff base ligands. The ES mass spectra of the free ligands give strong peaks due to singly protonated intact ions which are formed by reaction with the acetic acid in the mobile phase used in the spectrometer. In the presence of alkali metal salts, the ES mass spectra show the corresponding adduct ions with one alkali metal attached to each Schiff base. The neutral Schiff base derivatives of Cu(II), Ni(II), Pd(II) and VO(II) all give intact ions corresponding to the singly protonated species in their ES mass spectra. Mixtures of different transition metal Schiff base derivatives give only ions for the protonated starting materials, with no evidence for metal exchange between the complexes. In the presence of alkali metal salts, the transition metal Schiff base complexes act as neutral ligands towards the alkali metals producing ions with one, two and three metal complexes coordinated to the alkali metal. Similar coordination of the transition metal Schiff base complexes to divalent transition metal ions is also observed, but in these cases acetate ions are involved so that the charge on the observed ions is always +1.