Study of diketone/metal ion complexes by electrospray ionization mass spectrometry: influence of keto-enol tautomerism and chelation

Abstract

The formation and collisionally activated dissociation (CAD) behavior of a series of complexes containing cyclic or linear diketone ligands and alkali, alkaline earth, or transition metal ions are investigated. Electrospray ionization (ESI) is utilized for introduction of the metal ion complexes into a quadrupole ion trap mass spectrometer. The proximity of the carbonyl groups is crucial for formation and detection of ion complexes by ESI. For example, no metal ion complexes are observed for 1,4-cyclohexanedione, but they are readily detected for the isomers, 1,2- and 1,3-cyclohexanedione. Although the diketones form stable doubly charged complexes, the formation of singly charged alkaline earth complexes of the type ( nL + M 2+ − H +) + where L = 1,3-cyclohexanedione or 2,4-pentanedione is the first evidence of charge reduction. CAD investigations provide further evidence of charge reduction processes occurring in the gas-phase complexes. The CAD studies indicate that an intramolecular proton transfer between two diketone ligands attached to a doubly charged metal ion, followed by elimination of the resulting protonated ligand, produces the charge reduced complex. For transition metal complexation, the preference for formation of doubly charged versus singly charged complexes correlates with the keto-enol distribution of the diketones in solution.