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

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 i onization (ESI) is utilized for introduction of the metal ion complexes int o a quadrupole ion trap mass spectrometer. The proximity of the carbonyl gr oups is crucial for formation and detection of ion complexes by ESI. For ex ample, no metal ion complexes are observed for 1,4-cyclohexanedione, but th ey are readily detected for the isomers, 1,2- and 1,3-cyclohexanedione. Alt hough the diketones form stable doubly charged complexes, the formation of singly charged alkaline earth complexes of the type (nL + M-24 - H+)(+) whe re L = 1,3-cyclohexanedione or 2,4-pentandlione is the first evidence of ch arge reduction. CAD investigations provide further evidence of charge reduc tion processes occurring in the gas-phase complexes. The CAD studies indica te that an intramolecular proton transfer between two diketone ligands atta ched to a doubly charged metal ion, followed by elimination of the resultin g protonated ligand, produces the charge reduced complex. For transition me tal complexation, the preference for formation of doubly charged versus sin gly charged complexes correlates with the keto-enol distribution of the dik etones in solution. (C) 1999 American Society for Mass Spectrometry.