Complexation of divalent metal ions with diols in the presence of anion auxiliary ligands: zinc-induced oxidation of ethylene glycol to glycolaldehyde by consecutive hydride ion and proton shifts

Ternary complexes of the type AHM2+L (AH?=?diol, including diethylene and triethylene glycol, M?=?Ca, Mn, Fe, Co, Ni, Cu and Zn and auxiliary anion ligand L?=?CH3COO, HCOO and Cl) have been generated in the gas phase by MALDI and ESI, and their dissociation characteristics have been obtained. Use of the auxiliary ligands enables the complexation of AH with the divalent metal ion without AH becoming deprotonated, although AM2+ is often also generated in the ion source or after MS/MS. For M?=?Ca, dissociation occurs to AH?+?M2+L and/or to AM2+ + LH, the latter being produced from the H-shifted isomer AM2+LH. For a given ligand L, the intensity ratio of these processes can be interpreted (barring reverse energy barriers) in terms of the quantity PA(A) Caaff(A), where PA is the proton affinity and Caaff is the calcium ion affinity. Deuterium labeling shows that the complex ion HOCH2CH2OHZn2+OOCCH3, in addition to losing acetic acid (60 Da), also eliminates glycolaldehyde (HOCH2CH=O, also 60 Da); it is proposed that these reactions commence with a hydride ion shift to produce the iondipole complex HOCH2CHOH+ HZnOOCCH3, which then undergoes proton transfer and dissociation to HOCH2CH=O?+?HZn+O?=?C(OH)CH3. In this reaction, ethylene glycol is oxidized by consecutive hydride ion and proton shifts. A minor process leads to loss of the isomeric species HOCH=CHOH. Copyright (C) 2012 John Wiley & Sons, Ltd.