ESI-MS and theoretical study on the coordination structures and reaction modes of the diperoxovanadate complexes containing histidine-like ligands

In order to study the coordination structures and the reaction modes of diperoxovanadate complexes in the gas phase, the interaction between K3[OV(O2)2(C2O4)]·H2O and a series of histidine-like ligands has been investigated by the combination of the electrospray ionization-mass spectrometry (ESI-MS) and the density functional theory (DFT) calculations. The experimental results proved the formation of both [OV(O2)2L]− (L = all histidine-like ligands) and [OV(O2)2L′2]−[OV(O2)2L′2]− (L′ = histidine and carnosine only) species. DFT calculations at the level of B3LYP/6-31+G* showed that [OV(O2)2L′2]−[OV(O2)2L′2]− is a hexa-coordinated complex, instead of a hepta-coordinated complex as proposed before. The unique coordination mode in the gas phase is for one ligand to bind to the oxygen atoms via hydrogen binding, rather than both ligands to the metal center. The L′2L′2 dimer formation and the maintenance of the hydrogen bonding within the dimer during the complex formation are two important factors that enhance the abundance of the [OV(O2)2L′2]−[OV(O2)2L′2]− species. The calculated bonding enthalpy and free energy changes provided an explanation on the reaction modes of the interaction systems, in agreement with the observations of the ESI-MS experiments.