Metal- and Ligation-Dependent Fragmentation of [M(1,10-Phenanthroline)1,2,3]2+ Cations with M = Mn, Fe, Co, Ni, Cu, and Zn: Comparison between the Gas Phase and Solution

Abstract

The core ions [ML(n)]2+ with n = 1-3, where L = 1,10-phenanthroline and M is a first-row transition metal, have been successfully transferred from aqueous solution into the gas phase by electrospraying and then probed for their stabilities by collision-induced dissociation in a triple quadrupole mass spectrometer. The triply ligated metal dications [ML3]2+ were observed to dissociate by the extrusion of a neutral ligand, while ligand loss from both [ML2]2+ and [ML]2+ was accompanied by electron transfer. Comparisons are provided between gas-phase stabilities and stabilities for ligand loss measured in aqueous solution at 298 K. The measured onset for ligand loss from [ML3]2+ is quite insensitive to the metal, while a distinct stability order has been reported for aqueous solution. Low level density functional theory (DFT) calculations predict an intrinsic stability order for loss of ligand from [ML2]2+, but it differs from that in aqueous solution. Substantial agreement was obtained for the stability order for the loss of ligand from [ML]2+ deduced from onset energies measured for charge separation, computed with DFT, and reported for aqueous solution where hydration seems less decisive in influencing this stability order. A qualitative potential-energy diagram is presented that allows the energy for charge separation to be related to the energy for neutral ligand loss from [ML]2+ and shows that IE(M+) is decisive in determining the intrinsic stability order for loss of ligand from [ML]2+.