Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation

Abstract

In contrast to the extensive knowledge of lithium cation affinities and basicities, the thermochemistry of microsolvated lithium cations is much less explored. Here, we determine the relative stabilities of Li(A,B)_n⁺ complexes, n = 2 and 3, by monitoring their gas-phase reactions with A and B substrate molecules, A/B = Me₂O, Et₂O, tetrahydrofuran, and MeCN, in a three-dimensional quadrupole-ion trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies. In addition, we use high-level quantum chemical calculations to predict the structures and sequential ligand dissociation energies of the homoleptic Li(A)_n⁺ complexes, n = 1–3. As expected, the ligands dissociate more easily from complexes in higher coordination states. However, the very nature of the ligand also matters. Ligands with different steric demands can, thus, invert their relative Li⁺ affinities depending on the coordination state of the metal center. This finding shows that microsolvation of Li⁺ can result in specific effects, which are not recognized if the analysis takes into account only simple lithium cation affinities and basicities.