Ion-pairing in polyoxometalate chemistry: impact of fully hydrated alkali metal cations on properties of the keggin [PWO] anion.

The counterions of polyoxometalates (POMs) impact properties and applications of this growing class of inorganic clusters. Here, we used density functional theory (DFT) to elucidate the impact of fully hydrated alkali metal cations on the geometry, electronic structure, and chemical properties of the polyoxotungstate anion [PWO]. The calculations show that the HOMO of the free anion [PWO] is a linear combination of the 2p AOs of the bridging oxygens, and the first few LUMOs are the 5d orbitals of the tungsten atoms. The S→ S electron excitation, near 3 eV, is associated with the O(2p) → W(5d) transition. Anion/cation complexation leads to formation of [PWO][M(HO)] ion-pair complexes, where with the increase of atomic number of M, the M(HO) cluster releases several water molecules and interacts strongly with the polyoxometalate anion. For M = Li, Na and K, [PWO][M(HO)] is characterized as a "hydrated" ion-pair complex. However, for M = Rb and Cs, it is a "contact" ion-pair complex, where the strong anion-cation interaction makes it a better electron acceptor than the "hydrated" ion-pair complexes. Remarkably, the electronic excitations in the visible part of the absorption spectrum of these complexes are predominantly solvent-to-POM charge transfer transitions (i.e. intermolecular CT). The ratio of the number of intermolecular charge transfer transitions to the number of O(2p)-to-W(5d) valence (i.e. intramolecular) transitions increases with the increasing atomic number of the alkali metals.