Keggin-Type Anions as Halogen Bond Acceptors

To study the potential of Keggin-type polyoxometalate anions to act as halogen bond acceptors, we have prepared a series of 10 halogen-bonded compounds starting from phosphomolybdic and phosphotungstic acid and halogenopyridinium cations as halogen (and hydrogen) bond donors. In all the structures, the cations and the anions were interconnected by halogen bonds, more often with terminal M=O oxygen atoms than with bridging oxygen atoms as acceptors. In four structures comprising protonated iodopyridinium cations capable of forming both hydrogen and halogen bonds with the anion, the halogen bond with the anion is apparently favored, whereas hydrogen bonds preferentially involve other acceptors present in the structure. In three obtained structures derived from phosphomolybdic acid, the corresponding oxoanion has been found in its reduced state [Mo₁₂PO₄₀]^4-, which has also led to a decrease in halogen bond lengths as compared to the fully oxidated [Mo₁₂PO₄₀]^3-. The electrostatic potential on the three types of anions involved in the study ([Mo₁₂PO₄₀]^3-, [Mo₁₂PO₄₀]^4-, and [W₁₂PO₄₀]^3-) has been calculated for optimized geometries of the anions, and it has been shown that the terminal M=O oxygen atoms are the least negative sites of the anions, indicating that they act as halogen bond acceptors primarily due to their steric availability.