Trapping the δ Isomer of the Polyoxometalate-Based Keggin Cluster with a Tripodal Ligand

We report the synthesis, structural, and electronic characterization of the theoretically predicted, but experimentally elusive δ-isomer of the Keggin polyanion. A family of δ-Keggin polyoxoanions of the general formula, (TEA)HpNaq [H2M12(XO4)O33(TEA)].rH2O where p, q, r = [2,3,8] for 1 and [4,1,4] for 2 were isolated by the reaction of tungstate(VI) and vanadium(V) with triethanolammonium ions (TEAH), acting as a tripodal ligand grafted to the surface of the cluster leading to the entrapment and stabilization of the elusive polyanhionic δ Keggin archetype. The δ-Keggin species were characterized by single-crystal X-ray diffraction, FT-IR, UV-vis, NMR and ESI-MS spectrometry. Electronic structure and structure-stability correlations were evaluated by means of DFT calculations. The compounds exhibited multi-electron transfer and reversible photochromic properties by undergoing single-crystal-to-single-crystal (SC-SC) transformations accompanied with colour changes under light