Weak charge-transfer polyoxoanion salts: the reaction of quinolin-8-ol (Hquin) with phosphotungstic acid and the crystal and molecular structure of [H2quin]3[PW12O40]·4EtOH·2H2O

Abstract

Different solvated charge-transfer salts between quinolin-8-ol (Hquin) and phosphotungstic acid have been synthesized and characterized. Spectroscopic data support the presence of a sizeable electronic interaction between electron-rich aromatic organics and the inorganic acid, both in the solid state and in solution, provided that a large excess of the organic is present. This interaction is responsible for the formation of photosensitive compounds and provides a rationale for the subsequent facile photo-oxidation of the organic substrate. The crystal and molecular structure of [H₂quin]₃[PW₁₂O₄₀]·4EtOH·2H₂O has been solved by the heavy-atom method and refined to a final R= 0.083, for 4 902 reflections with I 3σ(I): triclinic space group P; with a= 11.721(3), b= 12.818(3), c= 12.878(3)Å, α= 89.20(2), β= 104.10(2), γ= 117.30(2)°, and Z= 1. The anion structure consists of an α-Keggin-type molecule in two-fold disorder. The 8-hydroxyquinolinium moieties (three per [PW₁₂O₄₀]^3–) are stacked approximately along the x direction, with alternative interplanar spacings of 3.25 and 3.57 Å. The crystal structure determination reveals the absence of any substantial, direct interaction between the organic cations and the phosphototungstate anions. Therefore no clear structural basis for the electronic interaction noted above can be given.