“Mixed-Valence Decatungstates” Revisited: Crystal Structure and Aqueous Solution Chemistry of [W₁₀O₃₂]^6– and an ESR Study on Electron Delocalization

A [ⁱPrNH₃]⁺/Na⁺-mixed salt of unprotonated two-electron-reduced decatungstate, [ⁱPrNH₃]₈[W₁₀O₃₂]·(NaCl₂)₂H₂O (1), is isolated from the photolyte obtained by the UV photolysis of the Na₂WO₄·2H₂O/[ⁱPrNH₃]Cl aqueous solution at pH 6.5 and characterized in the solid state (X-ray diffraction and elemental analysis) and solution (UV–vis absorption and electrochemistry). [W₁₀O₃₂]^5–, as a paramagnetic contaminant in the crystallite of diamagnetic 1, shows only a broadened singlet-like electron spin resonance (ESR) line around g = 1.82 with an unresolved ¹⁸³W-hyperfine interaction (A_W) at low temperatures. To determine the effective electronic delocalization in the reduced decatungstates consisting of two polar WO₆ (W_pO₆) and eight equatorial WO₆ (W_eqO₆) octahedra with an approximate D_4h symmetry, we calculated isotropic ESR spectral patterns of the protonated reduced decatungstate, [HW₁₀O₃₂]^4–, for a variety of |A_W| on the basis of interactions of the 5d¹-electronic spin with eight magnetically equivalent nuclear spins of W_eq and the proton ¹H (I_H = 1/2) and took |A_W| = 2|A_H| (A_H = ¹H-superhyperfine interaction) as the best choice for direct comparison with the experimental single-crystal ESR spectral pattern of [HW₁₀O₃₂]^4– at 77 K. Electronically fully delocalized over eight W_eqO₆ octahedron mixed-valence sites of class-III-type clusters, [W₁₀O₃₂]^5– ≡ [W_p⁶⁺₂(W_eq^5.875+)₈O₃₂]^5– and [W₁₀O₃₂]^6– ≡ W_p⁶⁺₂(W_eq^5.75+)₈O₃₂]^6–, are demonstrated with the help of ESR spectroscopy of [HW₁₀O₃₂]^4– ≡ [W_p⁶⁺₂(H_0.125W_eq^5.875+)₈O₃₂]^4– which revealed that both the 5d¹-electron and proton are situated at each equatorial WO₆ octahedron site with a 1/8 occupancy at low temperatures (≤77 K). The nucleation process to [W₁₀O₃₂]^6– through the UV photolysis of [W₇O₂₄]^6– at pH 6.5 is also discussed based on the electrospray ionization mass spectrometry of the photolyte.