A V₁₆-type Polyoxovanadate Structure with Intricate Electronic Distribution: Insights from Magnetochemistry

The black-green solid (NEt₄)₅­[V₁₆O₃₈(Br)]­·2H₂O (<b>1</b>) was synthesized by the pH-controlled reaction of a mixed-valence precursor (NH₄)₈­[H₉V^IV₁₂V^V₇O₅₀]­·11H₂O with Et₄NBr in water under aerobic conditions. Compound <b>1</b> crystallizes as pseudomerohedral three-domain twins with pronounced pseudosymmetry and very large voids accommodating the majority of the countercations and solvent water molecules. The central structural motif of <b>1</b> is represented by a spherical, mixed-valence, host–guest vanadium-oxo cluster [V^IV/V₁₆O₃₈(Br)]<i>^q</i> with <i>q</i> = 5–, 4–, or 6–, exhibiting dominant antiferromagnetic and weaker ferromagnetic exchange interactions. The intriguing valence-state and dependent magnetic behavior of this compound have been unraveled by weighted model Hamiltonian calculations combined with diffraction, quantum mechanical, spectroscopic, and spectrometric techniques. It appears that <b>1</b> features a hitherto not identified and initially not evident V^IV/V^V average ratio of 8:8 which corresponds to an average charge <i>q</i> = 5– of the polyoxovanadate. Our study makes a substantial contribution to the further development of methods improving the understanding of poorly soluble mixed-valence polyoxometalates with complex spin architectures