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

The black-green solid (NEt4)5[V16O38(Br)]·2H2O (1) was synthesized by the pH-controlled reaction of a mixed-valence precursor (NH4)8[H9VIV12VV7O50]·11H2O with Et4NBr in water under aerobic conditions. Compound 1 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 1 is represented by a spherical, mixed-valence, host–guest vanadium-oxo cluster [VIV/V16O38(Br)]q with q = 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 1 features a hitherto not identified and initially not evident VIV/VV average ratio of 8:8 which corresponds to an average charge q = 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.