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

Molecular Characteristics of a Mixed-Valence Polyoxovanadate {V^IV/V₁₈O₄₂} in Solution and at the Liquid–Surface Interface

The understanding of the molecular state of vanadium-oxo clusters (polyoxovanadates, POVs) in solution and on surface is a key to their target application in catalysis as well as molecular electronics and spintronics. We here report the results of a combined experimental and computational study of the behavior of nucleophilic polyoxoanions [V^IV₁₀V^V₈O₄₂(I)]^5– charged balanced by Et₄N⁺ in water, in a one-phase organic solution of <i>N</i>,<i>N</i>-dimethylformamid (DMF) or acetonitrile (MeCN), in a mixed solution of MeCN–water, and at the hybrid liquid–surface interface. The molecular characteristics of the compound (NEt₄)₅[V₁₈O₄₂(I)] (<b>1</b>) in the given environments were studied by microspectroscopic, electrochemical, scattering, and molecular mechanics methods. Contrary to the situation in pure water, where we observe great agglomeration with a number of intercalated H₂O molecules between POVs that are surrounded by the Et₄N⁺ ions, no or only minor agglomeration of redox-active POVs in an unprecedented cation-mediated fashion was detected in pure DMF and MeCN, respectively. An inclusion of 1% water in the MeCN solution does not have an effect significant enough to reinforce agglomeration; however, this leads to the POV···POV interface characterized by the presence of the Et₄N⁺ ions and a small number of H₂O molecules. Water amounts of ≥5% trigger the formation of higher oligomers. The deposition of compound <b>1</b> from MeCN onto an Au(111) surface affords nearly round-shaped particles (∼10 nm). The use of DMF instead of MeCN results in bigger, irregularly shaped particles (∼30 nm). This change of solvent gives rise to more extensive intermolecular interactions between polyoxoanions and their countercations as well as weaker binding of ion-pairing induced agglomerates to the metallic substrate. Lower concentration of adsorbed molecules leads to a submonolayer coverage and an accompanied change of the POV’s redox state, whereas their higher concentration results in a multilayer coverage that offers the pristine mixed-valence structure of the polyoxoanion. Our study provides first important insights into the reactivity peculiarities of this redox-responsive material class on a solid support

Molecular Characteristics of a Mixed-Valence Polyoxovanadate {V^IV/V₁₈O₄₂} in Solution and at the Liquid–Surface Interface

The understanding of the molecular state of vanadium-oxo clusters (polyoxovanadates, POVs) in solution and on surface is a key to their target application in catalysis as well as molecular electronics and spintronics. We here report the results of a combined experimental and computational study of the behavior of nucleophilic polyoxoanions [V^IV₁₀V^V₈O₄₂(I)]^5– charged balanced by Et₄N⁺ in water, in a one-phase organic solution of <i>N</i>,<i>N</i>-dimethylformamid (DMF) or acetonitrile (MeCN), in a mixed solution of MeCN–water, and at the hybrid liquid–surface interface. The molecular characteristics of the compound (NEt₄)₅[V₁₈O₄₂(I)] (<b>1</b>) in the given environments were studied by microspectroscopic, electrochemical, scattering, and molecular mechanics methods. Contrary to the situation in pure water, where we observe great agglomeration with a number of intercalated H₂O molecules between POVs that are surrounded by the Et₄N⁺ ions, no or only minor agglomeration of redox-active POVs in an unprecedented cation-mediated fashion was detected in pure DMF and MeCN, respectively. An inclusion of 1% water in the MeCN solution does not have an effect significant enough to reinforce agglomeration; however, this leads to the POV···POV interface characterized by the presence of the Et₄N⁺ ions and a small number of H₂O molecules. Water amounts of ≥5% trigger the formation of higher oligomers. The deposition of compound <b>1</b> from MeCN onto an Au(111) surface affords nearly round-shaped particles (∼10 nm). The use of DMF instead of MeCN results in bigger, irregularly shaped particles (∼30 nm). This change of solvent gives rise to more extensive intermolecular interactions between polyoxoanions and their countercations as well as weaker binding of ion-pairing induced agglomerates to the metallic substrate. Lower concentration of adsorbed molecules leads to a submonolayer coverage and an accompanied change of the POV’s redox state, whereas their higher concentration results in a multilayer coverage that offers the pristine mixed-valence structure of the polyoxoanion. Our study provides first important insights into the reactivity peculiarities of this redox-responsive material class on a solid support