Experimental and Theoretical Investigations of the Sulfite‐Based Polyoxometalate Cluster Redox Series: α‐ and β‐[Mo₁₈O₅₄(SO₃)₂]^{4−/5−/6−}

Abstract

The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D(3h))(C(20)H(44)N)(4){alpha-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN and beta-(D(3d))(C(20)H(44)N)(4){beta-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo(18)O(54)(SO(3))(2)](4-/5-) and [Mo(18)O(54)(SO(3))(2)](5-/6-) processes produce stable [Mo(18)O(54)(SO(3))(2)](5-) and [Mo(18)O(54)(SO(3))(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta-->alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the alpha- and beta-[Mo(18)O(54)(SO(3))(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the alpha cluster than the beta form and also provide insight into the driving force for beta-->alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo(18)O(54)(SO(4))(2))](4-) sulfate cluster.

Polyoxometalates: Building Blocks for Functional Nanoscale Systems

“…By utilizing sulfite, selenite, tellurite, and periodate anions as templates, several new types of redox‐active heteroPOMs have been isolated. These POMs exhibit interesting thermochromic behavior or electrochemical properties that are related to the electron‐transfer reactions within the clusters 20. 21 The POM cluster [Mo 18 O 54 (SO 3 ) 2 ] 4− , which contains two embedded redox‐active sulfite templates, can be activated by a metallic surface and can reversibly interconvert between two electronic states 16.…”

“…Electrochemical investigations of [Mo 18 O 54 (SO 3 ) 2 ] 4− show an extensive series of six one‐electron reduction processes for both isomers of this cluster 20. The tungstate analogues and one‐electron reduced species [W 18 O 54 (SO 3 ) 2 ] n − have been synthesized and their electrochemical properties studied 21.…”

Using a simple high‐performance liquid chromatography separation and fraction collection methodology to achieve compound‐specific isotopic analysis for dissolved organic compounds

“…Mit redoxaktiven Anionen wie Sulfit, Selenit, Tellurit, Periodat usw. wurden mehrere neue Hetero‐POMs isoliert, die als Folge von Elektronentransfer‐Reaktionen innerhalb der Cluster eine interessante Thermochromie oder interessante elektrochemische Eigenschaften zeigen 20. 21 Beispielsweise kann [Mo 18 O 54 (SO 3 ) 2 ] 4− , ein POM‐Cluster mit zwei eingeschlossenen Sulfit‐Templaten, durch eine Metalloberfläche aktiviert werden und reversibel zwischen zwei elektronischen Zuständen wechseln 16.…”

“…Bei elektrochemischen Untersuchungen an [Mo 18 O 54 (SO 3 ) 2 ] 4− wurde für beide Isomere des Clusters eine Reihe von sechs Einelektronenreduktionen beobachtet 20. Die Wolframat‐Analoga und die reduzierten Spezies [W 18 O 54 (SO 3 ) 2 ] n − wurden ebenfalls synthetisiert, und ihre elektrochemischen Eigenschaften wurden untersucht 21.…”

Cation-Anion Interactions and Synergistic Catalysis by Supramolecular Polyoxometalate Complexes [C10H18N]n[XM12O40]

“…Cations are crucial not only in the construction and selective crystallization of novel POM clusters,, but also in the catalytic activity of POMs . Organic amines, mostly acting as encapsulating cations, are widely used as “shrink‐wrapping” agents in the synthesis of a range of new POMs by Cronin et al., e. g. (C 10 H 18 N) 4 (H 2 V 10 O 28 )⋅2DMF⋅2H 2 O, (C 6 H 13 N 4 ) 10 [H 2 Mo 16 O 52 ], (DMAH) 8 [W 18 O 56 (SO 3 ) 2 (H 2 O) 2 ]⋅9H 2 O, (TEAH) 6 [Mo 18 O 54 (SO 3 ) 2 ]⋅4H 2 O, (TEAH) 6 [H 4 W 19 O 62 ], (TEAH) 9 Na 2 {(H 2 O) 4 K[H 12 W 36 O 120 ]}⋅17H 2 O, (H 2 BHEP) 5 (HBHEP) 2 [(PMnW 11 O 39 ) 2 {PO 4 }]⋅18H 2 O . The use of bulky organic amine cations directs the self‐assembly process and isolates the self‐assembled POM species in the reaction systems.…”