A Host–Guest Supercapacitor Electrode Material Based on a Mixed Hexa-Transition Metal Sandwiched Arsenotungstate Chain and Three-Dimensional Supramolecular Metal–Organic Networks with One-Dimensional Cavities

The mixed hexa-transition metal (hexa-TM) sandwiched arsenotungstate derivative, [Cu^I₃(pz)₂(phen)₃]₂[Cu^I(phen)₂][{Na(H₂O)₂}{(V^IV₅Cu^IIO₆)(As^IIIW₉O₃₃)₂}]·6H₂O (1) (pz = pyrazine; phen = 1,10-phenanthroline), has been hydrothermally synthesized and structurally characterized. In compound 1, two {AsW₉O₃₃} clusters are connected by mixed hexa-TM ring unit {V^VI₅Cu^IIO₆} to form a sandwich-type dimer, which are further bonded in "ABAB" mode by the {Na(H₂O)₂} linker resulting in pure inorganic chains. The unique "L-shaped" trinuclear complex {Cu₃(phen)₃(pz)₂} is supported together via staggered π-π interactions to generate extending waveform two-dimensional supramolecular layers, which are further aggregated with their adjacent analogues by complexes {Cu(phen)₂} via H-bonding interaction to yield an unprecedented three-dimensional (3D) metal-organic networks with one-dimensional (1D) cavities. The pure inorganic 1D sandwich chains are implanted in the cavities as guest units via supramolecular interactions to form a POMOF 3D framework. Compound 1, as the electrode of the supercapacitor, exhibits higher specific capacitances (825 F g^-1 at a current density of 2.4 A g^-1), better rate capability, more durable cyclic stability (91.4% of cycle efficiency after 3000 cycles), and improved conductivity and electroactivity compared to those of parent polyoxometalate (POM) Na₉[AsW₉O₃₃]·19H₂O (2) and 6-Cu-substituted POM [Cu₆(imi)₆{As^IIIW₉O₃₀Cl₃}₂]·6H₂O (3), which may be attributed to the introduction of V⁴⁺, the unique host-guest structure, and the rich π electron system. In addition, compound 1 exhibits dual-function electrocatalytic behavior in reducing inorganic salt IO₃^- and oxidizing the organic molecule dopamine.