Self-Assembly and Properties of a Discrete Water-Soluble Prussian Blue Analogue FeII/CoIII Cube: Confinement of a Water Molecule in Aqueous Solution

Novel types of water-soluble anionic cubic cages, K4[{CoIII(Me3-Tacn)}4{FeII(CN)6}4] and Na4[{CoIII(Me3-Tacn)}4{FeII(CN)6}4] (Me3-Tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), were prepared by means of a mechanistically designed self-assembly process between [Co­(Me3-Tacn)­Cl3] and A4[FeII(CN)6] (M = Na or K), consisting of a rate-limiting outer-sphere redox step, followed by a fast substitution/inner-sphere redox reaction sequence. These compounds show remarkable stability in aqueous solution at different pH ranges, displaying neat protonation processes and reversible oxidation with peroxodisulfate to its neutral {FeIII 4CoIII 4} form. Furthermore, the cages behave as a robust and water-soluble molecular Prussian Blue analogue capable of encapsulating {Na-OH2}+ pairs and K+ cations in aqueous solution, with the cubic structure of the complex being preserved. Substitution of the {Na-OH2}+ pairs by K+ is easily accomplished, and the electrochemical properties of the sodium and potassium salts of the new cages have been found to be dramatically dependent on the encapsulated units.