Expansion of the porous solid Na2Zn3[Fe(CN)6]2•9H2O:: Enhanced ion-exchange capacity in Na2Zn3[Re6Se8(CN)6]2•24H2O

A technique for increasing the porosity in solid structures by replacing octahedral metal ions with hexanuclear cluster cores is extended with the expansion of the ion-exchange material Na2Zn3[Fe(CN)(6)](2). 9H(2)O. At high relative cluster concentrations, the reaction between [Zn(H2O)(6)](2+) and Na-4[Re6Se8(CN)(6)] produces Na2Zn3[Re6Se8(CN)(6)](2). 24H(2)O (1), a compound exhibiting a porous three-dimensional framework isotypic with that of Na2Zn3[Fe(CN)(6)](2). 9H(2)O. Its framework is characterized by hexagonal bipyramidal cages, each enclosing two Na+ ions and 24 water molecules in a volume of 1340 Angstrom(3)-more than triple the volume of the cages in the original structure. The expanded cavities and framework openings in compound 1 are shown to facilitate absorption of larger cationic complexes, specifically [M(H2O)(6)](n+) (M = Mg2+, Cr3+, Mn2+, Ni2+, Zn2+) and [Cr(en)(3)](3+), via exchange for the Na+ ions. Indeed, when the preparation of 1 is attempted using a slight excess of the zinc reactant, [Zn(H2O)(6)](2+) incorporates instead of Na+, leading to direct formation of the ion exchanged compound [Zn(H2O)(6)]Zn-3[Re6Se8(CN)(6)](2). 18H(2)O (2). The crystal structure Of 2 reveals the [Zn(H2O)(6)](2+) complexes to reside at the exact center of the hexagonal bipyramidal cages. With prolonged exposure to air, compound 1 and its ion-exchanged variants undergo a color change from orange to green, which is attributed to the one-electron oxidation of the [Re6Se8](2+) cluster cores in the solid framework. In a further: parallel with ferrocyanide chemistry, the reaction between [Zn(H2O)(6)](2+) and Na-4[Re6Se8(CN)(6)] at low relative cluster concentrations is found to yield [Zn(H2O)](2)[Re6Se8CN)(6)]. 13H(2)O (3), a phase exhibiting a two-dimensional framework structure isotypic with that of [Zn(H2O)](2)[Fe(CN6)]. 0.5H(2)O.