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 Na 2 Zn 3 [Fe(CN) 6 ] 2 ·9H 2 O. At high relative cluster concentrations, the reaction between [Zn(H 2 O) 6 ] 2+ and Na 4 [Re 6 Se 8 (CN) 6 ] produces Na 2 Zn 3 [Re 6 Se 8 (CN) 6 ] 2 ·24H 2 O ( 1 ), a compound exhibiting a porous three-dimensional framework isotypic with that of Na 2 Zn 3 [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 Å 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(H 2 O) 6 ] n + (M = Mg 2+, Cr 3+, Mn 2+, Ni 2+, Zn 2+ ) 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(H 2 O) 6 ] 2+ incorporates instead of Na +, leading to direct formation of the ion-exchanged compound [Zn(H 2 O) 6 ]Zn 3 [Re 6 Se 8 (CN) 6 ] 2 ·18H 2 O ( 2 ). The crystal structure of 2 reveals the [Zn(H 2 O) 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 [Re 6 Se 8 ] 2+ cluster cores in the solid framework. In a further parallel with ferrocyanide chemistry, the reaction between [Zn(H 2 O) 6 ] 2+ and Na 4 [Re 6 Se 8 (CN) 6 ] at low relative cluster concentrations is found to yield [Zn(H 2 O)] 2 [Re 6 Se 8 CN) 6 ]·13H 2 O ( 3 ), a phase exhibiting a two-dimensional framework structure isotypic with that of [Zn(H 2 O)] 2 [Fe(CN) 6 ]·0.5H 2 O.