Expanded Prussian Blue Analogues Incorporating [Re₆Se₈(CN)₆]^3-/4- Clusters: Adjusting Porosity via Charge Balance

Face-capped octahedral [Re6Se8(CN)6]3-/4- clusters are used in place of octahedral [M(CN)6]3-/4- complexes for the synthesis of microporous Prussian blue type solids with adjustable porosity. The reaction between [Fe(H2O)6]3+ and [Re6Se8(CN)6]4- in aqueous solution yields, upon heating, Fe4[Re6Se8(CN)6]3·36H2O (4). A single-crystal X-ray analysis confirms the structure of 4 to be a direct expansion of Prussian blue (Fe4[Fe(CN)6]3·14H2O), with [Re6Se8(CN)6]4- clusters connected through octahedral Fe3+ ions in a cubic three-dimensional framework. As in Prussian blue, one out of every four hexacyanide units is missing from the structure, creating sizable, water-filled cavities within the neutral framework. Oxidation of (Bu4N)4[Re6Se8(CN)6] (1) with iodine in methanol produces (Bu4N)3[Re6Se8(CN)6] (2), which is then metathesized to give the water-soluble salt Na3[Re6Se8(CN)6] (3). Reaction of [Co(H2O)6]2+ or [Ni(H2O)6]2+ with 3 in aqueous solution affords Co3[Re6Se8(CN)6]2·25H2O (5) or Ni3[Re6Se8(CN)6]2·33H2O (6). Powder X-ray diffraction data show these compounds to adopt structures based on the same cubic framework present in 4, but with one out of every three cluster hexacyanide units missing as a consequence of charge balance. In contrast, reaction of [Ga(H2O)6]3+ with 3 gives Ga[Re6Se8(CN)6]·6H2O (7), wherein charge balance dictates a fully occupied cubic framework enclosing much smaller cavities. The expanded Prussian blue analogues 4−7 can be fully dehydrated, and retain their crystallinity with extended heating at 250 °C. Consistent with the trend in the frequency of framework vacancies, dinitrogen sorption isotherms show porosity to increase along the series of representative compounds 7, Ga4[Re6Se8(CN)6]3·38H2O, and 6. Furthermore, all of these phases display a significantly higher sorption capacity and surface area than observed in dehydrated Prussian blue. Despite incorporating paramagnetic [Re6Se8(CN)6]3- clusters, no evidence for magnetic ordering in compound 6 is apparent at temperatures down to 5 K. Reactions related to those employed in preparing compounds 4−6, but carried out at lower pH, produce the isostructural phases H[cis-M(H2O)2][Re6Se8(CN)6]·2H2O (M = Fe (8), Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-dimensional framework in which [Re6Se8(CN)6]4- clusters are interlinked through a combination of protons and Fe3+ ions.