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

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Abstract

Face-capped octahedral [Re(6)Se(8)(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(H(2)O)(6)](3+) and [Re(6)Se(8)(CN)(6)](4-) in aqueous solution yields, upon heating, Fe(4)[Re(6)Se(8)(CN)(6)](3).36H(2)O (4). A single-crystal X-ray analysis confirms the structure of 4 to be a direct expansion of Prussian blue (Fe(4)[Fe(CN)(6)](3).14H(2)O), with [Re(6)Se(8)(CN)(6)](4-) clusters connected through octahedral Fe(3+) 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 (Bu(4)N)(4)[Re(6)Se(8)(CN)(6)] (1) with iodine in methanol produces (Bu(4)N)(3)[Re(6)Se(8)(CN)(6)] (2), which is then metathesized to give the water-soluble salt Na(3)[Re(6)Se(8)(CN)(6)] (3). Reaction of [Co(H(2)O)(6)](2+) or [Ni(H(2)O)(6)](2+) with 3 in aqueous solution affords Co(3)[Re(6)Se(8)(CN)(6)](2).25H(2)O (5) or Ni(3)[Re(6)Se(8)(CN)(6)](2).33H(2)O (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(H(2)O)(6)](3+) with 3 gives Ga[Re(6)Se(8)(CN)(6)].6H(2)O (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 degrees 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, Ga(4)[Re(6)Se(8)(CN)(6)](3).38H(2)O, 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 [Re(6)Se(8)(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(H(2)O)(2)][Re(6)Se(8)(CN)(6)].2H(2)O (M = Fe (8), Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-dimensional framework in which [Re(6)Se(8)(CN)(6)](4-) clusters are interlinked through a combination of protons and Fe(3+) ions.

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