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 Pr ussian blue type solids with adjustable porosity. The reaction between [Fe( H2O)(6)](3+) and [Re6Se8(CN)(6)](4-) in aqueous solution yields, upon heati ng, Fe-4[Re6Se8(CN)(6)](3). 36H(2)O (4). A single-crystal X-ray analysis co nfirms the structure of 4 to be a direct expansion of Prussian blue (Fe-4[F e(CN)(6)](3). 14H(2)O), with [Re6Se8(CN)(6)](4-) clusters connected through octahedral Fe3+ ions in a cubic three-dimensional framework. As in Prussia n blue, one out of every four hexacyanide units is missing from the structu re, creating sizable, water-filled cavities within the neutral framework. O xidation 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-s oluble salt Na-3[Re6Se8(CN)(6)] (3). Reaction of [Co(H2O)(6)](2+) or [Ni(H2 O)(6)](2+) with 3 in aqueous solution affords CO3[Re6Se8(CN)(6)](2). 25H(2) O (5) or Ni-3[Re6Se8(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 mi ssing as a consequence of charge balance. In contrast, reaction of [Ga(H2O) (6)](3+) with 3 gives Ga[Re6Se8(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 reta in their crystallinity with extended heating at 250 degreesC. Consistent wi th the trend in the frequency of framework vacancies, dinitrogen sorption i sotherms show porosity to increase along the series of representative compo unds 7, Ga-4[Re6Se8(CN)(6)](3). 38H(2)O, and 6. Furthermore, all of these p hases display a significantly higher sorption capacity and surface area tha n 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 empl oyed in preparing compounds 4-6, but carried out at lower pH, produce the i sostructural phases H[cis-M(H2O)(2)][Re6Se8(CN)(6)]. 2H(2)O (M = Fe (8). Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-d imensional framework in which [Re6Se8(CN)(6)](4-) clusters are interlinked through a combination of protons and Fe3+ ions.