Face-capped octahedral clusters of the type [Re6Q8(CN)6]4- (Q = S, Se) are used to space apart partially hydrated Co2+ ions in extended solid frameworks, creating porous materials that display dramatic color changes upon exposure to certain organic solvents. The clusters react with cobaltous ions in aqueous solution to precipitate the new solid phases [Co2(H2O)4][Re6S8(CN)6]·10H2O (1), Cs2[Co(H2O)2][Re6S8(CN)6]·2H2O (2), and [Co(H2O)3]4[Co2(H2O)4][Re6Se8(CN)6]3·44H2O (3). The structures of 1·2H2O and 3 were determined by single-crystal X-ray analysis. The former consists of an expanded Prussian blue type framework with [Re6S8]2+ and [Co2(μ-OH2)2]4+ cluster cores occupying alternate metal ion sites, and features cubelike cages enclosing water-filled cavities approximately 258 Å3 in volume. The latter structure exhibits a network of Co2+ ions and [Co2(μ-OH2)2]4+ cores connected through [Re6Se8(CN)6]4- clusters, defining an array of one-dimensional channels with minimum internal diameters of 4.8 Å. A Rietveld refinement against X-ray powder diffraction data established compound 2 as isostructural to an analogous Fe-containing phase with a two-dimensional framework reminiscent of the Hoffman clathrates. Thermogravimetric analyses show that all three compounds are fully dehydrated by ca. 100 °C, with no further significant loss of mass below 500 °C. Upon exposure to diethyl ether vapor, the color of compounds 1 and 3 immediately changes from orange to an intense blue-violet or blue; other polar solvents induce somewhat different colors. These (reversible) changes are associated with the emergence of an envelope of new absorption features at wavelengths between 500 and 650 nm, and the magnitude of the response to a solvent can be estimated by measuring the relative intensity of a band with a maximum near 600 nm. We propose that the vapochromic response is due to solvent molecules entering the pores of the solid, where they disrupt the hydrogen-bonded water network, prompting the release of bound water from the [Co2(H2O)4]4+ clusters and conversion of their Co centers from octahedral to tetrahedral coordination. Significantly, this process does not destroy the three-dimensional connectivity in either structure, but rather creates a much more flexible framework that can expand to accommodate the incoming solvent molecules. Spectroscopic and magnetic data confirm the change in coordination geometry, and the trends in solvent responses (e.g., methanol < ethanol < n-propanol < i-propanol) are consistent with a decreased ability to support the bridging water ligands of the clusters as steric bulk increases. Size-selective sensing is demonstrated with methyl tert-butyl ether, which causes a color change in compound 3, but not in compound 1. X-ray powder diffraction experiments indicate that the vapochromic response in both compounds is affiliated with a reversible change in the bulk crystal structure of the material. Variable-temperature magnetic susceptibility data for compound 1 suggest a weak antiferromagnetic coupling int...
Soluble methane monooxygenase (sMMO) isolated from Methylococcus capsulatus (Bath) utilizes a carboxylate-bridged diiron center and dioxygen to catalyze the conversion of methane to methanol. Previous studies revealed that a di(mu-oxo)diiron(IV) intermediate termed Q is responsible for the catalytic activity with hydrocarbons. In addition, the peroxodiiron(III) intermediate (H(peroxo)) that precedes Q formation in the catalytic cycle has been demonstrated to react with propylene, but its reactivity has not been extensively investigated. Given the burgeoning interest in the existence of multiple oxidants in metalloenzymes, a more exhaustive study of the reactivity of H(peroxo) was undertaken. The kinetics of single turnover reactions of the two intermediates with ethyl vinyl ether and diethyl ether were monitored by single- and double-mixing stopped-flow optical spectroscopy. For both substrates, the rate constants for reaction with H(peroxo) are greater than those for Q. An analytical model for explaining the transient kinetics is described and used successfully to fit the observed data. Activation parameters were determined through temperature-dependent studies, and the kinetic isotope effects for the reactions with diethyl ether were measured. The rate constants indicate that H(peroxo) is a more electrophilic oxidant than Q. We propose that H(peroxo) reacts via two-electron transfer mechanisms, and that Q reacts by single-electron transfer steps.
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.
A method for expanding the pores in crystalline frameworks is demonstrated with the substitution of face-capped octahedral [Re 6 Q 8 (CN) 6 ] 4-(Q ) Se, Te) clusters into the Prussian blue structure. The clusters react in aqueous solution with Ga 3+ and Fe 3+ , respectively, to precipitate Ga 4 [Re 6 Se 8 (CN) 6 ] 3 ‚xH 2 O and Fe 4 [Re 6 -Te 8 (CN) 6 ] 3 ‚xH 2 O. Rietveld analysis of X-ray powder diffraction data for the former compound confirms the expanded crystal structure, which features large water-filled cavities more than twice the volume of those in Prussian blue. The new materials can be dehydrated without loss of integrity, and maintain crystallinity at temperatures up to 250 or 300°C. Further, the increase in void volume is shown to significantly enhance their capacities as molecular sieves and enable absorption of larger alcohol molecules such as ethanol and n-propanol. A soluble form of the black iron-containing phase exhibits a cluster-to-metal charge-transfer band at 736 nm, slightly lower energy than the metal-to-metal charge-transfer band responsible for the color of Prussian blue.
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