Cyano-Bridged Re6Q8(Q = S, Se) Cluster-Cobalt(II) Framework Materials:  Versatile Solid Chemical Sensors

Face-capped octahedral clusters of the type Re 6 Q 8 (CN) 6 4 - (Q = S, Se) are used to space apart partially hydrated Co 2+ 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 Co 2 (H 2 O) 4 Re 6 S 8 (CN) 6 ·10H 2 O ( 1 ), Cs 2 Co(H 2 O) 2 Re 6 S 8 (CN) 6 ·2H 2 O ( 2 ), and Co(H 2 O) 3 4 Co 2 (H 2 O) 4 Re 6 Se 8 (CN) 6 3 ·44H 2 O ( 3 ). The structures of 1 ·2H 2 O and 3 were determined by single-crystal X-ray analysis. The former consists of an expanded Prussian blue type framework with Re 6 S 8 2+ and Co 2 ( μ -OH 2 ) 2 4+ cluster cores occupying alternate metal ion sites, and features cubelike cages enclosing water-filled cavities approximately 258 Å 3 in volume. The latter structure exhibits a network of Co 2+ ions and Co 2 ( μ -OH 2 ) 2 4+ cores connected through Re 6 Se 8 (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 Co 2 (H 2 O) 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 interaction between the water-bridged Co 2+ ions of the dinuclear cluster units. Finally, a simple chemical sensing device employing these solids is described, along with some properties relevant to its function.