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Porous
Magnetic Materials |
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Our group has had a long standing interest in the synthesis and properties of microporous magnets. The reduced number of bonds per unit volume associated with a microporous solid is at direct odds with the correlation between the number of exchange pathways and the ordering temperature of a magnet. Consequently, the synthesis of a microporous solid that behaves as a magnet at room temperature remains an open challenge. Such a material could potentially be used to perform magnetic separations, in which the strong magnetic field within the pores of the solid will selectively attract paramagnetic molecules. Ensuing applications might include the extraction of dioxygen from air through a non-cryogenic process. Due to their potential for both high surface area1 and high-temperature magnetic ordering2, work in our group has focused on Prussian blue analogues. Recently, we have synthesized two new microporous magnets, CsNi[Cr(CN)6] and Cr3[Cr(CN)6]2·6H2O, with surface areas of 370 m2/g and 390 m2/g, respectively. These two frameworks magnetically order at 65 K and 220 K, respectively, the highest ordering temperatures yet observed for a microporous magnet.3 These high-temperature microporous magnets are being used to investigate the effects of magnetic field on the adsorption of paramagnetic molecules. Below the ordering temperature of a microporous magnet, the enthalpy of adsorption of a paramagnetic molecule should increase due the alignment of its magnetic moment with the magnetic field of the material. This increase in adsorption enthalpy is being studied using gas adsorption calorimetry and SQUID magnetometry. Work is also underway to synthesize new magnetic frameworks with high ordering temperature and larger magnetic moment. |
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Structure and magnetization as a function of temperature for Cr3[Cr(CN)6]2·6H2O. Green, red, blue, and grey atoms represent chromium(III), chromium(II), nitrogen, and carbon, respectively. Coordinated H2O molecules have been omitted for clarity. |
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References
(1) Kaye, S. S.; Long, J. R. J. Am. Chem. Soc. 2005,
127, 6506. (2) (a) Gadet,
V.; Mallah, T.; Castro, I. ; Verdaguer,
M.; Veillet, P. J. Am. Chem. Soc. 1992, 114, 9213. (b) Mallah, T.; Thiebaut,
S.; Verdaguer, M.; Veillet,
P. Science, 1993, 262, 1554. (c) Holmes,
S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999,
121, 5593. (3) Beauvais,
L. G.; Long, J. R. J. Am.
Chem. Soc. 2002,
124, 12096-12097 and references therein. |
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