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Article

Nanoporous Carbohydrate Metal–Organic Frameworks

Ross S. Forgan†, Ronald A. Smaldone†, Jeremiah J. Gassensmith†, Hiroyasu Furukawa

, David B. Cordes

, Qiaowei Li

, Christopher E. Wilmer§, Youssry Y. Botros†‡#

, Randall Q. Snurr§, Alexandra M. Z. Slawin

, and J. Fraser Stoddart*†

^†Center for the Chemistry of Integrated Systems, Department of Chemistry, ^‡Department of Materials Science, and ^§Department of Chemical & Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3133, United States

The Center for Reticular Chemistry, Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, California 90095-1569, United States

School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St Andrews, Fife, Scotland, KY16 9ST, United Kingdom

^# Intel Laboratories, Building RNB-6-61, 2200 Mission College Boulevard, Santa Clara, California 95054-1549, United States

National Center for Nano Technology Research, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia

NanoCentury KAIST Institute and Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong Dong, Yuseong Gu, Daejeon 305-701, Republic of Korea

J. Am. Chem. Soc., 2012, 134 (1), pp 406–417

DOI: 10.1021/ja208224f

Publication Date (Web): November 17, 2011

stoddart@northwestern.edu

Section:

Inorganic Chemicals and Reactions

Abstract

The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host–guest supramolecular chemistry in the 1970s and 1980s. The arrangement of −OCCO– and −OCCN– chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this −OCCO– bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal–organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbOH, respectively, form body-centered cubic arrangements of (γ-CD)₆ cubes linked by eight-coordinate alkali metal cations. These cubic CD-MOFs are (i) stable to the removal of solvents, (ii) permanently porous, with surface areas of 1200 m² g^–1, and (iii) capable of storing gases and small molecules within their pores. The fact that the −OCCO– moieties of γ-CD are not prearranged in a manner conducive to encapsulating single metal cations has led to our isolating other infinite frameworks, with different topologies, from salts of Na⁺, Cs⁺, and Sr²⁺. This lack of preorganization is expressed emphatically in the case of Cs⁺, where two polymorphs assemble under identical conditions. CD-MOF-3 has the cubic topology observed for CD-MOFs 1 and 2, while CD-MOF-4 displays a channel structure wherein γ-CD tori are perfectly stacked in one dimension in a manner reminiscent of the structures of some γ-CD solvates, but with added crystal stability imparted by metal–ion coordination. These new MOFs demonstrate that the CDs can indeed function as ligands for alkali and alkaline earth metal cations in a manner similar to that found with crown ethers. These inexpensive, green, nanoporous materials exhibit absorption properties which make them realistic candidates for commercial development, not least of all because edible derivatives, fit for human consumption, can be prepared entirely from food-grade ingredients.