Pore Surface Engineering with Controlled Loadings of Functional Groups via Click Chemistry in Highly Stable Metal-Organic Frameworks Hai-Long Jiang, Dawei Feng, Tian-Fu Liu, Jian-Rong Li, and Hong-Cai Zhou* Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States * S Supporting Information ABSTRACT: Reactions of ZrCl 4 and single or mixed linear dicarboxylic acids bearing methyl or azide groups lead to highly stable isoreticular metal-organic frame- works (MOFs) with content-tunable, accessible, reactive azide groups inside the large pores. These Zr-based MOFs offer an ideal platform for pore surface engineering by anchoring various functional groups with controlled loadings onto the pore walls via the click reaction, endowing the MOFs with tailor-made interfaces. Signifi- cantly, the framework and crystallinity of the function- alized MOFs are well-retained, and the engineered pore surfaces have been demonstrated to be readily accessible, thus providing more opportunities for powerful and broad applications of MOFs. A s highly ordered porous materials, metal-organic frame- works (MOFs) have attracted great interest in the last 20 years because of their crystalline nature, pore tunability, and structural diversity as well as numerous potential applications such as gas storage/separation, catalysis, sensing, and drug delivery. 1-5 The permanent porosity and chemical environment of the internal surfaces of MOFs are crucial for such applications. However, modification of the pore surfaces with desired functional groups in well-defined MOFs remains a significant challenge. Currently, the modification of MOF pore surfaces is mostly based on the use of predesigned ligands with specific functional groups. 6 This approach is somewhat limited because of the cumbersome multistep process of ligand synthesis and the often unpredictable coordination between reactive functional groups (e.g., -OH, -COOH, N-donating groups, etc.) and metal centers during the MOF assembly process. Moreover, it is generally difficult to obtain MOFs with long and/or large groups appended on the pore walls by direct solvothermal reactions. Therefore, the development of a general strategy for systematic pore surface engineering of MOF pore walls is imperative, as it would endow MOFs with tailor-made internal surfaces to meet specific application requirements. Postsynthetic modification (PSM) represents a powerful tool for anchoring functional groups onto MOFs. For instance, Cohen and co-workers employed MOFs bearing -NH 2 groups as platforms to graft various functional groups such as aldehydes, isocyanates, and anhydrides. 7 Recently, Sharpless click chemistry has also been demonstrated to be an alternative route for enriching the chemical diversity of MOFs. 8-10 Sada and co- workers employed a Zn-based MOF bearing azide groups for PSM through click chemistry, although the MOF dissolved upon soaking in a solution with “reactive” molecules, such as reactants bearing an amine or carboxylic acid group, significantly limiting the utility of the approach. 9 The Hupp and Nguyen groups and Farrusseng and co-workers also applied click reactions in PSM of MOFs, where very careful deprotection of an acetylene group or transformation of an amine to an azide group was a necessary step prior to the click reaction. 10a-e Multiple steps in PSM often cause partial or complete framework collapse, especially when the MOF is not robust. In addition, grafting functional groups with controlled loadings in MOFs has not been achieved to date. Herein we report the preparation of highly stable isoreticular Zr- based MOFs with accessible, reactive azide groups in large pores that enable the MOFs to undergo a quantitative click reaction with alkynes to form triazole-linked groups on the pore wall surfaces. Significantly, our synthetic route allows accurate control of the loading of azide groups on the internal surface of the MOF for the first time. The highly stable Zr-based MOFs offer an ideal platform for pore surface engineering. A variety of functional groups can be anchored onto the pore walls of the MOFs with precise control over the loading, density, and functionality. To design an azide-appended MOF material with large enough cavities whose openings can be fully accessed by various molecules with an alkyne group for the click reaction, we designed four elongated linear dicarboxylic acids with three benzene rings in each, 2′,5′-dimethylterphenyl-4,4″-dicarboxylic acid (TPDC-2CH 3 ) and 2′,3′,5′,6′-tetramethylterphenyl-4,4″- dicarboxylic acid (TPDC-4CH 3 ), and their corresponding azide derivatives (TPDC-2CH 2 N 3 and TPDC-4CH 2 N 3 ) as organic linkers [section 2 in the Supporting Information (SI)]. To ensure structural integrity during the click reaction, we aimed to construct Zr-based MOFs, which are well-known for their superior stability compared with common Zn/Cu-centered MOFs. 11 It is especially difficult to obtain single crystals of Zr- based MOFs because the inert coordination bonds between Zr 4+ cations and carboxylate anions make ligand exchange reactions extremely slow, which is unfavorable for defect repair during crystal growth. 11 To overcome this difficulty, a modulated synthetic strategy was adopted, and benzoic acid was introduced into the synthetic system. 11c To our delight, octahedron-shaped crystals suitable for single-crystal X-ray diffraction (XRD) were obtained from a reaction mixture containing zirconium(IV) chloride, the elongated organic linkers, benzoic acid, and N,N- Received: July 1, 2012 Published: August 20, 2012 Communication pubs.acs.org/JACS © 2012 American Chemical Society 14690 dx.doi.org/10.1021/ja3063919 | J. Am. Chem. Soc. 2012, 134, 14690-14693