Kristine K. Tanabe

Systematic Functionalization of a Metal−Organic Framework via a Postsynthetic Modification Approach

The pendant amino groups in isoreticular metal-organic framework-3 (IRMOF-3) were subjected to postsynthetic modification with 10 linear alkyl anhydrides (O(CO(CH2)nCH3)2 (where n = 1 to 18) and the extent of conversion, thermal and structural stability, and Brunauer-Emmett-Teller (BET) surface areas of the resulting materials were probed. (1)H NMR of digested samples showed that longer alkyl chain anhydrides resulted in lower conversions of IRMOF-3 to the corresponding amide framework (designated as IRMOF-3-AM2 to IRMOF-3-AM19). Percent conversions ranged from essentially quantitative (approximately 99%, -AM2) to approximately 7% (-AM19) with IRMOF-3 samples. Modified samples were thermally stable up to approximately 430 degrees C and remained crystalline based on powder X-ray diffraction (PXRD) measurements. Under specific reaction conditions, significant conversions were obtained with complete retention of crystallinity, as verified by single-crystal X-ray diffraction experiments. Single crystals of modified IRMOF-3 samples all showed that the F-centered cubic framework was preserved. All single crystals used for X-ray diffraction were analyzed by electrospray ionization mass spectrometry (ESI-MS) to confirm that these frameworks contained the modified 1,4-benzenedicarboxylate ligand. Single crystals of each modified IRMOF-3 were further characterized by measuring the dinitrogen gas sorption of each framework to determine the effects of modification on the porosity of the MOF. BET surface areas (m(2)/g) confirmed that all modified IRMOF-3 samples maintained microporosity regardless of the extent of modification. The surface area of modified MOFs was found to correlate to the size and number of substituents added to the framework.

Postsynthetic Modification: A Versatile Approach Toward Multifunctional Metal-Organic Frameworks

An isoreticular metal-organic framework (IRMOF-3) containing 2-amino-1,4-benzenedicarboxylic acid (NH(2)-BDC) as a building block is shown to undergo chemical modification with a diverse series of anhydrides and isocyanates. The modification of IRMOF-3 by these reagents has been evidenced by using a variety of methods, including NMR and electrospray ionization mass spectrometry, and the structural integrity of the modified MOFs has been confirmed by thermogravimetric analysis, powder X-ray diffraction, and gas sorption analysis. The results show that a variety of functional groups can be introduced onto the MOF including amines, carboxylic acids, and chiral groups. Furthermore, it is shown that tert-butyl-based asymmetric anhydrides can be used to selectively deliver chemical payloads to the IRMOF. Finally, the results demonstrate that at least four different chemical modifications can be performed on IRMOF-3 and that the reaction conditions can be modulated to control the relative abundance of each group. The findings presented here demonstrate several important features of postsynthetic modification on IRMOF-3, including (1) facile introduction of a wide range of functional groups using simple reagents (e.g., anhydrides and isocyanates), (2) the introduction of multiple (as many as four different) substituents into the MOF lattice, and (3) control over reaction conditions to preserve the crystallinity and microporosity of the resultant MOFs. The findings clearly illustrate that postsynthetic modification represents a powerful means to access new MOF compounds with unprecedented chemical complexity, which may serve as the basis of multifunctional materials.

Tuning hydrogen sorption properties of metal-organic frameworks by postsynthetic covalent modification.

Postsynthetic modification is presented as a means to tune the hydrogen adsorption properties of a series of metal-organic frameworks (MOFs). IRMOF-3 (isoreticular metal-organic framework), UMCM-1-NH(2) (University of Michigan crystalline material), and DMOF-1-NH(2) (DABCO metal-organic framework) have been covalently modified with a series of anhydrides or isocyanates and the hydrogen sorption properties have been studied. Both the storage capacities and isosteric heats of adsorption clearly show that covalent postsynthetic modification can significantly enhance the sorption affinity of MOFs with hydrogen and in some cases increase both gravimetric and volumetric uptake of the gas as much as 40 %. The significance of the present study is illustrated by: 1) the nature of the substituents introduced by postsynthetic modification result in different effects on the binding of hydrogen; 2) the covalent postsynthetic modification approach allows for systematic modulation of hydrogen sorption properties; and 3) the ease of postsynthetic modification of MOFs allows a direct evaluation of the interplay between MOF structure, hydrogen uptake, and heat of adsorption. The findings presented herein show that postsynthetic modification is a powerful method to manipulate and better understand the gas sorption properties of MOFs.

Modular, Active, and Robust Lewis Acid Catalysts Supported on a Metal−Organic Framework

Metal-organic frameworks (MOFs) have shown promise as heterogeneous catalysts because of their high crystallinity, uniform pores, and ability to be chemically and physically tuned for specific chemical transformations. One of the challenges with MOF-based catalysis is few systems achieve all of the desired features for a heterogeneous catalyst, including high activity, robustness (recyclability), and excellent selectivity. Herein, postsynthetic modification (PSM) of a MOF is used to synthesize a series of MOF catalysts that are highly robust and active for epoxide ring-opening reactions. In the following study, four metalated MOFs (UMCM-1-AMInpz, UMCM-1-AMInsal, UMCM-1-AMFesal, and UMCM-1-AMCupz) are examined as catalysts for beta-azido and beta-amino alcohol synthesis with epoxides of varying sizes and shapes using two different nucleophiles (TMSN(3) and aniline). The four MOFs are isostructural, exhibit good thermal and structural stability, and display different catalytic activities based on the combination of metal ion and chelating ligand immobilized within the framework. In particular, UMCM-1-AMInpz and UMCM-1-AMInsal act as robust, single-site catalysts with distinct selectivity for ring-opening reactions with specific nucleophiles. More importantly, one of these catalysts, UMCM-1-AMInpz, selectively promotes the ring-opening of cis-stilbene oxide in the presence of trans-stilbene oxide, which cannot be achieved with a comparable molecular Lewis acid catalyst. The results show that PSM is a promising, modular, and highly tunable approach for the discovery of robust, active, and selective MOF catalysts that combine the best aspects of homogeneous and heterogeneous systems.

Postsynthetic diazeniumdiolate formation and NO release from MOFs

Two different metal–organic frameworks (MOFs) have been modified with nitric oxide (NO) through covalent postsynthetic modification (PSM) to form diazeniumdiolate-functionalized and releasing MOFs.