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. Author manuscript; available in PMC: 2012 Feb 7.
Published in final edited form as: Inorg Chem. 2011 Jan 5;50(3):729–731. doi: 10.1021/ic102436b

Microwave-Assisted Cyanation of an Aryl Bromide Directly on a Metal-Organic Framework

Min Kim 1, Sergio J Garibay 1, Seth M Cohen 1,*
PMCID: PMC3076622  NIHMSID: NIHMS262777  PMID: 21207935

Abstract

A microwave-assisted postsynthetic modification (PSM) reaction on a metal-organic framework (MOF) has been realized. Cyanation of the Zr4+-based UiO-66-Br was achieved with CuCN and microwave irradiation to produce UiO-66-CN. This protocol represents a significant example of PSM modification on an aryl halide MOF producing a cyano-functionalized MOF.

Metal-organic frameworks (MOFs) are porous materials that are constructed from inorganic metal ion clusters and tunable organic linkers.1 An immense range of MOF materials have been developed to date, mostly under solvothermal reaction conditions.2 In the last decade, MOFs have emerged as an exciting area in inorganic and materials chemistry for use in gas adsorption, separation, catalysis, and drug delivery.3

Recently, we4–7 and others8–15 have described postsynthetic modification (PSM) as method for manipulating the organic linkers in MOFs. PSM is versatile with regard to the scope of MOF substrates to which it can be applied, and new physical and chemical properties can be realized in MOFs via this approach.7,16 Despite significant advances, the majority of PSM reactions have been confined to MOFs bearing a limited variety of functional group ‘handles’ including amines, aldehydes, alkynes, and a few others.17,18 For example, amino-containing MOFs have been converted to amides,4 iso(thio)cyanates,6 ureas,19 imines,10,11 and azides.13 Novel PSM reactions, which originate from alternative functional group handles, is of great value for advancing the utility of PSM and creating MOF materials that are otherwise inaccessible by conventional synthetic approaches.

Described herein is the establishment of a new PSM route using aryl bromide-functionalized MOFs. Aryl bromides are well-known for undergoing a wide variety of chemical transformations.20 Aryl bromide-containing MOFs have been known for some time, since the description of isoreticular metal-organic framework-2 (IRMOF-2) in 2002.2 IRMOF-2 is comprised of octahedral Zn4O clusters linked by 2-bromo-1,4-benzenedicarboxylate (Br-BDC) ligands. However, most transformations described with aryl bromides require reaction conditions or reagents (base) that have the possibility for adverse effect with the chemical stability of IRMOFs.21 Therefore, to achieve PSM on MOFs containing the Br-BDC ligand, more stable MOF compositions are required.

Among the more chemically robust MOFs that have been reported, the Zr4+-based MOFs developed by Lillerud (so-called UiO materials) have attracted increasing attention.22 UiOs, based on Zr6O6 clusters and linear dicarboxylate ligands (such as 1,4-benzenedicarboxylate, BDC), exhibit significant chemical and thermal stability, and we recently reported that the synthesis of UiO-66 analogues based on functionalized BDC ligands was quite straightforward.23,24 Furthermore, amino-containing UiO-66-NH2 was readily modified by PSM in a manner similar to that used on other MOF systems.23,24 Herein, we show that the bromo-containing UiO-66-Br can undergo an unparalleled PSM transformation that has not been described for any other MOF, producing a new class of nitrile-bearing crystalline, porous solids.

UiO-66-Br was synthesized by the combination of ZrCl4 with Br-BDC as previously described (Scheme 1).23 The chemical stability of UiO-66-Br is quite similar to those of other UiO-66 derivatives, including a high tolerance to polar solvents like alcohols, DMSO, and even aqueous solutions. In attempt to realize novel PSM reactions, UiO-66-Br was subject to cyanation. The nitrile group is an important precursor for various other functional groups, such as carboxylic acids, aliphatic amines, and heterocyclic compounds.24 Established named reactions, such as the Rosenmund-von Braun reaction25 and Sandmeyer26 reaction, are generally used for preparation of aryl nitriles.

Scheme 1.

Scheme 1

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Synthesis of UiO-66-Br (top) and PSM cyanation to UiO-66-CN (bottom). A representation of the UiO-66 framework is shown (top right) with the Zr6O6 cuboctahedron SBUs highlighted in green.

Our first attempts at PSM on UiO-66-Br utilized CuCN (1.3 equiv) in a N,N-dimethylformamide (DMF) suspension of UiO-66-Br for 24 h at 140 °C (Scheme 1).27 After removal of excess copper (Table S1) by washing with DMF, an aqueous KCN solution, and methanol, the bromo groups of UiO-66-Br were found to be ca. 43% converted to nitrile groups. The percent conversion from bromo to nitrile was determined by 1H NMR upon digestion of the MOF in HF in CD3OD,23 a method now commonly used for characterization of PSM on MOFs. In the 1H NMR spectra, the 2-cyano-1,4-benzenedicarboxylate (CN-BDC) resonances generated from the UiO-66-CN product (Figure 1, Figure S1) are shifted downfield relative to the Br-BDC starting material.

Figure 1.

Figure 1

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1H NMR spectra of UiO-66-CN produced by thermal and microwave procedures (spectra obtained by digestion in HF in CD3OD). Filled circles indicate Br-BDC and open squares indicate CN-BDC. A spectrum of unmodified UiO-66-Br is shown for reference.

Encouraged by these preliminary results, we sought to improve the cyanation PSM reaction conditions. The high chemical and thermal stability of UiO-66-Br led us to attempt microwave-assisted cyanation.28 UiO-66-Br was suspended in N-methyl-2-pyrrolidone (NMP) and subjected to microwave irradiation (170 °C, 10 min) producing UiO-66-CN in ca. 90% yield (as determined by 1H NMR analysis, Figure 1, Figure S2). Electrospray ionization mass spectrometry (ESI-MS) and infrared spectroscopy (ATR-FTIR) also confirmed conversion to the modified CN-BDC ligand (Figures S3, S4). To the best of our knowledge this is the first time microwave irradiation has been utilized in the PSM of a MOF. Importantly, microwave irradiation resulted in a much faster and higher yielding PSM reaction when compared to standard thermal conditions.

Several studies on the microwave-assisted synthesis of MOFs have been described.15,29,30 In light of these reports, we attempted the microwave synthesis of UiO-66-Br and UiO-66-CN from ZrCl4 with Br-BDC or CN-BDC, respectively. Using the same microwave heating conditions used in the PSM cyanation (in either DMF or NMP), neither UiO-66-Br nor UiO-66-CN was produced. These important control reactions demonstrate that the microwave-assisted PSM cyanation is operating heterogeneously on UiO-66-Br and not on soluble species that are assembling in situ to form the MOF. These experiments strongly suggest that the microwave PSM reaction conditions used here are truly a heterogeneous transformation of the UiO-66-Br framework, not involving a series of dissolution, modification, and reformation steps.

Complete characterization on UiO-66-CN confirmed that it is a stable, microporous material. Powder X-ray diffraction (PXRD) shows that the crystallinity of UiO-66-CN is retained (Figure 2). Thermogravimetric analysis (TGA) indicated that UiO-66-CN possesses good thermal stability comparable to the other UiO-66 derivatives (Figure S5). Scanning electron microscopy (SEM) images showed that the crystallites were identical in appearance before and after the PSM reaction (Figure S6). Finally, the surface area of UiO-66-CN was measured by dinitrogen gas sorption (77 K), which gave a Brunauer Emmett Teller (BET) surface area of 661±53 m2/g (three independent samples). This is comparable to that of the parent UiO-66-Br (~851 m2/g),23 indicating that the material remains microporous even after the microwave modification procedure.

Figure 2.

Figure 2

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PXRD patterns of UiO-66-Br and UiO-66-CN. The UiO-66-CN sample was produced under microwave irradiation reaction conditions.

Fundamentally, introduction of labile and reactive functional groups on an organic ligand those are not intended for constructing a framework can complicate the solvothermal synthesis of MOFs. The ability of nitrile groups to bind metal ions, as well as their susceptibility to hydrolysis under forcing conditions, prompted us to explore whether UiO-66-CN could be prepared directly from CN-BDC under solvothermal conditions. The CN-BDC ligand was prepared starting from Br-BDC in three steps (Scheme S1). Use of CN-BDC with ZrCl4 in DMF under standard solvothermal conditions (120 °C, 24 h) for preparing UiO-66 failed to generate UiO-66-CN. After testing a number of reaction conditions (Table S2), UiO-66-CN was eventually obtained directly from CN-BDC by lowering the reaction temperature (85 °C, Figure S7). These control experiments clearly illustrate the utility of PSM in MOF functionalization. Even in a MOF system highly tolerant of ligand derivatization, CN-BDC did not produce UiO-66-CN under the same reaction conditions as used for UiO-66 and many UiO-66 derivatives.23 More importantly, synthesis of UiO-66-CN directly from CN-BDC requires four steps, including the synthesis of the MOF, giving an overall yield of 31% and requiring ~98 h. However, by using PSM, the same MOF can be prepared in only two steps, in ~24 h and in 90% yield. Therefore, it is clear that for UiO-66-CN, PSM is a more efficient and higher-yielding route to obtain the desired MOF.

In conclusion, we have reported the first PSM reaction using an aryl halide chemical handle within a MOF. Furthermore, for the first time, microwave irradiation was used to obtain rapid and clean PSM, which performed better than standard thermal conditions. The use of both aryl halides and microwave irradiation opens up new tools and hence new possibilities for tuning the pore environment in MOFs.

Supplementary Material

1_si_001

Acknowledgment

We thank Dr. Y. Su (U.C.S.D.) for performing the mass spectrometry experiments and C. Allen and K. Tanabe (U.C.S.D.) for obtaining SEM images, and the reviewers of this manuscript for several helpful suggestions. This work was supported by U.C.S.D., the NSF (new MOF synthesis CHE-0952370; instrumentation grants CHE-9709183, CHE-0116662 and CHE-0741968), and the Department of Energy (DE-FG02-08ER46519, MOF modification for gas sorption, PXRD instrumentation). S.J.G. was supported by a supplement to NCI grant 3R01 CA095298-07S1.

Footnotes

Supporting Information Available: Experimental details and Scheme S1, Figures S1–S6. This materials is available free of charge via the Internet at http://pubs.acs.org.

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Supplementary Materials

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