Organic amines as templates

Volume 54 Number 80 14 October 2018 Pages 11215–11362

ChemComm Chemical Communications rsc.li/chemcomm

ISSN 1359-7345

COMMUNICATION Guangshan Zhu et al. Organic amines as templates: pore imprints with exactly matching sizes in a series of metal–organic frameworks

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Cite this: Chem. Commun., 2018, 54, 11264

Published on 30 August 2018. Downloaded on 10/31/2018 1:50:53 AM.

Received 5th July 2018, Accepted 24th August 2018

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Organic amines as templates: pore imprints with exactly matching sizes in a series of metal–organic frameworks† Nian Zhao, a Lun Yang, a Bo Xie, a Juanjuan Han,a Qiyun Pan,a Xiang Li,a Meifeng Liu,a Yu Wang,a Xiuzhang Wanga and Guangshan Zhu *b

DOI: 10.1039/c8cc05404e rsc.li/chemcomm

Three kinds of organic amines have been used as soft templates for constructing a series of metal–organic frameworks with variable pores. The pore sizes of these MOFs exactly matched those of amines, confirming the template effect of these amines.

Molecular templating was first proposed in the synthesis of zeolites in the 1970s. After that, it has been widely used to prepare porous materials, from microporous to macroporous, as well as nanomaterials. In general, templates can be divided into two kinds, which are the so-called hard and soft templates.1 Hard templates are always structured porous solids such as mesoporous silica. The precursors merely fill up and react in the pores or cavities present in these templates, and the templates can be removed by calcination or acid/base treatment after synthesis. Another kind of template is the soft template. In this case, the precursors usually interact with templates via weak intermolecular interactions including electrostatic forces, hydrogen bonds and/or van der Waals forces, which can directly arrange the framework precursors into specific building units or packing modes, leading to final different frameworks. More subtle and mutual interactions between the framework precursors and templates can lead to a much better control over the material properties and a variety of structures.2 These are usually observed in the synthesis of zeolites and mesoporous materials. Metal–organic frameworks (MOFs) are a class of crystalline porous materials composed of metal ions or inorganic clusters as nodes and organic ligands as linkers. They have attracted great attention of materials researchers all over the world due to their tunable channels,3 elegant structures,4 varieties of topologies5 a

Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China b Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Synthesis in detail, the crystallographic data for MCIF-2–4, complementary structure figures. CCDC 1851345–1851347. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc05404e

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and great application potential in many fields such as gas sorption and separation,6 catalysis,7 molecular sensors,8 drug delivery9 and so on. The permanent porosity of MOFs is mainly decided by the rigidity of the linkers and the coordination modes of the metal ions. In the most rigid frameworks, the additives usually just play the role of filling agents rather than structure directing agents. However, in other semi-rigid or soft frameworks, the additives indeed acted as templates and could usually induce the formation of different structures with various topologies and pore sizes. Although there are a few reports on using organic solvents or additive templating routes, the use of organic amines as templates in MOF synthesis is relatively rare compared to the synthesis of zeolites. This is possibly caused by the fact that the additional amines might complicate the MOF reaction system and diminish the possibility of obtaining good quality single crystals required for structural analysis.10 In addition, organic amines in cavities or channels are usually highly disordered which cause great trouble in structure determination, let alone host–guest interaction analysis. In 2005, Burrows et al. reported one Zn-BDC MOF [NH2Et2]2[Zn3(m-bdc)4], in which the counter ion [NH2Et2]+ was derived from the hydrolysis of DEF.11 In the year 2009, Su et al. reported two MOFs, [Me2NH2]2[Cd2(bpdc)3] and [Me2NH2]2[Cd2(NH2bdc)3], in which the counter ion [Me2NH2]+ was derived from the hydrolysis of DMF.12 Adding [Me2NH2]Cl under the same reaction conditions with fresh DMF could still give the crystals mentioned above, confirming the templating effect of [Me2NH2]+. To the best of our knowledge, among all the reported MOFs with amines as templates, most of these amines were hydrolysed from alkylamine solvents, which greatly limited the species of amine templates. In the year 2007, Qiu and Zhu et al. reported seven MOFs, JUC-49, -50, -51, -52, -53, -54, and -55, with a variety of channels and topologies that resulted from the use of five kinds of alkylamines as templates.13 These organic amines resided in the interlayer or channel space, which indicates that they played important roles in templating, space filling and charge-balancing. To the best of our knowledge, this should be the only work discussing the organic amine templating effect in detail to date. Herein, we have chosen

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another four new organic amines and successfully synthesized four metal–cyanoimidazole frameworks (MCIFs), namely, [Cu4I4(DCI)(HDEA)] (denoted as MCIF-2) (DCI = 4,5-dicyanoimidazole, DEA = N,N-diethylamine), [Cu4I4(DCI)(HDPPA)] (denoted as MCIF-3) (DPPA = N,N-dipropyl-1-propanamine), and [Cu16I16(DCI)4(HTMHDA)2(H2O)] (denoted as MCIF-5) (TMHDA = N,N,N0 ,N0 -tetramethyl-1,6-hexanediamine) via the amine-templating strategy by adding different amines under the same reaction conditions as MCIF-1, which was reported in our previous work.14 Single crystal analysis suggested that these protonated organic amines were confined in the channel space, playing important roles in templating, space filling and chargebalancing. All these MCIFs were in different space groups and possessed different channel structures. The sizes of the channels exactly matched those of corresponding amines, thus confirming the templating effect of these amines. The MCIFs were typically synthesized as following: CuI and H-DCI were dissolved in MeCN, and then one drop of the corresponding organic amine (DEA for MCIF-2, DPPA for MCIF-3 and TMHDA for MCIF-4) was added into the solution. After the resulting precipitates were dissolved by adding HCOOH, the clear solution was left standing still at room temperature for 3 to 5 days to give colourless crystals of the corresponding MCIFs. The space groups of MCIFs were different from each other: MCIF-2 was in the orthorhombic Pnna space group; MCIF-3 in the monoclinic P21/n space group and MCIF-4 in the orthorhombic Pmmn space group. All the structures of these MCIF materials were analogous. The asymmetric unit of each MCIF material contained one classic Cu4I4 cluster,15 one DCI ligand and one corresponding protonated amine molecule except for MCIF-4, which possessed half HTMHDA and a quarter water molecule as guests. In each MCIF, the connection of tetrahedral Cu4I4 and square DCI generated a (4,4)-connected pts net. Powder X-ray diffraction (PXRD) experiments indicated their phase purities, respectively (Fig. S1–S3, ESI†). Thermal gravimetric analysis (TGA) experiments suggested the relatively poor thermal stabilities of MCIFs and that they had continuous weight losses before 300 1C followed by rapid framework collapse (Fig. S4–S6, ESI†). Pore structure characterization via 77 K N2 sorption experiments failed as the samples of MCIFs were all collapsed after activation, which was confirmed by XRD patterns (Fig. S14, ESI†). The theoretical maximum solvent accessible volumes were 46.9%, 47.5% and 44.8% for MCIF-2, -3 and -4 calculated using the PLATON software package, respectively. In order to investigate the structure differences of these MCIFs, we picked out the Cu4I4 cluster coordinated with four DCI ligand molecules from each MCIF as the research object, respectively. As expected, the conformations of the Cu4I4 cluster and DCI ligands could not be completely overlapped in these MCIFs (Fig. S13, ESI†). On one hand, the N–Cu coordination bonds between the –CN moiety and Cu4I4 cluster showed certain flexibilities, that is the C–N–Cu bond angles differed from each other, which are listed in Fig. 1a. The related bond angles in MCIF-2 were 169.071 and 169.071; in MCIF-3 157.711 and 173.911 and in MCIF-4 176.341 and 176.341, respectively.


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One the other hand, the rotation degrees of the imidazole moieties also differed. For quantitative analysis of these differences, we added a division plane in the Cu4I4 cluster and measured the dihedral angles between the division plane and the imidazole ring which are listed in Fig. 1b. The dihedral angles in MCIFs were 46.201 in MCIF-2, 29.041 in MCIF-3 and 12.841 in MCIF-4, respectively. These derivations of coordination bonds and dihedral angles lead to the difference of distances and dihedral angles between the imidazole moieties which are listed in Fig. 1c. The differences of dihedral angles y, which were 63.861 in MCIF-2, 55.221 in MCIF-3 and 88.981 in MCIF-4, respectively, contributed to the different pore sizes of MCIFs, whose pore sizes were 5.2 Å 7.8 Å in MCIF-2, 5.2 Å 8.6 Å in MCIF-3 and 7.1 Å 8.3 Å in MCIF-4, as shown in Fig. 1d, respectively. There were protonated organic amine molecules confined in the pores of MCIFs which interacted with the host frameworks via weak intermolecular forces, as shown in Fig. 1e. In MCIF-2, the H-DEA molecules interacted with the framework via the C4A–H4A3 I2 halogen bonds (C4A–H4A3 I2: 2.92 Å, +DHA = 1521) (Fig. S7, ESI†). In MCIF-3, the H-DPPA molecules interacted with the framework via C6–H6B I4 halogen bonds (C6–H6B I4: 3.31 Å, +DHA = 130.81), C7–H7B I3 halogen bonds (C7–H7B I3: 3.18 Å, +DHA = 140.71), C12–H12A I4 halogen bonds (C12–H12A I4: 3.19 Å, +DHA = 148.71), C12–H12B I3 halogen bonds (C12–H12B I3: 3.23 Å, +DHA = 113.61), and N5–H5 I3 halogen bonds (N5–H5 I3: 2.92 Å, +DHA = 139.21) (Fig. S9, ESI†). In MCIF-4, the H-TMHDA molecules interacted with the framework via C6–H6 I2 halogen bonds (C6–H6 I2: 3.08 Å, +DHA = 125.61) and C16–H16C I1 halogen bonds (C16–H16C I1: 3.27 Å, +DHA = 138.81). In addition, one water molecule is connected to two TMHDA molecules as a bridge by the O1–H1W N6 hydrogen bonds (O1–H1W N6: 2.17 Å, +DHA = 1671) (Fig. S11, ESI†). To understand the template effect of organic amines, we compared the sizes of the guest amines (Fig. 1f) with those of channels of the corresponding host framework MCIFs. The sizes of H-DEA and pores of MCIF-2 were 6.3 Å 2.7 Å and 5.2 Å 7.8 Å, of H-DPPA and pores of MCIF-3 were 7.4 Å 7.7 Å and 5.2 Å 8.6 Å, and of H-TMHDA and pores of MCIF-4 were 10.5 Å 3.4 Å and 8.3 Å 7.1 Å, respectively. Approximately the same sizes between the organic amines and pores of the MCIFs suggested the templating effect. For further understanding the templating ability of these amines, we calculated the nonbonding interaction energies between the host frameworks and the corresponding amine templates based on the experimental structure data.16 The results are listed as the following: 75.30 kcal mol1, 115.30 kcal mol1 and 7.69 kcal mol1 per unit cell of MCIF-2, MCIF-3 and MCIF-4, respectively. The stronger host–guest interaction could indicate more significant confinement effect of the cavities and greater structuredirecting abilities. In conclusion, we have synthesized three metal–cyanoimidazole frameworks with organic amines as templates. These amines were all trapped in the channels and the similarity between the sizes of the channels and those of the corresponding amines suggested the templating effect. Theoretical calculations evaluated the structuredirecting abilities of the amines in the formation of MOF

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Fig. 1 (a) The C–N–Cu bond angles in MCIFs. (b) The dihedral angles between the imidazole ring and division plane of Cu4I4 cluster. The cyan plane represents the division plane of Cu4I4 and the purple plane represents the imidazole plane. (c) The dihedral angles and distances between imidazole planes. (d) The pore sizes in MCIFs. (e) The host–guest interactions between MCIFs and the corresponding organic amines. (f) The molecule sizes for protonated amines.

structures. Further investigation on the induction mechanism is currently underway. We believe that the results might guide to direct synthesis of novel structures. We are grateful for the financial support from the NSFC (Grant No. 21801072, 20831002, 11704109, 51801059), the National Basic Research Program of China (973 Program, Grant No. 2014CB931804), and the Research Project of Hubei Key Laboratory of Pollutant Analysis & Reuse Technology (Grant No. PA20170203).

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Conflicts of interest There are no conflicts to declare.

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