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Aug 8, 2018 - Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar. # ... Metal−organic frameworks (MOFs) are an emer...
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Expose Equatorial Positions of Metal Centers via Sequential Ligand Elimination and Installation in MOFs Shuai Yuan, Peng Zhang, Liangliang Zhang, Angel T Garcia-Esparza, Dimosthenis Sokaras, Junsheng Qin, Liang Feng, Gregory S. Day, Wenmiao Chen, Hannah F. Drake, Palani Elumalai, Sherzod Madrahimov, Daofeng Sun, and Hong-Cai Zhou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04886 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Journal of the American Chemical Society

Expose Equatorial Positions of Metal Centers via Sequential Ligand Elimination and Installation in MOFs Shuai Yuan,1‡ Peng Zhang,1‡ Liangliang Zhang,2 Angel T. Garcia-Esparza3,4, Dimosthenis Sokaras3, Jun-Sheng Qin,1 Liang Feng,1 Gregory S. Day,1 Wenmiao Chen,1 Hannah F. Drake,1 Palani Elumalai,5 Sherzod Madrahimov,5 Daofeng Sun,2* and Hong-Cai Zhou1,6* 1

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States

2

College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China

3

Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States 4 5

Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States

Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar

6

Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United States

ABSTRACT: Metal-organic frameworks (MOFs) provide highly designable platforms to construct complex coordination architectures for targeted applications. Herein, we demonstrate that trans-coordinated metal centers with exposed equatorial positions can be placed in a MOF matrix. A Zr-based MOF, namely PCN-160, was initially synthesized as a scaffold structure. Postsynthetic linker labilization was subsequently implemented to partially remove the original dicarboxylate linkers and incorporate pyridinecarboxylates. A pair of neighboring pyridyl groups was arranged at proper proximity within the framework 2+ 2+ 2+ 2+ 2+ 2+ to form trans-binding sites that accommodate different metal cations including Mn , Fe , Co , Ni , Cu , and Pd . 2+ Furthermore, the trans-coordinated Ni sites in porous frameworks can be readily accessed by substrates along the equatorial plane, facilitating the catalysis as manifested by the superior activity in ethylene dimerization than that observed for a cischelated catalyst.

INTRODUCTION Metal-organic frameworks (MOFs) are an emerging class of crystalline porous materials constructed from metal1 containing nodes and organic linkers. The structures and structure-related properties of MOFs can be precisely controlled at atomic precision by judicious design of both 2 constituents. Due to their structural and functional tunability as well as their ever-expanding application scope, MOFs have become one of the most fascinating classes of 3 materials for both scientists and engineers. The ability to control the framework structures in three dimensions has led to fascinating supramolecular assemblies with unique properties. For example, MOFs constructed with flexible 4 5 6 frameworks, exposed metal sites, and synergistic moieties 7 exhibited unique cooperative gas adsorption behaviors. Despite numerous advantages, the rational design and synthesis of targeted MOF structures are ultimately limited by the lack of control over the framework assembly in a conventional “one-pot” reaction. To overcome the inherent limitations of the “one-pot” strategy, postsynthetic 8 modification (PSM) methods were developed. Functional groups and metal complexes have been postsynthetically anchored on the organic linkers through covalent or

coordinative bonds, giving rise to topologically identical 9 MOFs with diverse functionalities. Furthermore, metal cations and organic linkers can be exchanged by the virtue of the framework dynamics. This allows the formation of metastable MOFs that cannot be synthesized in a “one-pot” 10 reaction. Recent developments of stable MOFs bring new opportunities to perform a wider range of PSM reactions 11 while maintaining the framework’s structural integrity. For example, as stable and versatile platforms, Zr-MOFs allow 12 the incorporation of terminal ligands, dicarboxylate 13 14 linkers, metal cations, and metal-organic complexes onto the coordinatively unsaturated Zr6 clusters. Most recently, our group proposed the concept of “MOF retrosynthesis” to construct complex MOFs by “layer-on” molecular elaborations to preformed Zr-MOFs using a series of 6b postsynthetic strategies. Along this line, we seek to use MOFs as the matrix to assemble coordination structures that are difficult to synthesize in other systems, such as the transcoordinated metal complexes. Trans-chelating or trans-spanning ligands are bidentate ligands that bind the opposite sites of a complex with square15 planar geometry at or near 180° angle. Although a far larger number and variety of cis-chelating ligands than transchelating ones have been developed for catalysis and their

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chemistry thoroughly explored, catalysts containing transchelating ligands may display alternative chemistry, activity and selectivity in various catalytic processes. However, many attempts to generate trans-chelating complexes have led to the formation of coordination polymers in which bidentate ligands act as bridging linkers. Although this problem can be 16 addressed by tuning the ligand rigidity and bulkiness, ligand synthesis remains a challenge. Herein, we show that the trans-coordinated metal centers with exposed equatorial positions can be created in three-dimensions using MOFs as the matrix.

into 4-amino benzoic acid and 4-formylbenzoic acid through hydrolysis by acetic acid/N,N-dimethylformamide (DMF) solution to create missing-linker defects (Figure 1e). Since isonicotinic acid (pKa = 4.96) exhibits similar acidity to acetic acid (pKa = 4.76), we hypothesized that it would replace the CBAB linker and form trans-binding sites (Figure 1f). In fact, metalloligands with similar trans-coordinated 19 geometries have been adopted in MOF synthesis. Therefore, we expect that the proposed structure is feasible.

RESULTS AND DISCUSSION

Figure 1. Structure of PCN-160 (a) and PCN-160-R%M with trans-chelating ligands (b). Transformation of ligand fragment in PCN-160 (c) by CBAB exchange (d), linker labialization (e), and installation of M-INA2 (f). These figures are based on respective single crystal structures after removal of the disordered fragments. Creating Trans-Coordination Sites in MOFs. In the literature, trans-chelating bis-pyridyl ligands are usually constructed by connecting two 2-pyridyl groups by judiciously designed spacers such as 1,2-dialkynylbenzene 16a, 16c (Figure S1a). We hypothesized that MOFs can act as three-dimensional spacers for the alignment of two pyridyl moieties in the trans-position (Figure S1b). For this purpose, we used a previously reported Zr-based MOF, Zr6-AZDC or 17 PCN-160, as the matrix. It is noteworthy that Zr-AZDC based MOFs were firstly reported in 2012 and reproduced 17 using different synthetic methods in many other literatures. By replacing the azobenzene-4,4′-dicarboxylate linker (AZDC) with a pair of 4-pyridinecarboxylates (also known as isonicotinate or INA), a bis-pyridyl sites at trans-position will be formed (Figures 1a, b). Direct exchange of AZDC by INA or M-INA2 was unsuccessful because of the inertness of the AZDC linker. Our previous works have shown that AZDC can be labilized by the exchange of an imine-based linker with identical length, denoted as CBAB (4-carboxybenzylidene-418 aminobenzate, Figures 1c, d). The CBAB can be dissociated

Figure 2. (a) Exchange process of CBAB by Ni-INA2 monitored by UV-vis. (b) PXRD patterns of PCN-160 and PCN-160R%Ni. (c) N2 adsorption-desorption isotherms of PCN-160 and PCN-160-R%Ni at 77 K, 1 bar. Monitoring Sequential Ligand Elimination and Installation. As anticipated, Ni-INA2 moieties were successfully incorporated into PCN-160 by CBAB exchange and subsequent treatment with a solution of INA and NiCl2. The final product was verified by single crystal X-ray crystallography (Figure 1f, Table S1), powder X-ray diffraction (PXRD), inductively coupled plasma mass spectrometry 1 (ICP-MS), and H-NMR digestion experiments (Table S3). The effect of CBAB linker and reaction time on the level of incorporation of M-INA2 to PCN-160 structure was systematically studied. For this purpose, first, a series of PCN-160 samples with variable CBAB linker content were synthesized through a linker exchange procedure reported in literature (Figures S2-4). These samples are denoted as PCN160-R%, where R% stands for the percentage of the CBAB

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Journal of the American Chemical Society linker to the total amount of linkers in the sample (Table S2). Crystals of PCN-160-R% were subsequently incubated in DMF solution with an excess amount of INA and NiCl2 at 75 °C. Incorporation of the INA linker to PCN-160 structure was monitored by ultraviolet–visible (UV-vis) spectroscopy as both AZDC and INA linkers possess distinctive UV-vis absorption peaks at 432 nm and 264 nm respectively. The NiINA2 exchange ratio in PCN-160, defined as the amount of Ni-INA2 divided by the total amount of all the linkers (CBAB+AZDC+Ni-INA2), was calculated based on the UV-vis data of the digested MOFs. As shown in Figure 2a, the NiINA2 content in the samples increased with time and leveled off after 4 h, indicating that a dynamic equilibrium reached between the MOF scaffold and the solution. The CBAB ligands are important for the incorporation of Ni-INA2 as manifested by the low exchange ratio and slow reaction kinetics of PCN-160. After the treatment by INA and NiCl2, the CBAB linker was completely replaced by Ni-INA2 1 according to UV-vis and H-NMR data. The maximum NiINA2 exchange ratios were roughly equal to the CBAB content in the initial sample (Table S3). The size and morphology of the MOF particles were maintained throughout the postsynthetic modification, suggesting a single crystal to single crystal transformation process. It is important to note that the direct synthesis of MOFs using CBAB (PCN-160-R%) or Ni-INA2 (PCN-160-R%Ni) as starting materials was not successful due to the labile imine bonds or Ni-pyridyl bonds that tend to be destroyed by the harsh solvothermal synthesis necessary for Zr-MOF synthesis (Figure S5). As revealed by the powder X-ray diffraction (PXRD) patterns (Figure 2b), PCN-160-R%Ni samples maintained crystallinity. They showed similar PXRD patterns to that of the parent PCN-160 with slight changes of the peak intensities and some shifting of the peak positions. The peak intensity at 6.0 degrees decreased while the peaks at 10.4 degrees increased with the incorporation of Ni-INA2 which matched well with simulations. The peaks of PCN-160-R%Ni were slightly shifted to a lower angle as compared to that of the parent PCN-160 (Figure S6), corresponding to a unit cell expansion which resulted from the replacement of the shorter AZDC linkers (13.1 Å) with longer Ni-INA2 linkers (13.7 Å). After the incorporation of the Ni-INA2 linkers, the PCN-160-R%Ni were still porous, although the total N2 uptake and BET surface area decreased (Figure 2c). This is attributed to the increased material density and decreased pore size after the Ni-INA2 ligand incorporation. In addition, the partial decomposition of the framework after solvent removal also accounts for the decreased surface area. Indeed, PCN-160-R%Ni samples showed decreased mechanical stability due to the relatively weak Ni-pyridyl bonds. Versatility of M-INA2. The method for incorporating NiINA2 linkers to PCN-160 structure can also be used to 2+ 2+ incorporate a series of M transition metals including Mn , 2+ 2+ 2+ 2+ 2+ Fe , Co , Ni , Cu , and Pd into the structure of a MOF. The successful incorporation of different metals into the framework can be directly observed by microscope images of the respective single crystals (Figure 3). The compositions of PCN-160-65%M (M = Mn, Fe, Co, Ni, Cu, and Pd) were 1 further determined by ICP-MS and H-NMR. Although the amount of incorporated M-INA2 varied for different samples, the metal to INA ligand ratio was approximately 1:2. (Table

S4). The PXRD patterns and N2 sorption isotherms further verified the maintained crystallinity and porosity. (Figures S7-8). Although different metal sizes can affect the length of M-INA2 linkers, the framework of PCN-160 possesses certain flexibility that can tolerate the change caused by different metals. X-ray photoelectron spectroscopy (XPS) showed that all the metals incorporated into MOFs are in +2 oxidation states (Figures S9-13).

Figure 3. Photos and microscope images of PCN-160 (a), PCN-160-65% (b), and PCN-160-65%M (M = Mn, Fe, Co, Ni, Cu, and Pd). Coordination Environment of Metal Sites. Single crystal structures of PCN-160-47%Ni and PCN-160-47%Pd were successfully obtained, providing direct structural evidence of the M-INA2 moieties in a trans-geometry (Table S1). The existence of Ni and Pd was clearly refined with an occupancy of 50%, consistent with the ICP-MS results. However, the terminal ligands cannot be precisely refined because of the disorder within the structure. To gain a better understanding of the structure of the M-INA2 moieties, single crystals of molecular M-INA2 complexes were isolated by slow cooling of saturated solutions containing the corresponding MCl2 salts and INA ligands. Both Ni-INA2 and Cu-INA2 show a six-coordinate octahedral metal center surrounded by two INA ligands, two Cl ions, and two DMSO molecules in a trans orientation (Figure S14). By adding 30% water to precipitate the Ni-INA2 and Cu-INA2 crystals, the Cl , and DMSO ligands could be replaced by OH and H2O respectively. The identity of the coordinated solvent on the metal center was further confirmed by thermogravimetric analysis/mass spectrometry (TGA-MS) plots, which revealed the loss of coordinated solvent molecules before 250 °C (Figure S15). The coordination environment of Ni in PCN160-47%Ni was further studied by extended X-ray absorption fine structure (EXAFS) at the Ni K-edge. EXAFS fitting confirms the presence of four direct O/N ligands and two Cl in the PCN-160-47%Ni MOF sample (Table S5 and Figure S16a). The EXAFS technique does not have the sensitivity to separate N from O, hence both ligands are treated with the same single scattering path. For comparison, a complementary fitting was performed on PCN-160-47%Ni assuming only six O/N ligands without Cl-coordination;

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however, the unsuitability of that model was clear (Figure S16b and Table S6). Furthermore, the pre-edge of the X-ray absorption near edge structure (XANES) spectra also indicates an octahedral local geometry (see comparison of the reference standards NiO and NiCl2∙6H2O, with PCN-16047%Ni at Figure S18). The single crystal structures, TGA-MS, XANES, and EXAFS manifest that the Ni center is six coordinated to INA ligands, two Cl ions, and two solvent molecules in octahedral geometry. In contrast to the firstrow transition metals, Pd was determined to be four coordinated with two INA and two Cl ligands, leading to a square planar geometry. Strictly speaking, only PCN-160R%Pd can be named as a trans-chelating complex, because the trans-chelating ligands are defined as bidentate ligands that span the trans-positions of square-planar complexes. Very recently, the Rosi group also reported Zr-MOFs built from Cu-(2-methyl-INA)2 complex as a supramolecular linear 19b dicarboxylate linker. This work is in line with our hypothesis that M-INA2 can act as a linker fragment to support the MOF structure. Compared with our stepwise synthetic method, the one-pot strategy by Rosi group allows facile preparation of heterometallic MOFs with various structures. However, these structures are supported by MINA2 ligands exclusively, resulting in relatively labile frameworks. PCN-160-R%M contains both stable AZDC linker to form a robust matrix and M-INA2 ligands as catalytic sites, which is more suitable for catalysis. Since the stability and porosity of PCN-160-R%Ni decrease as R% escalates, the exchange ratio was controlled up to 47% for catalytic applications. Ethylene Dimerization Reaction. With a series of transcoordinated metal complexes with exposed equatorial positions incorporated into MOFs, we further explored their catalytic properties. The Ni catalyzed ethylene dimerization reaction was selected as a model reaction in this study. The ethylene dimerization reactions were carried out in a 25-mL stainless steel reactor under 40 bar of ethylene in toluene at 25 °C with diethylaluminum chloride (Et2AlCl) as the activator (Table 1, Table S7, and Figure S19). Both PCN-160 and the activator Et2AlCl showed no catalytic activity for ethylene dimerization as controls, thus ruling out the 2+ possibility of any background reaction in the absence of Ni 2+ (entries 1 and 2). To eliminate the influence of Ni trapped in the MOF cavity, PCN-160 was incubated in NiCl2 solution without INA ligands (NiCl2@PCN-160). It showed very low catalytic activity (entry 3). The molecular Ni-INA2 and (bpy)NiCl2 complexes displayed poor catalytic activity, due to the low solubility and limited surface area which prevent the exposure of active sites (entries 4 and 5). For comparison, the catalytic performance of UiO-67-50%Ni with cis-chelating 20 Ni-2,2'-bipyridyl centers was also tested. Notably, we found -1 that PCN-160-47%Ni has an intrinsic activity of 3360 h for ethylene dimerization reaction, which is ~7 times higher than that of UiO-67-50%Ni. The PCN-160-47%Ni and UiO67-50%Ni showed butene selectivity comparable to other 21 previously reported catalysts (entries 6 and 7). Since PCN160-47%Ni and UiO-67-50%Ni have similar Ni content, pore size, and BET surface area (Figure S20), the superior catalytic activity of PCN-160-47%Ni was attributed to its unique transcoordination environment of the Ni. The trans-coordinated Ni sites in PCN-160-47%Ni are more sterically accessible than those of the Ni-2,2'-bipyridyl sites in UiO-67-50%Ni. As such,

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substrates can more readily reach the trans-coordinated Ni center from any direction within the equatorial plane, facilitating the ethylene binding and accelerating the reaction. Moreover, PCN-160-47%Ni can be recycled for at least three runs by separation from the reaction mixture through centrifugation at the end of each reaction without significant loss of Ni content and crystallinity (Figure S21). Elemental mapping at the microstructural level by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDX) shows uniform distribution of Ni throughout the crystal (Figure S22). The catalytic activity evaluated by the amount of generated C4 only reduced by 14% after three cycles. (Table S7) Table 1. Ethylene dimerization a containing catalysts

catalyzed

by Ni-

selectivity (%)

entry

catalysts

C4 intrinsic c -1 activity (h )

1-C4

2-C4

C6 + C8

1d

PCN-160

0

0

0

0

2e

none

0

0

0

0

3d

NiCl2@PCN-160

171±30

81.1

9.9

9.0

4

Ni-INA2

304±50

75.9

21.1

3.0

5

(bpy)NiCl2

288±20

74.1

19.5

6.4

UiO-67-50%Ni

455±40

66.7

29.0

4.3

PCN-160-47%Ni

3360±240

82.4

15.8

1.8

6 7

e

b

a

Reactions were performed in toluene under 40 bar of ethylene at 25 °C. Et2AlCl (Al/Ni = 300) was used as activator and reaction time was 1 h. b Catalysts containing 5 μmol Ni were used unless otherwise notified. c Calculated based on the moles of butenes generated and the Ni content in the catalyst. Standard deviations were calculated based on three runs. d MOF catalysts (5 mg) were used. The Ni loading in NiCl2@PCN-160 is 0.4 wt%, as determined by ICP-MS. e Et2AlCl (1.5 mmol) was added. e The first cycle of PCN-160-47%Ni catalyst.

CONCLUSIONS In conclusion, trans-coordinated metal centers were created in MOFs by sequential ligand elimination and installation of M-INA2 moieties. This method enables the 2+ convenient incorporation of different M transition metals of various ratios into trans-binding sites within a porous framework. Furthermore, we have shown that the transcoordinated metal sites in MOFs have intriguing properties in catalytic processes due to the exposed equatorial positions, as demonstrated by the remarkably improved activity of PCN-160-47%Ni in catalytic ethylene dimerization. Considering the versatility of this method and the unlimited tunability of MOFs, many more trans-coordinated metal complexes are envisioned. Apart from providing a method to construct trans-coordinated complexes, this work also highlights the vast opportunities of using a MOF matrix to assemble coordination architectures that are otherwise difficult or impossible to realize.

ASSOCIATED CONTENT Supporting Information

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Journal of the American Chemical Society Text, tables, and figures giving experimental procedures for the syntheses of the ligand, MOFs, PXRD, N2 adsorption isotherms, TGA, and crystallographic data of the structure. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]

Author Contributions ‡S.Y. and P.Z. contributed equally. Original idea was conceived by H.-C.Z. and S.Y.; experiments and data analysis were performed by S.Y., P.Z., L.F., G.S.D., W.C. and L.Z.; single crystal structures were refined by J.-S.Q.; catalysis was performed by P.Z. and P.E.; EXAFS was performed by A.T.G.E. and D.S., manuscript was drafted by H.-C.Z, S.Y., P.Z., H.F.D., S.M., and D.S. All authors have given approval to the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The gas adsorption-desorption studies of this research were supported by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001015. Structural analyses were supported by the Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-0030). The authors also acknowledge the financial supports of U.S. Department of Energy Office of Fossil Energy, National Energy Technology Laboratory (DE-FE0026472), National Science Foundation Small Business Innovation Research (NSF-SBIR) under Grant No. (1632486), and NPRP award (NPRP9-377-1-080) from the Qatar National Research Fund. The EXAFS experiments were performed at the Stanford Synchrotron Radiation Lightsource (SSRL) of the SLAC National Accelerator Laboratory, and use of the SSRL is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. The EXAFS fitting used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE: 1252521.

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