DOI: 10.1002/ejic.201600627 Communication

Post-Synthetic Transformation

Adding to the Arsenal of Zirconium-Based Metal–OrganicFrameworks: the Topology as a Platform for Solvent-AssistedMetal IncorporationTian-Fu Liu,[a] Nicolaas A. Vermeulen,[a] Ashlee J. Howarth,[a] Peng Li,[a] Amy A. Sarjeant,[b]Joseph T. Hupp,[a] and Omar K. Farha*[a]

Abstract: Gaining control over the assembly of mesoporous Zr-based metal–organic frameworks (MOFs) with modifiable metalnodes remains a challenge. The the topology is intriguing, par-ticularly if considering postsynthetic modification, as it presentslarge sodalite-like cages surrounded by 12 interconnected 1Dchannels that allow for efficient diffusion of guest molecules

Introduction

Metal–organic frameworks (MOFs) are well-ordered and atomi-cally defined porous materials that can act as heterogeneousscaffolds to bind small organic molecules, biomolecules, andmetals for a number of applications ranging from gas storageto catalysis.[1] The structural diversity of MOFs, owing to themultitude of unique combinations of organic and inorganicbuilding blocks that can be used to construct these materials,can be exploited to prepare frameworks with (1) high structuralstability towards water and high temperatures, (2) exposedbinding sites that can be used to anchor a variety of molecularentities,[2] and (3) interconnected mesoporous channels to pro-mote rapid diffusion of molecules throughout the structure.[3]

MOFs containing Zr6 nodes are particularly intriguing, asthey are stable at high temperatures and to aqueous solutionsover a wide pH range.[4] The Zr6 cluster can be 12-, 10-, 8-, or6-connected depending on the number of carboxylate linkerstethered to the Zr6 building unit (Figure 1).[4c,4d,5] Significanteffort has been dedicated to the exploration of MOFs with par-tially unsaturated 8- or 6-connected Zr6 nodes, as the remainingcoordination sites are occupied by terminal, labile hydroxy andaqua groups (–OH/H2O), which are available for substitution orsite-limiting reactivity.[5g,6] Currently, only a limited number of8- and 6-connected Zr6-based MOFs are suitable for postsyn-thetic node modification, and these MOFs are restricted to only

[a] Department of Chemistry, Northwestern University,2145 Sheridan Road, Evanston, IL 60208, USAE-mail: o-farha@northwestern.eduhttp://faculty.wcas.northwestern.edu/omarfarha/index.html

[b] Cambridge Crystallographic Data Centre, Center for Integrative ProteomicsResearch,174 Frelinghuysen Road, Piscataway, NJ 08854, USASupporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/ejic.201600627.

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throughout the structure. Herein, we demonstrate the first ZrMOF with the topology, NU-1200, which was synthesized bycontrolling the geometry of the organic building unit. In addi-tion, by using NU-1200 as a platform, we perform solvent-as-sisted metal incorporation to form single-site metalated nodes,a process that is characterized by single-crystal X-ray diffraction.

a few topologies including csq,[4a,5g,7] bcu,[5c,8] and spn.[4c,5i]

Therefore, the design and construction of new mesoporousframeworks,[9] especially those with new topologies, based onmodifiable unsaturated Zr6 nodes continues to be of interest.

Figure 1. The “top” and “side” views of a 12-, 8-, and 6-connected Zr6 oxidenode. The unsaturated 8- and 6-connected nodes have potential anchoringpoints, as indicated by the arrows.

The use of di- and tetracarboxylic acid linkers with Zr6 clus-ters for the construction of MOFs has been well ex-plored.[4c,4d,5g] For example, a number of isoreticular structures,expanded in size or containing peripheral covalent chemicalmodification, have been prepared for the fcu (2,12)-connectedand ftw (4,12)-connected[10] topologies.[4d,5a,11] Tricarboxylic

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acid linkers are not as commonly used to construct Zr6-basedMOFs but have been utilized to construct MOF-808 and PCN-777 (Figure 2, a). Using a 6-connected Zr6 node with 1,3,5-benzenetricarboxylic acid (BTC) or 4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoic acid (TATB) gives the spn (3,6)-connected topology ofMOF-808[4c] or PCN-777 (Figure 2, a),[5i] respectively. Obtaininga Zr6-based MOF by using a threefold symmetric linker and an8-connected node to prepare a the (3,8)-connected networkshould be possible but has not yet been achieved. The the

Figure 2. Cartoon and line drawing of the overall structure and linker of(a) the 3,6-connected PCN-777 and (b) the 3,8-connected NU-1200.

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topology consists of sodalite-like cages with postsyntheticallymodifiable nodes next to interconnected one-dimensional (1D)channels, which make it an ideal candidate for postsynthesismodification, in which every node is easily accessible. Herein,we control the geometry of an organic linker by introducingsteric constraints to form a triangular prism, instead of a flattriangular, linker to obtain a new mesoporous, 8-connected Zr-based MOF with the topology, referred to as NU-1200 (Figure 2,b).[12] We also demonstrate the potential of NU-1200 for post-synthetic modification by using a reactive Ti4+ precursor in solu-tion.

Results and Discussion

The fully saturated Zr6 node [Zr6O4OH4(COO)12] possesses Ohsymmetry (e.g., UiO-66) and is compatible with many carboxyl-ate-containing organic linkers. Careful inspection of the linker/node interactions in the tricarboxylic acid based frameworksMOF-808 and PCN-777 (Figure 2, a) reveals that all of the carb-oxylate groups of a single linker are coplanar, which rendersthe linker flat. From this it becomes apparent that if a tritopiclinker is used that cannot be flat, a different topology with dif-ferent node connectivity will form. Notably, there are very fewexamples of MOFs with the topology and those that do existare mostly based on square-planner metals (Cu, Co, Mn, Zn, Cd,Ni, Fe, Ln) in 8-connected nodes and co-planer triangular 3-connected linkers.[13] Obtaining a material with the topologyby using an 8-connected Zr6 cluster and a triangular linker hasnot yet been realized.

With these considerations in mind, 4,4′,4′′-(2,4,6-trimethyl-benzene-1,3,5-triyl)tribenzoic acid (TMTB) was synthesized. Thesteric demand imparted by the inclusion of three methylgroups forces the appended benzoic acid groups to rotate andstay perpendicular to the plane of the benzenoid core. TMTBretains D3h symmetry but forms a prismatic, not flat, structurewith each carboxylic acid group parallel to the others. If mixedwith ZrCl4 and benzoic acid (as a modulator) under solvother-mal conditions in DMF, this ligand directs the formation of anew Zr-based MOF with the topology. Large cubic single crys-tals (>20 μm) of NU-1200 were grown and found to be suitablefor single-crystal X-ray diffraction (SC-XRD). In general, it is verychallenging to obtain large, single crystals of Zr-based frame-works that are suitable for analysis by SC-XRD. The ability toobtain large crystals of NU-1200 adds to the appeal of thisframework for studying postsynthetic modification techniques,as characterization by X-ray diffraction can be achieved. SC-XRDreveals that NU-1200 consists of sodalite-like cages and inter-connected 1D channels, which is consistent with the topology(Figure 2, b). Interestingly, the terminal –OH/H2O groups on theunsaturated 8-connected Zr6 nodes in NU-1200 all point intothe large (20 Å, nucleus to nucleus) mesoporous 1D channels,which makes this MOF particularly intriguing for postsyntheticnode modifications. Nitrogen adsorption studies reveal that NU-1200 has a Brunauer–Emmett–Teller (BET) surface area of2400 m2 g–1 (Figure 3) and a distinctive mesoporous step at arelative pressure (p/p0) of 0.1. NU-1200 has a total pore volumeof 1.42 cm3 g–1, and the DFT pore-size distribution indicates

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pores with diameters of ca. 14 Å (consistent with the diameterof sodalite-like cage) and 20 Å (1D channel) (Figure S5, Support-ing Information). Thermogravimetric analysis (TGA) and varia-ble-temperature powder X-ray diffraction (VT-PXRD) demon-strate that NU-1200 is stable up to 300 °C. After activation withHCl to remove the excess amount of the benzoic acid modula-tor coordinated to the Zr6 node (Section S3), the presence ofthe terminal –OH/H2O groups in NU-1200 was confirmed bydiffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) (Figure S7). The sharp band at 3670 cm–1 is assignedto the terminal –OH groups on the Zr6(μ3-OH)8(OH)8 node,whereas the band at 2750 cm–1 corresponds to O–H stretchesfrom hydrogen bonding of the terminal water ligands on thenode.[6]

Figure 3. N2 sorption isotherm at 77 K for NU-1200 after activation with 4 MHCl. The inlay shows the large >20 μm cubic crystals of NU-1200.

To demonstrate the ease by which the exposed –OH/H2Ogroups on the unsaturated 8-connected Zr6 nodes can be modi-fied, we turned to a solution-based technique we term solvent-assisted metal incorporation (SAMI).[14] For efficient metal incor-poration, several criteria should be considered: (1) the metalsource should be very reactive to the –OH and H2O functionali-ties, (2) the solvent must dissolve the reactive metal complexbut must not make strong coordination complexes with themetal source, (3) the conjugate acid, following ligand exchangeon the desired metal ion, must not destroy the framework, andfinally (4) metalation should not result in significant node dis-tortion,[15] as this would hinder the use of the framework as ageneral heterogeneous support. We chose titanium isopropox-ide [Ti(iPrO)4] to modify NU-1200 in dichloromethane (CH2Cl2)(Section S4). The noncoordinating solvent readily dissolves themetal precursor and the 2-propanol product formed duringSAMI is nondestructive. PXRD measurements show that NU-1200 remains crystalline throughout the SAMI process. To fur-ther uncover the atomic structure, the post-SAMI NU-1200 wasexamined by SC-XRD. The crystal structure reveals the success-ful preparation of NU-1200–Ti with two Ti4+ atoms per Zr6 node(Figure 4). Specifically, each node contains four distinct Ti4+ sitesthat are each 50 % occupied. Each Ti4+ is attached to two oxy-

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gen atoms from the Zr6 cluster and four oxygen atoms fromhydroxy/aqua ligands. The average loading of two Ti atoms perZr6 node was corroborated for a bulk sample by using induc-tively coupled plasma optical emission spectrometry (ICP-OES)(Section S10). Additionally, the metalation occurs with insignifi-cant distortion of the Zr6 node, as confirmed by SC-XRD. SAMIholds great promise as an approach to form immobilized single-site metal atoms/clusters by using reactive metal precursors,and we are currently working to generalize this technique.

Figure 4. The metalation of unsaturated NU-1200 with exposed –OH/H2Ogroups by using a reactive Ti precursor to form NU-1200–Ti containing, onaverage, two Ti atoms per node.

ConclusionIn summary, we designed a new Zr-MOF, NU-1200, by carefullycontrolling the steric environment of a threefold symmetric or-ganic linker. NU-1200 displays a the topology with sodalite-likecages next to large interconnected 1D channels. Exposed –OHand H2O groups attached to the unsaturated Zr6 nodes andpointing into the mesoporous channels were readily functional-ized with a reactive Ti4+ species, and the product, NU-1200–Ti,was characterized with atomic precision by using single-crystalX-ray diffraction. NU-1200 represents the first Zr-based MOFwith an open and easily modifiable the topology. These struc-tural characteristics allow NU-1200 to serve as an atomicallydefined and robust platform for future targeted applicationsrequiring postsynthetic modifications.

Experimental SectionSynthesis of NU-1200: A 4 mL Pyrex vial was charged with ZrCl4(0.05 mmol, 12 mg), 4,4′,4′′-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoic acid (TMTB) (0.02 mmol, 10 mg), benzoic acid(2.87 mmol, 350 mg) and DMF (2 mL). The mixture was heated in a120 °C oven for 48 h. The solution was removed immediately, andthe solid residue was washed with fresh DMF (3×). Colorless cubic-shape crystals were harvested with a yield of 70 % (8 mg) based onTMTB ligand.

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SAMI of Ti Inside NU-1200: Residual DMF was removed from theas-synthesized NU-1200 (20 mg) through solvent exchange withCH2Cl2 (5 mL) at room temperature (18 °C) over the course of 24 h.The solvent was decanted, and the NU-1200 was suspended inCH2Cl2 (5 mL). Ti(iPrO)4 (0.15 mL) was added to this suspensionunder an atmosphere of nitrogen, and the sample was heated at60 °C for 24 h. After cooling to room temperature, the sample waswashed with CH2Cl2 (3 × 10 mL) and dried at room temperature.

CCDC 1481852 (for NU-1200) and 1481853 (for NU-1200–Ti) containthe supplementary crystallographic data for this paper. These datacan be obtained free of charge from The Cambridge Crystallo-graphic Data Centre.

AcknowledgmentsO. K. F. and J. T. H. gratefully acknowledge support from theInorganometallic Catalyst Design Center, an Energy Frontier Re-search Center, funded by the U. S. Department of Energy, Officeof Science, Basic Energy Sciences, under Award DESC0012702.This work made use of the J. B. Cohen X-ray Diffraction Facilitysupported by the MRSEC program of the National ScienceFoundation (DMR-1121262) at the Materials Research Center ofNorthwestern University.

Keywords: Metal–organic frameworks · Clustercompounds · Zirconium · Geometry · Topology

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[12] While this paper was under review, a MOF with the same structure asNU-1200 was reported and called BUT-12, see: B. Wang, X.-L. Lv, D. Feng,L.-H. Xie, J. Zhang, M. Li, Y. Xie, J.-R. Li, H.-C. Zhou, J. Am. Chem. Soc.2016, 138, 6204–6216.

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[14] Solution-phase metalation for the Zr6 node by using SAMI is comple-mentary to gas-phase metalation of the node by using atomic layer dep-osition (ALD) with volatile metal precursors. ALD in MOFs (AIM) is anongoing field of study, see ref.[5g]

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Received: April 25, 2016Published Online: ■

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Post-Synthetic Transformation NU-1200 is a Zr-based metal–organicframework (MOF) with the topology.T.-F. Liu, N. A. Vermeulen,Consisting of unsaturated Zr6 oxideA. J. Howarth, P. Li, A. A. Sarjeant,nodes and triangular linkers with largeJ. T. Hupp, O. K. Farha* .................... 1–5interconnected channels, this MOF is

Adding to the Arsenal of Zirconium- an ideal candidate for postsynthesisBased Metal–Organic Frameworks: metalation.the Topology as a Platform for Sol-vent-Assisted Metal Incorporation

DOI: 10.1002/ejic.201600627

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