Architectural Diversity in Multicomponent Metal–Organic Frameworks

Article pubs.acs.org/crystal

Architectural Diversity in Multicomponent Metal−Organic Frameworks Constructed from Similar Building Blocks Published as part of a Crystal Growth and Design virtual special issue on Crystal Engineering of Nanoporous Materials for Gas Storage and Separation Seok J. Lee, Celine Doussot, and Shane G. Telfer* MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North 5301, New Zealand S Supporting Information *

ABSTRACT: The architecture of metal−organic frameworks (MOFs) is intimately related to their functional properties. In this light, methods that control the topology that is produced by the combination of a given metal cluster and organic linker (or set of linkers) are valuable. Previously, it has been established that 4,4′,4″-nitrilotribenzoate (ntb) and benzene-1,4-dicarboxylate (bdc) combine with Zn4O clusters to produce [Zn4O(ntb)(bdc)3/2] (UMCM-4). Here, we report frameworks with a different architecture are produced when certain, typically bulky, substituents are introduced to the bdc linker. These MOFs are designated as MUF-8, and they adopt the ith-d topology. The general formula of the MUF-8 family is [Zn4O(ntb)4/3(bdc-X)], where bdc-X is a substituted bdc linker. Frameworks that are isoreticular to UMCM-4, termed the MUF-84 family, were observed when small substituents were appended to the bdc linker. Many MUF-8 and MUF-84 materials are accessible by direct synthesis. To generate frameworks that could not be synthesized directly, postsynthetic exchange reactions of the bdc linkers were employed.

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INTRODUCTION While it is conventional to refer to the metal−organic framework (MOF) that is produced from a given combination of organic linkers and metal nodes, it is often more appropriate to refer to a MOF that is produced from that set of components. This is because the same building blocks can give rise to multiple frameworks that are polymorphs (either topological or supramolecular isomers) of each other or they can lead to frameworks that are entirely different from one another. Many factors govern the observed outcome, including the nature of the building blocks, reaction solvent, concentration, and temperature, the addition of modulators, and the relative ratios of the components. Strategies to control and direct systems toward preferred outcomes are highly valuable.1 A related point is the relationship between the structure of the linker(s) and the structure and topology of the resulting framework. The principle of isoreticular chemistry, which posits that extending the linkers or introducing substituents can © XXXX American Chemical Society

produce frameworks with the same topology as the parent, is often of high predictive power.2 Cases that deviate from the isoreticular principle often reveal interesting underlying chemistry. One notable example is the linking of C3-symmetric carboxylate linkers by copper(II) paddlewheel SBUs into 3,4connected nets. [Cu2(btc)4/3] (HKUST-1, btc = 1,3,5benzenetricarboxylate) adopts the tbo topology,3 while the extended-linker analogue [Cu2(btb)4/3] (MOF-14, btb = 1,3,5benzenetribenzoate) describes a pto net.4 This has been rationalized as a trade-off between the bowing of the linker and the rotation of the carboxylate groups out of the plane of the adjacent aromatic ring.5 Certain substituents on the btc linkers lead to MOFs that are isoreticular to HKUST-1 (tbo topology), while others generate pseudopolymorphs with the fmj Received: January 30, 2017 Revised: April 4, 2017

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topology.6 Both pto and tbo topologies are accessible to the combination of copper(II) paddlewheel SBUs and 1,3,5benzene-trisethynylbenzoate linkers.7 The concepts of isoreticular chemistry can often be applied to frameworks that are built up using two or more linkers that are geometrically distinct from each other (multicomponent MOFs). For example, functional groups can be introduced to [Zn4O(btb)4/3(bpdc)1/2(bdc)1/2] (MUF-7, bpdc = biphenyl4,4′-dibenzoate)8 and [Zn4O(btb)4/3(bdc)] (UMCM-1)9 without perturbing the topology of the parent framework. Further, the isoreticular relationship between UMCM-210 and DUT3211 demonstrates that the topology can be maintained if the constituent linkers of the framework are extended proportionately. Where the two linkers of a multicomponent MOF are extended disproportionately different topologies usually result. This is exemplified by the UMCM-1−5 series in which tritopic and ditopic linkers with different metrics engender frameworks with various topologies.12 In the present report we focus on multicomponent MOFs built up from tritopic 4,4′,4″-nitrilotribenzoate (ntb) and ditopic benzene-1,4-dicarboxylate (bdc) and its derivatives. The parent member of this family is UMCM-4, [Zn4O(ntb)(bdc)3/2], which was first described by Matzger’s group.12 If the isoreticular principle is followed, then substituted bdc ligands will produce MOFs with the same topology as UMCM-4. We set about exploring whether this would be observed in practice. Our key finding is that the isoreticular principle is only followed in same cases and that an alternative framework architecture is possible. The observed topology largely depends on the size and arrangement of the substituents on the bdc linker. To generate frameworks that could not be synthesized directly, postsynthetic exchange reactions of the bdc linkers were employed.

Figure 1. Views of the structure of MUF-8a ([Zn4O(ntb)4/3(adc)]) as determined by single-crystal X-ray diffraction. (a) A close-up view of the Zn4O SBU, (b) the small tetrahedral pore with a gray sphere highlighting the void volume, (c) the large dodecahedral pore with a yellow sphere highlighting the void volume, and (d) the spatial arrangement of the pores. The adc ligand is shown in orange and the ntb ligand in green.

feed ratio of tritopic to ditopic linkers while keeping the reaction temperature (85 °C) and solvent (N,N-diethylformamide, DEF) the same. After screening the reaction products by PXRD, it became evident that MUF-84a could not be produced under these conditions. We subsequently turned our attention to the ditopic linker ndc (1,4-naphthalenedicarboxylate). Since its structure is intermediate between bdc and adc, we reasoned that it may be capable of producing both [Zn4O(ntb)4/3(ndc)] (MUF-8f) and [Zn4O(ntb)(ndc)3/2] (MUF-84f). We explored a number of parameters such as the reaction temperature, the concentration of benzoic acid as a modulator, and the H3ntb/H2ndc feed ratio. We found the feed ratio to have the most profound impact on the outcome of the reaction (Figure 2). At a H3ntb/H2ndc ratio of ∼1:6 a mixture of MOFs was produced, as deduced from PXRD patterns. The diffractogram of one of these phases corresponds to MUF-8f, while the other was ascribed to MUF-84f on the basis of its similarity to the pattern of UMCM-4. At higher H3ntb/H2ndc ratios PXRD patterns indicated that MUF-84f, [Zn4O(ntb)4/3(ndc)], could be produced as the major product (Figure S9). The observed preference for MUF84f at the expense of MUF-8f as the relative amount of H2ndc is raised is consistent with the stoichiometry of these materials. Unfortunately, traces of a contaminant (presumably MUF-8f) persisted in the synthesis of MUF-84f. In line with intuitive expectations, MUF-8f dominated when the H3ntb/H2ndc ratio was lowered (Table 1). In this case, 1H NMR spectroscopy on dissolved samples showed that MUF-8f was phase pure (Figure S3) The structure of MUF-8f was confirmed to be isoreticular to MUF-8a by single-crystal X-ray diffraction. To verify the structure of MUF-84f we attempted to grow high quality single crystals. Unfortunately, the exploration of a wide range of solvothermal crystallization conditions proved fruitless. We subsequently explored the combination of H3ntb and a range of

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RESULTS AND DISCUSSION Direct Synthesis. [Zn4O(ntb)(bdc)3/2] (UMCM-4) is reported to form when the molar feed ratio of H3ntb to H2bdc in the synthesis mixture is between 3:7 and 4:6 mol.12 While the original publication does not explicitly state what is formed outside of this range, it is likely that [Zn4O(ntb)2] (MOF-15013) is produced when the amount of H3ntb is too high. In initial screening reactions, we found that replacing H2bdc by H2adc (9,10-anthracenedicarboxylic acid), while keeping the tritopic/ditopic linker ratio ∼2:3, led to a crystalline material. However, the PXRD pattern of this material differed from UMCM-4. This prompted us to analyze its structure by single-crystal X-ray diffraction, which revealed the framework depicted in Figure 1. This MOF, henceforth referred to as MUF-8a, has the formula [Zn4O(ntb)4/3(adc)]. The framework is built up from classical Zn4O SBUs that coordinate to the carboxylate groups of ntb and adc to produce a well-ordered lattice. The topology of the MUF-8a network is ith-d. It is therefore isoreticular with other multicomponent MOFs such as MUF-7,8 MUF-77,14 MUF-32,15 and MOF-205/ DUT-6,16,17 and displays the same combination of smaller tetrahedral pores and larger dodecahedral pores (Figure 1). The observation of MUF-8a is at odds with the isoreticular principle, which would have predicted the formation of a MOF with the formula [Zn4O(ntb)(adc)3/2] and isostructural to UMCM-4. We refer to MOFs with the UMCM-4 topology as MUF-84, and hence this MOF would be MUF-84a (Chart 1). To explore whether the putative MUF-84a framework could be formed under alternative synthesis conditions, we varied the B

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Chart 1. Ligand Structures and Categorization into Groupsa

architecture; MUF-84 analogues were not observed starting from Group I ligands. The situation is reversed for Group II ligands. Here, the ligands bear only one substituent (or no substituents in the case of H2bdc) on the phenyl ring. MOFs with the MUF-84 architecture are produced by these ligands at the exclusion of MUF-8-type frameworks. The ligands in the third category, Group III, bear two substituents on the phenyl ring in either a 2,3 or a 2,5 arrangement relative to the two carboxyl groups. By tuning the synthesis conditions, these ligands are able to form either MUF-8 or MUF-84 type frameworks (Figures S8−S9). The MUF-8 materials can be produced with complete phase purity (Figures S3−S5); however, minor impurities persisted in the synthesis of the MUF-84 materials (Table 1). Postsynthetic Ligand Exchange. To access MOFs that are not accessible by direct synthesis we attempted postsynthetic ligand exchange reactions.18 In initial trials of this method we observed that soaking crystals of MUF-8f in a solution of H2bdc led to the gradual replacement of the ndc linkers in the framework by bdc. This process could be monitored by several complementary techniques. First, 1H NMR spectroscopy on dissolved samples provided a quantitative ratio of the ndc and bdc linkers that comprise the framework (Figure 4a). Second, PXRD demonstrated that the global lattice structure was unaffected by the ligand substitution reaction (Figure 4b). Third, optical microscopy of the crystals showed that they retained their external form and transparency (Figure 4b). Over a period of 30 days complete displacement of the ndc linkers in MUF-8b occurred to produce MUF-8c* (MOFs prepared by postsynthetic ligand exchange reactions are denoted with an asterisk). The structure of MUF-8c* was determined by single-crystal X-ray diffraction (Table S2). The high quality of the diffraction data confirms the single-crystal-to-single-crystal nature of the linker exchange process. It also underscores the utility of linker exchange as a route to frameworks that cannot be synthesized directly. Following the successful realization of the postsynthetic linker exchange strategy, we expanded its scope (Table 2). The standard experimental procedure that we developed was to soak ∼20 mg of the starting MOF crystals in a solution of the target ligand in DMF (0.01 M) at ambient temperature. The supernatant was replaced with the fresh ligand solution every 2 days at the early stage of the exchanging process, and then the time gap was increased to every 4−5 days once the exchange ratio exceeded ∼70%. The 1H NMR spectra and PXRD patterns of frameworks undergoing postsynthetic linker exchanges are presented in Figures S24−S35. We established the following general rules that govern postsynthetic linker exchange reactions in these materials: (i) Group I linkers in frameworks with the MUF-8 architecture can be replaced by Group II or Group III linkers. This provides access to MUF-8 materials that cannot be synthesized directly, such as MUF-8c ([Zn4O(ntb)4/3(bdc)]). Linker exchange is relatively rapid, proceeding to completion over ∼14 days. (ii) Group II linkers can replace other Group II linkers in MUF-84 type frameworks. This allowed for the conversion of MUF-84c ([Zn4O(ntb)(bdc)3/2]) to MUF-84d* ([Zn4O(ntb)(mdc)3/2]). Postsynthetic linker exchange processes in are typically much slower in MUF84 lattices than their MUF-8 counterparts.

a

The letters in brackets for the ditopic linkers refer to the suffixes employed for the MUF-8 and MUF-84 materials.

Figure 2. Influence of the H2ndc/H3ntb feed ratio on the production of MUF-8f and MUF-84f. MOF-150 ([Zn4O(ntb)2]) forms as an impurity phase with high levels of H3ntb.

other ditopic bdc linkers that bear substituents on their central phenyl ring (Chart 1). Introducing the dmbdc linker gave high quality crystals of MUF-84g ([Zn4O(ntb)(dmdc)3/2]). Singlecrystal XRD of this framework verified that the topology of the MUF-84 series is identical to that of UMCM-4. At lower H3ntb/H2dmbdc ratios, this linker set is also capable of forming MUF-8g ([Zn4O(ntb)4/3(dmbdc)]). The contrasting architectures of MUF-8 and MUF-84 are highlighted in Figure 3. In total eight ligands were tested and we found that they fell into three categories: Group I comprises adc and tmbdc. These ligands have substituents on all four carbon atoms of the phenyl ring, and they exclusively produce frameworks with the MUF-8 C

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Table 1. Summary of MUF-8 and MUF-84 Frameworks Prepared by Direct Synthesis ditopic group

ditopic ligand

ntb/ditopic ligand ratio in synthesis

producta

phase pure?b

space group

BET surface area (m2 g−1)

pore volume (cm3 g−1)

Group I

H2adc H2tmbdc H2bdc H2mbdc H2ambdc H2ndc

1:1.5 1:2.4 1:2.1 1:3.4 1:3.4 1:3.1 1:6 1:3.0 1:6 1:3.3 1:8

MUF-8a MUF-8b MUF-84cc MUF-84d MUF-84e MUF-8f MUF-84f MUF-8g MUF-84g MUF-8h MUF-84h

Y Y Y Y N Y N Y N Y N

Pm3̅n P42/mmc Pnma Pnma Pnma Pm3̅n d P42/mmc P21/c Pm3̅n P21/c

3380 3560 3720 3130

1.25 1.32 1.39 1.28

3590

1.33

3720

1.39

Group II

Group III

H2dmbdc H2dmobdc

a The general formula of MUF-8 is [Zn4O(ntb)4/3(ditopic)] while that of MUF-84 is [Zn4O(ntb)(ditopic)3/2]. bPhase purity evidenced by 1H NMR spectroscopy of dissolved samples. MUF-84f, g, and h had a persistent contaminant with the MUF-8 structure, and phase pure materials could not be obtained by adjusting the reaction parameters. cFirst reported as UMCM-4. The textural data presented here match those in the original report. d This material could only be obtained as a microcrystalline powder.

Figure 3. A comparison of the topologies of the MUF-8 series and the MUF-84 series as exemplified by the structures of MUF-8g and MUF84g determined by single-crystal X-ray diffraction. The ditopic linkers are shown in orange, the ntb linkers are in green, and the Zn4O SBUs are in turquoise.

(iii) Group III linkers can replace Group II linkers in MUF-84 type frameworks. By way of example, ndc can quantitatively replace bdc in MUF-84c to give MUF84f* ([Zn4O(ntb)(ndc)3/2]). (iv) Group I linkers cannot replace the Group II linkers in MUF-84. Instead, we found a third phase to form on the surface of MUF-84 crystals. Although the structure of

Figure 4. (a) The ratio of ndc to bdc in crystals of MUF-8f as determined by 1H NMR spectroscopy on dissolved samples during its conversion to MUF-8c by postsynthetic linker exchange. (b) PXRD patterns an optical micrographs recorded during the ligand exchange process.

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Table 2. Summary of Postsynthetic Ligand Exchange Reactions Leading to MUF-8* and MUF-84* starting MOF

original linker

replacement linker

final MOFa

reaction time

MUF-8b MUF-8b MUF-8f MUF-84c MUF-84c MUF-84c

tmbdc tmbdc ndc bdc bdc bdc

bdc ndc bdc mbdc ndc adc

MUF-8c* MUF-84f* MUF-8c* MUF-84d* MUF-84f* b

14 days 14 days 30 days 33 days 21 days n/a

a

MOFs prepared by postsynthetic ligand exchange reactions are denoted with an asterisk. bLinker exchange was not observed. Instead, crystals of a new phase grow on the surface of the original MOF crystals. n/a = not applicable.

this material was not examined in detail, 1H NMR spectroscopy indicated that it comprises primarily adc and bdc (Figures S34, S35). Gas Adsorption Isotherms. A selection of gas adsorption isotherms were measured on the MUF-8 and MUF-84 materials that could be produced in a phase-pure form. Prior to these measurements they were activated by exchange with dry CH2Cl2 followed by the removal of all occluded guest molecules under reduced pressure. The materials are highly porous. N2 adsorption isotherms showed Type I behavior with a steep rise in the low P/Po region followed by plateau above P/Po ≈ 0.2 due to saturation of the framework pores (Figures S10−S16). Surface areas were estimated using the BET model and pore volumes from the saturation uptake level (Table 1). MUF-8c* prepared by the postsynthetic linker exchange method has a BET surface area of 3850 m2 g−1 (Figure S14). This value is the highest measured among the materials reported herein and highlights the tendency for smaller ditopic linkers to lead to higher gravimetric surface areas. Pore size distributions were calculated from the X-ray crystal structures using RASPA19 and are presented in Figure 5. Two distinct pore sizes are apparent in the MUF-8 series. These correspond to the smaller tetrahedral pores (∼5−9 Å) and larger dodecahedral pores (∼16−19 Å) (Figure 1). In contrast, the pores in the MUF-84 frameworks are more uniform in size and fall in the range ∼8−13 Å. The presence of large accssible micropores in the MUF-8 frameworks is confirmed by the inflection around P/Po = 0.025 in the N2 adsorption isotherms, which is absent from the MUF-84 isotherms. CO2 adsorption isotherms were measured at 273 K (Figure S17). The linearity of these isotherms indicates a relatively weak interaction between these MOFs and CO2, which is due to their large pore size. The MUF-8 series adsorbs CO2 to a greater extent than the MUF-84 series. For example, MUF-8c* takes up around 56 cm3(STP) g−1 at a pressure of 1 bar, while MUF84c has a capacity of 31 cm3(STP) g−1. This enhancement is not simply due to increased surface area nor pore volume nor pore chemistry, but rather the pore shape, by way of the framework architecture, has the most distinctive influence. This underscores the benefits of being able to access different framework types from the same set of building blocks. The MOF-905 series, multicomponent MOFs with an ith-d topology built up from a combination of tritopic and ditopic carboxylate linkers, has recently been identified as a promising class of high-pressure methane storage materials.20 The key metrics of the MOF-905 family in this regard are a pore volume

Figure 5. Pore size distributions calculated from SCXRD structures.

of ∼1.3 cm3 g−1, pore diameters centering around 6 and 17 Å, and moderate (up to 11 cm3(STP) g−1) uptake of CH4 at 1.1 bar and 298 K. The MUF-8 series shares many of the structural characteristics of the MOF-905 series, and therefore they also have promise in methane storage applications. CH4 adsorption isotherms were measured around room temperature and show uptakes at 1 bar in the region 14−16 cm3(STP) g−1 at 273 K and 4−8 cm3(STP) g−1 at 298 K (Figures S18−S21). Future work will focus on the application of MUF-8 frameworks to high-pressure methane adsorption. Relationship between Linker and Framework Structures. In the direct synthesis of MUF-8 and MUF-84, it is clear that the ditopic ligand structure plays a key role in determining which of the two frameworks is preferred (Table 3). One key structural feature that differentiates the groups of ditopic ligands is the position and size of their substituents. In addition to presenting various degrees of steric bulk, these substituents influence the preferred dihedral angle between the phenyl ring and the carboxyl groups of the linker.21,22 The linkers in Group II have either one small or no substituents. They are expected to preferentially adopt a dihedral angle close to zero and the ligand will be planar overall. An increased number and/or bulk of the substituents, as present in Groups I E

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Table 3. Structural and Conformational Characteristics of the Ditopic Ligands That Are Components of MUF-8 and/ or MUF-84 Group

substituted positions

preferred geometrya

carboxyl group rotational barrier

preferred topology

I II III

2,3,5,6 2/unsubst 2,3 or 2,5

nonplanar planar nonplanar

low high/medium low/medium

MUF-8 MUF-84 either

a

Experimental details, additional characterization data (PDF) Accession Codes

CCDC 1530034−1530038 and 1530138−1530143 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Torsion angle between the phenyl ring and the carboxyl group.

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and III, will induce a twist in the aryl-carboxyl group bond. The ligand groups are also differentiated by the strength of these conformational preferences. Specifically, the energetic barrier to the rotation of the carboxyl group23 is highest for the Group II linkers. That is, these ligands strongly prefer planarity. The rotational barrier is lower for Groups I and III, implying that these linkers have weaker conformational preferences than those in Group II. A clear correlation may be drawn from the data in Table 3: heavily substituted ditopic ligands prefer the MUF-8 architecture, and lightly substituted linkers prefer MUF-84. Pinpointing the factors that underlie this correlation is not straightforward, especially given that crystallography disorder obscures the conformation adopted by the ditopic linkers in the MUF-8 frameworks. Furthermore, the energetic difference between the lattices is small given the existence of ligands that can form either architecture depending on the synthesis conditions and that, in many cases, the linkers are interchangeable by postsynthetic exchange. As for other cases where similar building blocks give rise to diverse framework architectures, we suggest that molecular mechanics calculations will be essential to deconvolute the complex interplay between ligand, SBU, and lattice structures.24

Corresponding Author

*E-mail: [email protected]. ORCID

Shane G. Telfer: 0000-0003-1596-6652 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the RSNZ Marsden Fund for supporting this research (Contract 14-MAU-024).

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ABBREVIATIONS BET, Brunauer−Emmett−Teller; MOF, metal−organic framework; MUF, Massey University Framework; SBU, secondary building unit

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REFERENCES

(1) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 974. (2) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 2002, 295, 469. (3) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A chemically functionalizable nanoporous material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148. (4) Chen, B.; Eddaoudi, M.; Hyde, S. T.; O’Keeffe, M.; Yaghi, O. M. Interwoven metal-organic framework on a periodic minimal surface with extra-large pores. Science 2001, 291, 1021. (5) Amirjalayer, S.; Tafipolsky, M.; Schmid, R. Exploring Network Topologies of Copper Paddle Wheel Based Metal−Organic Frameworks with a First-Principles Derived Force Field. J. Phys. Chem. C 2011, 115, 15133. (6) Cai, Y.; Kulkarni, A. R.; Huang, Y.-G.; Sholl, D. S.; Walton, K. S. Control of Metal−Organic Framework Crystal Topology by Ligand Functionalization: Functionalized HKUST-1 Derivatives. Cryst. Growth Des. 2014, 14, 6122. (7) Zhu, N.; Lennox, M. J.; Duren, T.; Schmitt, W. Polymorphism of metal-organic frameworks: direct comparison of structures and theoretical N2-uptake of topological pto- and tbo-isomers. Chem. Commun. 2014, 50, 4207. (8) Liu, L.; Konstas, K.; Hill, M. R.; Telfer, S. G. Programmed Pore Architectures in Modular Quaternary Metal−Organic Frameworks. J. Am. Chem. Soc. 2013, 135, 17731. (9) Kim, M.; Boissonnault, J. A.; Allen, C. A.; Dau, P. V.; Cohen, S. M. Functional tolerance in an isoreticular series of highly porous metal-organic frameworks. Dalton Transactions 2012, 41, 6277. (10) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. A Porous Coordination Copolymer with over 5000 m2/g BET Surface Area. J. Am. Chem. Soc. 2009, 131, 4184.

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CONCLUSION The major finding of the present work is that sets of linkers comprising the tritopic linker ntb and the ditopic linker bdc and its substituted homologues can lead to two different framework types: [Zn4O(ntb)4/3(ditopic)] (MUF-8) and [Zn4O(ntb)(ditopic)3/2] (MUF-84). Heavily substituted bdc linkers exhibit a preference for the former, while bdc itself and analogues with one small substituent prefer the latter. In many cases these MOFs readily exchange their ditopic linkers in postsynthetic reactions to provide access to frameworks that cannot be synthesized directly. These findings add to the growing body of knowledge of how the copolymerization of tritopic and ditopic linkers in combination with Zn4O SBUs can generate multicomponent MOFs with interesting architectures and functional properties. In addition to varying a given framework structure in an isoreticular way by lengthening the ligands or introducing functional groups, we have shown that is possible to produce entirely different frameworks starting from linkers with the same metrics. Variations in the shapes, sizes, and chemical characteristics of the pores in these materials will allow their optimization for targeted functional properties.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00153. F

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.7b00153 Cryst. Growth Des. XXXX, XXX, XXX−XXX