Enhancing Strategies for the Assembly of Metal–Organic Systems with

Communication pubs.acs.org/crystal

Enhancing Strategies for the Assembly of Metal−Organic Systems with Inherent Cavity-Containing Calix[4]arenes Piotr P. Cholewa,† Christine M. Beavers,‡ Simon J. Teat,‡ and Scott J. Dalgarno*,† †

Institute of Chemical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K. Advanced Light Source, Berkeley Lab, 1 Cyclotron Road, MS6R2100, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: p-Carboxylatocalix[4]arenes have recently emerged as useful building blocks in the assembly of both discrete and polymeric coordination compounds. Steric effects of coligands used are now shown to dramatically influence the assembly process, dictating the assembly of one-dimensional (1D)-three-dimensional (3D) systems. Solvothermal techniques have also been found to promote formation of 3D systems with a sterically undemanding coligand.

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synthesis with a series of tertiary bipyridyl components afforded the anticipated 1D coordination polymer (CP) chains in line with literature precedent for benzoate analogues. We moved on to study the metal-directed assembly of C2-symmetric, cavity containing di-O-alkylcalix[4]arene dicarboxylic acids, in particular investigating the effects of placing the upper-rim functional groups at different distal positions; these are labeled Types I and II in Figure 1. In that study, we used a general strategy of

etal−organic frameworks and polyhedra (MOFs and MOPs) continue to attract attention due to the diverse range of applications they exhibit; recent examples include the use of MOFs in heterogeneous catalysis,1 selective separation of hydrocarbons,2 and gas storage.3 In addition, inherent cavitycontaining molecules are emerging as important new materials in gas storage applications.4−6 Calix[4]arenes possess cavities when they adopt cone conformations, and gas uptake in the organic solid state is well-documented for the parent p-tBucalix[4]arene.7−10 Given that the benzoate moiety is ubiquitous in MOFs, it is surprising that the p-carboxylatocalix[n]arenes (general notation herein pCO2[n]) have received relatively little attention in MOF/MOP synthesis. We have been looking to establish control over pCO2[n] assembly with a view to marrying all of the above topics to afford new one-dimensional (1D)−three-dimensional (3D) coordination systems for ultimate study in the aforementioned areas. Notwithstanding our own work (and to our knowledge) there are only two examples that have made inroads in this burgeoning area. In the first example, de Mendoza and co-workers reported the formation of giant regular polyhedra through the metal-directed assembly of pCO2[4]s and pCO2[5]s with the uranyl ion.11 In the second example, Burrows and co-workers reported a series of MOFs formed with a tetra-O-alkylcalix[4]arene dicarboxylic acid.12 The calixarenes are in the pinched-cone conformation, with upper-rim CO2− functionality directed away from the C[4] centroid, with the cavity blocked off due to steric interactions associated with tetra-O-alkylation. The authors reported difficulty in activation of the MOFs by removal of ligated solvent but suggest that the calixarene cavities may become accessible to gases based on computational analysis. We began our investigations in this area by employing tetraO-alkylcalix[4]arene monocarboxylic acids as building blocks in the targeted synthesis of transition metal/pCO2[4] panels;13 © XXXX American Chemical Society

Figure 1. Type I and II di-O-alkylcalix[4]arene dicarboxylic acids.14

incorporating 1,10-phenanthroline (Phen) as a coligand to block off coordination positions around directing Cd(II) centers, thus preventing formation of systems akin to those reported by Burrows and co-workers.12 Type I and II pCO2[4] diacids display cone angles between functional groups of ∼110° and ∼90°, respectively. Assembly of the molecular components was found to rely on Type I or II cone angles; ambient reaction of 1 or 2 with Cd(II) and Phen gave interwoven CP chains (Figure 2A), while Received: September 13, 2013 Revised: October 17, 2013

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the extended structure, one can immediately see by inspection that the ligated TMePhen is too large to reside in the cavity of a symmetry equivalent (s.e.) 1 from a neighboring chain. The crystallographically unique cavity of 1 is now occupied by a dmf of crystallization as was the goal of using a bulky coligand. Neighboring chains pack through (a) a series of π-stacking interactions between s.e. TMePhens as shown in Figure S1 of the Supporting Information and (b) two crystallographically unique H-bonding interactions between an aquo ligand and pCO2[4] carboxylic oxygen atoms. Reaction of 2 with TMePhen also afforded single crystals of formula [Cd4(2-2H)4(TMePhen)4] (dmf)5 (6), but in contrast to 5, these were found to be a complex 3D CP based on a binuclear secondary building unit (Figure 4A).15 The marked

Figure 2. (A) Metal−organic coordination polymer and (B) capsule systems formed by reaction of Type I and II pCO2[4]s with cadmium(II) nitrate and phen in dmf.14

3 or 4 gave discrete, tilted metal−organic capsules from analogous conditions (Figure 2B).14 As Phen was found to reside in the Type I pCO2[4] cavity, and that it could potentially influence the formation of capsules, the introduction of small alkyl groups around the coligand framework offered a route toward potentially controlling the process of metaldirected assembly. We anticipated that a substituted Phen would (a) not be able to reside in an adjacent Type I pCO2[4] cavity, (b) potentially alter capsule shape with Type II pCO2[4] diacids depending on the substituent position, and (c) that C[4] lower-rim alkyl chain variation would not influence the assembly greatly. To begin this structural investigation into coligand control over Cd(II) systems, we chose 2-, 3-, 4methyl-1,10-phenanthroline (2-, 3-, 4-MePhen), and 3,4,7,8tetramethyl-1,10-phenanthroline (TMePhen) as suitable candidates. Reaction of 1 or 2 with Cd(II) nitrate hydrate and 2-, 3-, or 4-MePhen did not return single crystals suitable for diffraction studies. Reaction of 1 with TMePhen did afford single crystals that were found to contain a 1D CP chain of formula [Cd(12H)(TMePhen)(H2O)⊂(dmf)] (dmf)(H2O)2 (5), as shown in Figure 3, with the pCO2[4] diacid linking Cd(II) centers. From

Figure 4. (A) Section of the extended structure in 6 with hydrogens, pCO2[4] lower-rim Bun chains, and TMePhens omitted for clarity. (B) Adamantoid topology found in 6.

difference in the outcome of assembly is associated with a series of differences in the Cd(II) coordination spheres in 5 and 6. The first notable difference is an increase in the nuclearity upon moving from 5 to 6, while the second is a lack of the crystallographically unique aqua ligand observed in 5 (Figure S2 of the Supporting Information). Furthermore, two carboxylates that form the SBU in 6 (comprising two Cd(II), four pCO2[4]s, and two TMePhen) are coordinated in a bridging fashion to both Cd(II) centers. Finally, the TMePhens coordinated to the two Cd(II) centers are suitably positioned to facilitate π-stacking interactions as shown in Figure S3 of the Supporting Information. In this position, a methyl group from each TMePhen (located at C-3) resides in a Type I pCO2[4] cavity, forming a crystallographically unique C−H···π interaction in each case (Figure S3 of the Supporting Information). Expansion of the asymmetric unit shows that each SBU is connected with four neighboring SBUs through pCO2[4]s. The

Figure 3. Partial extended structure of 5 showing neighboring 1D CP chains. Solvent omitted for clarity. B

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of formula [Cd2(4-2H)2(TMePhen)2⊂(dmf)2] (dmf)5(H2O)9 (8, Figure 6A) that is akin to that observed in 7; the TMePhens

extended structure is difficult to show due to the shape of the constituent building blocks, but analysis found the 3D complex CP to conform to adamantoid topology as shown schematically in Figure 4B. Each pCO2[4] cavity is occupied by a TMePhen methyl group, and the solvent molecules form channels (Figure S4 of the Supporting Information). As stated above, reaction of 3 or 4 with Cd(II) nitrate and Phen afforded tilted metal−organic capsules in both cases.14 Reaction of 3 or 4 with Cd(II) nitrate and 2-, 3-, or 4-MePhen only afforded single crystals suitable for diffraction studies in the case of 4/Cd(II/)2-MePhen. Structural analysis reveals that the components assemble to form a new head-to-head metal− organic capsule of formula [Cd2(4-2H)2(2-MePhen)2⊂(dmf)2] (dmf)2(H2O)6 (7, Figure 5A). Each capsule consist of two

Figure 6. (A) Head-to-head metal−organic capsule found in 8. (B) Space-filling representation of cavity-bound dmf molecules.

are coordinated to the Cd(II) centers and point away from the capsule periphery, and the interior is also occupied by two dmf molecules of crystallization (Figure 6B). Analysis of the extended structure shows that the packing is near identical to that found in 7, with a series of π-stacking interactions occurring between TMePhens and pCO2[4] arenes (Figure S7 of the Supporting Information). Solvent channels that are similar to those observed in 7 are also present (Figure S8 of the Supporting Information), indicating that this behavior is tolerant toward introduction of various groups around the general Phen coligand framework. As stated above, 7 and 8 are essentially isostructural despite their possessing different numbers of methyl groups in the Phen derivative employed. Careful analysis of the solid state packing for the tilted metal−organic capsules reported previously (Figure 2B) reveals interesting features that may dominate the overall capsule arrangement (i.e., head-to-head vs tilted). In the tilted capsules, the distances between Phen H atoms occupying the 2- or 3- and 8- or 9-positions and closest s.e. atoms (from either a neighboring assembly or ligated water/dmf) are too short to permit the introduction of a methyl substituent. Given this, we propose that rearrangements in the coordination sphere are dictated by these steric factors; in essence tilting is required to allow for ligation of water and or dmf molecules, and this is dictated by the nature of the Phen derivative. As we had investigated various aspects of coligand control over assembly, and given that we were able to obtain a 3D CP possessing higher metal nuclearity by means of ligand variation, we investigated the use of solvothermal synthesis across a range of our component combinations. Reaction of 1−4 with either Phen, 2-, 3-, 4-MePhen, or TMePhen afforded single crystals in only one case. To our surprise, this was found to be the 3/ Cd(II)/Phen combination that we would have expected to produce the known metal−organic capsule.14 Structural analysis reveals that the asymmetric unit is also a complex 3D CP of formula [Cd(3-2H)(Phen)⊂(dmf)] (dmf)2(H2O) (9) with a similar binuclear Cd(II) SBU to that found in 6 (Figure 7A). Two Phens are coordinated to the Cd(II) centers, and these form π-stacking interactions as would be expected. Each SBU is linked to four others through the pCO2[4]s, but structural analysis of 9 shows a difference in the orientation of the calixarenes around the Cd(II) centers. As a result, there is a change of direction in which the carboxylic moieties are pointing, thus influencing the assembly of the 3D system, which is found to have a distorted adamantoid topology (Figure 7B). Perhaps most importantly, the absence of methyl substituents

Figure 5. (A) Head-to-head metal−organic capsule found in 7. (B) Space-filling representation of cavity-bound dmf molecules. (C) Solvent channels found along the three unit cell axes in the extended structure. H atoms (except those of solvent) omitted in A and B for clarity. Solvent molecules omitted for clarity in C.

Cd(II) centers, two molecules of 4 and two 2-MePhen; each 2MePhen is coordinated to a Cd(II) center and points away from the capsule periphery. The 2-MePhens are involved in a series of π-stacking interactions taking place between s.e. capsules, as shown in Figure S5 of the Supporting Information. They are also involved in further association via a unique πstacking interaction with an arene from a pCO2[4] in a s.e. capsule. Analysis of the crystal structure showed the presence of two dmf molecules of crystallization within the capsule (Figure 5B); these are found to reside in the available pCO2[4] cavities, forming CH···π interactions with the aryl rings as expected. The new head-to-head pCO2[4] arrangement in 7, in conjunction with capsule alignment through packing, gives rise to solvent channels that run along all axes within the crystal (Figure 5C and Figure S6 of the Supporting Information). This suggests that these materials may be suitable for (potentially selective) guest exchange/uptake in the solid state. Furthermore, should these materials be stable toward desolvation, gas and solvent vapor uptake can also be explored; in this regard, we have calculated the capsule volume to be ∼210 Å3.16 Reaction of 3 or 4 with Cd(II) nitrate and TMePhen afforded single crystals suitable for diffraction studies in the case of 4/Cd(II)/TMePhen. Structural analysis shows that this combination also affords a head-to-head metal−organic capsule C

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +44 (0)131 451 8025. Fax: +44 (0)131 451 3180. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EPSRC for financial support of this work. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC0205CH11231.

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Figure 7. (A) Complex 3D coordination polymer 9 based on (B) an adamantoid topology.

on the Phen coligand frees up the Type II pCO2[4] cavities, thereby increasing the overall size of the solvent channels running through the crystal (Figure S9 of the Supporting Information). These channels are occupied by dmf molecules of crystallization that form CH···π interactions with the aryl rings as expected (Figure S10 of the Supporting Information). There are clear challenges associated with the controlled assembly of complex ternary systems that can also involve numerous intermolecular interactions between the constituent components. This can be further exacerbated by the use of large/bulky, cavity-containing building blocks that have associated solubility issues (e.g., calixarenes). In this structural study, we have shown that cavity to coligand size mismatches can influence the assembly of challenging metal−organic systems. Furthermore, solvothermal synthesis has allowed access to an analogous MOF topology in a case where one might expect to isolate a discrete capsule based on ambient reaction conditions. We have also (somewhat unexpectedly) discovered that small changes in the C[4] lower-rim chain length can also influence assembly. Although there is this exception to the rule, a statistical degree of understanding can be accepted, given the number of successful structural characterizations. We have made a significant step toward the controlled assembly of new cavity containing systems that should not require activation by removal of ligated solvent in order to study porosity; by using our strategy, we have reported a number of structures in which the cavity is preserved and occupied by solvent of crystallization. Work continues in this area with a view to examining the potential use of these systems as new sorbent materials.

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REFERENCES

(1) Zhang, Z.; Zhang, L.; Wojtas, L.; Nugent, P.; Eddaoudi, M.; Zaworotko, M. J. J. Am. Chem. Soc. 2012, 134, 924. (2) Bae, Y. S.; Lee, C. Y.; Kim, K. C.; Farha, O. K.; Nickias, P.; Hupp, J. T.; Nguyen, S. T.; Snurr, R. Q. Angew. Chem., Int. Ed. 2012, 51, 1857. (3) (a) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 14176. (b) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. (4) Mastalerz, M.; Schneider, M. W.; Oppel, I. M.; Presly, O. Angew. Chem., Int. Ed. 2011, 50, 1046. (5) Wang, W.; Bray, C. L.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 11608. (6) Tian, J.; Thallapally, P. K.; Dalgarno, S. J.; McGrail, P. B.; Atwood, J. L. Angew. Chem., Int. Ed. 2009, 48, 5492. (7) Thallapally, P. K.; McGrail, P. B.; Dalgarno, S. J.; Schaef, H. T.; Tian, J.; Atwood, J. L. Nat. Mater. 2008, 7, 146. (8) Ananchenko, G. S.; Moudrakovski, I. L.; Coleman, A. W.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2008, 47, 5616. (9) Atwood, J. L.; Barbour, L. J.; Jerga, A. Angew. Chem., Int. Ed. 2004, 43, 2948. (10) Atwood, J. L.; Barbour, L. J.; Jerga, A.; Schottel, B. L. Science 2002, 298, 1000. (11) Pasquale, S.; Sattin, S.; Escudero-Adan, E. C.; MartinezBelmonte, M.; de Mendoza, J. Nat. Commun. 2012, 3, 785. (12) Bew, S. P.; Burrows, A. D.; Duren, T.; Mahon, M. F.; Moghadam, P. Z.; Sebestyen, V. M.; Thurston, S. Chem. Commun. 2012, 48, 4824. (13) Cholewa, P. P.; Beavers, C. M.; Teat, S. J.; Dalgarno, S. J. Cryst. Growth Des. 2013, 13, 2703. (14) Cholewa, P. P.; Beavers, C. M.; Teat, S. J.; Dalgarno, S. J. Chem. Commun. 2013, 49, 3203. (15) Li, H. J.; Gao, Z. Q.; Gu, J. Z. Acta Crystallogr. 2011, E67, m919. (16) Interior volume calculated with a probe radius of 1.5 Å.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic Information Files (CIFs) and experimental details for 5−9. This material is available free of charge via the Internet at http://pubs.acs.org. D

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