Three Porphyrin-Encapsulating Metal–Organic Materials with

Communication pubs.acs.org/crystal

Three Porphyrin-Encapsulating Metal−Organic Materials with Ordered Metalloporphyrin Moieties Zhenjie Zhang,† Lukasz Wojtas,† and Michael J. Zaworotko*,†,‡ †

Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, FL 33620, United States Department of Chemical & Environmental Sciences, University of Limerick, Limerick, Republic of Ireland

‡

S Supporting Information *

ABSTRACT: 1,3,5-benzenetricarboxylate, biphenyl-3,4′,5-tricarboxylate, and [1,1′:3′,1″-terphenyl]-4,4″,5′-tricarboxylate react with Cd(II) cations in the presence of meso-tetra(Nmethyl-4-pyridyl) porphine cations to afford three metalloporphyrin-encapsulating metal−organic materials, porph@ MOM-11, -12 and -13, respectively. The metalloporphyrin moieties are ordered within channels and closely fit hexagonal or rectangular cavities, thereby facilitating a better understanding of the structure-directing effect that can be promoted by metalloporphyrins. Porph@MOM-12 is noteworthy because it exhibits two distinct types of hexagonal channels and it represents the first example of a net that exhibits mzz topology.

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INTRODUCTION Porphyrins are versatile enough to serve as dyes,1 catalysts,2 and sensors,3 and can be incorporated into metal−organic materials (MOMs) that combine the physicochemical properties of porphyrins and the permanent porosity of MOMs.4 Two design approaches have come to the forefront: porphMOMs, in which decorated porphyrins serve as nodes or linkers5 and porph@ MOMs, in which porphyrins are encapsulated by MOM frameworks.6 Until recently, porph@MOMs were limited to just three examples6 because of the dearth of MOMs with cavities that exhibit the right size and symmetry. However, by reacting polycarboxylate ligands with metals that exhibit a diverse range of coordination modes, our group has been able to invoke a template strategy for synthesis of a new generation of [email protected] In effect, porphyrin moieties serve as structure-directing agents to build the cages or supermolecular building blocks (SBBs) that encapsulate the porphyrin moiety and sustain three-dimensional (3D) networks. For example, meso-tetra(N-methyl-4-pyridyl) porphine tetratosylate (TMPyP) can template the reaction of 1,3,5-benzenetricarboxylate (H3BTC) with transition metal cations to generate octahemioctahedral cages that result in tbo nets with high symmetry polyhedral cages.7 The porphyrin moieties in these structures are highly disordered in the octahemioctahedral cages, meaning that the nature of their interactions with the MOM networks remains unclear. In this contribution, we reveal how lower symmetry cages can be generated from H3BTC or lower symmetry derivatives8 such as biphenyl-3,4′,5-tricarboxylate (H3BPT) and [1,1′:3′,1″terphenyl]-4,4″,5′-tricarboxylate (H 3 TPT) (Scheme 1). H3BTC possesses approximate D 3h symmetry, whereas © 2014 American Chemical Society

Scheme 1. H3BPT, H3BTC, and H3TPT, the Ligands Used Herein

H3BPT and H3TPT exhibit approximate C2v symmetry. Reaction of H3BPT, H3BTC, and H3TPT with Cd(II) salts afforded three cadmium porph@MOMs, porph@MOM-11, -12 and -13, in which porphyrin moieties are ordered within the frameworks, thereby providing opportunity to study the nature of the interactions between the porphyrin moieties and the MOM frameworks.

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EXPERIMENTAL SECTION

Materials and Methods. All substrates were purchased as highpurity grade reagents from Fisher Scientific and used without further purification. Solvents were purified according to standard methods and stored in the presence of molecular sieves. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance X-ray diffractometer at 20 kV, 5 mA, for Cu Kα (λ = 1.5418 Å). The Received: February 5, 2014 Revised: March 7, 2014 Published: March 10, 2014 1526

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calculated PXRD patterns were generated using Mercury. AA spectrophotometry was conducted on a Varian AA SpectrLab 100 instrument. UV−vis spectra were measured on a PerkinElmer Lambda 35 UV/vis/NIR Spectrometer. GC data were collected on an HP 6890 series GC system and MS on a 5971A mass selective detector. Gas adsorption isotherms were measured on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Synthesis of Porph@MOM-11. Cd(NO3)2·4H2O (15.4 mg, 0.050 mmol), H3BPT (14.8 mg, 0.050 mmol), and TMPyP (15.0 mg, 0.011 mmol) were added to 2.3 mL of DMF (2.0 mL) and H2O (0.3 mL) in a 7.0 mL scintillation vial. After the solution was heated at 85 °C for 12 h, dark prismatic crystals were harvested and washed with methanol (yield: ∼75% based on Cd(NO3)2·4H2O). Synthesis of Porph@MOM-12. CdCl2 (27.5 mg, 0.15 mmol), H3BTC (10.5 mg, 0.050 mmol), and TMPyP (6.0 mg, 0.0045 mmol) were added to a 2.3 mL of DMF (2.0 mL) and H2O (0.3 mL) in a 7.0 mL scintillation vial. The reaction mixture was heated to 85 °C for 48 h. The resulting dark prismatic crystals were harvested and washed with methanol (yield: ∼30% based on CdCl2). Synthesis of Porph@MOM-13. Cd(NO3)2·4H2O (46.3 mg, 0.15 mmol), H3TPT (18.1 mg, 0.050 mmol), and TMPyP (6.0 mg, 0.0045 mmol) were added to a 2.3 mL solution of DMF (2.0 mL) and H2O (0.3 mL) in a 7.0 mL scintillation vial. The reaction mixture was heated to 85 °C for 48 h. Dark block crystals were harvested and washed with methanol (yield: ∼25% based on Cd(NO3)2·4H2O). X-ray Crystallography. Data for single crystals of porph@MOM12 and porph@MOM-13 were collected on a Bruker-AXS SMART APEX/CCD diffractometer using Cu Kα radiation [λ = 1.5418 Å, T = 100(2) K]. Indexing was performed using APEX2 (difference vectors method). Data integration and reduction were conducted using SaintPlus 6.01. Scaling and absorption correction were performed by a multiscan method implemented in SADABS.9 Space groups were determined using XPREP implemented in APEX2. The crystal structures were solved using SHELXS-97 (direct methods) and refined using SHELXL-97 (full-matrix least-squares on F2) as contained in APEX2 and WinGX v1.70.01.10 Metal atoms of the porphyrin moieties were located via difference Fourier map inspection and refined anisotropically. Site occupancy was determined through refinement. In porph@MOM-12 and porph@MOM-13, the contribution of disordered solvent molecules was treated as diffuse using the Squeeze procedure implemented in Platon,11 whereas for the coordinated solvent molecules, only O atoms were refined.

Figure 1. (a) Coordination environments of the Cd(II) cations in porph@MOM-12. (b) A view of the mzz net of porph@MOM-12 that reveals the presence of two types of hexagonal channels.

types of hexagonal channels. CdTMPyP moieties are located within one of these channels, whereas solvent molecules occupy the second channel (Figure 1b). The solvent occupied channels are surrounded by six porphyrin occupied channels. Nmethylpyridyl groups of CdTMPyP cations are oriented through the windows that connect the two types of channels. Each porphyrin moiety binds a third Cd(II) cation, Cd3, which is five-coordinate through four porphyrin nitrogen atoms and one terminal oxygen atom from solvent. Cd3 lies out of the porphyrin plane with ΔCβ, the average deviation of β-carbon atoms from the porphyrin plane, being 0.79 Å. Average Cd−N bond distances are 2.251 Å.14 The expanded but lower symmetry ligands H3BPT and H 3 TPT afforded [Cd 4 (BPT) 4 ]·[C 44 H 36 N 8 Cd]·[solvent], porph@MOM-11, and [Cd4(TPT)4]·[C44H36N8Cd]·[solvent], porph@MOM-13, respectively. That a template effect was promoted by TMPyP was supported by control reactions in the absence of TMPyP, which afforded a clear solution in the case of porph@MOM-13 and colorless prismatic crystals with a different PXRD pattern for porph@MOM-11 (Figure S3 of the Supporting Information). Porph@MOM-13 adopts the same 3,6-connected rtl topology as porph@MOM-11, a structure that we communicated previously.7a [Cd2(COO)6]2− MBBs serve as 6-connected MBBs linked by 3-connected TPT ligands. Figure 2 reveals that Cd porphyrin moieties lie in alternate channels along the a axis in porph@MOM-11, whereas in porph@MOM-13 the porphyrin moieties occupy every other cavity in all channels. That TPT has an additional phenyl group versus BPT means that there are larger windows (Figure 3) in porph@MOM-13. Figure 3 and Figure S4 of the Supporting Information reveal how the N-methyl moieties of CdTMPyP cations are oriented through the 7.6 × 10.5 Å

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RESULTS AND DISCUSSION Reaction of H3BTC and CdCl2 with TMPyP in DMA/H2O at 85 °C afforded [Cd 14(BTC) 12]·3CdTMPyP·4Cl·solvent, porph@MOM-12, as dark prismatic crystals [trigonal space group P3̅ with a = b = 30.4643(6) Å, c = 10.0841(4) Å; V = 8104.9(4) Å3]. That a template effect was promoted by TMPyP was supported by a control reaction conducted without adding TMPyP, which afforded colorless crystals with a different PXRD pattern (Figure S1 of the Supporting Information). Figure 1a reveals the two independent Cd(II) cations in the network: Cd1 is 6-coordinate with six oxygen atoms from three bidentate chelating (μ1-η1η1) carboxylates, Cd2 is 5-coordinate with five oxygen atoms from one μ1-η1η1 carboxylate and three bridging μ2-η1η1 carboxylates. Cd2 and its symmetry equivalent form 5-connected molecular building blocks (MBBs) of formula [Cd2(COO)5]−. Cd−O bond distances range from 2.212 Å to 2.590 Å, which is consistent with reported values.12 If the BTC ligand is treated as a 3-connected node, the [Cd(COO)3]− MBB as a 3-connected node, and the [Cd2(COO)5]− MBB as a 5-connected node, then porph@ MOM-12 exhibits a 3,3,3,5-connected 3D honeycomb-like structure (Figure S2 of the Supporting Information) with point symbol {4.62}3{4.82}3{42.65.83}3{83}. This new net is classified as mzz in the RCSR database.13 Porph@MOM-12 exhibits two 1527

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Figure 4. The hexagonal macrocycle in porph@MOM-12 offers a good fit for CdTMPyP cations and can be regarded as an SBB. Hydrogen bonds are highlighted by purple dashed lines, whereas π···π interactions are illustrated by black dashed lines.

moieties in the hexagonal SBBs is seen through a series of short contacts: offset face-to-face π···π interactions (∼3.4 Å) between the porphyrin arms (pyridyl groups) and phenyl groups of adjacent BTC ligands, weak hydrogen-bonding interactions between the methyl group (C26) of CdTMPyP and an adjacent carboxylate oxygen atom, O6 (3.32 Å, C−H···O = 142°),15 hydrogen-bonding interactions between C39 from CdTMPyP and an adjacent carboxylate oxygen O11 (3.16 Å, C−H···O = 140°), and electrostatic interactions between the anionic framework and cationic porphyrin moieties. The ligands in porph@MOM-11 and porph@MOM-13 are larger than BTC and can accommodate CdTMPyP cations in rectangular cavities. The BPT ligands in porph@MOM-11 have one extended arm and dimensions of 6.1 × 9.9 Å. Four H3BPT ligands linked by four [Cd2(COO)6]2− MBBs afford a rectangle with diagonal distances of 18.2 × 22.0 Å (Figure 5). This rectangle serves as a cavity for CdTMPyP cations (diagonals of 18.4 Å), which engage in the following close

Figure 2. The structures of (a) porph@MOM-11 and (b) porph@ MOM-13 viewed along the a axis.

Figure 3. Porphyrin N-methyl arms orient through square windows in both (a) porph@MOM-11 and (b) porph@MOM-13.

square windows of porph@MOM-11, whereas the 11.7 × 10.5 Å square windows of porph@MOM-13 are large enough to accommodate two N-methyl moieties. The three porph@MOMs detailed herein exhibit structures that can be rationalized from the size and shape of the CdTMPyP cations, which exhibit approximate D4h symmetry, a cross-section of 13.4 × 13.4 Å, and a diagonal of 18.4 Å (the distance between carbons of methyl groups). H3BTC is only 6.1 × 6.1 Å, the distance between adjacent carboxylates. To accommodate CdTMPyP moieties into BTC frameworks, six BTC ligands assemble with six MBBs to form a hexagonal cavity with a diagonal of 17.6 Å [i.e., close to the dimensions of CdTMPyP (Figure 4)]. The resulting hexagonal cavities then serve as SBBs linked by BTC ligands to build the resulting mzz topology network. The tightness of the fit of the porphyrin

Figure 5. The rectangular macrocycle in porph@MOM-11 and porph@MOM-13 is a good fit for CdTMPyP cations. Hydrogen bonds are highlighted by purple dashed lines and π···π interactions with black dashed lines. 1528

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contacts: π···π interactions (∼3.5 Å) between porphyrin pyridyl groups and phenyl groups from adjacent BPT ligands, hydrogen bonding interactions between the terminal methyl groups of CdTMPyP cations and oxygen atoms from μ1-η1η1 bidentate carboxylates (3.35 Å, C−H···O = 144° and 3.08 Å, C−H···O = 137°), electrostatic interactions between the anionic framework and CdTMPyP cations. The TPT ligands in porph@MOM-13 exhibit dimensions of 9.9 × 13.3 Å. When linked by four [Cd2(COO)6]2− MBBs, a rectangle of dimensions 22.2 × 22.4 Å is generated. This cavity (Table 1 and Figure 5), which is very similar to that in porph@MOM11, is also well-suited to accommodate CdTMPyP cations.

respectively, at 273 K and 1 bar. However, porph@MOM-12 and -13 did not adsorb N2 at 77 K. Power X-ray diffraction (PXRD) revealed that, upon activation, porph@MOM-12 became amorphous, whereas porph@MOM-13 retained its crystallinity with peaks shifting only slightly (Figure S5 of the Supporting Information). These observations indicate that the framework of porph@MOM-12 is unstable to loss of guest but that porph@MOM-13 retains its structure. The low surface area and gas uptakes measured for porph@MOM-13 can be explained by a lack of open channels resulting from the arrangement of CdTMPyP cations. We also investigated if the CdTMPyP cations are amenable to metal exchange of Cd in the presence of Mn2+. Crystals of Cd-porph@MOM-12 were soaked for four days in an MeOH solution of MnCl2 (25.0 mmol/L) and metal exchange was verified by disappearance of the UV−vis Soret band of CdTMPyP at ∼430 nm and appearance of strong Soret bands for Mn(III)TMPyP at ∼460 nm (Figure 7). However, porph@MOM-13 did not undergo

Table 1. Ligand Size and Topology of porph@MOMs Formed by H3BTC, H3BPT, and H3TPTa

a

Distance between the two adjacent carboxylates. Figure 7. UV−Vis spectra (in water) of Mn-porph@MOM-12 compared with Cd-porph@MOM-12 and a commercial sample of Mn(III)TMPyP.

PLATON16 indicates ∼47%, 39%, and 44% free volume in the unit cells of porph@MOM-11, -12, and -13, respectively. Activation of these porph@MOMs for gas sorption studies was accomplished by soaking crystals in MeOH for 5 days followed by exposure to vacuum at room temperature for 10 h. The gas sorption properties of porph@MOM-11 have already been reported.7a Porph@MOM-12 and -13 were tested for CO2 and N2 sorption. As revealed by Figure 6, porph@MOM-11, -12, and -13 exhibit CO2 uptakes of 90.7, 10.1, and 17.1 cc/g,

exchange by Mn according to UV−vis spectroscopy and single crystal X-ray diffraction. Mn-porph@MOM-12 was found to catalyze styrene oxidation more efficiently than Mn(III)TMPyP in solution and Cd-porph@MOM-12: 61% conversion (10 h, TOF = 404 h−1) versus 45% and 5% conversion, respectively. Benzaldehyde and styrene oxide were the major products for Mn-porph@MOM-12 with 57% and 21% yields, respectively. The filtrant was recycled and even after four 10 h cycles we observed >55% conversion of styrene (see the Supporting Information for more details). We note that the structures of the porph@MOMs are quite different in terms of the building blocks and the channel structures and this could explain their different stabilities. First, there are large channels occupied by only solvent molecules in porph@MOM-12, whereas all channels are partially occupied by porphyrins in porph@MOM-13. We presume that the porphyrins in porph@MOM-13 might support the 3D structure more effectively than in porph@MOM-12. Second, porph@MOM-12 is based on monometallic 3-connected [Cd(COO)3]− building blocks, whereas porph@MOM-13 is built from bimetallic 6-connected [Cd2(COO)6]2− building blocks. The higher connectivity and nuclearity of the building blocks might lead to the higher relative stability of porph@ MOM-13.

Figure 6. CO2 adsorption isotherms of porph@MOM-11, -12, and -13 collected at 273 K. 1529

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(13) (a) O’Keeffe, M. Reticular Chemistry Structure Resource (http://rcsr.anu.edu.au/) (accessed Feb 5, 2014); (b) Blatov, V. A.; Proserpio, D. M. Acta Crystallogr. 2009, A65, 202. (c) Blatov, V. A. IUCr CompComm Newsletter 2006, 7, 4. (14) Berezin, D. B.; Shukhto, O. V.; Reshetyan, M. S. Russ. J. Gen. Chem. 2010, 80, 518. (15) Gu, Y.; Kar, T.; Scheiner, S. J. Am. Chem. Soc. 1999, 121, 9411. (16) Spek, A. L. Acta Crystallogr. 1990, A46, c34.

CONCLUSION In summary, three rigid tricarboxylate ligands of varying scale were used to study the structure-directing effect of TMPyP during the formation of porph@MOMs. The template effect of TMPyP was supported by control reactions in the absence of TMPyP that afforded different products. The size of the ligands and the resulting windows impacted the geometry of the cavities and/or the manner in which CdTMPyP cations are arranged in channels. These results suggest that porph@ MOMs will be accessible from a wide range of ligands and metal-based MBBs.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Crystal data, PXRD, additional structural figures, tables, and figures that detail the catalysis reactions. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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