Expansion of Pore Windows and Interior Spaces of Microporous Porphyrin-based MetalCarboxylate Frameworks: Synthesis and Crystal Structure of [Cu2(ZnBDCBPP)]

Satoshi Matsunaga,* Shigeki Kato, Nanako Endo, and Wasuke Mori*Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293

(Received December 4, 2012; CL-121205; E-mail: matsunaga@kanagawa-u.ac.jp, wmori@kanagawa-u.ac.jp)

We successfully synthesized a novel porphyrin-basedmetal carboxylate framework, [Cu2(ZnBDCBPP)]¢5DMF¢5H2O, with expanded pore windows and interior spaces,by using a ZnBDCBPP building block. Single-crystal X-rayanalysis revealed that the framework of [Cu2(ZnBDCBPP)]formed a three-dimensional network. The cavity was surroundedby a total of 16 accessible metal sites with two distinct metals,and large pore windows were present. These large pore windowsappear to be crucial for an enhanced catalytic performancebecause they facilitate the diffusion of substrates and products.

Porous metal­organic frameworks (MOFs) have attractedconsiderable attention owing to their potential applications invarious fields such as gas storage,1 gas separation,2 ionexchange,3 and catalysis.4,5a­5d The incorporation of accessiblemetal sites (AMSs) into porous MOFs is important because theresulting AMS-incorporated MOFs exhibit enhanced gas-ad-sorption capacities and can be applied to specific catalysis.6 Oneapproach to the incorporation of AMSs into MOFs is the use ofAMS-containing building blocks. In this approach, the rationaldesign of a framework structure is possible through comparisonwith the method of formation of AMSs at structural nodes in theself-assembly process. In addition, because molecules that arealready known to be effective homogeneous catalysts can beused as building blocks, this approach is useful for the moleculardesign of heterogeneous catalytically active MOFs. The useof AMS-containing building blocks such as metallosalens,7

N-heterocyclic carbene (NHC) complexes,8 and metalloporphy-rins5a,5e,5f,9 has been reported in many studies.

Metalloporphyrins, in particular, can be found throughoutnature, and they exhibit unique catalytic, electronic, and opticalproperties.10 When metalloporphyrins are used as buildingblocks for MOFs, the facile molecular modification of porphy-rins expands the possibilities of structural design. Because mostof the elements in the periodic table can be inserted intoporphyrins,10 it is possible to introduce various AMSs into thepore surface of MOFs without alteration of the frameworktopology. In addition, the physical and chemical properties ofmetalloporphyrins can be controlled through appropriate func-tionalization of the porphyrin core using standard modifications.The use of metalloporphyrins as MOF building blocks offers thepotential to tailor the photochemical and redox properties of theframeworks. Therefore, metalloporphyrins are one of the bestbuilding blocks for the incorporation of AMSs into MOFs.

We have previously reported the synthesis of porphyrin-based MOFs in which 5,15-bis(dicarboxyphenyl)porphyrin(BDCPP: Figure 1, left) was linked via a paddle-wheel Cu2unit to form [Cu2(MBDCPP)] (M = Zn2+, Ni2+, Pd2+,Mn3+(NO3), or Ru2+(CO)), and have described their crystalstructures and N2/H2 adsorption properties.5f These MOFs

exhibited permanent porosity and had an interior space that wassurrounded by two distinct types of AMSs: the metal centers ofthe porphyrin moieties and the axial sites of the paddle-wheelCu2 units. In addition, we were able to incorporate variousAMSs (Zn2+, Ni2+, Pd2+, Mn3+, or Ru2+) into the frameworkwithout altering the structure. Although the BDCPP-basedMOFs contained large interior spaces (ca. 20¡ in diameter),the pore window was too small to allow the access of bulkyguest molecules. The small pore window negatively affected theperformance of the catalysts by slowing down substrate andproduct diffusion through the window, and only small substratescould be used for catalytic reactions.

To address this issue, we designed an elongated butgeometrically equivalent BDCPP analogue, i.e., 5,15-bis(dicar-boxybiphenyl)porphyrin (BDCBPP: Figure 1, right), in whichthe two 3,5-dicarboxybiphenyl groups are attached to theporphyrin core via a phenylene group. We report the construc-tion of a novel porphyrin-based metal carboxylate framework,[Cu2(ZnBDCBPP)]¢5DMF¢5H2O, with expanded pore win-dows and interior spaces, achieved through the use of aZnBDCBPP building block, and discuss its synthesis, crystalstructure, and gas-adsorption properties.

The synthetic routes to ZnBDCBPP are shown in Scheme 1.5-Aminoisophthalic acid (1) was iodized by using Sandmeyerconditions followed by isobutyl esterification to yield5-iodoisophthalic acid isobutyl ester (2). The porphyrin pre-cursor aldehyde 3 was synthesized via the Suzuki­Miyauracoupling of 2 with 4-formylphenylboronic acid. The synthesisof free-base porphyrin tetraisobutyl ester, H2BDiBuCBPP, wasachieved through a standard Lindsey method by condensing

17.8 Å 26.2 Å

BDCPP BDCBPP

Figure 1. BDCPP and BDCBPP ligands.

Published on the web March 2, 2013298doi:10.1246/cl.2013.298

© 2013 The Chemical Society of JapanChem. Lett. 2013, 42, 298­300 www.csj.jp/journals/chem-lett/

dipyromethane with aldehyde 3 in CHCl3 in the presence ofBF3¢OEt2 as a catalyst.11 The free-base porphyrin H2BDi-BuCBPP was metalated with Zn(OAc)2¢2H2O to provide thezinc porphyrin tetraisobutyl ester ZnBDiBuCBPP. Basic hydrol-ysis of the tetraester ZnBDiBuCBPP gave the tetracarboxylicacid building block ZnBDCBPP. In the steps of the condensationand metalation reactions, the isobutyl esters were used toimprove the yield and solubility in organic solvents.5f Thereaction of ZnBDCBPP with Cu(NO3)2¢3H2O in DMF/H2Oat 60 °C for 24 h gave the BDCBPP-based porous complex[Cu2(ZnBDCBPP)]. Large, high-quality crystals of [Cu2-(ZnBDCBPP)] suitable for single-crystal X-ray diffraction wereobtained from a mixture of [Cu2(ZnBDCBPP)] and Cu(NO3)2¢3H2O in DMF/H2O that was left undisturbed at room temper-ature for one month. Thermogravimetric analysis (TGA,Figure S1)15 of [Cu2(ZnBDCBPP)] under atmospheric condi-tions revealed a weight loss of 30.18% at temperatures of lessthan 275 °C (calculated 31.76% for a total of five DMF and fivewater molecules), and the weight loss due to the decompositionof the framework was observed at approximately 280 °C. Theformula for this BDCBPP-based porous complex was deter-mined on the basis of TGA to be [Cu2(ZnBDCBPP)]¢5DMF¢5H2O. The composition of this complex was also confirmed byelemental analysis (see Supporting Information).15

Single-crystal X-ray analysis revealed that [Cu2-(ZnBDCBPP)] crystallizes in the tetragonal P42/mnm spacegroup.12 The carboxylate groups of the ZnBDCBPP buildingblock form four different Cu2 paddle-wheel units, and the axialpositions of the Cu2 paddle-wheel units are occupied bycoordinated water molecules (Figure 2). If the paddle-wheelunits are considered to be four-connecting nodes and the twocarboxybiphenyl groups of the ZnBDCBPP ligand are consid-ered to be three-connecting nodes, the overall structure of[Cu2(ZnBDCBPP)] is three-dimensional and can be symbolizedas a net with (62 82 102)(62 8)2, as calculated using the TOPOSsoftware package. This topological net is recognized as net

sqc3895 (EPINET database).13 The three-dimensional structureof [Cu2(ZnBDCBPP)] is very similar to that of the BDCPP-based MOF reported previously5f and may also be regarded asan assembly of cage structures that consist of eight ZnBDCBPPbuilding blocks and eight Cu2 paddle-wheel units (Figure 3).This cage has an ellipsoidal cavity elongated along the c axisbecause the ZnBDCBPP building block exhibits a unidirection-ally elongated molecular structure. The ellipsoidal cage in[Cu2(ZnBDCBPP)] is elongated with a diameter of 19.7¡ alongthe a and b axes (minor axes), which is similar to the elongationin [Cu2(ZnBDCPP)] but with a diameter along the c axis (majoraxis) of 27.2¡. The void volume, which was calculated from thesingle-crystal structures using PLATON/VOID,14 is 57.4% andis larger than that of the DDCPP-based MOF (53.5%).Compared with [Cu2(ZnBDCPP)], [Cu2(ZnBDCBPP)] containslarger pore windows when viewed along the a and b axesbecause of the elongated molecular size and the differentorientation of the BDCBPP units. These large pore windowsappear to be crucial for achieving an enhanced catalyticperformance owing to their facilitation of the diffusion ofsubstrates and products. In fact, unlike [Cu2(ZnBDCPP)], theTGA of the [Cu2(ZnBDCBPP)] sample after treatment with THFshowed no weight loss corresponding to the loss of DMFmolecules (Figure S2).15 This result indicates that the guestDMF molecules in the cavity can be removed because of thelarge pore window, and the elongation of the pore windows canfacilitate the diffusion of the guest molecules in the cavity.

To confirm the structural stability of [Cu2(ZnBDCBPP)],we performed X-ray powder diffraction studies (PXRD,

N

N N

N

Zn

OH

O

O

OH

OH

O

O

HO

NH2

OH

O

O

OH

I

Oi Bu

O

Oi Bu

O

Oi Bu

O

Oi Bu

O

CHO

N

N N

N

Zn

Oi Bu

O

O

Oi Bu

Oi Bu

O

O

i BuO

N

NH N

HN

Oi Bu

O

O

Oi Bu

Oi Bu

O

O

1) NaNO2, HCl / H2O2) KI3) H2SO4 / i BuOH

4-Formylbenzeneboronic acid,[PdCl2(PPh3)2], K2CO3/ DMF, H2O

1) dipyrrolemethane, BF3·OEt2 / CHCl32) DDQ, Et3N

Zn(OAc)2·2H2O/ CHCl3

1) NaOH / THF, H2O2) 1M HClaq.

1 2 3

H2BDi BuCBPP ZnBDi BuCBPP ZnBDCBPP

ZnBDCBPP

Cu(NO3)2·3H2O/DMF, H2O, HCl

[Cu2(ZnBDCBPP)]·5DMF·5H2O

i BuO

Scheme 1.

19.7 Å 27.2 Å

8.1 Å

(a) (b)

C

N

O

Cu

Zn

Figure 2. (a) ZnBDCBPP moiety of [Cu2(ZnBDCBPP)];(b) cage consisting of eight ZnBDCBPP ligands and eightpaddle-wheel Cu2 nodes. H atoms are omitted for clarity.

c

a

Figure 3. Extended structure of [Cu2(ZnBDCBPP)] viewedalong the crystallographic b axis.

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Figure S3).15 The PXRD pattern matched the simulated patternbased on single-crystal diffraction; however, the peaks in thepattern appeared to be slightly broadened and weakened. Thisbroadening and weakening suggested that when the sample of[Cu2(ZnBDCBPP)] was removed from the solution, the micro-porous framework structure gradually collapsed owing to thedesolvation of guest DMF or H2O molecules.

Nitrogen gas-adsorption measurements were performed on[Cu2(ZnBDCBPP)] at 77K (Figure S4).15 The sample wasimmersed in THF for several days to allow exchange of theincluded nonvolatile solvates (i.e., DMF and H2O), and thesample was subsequently dried under vacuum at room temper-ature for 24 h. A limited amount of N2 (3 cm3 g¹1) was adsorbedat ca. 760mmHg (the BET surface area was 3.90m2 g¹1, and theLangmuir surface area was 5.81m2 g¹1). The void volumecalculated from the N2 isotherm was 0.00170 cm3 g¹1 and0.127% at a relative pressure of 0.200, which is significantlysmaller than that calculated from the single-crystal structuresusing PLATON/VOID (57.4%). These facts suggest that N2

molecules may be adsorbed only on the external surface of[Cu2(ZnBDCBPP)] because of the collapse of the microporousframework structure.

The PXRD and N2 adsorption measurements indicated thatthe microporous framework structure of [Cu2(ZnBDCBPP)] wasunstable with respect to the removal of the guest molecules inthe cavity. This structural weakness of the [Cu2(ZnBDCBPP)]framework may be related to the elongation of the ZnBDCBPPbuilding block. Therefore, the BDCBPP-based framework of[Cu2(ZnBDCBPP)] may not be suitable for gas-phase heteroge-neous catalysis. However, the persistence of permanent porosityafter solvent evacuation is not required for a heterogeneouscatalyst in a solution;7b therefore, the interior space that issurrounded by two distinct types of AMSs and the large porewindows are useful for solution-based heterogeneous catalysisapplications.

In conclusion, we have succeeded in the construction of anovel porphyrin-based metal carboxylate framework, [Cu2-(ZnBDCBPP)]¢5DMF¢5H2O, with expanded interior spacesand pore windows, by using a ZnBDCBPP building block. Thecomplex has a three-dimensional porous structure with aninternal ellipsoidal cavity, which is surrounded by a total of 16AMSs of two types. This porous complex is isostructural withpreviously reported BDCPP-based frameworks; it therefore hasthe potential for systematic incorporation of a wide variety ofAMSs into the framework without alteration of the frameworktopology. Compared with BDCPP-based frameworks, [Cu2-(ZnBDCBPP)]¢5DMF¢5H2O contains large pore windows,which appear to be crucial for achieving an enhanced catalyticperformance because they facilitate the diffusion of substratesand products. We are currently working on the synthesis ofBDCBPP-based MOFs containing other metals, and efforts areunderway to evaluate the catalytic activities of these MOFs.

This work was supported by the Ministry of Education,Culture, Sports, Science and Technology, Japan, through aGrant-in-Aid for Young Scientists (B) No. 24750138. Theauthors would like to thank Enago (www.enago.jp) for theEnglish language review.

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11 J. S. Lindsey, R. W. Wagner, J. Org. Chem. 1989, 54, 828.12 A deep-purple plate crystal of [Cu2(ZnBDCBPP)] (0.15 ©

0.15 © 0.01mm3) was surrounded with liquid paraffin (Paratone-N) and analyzed at 90(2)K. Data: tetragonal, space group P42/mnm; a = 18.5837(7)¡, c = 53.135(3)¡, V = 18350.5(14)¡3,Z = 8, dcalcd = 0.747 g cm¹3; ®(MoK¡) = 0.751mm¹1. Solventmolecules in the structure were highly disordered and impossibleto refine using conventional discrete-atom models. To resolvethese issues, the contribution of the solvent electron density wasremoved using the SQUEEZE routine in PLATON. CCDC-910789 contains the supplementary crystallographic data for thispaper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

13 S. Ramsden, V. Robins, S. T. Hyde, S. Hungerford, EPINET:Euclidean Patterns in Non-Euclidean Tilings, 2006, http://epinet.anu.edu.au.

14 A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7.15 Supporting Information is available electronically on the CSJ-

Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.

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