Remarkably Stable Porous Assembly of Nanorods Derived from a SimpleMetal-Organic Framework

D. Krishna Kumar, Amitava Das,* and Parthasarathi Dastidar*

Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute (CSIR), G B Marg,BhaVnagar 364 002, Gujarat, India

ReceiVed October 3, 2006; ReVised Manuscript ReceiVed NoVember 21, 2006

ABSTRACT: A new strategy has been described to create a microporous metal-organic framework by assembling nanorodlike constructsequipped with a complementary hydrogen-bonding surface. Guest-accessible channels are located in the internanorod space generated bythe self-assembly of the nanorodlike constructs via complementary hydrogen bonding. Thus, reaction of a hydrogen-bond-equipped andconformationally flexible ligand such asN,N′-bis(3-pyridyl)urea with CuSO4 resulted in the formation of a stable, guest-free microporousMOF wherein a nanorodlike construct is formed that is further self-assembled into a microporous metal-organic framework via self-complementary hydrogen bonding involving a urea backbone and sulfate anion. Because of conformational flexibility, a one-dimensionalzigzag coordination polymeric network is also formed. The structures are characterized mainly by single-crystal X-ray diffraction techniques.

Porous networks of MOF origin have attracted extensive researchattention because of their various potential applications.1 However,it is difficult to control the size and shape of the pores as well asthe pore-volume availability. Moreover, collapse of porous networksdue to guest removal or high temperature is a serious problem indesigning useful porous MOFs. Thus, creation of stable porousnetwork of MOF origin always remains a major challenge.

Inorganic and organic nanotubes have received much impetusdue to their various potential applications.2 It may be envisagedthat assembly of nanotubes devoid of a sticky surface (e.g., absenceof complementary hydrogen-bonding sites) are expected to packin a hexagonal close packing manner, thereby leaving no intertubespace in the crystal lattice for occluding any guest species. However,nanotubes with complementary hydrogen-bonding functionality onthe surface are expected to self-assemble in a fashion directed bythe specific hydrogen-bonding interactions, thereby leaving intertubespace for guest molecules to accommodate (Scheme 1). Thisstrategy of creating a stable porous network by assembling stickynanotubes of MOF origin is hitherto unknown.

As a part of our research program in the area of MOFs,3 wehave recently reported that the ligandN,N′-bis(3-pyridyl)ureaL14

gave a square-grid network, whereas its structural isomer, namelyN,N′-bis(4-pyridyl)urea, resulted in a 5-fold interpenetrated dia-mondoid network in the resulting MOFs.3d It is readily envisagedthatL1 is able to display various ligating topologies depending onthe relative orientation of the pyridyl rings with respect to the ureafunctionality.5 Consequently, the resulting MOF architecture isdependent on the ligand conformation. For example, syn-anticonformation should give rise to 1D zigzag polymeric chain,whereas syn-syn or anti-anti conformation may result in theformation of a metal-organic macrocycle (Scheme 2).

In the latter case, the metal-organic macrocycle can be furtherlinked to form a nanotubular construct using a suitable bridgingligand such as sulfate.6 Because the ligandL1 has a hydrogen-bonding functionality such as urea, which is capable of formingchelate hydrogen-bonding interactions with an oxoanion such assulfate (urea-sulfate synthon), it is expected that the resultingnanotubular constructs would self-assemble further, directed bythese complementary hydrogen-bonding interactions and therebygenerating a microporous MOF. Thus, we have reactedL1 withCuSO4 in a 1:1 molar ratio with the hope of creating a nanotubularconstruct that would self-assemble into a porous network. Crystalsof different morphologies (needle1 and block2) were obtainedfrom the reaction mixture7 (see the Supporting Information). X-raystructural analyses of these two crystals8 revealed that the crystals

of 1 and 2 are MOFs of nanorod and zigzag architectures,respectively, as envisaged in Scheme 2.

In 1, the ligand adopts a syn-syn conformation with significantmolecular planarity (urea/pyridyl dihedral angles of 2.2 and 3.3°).Two such ligands connect the Cu(II) metal center in such a waythat a metal-organic macrocyle is formed. The metal center isfound to be slightly distorted square-pyramidal (∠N-Cu-O )

* To whom correspondence should be addressed. E-mail: dastidar@csmcri.org, parthod123@rediffmail.com (P.D); amitava@csmcri.org (A.D.).Fax: 91-278-2567562.

Scheme 1

Scheme 2

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85.2(2)-100.1(2)°). The two pyridyl N atoms ofL1 occupy theequatorial positions in a trans fashion. One water molecule is foundto be coordinated to the other equatorial position. Sulfate, on theother hand, acts as a bridging ligand by coordinating equatorialand apical positions of the metal centers of the adjacent metal-organic macrocycles, thereby forming a 1D infinite nanotubularconstruct. A space of∼1.2 × 6.2 Å2 (considering van der Waalsradii) is present in the metal-organic macrocycle. However, themacrocycles are arranged on top of each other in a slightly offsetfashion, presumably due to the coordinated water molecule thatoccupies one of the equatorial positions. As a result, a close-endednanotubular MOF is formed and the space within the nanotube isoccupied by the metal-bound water molecule, which is furtherhydrogen bonded with the CdO oxygen atoms of the urea moietyof each macrocycle (O‚‚‚O ) 2.757(5) Å;∠O-H‚‚‚O ) 157(9)°).Therefore, the space within the nanotubular construct is not availablefor further guest occlution. Thus, this nanotubular assembly canbest be described as comprising nanorods. However, the outersurface of the nanorod is equipped with complementary hydrogen-bonding functionalities arising from urea N-H and sulfate O atoms.In the crystal structure, it can be seen that each nanorod issurrounded by four neighboring nanorods sustained by comple-mentary N-H‚‚‚O hydrogen bonding involving urea and sulfatemoieties (N‚‚‚O ) 2.715(6)-2.833(5) Å;∠N-H‚‚‚O ) 152.3-165.3°), resulting in the formation of a 2D hydrogen-bondednetwork with a continuous internanorod channel running down thea-axis (Figure 1). No electron density peaks could be found withinthe channel. PLATON SQUEEZE9a calculations indicate that thereare no electron densities left in the unit cell.7 This indicates thatthe assembly of the nanorods through specific hydrogen-bondinginteractions results in the formation of a stable, guest-free mi-croporous network. PLATON9b calculations show that the solvent-accessible void is 218.4 Å3 per unit-cell volume. A carefulexamination of the internanorod channel space reveals that the innerwall of the channel is highly hydrophobic. Because the crystals of1 are obtained from EtOH/water, it is expected that the solventmolecules are not strongly bound in the hydrophobic space of theinternanorod channel, thereby leaving the crystal lattice before orduring data collection. Thermal analyses7 of 1 confirmed theabsence of any guest species that corroborates well with the X-raystructure and the framework in1 collapses at a temperature as highas 260°C. On the other hand, block-shaped crystal2 turned out tobe a 1D zigzag polymeric network,7 understandably arising becauseof the syn-anti conformation ofL1 (Figure 2).

It is interesting to note that the CdO stretching bands of1 and2 appear at 1673 and 1703 cm-1, respectively. The 30 cm-1 shiftof the CdO band may be attributed to the difference in hydrogen-bonding environment of the urea carbonyl in these two MOFs; in1, the urea O atom is hydrogen bonded to two coordinated watermolecules in a bifurcated fashion, whereas that in2 is hydrogenbonded with one solvate water molecule. This is further cor-roborated with the thermal analyses,7 wherein the water moleculesof 1 are released at much higher temperature (136.4°) than that ofin 2 (87.2°).

It may be noted that both MOFs1 and2 are obtained in a singlepot. To find out the experimental conditions that result in selectivepreparation of one of the MOFs, we reacted the components usingan instant mixing method.10 XRPD of the resultant microcrystals7

shows an excellent match with that of the simulated pattern obtainedfrom single-crystal data of2, indicating the exclusive formation ofzigzag MOF under this experimental condition, whereas the layeringmethod always resulted in the formation of both nanorod and zigzagMOFs.

The complementary hydrogen-bonding surface (urea-sulfatesynthon) of the nanorods helps them self-assemble into a highlystable guest-free microporous MOF. Thus, by exploiting theconformational flexibility and hydrogen-bonding backbone (urea)of the ligandL1 and judicial choice of the counterion cum linkerligand (sulfate), we have been able to self-assemble the resultingnanorodlike MOF via complementary hydrogen-bonding interac-tions involving the urea moiety and sulfate ion into a highly stable,guest-free microporous framework. This strategy of assembling thediscrete sticky nanorodlike constructs into a hierarchical mi-croporous framework is the first example in the literature.

Acknowledgment. We thank the Department of Science &Technology (DST), New Delhi, India, for financial support. D.K.K.thanks CSIR, New Delhi, India, for a SRF.

Supporting Information Available: Syntheses, FT-IR data, crystal-lographic data (CIF) in CIF format, thermal analyses plot for1 and2, XRPDcomparison plot for2. This material is available free of charge via theInternet at http://pubs.acs.org.

References

(1) (a) Saalfrank, R. W.; Demleitner, B. InTransition Metals inSupramolecular Chemistry; Perspectives in Supramolecular Chem-istry; J.-P. Sauvage, Ed.; Wiley-VCH: Weinheim, Germany, 1999;Vol. 5, p 1. (b) Fujita, M.Chem. Soc. ReV. 1998, 27, 417. (c) Stang,P. J.; Olenyuk, B.Acc. Chem. Res.1997, 30, 502. (d) Schnebeck,R.-D.; Freisinger, E.; Lippert, B.Angew. Chem., Int. Ed.1999, 38,168. (e) Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kubota, Y.;Belosludov, R. V.; Kobayashi, R. C.; Kakamoto, H.; Chiba, T.;Takata, M.; Kawazoe, Y.; Mita, Y.Nature2005, 436, 238. (f) Rosi,N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe,M. Yaghi, O. M. Science2003, 300, 1127. (g) Pan, L.; Olson, D.H.; Ciemnolonski, L. R.; Heddy, R.; Huang, X.-Y.; Li, J.Angew.Chem., Int. Ed.2006, 45, 616. (h) Li, Y.; Yang, R. T.J. Am. Chem.Soc.2006, 128, 726. (i) Kepert, C. J.Chem. Commun.2006, 695.

(2) (a) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; Pellechia, P. J.; zurLoye, H.-C.J. Am. Chem. Soc.2004, 126, 3576 and references citedtherein. (b) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R.Angew. Chem., Int. Ed.2001, 40, 988.

(3) (a) Krishna Kumar, D.; Ballabh, A.; Jose, D. A.; Dastidar, P.; Das,A. Cryst. Growth Des.2005, 5, 651. (b) Krishna Kumar, D.; Das,A.; Dastidar, P.Cryst. Growth Des.2006, 6, 216. (c) Krishna Kumar,

Figure 1. Optical micrograph of the crystal of1 used for data collection(left); crystal structure illustration of1; (a) metal-organic macrocycle; (b)polymeric network of nanorod-like construct displaying sulfate (orange) aslinker of the macrocycles; (c) self-assembly of the nanorodlike constructsdisplaying microporous architecture; (d) titled view of assembly of fourindependent nanorodlike constructs.

Figure 2. Optical micrograph of the crystal of2 used for data collection(left); 1D polymeric zigzag network of MOF2 displaying hydrogen-bondedwater tetramer (right).

206 Crystal Growth & Design, Vol. 7, No. 2, 2007 Communications

D.; Das, A.; Dastidar, P.J. Mol. Struct.2006, 796, 139. (d) KrishnaKumar, D.; Jose, D. A.; Das, A.; Dastidar, P.Inorg. Chem.2005,44, 6933. (e) Krishna Kumar, D.; Das, A.; Dastidar, P.Cryst. GrowthDes.2006, 6, 1903.

(4) Krishna Kumar, D.; Jose, D. A.; Das, A.; Dastidar, P.Chem.Commun. 2005, 4059.

(5) Custelcean, R.; Moyer, B. A.; Bryantsev, V. S. Hay, B. P.Cryst.Growth Des.2006, 6, 555.

(6) CSD 5.27, Nov. 2005 release: A search motif having sulphateconnected to any transition metal in a polymeric fashion resulted in162 hits.

(7) See the Supporting Information.(8) Crystal data for1: C11H12CuN4O6S, fw ) 391.85, 0.34× 0.10×

0.05 mm3, monoclinic,P21/c, a ) 4.9618(14) Å,b ) 16.979(5) Å,c ) 18.354(5) Å,â ) 91.968(5)°, V ) 1545.3(8) Å3, T ) 298(2) K,Z ) 4, Fcalcd) 1.684 g cm-3, µ ) 1.584 mm-1, F(000)) 796; 8498reflections collected. Final residuals (for 213 parameters) were R1) 0.0643, wR2) 0.1426 for 2534 reflections withI > 2σ(I), andR1 ) 0.0958, wR2) 0.1672, GOF) 1.031 for all 3494 reflections.

Max/min residual electron density: 0.930/-0.603 e/Å3. Crystal datafor 2: C11H18CuN4O9S, fw ) 445.89, 0.36× 0.24 × 0.18 mm3,monoclinic,P21/c, a ) 7.5168(8) Å,b ) 21.771(2) Å,c ) 12.1053-(10) Å, â ) 122.872(5)°, V ) 1663.8(3) Å3, T ) 100(2) K,Z ) 4,Fcalcd ) 1.780 g cm-3, µ ) 1.496 mm-1, F(000) ) 916; 6502reflections were collected. Final residuals (for 235 parameters) wereR1 ) 0.0478, wR2) 0.1162 for 1754 reflections withI > 2σ(I),and R1 ) 0.0607, wR2) 0.1230, GOF) 1.036 for all 2163reflections. Max/min residual electron density: 0.749/-0.662 e/Å3.

(9) (a) Van der Sluis, P.; Spek, A. L.Acta Crystallogr., Sect. A1990,46, 194. (b) Spek, A. L.PLATON-97; University of Utrecht: Utrecht,The Netherlands, 1997.

(10) Reaction was performed by immediate mixing of an ethanolic solution(10 mL) of the ligandL1 (42.8mg, 0.2mmol) with an aqueoussolution of Cu(SO4)2‚5H2O (49.8 mg, 0.2 mmol) in a 50 mL RBflask. The mixture was stirred at room temperature for 10 minutesand filtered.

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