Paper

Construction of biporous coordination networks via p–p interaction{

Osamu Ohmori, Masaki Kawano* and Makoto Fujita*

Department of Applied Chemistry, School of Engineering, The University of Tokyo,

and Genesis Research Institute, Inc., Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.

E-mail: mkawano@appchem.t.u-tokyo.ac.jp; mfujita@appchem.t.u-tokyo.ac.jp

Received 7th February 2005, Accepted 22nd March 2005

First published as an Advance Article on the web 30th March 2005

Biporous 3D-coordination networks {[(ZnI2)3(1)2(G)]?solvents}z (G ~ triphenylene or perylene) were

synthesized by the electron-deficient triazine ligand (1) with ZnI2 in the presence of an electron-rich guest (G)

via charge-transfer interaction. The frameworks of networks include predictable infinite aromatic stacking

between 1 and G. Also, single-crystal-to-single-crystal guest-exchange was successfully performed in the

channels without loss of intercalated guest molecules.

Introduction

Crystallographic studies of a number of coordination networkswere reported because of useful applications.1 Self-assembly oftransition metals and organic bridging ligands via coordinationbonds or hydrogen bonds or these combinations is a practicalmethod to prepare both infinite1 and discrete2 frameworks. Forthe designer assembly of solid materials, we focused on thecharge-transfer p–p interaction between organic bridgingligands and guest molecules.3 Generally, charge-transfercomplexes such as BET-TTF form stable infinite networkstructures via a p–p interaction. Therefore, we designed acoordination network structure possessing a donor–acceptorcolumnar stacking between aromatic groups which canpotentially play the role of a part of a host framework.We focussed on 2,4,6-tris(4-pyridyl)triazine (1)4 because of

the following features: (a) 1 behaves as an electron acceptorligand owning to the presence of a triazine ring;5 (b) theacceptor character can be controlled by coordination metals;(c) 1 possesses high planarity as well as three coordination sites.Coordination of an electron deficient metal to 1 enhances theacceptor character. In fact, a mono-porous 3D-coordinationnetwork composed of 1 and ZnI2 (Scheme 1)6 shows single-crystal-to-single-crystal guest-exchange of large organic mole-cules such as triphenylene (G1) and perylene (G2) having adonor nature.7 Crystallographic analysis of the inclusioncomplexes (3: triphenylene inclusion complex; 4: peryleneinclusion complex) revealed efficient p–p stackings betweenguest molecules (G1 and G2) and the ligand (1). The inclusioncomplexes promptly showed drastic color change by guestexchange because of charge transfer interactions betweenthe electron-deficient ligand (1) and electron-rich guests

(G1 and G2). In addition to the coordination network, discretecoordination cages such as {[Pd(en)]6(1)4}

121 also have highcapability of aromatic guest inclusion in water.8

On the basis of the above knowledge, we expect that thecombination of the electron-deficient ligand (1) and ZnI2 withelectron-rich guests (G1 or G2) leads to an infinite aromaticstacking (1??G1??1??G1?? or 1??G2??1??G2??) to form a part of theframework. Here we report the preparation and property ofbiporous 3D-coordination networks including predictableinfinite aromatic stacking (Scheme 1). Although designablebiporous coordination networks can be highly useful forvarious applications, e.g. selective separation, transportation,and so on, there are just a few reports on the preparationand characterization of biporous coordination networks.9,10

Recently we reported a specific guest inclusion process inthe biporous network. The stability of the framework enabledus to directly observe the guest exchange process by X-raycrystallography.

Results and disccussion

Synthesis and structure

The layering diffusion of a methanol solution of ZnI2 into anitrobenzene/methanol solution of 1 and triphenylene (G1)at room temperature for a week resulted in single crystalformation of the complex {[(ZnI2)3(1)2(G1)]?x(nitrobenzene)?y(methanol)}z (5; x ~ ca. 4, y ~ ca. 2). Crystallographicanalysis revealed an interpenetrated 3-D structure in whichtriphenylene (G1) is tightly intercalated between two triazineligands (1A and 1B) (Fig. 1). The Zn atom has a tetrahedralgeometry composed of two iodide and two pyridyl groups of 1.The iodide atoms function as a protective group. Macrocyclesin the 3D-network consist of ten panels of 1 and ten hinges ofZnI2 moieties. The macrocycles are catenated through p–pstacking between two of 1 and G1 (Fig. 1b). In addition, aninfinite aromatic stacking (1A??G1??1B??G1??) is formed whichstabilizes the structure as a pillar (Fig. 1c). The interplanedistances between 1 and G1 are ca. 3.4 A, which is slightlyshorter than the sum of van der Waals radii (3.5 A).On the basis of the structural knowledge of 5, we attempted

to replace intercalated guest molecules of G1 with severalplaner ligands. The use of perylene in place of triphenyleneresulted in a similar intercalated complex, {[(ZnI2)3(1)2(G2)]?x(nitrobenzene)}z (6; x ~ ca. 4) which, as expected, containsthe same infinite aromatic stacking pattern (1A??G2??1B??G2??)as that of 5 (Fig. 1c). The interplane distances between 1 andG2

{ Electronic supplementary information (ESI) available: Structureinformation on the compounds studied (Figs. S1–S11). See http://www.rsc.org/suppdata/ce/b5/b501928a/

Scheme 1

DOI: 10.1039/b501928a CrystEngComm, 2005, 7(40), 255–259 255

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are ca. 3.4 A—similar to that of 5. DFT calculations showedthat the phases of the HOMO of guests (G1 and G2) effectivelymatched those of the LUMO of the simplified model frame-work11 (Fig. 2), allowing efficient donor–acceptor interactions.In the case of anthracene having a smaller cross-section, thecomplexation was unsuccessful, probably because the p–pinteraction was too weak to induce the pillar structure.The salient features of complexes 5 and 6 are the presence of

two kinds of distinct channels (A and B) within the framework.

Channel A is roughly cylindrical and mainly surrounded by thehydrogen atoms of infinitely stacked 1 and G1 (or G2). On theother hand, channel B is composed of three walls which areroughly trigonal prismatic. Two of the three walls are p-faces ofligand (1) and the third—the edges of 1 and G1 (or G2) (Fig. 3).Therefore, the atmosphere inside channel A is quite differentfrom that of channel B. The approximate dimensions of thechannels are 7.46 5.5 A (channel A) and 7.36 5.5 A (channelB). There are slight structural differences between 5 and 6induced by intercalated guest molecules, G1 and G2, because oftheir different HOMO phases, as shown in Fig. 2b. Theinduced distortion is partly reduced by coordination bonds toZn atoms owing to their flexibility, which is characteristic ofcoordination networks.

Single-crystal-to-single-crystal guest-exchange6,7,9,12

The salient feature of 5 and 6 is that intercalated planar guestmolecules play a role of a pillar as a part of framework and arenot extracted by the guest exchange process. The channels of 5are filled with nitrobenzene and methanol (Fig. 4a). Arrays ofguest molecules are different in each channel. In channel Asolvent molecules are disordered too severely to locate bycrystallographic analysis. In channel B they are ordered ascompared with channel A. When single crystals of 5 wereimmersed in cyclohexane at room temperature for 7 d, thecrystallographic analysis indicated that some guest moleculeswere exchanged with cyclohexane to result in {[(ZnI2)3(1)2(G1)]?x(nitrobenzene)?y(cyclohexane)}z (5’; x ~ ca. 0.66, y ~

ca. 3.2).13 The exchange abilities of two distinct channels aredifferent. Channel A was filled with cyclohexane (occupancies:64%) and nitrobenzene (occupancies: 36%). Also, channel Bwas filled with cyclohexane (occupancies: 100%) (Fig. 4b). Thedifference of the exchange ability was clearly elucidated byusing a mixture of cyclohexane and naphthalene. Channels Aand B selectively adsorbed naphthalene (100% occupancy) andcyclohexane (82% occupancy), respectively (crystal 5@, Fig. 4c).This guest exchange process was also confirmed in 6 bycrystallographic analysis. Channels A and B of as-synthesized6 were filled with nitrobenzene, which were considerablydisordered and not fully located by X-ray analysis (Fig. 5a).The guest-exchange with cyclohexane is similar to that for 5.Channels A were filled with cyclohexane (occupancies: 63%)and nitrobenzene (occupancies: 37%), whereas channel B wasfilled with cyclohexane (occupancy: 100%) (crystal 6’, Fig. 5b).However, in the case of the guest-exchange with a mixture ofcyclohexane and naphthalene, the selectivity of guest inclusionwas different from that for 5. Channel A was filled withnaphthalene (occupancies: 100%), whereas channel B was filledwith naphthalene (occupancies: 50%) and cyclohexane (occu-pancies: 50%) (crystal 6@, Fig. 5c). During the guest-exchangeprocess, the superposition of intercalated molecules (G1 andG2) with the triazine ligand (1) was almost the same. Theseresults indicate that intercalated molecules behave as a part ofthe host framework rather than as a guest.

Fig. 2 (a) Partial structures of 5 (left) and 6 (right): The occupancy ofeach guest molecule: 84% for 5; 72% for 6. (b) Superposition of theHOMO and the LUMO.

Fig. 3 Two distinct channels exhibited in complex 5.

Fig. 1 Crystal structure of 5 and 6: (a) view of the interpenetratednetworks along the b-axis; (b) catenated macrocycles in 3D-network;(c) an infinite aromatic stacking throughout ligands (1A and 1B) andplanar guest molecules [triphenylene G1 (left) or perylene G2 (right)] atan inserted square in (a). Click here to access a 3-D image of Fig. 1a.

256 CrystEngComm, 2005, 7(40), 255–259

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Spectroscopic properties

The absorption spectral tendencies of 5 and 6 are similarto those of the guest-exchanged mono-porous complex{[(ZnI2)3(1)2]?x(G)}z of 3 and 4, respectively (Figs. 6). Thecrystal color of 5 is pale yellow, on the other hand, that of 6 isdeep-red and the absorption band was significantly red-shifted,

reaching 900 nm. This result clearly indicates the presence ofcharge-transfer interactions between the triazine ligand (1) andtriphenylene (G1) [or perylene (G2)]. The perylene complex of 6shows CT bands in the lower energy region than 5 because theHOMO–LUMO gap of perylene is smaller than that oftriphenylene, as expected from their half-oxidation poten-tials.14 The absorption edges of both 5 and 6 are red-shifted by100 nm as compared with those of the corresponding guest-exchanged complex. These spectral shifts are attributable to theformation of infinite p–p stacking chains via intercalation ofplanar guest molecules.The emission spectrum for 5 in a powder state showed a

broad band with maximum at 526 nm as shown in Fig. 7.15 Theemission band is red-shifted by 130 nm as compared with thatof G1. This shift is attributed to the formation of the exciplex.The emission band maximum for 5 is almost the same as thatfor 3. This fact indicates that the excited state structure of 5having infinite stacking may be close to that of 3 having discrete1 : 1 p–p stacking. It is very likely that the emission of 5may bederived from a discrete exciplex, although we need furtherinvestigation of the electronic properties.

Thermogravimetric analysis

Thermogravimetric analysis of the crystalline sample of 5showed the release of guest molecules in two steps. Guestmolecules, weight loss obsd (calcd), and temperature areas follows: nitrobenzene and methanol, 21.82% (22.90%),100–230 uC; triphenylene, 9.63% (9.71%) ca. 400 uC. Also, the

Fig. 4 Arrays of guest molecules in channels A (left) and B (right) for 5: (a) in 5 before guest-exchange; (b) in 5’, (c) in 5@ after guest-exchange.Partially disordered molecules were omitted for clarify. The crystallographic details are shown in the supplementary information.{

Fig. 5 Arrays of guest molecules in channels A (left) and B (right) for 6: (a) in 6 before guest-exchange; (b) in 6’, (c) in 6@ after guest-exchange.Partially disordered molecules were omitted for clarify. The crystallographic details are shown in the supplementary information.{

Fig. 6 Diffuse reflectance UV-vis spectra of 5, 3, G1, 6, 4 andG2 in BaSO4.

CrystEngComm, 2005, 7(40), 255–259 257

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crystalline sample of 6 showed the release of guest molecules intwo steps: nitrobenzene and methanol, 20.72% (21.14%), 100–230 uC; perylene, 10.36% (10.92%) ca. 430 uC. These resultsagreed with elemental analyses. Note that the crystals oftriphenylene or perylene showed weight loss at lower tem-perature (triphenylene: 210–330 uC; perylene: 270–390 uC).This fact indicates that the coordination network stronglyholds intercalated molecules.

Conclusion

We proposed a new approach to construct a 3-D coordinationnetwork via p–p interaction rather than covalent or hydrogenbonds. In this study the framework construction waspredictable, depending on matching of both HOMO–LUMOphases and energy gap. The infinite p–p stacking in intercalatednetworks of 5 and 6 constitute a pillar as a part of hostframeworks. The advantage of this method is that functionalgroups can be readily introduced in a cavity by modificationof intercalated planar molecules. In addition, we found thatcomplexation of 1 and ZnI2 with intercalated moleculesproduced biporous networks. Since biporous materials witha functionalized cavity are promising for application, furtherdevelopment is in progress.

Experimental

General

Reagents and solvents were obtained from TCI Co., Ltd.,WAKO Pure Chemical Industries Ltd., and Aldrich Chemical.,Ltd. and used without further purification. Diffuse reflectanceUV-vis spectra were measured on ShimadzuUV-3150 equippedwith an integrating sphere and were converted from reflectionto absorbance by the Kubelka-Munk method. IR spectra weremeasured on DIGILAB FTS2000S by ATR method. Emissionspectra were measured on JASCO FP-6500. Thermogravi-metric analyses were carried out on Netzsch STA-409 PG Luxxat a heating rate of 10 K min21 in a dynamic nitrogenatmosphere (gas flow rate 70 ml min21).

Single crystal X-ray crystallography

Single crystal X-ray diffraction data were collected on a BrukerSmart 1000 CCD diffractometer with a cryostat systemequipped with a N2 generator (Japan Thermal Eng. Co.,Ltd.) using graphite-monochromated Mo Ka radiation (l ~0.71073 A). Face indexing absorption corrections were appliedto crystals 5 and 6 using the XPREP and SADABS. Empiricalabsorption corrections were applied to crystals 5’, 5@, 6’, and 6@

Fig. 7 Emission spectra of 5, 3 and G1 with 320 nm excitation.

Table

1Crystaldata

for5–6@

56

5’

5@

6’

6@

Empiricalform

ula

C79.33H

55.56I 6N

15.91O

9.68Zn3

C72.38H

49.63I 6N

14.75O

5.50Zn3

C77.13H

77.65I 6N

12.66O

1.32Zn3

C85.38H

71.07I 6N

12Zn3

C79.31H

77.74I 6N

12.69O

1.39Zn3

C86.92H

67.93I 6N

12Zn3

Form

ula

weight

2344.00

2171.40

2160.47

2222.62

2188.44

2237.99

Crystalsize/m

m0.176

0.116

0.06

0.166

0.116

0.08

0.136

0.096

0.08

0.126

0.116

0.10

0.146

0.126

0.10

0.226

0.146

0.13

Crystalsystem

Orthorhombic

Orthorhombic

Orthorhombic

Orthorhombic

Orthorhombic

Orthorhombic

Space

group

Pbca

Pbca

Pbca

Pbca

Pbca

Pbca

a/A

27.268(3)

28.253(3)

27.873(2)

27.6479(19)

27.865(3)

28.519(2)

b/A

13.6992(16)

13.8524(15)

13.7073(10)

13.7500(9)

13.7117(15)

13.8219(11)

c/A

45.888(5)

44.966(5)

45.852(3)

45.714(3)

45.650(5)

44.097(3)

V/A

317141(3)

17598(3)

17518(2)

17379(2)

17442(3)

17383(2)

Z8

88

88

8m/m

m21

3.058

2.968

2.977

3.002

2.991

3.002

T/K

80(2)

80(2)

80(2)

80(2)

80(2)

80(2)

R1[I

w2s(I)]

0.0821

0.0893

0.0739

0.0517

0.1034

0.0773

wR2[I

w2s(I)]

0.1478

0.2230

0.1321

0.1174

0.1792

0.1589

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using the SADABS. The structures were solved by directmethods (SHELXS 97) and refined by full-matrix least-squarescalculations on F2 (SHELXL-97) using the SHELX-TLprogram package. The crystal data for 5–6@ are summarizedin Table 1. CCDC 251671 (5), CCDC 258684 (6), CCDC251674 (5’), CCDC 261642(6’), CCDC 251672 (5@), and CCDC261643 (6@) are the CCDC reference numbers. These datacan be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk,or by contacting The Cambridge Crystallographic DataCentre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:144 1223 336033. See http://www.rsc.org/suppdata/ce/b5/b501928a/ for crystallographic data in CIF or other electroncformat.

Synthesis of 5

A buffer zone of methanol (0.5 mL) was layered on the top of anitrobenzene/methanol solution (5 mL, 4/1) of ligand (1)(6.3 mg, 0.02 mmol) and triphenylene (G1, 22.8 mg, 0.1 mmol)and then a methanol solution (0.5 mL) of ZnI2 (9.6 mg,0.03 mmol) was carefully layered on the top of that. After thesolution was allowed to stand for a week, the crystalsformed were isolated in 54% yield (12.7 mg, 0.0054 mmol) byfiltration. Anal Calcd for {[(ZnI2)3(1)2(G1)]?3.9(nitrobenzene)?1.8(methanol)}z: C, 40.51; H, 2.69; N, 9.48. Found: C, 40.89;H, 2.60; N, 9.49. IR (ATR, cm21) 3065, 1620, 1578, 1515, 1424,1376, 1344, 1316, 1244, 1215, 1162, 1107, 1060, 1027, 936,868, 851.

Synthesis of 6

A buffer zone of methanol (0.5 mL) was layered on the top ofa nitrobenzene/methanol solution (6 ml, 2/1) of ligand 1(0.02 mmol) and perylene (G2, 5.1 mg, 0.02 mmol) and then amethanol solution (0.5 mL) of ZnI2 (9.6 mg, 0.03 mmol) wascarefully layered on top of that. After the solution had beenallowed to stand for a week, the crystals formed were isolatedin 45% yield (10.5 mg, 0.0045 mmol) by filtration. Anal Calcdfor {[(ZnI2)3(1)2(G2)]?4(nitrobenzene)}z: C, 41.29; H, 2.66; N,9.63. Found: C, 41.56; H, 2.51; N, 9.35. IR (ATR, cm21) 3053,1620, 1575, 1516, 1423, 1375, 1345, 1315, 1214, 1160, 1107,1059, 1027, 935, 869, 851, 819.

Guest exchange procedure

The crystals of 5/6 were immersed in a solution of guestmolecules for 1–7 days.

Elemental analyses of 5’–6@. Calcd for {[(ZnI2)3(1)2(G1)]?2.0(cyclohexane)?0.4(nitrobenzene)}z (5’): C, 40.51; H, 3.08; N,8.56. Found: C, 40.51; H, 3.03; N, 8.18.Calcd for {[(ZnI2)3(1)2(G1)] ?1.3(cyclohexane)?2.3(naphtha-

lene)}z (5@): C, 45.99; H, 3.19; N, 7.59. Found: C, 45.71; H,2.92; N, 7.29. Calcd for {[(ZnI2)3(1)2(G2)]?1.8(cyclohexane)?0.2(nitrobenzene)}z (6’): C, 40.58; H, 3.03; N, 8.49. Found:C, 40.44; H, 3.19; N, 8.20. Calcd for {[(ZnI2)3(1)2(G2)]?2.8(naphthalene)}z (6@): C, 45.95; H, 2.77; N, 7.66. Found:C, 45.78; H, 2.86; N, 7.29.

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11 The simplified model of 5/6, [(ZnI2)3(1)(pyridine)3], was optimizedat the B3LYP level of theory with the SDD basis set for Zn and I.

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13 The cyclohexane-exchanged crystals were immersed in nitroben-zene at room temperature. The crystallographic analysis revealedre-inclusion of nitrobenzene.

14 E1/2 (vs. SCE) ~ 1.55 mV (triphenylene), 0.85 mV (perylene).E. S. Pysh and N. C. Yang, J. Am. Chem. Soc., 1963, 85,2124–2130.

15 Unfortunately, the emission bands of 6 were not clearly observedin the region of 600 to 700 nm because of overlapping with anexcitation band.

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