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Polyhedron 25 (2006) 2483–2490

Photochromic Rh(II) complexes based on1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene

Jing Han, Atsuhiro Nabei, Yusaku Suenaga, Masahiko Maekawa, Hiromichi Isihara,Takayoshi Kuroda-Sowa, Megumu Munakata *

Department of Chemistry, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

Received 12 November 2005; accepted 15 February 2006Available online 16 March 2006

Abstract

Reaction of [Rh2(O2CC(CH3)3)4] with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (cis-dbe) or closed-dbe in benzene anddichloromethane yielded three novel Rh(II) complexes. Their structures were characterized and photochromic properties were studied.X-ray crystallographic analyses revealed that the coordination environments of the three metal complexes are quite different. Complex 2

exhibits a 1-D infinite chain structure with two cyano groups of the ligand bridging two metal ions while in complexes 1 and 3 the metalions have a direct interaction with at least one of the sulfur atoms of the two bisthienylethene molecules. Closed-dbe of complex 3 wastransferred to the ring-open form in the crystalline phase upon photoexcitation with 529 nm light. It underwent a photocycloreversionreaction although the metal atom is coordinated to the thienyl group. The irreversible cyclization reaction was presumably attributed tothe fixed rotation of thienyl rings, short RhAS distance and steric hindrance of anions.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Bisthienylethene; cis-dbe; Rhodium(II); Photochromism; Crystalline phase

1. Introduction

Considerable research efforts have been devoted toorganic photochromic compounds that exhibit two differ-ent chemical forms and are reversibly interconverted uponirradiation with light of the appropriate wavelength. Oneof the most promising classes of photochromic materialsis diarylethenes with attached thiophene rings (i.e., bisthie-nylethenes). These bisthienylethene derivatives are promis-ing candidates for optical memory and photoswitchingmolecules because of their outstanding fatigue resistantand high thermal stabilities [1–14].

cis-1,2-Dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene(cis-dbe) was first synthesized and studied by Irie [9,15,16].It undergoes the following photochromism in solution andin the solid phase (Scheme 1). The original yellow open-

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doi:10.1016/j.poly.2006.02.013

* Corresponding author. Tel.: +81 6 6723 2332/4119; fax: +81 6 67232721.

E-mail address: munakata@chem.kindai.ac.jp (M. Munakata).

ring form is transformed into the red closed-ring formwhen irradiated with 405 nm light, and the red isomer con-verted back the initial color upon exposure to 546 nm light.Various properties, including optical and electrochemical,of cis-dbe have also been investigated, which show itspromising application to novel function photomemoryand photoswitching devices [17,18]. Recently much interesthas been directed to the photoisomerization of cis-dbemolecularly doped in amorphous polymer thin filmsformed by varied techniques such as casting, spin-coatingand the vapor transportation method [19–25]. But studiesof cis-dbe in the crystalline phase are still extremely rare[26–28].

We have succeeded in the preparation of Ag(I), Cu(I),Mo(II) and Rh(II) complexes with cis-dbe or closed-dbeand their photochromic behaviors were investigated inthe crystalline phase [26–28]. Among these complexes onlydirhodium(II) trifluoroacetate complexes did not displayreversible photochromic reactions. In a continuation ofour work and with the aim of changing the coordination

CNNC

SS

CNNC

S S

405 nm

546 nm

Scheme 1. Photochromic reaction of cis-dbe.

2484 J. Han et al. / Polyhedron 25 (2006) 2483–2490

properties of Rh(II), dirhodium pivalate was selected as themetal salt in this work and three novel Rh(II) complexeswere prepared, which are different from the previouslydirhodium trifluoroacetate complexes [28] both in coordi-nation modes and in conformation. Furthermore it wasfound that the dirhodium complex with the closed-dbe,where the metal atom has a direct coordination with thethienyl ring, is transferred to the open-form upon excita-tion with 529 nm light during the cycloreversion reaction.

2. Experimental

2.1. General methods

Unless otherwise indicated, all starting materials wereused as received from commercial suppliers (Aldrich andWako Chemicals). All reactions and manipulations werecarried out under an argon atmosphere prior to use. Sol-vents were dried using standard procedures and distilledunder an argon atmosphere. [Rh2(O2CC(CH3)3)4] was pre-pared according to the known literature procedure [29].Closed-dbe was obtained by separating a cis-dbe hexanesolution irradiated with 405 nm light.

Infrared spectra were recorded using KBr disks on aJASCO FT/IR-430 spectrometer. Absorption spectra inthe crystalline state were measured by diffuse reflectionusing the Kubelka–Munk method on a SHIMADZUUV-2450 spectrometer, and barium sulfate was used as areference. Photoirradiation was carried out using a 150 WXe lamp and monochromatic light was obtained by passingthe light through a monochromator.

2.2. Syntheses

2.2.1. [Rh2(O2CC(CH3)3)4(cis-dbe)2] (1)

[Rh2(O2CC(CH3)3)4] (15.3 mg, 0.03 mmol) and cis-dbe(8.2 mg, 0.03 mmol) were dissolved in 5 mL benzene, andthe resultant solution was carefully introduced into a7 mm glass tube, which afforded complex 1 as green brickcrystals after standing in the dark at an ambient temperaturefor 1 week. IR (KBr pellet, range 4000–400 cm�1): 2965 (m),2920 (m), 2864 (m), 2214 (m), 1578 (s), 1547 (w), 1484 (w),1457 (w), 1439 (w), 1416 (m), 1376 (w), 1363 (w).

2.2.2. [Rh2(O2CC(CH3)3)4] (trans-dbe)](CH2Cl2)4 (2)

Red-brown brick crystals were obtained similarly tothose of complex 1 using dichloromethane instead of ben-zene. Yield: 63.8%. Rh2S2O8N2C39.5H57Cl3: C, 44.58; H,

5.40; N, 2.63. Found: C, 44.42; H, 5.33; N, 2.61%. IR(KBr pellet, range 4000–400 cm�1): 2987 (m), 2958 (m),2923 (m), 2867 (m), 2237 (w), 1585 (s), 1484 (m), 1454(w), 1415 (s), 1376 (m), 1363 (w).

2.2.3. [Rh2(O2CC(CH3)3)4] (closed-dbe) (3)Red brick crystals were obtained similarly to those of

complex 1 using closed-dbe in place of cis-dbe. Yield:70.0%. Anal. Calc. for Rh2S2O8N2C38H54: C, 48.72; H,5.81; N, 2.99. Found: C, 47.78; H, 5.51; N, 2.90%. IR(KBr pellet, range 4000–400 cm�1): 3446 (m), 2959 (m),2928 (m), 2904 (m), 2871 (m), 1584 (s), 1540 (w), 1505(m), 1483 (s), 1457 (w), 1444 (w), 1414 (s), 1375 (m), 1363(m).

2.3. X-ray data collection and structure solution and

refinement

Diffraction data for complexes 1–3 were collected on aRigaku MSC Mercury CCD diffractometer with graphitemonochromated Mo Ka radiation (k = 0.71069 A). Inten-sity data were collected at 150 K using the x scanning tech-nique and a total of 6623, 6235 and 12071 reflections werecollected for complexes 1–3, respectively. No decay correc-tion was applied. The linear absorption coefficients l forthe Mo Ka radiation were 7.28, 10.62 and 7.38 cm�1,respectively.

The structures were solved by direct methods followedby subsequent Fourier calculations [30]. The non-hydrogenatoms were refined anisotropically for all complexes. Forcomplex 1 the hydrogen atoms were located from differencedensity maps and their coordinates were refined. The CAHand OAH bonds lengths are 0.69(3)–1.14(3) A. For com-plexes 2 and 3 the hydrogen atoms were introduced as fixedcontributors. The final cycle of the full-matrix least squaresrefinement was based on 6059, 5809 and 9635 observedreflections and 442, 307 and 470 variable parameters for1–3, respectively, converged with the unweighted andweighted agreement factors of R ¼

PkF oj � jF ck=

PjF oj

and Rw ¼ ½P

wðF 2o � F 2

cÞ2=P

wðF 2oÞ

2�1=2. The atomic scat-tering factors and anomalous dispersion terms were takenfrom the International Tables for X-ray Crystallography,vol. IV [31]. All calculations were performed using the TEX-

SAN crystallographic software package [32]. Details of theX-ray experiments and crystal data are summarized inTable 1. Selected bond lengths and bond angles are givenin Table 2.

3. Results and discussion

3.1. Structural characterizations

The structures of the three Rh(II) coordination com-pounds were determined by X-ray crystallographic analy-ses and are showed in Figs. 1–3. The crystallographicdata and lengths and angles of selected bonds of complexes1–3 are listed in Tables 1 and 2.

Table 1Crystallographic data for complexes 1–3

1 2 3

Formula Rh2S4O8N4C56H72 Rh2S2O8N2C42H62Cl8 Rh2S2O8N2C38H54

FW 1263.26 1276.52 936.78Crystal system triclinic monoclinic orthorhombicSpace group P1(#2) C2/c(#15) Fdd2(#43)a (A) 10.212(4) 17.992(2) 36.112(3)b (A) 12.968(8) 17.224(2) 51.701(5)c (A) 13.042(6) 18.549(2) 11.379(1)a (�) 93.40(2)b (�) 104.29(3) 88.817(6)c (�) 112.20(2)V (A3) 1527(1) 5746(1) 21245(3)Z 1 4 16D (g cm�3) 1.374 1.475 1.171l (Cu Ka) 7.28 10.62 7.38Observed reflections (I > 2r(I)) 6059 5809 9635Ra 0.034 0.046 0.068Rw

b 0.088 0.120 0.204Goodness-of-fit 1.05 1.08 1.05

a R ¼PkF oj � jF ck=

PjF oj.

b Rw ¼ fP

wðF 2o � F 2

cÞ2=P

wðF 2oÞ

2g1=2.

Table 2Selected bond lengths (A) and angles (�) for complexes 1–3

Complex 1

Rh(1)ARh(1*) 2.394(1) Rh(1)AO(2*) 2.030(2)Rh(1)AS(1) 2.544(1) Rh(1)AO(3) 2.042(2)Rh(1)AO(1) 2.033(2) Rh(1)AO(4*) 2.032(2)

Rh(1*)ARh(1)AS(1) 175.40(2)

Complex 2

Rh(1)ARh(1*) 2.3802(4) Rh(1)AO(4*) 2.037(2)Rh(1)AO(1) 2.039(2) N(1)AC(6) 1.140(4)Rh(1)AO(2*) 2.029(2) Rh(1)AN(1) 2.137(3)Rh(1)AO(3) 2.034(2)

Rh(1)AN(1)AC(6) 163.9(3) Rh(1*)ARh(1)AN(1) 176.46(7)

Complex 3

Rh(1)ARh(2) 2.3987(8) Rh(1)AO(1) 2.041(6)Rh(1)AS(1) 2.585(2) Rh(1)AO(3) 2.044(6)Rh(2 0)AN(2) 2.155(7) Rh(1)AO(5) 2.048(6)C(3)AN(1) 1.16(1) Rh(1)AO(7) 2.021(6)C(4)AN(2) 1.14(1)

Rh(2)ARh(1)AS(1) 176.30(5) Rh(20)AN(2)AC(4) 173.1(7)

J. Han et al. / Polyhedron 25 (2006) 2483–2490 2485

In complex 1 it was found that two Rh(II) ions are coor-dinated with two S atoms of thienyl groups from two dif-ferent ligands to give a 2:1 mononuclear complex with adirhodium unit and the two thienyl groups of the ligandare arranged in a parallel conformation (Fig. 1). As inRh(II) trifluoroacetate complexes the metal ions have nointeraction with the thieophene rings of the bisthienyleth-enes, this implies the coordination mode and conformationof the crystals are influenced by the carboxylates. Complex1 is also quite distinguished from the structures of Ag(I)complexes with cis-dbe [27] where the metal center is coor-dinated with one cyano group as well as one thienyl S atomof different bisthienylethene molecules.

The crystals of complex 2 were synthesized similarly tocomplex 1 using dichloromethane instead of benzene. TheX-ray crystallographic analyses show cis-dbe is changedto trans-dbe to coordinate with Rh(II). Each Rh(II) atomis coordinated to one cyano group of the trans-dbe witha bond length of 2.173(3) A, which lead to an almoststraight 1-D chain (Fig. 2). The coordination environmentaround the rhodium ions of complex 2 is notably differentfrom that of complex 1. In comparison with a Cu(I) com-plex with trans-dbe [26], both metal atoms are coordinatedwith two cyano groups from two different trans-dbe. TheRhARh bond length has a slightly change from2.394(1) A in complex 1 to 2.3802(4) A in complex 2.

Complex 3 was synthesized in the same manner as com-plex 1 using rigid closed-dbe, of which two thienyl groupsare almost in the same plane, in the place of the flexiblecis-dbe. The X-ray crystallographic analyses found themetal atoms are coordinated with one cyano group andone thienyl S atom from two different closed-dbe moleculesto give a 1-D chain structure (Fig. 3). The RhAS bond dis-tance in both complexes coordinated to at least one thienylgroup, namely complexes 1 and 3 is 2.544(1) A and2.588(2) A, respectively. The short RhAS bond distancein complex 3 is considered less favorable in photogeneratedreactions based on the free rotation of the two thienylgroups. The RhARh distance is 2.394(1) A in complex 1

and 2.3987(8) A in complex 3, which are slightly longer val-ues than that of the parent compound [Rh2(O2CC(CH3)3)4](2.371(1) A).

In conclusion, three Rh(II) complexes with cis, trans andclosed-dbe were synthesized in which the coordinationmodes are different. In addition there are some differencesin structures and conformations of three complexes: com-plexes 2 and 3 are coordination polymers whereas complex1 is a mononuclear complex. In fact only complex 1 in this

Fig. 1. Crystal structure of complex 1 ([Rh2(O2CCMe3)4(cis-dbe)2]). (a) ORTEP view with the atomic labelling scheme, showing 50% thermal ellipsoids.Hydrogen atoms have been omitted for clarity. (b) Schematic figure.

Fig. 2. Crystal structure of complex 2 ([Rh2(O2CCMe3)4(trans-dbe)](CH2Cl2)4). (a) ORTEP view with the atomic labelling scheme, showing 50% thermalellipsoids. Hydrogen atoms have been omitted for clarity. (b) Schematic figure.

2486 J. Han et al. / Polyhedron 25 (2006) 2483–2490

work is not a chain structure, unlike the five dirhodium(II)trifluoroacetate and other pivalate complexes. A detailedcomparison of Rh(II) complexes with cis-dbe or closed-dbe is summarized in Table 3.

3.2. Photochromism in the crystalline phase

The green complex [Rh2(O2CC(CH3)3)4] (cis-dbe) 1 wassynthesized in benzene. The most remarkable feature of 1 is

Fig. 3. Crystal structure of complex 3 ([Rh2(O2CCMe3)4(closed-dbe)]). (a) ORTEP view with the atomic labelling scheme, showing 50% thermal ellipsoids.Hydrogen atoms have been omitted for clarity. (b) Schematic figure.

Table 3Rh(II) complexes with dbe

Complex Anion Ligand Bridging Conformation Reference

[Rh2(O2CCF3)4(cis-dbe)](benzene) O2CCF32� cis-dbe two cyano groups three kinds [28]

[Rh2(O2CCF3)4(closed-dbe)](p-xylene) O2CCF32� closed-dbe two cyano groups antiparallel [28]

[Rh2(O2CC(CH3)3)4(cis-dbe)] O2CCðCH3Þ32� cis-dbe two thienyl groups parallel this work[Rh2(O2CC(CH3)3)4 (trans-dbe)](CH2Cl2)4 O2CCðCH3Þ32� cis-dbe two cyano groups trans this work[Rh2(O2CC(CH3)3)4 (closed-dbe)] O2CCðCH3Þ32� closed-dbe one thienyl group and one cyano group antiparallel this work

J. Han et al. / Polyhedron 25 (2006) 2483–2490 2487

one thienyl group of cis-dbe is coordinated to Rh to givetwo thienyl groups in a parallel conformation, which sup-press the photocyclization reaction proceeding (Scheme2). A bisthienylethene molecule could exist in two confor-mations: (i) parallel, in which the two thiophene rings arein mirror symmetry; (ii) antiparallel, in which the ringsare in C2 symmetry. According to the Woodward–Hoffmann rules, photochromic performance of diaryleth-ene derivatives in crystals strongly depends on the

antiparallel parallel

SS

NN

SS

NN

Scheme 2. Antiparallel and parallel conformations.

conformation of the molecules and the photocyclizationreaction can only occur from the antiparallel conformation[1]. Further the distance between the reactive carbon atomsC(5) and C(9) in parallel complex 1 is 4.174 A, which is toolong for a covalent CAC single bond formation [1]. Thus incomplex 1 a photocyclization reaction could not happenwithout the necessary antiparallel conformation and shortdistances between the reactive carbons. In contrast toRh(II) trifluoroacetates complexes [28], of which eachmetal ion of the dimetal trifluoroacetates is coordinatedwith a cyano group of cis-dbe and three kinds of conforma-tions coexist in the compound, each Rh(II) ion is coordi-nated with one thienyl group from each ligand and onlya parallel conformation is packed in complex 1. This phe-nomenon shows that the carboxylates influence not onlythe coordination mode but also the conformation of theligand, and subsequently the photoinduced behavior.

Complex 2 was synthesized in dichloromethane usingthe same ligand. cis-dbe is changed to trans-dbe accompa-nying the coordination of two cyano groups to differentRh(II) atoms and the formation of molecular packing

Fig. 4. Absorption spectral changes of complex 3 ([Rh2(O2CC(CH3)3)4

(closed-dbe)]) by photoirradiation in the crystalline phase: (a) the closed-form was transformed to the open-form with 529 nm light and irradiationtimes are 0, 1, 2, 3, 4, 5, 10, 20, 30, 60 and 300 min; (b) the open-form wasnot transformed to the closed-form with 405 nm light and irradiationtimes are 0, 30, 60 and 240 min.

2488 J. Han et al. / Polyhedron 25 (2006) 2483–2490

including dichloromethane molecules. This shows the sol-vent has a close correlation with the coordination modeand conformation of dbe in the complex. Further investiga-tion on NMR spectra of the free cis-dbe ligand in deuter-ated benzene and deuterated dichloromethane shows thesignals of the methyl protons in different solvents are quitedifferent (see Supplementary Information). This result sug-gests the existence of both cis and trans conformations insolution, which is agreeable with the previous examinationof cis-dbe in deuterated CCl4 solution [9], but there is dif-ferent equilibrium between the two conformers in bothsolvents. The interesting solvent effect responsible for thedifferent conformation in complexes 1 and 2 may bedecided by which conformer, cis or trans, is superior inbenzene or dichloromethane, and in which the metalcomplex is predominantly crystallized. Similarly to the pre-viously reported Cu(I) complex with trans-dbe [26], com-plex 2 shows no visible color change upon any reasonableperiod of irradiation. The kmax of complex 2 is 539 nmand is shifted to 570 nm after irradiation with 405 nm lightfor 30 min, indicating that the structure of complex 2 ischanged with the irradiation.

The reddish purple complex 3 was prepared by usingclosed-dbe to avoid the rotation of cis-trans isomerizationand the parallel conformation of two thienyl groups ofcis-dbe. The most interesting feature of this complex is thatone thienyl group, as well as one cyano group of cis-dbe, iscoordinated to the rhodium atoms. Fig. 4 shows theabsorption spectra changes of complex 3 in the crystallinephase. Before irradiation complex 3 has strong absorptionsin the UV region and at 529 nm. Upon irradiation, the red-dish purple complex 3 of closed-ring form is transformed tothe green open-ring form and the 529 nm peak graduallyflattens. The spectrum changed showing two constantisosbestic points (456 and 632 nm), and subsequently indi-cates that the transformation is occurring without thedecomposition of the original and resulting complexes.The shoulder band around 529 nm did not disappear byfurther irradiation, indicating completeness of the conver-sion. The open-form of complex 3 shows a new absorptionband with a kmax value of 593 nm due to a p*! r* transi-tion band, which is at 683 nm in [Rh2(O2CC(CH3)3)4], asshown in Fig. 5.

It is noted that in this work the closed-dbe of antiparal-lel conformation in complex 3 is transferred to thering-open isomer in spite of the special coordinationenvironment of Rh(II). It is known that the photocyclore-version reaction of bisthienylethene in the crystalline phasevaries depending on the molecular structure [2], but thedetails have not been clarified at present. The conversionof the closed-form to the open-form in complex 3 clearlysuggests that the photocycloreversion reaction of bisthieny-lethene complexes could proceed with metal atoms beingcoordinated with a thienyl group. It is expected that thisfinding is useful to further understand the photochromicbehavior of bisthienylethene coordination compounds inthe crystalline phase.

However, the resultant open-form of complex 3 is notconverted to the initial closed-form on exposure to 405 nmlight. The precise mechanism of the cyclization reactionremains unknown, while some consideration is necessary.Generally, a rotational motion encounters steric hindrancein the crystal. Therefore, the cyclization reaction in crystal-line phases is dependent on the conformation and crystalpacking [33]. In the previous report [27] we found thatAg(I) complexes with cis-dbe were transferred to the closedform where two thienyl S atoms were coordinated withmetal atoms. No transformation of the open-form to theclosed-form in complex 3 may be attributable to the sterichindrance of pivalate to the rotation of the thiophene rings.

It is well established that UV–Vis spectroscopy is themost common and effective detection method used in thesolid state by far, where the photochromic states are iden-tified by measuring the absorption bands characteristic ofthe open and closed isomers [4,14,26–28]. The photoreac-tion of the closed-dbe of complex 3 with irradiation of light

Fig. 5. Absorption spectral changes of complex 3 by photoirradiation in the crystalline phase: before irradiation (—), irradiation with 529 nm light for300 min (- - -) and in the photostationary state under irradiation with 405 nm light for 240 min (- Æ -). Absorption spectrum of [Rh2(O2CC(CH3)4) (- Æ Æ -),kmax = 311, 440, 683 nm.

J. Han et al. / Polyhedron 25 (2006) 2483–2490 2489

occurs only on the surface of the crystals and subsequentlythe small amount of product (open-ring form) may not bedetected by MAS NMR spectroscopy. Despite the limitedmonitoring method the green open form of complex 3 isconsidered not identical to complex 1. The crystals of com-plex 3 are packed in a fixed orientation in the crystallinephase and a marked transformation from antiparallel toparallel is unacceptable. Also the open form of complex 3seems not to decompose on exposure to 405 nm light whichshows two constant isosbestic points judging from the UVspectra (Fig. 4(b)).

In contrast, kmax of the metal-free closed-dbe (484 nm) <complex 3 (529 nm) < [Rh2(O2CCF3)4 (closed-dbe)](574 nm). The absorption maximum is shifted much withcoordination of both cyano groups compared to coordina-tion with one cyano group and one thienyl group.

4. Conclusions

Three novel Rh(II) pivalate complexes were preparedwhere the dirhodium moieties are linked via 1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene of cis, trans andclosed forms. X-ray crystallographic analyses demon-strated that dirhodium pivalate leads to three kinds ofcoordination mode, that is the metal ions can be coordi-nated with two thienyl groups (complex 1), two cyanogroups (complex 2) and one cyano group as well as one thi-enyl group (complex 3). The three coordination modes are

of interest in connection with the syntheses of photochro-mic complexes with bisthienylethene. cis-dbe is coordinatedto Rh(II) accompanied by parallel conformation and trans

isomerization to generate complex 1 and 2, respectively. Onthe other hand, the closed-dbe of complex 3 shows typicalspectral changes to the open-form on irradiation at 529 nmin spite of the Rh(II) being coordinated to both thienyl andcyano groups.

Acknowledgments

This work was partially supported by a Grant-in-Aid forScience Research (Nos. 14340211 and 13874084) from theMinistry of Education, Science, Sports and Culture inJapan.

Appendix A. Supplementary data

NMR spectra of the free ligand in benzene and dichloro-methane can be obtained as supplementary material. Crys-tallographic data for the X-ray crystal structural analysishave been deposited with Cambridge CrystallographicData Center, Nos. (0580) 289367–289369 for complexes1–3, respectively. Copies of this information may beobtained free of charge from The Director, CCDC,12 Union Road, Cambridge, CB2 1EZ, UK (fax: +441233 336033); e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk. Supplementary data associated with

2490 J. Han et al. / Polyhedron 25 (2006) 2483–2490

this article can be found, in the online version, atdoi:10.1016/j.poly.2006.02.013.

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