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Inorganica Chimica Acta 361 (2008) 2335–2342

Zinc(II) complexes with 1,8-naphthyridine-based ligand:Crystal structures and luminescent properties

Yong Chen a, Xi-Juan Zhao a, Xin Gan b, Wen-Fu Fu a,b,*

a Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR Chinab College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, PR China

Received 30 June 2007; received in revised form 21 November 2007; accepted 25 November 2007Available online 9 April 2008

Abstract

The reactions of 2,4-dimethyl-7-(2-pyridylamino)-1,8-naphthyridine (L1) with Zn(ClO4)2 � 6H2O, and bis(5,7-dimethyl-1,8-naphthyrid-2-yl)amine ligand (L2) with Zn(OAc)2 � 2H2O, ZnCl2 or Zn(ClO4)2 � 6H2O afforded four blue luminescent zinc(II) complexes,[Zn(L1)2](ClO4)2 � 2CH2Cl2 (1), [Zn(L2)(OAc)2] � CH2Cl2 (2), [Zn(L2)2][ZnCl4] � 3.5CH2Cl2 (3) and [Zn(L2)2](ClO4)2 (4), respectively.Crystal structures of complexes 1–3 have been determined by X-ray structural analyses as mononuclear complexes with pseudo-tetrahe-dral geometry. The crystal packing of 1 reveals the coordination cation which is self-assembled to stair chains through aromatic p–pinteractions. The intermolecular N–H� � �O hydrogen bond in 2 generates a centrosymmetric H-bonded dimer. However, the crystal latticeof 3 shows that the molecules are linked by extensive intermolecular hydrogen bonds between the amino groups and the ZnCl4

2� anions,resulting in a one-dimensional zigzag chain. Furthermore, these molecular pairs or chains were self-assembled to two-dimensional sheetsor three-dimensional networks through aromatic p–p interactions. All the zinc(II) complexes display intense intraligand 1(p–p*) fluores-cence with kmax at 380 and 393 nm for 1, 385 and 404 nm for 2–4 in methanol at room temperature, respectively. Emission quantumyields of these complexes are in the range from 0.41 to 0.57. The broad emission bands in their solid-state emission spectra are attributedto intraligand 1(p–p*) transition and aromatic p–p interactions as well.� 2007 Elsevier B.V. All rights reserved.

Keywords: Zinc(II) complex; Naphthyridine; Self-assemble; Crystal structures; Photoluminescence

1. Introduction

1,8-Naphthyridine and its derivatives have been exten-sively investigated and remain an ongoing interest in coor-dination chemistry and biomedical science, due to theirfavorable coordinate ability and biological activities [1,2].Since the first 1,8-naphthyridine compound was preparedin the 1920s, sustaining efforts have been dedicated to thesynthesis of 1,8-naphthyridine derivatives [3]. Numerouscomplexes with naphthyridine-based ligands in differentcoordination modes have also been reported since 1969

0020-1693/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.11.035

* Corresponding author. Address: Technical Institute of Physics andChemistry, Chinese Academy of Sciences, Beijing 100080, PR China. Tel.:+86 10 8254 3519; fax: +86 10 6255 4670.

E-mail address: wf_fu@yahoo.com.cn (W.-F. Fu).

[1]. Hendricker and co-workers have done outstandingwork in this field [4]. More recently, Lippard and co-work-ers synthesized some stable dinuclear complexes based on1,8-naphthyridine derivatives as models to investigate thereactivity of (l-hydroxo)metal centers in metallohydrolases[5].

On the other hand, many zinc(II) complexes with nitro-gen-containing ligands are known to exhibit intense emis-sion at room temperature and there has been substantialresearch into the potential of zinc(II) complexes for light-emitting diode devices [6]. Notable examples are those con-taining either 8-hydroxyquinoline or 7-azaindolate ligands[7]. Che et al. have developed new blue luminous materialsof zinc(II) complexes based on di-2-pyridylamine with asupramolecular structure through intermolecular hydrogenbonding and/or p–p stacking interaction [8]. However,

2336 Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342

zinc(II) complexes containing naphthyridine-based ligandsare surprisingly less investigated [9].

Herein, we describe a full account on the preparation andstructural characterization of four zinc(II) complexes contain-ing 2,4-dimethyl-7-(2-pyridylamino)-1,8-naphthyridine (L1)and bis(5,7-dimethyl-1,8-naphthyrid-2-yl)amine (L2) [10],which were found to exhibit intense blue photoluminescencein methanol solution. In general, these complexes show self-assembled supramolecular structures in the solid state viaco-operative hydrogen bonds and/or aromatic p–p stackinginteractions to generate chains, sheets or networks.

2. Experimental

2.1. Materials and reagents

All the starting materials were used as received, and sol-vents were purified according to the literature methods [11].2-Amino-5,7-dimethyl-1,8-naphthyridine [12] and 2-chloro-5,7-dimethyl-1,8-naphthyridine [13] were preparedaccording to the literature methods.

2.2. Instrumentation

UV–Vis absorption spectra were recorded using HitachU-3010 spectrophotometer. Emission spectra wereobtained on a Hitachi F-4500 fluorescence spectrofluorom-eter. 1H NMR and 13C NMR spectra were measured on aBruker Avance DPX-400 MHz resonance spectrometer,using TMS (SiMe4) as an internal reference at room tem-perature. Elemental analysis was performed on CarloErba1106 elemental analyzer. ESI-MS spectrum wasobtained on an APEXII FT-ICR mass spectrometer.

2.3. Synthesis

L1: A suspension of 2-amino-5,7-dimethyl-1,8-naph-thyridine (1.73 g, 0.01 mol), 2-bromopyridine (1.57 g,0.01 mol), powdered KOH (0.7 g, 0.0125 mol) in 100 mLtoluene was refluxed for 24 h. Upon removal of the solvent,the residue was washed with water until the washing wasneutral. The product was purified by column chromatogra-phy over silica gel column using CHCl3/CH3CH2OH as theeluent. The pale yellow fraction was collected and evapo-rated to afford a pale yellow solid. Yield: 0.62 g, 25%. 1HNMR (CDCl3, 400 MHz): d = 2.60 (s, 3H, CH3), 2.70 (s,3H, CH3), 6.95 (t, J = 6.7 Hz, 1H), 7.02 (s, 1H), 7.42 (d,J = 8.8 Hz, 1H), 7.73 (t, J = 7.8 Hz, 1H), 8.14 (d,J = 8.9 Hz, 1H), 8.33 (d, J = 4.2 Hz, 1H), 8.42 (br s, 1H),10.90 (s, 1H). Anal. Calc. for C15H14N4: C, 71.98; H,5.64; N, 22.38. Found: C, 71. 86; H, 5.58; N, 22.56%.

L2: The procedure was similar to that for L1 except that2-chloro-5,7-dimethyl-1,8-nathphyridine was used insteadof 2-bromopyridine. Yield 0.72 g, 22%. 1H NMR(400 MHz, DMSO-d6): d = 2.60 (s, 6H, CH3), 2.62 (s,6H, CH3), 7.17 (s, 2H), 8.28 (d, J = 9.0 Hz, 2H), 8.45 (d,J = 9.0 Hz, 2H), 10.68 (s, 1H). Anal. Calc. for C20H19N5:

C, 72.93; H, 5.81; N, 21.26. Found: C, 73.02; H, 5.76; N,21.22%.

[Zn(L1)2](ClO4)2 � 2CH2Cl2 (1): A methanol solution(10 mL) of Zn(ClO4)2 � 6H2O (74.5 mg, 0.2 mmol) wasadded to a dichloromethane solution (20 mL) of L1

(100 mg, 0.4 mmol). The solution was stirred for 5 h atroom temperature and then filtrated. Crystals suitable forX-ray structural analysis were obtained by vapor diffusionof diethyl ether into the filtrate. Yield: 0.17 g, 66.5%. 1HNMR (400 MHz, CDCl3): d = 2.22 (6H, s, CH3), 2.57(6H, s, CH3), 7.18–7.22 (4H, br s), 7.56 (2H, d,J = 8.9 Hz, napy-H), 7.66 (2H, br s), 8.14 (2H, br s), 8.34(2H, d, J = 4.7 Hz, py-H), 8.67 (2H, br s), 11.95 (2H, brs, NH). Anal. Calc. for C30H28Cl2N8O8Zn: C, 47.11; H,3.69; N, 14.65. Found: C, 47.26; H, 3.81; N, 14.88%.

Zn(L2)(OAc)2 � CH2Cl2 (2): A methanol solution(10 mL) of Zn(OAc)2 � 2H2O (0.13 mg, 0.60 mmol) wasadded to a dichloromethane solution (20 mL) of L2

(0.20 mg, 0.60 mmol). The green solution was stirred for5 h at room temperature and then filtrated. Crystals suitablefor X-ray structural analysis were obtained by vapor diffu-sion of diethyl ether into the filtrate. Yield: 0.25 g, 80.2%.1H NMR (400 MHz, CDCl3): d = 2.10 (6H, s, CH3), 2.11(6H, s, CH3), 2.50 (6H, s, CH3), 6.82 (2H, s, napy-H),8.24 (2H, d, J = 9.1 Hz, napy-H), 8.31 (2H, d, J = 9.1 Hz,napy-H). Anal. Calc. for C24H25N5O4Zn: C, 56.21; H,4.91; N, 13.66. Found: C, 56.02; H, 4.88; N, 13.56%.

[Zn(L2)2](ZnCl4) � 3.5CH2Cl2 (3): The procedure wassimilar to that for complex 2 except that ZnCl2 was usedinstead of Zn(OAc)2 � 2H2O. Yield: 0.21 g, 74.2%. 1HNMR (400 MHz, DMSO-d6): d = 2.05 (12H, s, CH3),2.55 (12H, s, CH3), 7.12 (4H, s, napy-H), 7.84 (4H, br s,napy-H), 8.77 (4H, br s, napy-H), 12.63 (2H, s, NH). Anal.

Calc. for C40H38Cl4N10Zn2: C, 51.58; H, 4.11; N, 15.04.Found: C, 51.38; H, 4.02; N, 14.96%.

[Zn(L2)2](ClO4)2 (4): The procedure was similar to thatfor complex 1 except that L2 was used instead of L1. Yield:0.12 g, 84.9%. 1H NMR (400 MHz, DMSO-d6): d = 2.07(12H, s, CH3), 2.57 (12H, s, CH3), 7.15 (4H, s, napy-H),7.77 (4H, d, J = 9.1 Hz, napy-H), 8.81 (4H, d,J = 9.1 Hz, napy-H), 12.42 (2H, s, NH). 13C{1H} NMR(400 MHz, DMSO-d6): d = 162.9, 154.2, 151.2, 147.7,139.8, 123.8, 117.0, 115.4, 24.1, 17.7. ESI-MS: 721.4[M2+�2] and 723.4 [M]2+. Anal. Calc. for C40H38Cl2-N10O8Zn: C, 52.05; H, 4.15; N, 15.17. Found: C, 50.20;H, 4.12; N, 15.32%.

2.4. X-ray structure determination

Crystals of 1 and 3 suitable for X-ray structure analyseswere grown from CH2Cl2/CH3OH mixed solution by theslow diffusion of diethyl ether over a period of several days.All of these crystal structures were of dichloromethane sol-vate. Crystal data and details of data collection as well asrefinement are summarized in Table 1. The diffraction datawere collected at 113(2) K with graphite-monochromatizedMo Ka radiation (k = 0.71073 A) on a Rigaku Saturn

Table 1Summary of X-ray crystallographic data for 1 and 3

1 3

Formula C32H32Cl6N8O8Zn C43.50H45Cl11N10Zn2

Formula weight 934.73 1228.58Space group C 1 2/c 1 P�1Crystal system monoclinic triclinica (A) 24.197(3) 14.155(3)b (A) 10.0122(14) 14.516(4)c (A) 18.292(2) 15.403(4)a (�) 90 59.495(7)b (�) 120.999(4) 85.933(8)c (�) 90 87.824(7)V (A3) 3798.6(8) 2719.9(12)Z 4 2T (K) 113(2) 113(2)qcalc (g cm�3) 1.634 1.498h Range (�) 1.96–27.78 3.06–27.40l (mm�1) 1.130 1.463Goodness-of-fit 1.12 1.056Number of unique 4469 11610Rint 0.0385 0.0307Number of parameters 280 622R1

a 0.0651 0.0849wR2

a 0.1889 0.2293R1

b 0.0707 0.1009wR2

b 0.1943 0.2470Maximum and minimum

peaks (e A�3)1.132, �1.091 2.087, �2.264

a I > 2r(I). R1 =PkFo| � |Fck

P|Fo|. wR2 ¼

Pw F 2

o � F 2c

� �2h i

=n

Pw F 2

o

� �2h io1=2

.b All data.

Fig. 1. (a) Perspective view of cation of 1 with 30% thermal ellipsoids. (b)The stair structure of 1 with 50% thermal ellipsoids. The hydrogen atoms,anions, and solvent molecules are omitted for clarity.

Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342 2337

diffractometer. An absorption correction was applied by thecorrection of symmetry-equivalent reflections using theABSCOR program [14]. The structure was solved by directmethods using the SHELXS-97 program [15] and refined byfull-matrix least squares on F2 using the SHELXL-97 software[16]. The hydrogen atoms were added using ideal geometries.

3. Results and discussion

3.1. Syntheses of ligands and complexes

Ligands L1 and L2 were synthesized from 2-amino-5,7-dimethyl-1,8-naphthyridine and 2-bromopyridine or2-chloro-5,7-dimethyl-1,8-naphthyridine in toluene underN2 atmosphere according to the modified literature methods[17]. The reaction of L1 and Zn(ClO4)2 � 6H2O in 2:1 molarratio affords a mononuclear complex, [Zn(L1)2](ClO4)2 �2CH2Cl2 (1). Similarly, [Zn(L2)(OAc)2] � CH2Cl2 (2) [10],[Zn(L2)2][ZnCl4] � 3.5CH2Cl2 (3) and [Zn(L2)2][ClO4]2 (4)were obtained by the treatment of L2 with Zn(OAc)2 �2H2O, ZnCl2 or Zn(ClO4)2 � 6H2O in mixed solvents ofmethanol and dichloromethane with high yields.

3.2. Crystal structures of complexes 1 and 3

Single-crystal X-ray analysis shows that the Zn(II)centers of mononuclear complexes 1 and 3 are in a pseudo-

tetrahedral geometry. Perspective views of the cations of 1

and 3 with atom-numbering scheme are illustrated in Figs.1 and 3, and abbreviated crystallographic data as well asselected bond distances and angles are listed in Tables 1and 2, respectively. Both pyridine and naphthyridine ringscoordinate to Zn(II) as a monodentate ligand in 1, and thetwo aromatic rings are near coplanar with the dihedralangles of 5.3�. The measured Zn–N(1) and Zn–N(3) dis-tances are 2.005(3) and 1.975(3) A, respectively, which arecompared to the corresponding bond distances of

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

Complex 1

Zn(1)–N(1) 2.005(3) Zn(1)–N(1A) 2.005(3)Zn(1)–N(3) 1.975(3) Zn(1)–N(3A) 1.975(3)

N(1)–Zn(1)–N(3) 94.51(1) N(1A)–Zn(1)–N(3A) 94.51(1)N(1)–Zn(1)–N(1A) 110.55(2) N(3)–Zn(1)–N(3A) 116.57(2)N(1)–Zn(1)–N(3A) 121.35(1) N(3)–Zn(1)–N(1A) 121.35(1)

Complex 3

Zn(1)–N(7) 2.012(4) Zn(1)–N(2) 2.024(4)Zn(1)–N(4) 2.029(4) Zn(1)–N(9) 2.064(5)

N(7)–Zn(1)–N(2) 124.55(2) N(7)–Zn(1)–N(4) 124.37(2)N(2)–Zn(1)–N(4) 91.66(2) N(7)–Zn(1)–N(9) 90.61(2)N(2)–Zn(1)–N(9) 110.99(2) N(4)–Zn(1)–N(9) 116.21(2)

Symmetry code of 1: A �x + 1, y, �z + 3/2.

Fig. 2. The crystal packing diagram of 2 with 50% thermal ellipsoids. Thehydrogen atoms and solvent molecules are omitted for clarity.

2338 Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342

2.043(2) and 2.050(2) A found for [Zn(bdan)(OAc)2](bdan = N,N0-bisbenzyl-2,7-diamino-1,8-naphthyridine) [9].The N(1)–Zn–N(3) bond angle of 94.51(1)� can be com-pared with that of 95.8(1)� found for [Zn(dpa)2][CF3SO3]2(dpa = 2,20-dipyridylamine) [8]. Then the molecules arelinked to form a one-dimensioned stair structure throughextensive p–p stacking interactions between the naphthy-ride rings (Fig. 1b). We cannot identify any significantp–p stacking interaction between the pyridyl substituentsas reflected by the rather large interplanar separation(>4.0 A). No extended intermolecular hydrogen bondsare recognized, though interaction between the non-coordi-nating NH group and one of the oxygen atoms of the per-chlorate anion is evident (N(2)� � �O(1) 2.906 A).

In a recent communication, we have obtained the crystalstructure of 2 [10]. The coordinate number of Zn(II) centerin complex 2 is four, with two nitrogen atoms from naph-thyridine ligand and two oxygen atoms from auxiliary g1-acetate ligands (Scheme 1). The bond lengths of Zn(1)–N(2)and Zn(1)–N(4) are 2.039 and 2.049 A, and the Zn(1)–O(1)and Zn(1)–O(3) distances are found to be 1.947(3) and1.961(3) A, respectively. The N(2)–Zn(1)–N(4) bond angleis 89.71(1)�, whereas the O(1)–Zn(1)–O(3) bond angle is103.50(1)�. In the crystal packing of 2, two adjacent mole-cules associate through intermolecular hydrogen bondsbetween the non-coordinating NH group and one oxygenatom of the g1-acetate group with the N� � �O distance of

O

O

N

N

HN

N

N

Zn

O

O

Scheme 1.

2.737 A to generate centrosymmetric H-bonded dimers.Furthermore, p–p stacking interactions between thesemolecular pairings lead to a two-dimensional sheet (Fig. 2).

In complex 3, the Zn(II) atom was coordinated by twoperpendicular naphthyridine ligands, and ZnCl4

2� acts asthe counterion (Fig. 3a). The Zn–N distances vary from2.012(4) to 2.064(5) A, which are similar to that found in1 and 2. The N(2)–Zn(1)–N(4) and N(7)–Zn(1)–N(9) bondangles of 91.66(2) and 90.61(2)� are slightly larger than89.71(1)� in 2, whereas smaller than 94.51(1)� in 1. Thereare two non-coordinating amino groups available forhydrogen bonding in complex 3. Its crystal packing dia-gram shows that each [Zn(L2)2]2+ cation associates withtwo neighbouring [ZnCl4]2� anions by extensive intermo-lecular hydrogen bond interactions, resulting in a poly-meric one-dimensional chain (Fig. 3b). At the same time,each cation interacts with the adjacent four cations throughp–p stacking between the aromatic rings of 1,8-naphthyri-dine forming a three-dimensional network (Fig. 3c). The X-ray crystal analysis showed that the crystal cell of complex3 contains seven CH2Cl2 solvent molecules. The losing sol-vent molecules in collected data and a highly disorderedmolecule of CH2Cl2 were responsible for the high residualelectron density, but this did not completely affect thedetermination of the molecular structure.

3.3. Absorption and emission spectra

The spectroscopic data of complexes 1–4 are listed inTable 3. Complex 1 shows two intense absorptions withkmax at 348 (e = 5.37 � 104 mol�1 dm3 cm�1) nm and363 (e = 6.31 � 104 mol�1 dm3 cm�1) nm, which closelymatches in energy with that of L1 (Fig. 4). The absorp-tion was assigned to intraligand p–p* transitions andthe spacing between two maxima is 1187 cm�1, whichcorresponds to C@N vibrational stretching frequency.Compared to the absorption spectrum of 1 in methanolsolution, those of complexes 2–4 exhibit multiple absorp-tion peaks being at 343, 359, 378, 413, and 438 nm andare similar to that of the free ligand L2. Therefore, theseabsorption can also be assigned to spin-allowed p ? p*

transition (Fig. 5).

Fig. 3. (a) Perspective view of cation of 3 with 30% thermal ellipsoids. (b) The crystal packing diagram of 3 with 50% thermal ellipsoids, showing a self-assembled 3-D network structure through extended intermolecular hydrogen bonds. (c) p–p interactions in 3. The hydrogen atoms are omitted forclarity.

Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342 2339

At room temperature, complex 1 and ligand L1 displaythe intense emissions with kmax at 393 and 410 nm inmethanol solution (Fig. 4). Upon excitation at 360 nm,complexes 2–4 exhibit intense structured emission withkmax at 385, 404 and 426 nm in methanol at room temper-

ature, with vibrational progressions of 1200 cm�1 (Fig. 5)and the emission quantum yields are 0.57, 0.45 and 0.41,respectively. The emission lifetimes of these complexesare less than 5 ns and the shape of emission spectrum issimilar to that of ligand L2. Therefore, the emissions are

Table 3Spectroscopic and photophysical properties of complexes 1–4

Medium (T (K)) kabs (nm) (e (mol�1 dm3 cm�1)) kem (nm) /ema

1 CH3OH (298) 363(63130) 380 0.43348(53750) 393

solid (298) 435

2 CH3OH (298) 438(4186) 385 0.57413(2558) 404378(54263) 426359(28682)343(10000)

solid (298) 447468

3 CH3OH (298) 438(1470) 386 0.45413(980) 405378(57850) 425359(32710)343(13832)

solid (298) 452472

4 CH3OH (298) 438(1292) 385 0.41413(646) 404378(65013) 426359(35194)343(11576)

solid (298) 414434462

a The fluorescence quantum yields were determined by the standardmethod with quinine sulfate in 1 N H2SO4 as reference (UFl = 0.546).

Fig. 5. The absorption and emission (inset) spectra of complexes 2 (dotline), 3 (solid line) and 4 (dash dot line) in methanol at room temperature.

Fig. 6. The solid-state emission spectra of complexes 2 (dot line), 3 (solidline) and 4 (dash dot line) with excitation at 380 nm at room temperature.

2340 Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342

assigned to come from intraligand 1(p–p*) excited state. Inthe solid state, all the complexes show broad emissionbands at >400 nm region, which are red-shifted from thesolution emission spectra (Figs. 4 and 6). As revealed bythe crystal packing diagram of the complexes, the aromaticp–p interactions of the 1,8-naphthyridine moieties in thesolid state should be responsible for the observed low-energy solid-state emissions.

Fig. 4. The absorption spectra of complex 1 (solid line) and L1 (dash dotline) in methanol at room temperature. Inset: The emission spectra ofcomplex 1 in the solid state (solid line) and in methanol (dot line) withexcitation at 370 and 350 nm at room temperature, respectively.

4. Conclusion

Four zinc(II) complexes were obtained by reacting 1,8-naphthyridine derivatives with Zn(OAc)2 � 2H2O, ZnCl2or Zn(ClO4)2 � 6H2O. Depending on the anions, these com-plexes were self-assembled to form molecular pairs or linesin the solid state via intramolecular hydrogen bonds, whichfurther formed sheets or networks through aromatic p–pstacking interactions. All of these complexes exhibit struc-tured absorption bands matching in energy with that of rel-evant ligands. The intense emissions of the obtainedcomplexes in methanol solution maximize in the range of380–430 nm, and the solid-state emissions are red-shifteddue to p–p interactions.

Acknowledgements

This work was supported by the National Basic Re-search Program of China (973 Programs 2005CCA06800and 2007CB613304), the National Natural Science Foun-dation of China (NSFC Grant Nos. 50463001, 20671094,90610034). We thank the foundation (50418010) forNSFC/RGC Joint Research.

Y. Chen et al. / Inorganica Chimica Acta 361 (2008) 2335–2342 2341

Appendix A. Supplementary material

CCDC 652455 and 652456 contain the supplementarycrystallographic data for 1 and 3. These data can beobtained free of charge from The Cambridge Crystallo-graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2007.11.035.

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