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Influence of chloro/chloro interaction and p–p stacking in 3Dsupramolecular framework construction†

Satirtha Sengupta, Arijit Goswami, Sumi Ganguly, Sukhen Bala, Manas Kumar Bhunia and Raju Mondal*

Received 22nd March 2011, Accepted 18th July 2011

DOI: 10.1039/c1ce05345k

A series of 1D and 2D coordination polymers have been synthesized under solvothermal conditions

using a terminal ligand, 3,5,6-trichlorosalicylic acid (H2TCSA) and different bipyridine based ligands,

2,20-bipyridyl (2,20-BIPY), 4,40-bipyridyl (4,40-BIPY), 4,40-trimethylene bipyridine (TMBP) and trans-

1,2-bis(4-pyridyl)-ethylene (TBPE). The major aim of this work was to study the influence of weak

interactions in constructing 3D supramolecular frameworks. A two-step crystal engineering strategy

has been adopted for generating 3D supramolecular frameworks. Firstly, various lower dimensional

coordination networks have been achieved and then 3D supramolecular frameworks were constructed

from the self-assembly of these networks via weak intermolecular interactions among the organic parts.

The latter was achieved with the usage of ligand systems that are devoid of any conventional hydrogen

bonding functional group. On the other hand, employment of a terminal ligand serves the purpose of

generating different lower dimensional architectures, ranging from discrete zero dimensional

coordination complexes to 1D, 2D coordination polymeric networks. We report herein, two 0D

complexes, [Zn(HTCSA)2(TBPE)2] (1), [{Zn(TCSA)(2,20-BIPY)}4] (2), five 1D coordination polymers,

[Zn(TCSA)(4,40-BIPY)0.5(DMF)]n(3), [Zn(HTCSA)2(4,40-BIPY)]n(4), [Zn(HTCSA)2(TMBP)]n(5), [Zn

(HTCSA)2(TMBP)]n (6), [Zn(HTCSA)2(TBPE)(H2O)]n (7), and two 2D polymeric networks, [Zn

(TCSA)(4,40-BIPY)]n (8) and [{Zn(TCSA)(TBPE)}$(TBPE)]n (9), and two related cobalt and nickel

complexes, [{Co(TCSA)(TBPE)}$(TBPE)]n (10) and [Ni(HTCSA)(TBPE)1.5$(NO3)]n (11). For all the

structures, p–p stacking and halogen bonding were found to be instrumental in bringing the additional

dimensions to the self-assembly of the lower dimensional networks and lead to diverse 3D

supramolecular frameworks.

Introduction

Rational design of coordination polymers with proper choice of

ligand and metal center has become an active area of research in

the field of crystal engineering and supramolecular chemistry.1

Significant efforts have been expanded towards the design and

synthesis of targeted multifunctional coordination polymers

because of their potential bulk properties that are intimately

related to their structures.2 Over the last few years, significant

progress has been achieved in constructing networks of desired

topologies and properties using the concept of crystal engi-

neering and supramolecular chemistry.3 Notwithstanding, pre-

dicting the structural framework constructed from a set of ligand

and metal center is still a distant aim, as it can be affected by

many subtle factors.4 One such widely accepted yet surprisingly

Department of Inorganic Chemistry, Indian Association for the Cultivationof Science, Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India;Tel: 91-33-2473 4971; Fax: (+)91-33-2473 2805. E-mail: icrm@iacs.res.in

† Electronic supplementary information (ESI) available: X-Raycrystallographic files in CIF format for 1–11. CCDC reference numbers774721–774729, 817005 and 817006. For crystallographic data in CIFor other electronic format see DOI: 10.1039/c1ce05345k

6136 | CrystEngComm, 2011, 13, 6136–6149

low-key factor, as far as the metal–ligand coordination

complexes are concerned, would be p–p stacking.5 Furthermore,

weak intermolecular interactions such as halogen bonding are

often considered as of secondary importance in metal–organic

frameworks.6 Strong metal–ligand coordinative interactions are

usually accepted as the all-important structure determining

interactions and accordingly maneuvered and manipulated for

designing targeted frameworks while other weak interactions

such as halogen bonding or p–p interactions merely appear on

an ornamentary basis.7 Nevertheless, one can hardly ignore the

importance of these interactions, especially p–p stacking. For

crystal engineering studies with organic molecules these inter-

actions often prove to be a useful, sometimes deciding factor.8

This led us to believe that weak interactions can be used as

a useful tool for generating 3D frameworks when the targeted

network is independent or least dependent on the coordinative

interactions. In other words, weak intermolecular interactions

can be of primary importance if the final structural topology is

directed by the self-assembly of organic parts.

Considering the above mentioned facts, we selected 3,5,6-tri-

chlorosalicylic acid (H2TCSA) in order to investigate the

This journal is ª The Royal Society of Chemistry 2011

Scheme 1

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View Article Online

influence of p–p stacking and halogen bonding on the resultant

structural framework. The H2TCSA molecule with a salicylic

acid group as the only coordinating site is expected to act as

a terminal ligand. This should help to reduce the dimensionality

of the metal dependent coordination network.9 A lower dimen-

sional coordination network will enhance the possibility of

supramolecular interactions among the organic parts to dictate

the final 3D framework. In other words, introduction of

a terminal ligand should make the final 3D framework less

dependent on metal–ligand coordinative interactions but on the

self-assembly of the organic parts i.e. H2TCSA and bipyridine

molecules which are used as secondary ligands. Furthermore, in

the absence of any conventional hydrogen bonding functional

group, the self-assembly process of the targeted lower dimen-

sional coordination/polymeric networks are expected to be

dominated by weak interactions, primarily by p–p stacking and

halogen bonding. The prospect of the structure determining role

of weak interactions certainly looks promising for the present set

of ligand molecules: the H2TCSA molecule with three chloro

substituents can be envisaged as an activated p system and

bipyridines with complementary metal coordinated p systems.

In this contribution, we report two metal coordination

complexes, [Zn(HTCSA)2(TBPE)2] (1), [{Zn(TCSA)(2,20-BIPY)}4] (2), five one-dimensional coordination polymers, [Zn

(TCSA)(4,40-BIPY)0.5(DMF)]n(3), [Zn(HTCSA)2(4,40-

BIPY)]n(4), [Zn(HTCSA)2(TMBP)]n(5), [Zn(HTCSA)2(TMBP)]n(6), [Zn(HTCSA)2(TBPE)(H2O)]n (7), and two two-dimensional

polymeric networks, [Zn(TCSA)(4,40-BIPY)]n (8)and [{Zn

(TCSA)(TBPE)}$(TBPE)]n (9) (where H2TCSA ¼ 3,5,6-tri-

chlorosalicylic acid, 2,20-BIPY ¼ 2,20-bipyridyl, TMBP ¼ 4,40-trimethylene bipyridine, TBPE ¼ trans-1,2-bis(4-pyridyl)-

ethylene and 4,40-BIPY ¼ 4,40-bipyridyl) (Scheme 1). While

compounds [{Co(TCSA)(TBPE)}$(TBPE)]n (10) and [Ni

(HTCSA)(TBPE)1.5$(NO3)]n (11) were prepared by varying the

metal centre. All these structures show a common trend, i.e.,

a 3D supramolecular framework dependent on p–p stacking and

halogen bonding and further reinforce our supposition that they

can be utilized for the design of self-assembly of lower dimen-

sional coordination polymers.

Experimental

Materials and general methods

All reagents and chemicals were purchased from commercial

sources and were used without further purification. FT-IR

spectra were obtained on a MAGNA-IR 750 spectrometer with

samples prepared as KBr pellets.

Syntheses of 1–9

Synthesis of [Zn(HTCSA)2(TBPE)2] (1) and [{Zn(TCSA)

(TBPE)}$(TBPE)]n (9). 3,5,6-Trichlorosalicylicacid (TCSA)

(0.0241 g, 0.1 mmol) and trans-1,2-bis(4-pyridyl)ethylene (TBPE)

(0.0182 g, 0.1 mmol) were dissolved in a 1 : 1 mixture of meth-

anol and DMF and to it an aqueous solution of Zn(NO3)2(0.0297 g, 0.1 mmol) was added. The whole mixture was added to

a sealed container and heated to 75 �C for 2 days. Afterwards, the

container was slowly cooled down to room temperature. A

mixture of colourless plate shaped crystals (1) and yellow block

This journal is ª The Royal Society of Chemistry 2011

shaped crystals (9) were obtained which were filtered, washed

with water, dried in air and manually separated.

IR (KBr, cm�1) for (1): 3429(br), 3068, 2926(w), 1674(s), 1607

(vs), 1566(w), 1443(s), 1410(m), 1306(w), 1234(br), 1128(s), 1020

(s), 840(br), 723, 553(s).

IR (KBr, cm�1) for (9): 3448(br), 3068, 3040(w), 2922(w), 1674

(s), 1607(vs), 1506(m), 1443(s), 1410(w), 1369(s), 1306(m), 1234,

1209(m), 1128(s), 1070(m), 1020(s), 840(m), 723(s), 660(m),

554(s).

Synthesis of [{Zn(TCSA)(2,20-BIPY)}4] (2). A procedure

similar to that for (1) was carried out with 2,20-bipyridyl (2,20-BPY) (0.0156 g, 0.1 mmol) in place of TBPE. Yellow block

shaped crystals (2) were obtained with 45% yield based on TCSA.

The crystals were washed with water and dried in air.

IR (KBr, cm�1) for C68H36N8O12Cl12Zn (2): 3439(br), 3059

(m), 2926(m), 2854(w), 1593, 1578, 1553(m), 1518(vs), 1491(m),

1437(s), 1385(s), 1358(m), 1315(s), 1223(s), 1171(m), 1153, 1124

(s), 1057(s), 1024(s), 766(s).

CrystEngComm, 2011, 13, 6136–6149 | 6137

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Synthesis of [Zn(TCSA)(4,40-BIPY)0.5(DMF)]n (3) and [Zn

(HTCSA)2(4,40-BIPY)]n (4). The synthesis was similar to the one

mentioned above but in place of TBPE, 4,40-bipyridyl (4,40-BPY)

(0.0156 g, 0.1 mmol) was added. A mixture of yellow block

shaped crystals (3) and colourless block shaped crystals (4) were

obtained which were filtered, washed with water, dried in air and

separated manually.

IR (KBr, cm�1) for (3): 3418(br), 3068(w), 2924(w), 1674(m),

1607(vs), 1555(m), 1506(w), 1440(s), 1411(w), 1371(m), 1234(m),

1130(s), 1022(s), 841(s), 723(s), 658(m), 553(s).

IR (KBr, cm�1) for (4): 3431(br), 3047, 2925, 2853(m), 1678(s),

1603(vs), 1564(w), 1489(s), 1444(s), 1404(s), 1371(m), 1223(s),

1172(w), 1128(s), 1070(m), 1043(m), 945, 815, 723, 632(s).

Synthesis of [Zn(HTCSA)2(TMBP)]n (5) and [Zn

(HTCSA)2(TMBP)]n (6). Similar to the above procedure for (1),

the reaction was carried out with 0.0198 g (0.1 mmol) of 4,40-trimethylenedipyridine (TMBP). A mixture of light brown plate-

shaped crystals (5) and colorless cubic crystals (6) were obtained

which were filtered, washed with water, dried in air and separated

manually.

IR (KBr, cm�1) for (5): 3483(br), 3211(m), 3072(s), 2928(m),

2864(w), 1763(w), 1622(vs), 1547(s), 1506(s), 1387(s), 1230(s),

1134(s), 1034(s), 824(s), 777(m), 663(w), 501(m), 401(m).

IR (KBr, cm�1) for (6): 3437(br), 3072(w), 2929(w), 2713(w),

1620(s), 1551(s), 1510(m), 1433(vs), 1355(s), 1323(m), 1230(s),

1134(s), 1070(m), 1031(s), 869, 831, 773(w), 511, 397(m).

Synthesis of [Zn(HTCSA)2(TBPE)(H2O)]n (7). Similar to the

above procedure, the synthesis was carried out by taking 0.0182 g

(0.1 mmol) of trans-1,2-bis(4-pyridyl)ethylene (TBPE) and

0.0482 g (0.2 mmol) of 3,5,6-trichlorosalicylic acid (H2TCSA) in

a 1 : 1 mixture of methanol and DMF and to it aqueous solution

of Zn(NO3)2 (0.0594 g, 0.2 mmol) was added. Light yellow block

shaped crystals (7) were obtained which were filtered, washed

with water and dried in air (35% yield based on TCSA).

IR (KBr, cm�1) for (7): 3456(br), 3093(w), 1674(s), 1606(vs),

1564(w), 1442 (vs), 1409(s), 1371(s), 1305(m), 1236(m), 1209(m),

1130(s), 1072(s), 1020(s), 840(m), 723(m), 657(m), 553(s).

Synthesis of [Zn(TCSA)(4,40-BIPY)]n (8). The synthesis was

similar to the above mentioned procedure for (2) except that the

reaction was carried out in a water–methanol (1 : 1) medium.

Yellow block shaped crystals (8) were obtained which were

filtered, washed with water and dried in air (40% overall yield

based on TCSA).

IR (KBr, cm�1) for (8): 3384(br), 3244(br), 1672, 1608(br),

1529(w), 1467, 1417(m), 1315(s), 1178(s), 1031(m), 796(m), 605

(w), 496(m).

Syntheses of [{Co(TCSA)(TBPE)}$(TBPE)]n (10) and [Ni

(HTCSA)(TBPE)1.5$(NO3)]n (11). The synthesis similar to that

for (1) was performed but in place of Zn(NO3)2, Co(NO3)2 (0.1

mmol) and Ni(NO3)2 (0.1 mmol) were taken. Purple block sha-

ped crystals of (10) and green block shaped crystals of (11) were

obtained with 40% and 45% yield respectively based on TCSA.

IR (KBr, cm�1) of (10): 3427(br), 3066, 3037(vw), 1597(vs),

1504(s), 1437(vs), 1406, 1369(s), 1300, 1240, 1130(s), 1070(m),

1018(m), 947, 721(s), 551(s).

6138 | CrystEngComm, 2011, 13, 6136–6149

IR (KBr, cm�1) of (11): 3427(br), 3045, 2927, 1610(vs), 1546

(m), 1502(s), 1439(s), 1344(s), 1271(m), 1224(m), 1128(s), 1020(s),

833(s), 557(vs), 406(s).

X-Ray crystallography. X-Ray diffraction intensities were

collected at 120 K on a Bruker APEX-2 CCD diffractometer

using Mo-Ka radiation and processed using SAINT. The struc-

tures were solved by direct methods in SHELXS and refined by

full matrix least-squares on F2 in SHELXL.10

The contents of the pore in 8 could not be assigned crystal-

lographically. At the final stage of the refinement, the excess

electron densities amounting to 14.65 e �A�3 (ranging from 3.91–

1.21 e�A�3) were left. The PLATON/SQUEEZE program11 was

used on the raw data to generate a new dataset that removed the

scattering contribution from any residual electron density found

in the lattice. The calculations indicate the presence of 40 elec-

trons per unit cell or 20 electrons per monomer unit of the

coordination polymer, which may be attributed to one methanol

molecule (18 e).

Crystallographic data are summarized in Table 1, and CIF files

for the structures reported in this paper have been deposited with

the Cambridge Crystallographic Data Centre (CCDC).† Depo-

sition numbers are given in Table 1. The MERCURY software,

a CCDC programme for crystal structure visualisation, was used

for drawing all the packing diagrams.12

Results and discussion

Complexes 1–11 were synthesized by a solvothermal method

under similar conditions using M(NO3)2 (M ¼ Zn, Co, Ni),

H2TCSA and the corresponding bipyridines in a 1 : 1 : 1 molar

ratio. Inspection of the infrared spectra of the metal complexes

shows a striking similarity in all of them. However IR spectra of

the parent ligand H2TCSA revealed certain characteristic

differences from its metal complexes. The most striking differ-

ence is the C]O stretch of the aromatic carboxylic acid observed

in the IR spectrum as a broad peak showing the maximum at

1660 cm�1 (characteristic of the chelated form of the acid) for the

free H2TCSA molecule which drifts to lower wavenumbers

centering around 1590–1620 cm�1 (vas (COO�)) and 1380–1450

cm�1 (vs (COO)) in metal complexes due to C]O participation

thus indicating acid dissociation in the presence of the metal.

Similarly, a small shoulder arises from the unchelated form of the

monomer in the free H2TCSA molecule at 1769 cm�1 which

wanders to wavenumbers centering around 1670–1680 cm�1 in

the metal complexes due to lengthening of the C–O bond caused

by the withdrawal of electron density from ligating oxygen atom

to metal atom.

The theme of the present work is based on a two-step crystal

engineering approach to generate a 3D supramolecular frame-

work from a lower dimensional coordination network. The

strategy was to achieve a lower dimensional metal dependent

coordination network (lower than 3Dmetal–organic framework)

with the use of a terminal ligand and then to generate the 3D

supramolecular frameworks from the self-assembly of these

lower dimensional coordination networks. Subsequently, the

objective of generating a 3D supramolecular framework can be

achieved by three different routes; (1) discrete metal complex or

zero dimensional coordination complex to 3D framework:

This journal is ª The Royal Society of Chemistry 2011

Table

1Crystaldata

andstructure

refinem

entinform

ationfor1–11

12

34

56

78

910

11

Empirical

form

ula

C38H

24Cl 6

N4O

6Zn

C68H

36Cl 12

N8O

12Zn4

C15H

12Cl 3

N2O

4Zn

C24H

12Cl 6

N2O

6Zn

C27H

18Cl 6

N2O

6Zn

C54H

35Cl 12

N4O

12Zn2

C26H

16Cl 6

N2O

7Zn

C17H

9Cl 3

N2O

3Zn

C25H

16Cl 3

N3O

3Zn

C25H

16Cl 3

CoN

3O

3

C25H

17Cl 3

N4NiO

6

Form

ula

weight

912.72

1843.93

455.99

702.43

744.50

1488.00

746.48

460.98

578.13

571.69

634.49

Crystal

system

Monoclinic

Triclinic

Triclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Triclinic

Triclinic

Triclinic

Triclinic

Space

group

C2/c

P� 1

P� 1

C2/c

P2(1)/c

P2(1)/n

P2(1)/c

P� 1

P� 1

P� 1

P� 1

a/� A

24.303(10)

10.9481(8)

8.3392(8)

20.5213(11)

14.4957(5)

11.8373(8)

13.6683(5)

8.2531(7)

9.1994(6)

9.1940(4)

9.8694(3)

b/� A

10.137(4)

13.2205(9)

10.5200(10)

9.4618(5)

8.8235(3)

13.5119(9)

13.6285(5)

10.6582(10)

10.5683(7)

10.5320(5)

10.1833(3)

c/� A

18.744(13)

13.9222(16)

11.0835(11)

16.9888(9)

23.1793(8)

18.5484(13)

15.1486(6)

12.4388(11)

13.8477(9)

13.8552(6)

13.5295(4)

a/�

90

106.761(3)

71.219(2)

90

90

90

90

73.757(2)

80.4862(2)

80.603(2)

92.154(1)

b/�

125.506(4)

102.853(3)

79.807(2)

126.518(2)

96.808(1)

106.514(2)

101.217(1)

73.122(2)

78.545(2)

78.290(2)

103.146(1)

g/�

90

108.249(2)

75.307(2)

90

90

90

90

76.445(2)

65.856(2)

65.9750(10)

104.038(1)

V/� A

33759(3)

1720.6(3)

885.63(15)

2651.1(2)

2943.79(18)

2844.3(3)

2767.95(18)

991.16(15)

1198.73(14)

1195.00(9)

1278.37(7)

Z4

12

44

22

22

22

Refls

collected

11707

23078

11686

18429

37166

30375

35396

11055

15678

20799

18784

Unique

reflections

3143

7561

3731

2733

6204

4527

6174

4527

5241

6677

4868

Obs.reflections

[I>2s(I)]

1762

5957

3118

2042

4758

2878

4713

3834

4471

5442

3836

R1

0.0504

0.0299

0.0385

0.0436

0.0395

0.0636

0.0428

0.0312

0.0396

0.0459

0.0363

wR2

0.1091

0.0681

0.1093

0.1002

0.0892

0.1559

0.0918

0.0765

0.0971

0.1113

0.0764

CCDC

numbers

774721

774722

774723

774724

774725

774726

774727

774728

774729

817005

817006

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supramolecular interactions expanded in all three directions, (2)

1D coordination network to 3D framework: horizontal and

vertical supramolecular expansion and (3) 2D coordination

network to 3D framework: supramolecular expansion in vertical

direction.

(A) From zero-dimensional molecular complex to 3D

supramolecular framework: expansion in all three dimensions

[Zn (HTCSA)2(TBPE)2] (1).Molecular complex 1 was formed

when TBPE, an excellent ditopic ligand, coordinates to the metal

center with only one of the two coordinating sites and effectively

acts as a terminal ligand. Its asymmteric unit consists of a zinc

atom, lies on a two fold axis, one monodentate HTCSAmolecule

and one TBPE molecule. Coordination of two perfectly planar

terminal ligands to the tetrahedral zinc metal center effectively

led to a molecular complex of tetrahedral geometry. Interest-

ingly, the 3D supramolecular structure of 1 can be better

explained in terms of the self-assembly of these tetrahedral

molecular units via p–p stacking (Fig. 1(a)). The tetrahedral unit

with two activated p systems (H2TCSA) and two extended

Fig. 1 Crystal packing of 1 showing (a) the self-assembly of a tetrahe-

dral molecular complex (central unit with carbon atoms in black) with

four other complexes with pair wise p–p stacking and (b) the parallel

array of infinitely stacked p rings.

6140 | CrystEngComm, 2011, 13, 6136–6149

conjugated p systems (TBPE) represents an interesting scaffold

for molecular recognition study. Apparently, the molecular

recognition of these scaffolds is more or less independent of the

metal center and mostly driven by pair wise p–p stacking of

TCSA and TBPE molecules. As illustrated in Fig. 1, each

tetrahedral unit is ‘glued’ to four other tetrahedral units by p–p

stacking and results in a 3D supramolecular framework.

[{Zn (TCSA)(2,20-BIPY)}4] (2). The crystal structure of 1 is

interesting and prompts us to explore the potential ofp–p stacking

in generating 3D supramolecular framework from a discrete

metal–ligand complex. Accordingly, we used 2,20-BIPY, a well

known chelating ligand, as the secondary ligand. Both H2TCSA

and 2,20-BIPY are terminal chelating ligands and should result in

a discrete or zero dimensional coordination complex. Indeed, the

crystal structure of 2 shows that an interesting tetranuclear zinc

complex has formed. The asymmetric unit contains two symmetry

independent zinc atoms, two 2,20-BIPY and two TCSA molecules.

Interestingly, the carboxylate groups adopt an unusual syn–anti

coordination mode (Scheme 1)13 and join the metal centres, which

eventually form a centrosymmetric dimer and result in a tetranu-

clear complex. The zinc atom adopts a square pyramidal geometry

with BIPY, phenoxy and one carboxylate oxygen atom taking the

basal position while the remaining carboxylate oxygen atom takes

the apical position.

As far as the supramolecular framework is concerned, complex

2 shows an interesting similarity to that of the tetrahedral scaf-

fold of complex 1. The spatial orientation of TCSA and 2,

20-BIPY molecules of the two adjacent zinc atoms resembles

closely the tetrahedral unit of 1. In other words, if we consider

two adjacent zinc atoms as a single metal center then complex 2

can be envisaged as two ‘extended’ tetrahedral units linked

together. This simply reduces complex 2 to a combination of two

sets of four p systems arranged in tetrahedral fashion. Subse-

quently, the 3D supramolecular framework results from two sets

of pair wise p–p stacking (Fig. 2). Four bipyridine rings of

a tetranuclear unit p–p stack with one of the bipyridine rings of

four neighboring tetranuclear units (Fig. 2(b)). The remaining

bipyridine rings of these tetranuclear units in turnp–p stack with

the surrounding bipyridine rings. Similarly, four TCSA mole-

cules of one tetranuclear unit p–p stack with four TCSA mole-

cules of the adjacent tetranuclear units (Fig. 2(c)) and

accordingly generate the 3D supramolecular framework, while

these arrangements are further sustained by several Cl/p

interactions.

(B) 1D coordination network to 3D supramolecular

framework: vertical and horizontal expansion

Five one-dimensional coordination polymers, [Zn(TCSA)(4,40-BIPY)0.5(DMF)]n(3), [Zn(HTCSA)2(4,4

0-BIPY)]n(4), [Zn

(HTCSA)2(TMBP)]n(5), [Zn(HTCSA)2(TMBP)]n (6), [Zn

(HTCSA)2(TBPE)(H2O)]n (7), with an excellent synergy to the

previous section, nicely demonstrate the influence of weak

interactions in generating 3D supramolecular frameworks.

However, unlike zero-dimensional discrete scaffolds of 1 and 2,

the structural framework of 3–7 can be better described in terms

of the self-assembly of 1D networks in horizontal and vertical

directions.

This journal is ª The Royal Society of Chemistry 2011

Fig. 2 Crystal packing of 2 illustrating how the tetranuclear zinc complex self-assemble to generate 3D supramolecular framework with pair wise p–p

stacking of (a) TCSA and (b) 202-bipy molecules.

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Basic differences in the one-dimensional coordination

networks of [Zn(TCSA)(4,40-BIPY)0.5(DMF)]n (3) and [Zn

(HTCSA)2(4,40-BIPY)]n (4), both with 4,40-BIPY as the

secondary ligand, can be attributed to the coordination mode of

the H2TCSA molecule. For 4, the H2TCSA molecule adopts

a monodentate coordination mode via one of the carboxylate

oxygen atoms while the phenolic hydroxyl group forms a char-

acteristic intramolecular O–H/O bond. The salicylic acid

group, however, adopts a chelating mode in 3 resulting in

a centrosymmetric binuclear cluster formation, facilitated by an

unusual syn–anti coordination mode of the carboxylate group

(Scheme 1). Coordination of two 4,4-bipyridyl molecules at the

two ends of the centrosymmetric cluster results in a linear one-

dimensional coordination network while one DMF molecule

occupies the apical position of the square pyramidal metal

Fig. 3 (a) 1D coordination polymeric networks of 3. (b) Self-assembly of the

help of p–p stacking.

This journal is ª The Royal Society of Chemistry 2011

center. The overall one-dimensional network resembles a stair-

case like topology wherein two TCSA molecules protrude from

each cluster and horizontally extend like wings. The p–p stack-

ing of these horizontally extended TCSA molecules with the

bipyridine rings is the essence of the supramolecular framework

formation of 3. As illustrated in Fig. 3, two aromatic rings of the

bipyridyl molecules are clipped with the adjacent one-dimen-

sional networks by p–p stacking with two TCSA molecules, one

from above and another from below the molecular plane of the

bipyridyl molecules. This effectively results in the supramolec-

ular self-assembly of one-dimensional networks in horizontal as

well as in vertical directions.

On the other hand, two terminally coordinated HTCSA

molecules lead to the formation of a one-dimensional zigzag

coordination network for 4 (Fig. 4(a)). Compound 4 crystallizes

coordination networks in horizontal as well as vertical direction with the

CrystEngComm, 2011, 13, 6136–6149 | 6141

Fig. 4 Crystal structure of 4 showing (a) 1D zig-zag coordination polymer network, (b) self-assembly of criss-crossing 1D networks sustained by p–p

stacking, (c) 2D grid network comprised of Type-I Cl/Cl interactions and (d) a space-filling model of interpenetrating grid networks.

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in the monoclinic C2/c space group and the asymmetric unit

contains a tetrahedral zinc atom coordinated to one HTCSA

molecule and one independent half molecule of BIPY molecule

lies on a crystallographic inversion centre.

The zig-zag chain can be envisaged as a combination of two

intersecting extended p systems, each made of three coplanar

aromatic rings, a bipyridyl molecule and two TCSA molecules

coordinated on the opposite side. The mutually perpendicular

orientation of the p systems (both TCSA and 4,40-bipyridyl) playsan important role in the self-assembly of the one dimensional

networks. As illustrated in Fig. 4(b), the crisscross networks align

themselves in such a way that the bipyridyl and TCSA molecules

form a mutually parallel orientation and result in strong p–p

stacking. Such self-assembly is further complimented by another

pair of criss-cross networks situated at the opposite side. As a result

of this, the TCSA molecules of the two oppositely faced networks

come very close to each other and form a strongp–p stacking. This

effectively leads to the formation of column of infinitely stacked p

rings propagated in two mutually perpendicular directions (Fig. 4

(b)). The mutually perpendicular orientation is further sustained by

several Cl/p interactions.

6142 | CrystEngComm, 2011, 13, 6136–6149

From a network point of view, crystal structure 4 forms an

undulated 2D grid architecture by joining the juxtaposed 1D

coordination networks by Type-I chloro/chloro interactions14

(Cl/Cl 3.11 �A, 165.65�, 165.65�) (Fig. 4(c)). The dimensions of

the grid are 11.17 �A � 16.99 �A. Interestingly, the 1D coordina-

tion networks interweave through the cavity created in the grid

network and lead to an interpenetrating network (Fig. 4(d)).

(i) Conformational polymorphism. Interestingly, the supra-

molecular framework of compound [Zn(HTCSA)2(TMBP)]n(5)

and [Zn(HTCSA)2(TMBP)]n (6) can also be described in terms of

p–p stacking and chloro/chloro interaction induced self-

assembly of 1D coordination polymers, similar to that of

complex 4.

Complexes 5 and 6 are obtained using a flexible 4,40-tri-methylene bipyridine (TMBP) molecule as the secondary ligand.

Conformational freedom of a flexible ligand can have a great

impact on the structural topology of the resultant coordination

polymer. Depending on the requirement, a flexible ligand can

adopt different conformation by bending or rotation when

coordinating to the metal center. In the present context, we were

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interested to see if TMBP adopts any particular conformation

that can facilitate the formation of stronger p–p stacking or

halogen bonding. Two conformational polymorphs,15 5 and 6,

are essentially resulted from two different conformations of the

TMBP molecule. The coordination environment around the

metal centre is the same for both the complexes, with a tetrahe-

dral zinc atom coordinated to two TMBP molecules and two

monodentate HTCSA molecules.

Similar to 4, the 1D coordination networks of 5 are joined

together with the formation of Type-I chloro/chloro interac-

tions (Cl/Cl 3.39 �A, 173.37�, 173.37�) resulting in a 2D grid

network with herringbone topology (Fig. 5). Subsequently, the

2D grids undergo five fold interpenetration to generate the 3D

supramolecular structure. However, a closer inspection of the

structure reveals that the supramolecular architecture is further

supported by several p–p interactions between the interwoven

networks. At the crossover points, the aromatic rings (both

TCSA and TMBP) of the interweaving networks come close to

each other with the formation of edge-to edge p–p stacking.

The crystal structure of 6 provides us with a nice example of

a case where weak interactions can influence the conformation of

Fig. 5 Crystal structure of 5 showing (a) 1D coordination polymeric networ

assembly of these interpenetrating supramolecular 2D networks by p–p stac

This journal is ª The Royal Society of Chemistry 2011

the flexible TMBP molecule. The self-assembly of 1D coordina-

tion networks using p–p stacking and chloro/chloro interac-

tion is a recurring theme. A comparative depiction of the

building blocks of 1D networks of 5 and 6 is shown in Fig. 6 to

emphasize that a flexible ligand molecule like TMBP can adopt

different conformations in order to facilitate better supramo-

lecular interactions. As illustrated in Fig. 6(b), complex 6 forms

an interesting 1D coordination network with two almost

coplanar wing like extensions of TCSA molecules on the either

side of the chain. Interestingly, the mean planes containing

extended TCSA molecules are arranged in perfectly parallel

fashion across the TMBP molecules. The resultant 1D coordi-

nation network therefore can be envisaged as an infinite array of

slabs of p rings joined together by TMBP molecules. The

generation of a 3D supramolecular framework with self-

assembly of these 1D coordination networks using chloro/chloro interactions and pair-wise p–p stacking as horizontal and

vertical expansion is illustrated in Fig. 7. A 2D grid network

results with dimensions of 18.96 �A � 13.51 �A when the TCSA

molecules on either side of the coordination networks come

close to each other and form Type-I chloro/chloro interactions

k. (b) Formation of 2D grid networks with Cl/Cl interactions. (c) Self-

king and (d) five-fold interpenetration in a space filling model.

CrystEngComm, 2011, 13, 6136–6149 | 6143

Fig. 6 (a) A superimposed depiction of two conformational poly-

morphs, 5 and 6, illustrating the importance of the flexible ligand towards

the final structure. (b) Basic polymeric unit of 6 showing parallel

arrangement of interdigitating p systems of TCSA molecules across the

TMBP molecule.

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(Cl/Cl 3.56 �A, 167.2�, 164.5�) (Fig. 7(b)). Similar to 4 and 5, the

2D grid networks undergo interpenetration in such a fashion that

the TCSA molecules sit on top of each other and result in strong

p–p stacking (Fig. 7(c)). However, unlike the previous two

structures, only the TCSA molecules are involved in p–p

stacking. The overall topology turns out to be an interesting four

Fig. 7 Crystal structure of 6 showing (a) 1D coordination network. (b) Form

these interpenetrating supramolecular 2D networks by face-to-face p–p st

framework with parallel array of infinitely stacked p rings of TCSA molecul

6144 | CrystEngComm, 2011, 13, 6136–6149

fold interpenetrating framework with parallel columns of infi-

nitely stacked p rings (Fig. 7(d)).

(ii) Two-in-one network with node sharing. Similar to 6, the

crystal structure of [Zn(HTCSA)2(TBPE)(H2O)]n (7) exhibits an

interesting 1D zig-zag coordination polymer with an almost

identical coplanar wing like extension of TCSA molecules. So

much so that the mean planes containing TCSA molecules are

also arranged in perfectly parallel fashion across the rigid

TBPE molecules. The asymmetric unit of 7 contains a zinc

atom in a trigonal bipyramidal geometry, two monodentate

HTCSA molecules, two bridging TBPE molecules, and a coor-

dinated molecule of water. The TBPE ligand molecules lie

about inversion centres and one of them exhibits some

disorder. The coordinated water molecule plays an important

role in supporting the coplanar arrangement of the two

contiguous TCSA molecules. The water molecule sits between

the two TCSA molecules and forms two strong O–H/O bonds

on either side with the uncoordinated oxygen atoms of the

carboxylate group.

The crystal structure of 7 further reinforces the case of

potential importance of p–p stacking and halogen bonding in

crystal engineering and supramolecular chemistry. As such the

crystal packing of 7 can be considered as a result of two different

types of nets, (4,4) grid and (6,3) nets, made of p–p stacking and

chloro/chloro interactions, respectively.

ation of 2D grid networks with Cl/Cl interactions. (c) Self-assembly of

acking between TCSA molecules and (d) the resultant supramolecular

es.

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The 1D zig-zag coordination polymers are arranged in such

a way that the two coplanar TCSA molecules of one network

come in close proximity to another network in a parallel fashion.

Consequently, the two pairs of coplanar TCSA molecules form

strong face-to-face p–p stacking. The overall topology can be

envisaged as a non-interpenetrative (4,4) grid network, consid-

ering the four p–p stacked TCSAmolecules as a node (Fig. 8(b)).

The 2D grids adopt an interesting interdigitating off-set

arrangement driven by p–p stacking (Fig. 8(c)). The networks

are arranged in such a way that the nodes of a lower grid (green)

penetrate upwards into another grid so that the upper pair of

TCSAmolecules become coplanar with the TBPEmolecule (light

grey). While the pair of TCSA molecules situated at the lower

end of the upper grid (yellow) becomes parallel with the former

pair as well as TBPE molecules. This results in a vertical

expansion of the 2D networks with a column of infinitely stacked

p systems.

On the other hand, the crystal structure of 7 can also be

envisaged as a 2D sheet network with (6,3) topology, considering

Fig. 8 (a) The coordinated water molecule placed itself between two TCSA

bonds. (b) A (4,4) grid network based on pair wise face-to-face p–p stacking b

stacking of (4, 4) grids sustained by p–p stacking. (d) A closer view of the h

This journal is ª The Royal Society of Chemistry 2011

each TCSA molecule as a node, using two pseudo Type-I

chloro/chloro interactions (Cl/Cl 3.26 �A, 165.77�, 121.34�;Cl/Cl 3.44 �A, 160.02�, 124.05�). The crystal structure of 7

therefore represents a unique 3D supramolecular framework

which is resulted from a combination of two different types of 2D

networks using two different types of weak interactions, viz, p–p

stacking and Cl/Cl interactions. Interestingly, a closer inspec-

tion of the 3D framework would reveal another unusual feature

of node sharing between p–p stacked (4,4) nets and a halogen

bonded (6,3) net, as illustrated in Fig. 9(b).

(C) 2D coordination network to 3D supramolecular

framework: vertical expansion

In excellent agreement with our supposition, weak interactions

evidently played a pivotal role in terms of bringing additional

dimensionality to the coordination complexes of 1–7. We

conclude this work with four more coordination polymers,

[Zn(TCSA)(BiPY)]n (8), {[Zn(TCSA)(TBPE)]$TBPE}n (9),

molecules in 7 and sustains their coplanar arrangement with O–H/O

etween the set of coplanar TCSAmolecules. (c) The interdigitating off-set

ighlighted p–p interactions among the TCSA and TBPE molecules.

CrystEngComm, 2011, 13, 6136–6149 | 6145

Fig. 9 (a) Crystal packing of 7 illustrating (a) the formation of a 2D (6,3) network with Cl/Cl interactions. (b) Formation of a 3D supramolecular

framework with an unusual event of node sharing between (6,3) net (black) and (4,4) nets.

Fig. 10 The (4,4) grid network of 8 considering dinuclear cluster as

a node.

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{[Co(TCSA)(TBPE)]$TBPE}n (10) and [Ni(HTCSA)

(TBPE)1.5$(NO3)]n (11), which clearly show the importance of

halogen bonding and p–p stacking in 3D supramolecular

network generation from a lower dimensional metal–organic

complex.

The chassis of the crystal structures of 8 and 9 shows a striking

similarity to that of 3, as far as the coordination geometry and

relative arrangement of organic molecules are concerned. All the

three structures show a similar coordination environment

around the metal centre, i.e., a square pyramidal geometry.

However, instead of a terminally coordinated DMF molecule in

3, the apical position of the dinuclear clusters of 8 and 9 is

occupied by the corresponding pyridyl molecule. For both the

structures, the pyridyl molecules lie about an inversion centre,

while the TBPE molecule in 9 shows a disorder similar to that of

compound 7. The simple change in the coordination geometry

transforms the 1D network of 3 to the (4,4) type grid networks

for 8 and 9, considering the dinuclear cluster as a node (Fig. 10).

One of the most interesting aspects of these frameworks would be

the large pore size of the non-interpenetrating grids. The

dimension of the grids can be assigned as 19.297 �A � 18.171 �A

6146 | CrystEngComm, 2011, 13, 6136–6149 This journal is ª The Royal Society of Chemistry 2011

Fig. 11 Vertical expansion of (4,4) grid networks with the help of p–p stacking for (a) 8 and (b) 9. Notice that for 9 the ethylene bond of the TBPE

molecule also takes part in p–p stacking. Chlorine atoms are shown in wireframe model for clarity.

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and 21.968 �A � 21.583 �A (based on the Zn/Zn distance) for 8

and 9, respectively. However, the effective pores in the actual

structures are reduced considerably due to the offset stacking of

two adjacent 2D single layers. A closer inspection of these

structures would reveal that the stacking of the 2D grids is not

random but is well directed by a particular spatial arrangement

of the organic molecules or more specifically the aromatic rings

involved in p–p stacking. The TCSA molecules are oriented in

up and down fashion at the node in both the structures, whilst the

aromatic rings of the pyridyl molecules become either coplanar

or perpendicular to the plane of the grid. The perpendicular rings

are therefore parallel with the TCSA molecules and promote the

inter-grid supramolecular connectivity along the vertical direc-

tion via p–p stacking. As illustrated in Fig. 11, the 2D grids are

stacked in such a fashion that one of the aromatic rings of the

Fig. 12 The (4,4) ‘templated’ 2D grid network of 9 sho

This journal is ª The Royal Society of Chemistry 2011

pyridyl molecule is locked with a TCSA ring of the upper 2D

layer while the second aromatic ring p–p stacks with the TCSA

molecules of the lower layer. For 9, additional support comes

from the p system of the ethylene bond.

However, there is a subtle difference between the p–p stacking

patterns of 8 and 9. An uncoordinated TBPE molecule brings in

some additional supramolecular features and the concept of

templating in 9.16 A template or a structure-directing agent is

commonly used in zeolite synthesis.17 The template is either

a molecule or ionic species around which a framework assembles

and crystallizes. For the present work, the (4,4) grid network of 9

can be envisaged as generated around an uncoordinated TBPE

molecule. The nitrogen atoms of the uncoordinated TBPE

molecule form two C–H/N bonds with the ethene and aromatic

hydrogen atoms on the either sides of the grid (Fig. 12). As far as

wn in (a) ball and stick and (b) space filling model.

CrystEngComm, 2011, 13, 6136–6149 | 6147

Fig. 13 Arrangement of ‘sandwiched’ uncoordinated TBPE molecules

with end-to-end p–p stacking in 9.

Fig. 14 Crystal structure of 11 showing (a) 1Dmolecular ladder type coordin

and (c) supramolecular self-assembly of interpenetrating grid networks in a s

6148 | CrystEngComm, 2011, 13, 6136–6149

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the 3D supramolecular framework is concerned, the crystal

structure is consistent with the theme of this paper. The unco-

ordinated TBPE molecules are also involved in vertical expan-

sion via p–p stacking.

As illustrated in Fig. 13, the uncoordinated TBPE molecules

(yellow) are expanded vertically with end-to-end p–p stacking

among themselves. This led to an interesting framework where

the uncoordinated TBPE molecules are sandwiched between the

grids forming some additional p–p stacking with the coordi-

nated TBPE molecules of the grid.

The crystal structure of 9 is intriguing and prompts us to

explore the generality of producing templated networks varying

the metal centres. Introduction of cobalt as the metal centre leads

to compound 10, showing an isostructural crystal packing to that

of 9. However, an interesting 1D ladder network is formed for

compound 11 ([Ni(HTCSA)(TBPE)1.5$(NO3)]n) when nickel was

used as the metal centre. Its asymmetric unit consists of one

nickel atom, one monodentate HTCSA molecule, one full and

ation polymer, (b) formation of 2D grid network with Cl/Cl interactions

pace filling model.

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one independent half molecule of TBPE molecule, which lie

about an inversion centre, and a coordinated nitrate ion. The

metal centre adopts an octahedral coordination and three coor-

dination sites, occupied by bipyridyls, are used for network

propagation. The other three coordination sites are occupied by

a nitrate molecule and a monodentate HTCSA molecule.

Although compound 11 with only one coordinated TCSA

molecule and a ladder type network differs considerably from the

other 1D networks, it exhibits a consistent p–p stacking and

chloro/chloro interaction induced 3D supramolecular frame-

work formation. As illustrated in Fig. 14, each ladder network

expands sidewise with the formation of Type-I chloro/chloro

interactions (Cl/Cl 3.68 �A, 138.95�, 138.95�) and results in

a network which closely resembles a (4,4) network. Similar to

previous structures, the network undergoes interpenetration with

the formation of p–p stacking between aromatic rings.

Conclusion

Crystal structures are always three-dimensional. A metal–ligand

complex/polymer, however, can form a lower dimensional (0D,

1D or 2D) network. In such cases, additional dimensionality

comes in the form of supramolecular interactions which trans-

form a lower dimensional coordination complex/network to a 3D

supramolecular framework. Indeed, we have successfully

demonstrated that with a proper choice of ligand system weak

interactions can play a pivotal role in transforming lower

dimensional networks to 3D supramolecular structures in

a comprehensible manner.

We have vindicated our two-step crystal engineering strategy

of generating a 3D supramolecular framework via self-assembly

of lower dimensional coordination polymers using weak inter-

actions. The synthesis of lower dimensional coordination poly-

mers was achieved by employing a terminal ligand.

Subsequently, employment of ligands devoid of any conven-

tional hydrogen bonding functional groups but with active p

systems and halogen bonding functionalities manifested in weak

interaction directed 3D supramolecular framework construction.

Acknowledgements

RM gratefully acknowledges the financial support of the SERC

Fast Track Proposal for Young Scientists Scheme (SR/FTP/CS-

36/2007), Department of Science and Technology (DST), India.

SSG and AG thank CSIR for a fellowship. We also thank DST

for National Single Crystal X-ray Diffractometer Facility at the

Department of Inorganic Chemistry, IACS.

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12 Mercury 2.3, supplied with Cambridge Structural Database; CCDC,Cambridge, UK, 2010–2011.

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