Synthesis, crystal structure and thermochromism of benzimidazoliumtetrachlorocuprate: (C7H7N2)2[CuCl4]

Rahul Bhattacharya a, Mau Sinha Ray a, Raja Dey b, Lara Righi c, Gabriele Bocelli c,Ashutosh Ghosh a,*

a Department of Chemistry, University College of Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, Indiab Department of Physics, University College of Science and Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India

c Centro di Studio per la Strutturistica Diffratometrica del CNR, 43100 Parma, Italy

Received 2 May 2002; accepted 3 September 2002

Abstract

The synthesis and crystal structure of thermochromic, yellow benzimidazolium tetrachlorocuprate(II), (C7H7N2)2CuCl4 (1), have

been reported. The compound crystallizes in the C2/c space group and contains discrete tetrahedral CuCl42� species. It absorbs one

molecule of water from humid atmosphere at room temperature to produce a hydrated green form, (C7H7N2)2CuCl4 �/H2O, which

upon heating loses the water molecule in two steps to form the anhydrous yellow compound 1. The role of the water molecule on the

solid state, yellowv/green thermochromic transformation is discussed.

# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Thermochromism; Crystal structures; Benzimidazolium; Tetrachlorocuprate

1. Introduction

The large structural variability of Cu(II) due to the

presence of an active Jahn�/Teller effect in the d9

electronic system and the relative flatness of the

potential surfaces make the thermochromism in chlor-

ocuprates of continual interest. It is found that in

various examples of thermochromism [1,2], different

types of driving forces come into play to determine the

colour of the substance at a particular temperature.

Willett and coworkers [3�/7] established that the effec-

tiveness of hydrogen bonding involving the protonated

cationic part and the chloride ion of the discrete anionic

CuCl42� species, i.e. N�/H� � �Cl hydrogen bond, has an

important role in the thermochromism. In general, the

stronger the hydrogen bonding, the closer the anions are

to a square planar coordination [8]. This is due to a

reduction of the effective charge on the chloride ions

thereby reducing the electrostatic repulsion between

them within the CuCl42� species. At lower temperature,

the hydrogen bonding is much more effective and the

square planar structure is preferred whereas at high

temperature, usually the onset of disorder of the organic

cations driven by the increased thermal energy weakens

the hydrogen bonding network which in turn becomes

responsible for the structural changes towards tetrahe-

dral geometry. The CuCl42� species being stereochemi-

cally non-rigid, can have any average trans Cl�/Cu�/Cl

angle, ranging from 1808 for square planar complexes to

109.58 for tetrahedral complexes. Its geometry as well as

the colour can typically be described by the average

trans angle. It exhibits yellow�/green colour at room

temperature when the trans angle is 1408. The colour

changes to deeper green as the angle increases and

towards orange as the angle decreases [9]. In a recent

study, it has been found that the thermochromism in

bis(dimethylaminopyridinium)tetrachlorocuprate(II) is

not associated with the onset of disorder of the organic

cations rather the different packing scheme associated

with the rearrangement of the C�/H� � �Cl interactions is

responsible [10]. Until now, however, the role of water

molecules in the thermochromism of chlorocuprates has

not been studied. We found that benzimidazolium

* Corresponding author. Tel.: �/91-33-350-8386; fax: �/91-33-351-

9755

E-mail address: ghosh_59@yahoo.com (A. Ghosh).

Polyhedron 21 (2002) 2561�/2565

www.elsevier.com/locate/poly

0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 2 3 7 - 8

tetrachlorocuprate(II) monohydrate is thermochromic

and the crystal structure of its hydrated green phase was

reported [11]. For a clear understanding of the mechan-

ism of thermochromism one needs the crystal structureanalysis of both the phases.

In this paper, we report the synthesis and crystal

structure of the anhydrous yellow phase; the role of the

water molecule in the thermochromism is also explored.

2. Experimental

2.1. Materials and measurements

High purity (98%) benzimidazole and copper(II)chloride, dihydrate were purchased from Emerck Ger-

many and used as received. Elemental analyses (carbon,

hydrogen and nitrogen) were performed using a Perkin�/

Elmer 240C elemental analyzer and the Cu contents in

all the complexes were estimated spectrophotometrically

[12]. IR spectra in KBr (4500�/500 cm�1) were recorded

using a Perkin�/Elmer RXI FT-IR spectrophotometer.

Electronic spectra of mulls (1200�/350 nm) were re-corded with a Hitachi U-3501 spectrophotometer where

Nujol was used as a medium as well as a reference. The

effective magnetic moments were calculated from mag-

netic susceptibility measurements with an EG&G PAR

vibrating sample magnetometer model 155 at room

temperature (r.t.) and diamagnetic corrections were

made using Pascal’s constants [13]. The thermal analyses

(TG�/DTA) were carried out on a Metler Toledo TGA/SDTA 851 thermal analyzer in a dynamic atmosphere of

dinitrogen (flow rate: 30 cm3 min�1). The sample was

heated in an alumina crucible at a rate of 5 8C min�1.

The X-ray powder diffraction experiments were carried

out using a Philips PW 3710 diffractometer with a Cu

tube anode.

2.2. Preparations

2.2.1. (HBz)2CuCl4 (yellow) (1)

CuCl2 �/2H2O (5 mmol) was dissolved in a minimumvolume of water. To this solution was mixed benzimi-

dazole (10 mmol) dissolved in 5 ml of 10 (M) HCl. The

resulting solution was concentrated in a water bath to

approximately 3 ml and kept in a desiccator. After a few

days yellow crystals suitable for X-ray diffraction were

obtained. Anal . Found (Calc.): C, 38.0 (37.9); H, 3.8

(3.6); Cu, 14.5 (14.3); N, 12.5 (12.6)%. meff 1.73 BM at

278 K. lmax 22 222, 8734 cm�1.

2.2.2. (HBz)2[CuCl4] �/H2O (green) (2)

The green crystals of the compound were prepared

according to the reported method [11]. Anal . Found

(Calc.): C, 36.2 (36.4); H, 4.2 (3.9); Cu, 13.6 (13.8); N,

12.3 (12.1)%. meff 1.80 BM at 278 K. lmax 24 155 (sh),

12 500 cm�1.

The concentration of HCl in the reaction medium

plays an important role in the synthesis of compound 1or 2. The green needle shaped crystals of 2 are readily

obtained if the acidity of the reaction mixture is less than

6 M, whereas yellow crystals of 1 is the only product

when the acidity is 9�/12 M. Both the green and yellow

compounds can be isolated from the 6 to 9 M acid

solutions depending upon the experimental conditions.

For instances, 2 is always resulted on cooling (10�/

12 8C) of a dilute solution (10 mmol benzimidazole, 5mmol CuCl2 �/2H2O in 10 ml of 6�/9 M HCl). If the

solution is more concentrated (approximately 5 ml) and

the temperature is higher (approximately 25 8C), yellow

crystals of 1 are obtained on keeping the solution

overnight without stirring. On stirring, however, 2

crystallizes immediately. It is very interesting to observe

that the seeding of green or yellow crystals in this

solution produce solely the respective crystals at r.t.

2.3. X-ray crystallography

A suitable single crystal of the complex 1 was

mounted on a Bruker AXS CCD area detector system

for data collection. The crystal data of the complex was

collected at 293(2) K. Intensity data were collected in the

Table 1

Crystal data and structure refinement details

Empirical formula C7H7Cl2Cu0.5N2

Formula weight 221.89

Crystal system monoclinic

Space group C2/c

Unit cell dimensions

a (A) 14.903(2)

b (A) 7.815(3)

c (A) 16.130(3)

b (8) 92.93(3)

Volume (A3) 1876.2(8)

Z 8

Dcalc (Mg m�3) 1.563

Absorption coefficient (mm�1) 1.735

F (000) 884

Crystal dimensions (mm) 0.26�0.19�0.14

Crystal colour yellow

u Range for data collection (8) 3.18�/27.00

Limiting indices �195h 519, 05k 59,

05 l 520

Reflections collected 11 396

Unique reflections 2038 [Rint�0.0226]

Completeness to u�28.098 92.2%

Refinement method full-matrix least-squares on F2

Data/restraints/parameters 2038/0/135

Final R indices [I� 2s (I )] R1�0.0376, wR2�0.0701

R indices (all data) R1�0.0992, wR2�0.0801

Goodness-of-fit on F2 0.758

Extinction coefficient 0.0003(2)

Largest difference peak and hole

(e A�3)

0.453 and �0.422

R. Bhattacharya et al. / Polyhedron 21 (2002) 2561�/25652562

v �/2u scan mode using graphite monochromated Mo

Ka radiation (0.71073 A). The empirical absorption

corrections were based on c scans. The structure was

solved by direct method using SIR-97 [14]. The derivedatomic coordinates were put through the full-matrix

least-squares method of refinement by the SHELXL-97

[15] program. The structure was first refined isotropi-

cally varying the positional parameters and isotropic

thermal parameters of the non-hydrogen atoms together

with the overall scale factor. Further least-square

refinements of the coordinates of non-hydrogen atoms

were done using anisotropic temperature factors. Thehydrogen atoms were placed geometrically and refined

in the ‘riding’ model with isotropic thermal parameters

1.2 times Ueq of the atom to which they were attached

and were treated as fixed contributors to the final

structure factor calculations. Complex neutral atom

scattering factors were used throughout the refinement.

All calculations were carried out using SHELX-97 [15],

PLATON-99 [16] and ORTEP [17] programs. Selectedcrystallographic data for compound 1 are displayed in

Table 1 while selected bond lengths and angles of the

compound are presented in Table 2. The H- and non-

bonding contacts of 1 are listed in Table 3.

3. Results and discussion

3.1. Description of the structure

An ORTEP view of compound 1 with the atom

numbering scheme is shown in Fig. 1. The crystal

structure of (HBz)2[CuCl4] (Bz�/benzimidazole) con-

sists of a discrete tetrachlorocuprate anion and two

benzimidazolium cations. The Cu atom, Cu1, occupies

the special position on the twofold axis of the C2/c

space group. The CuCl42� anion assumes a distorted

tetrahedral geometry of D2d symmetry, consistent with

the anticipated distortions predicted by the Jahn�/Teller

effect. These distortions are typically measured by the

value of the trans Cl�/Cu�/Cl angle, which is 132.898 [8]

in the present compound. The observed yellow colour of

compound 1 is also consistent with the value of the trans

angle [9]. In the organic cation, both the benzene and

imidazole rings are almost planar with deviations

ranging between �/0.0071 and 0.0073 A and are fused

to lie in the same plane. Here, both the hydrogen atoms

H1 and H3 attached to nitrogen atoms N1 and N3,

respectively play an important role in forming the

extended molecular association through hydrogen bond-

ing where only one Cl atom, Cl1, acts as an acceptor of

these N�/H� � �Cl hydrogen bonds (Fig. 2). It is interest-

ing to note that the other Cl atom, Cl2, is not

participating in the hydrogen bonding network. The

Table 2

Bond lengths (A) and angles (8) with e.s.d.’s in parentheses

Bond lengths

Cu(1)�Cl(2) 2.1951(10)

Cu(1)�Cl(1) 2.3040(11)

Bond angles

Cl(2)(1)�Cu(1)�Cl(2) 132.86(7)

Cl(2)(1)�Cu(1)�Cl(1) 99.04(4)

Cl(2)�Cu(1)�Cl(1) 108.42(4)

Cl(1)�Cu(1)�Cl(1)(1) 107.47(7)

Symmetry transformation used to generate equivalent atoms: (1)

�x�1, y , �z�1/2.

Table 3

Hydrogen bonding distances (A) and angles (8)

D�H� � �A D�H D� � �A H� � �A �D�H� � �A

N(1)�H(1)� � �Cl(1) 0.909 3.186(3) 2.324 158.17

N(3)�H(3)� � �Cl(1)(1) 0.716 3.169(4) 2.458 172.27

Symmetry equivalents: (1) x�1/2, y�1/2, z .

Fig. 1. ORTEP drawing of complex 1 showing the atom numbering

scheme.

Fig. 2. Crystal packing arrangement showing the hydrogen bonding

scheme and the stacking of benzimidazolium moieties. The dotted lines

indicate hydrogen bonds.

R. Bhattacharya et al. / Polyhedron 21 (2002) 2561�/2565 2563

Cu�/Cl distances differ appreciably (0.109 A) with the

longer Cu�/Cl distance associated with the Cl atom, Cl1,

which is engaged in hydrogen bonding and is consistent

with those found in other chlorocuprates [2]. Molecularpacking and stacking interaction plays the major role in

structural association. Cohesion of the crystal occurs

due to van der Waals’ interaction. An intermolecular

stacking is observed between the planes containing the

fused aromatic rings. The imidazole ring of one cationic

moiety is stacked with the benzene ring of two other

cationic moieties (1/2�/x , 1/2�/y , �/z ; �/x , 1�/y , �/z )

where the perpendicular distances between the stackedrings are 3.429 and 3.166 A, respectively. The benzene

ring of the same cationic moiety was found to be stacked

with the imidazole rings of two other likewise symmetry-

related cations with the same intervals. Thus one

cationic moiety forms antiparallel stacking with both

the symmetry related cations. The shortest C�/H� � �Cl

distance is more than 3.75 A, which excludes any C�/

H� � �Cl interactions [10].Since compound 1 can be obtained by solid state

transformation of 2, a structural comparison between

them is relevant. The crystal structure of compound 2

consists of alternating polymeric [CuCl42�]� and

[CuCl2(H2O)2]� chains along the y -axis [11]. The

structure of both chains may be described as elongated

square bipyramids with four very short Cu�/Cl(H2O)

bonds and two very long Cu�/Cl bonds of 3.154(1) and3.330(1) A. The four cationic columns surrounding each

polymeric chain form an extensive net of hydrogen

bonds. Unlike the yellow phase, here all the Cl atoms of

the CuCl42� units as well as the coordinated water

molecules participate in the hydrogen bonding. The H

atom of the imidazole carbon (C2) is also involved

significantly in the hydrogen bonding network in addi-

tion to both the hydrogens attached to the N atoms. Theextensive hydrogen bonding stabilises the square bipyr-

amid geometry and makes the aromatic ring much more

puckered compared to that of 1. Bezimidazolium cations

are stacked in parallel one upon another (compared to

the antiparallel stacking in the yellow phase) along the

y -axis. The colour of the square bipyramidal tetrachlor-

ocuprate is also found to be green in other similar types

of complexes [9].

3.2. Thermal analysis and mechanism of thermochromism

The simultaneous TG�/DTA curves of compound 2

(Fig. 3) show that it loses the coordinated water

molecule in two steps. The first step starts rather

abruptly at approximately 43 8C and continues up to

approximately 60 8C (obs. loss�/1.49/0.1%). The re-

maining portion of water is lost gradually up toapproximately 145 8C (obs. loss�/2.59/0.1%). The

observed total loss in the two steps (3.99/0.1%) corre-

sponds to the calculated loss of one molecule of water

(4.06%). The colour of 2, however, changes from green

to yellow at approximately 43 8C. This yellow com-

pound (and compound 1 also) on keeping in open

atmosphere at room temperature for several hours

absorbs exactly one molecule of water and transforms

to the green compound. The X-ray powder diffraction

pattern and the thermal profile of this transformed

species are found to be identical to those of 2.

In the hydrated green phase (2), an extended copper

halide network exists along with the extensive hydrogen

bonding network. In such a structure, usually, there is a

competition between the hydrogen bonding interaction

and the bridging Cu�/Cl�/Cu interaction. At higher

temperature, the increased thermal vibration of the

water molecules and organic cations weaken the hydro-

gen bonding which in turn increases the electrostatic

repulsion between the chloride ions [8]. Consequently,

the Cl�/Cu�/Cl angles which are close to 908 in the

square bipyramid structure tend to increase causing

changes in the coordination number and geometry

around Cu(II). The results of thermal analysis suggest

that the sequence of events starts at 43 8C with the loss

of one water molecule per Cu in the hydrated chain

(leaving the chain intact), followed by slow break up of

the hydrated chain at higher temperature corroborating

the gradual loss of water molecule until 145 8C.

Alternatively, both the water molecules of the hydrated

chlorocuprate chain may be thrown out of the coordi-

nation sphere at 43 8C leading to the formation of

discrete CuCl42�. For water molecules, the temperature

43 8C is too low to be evaporated instantly. Moreover,

at least part of the water molecules should remain in the

crystal lattice by forming hydrogen bonds with the

CuCl42�. As a result, in the TG curve a partial weight

loss, approximately 1.4% is recorded at 43�/60 8C and

Fig. 3. The simultaneous TG (*/) and DTA (----) curves for

compound 2 (sample mass 16.4 mg).

R. Bhattacharya et al. / Polyhedron 21 (2002) 2561�/25652564

the rest water is lost slowly upon heating. The different

structural parameters of the green and yellow phases

suggest that a phase transition should be accompanied

with the colour change as in related systems [4]. Theendothermic peak, found in the DTA curve at approxi-

mately 51 8C, is probably a combined effect of phase

transition and loss of water. This is further supported by

the fact that the compound 2 turns completely yellow at

approximately 43 8C, the temperature where the loss of

water just begins. The loss of water molecule is therefore

not the cause of the colour change, rather it is the effect

of some thermochromic phase transitions at this tem-perature. On the other hand, compound 2 on keeping in

a desiccator at room temperature turns slowly into

yellow. The complete transformation takes place over

several days in a P2O5 desiccator. The proportion of

yellow phase increases gradually with the loss of water

from compound 2 at this temperature indicating that the

bridging copper chloride framework is stabilized only in

the presence of coordinated water molecule. The loss ofwater molecule is therefore the cause of the green to

yellow transformation at any temperature below 43 8C.

The mechanism of the reversal of the yellow to green

phase is also very interesting. One discrete tetrachlor-

ocuprate of compound 1 absorbs two molecules of water

and selectively forms a one dimensional polymeric

[CuCl2(H2O)2]� chain with the similarly hydrated

species. As a requirement of the crystal lattice ofcompound 2, another chloro bridged chain of anhy-

drous tetrachlorocuprates, [CuCl42�]�, should also be

formed simultaneously. In thermochromic chlorocup-

rates, thermally induced change of coordination number

and consequent changes in the connectivity of the

extended copper halide framework is known [1]. To

our knowledge, to date the present system is the only

example of thermochromic tetrachlorocuprates contain-ing cations of organic base where deaquation�/aquation

is connected with monomericv/polymeric transforma-

tion associated with thermochromism.

4. Supplementary material

Crystallographic data for the structural analysis havebeen deposited with the Cambridge Crystallographic

Data Centre, CCDC No. 181606 (C14H14Cl4CuN4).

Copies of this information may be obtained free of

charge from The Director, CCDC, 12 Union Road,

Cambridge, CB2 1EZ, UK (fax: �/44-1223-336033;

e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

We would like to thank the University Grant Com-mission, India, for the financial support (Sanction No.

UGC/2976/GRW(UNG) 99-00).

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