x-ray diffraction and raman spectroscopic study of bis-(pyridine base) complexes of cadmium(ii)...

$: X-Ray Diffraction and Raman Spectroscopic Study of Bis-(Pyridine Base) Complexes of Cadmium(II) Halogenides$
Monatshefte fuÈr Chemie 132, 1145±1155 (2001)

X-Ray Diffraction and Raman SpectroscopicStudy of Bis-(Pyridine Base) Complexesof Cadmium(II) Halogenides

Keiichi Satoh�, Toshio Suzuki, and Kiyoshi Sawada

Laboratory of Analytical Chemistry, Faculty of Science, Niigata University,

Niigata 950-2181, Japan

Summary. The molecular structures of bis-(pyridine base) complexes of cadmium(II) chloride and

bromide, where the pyridine base is pyridine ( py), 3-methylpyridine (3-Me-py), 4-methylpyridine

(4-Me-py), and 4-ethylpyridine (4-Et-py), were investigated by means of single-crystal X-ray dif-

fraction and Raman spectroscopy. The crystal structures of CdCl2 py2 (1), CdCl2(3-Me-py)2 (2), and

CdCl2(4-Me-py)2 (3) were determined. All crystals are monoclinic; 1: a� 17.784(2), b� 8.666(1),

c� 3.8252(7) AÊ , �� 91.54(1)�, space group: P21/n; 2: a� 11.89(1), b� 14.41(1), c� 3.874(6) AÊ ,

�� 92.3(1)�, space group: P21/a; 3: a� 21.091(2), b� 3.8884(5), c� 18.2317(3) AÊ , �� 113.64(1)�,space group: C2/c. The structures were re®ned to R/Rw values (%) of 3.2/5.5, 3.0/5.0, and 3.4/5.1

for 1±3. All cadmium atoms are octahedrally coordinated with the chloride ions forming in®nite

di-�-chloro polymeric linear chains and the nitrogen atoms of the pyridine base in trans con®gura-

tion. The Cd chains are oriented along the c-axis in 1 and 2 and along the b-axis in 3. The crystal

structures indicate the absence of a peculiar interaction between the polymeric chains. The Raman

spectra of eight complexes were measured in the range of 550±50 cmÿ1, and the Raman peaks

originating from cadmium-halogen vibrations were assigned. The Raman spectra of 1 and 2 are

quite alike in the lattice mode vibration region. The resemblance of the cadmium-halogen vibration

peaks indicates the same halogenide ion bridged octahedral structure for all complexes.

Keywords. Cadmium(II) complexes; Pyridine base complexes; Raman spectroscopy; X-Ray struc-

ture determination.

Introduction

Crystal and molecular structures of pyridine and substituted pyridine (R-py)complexes of bivalent metal halogenides have been extensively studied, and varioustypes of structures have been reported [1±3]. The bis-(pyridine base) complexes ofcopper(II) chloride, CuCl2(R-py)2, show a wide variety of structures depending onthe pyridine base, the coordination number (four, ®ve, and six), and the existence ofmonomeric, dimeric, and polymeric forms [4±12]. The structure of bis-pyridinecomplexes of cadmium(II) and mercury(II) chloride, CdCl2 py2 [13] and HgCl2 py2

� Corresponding author. E-mail: [email protected]

[14], is characterized by a polymeric linear chain like that in cobalt(II) [15] andcopper(II) complexes [4, 5]. On the other hand, a series of bis-(pyridine base)complexes of zinc(II) chloride, ZnCl2(R-py)2, exhibits only one type of structure, i.e.the monomeric pseudo-tetrahedral arrangement [16±18]. The zinc-nitrogen bondlengths shorten linearly with increasing basicity of the pyridine base. In contrast, thecrystal structure of the cadmium analogues has been only studied for the pyridinevariety. Thus, it is interesting to determine the structures of the cadmium(II) com-plexes for various pyridine bases and to compare them with other metal complexes.

Raman spectroscopy provides information on structure and bond strength aroundthe metal ions since it covers a wide range of vibration frequencies, extending evento the lattice mode frequency. Thus, the combination of Raman spectroscopy andcrystallography is useful for assessing the structure of analogous complexes.

In the present investigation, four kinds of pyridine bases (pyridine ( py), 3-methylpyridine (3-Me-py), 4-methylpyridine (4-Me-py), and 4-ethylpyridine (4-Et-py)) were used to prepare the cadmium(II) chloride and bromide complexesCdCl2( py)2 (1), CdCl2(3-Me-py)2 (2), CdCl2(4-Me-py)2 (3), CdCl2(4-Et-py)2 (4),CdBr2( py)2 (5), CdBr2(3-Me-py)2 (6), CdBr2(4-Me-py)2 (7), and CdBr2(4-Et-py)2

(8). The crystal and molecular structures of 1±3 were determined by means ofthe single-crystal X-ray diffraction. The Raman spectra of 1±8 were measured,and the Raman peaks originating from the cadmium-halogen vibrations wereassigned.

Results and Discussion

Crystal structure

As can be seen from the representative examples of 1±3 (Figs. 1±3), the cadmiumatom in all complexes has an octahedral con®guration. All cadmium atoms arecoordinated by two nitrogen atoms of the pyridine base and four chloride ions. Itis noteworthy to state that a change of the pyridine base does not affect the octa-hedral geometry of the cadmium atom. This is in contrast to the structures of the

Fig. 1. Molecular structure of 1; atom numbering scheme; hydrogen atoms are omitted for clarity

1146 K. Satoh et al.

analogous cobalt(II) and copper(II) halogenide complexes [4, 5, 15] but similar tothe behaviour of the zinc(II) complexes [16±18]. This may be due to the relativelylarge ionic radius of the cadmium(II) ion compared to that of the zinc(II) ion. Thechloride of cadmium(II) has a layered octahedral structure, that of zinc(II) atetrahedral con®guration in the solid state [1]. This tendency may also hold for thepyridine complexes.

The cadmium atoms of 1 and 2 are located at the center of symmetry linedalong the c-axis; those of 3 lie on the two-fold axis lined along the b-axis. Alongthese directions, the adjacent cadmium atoms are combined by sharing two chlo-rine atoms to form in®nite di-�-chloro bridges. The intermetallic distances alongthe polymeric chains are thus equal to the b- or c-axis length. This polymeric chainstructure is similar to that found in CoCl2 py2, CuCl2 py2, and HgCl2 py2. In contrastto the thiocyanate complexes of cadmium(II) [19, 20], the coordination environ-ments are unique for the chloro complexes.

The formation of pyridine base complexes of metal(II) halogenides has beeninvestigated for various metal ions (Co [21, 22], Cu [23], Zn [24], and Cd [25±27])



Structure of Cadmium Complexes 1147

in the aprotic solvent 1,2-dichloroethane. The formation constants of pyridinecomplexes linearly increase with increasing pKa of the conjugate acid of thepyridine base. This fact indicates that the bond between the metal ion and thepyridine base is a in principle �-bond and the contribution of �-back donation isminimal. This bond nature is effective even in the relatively soft cadmium(II) ion.The effect of basicity of the pyridine base on the metal-nitrogen bond length (M±N)in the solid state has been investigated for the tetrahedral zinc(II) complexes [17].The M±N bond length of zinc(II) complexes linearly shortens with increasing pKa

of the pyridine base. As the coordination number and the geometry of other com-plexes such as those of cobalt(II) and copper(II) vary with the kind of pyridinebase, it is impossible to investigate systematically the in¯uence of ligand basicityon the metal-pyridine bond length in the solid state. Since the cadmium complexesin the present study have the same geometry, it is interesting to compare the resultwith zinc(II) complexes.

As shown in Table 1, the Cd±N distance of bis-(pyridine base) complexes doesnot vary essentially by changing the base in contrast to the Zn±N distances ofzinc(II) complexes. This may be due to the difference of the coordination geometrybetween the cadmium(II) and zinc(II) complexes in the solid state. In the case ofcadmium complexes, the pyridine base molecules coordinate trans to each other.Increasing basicity of the pyridine base results in a weakening of the M±N bondof another trans pyridine base. Thus, the effect of the basicity of the pyridine baseon the metal-nitrogen bond, i.e. the M±N bond length, might be largely cancelled.A thermal decomposition study on the series of pyridine base complexes of cad-mium(II) chloride indicated that the enthalpy of decomposition of the complexesscarcely depends on the basicity of the ligand [28]. This result is reasonably ex-plained by the structures of the complexes.

The metal-chloride (M±Cl) and metal-nitrogen (M±N) bond lengths of the non-substituted pyridine complexes for some metal ions are listed in Table 2. All metalcomplexes except for the tetrahedral zinc(II) complex have the same octahedralcon®guration. The M±Cl bond length is almost equal to the M±Cl0 bond length foreach metal. The large difference of bond lengths between Cu±Cl and Cu±Cl0 forcopper(II) may be due to the Jahn-Teller effect. The order of the average values of

Table 1. Bond distances (AÊ ) and angles (�) around the cadmium atoms

1 2 3

Bond Distances

Cd±Cl 2.6617(9) 2.660(4) 2.6596(8)

Cd±Cl0 2.6625(9) 2.669(4) 2.6795(8)

Cd±N 2.347(3) 2.352(4) 2.358(3)

Bond Angles

N±Cd±Cl 88.52(7) 89.59(7) 90.24(6)

N±Cd±Cl0 90.49(7) 90.11(7) 89.27(6)

Cl±Cd±Cl0 92.69(3) 93.27(11) 93.35(3)

Key to symmetry operations: x, y, ÿ1� z for 1 and 2; x, ÿ1� y, z for 3


the M±Cl and M±Cl0 bond lengths parallels that of the ionic radii of the metal ions.This fact suggests the same bond nature of the M±Cl bond for all metal complexes.On the other hand, the M±N bond length of the mercury(II) complex is shorter thanthat of its cadmium counterpart, in contrast to the order of ionic radii. This factmay be interpreted by the contribution of �-back bonding from the metal ion to thenitrogen atom in mercury complexes.

Figures 4±6 show the crystal structures of 1±3. The cadmium atoms of allcomplexes occupy crystallographically special positions. Accordingly, the cadmiumatoms of 1 and 2 are located at the center of symmetry lined along the c-axis,whereas those of 3 lie on the two-fold axis lined along the b-axis. The position ofcadmium atoms in the adjacent chain are translated by 0.5 unit cell lengths towardsthe c-axis for 1 and 2 and towards the b-axis for 3. Thus, the interaction of pyridinebases between the discrete chains is minimal, and no �-� stacking exists.

Table 2. Bond distances (AÊ ) of bis-pyridine complexes of metal chlorides

M±Cl av. M±Cla M±N av. M±Nb Ref.

MnCl2 py2 (oct) 2.56, 2.57 2.565 2.24 2.24 [6]

CoCl2 py2 (oct)c 2.485(7), 2.506(7) 2.504 2.11(1) 2.14 [15]

2.521(7), 2.503(7) 2.17(1)

CuCl2 py2 (oct) 2.299(2), 3.026(2) 2.663 2.004(5) 2.004 [5]

ZnCl2 py2 (tet) 2.215(2), 2.228(2) 2.222 2.046(5),

2.052(6)

2.049 [16]

CdCl2 py2 (oct) 2.661(1), 2.663(1) 2.662 2.346(3) 2.346 This work

HgCl2 py2 (oct) 2.754(2), 2.765(2) 2.760 2.266(6) 2.266 [14]

a Average value of M±Cl and M±Cl0 lengths; b average value for cobalt and zinc complexes; c since

there are two different molecules with different pyridine ring geometries, the values are listed for both

molecules

Fig. 4. Crystal structure of 1 (projection to (001) plane)


Raman spectroscopy

The molecular structure of pyridine base containing complexes of Cd(II) has beenevaluated by ESR for Mn(II) doped Cd(II) complexes [29, 30]. However, a syste-matic evaluation of CdX2(R-py)2 complexes has not yet been performed.

The Raman spectra of 1±8 are shown in Fig. 7. The Raman peaks below200 cmÿ1 usually contains some contribution of lattice mode vibrations. Thus, the




identi®cation and the assignment of the peaks in this region are dif®cult. It can,however, easily be seen that the spectroscopic features for 1 and 2 are quite similarbelow 200 cmÿ1. The resemblance of these spectra is due to the same symmetryelement. The spectrum of 4 resembles that of 3 in this region. This may indicatethat 4 has the same symmetry elements as 3 and that the structure of 4 is notaffected by the change of the substituent of the pyridine base.

Anhydrous cadmium(II) chloride and bromide show two intense Raman peaksat 233 and 131 cmÿ1 for chloride and 148 and 77 cmÿ1 for bromide. The higher fre-quency bands at 233 and 148 cmÿ1 were assigned to lattice vibrations, which havebeen shown to occur in the halogen-cadmium-halogen sandwich layer [31]. Inthese cadmium salts, the cadmium atoms are octahedrally surrounded by halo-genide ions and are connected by halogenide ion bridges. As can be seen fromFig. 7, a relatively strong peak at about 200 cmÿ1 (indicated by an arrow) is ob-served for all kinds of chloro complexes. Thus, by considering the crystal structurethis Raman peak can be assigned to the lattice vibration of the chloride-bridgedcadmium moieties.

Fig. 7. Raman spectra of 1±8; the Raman peaks marked with an arrow are assigned to halogen-

bridged halogen-cadmium-halogen vibrations; see text for details


The major Raman peaks for the bromide complexes appear at lower fre-quencies than those of the chloride complexes (Fig. 7). For the bromide complexes,the peak of medium intensity appears at about 150 cmÿ1 (indicated by an arrow inFig. 7) for any kind of pyridine base. The frequencies of these peaks agree with thefrequencies calculated from the corresponding chloride complex using theharmonic oscillator model. This fact indicates that the coordination environmentsof the bromide complexes are essentially the same as in the chloride complexes.Thus, it may be concluded that the structures of the bromide complexes are almostthe same as those of the chloride complexes, i.e. a bromide ion bridged octahedron.

For a given pyridine base complex, the Raman spectrum of the bromidecomplex in the frequency range above 200 cmÿ1 is essentially similar to that of thechloride complex. This fact indicates that the Cd±N bond strength is similar inchloride and bromide complexes. However, since the Raman frequencies of somepeaks are shifted from the positions of the neat pyridine base [32], an unequivocalassignment is dif®cult.

Experimental

Materials

Anhydrous cadmium(II) chloride and bromide (Nakarai Tesque Co. Ltd.) were used as received. The

bis-(pyrdine base) complexes were prepared according to Refs. [3, 28]. An appropriate amount of an

ethanolic solution of the pyridine base was added dropwise to a hot ethanolic solution of cadmium(II)

Table 3. Crystallographic data and experimental conditions of the X-ray measurements

1 2 3

Formula CdCl2C10H10N2 CdCl2C12H14N2 CdCl2C12H14N2

MW 341.51 369.57 369.57

Crystal System monoclinic monoclinic monoclinic

Space group P21/n P21/a C2/c

a/AÊ 17.784(2) 11.89(1) 21.091(2)

b/AÊ 8.666(1) 14.41(1) 3.8884(5)

c/AÊ 3.8252(7) 3.874(6) 18.2317(3)

�/� 91.54(1) 92.3(1) 113.64(1)

U/AÊ 3 589.3(1) 672.8(15) 1367.2(3)

Z 2 2 4

F(000) 332 364 728

Dc/g � cmÿ3 1.92 1.82 1.79

�(MoK�)/mmÿ1 2.23 2.01 1.95

Re¯ections measured 2543 2829 4666

Re¯ections with I> 3�(I ) 1518 2162 1840

2� range/� 3±65 3±65 3±65

R 0.032 0.030 0.034

R0 0.055 0.050 0.051

ga 0.052 0.015 0.066

Size of crystal/mm3 0.27� 0.18� 0.08 0.18� 0.52� 0.18 0.38� 0.17� 0.07

Scan width/� 1.3� 0.5tan� 1.2� 0.5tan� 1.3� 0.5tan�

a Weighting scheme: 1/w��2(F)� (g� jFoj)2


halogenide. Crude crystals of bis-(pyridine base) complexes were obtained upon cooling. Thermo-

gravimetry con®rmed the formation of bis-(pyridine base) complexes [28]. The crude product

was slowly recrystallized from an ethanolic solution of 0.1 mol � dmÿ3 pyridine base in a closed-

system evaporation crystallizer [33].

Crystal structure determination

Crystals (colorless needles) were cut to suitable length and mounted on top of a glass ®ber. Intensity

data were collected at room temperature on Rigaku AFC-5S and AFC-5R four-circle automated

diffractometers with graphite-monochromated MoK� radiation (�� 0.71073 AÊ ). The intensity data

were corrected for Lorentz and polarization factors and for absorption but not for extinction. Crystal

data and experimental conditions are listed in Table 3.

The structures were solved by the heavy atom method with the UNICS-III program [34]. The

positions of cadmium and chloride atoms were deduced from three-dimensional Patterson maps;

other atoms were located by successive Fourier syntheses and re®ned by a block-diagonal least-

squares method. Non-H atoms were re®ned with positional and anisotropic thermal parameters, and

Table 4. Fractional atomic coordinates of non-H atoms with estimated standard deviations for 1±3

x y z

CdCl2 py2 (1)

Cd 0.0 0.0 0.0

Cl 0.07807(4) 0.13900(10) 0.50955(20)

N 0.0878(2) ÿ0.2023(4) 0.0253(7)

C(1) 0.1594(2) ÿ0.1764(5) ÿ0.0536(10)

C(2) 0.2140(2) ÿ0.2891(6) ÿ0.0383(12)

C(3) 0.1944(3) ÿ0.4341(7) 0.0664(12)

C(4) 0.1211(4) ÿ0.4630(6) 0.1457(14)

C(5) 0.0698(2) ÿ0.3448(5) 0.1256(11)

CdCl2(3-Me-py)2 (2)

Cd 0.0 0.0 0.0

Cl ÿ0.14826(5) ÿ0.03389(5) 0.47992(14)

N ÿ0.0543(2) 0.1547(2) ÿ0.0030(5)

C(1) 0.0126(2) 0.2205(2) 0.1280(7)

C(2) ÿ0.0165(2) 0.3128(2) 0.1316(7)

C(3) ÿ0.1219(3) 0.3362(2) ÿ0.0101(8)

C(4) ÿ0.1914(3) 0.2693(2) ÿ0.1428(8)

C(5) ÿ0.1558(2) 0.1794(2) ÿ0.1379(7)

C(6) 0.0645(3) 0.3828(2) 0.2833(9)

CdCl2(4-Me-py)2 (3)

Cd 0.0 0.07019(7) 0.25

Cl ÿ0.07527(4) 0.56667(18) 0.15393(4)

N 0.0736(1) 0.0631(7) 0.1809(2)

C(1) 0.1374(2) ÿ0.0718(9) 0.2138(2)

C(2) 0.1798(2) ÿ0.1006(9) 0.1725(2)

C(3) 0.1575(2) 0.0126(8) 0.0943(2)

C(4) 0.0916(2) 0.1567(10) 0.0609(2)

C(5) 0.0523(2) 0.1752(9) 0.1054(2)

C(6) 0.2003(3) ÿ0.0235(14) 0.0465(3)


H atoms, located from difference maps, were re®ned isotropically. All calculations were carried out

on a HITAC M-680H computer at the computer center of the Institute for Molecular Science. The

atomic scattering factors were taken from Ref. [35]. The fractional atomic coordinates of non-H

atoms are listed in Table 4. The ORTEP program [36] was employed for drawing the structures. For

bond lengths and angles around cadmium atoms, see Table 1. Details of the crystal structures have

been deposited at the Cambridge Crystallographic Data Center (CCDC 166164±166166).

Raman spectroscopy

Raman spectra of 1±8 as well as of anhydrous cadmium(II) chloride and bromide and neat pyridine

bases were recorded on a Jobin Yvon Ramanor U-1000 spectrometer, the exciting line (514.5 nm)

being provided by an NEC GLC 3200 argon ion laser. Raman spectra were measured for powder

samples in the range of 550±50 cmÿ1 and for neat pyridine bases in the range of 550±150 cmÿ1 at

0.5 cmÿ1 resolution.

Acknowledgements

The authors greatly acknowledge assistance and suggestions for crystallographic work from Prof.

K. Toriumi at the Himeji Institute for Technology.

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Received March 27, 2001. Accepted (revised) June 19, 2001


x-ray diffraction and raman spectroscopic study of bis-(pyridine base) complexes of cadmium(ii)...

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