Pyridine-type complexes of transition-metal halides XIV. Part III. Complexes with 2,4-, 2,6-, 3,4- and 3,5-lutidines

Article abstract of DOI:10.1023/A:1014972517695

Compounds obtained by a solid-gas phase reactions between copper(II) chloride and bromide and 2,4-, 2,6-, 3,4- and 3,5-lutidines were studied using thermogravimetry, far-infrared, electronic spectroscopy and X-ray diffraction. The results were compared with the corresponding data for the similar compounds with methylpyridines and 2,4,6-collidine. A special attention was paid to the host-guest phenomenon, a new structural feature of transition-metal halide complexes.

Full text of DOI:10.1023/A:1014972517695

Journal of Thermal Analysis and Calorimetry, Vol. 68 (2002) 81–90
PYRIDINE-TYPE COMPLEXES OF
TRANSITION-METAL HALIDES XIV
Part III. Complexes with 2,4-, 2,6-, 3,4- and 3,5-lutidines
K. Mészáros Szécsényi1, T. Wadsten2, B. Carson3, É. Bencze4 and
G. Liptay2
1Institute of Chemistry, Faculty of Sciences, University of Novi Sad, 21000 Novi Sad,
Trg D. Obradoviæa 3, Yugoslavia
2Department of Inorganic Chemistry, Technical University of Budapest, Budapest H-1521,
Hungary
3International Recruitment Office, Recruitment and Admissions Service, Heriot-Watt University,
Edinburgh, Scotland, EH14 4AS
4Institute of Isotopes and Surface Chemistry, Chemical Research Center, Hungarian Academy of
Sciences, P.O. Box: 77, Budapest, H-1525, Hungary
(Received July 30, 2001)
Abstract
Compounds obtained by a solid–gas phase reactions between copper(II) chloride and bromide and
2,4-, 2,6- 3,4- and 3,5-lutidines were studied using thermogravimetry, far-infrared, electronic spec-
troscopy and X-ray diffraction. The results were compared with the corresponding data for the simi-
lar compounds with methylpyridines and 2,4,6-collidine. A special attention was paid to the
host-guest phenomenon, a new structural feature of transition-metal halide complexes.
Keywords: copper(II) halide–lutidine (dimethylpyridine) complexes, host-guest phenomenon
Introduction
In our previous publications [1, 2] we studied copper(II) halide complexes with methyl-
pyridines. This paper deals with 2,4-, 2,6- 3,4- and 3,5-lutidine (2,4-L, 2,6-L, 3,4-L and
3,5-L, respectively) complexes of copper(II) chloride and bromide. The aim of the study
was to find the relationship between the methyl group(s) position and the newly observed
host-guest phenomenon in copper(II) halide–methylpyridine complexes using spectro-
scopic and X-ray diffraction data of the originally obtained compounds and the interme-
diates which were formed during the thermal decomposition of the complexes.
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
Experimental
The preparation of the compounds, the methods for analysis and the instrumentation
are described in detail in our previous paper [1].
Results and discussion
The compounds were synthesised by a solid–gas phase reaction between the appro-
priate copper(II) halide and lutidine. As the atmosphere above the copper(II) salt was
always saturated with the vapour of the given lutidine, always a compound with the
highest co-ordination number was obtained instead of the most stable one. This
means that the composition of some compounds has changed with time. The elemen-
tal analyses data of the freshly prepared samples and the intermediates are given in
Table 1. It is apparent that the number of the ligands with a 2- or 6-methyl substitu-
ents, near to the co-ordinating nitrogen atom, is only two with both halide salts refer-
ring to a steric hindrance during the complex formation. With methyl substituents in
position 3,4- and 3,5-, complexes with higher co-ordination number are forming.
With 3,4-dimethylpyridine tetrakis(ligand) complexes were obtained with both of the
copper(II) halides. With 3,5-dimethylpyridine and CuCl2 the number of the ligands of
the freshly prepared compound was somewhat more than 3, while with CuBr2 this
ligand gave a tris(ligand) complex.
Table 1 Elemental analysis data for the starting compounds and the intermediate (The intermedi-
ates are marked with*)
%C
%H
%N
Compound
calc.
48.21
48.21
59.72
48.21
48.21
38.42
38.42
51.58
46.29
32.83
found
47.63
48.23
60.03
48.04
48.01
38.54
38.03
51.50
45.19
34.97
calc.
5.20
5.20
6.44
5.02
5.20
4.15
4.15
5.57
5.00
3.54
found
5.00
5.17
6.50
5.24
5.17
3.94
3.92
5.57
4.76
3.62
calc.
8.03
8.03
9.95
8.03
8.03
6.40
6.40
8.60
7.71
5.47
found
7.93
8.04
10.04
7.94
7.82
6.49
6.32
8.55
7.52
5.80
Cu(2,4-L)2Cl2
Cu(2,6-L)2Cl2
Cu(3,4-L)4Cl2
*Cu(3,4-L)2Cl2
*Cu(3,5-L)2Cl2
Cu(2,4-L)2Br2
Cu(2,6-L)2Br2
Cu(3,4-L)4Br2
Cu(3,5-L)3Br2
*Cu(3,5-L)1.5Br2
All stable compounds contain two ligands except the tris(3,5-methylpyridine)
copper(II) bromide complex which is stable in a temperature range up to 350 K where
it decomposes to a compound with a composition of Cu(3,5-L)1.5Br2. The degradation
of the intermediate starts almost immediately. The decomposition of the other stable
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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
83
complexes takes place in a temperature range of 400–450 K. The decomposition of
the Cu(3,5-L)2Cl3.25 begins almost at room temperature, so the composition of the
freshly prepared sample is not presented in Table 1 and was determined on the basis
of the thermal decomposition curve according to the composition of the stable com-
pound. In case of Cu(3,4-L)4Cl2, Cu(3,5-L)2Cl3.25 and Cu(3,5-L)3Br2 complexes, inter-
mediates could be isolated and the composition of the obtained compounds was de-
termined by elemental analysis. The results show that the composition of the interme-
diates corresponds to the complexes with two ligand molecules except in the case of
Cu(3,5-L)3Br2 where the composition of the intermediate agree with Cu(3,5-L)1.5Br2.
The thermal stability of this compound and the Cu(3,4-L)2Cl2 intermediate is poor
and they were isolated by isothermal heating at the beginning of the decomposition of
the parent compound (around 350 K in both cases).
Fig. 1 DSC curves for the copper(II) chloride complexes with lutidines
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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
Fig. 2 DSC curves for the copper(II) bromide complexes with lutidines
The thermal decomposition curves of the compounds are presented in Figs 1–4.
DSC curves (Figs 1 and 2) refer to different decomposition courses: the decomposi-
tion of Cu(3,5-L)3Br2, Cu(3,4-L)4Br2, Cu(3,4-L)4Cl2 and Cu(2,6-L)2Cl2 is endother-
mic up to 550 K. In case of other compounds during the decomposition an exothermic
effect was observed which is very explicit at the decomposition of the Cu(3,4-L)4Cl2,
referring probably to a structural rearrangement. In bromide complexes melting was
observed in the first decomposition step, while there is no melting in case of chloride
complexes.
The corresponding TG curves are presented in Figs 3 and 4 (DTG curves are
omitted for the sake of a better introspection). The decomposition of Cu(3,5-L)2Cl3.25
,
calculated on the basis of the mass loss data can be presented by the following
scheme,
Cu(3,5-L)2Cl3.25 → Cu(3,5-L)2Cl2+1.5 (3,5-L)
Cu(3,5-L)2Cl2 → Cu(3,5-L)Cl2+(3,5-L)
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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
85
Cu(3,5-L)Cl2 → Cu(3,5-L)1/3Cl2+2/3 (3,5-L)
Cu(3,5-L)1/3Cl2 → CuCl2+1/3 (3,5-L),
while the decomposition of Cu(3,5-L)3Br2 takes place in two step to CuBr2
Cu(3,5-L)3Br2 → Cu(3,5-L)1.5Br2+1.5 (3,5-L)
Cu(3,5-L)1.5Br2 → CuBr2+1.5 (3,5-L).
The expected end product of the decomposition should be the corresponding
copper(II) halide [1, 2] (or at higher temperatures CuO in curves recorded in an air at-
mosphere) in all the complexes. However, this is the case only with the compounds of
Cu(3,5-L)3Cl3.25 and Cu(3,5-L)3Br2. Even the stability of these halides is poor. The de-
Fig. 3 TG curves for the copper(II) chloride complexes with lutidines
J. Therm. Anal. Cal., 68, 2002

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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
Fig. 4 TG curves for the copper(II) bromide complexes with lutidines
composition of the other compounds is continuous with a CuO end product around
800 K, except of the chloride complex with 3,4-lutidine where almost no residue is
left. The chloride complex with 2,6-lutidine decomposes to CuCl (even in air), which
is stable in a temperature range of about 50 K.
The FTIR spectra of the samples refer to a different geometry of the complexes [3].
Both the chloride and the bromide derivatives of 2,4-lutidine show a square planar ar-
rangement [3–5]. In case of 2,6-lutidine analogues the bromide complex show a distorted
octahedral, while the chloride one shows up as having a tetrahedral symmetry. For the
chloride complex with 3,5-lutidine, the spectrum of the parent compound refers to an oc-
tahedral structure. Besides, bands of the free ligand were also observed [6] (531 sh, 284
sh, 209 w) indicating a host-guest interaction with the stable bis(ligand) compound which
is forming during the thermal decomposition. According to the FTIR data, with losing the
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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
87
excess ligand the geometry of the parent compound changes to a square planar structure
[3, 5]. The corresponding bromide intermediate appears to have a distorted octahedral
symmetry. In the spectrum of the parent chloride complex with 3,4-lutidine [3, 5], the
bands belonging to the ligand were splitted showing the presence of the free ligand ([6],
532 m, 421 sh, 257 s with participation of ν(CuN)) and the molecule shows an octahedral
arrangement. The chloride intermediate spectrum indicates a tetrahedral, while the bro-
mide analogue a distorted octahedral symmetry. The assignation of CuX, CuN
stretchings and that of the ligand bands is given in Table 2.
Table 2 The assignation of the FTIR vibration bands
Compound
ν(CuX)
ν(CuN)
252 s
Ligand bands
Cu(2,4-L)2Cl2
Cu(2,6-L)2Cl2
317 vvs
543 m, 447 vs, 432 w, 298 s, 212 w
542 vw, 447 m, 284 m, 202 w
313 vvs,
296 vvs
240 vs
Cu(2,4-L)2Br2
Cu(2,6-L)2Br2
Cu(3,4-L)2Cl2
242 vs
229 vs
253 sh
245 s
257 s
543 m, 446 vs, 432 w, 298 m, 208 w
541 w, 448 m, 287 vs, 203 w
358 m, 428 s, 191 vs, 164 m
295 vvs,
286 vvs
Cu(3,5-L)2Cl2
Cu(3,4-L)2Br2
Cu(3,5-L)2Br2
294 vvs
220 m
217 m
249 s
542 w, 482 vw, 422 vw, 292 m, 224 w, 181 vs
542 vs, 433 vs, 187 vw
243 vs
241 vs
540 wm, 422 w, 292 m, 209 m
The electronic spectral data for the parent compounds and the intermediates are
presented in Table 3 together with the type of the d→d transition. The geometry of
the compounds determined on this basis is in accordance with the geometry sug-
gested on the basis of the FTIR data.
Table 3 Electronic spectral data of the compounds
Compound
Peak position cm–1
17.391
d → d transition
2B1g → 2A2g
2B1g → 2A2g
2T2 → 2E
Geometry
Cu(2,4-L)2Cl2
Cu(2,4-L)2Br2
Cu(2,6-L)2Cl2
Cu(2,6-L)2Br2
Cu(3,4-L)4Cl2
*Cu(3,4-L)2Cl2
Cu(3,4-L)4Br2
Cu(3,5-L)4Cl2
*Cu(3,5-L)2Cl2
Cu(3,5-L)3Br2
square planar
square planar
tetrahedral
16.474
14.705
2
16.000
2Eg → T2g
distorted octahedral
octahedral
2
16.000
2Eg → T2g
14.706
2T2 → 2E
tetrahedral
2
16.520
2Eg → T2g
distorted octahedral
octahedral
2
14.598
2Eg → T2g
14.598
2B1g → 2A2g
square planar
distorted octahedral
2
15.552
2Eg → T2g
J. Therm. Anal. Cal., 68, 2002

Table 4 The calculated and refined unit cell dimensions of the chloride and bromide complexes with 2,4- and 2,6-lutidine
Unit cell parameters
Density
Dobs. Dcalc
Compound
Symmetry
a/
b/
Å3
c/
α/
β/
γ/
V/
Z
deg
Å3
g cm–3
Cu(2,4-L)2Cl2
Cu(2,4-L)2Br2
Cu(2,6-L)2Cl2
Cu(2,6-L)2Br2
monocl.
tric.
2
1
2
1
11.08
11.34
8.05
14.81
14.75
7.65
8.44
5.31
5.50
8.03
7.85
90.00
90.00
93.81
63.94
114.58
114.50
62.93
67.14
90.00
90.00
66.55
57.85
792.4
835.6
385.6
394.2
1.472
1.46
1.73
1.50
1.84
1.756
1.534
1.865
monocl.
tric.
8.01
Table 5 The calculated and refined unit cell dimensions of the chloride and bromide complexes with 3,4- and 3,5-lutidine
Unit cell parameters
Density
Compound
Symmetry
a/
b/
Å3
c/
α/
β/
γ/
V/
Å3
Dobs.
Dcalc
Z
deg
g cm–3
Cu(3,4-L)4Cl2
Cu(3,4-L)2Cl2
Cu(3,4-L)4Br2
Cu(3,5-L)2Cl2
Cu(3,5-L)2Br2
orto
tetr.
4
8
4
2
2
9.79
14.64
9.96
14.80
14.64
14.79
14.14
14.43
9.78
12.54
9.96
90.00
90.00
90.00
90.00
90.00
90.00
90.00
90.00
93.36
93.49
90.00
90.00
90.00
90.00
90.00
1409.2
2688.0
1458.7
760.9
818.9
1.326
1.722
1.483
1.52
orto
monocl.
monocl.
13.66
13.89
3.94
1.57
4.09
1.78

MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
89
X-ray diffraction data on the bis(ligand) copper(II) halide compounds with 2,4- and
2,6-lutidines (the observed and the calculated densities within reasonable accuracy) cor-
respond well to the given compositions. The index calculation of young samples is rather
poor. Maximum one (normally 0) observed peak is not indexed. However, after storing
several weeks or months at room temperature new sets of diffraction peaks are registered.
The true history of this ageing is not yet understood. The observed and the calculated
density data for these complexes are given in Table 4.
As the decompositions of 3,5-lutidine complexes starts almost at room tempera-
ture, the phase analysis of the stable compounds was performed. X-ray diffraction
data of these stable intermediates show very large similarities, which is also reflected
after indexing and refinements. According to TG-data the structure of 3,5-lutidine
complexes seems to swallow at least three ligands. The quality of the diffraction char-
acters of these two room temperature complexes is rather poor. The number of the
measurable signals, apart from sets of the high-temperature type ones is low, which
makes a throughout study worthless. Concerning the high-temperature composition,
the number of ligands seems to correspond to two before a complete breakdown into
solid halides. The amount of CuBr2 complex was too small to make a true density
measurement. Cu(3,5-L)2Cl2 complex has an observed density of 1.56 g cm–3. With
two ligands the density, calculated from X-ray data is 1.52 g cm–3. The calculated and
refined unit cell dimensions of the compounds are given in Table 5. The same data for
the parent and the intermediate CuCl2 complex with 3,4-lutidine and the CuBr2 com-
pound with the same ligand are presented in the same Table. The refined and indexed
monophase X-ray patterns can be obtained upon request from the corresponding au-
thor or found in the near future in the JCPDS database.
Conclusions
Most of the copper(II) chloride and bromide complexes with mono- and dimethyl-
substituated pyridines have a zeolite structure and should be regarded as having
stoichiometries [Cu(L)2Cl2] or [Cu(L)2Br2] with a variable increased composition up
to two extra non-coordinated ligands, [1, 2]. In case of parent compounds the X-ray
measurements as well as FTIR spectra beside the co-ordinated ligand molecules indi-
cate the presence of free, non-coordinated ligands.
The number of the excess ligand molecules depends on the position of the
methylsubstituent(s). With ligands having 2-methylsubstituents no host-guest phe-
nomenon was observed, while ligands with a methyl group in position 4 give com-
pounds with the highest co-ordination number. This is in accordance with the extent
of the steric repulsion between the halides and the ring substituent(s).
From the comparison of the thermal decomposition curves it is evident that the
corresponding bromido and chlorido complexes display a similar decomposition
mechanism, suggesting that the chemical and structural properties of the complexes
are dependent on the ligand structure rather than of the nature of the halide. In case of
complexes with 4-methyl substituents the exothermic effect in DSC curves suggests a
structural rearrangement during the decomposition. Besides, the very sharp endother-
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MÉSZÁROS SZÉCSÉNYI et al.: COPPER(II) CHLORIDE AND BROMIDE COMPLEXES
mic maximum in bromide complexes refer to the melting of the compound. In case of
chloride complexes no melting was observed.
2,4,6-collidine with copper(II) chloride gives a complex only with loosely bond
ligand molecules, while with copper(II) bromide no complex formation takes place.
* * *
The authors would like to thank the Royal Swedish Academy of Sciences and the Domus Hungarica
Scientiarum Artiumque Foundation for their support. The project was supported also by the Hungar-
ian OTKA Foundation (T-019555).
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