Dimorphism of a new Cul coordination polymer: Synthesis, crystal structures and properties of catena[Cul(2-iodopyrazine-N)] and poly[Cul(μ2-2-iodopyrazine-N,N′)]

Article abstract of DOI:10.1021/ic026007u

Two modifications of the new copper(I) iodide coordination polymer Cul(2-iodopyrazine) were obtained by the reaction of Cul and 2-iodopyrazine in acetonitrile. During this reaction, intensely yellow crystals of form I appear first which transform within several minutes to intensely red crystals of form II which is the thermodynamically most stable form at room temperature. In catena[Cul(2-iodopyrazine-N)] (form I; a = 4.1830 (6) A; b = -10.814 (1) A; c = 17.961 (4) A; V = 812.5 (2) A³; orthorhombic; P2₁2₁2₁; Z = 4), corrugated Cul double chains are found in which each copper atom is coordinated by one additional 2-iodopyrazine ligand. In poly[Cul(μ-2-iodopyrazine-N, N′)] (form II; a = 4.2679 (5) A; b = 13.942 (2) A, c = 13.017 (2) A, b = 92.64 (1)°; V = 773.76 (2) A³; monoclinic; P2₁/c; Z = 4), Cul single chains occur which are connected via μ-N,N′ coordination by the 2-iodopyrazine ligands to layers parallel to (010). The thermal behavior of both forms was investigated using simultaneous differential thermoanalysis, thermogravimetry, and mass spectrometry as well as differential scanning calorimetry and temperature resolved X-ray powder diffraction. On heating, both forms decompose to copper(I) iodide, and the decomposition temperature of form I is significantly lower than that of form II. From all experiments, there is no indication of a phase transition of one form into the other or for the formation of a phase with lower amine content.

Full text of DOI:10.1021/ic026007u

Inorg. Chem. 2003, 42, 2391−2397
Dimorphism of a New CuI Coordination Polymer:Synthesis, Crystal
Structures and Properties of Catena[CuI(2-Iodopyrazine-N)] and
Poly[CuI(µ2-2-Iodopyrazine-N,N′)]
Christian Na1ther,* Mario Wriedt, and Inke Jess
Kiel, Institut fu¨r Anorganische Chemie der Christian-Albrechts-UniVersita¨t,
Olshausenstr. 40, D-24098 Kiel, Germany
Received September 6, 2002
Two modifications of the new copper(I) iodide coordination polymer CuI(2-iodopyrazine) were obtained by the
reaction of CuI and 2-iodopyrazine in acetonitrile. During this reaction, intensely yellow crystals of form I appear
first which transform within several minutes to intensely red crystals of form II which is the thermodynamically most
stable form at room temperature. In catena[CuI(2-iodopyrazine-N)] (form I; a ) 4.1830 (6) Å; b ) 10.814 (1) Å;
3
c ) 17.961 (4) Å; V ) 812.5 (2) Å ; orthorhombic; P212121; Z ) 4), corrugated CuI double chains are found in
which each copper atom is coordinated by one additional 2-iodopyrazine ligand. In poly[CuI(µ-2-iodopyrazine-
3
N,N′)] (form II; a ) 4.2679 (5) Å; b ) 13.942 (2) Å; c ) 13.017 (2) Å; b ) 92.64 (1)°; V ) 773.76 (2) Å ;
monoclinic; P21/c; Z ) 4), CuI single chains occur which are connected via µ-N,N′ coordination by the 2-iodopyrazine
ligands to layers parallel to (010). The thermal behavior of both forms was investigated using simultaneous differential
thermoanalysis, thermogravimetry, and mass spectrometry as well as differential scanning calorimetry and temperature
resolved X-ray powder diffraction. On heating, both forms decompose to copper(I) iodide, and the decomposition
temperature of form I is significantly lower than that of form II. From all experiments, there is no indication of a
phase transition of one form into the other or for the formation of a phase with lower amine content.
Introduction
is the development of synthetic strategies for a more directed
design of the structures of these solids in order to influence
their physical properties in a desired way.1-14 One interesting
class of coordination polymers is based on copper(I)halides
or pseudo-halides and multidendate amino ligands. These
compounds are mostly intense red to yellow with some of
them showing luminescence and thermoluminescence. The
crystal structures of these coordination polymers are based
on inorganic CuX substructures (X ) halide or pseudo
halide) which are linked by the organic ligands to multi-
dimensional coordination polymers. Dependent on the cop-
per(I) halide or pseudo-halide and the nature of the organic
ligand, different CuX substructures such as dimers, tetramers,
The design of new coordination polymers based on
transition metal atoms and organic ligands has become of
increasing interest recently.1-14 One major goal in this area
* Corresponding author. E-mail: cnaether@ac.uni-kiel.de.
(1) Batten, S. R.; Robson, R. Angew. Chem. 1998, 110, 1558-1595;
Angew. Chem., Int. Ed. 1998, 37, 1460-1467.
(2) Robson, R.; Abrahams, B. F.; Batten, S. R.; Grable, R. W.; Hoskins,
B. F.; Liu, J. In Supramolecular Architecture; American Chemical
Society: Washington, DC, 1992; Chapter 19.
(3) Moulton, B.; Zaworotko, M. J. Chem. Soc. ReV. 2001, 101, 1629-
1658.
(4) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc.
Chem. Res. 1998, 31, 474-484.
(5) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem. 1999, 111,
2798-2848; Angew. Chem., Int. Ed. 1999, 38, 2638-2688.
(6) Robson, R. In ComprehensiVe Supramolecular Chemistry; Perga-
mon: New York, 1996; Chapter 22, pp 733-748.
(11) Manson, J. L.; Arif, A. M.; Miller, J. S. Chem. Commun. 1999, 1479-
1480.
(7) Kutasi, A. M.; Batten, S. R.; Harris, A. R.; Moubaraki, B.; Murray,
K. S. CrystEngComm. 2002, 202-204.
(12) Reineke, T. M.; Eddaoudi, M.; O’Keeffe, O.; Yaghi, O. M. Angew.
Chem. 1999, 111, 2712-2716; Angew. Chem., Int. Ed. 1999, 38,
2590-2594.
(8) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W.-S.; Schro¨der,
M. Coord. Chem. ReV. 1999, 183, 17-33.
(9) Holiday, B. J.; Mirkin, C. A. Angew. Chem. 2001, 113, 2076-2098;
Angew. Chem., Int. Ed. 2001, 40, 2022-2044.
(13) Zaworotko, M. J. Angew. Chem. 2000, 112, 3180-3182; Angew.
Chem., Int. Ed. 2000, 39, 3052-3054.
(10) Chesnut, D. J.; Kusnetzow, A.; Zubieta, J. J. Chem. Soc., Dalton Trans.
1998, 4081-4083.
(14) Batten, S. R.; Hoskins, B. F.; Robson, R. Chem. Eur. J. 2000, 6 (1),
156-161.
10.1021/ic026007u CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/05/2003
Inorganic Chemistry, Vol. 42, No. 7, 2003 2391

Na1ther et al.
and single, double, or helical single chains are observed.15-36
The dimensionality of the coordination network can be
influenced predominantly by the coordination properties of
the organic ligands. For one definite copper(I) halide or
pseudo-halide and one specific ligand, frequently several
compounds are found, which differ in the ratio between the
inorganic and organic parts. We have found that most of
the amine rich compounds can be quantitatively transformed
to amine poorer compounds via controlled thermal decom-
position. In some cases, decomposition to the copper(I)
halides or pseudo-halides is observed without the formation
of an intermediate phase.30-37 Nevertheless, this method is
an attractive strategy for the preparation of new coordination
polymers which cannot be synthesized directly, which are
obtained as mixtures, or which are frequently overlooked if
the reaction is performed in solution. The number of
intermediate compounds that can be observed depends, for
example, on the experimental conditions and the heating rate
used in the experiments. That shows that the kinetics of all
reactions involved may play an important role.34 To under-
stand such thermal reactions in more detail, we have started
systematic investigations on the synthesis, preparation and
the thermal behavior of these compounds. In order to obtain
strategies for a more directed design of specific structures,
to define structure property relationships, and to investigate
thermodynamic and kinetic properties of such compounds,
one has to be aware of the phenomena of polymorphism or
isomerism. This is frequently found in organic or metal
organic compounds.3,29,38-40 These phenomena are of interest
from several points of view. First of all, the structural aspects
of polymorphism and isomerism provide information on
intermolecular interactions in crystals and, therefore, can be
used for a more rational crystal design. Moreover, investiga-
tions on the thermodynamic and kinetic aspects, e.g., of
polymorphism provide important information about the
stability of each modification and the transformation
behavior.41-43
During our investigations on CuX coordination polymers,
we have found two forms of CuI(2-iodopyrazine) which
differ in the arrangement of the building blocks. Here we
report on the synthesis, the crystal structures, and the thermal
behavior of both forms. The structural and thermodynamic
aspects as well as the properties of these modifications are
discussed and compared with those obtained for related CuX
coordination polymers.
Experimental Section
Synthesis of Catena[CuI(2-iodopyrazine-N)] (I). Copper(I)
iodide (518.7 mg (5.24 mmol)) and 600 mg (5.24 mmol) of
2-iodopyrazine were stirred in 9 mL of acetonitrile at room
temperature. Within a few minutes, yellow crystals of I appear
which were filtered off before the transformation into the red form
II started and were washed with ethanol and diethyl ether. In some
of the isolated powders, extremely small single crystals have formed
which were suitable for X-ray single crystal determination. Yield:
88.1%. Anal. (%) Calcd: C, 12.20; N, 6.99; H, 0.67. Found: C,
12.12; N, 7.07; H, 0.76. X-ray powder diffraction: phase pure.
Synthesis of Poly[CuI(µ-2-iodopyrazine-N,N′)] (II). If the
reaction mixture described in the preceding paragraph were stirred
for about 15 min, all yellow crystals of I would transform into the
red crystals of II. Yield: 92.3%. Anal. (%) Calcd: C, 12.20; N,
6.99; H, 0.67. Found: C, 12.21; N, 7.00; H, 0.70. X-ray powder
diffraction: phase pure.
Single Crystal Structure Determination. For both compounds,
a face-indexed absorption using X-SHAPE44 and XEMP45 was
applied. The structure solution was performed using SHELXS-97,46
and structure refinement was done against F2 using SHELXL-97.47
All non-hydrogen atoms were refined using anisotropic displace-
ment parameters. The hydrogen atoms were positioned with
idealized geometry and refined with isotropic displacement param-
eters using the riding model. For compound I, the absolute structure
(15) Batten, S. R.; Jeffery, J. C.; Ward, M. D. Inorg. Chim. Acta 1999,
292, 231-237.
(16) Blake, A. J.; Brooks, N. R.; Champness, N. R.; Cook, P. A.; Deveson,
A. M.; Fenske, D.; Hubberstey, P.; Li, W.-S.; Schro¨der, M. J. Chem.
Soc., Dalton Trans 1999, 2103-2110.
(17) Rossenbeck, B.; Sheldrick, W. S. Z. Naturforsch. 2000, 55b, 467-
472.
(18) Kromp, T.; Sheldrick, W. S. Z. Naturforsch, 1999, 54b, 1175-1180.
(19) Teichert, O.; Sheldrick, W. S. Z. Anorg. Allg. Chem. 1999, 625, 1860-
1865.
(20) Teichert, O.; Sheldrick, W. S. Z. Anorg. Allg. Chem. 2000, 626, 1509-
1513.
(21) Teichert, O.; Sheldrick, W. S. Z. Anorg. Allg. Chem. 2000, 626, 2196-
2202.
(22) Blake, A. J.; Brooks, N. R.; Champness, N. R.; Cooke, P. A.; Crew,
M.; Deveson, A.; M. Hanton, L. R.; Hubberstey, P.; Fenske, D.;
Schro¨der, M. Cryst. Eng. 1999, 2, 181-195.
(23) Persky, N. S.; Chow, J. M.; Poschmann, K. A.; Lacuesta, N. N.; Stoll,
S. L. Inorg. Chem. 2001, 40, 29-35.
(24) Graham, P. M.; Pike, R. D.; Sabat, M.; Bailey, R. D.; Pennington, W.
T. Inorg. Chem. 2000, 39, 5121-5132.
(38) Batten, S. R.; Murray, K. S. Aust. J. Chem. 2001, 54, 605-609.
(39) Braga, D.; Maini, L.; Polito, M.; Scaccianoce, L.; Cojazzi, G.; Grepioni,
F. Coord. Chem. ReV. 2001, 216, 783-788.
(40) Desiraju, G. R. Crystal EngineeringsThe Design of Organic Solids.
Material Science Monographs; Elsevier: New York, 1989; p 54 and
literature cited therein.
(25) Blake, A. J.; Brooks, N. R.; Champness, N. R.; Hanton, L. R.;
Hubberstey, P.; Schro¨der, M. Pure Appl. Chem. 1998, 70, 2351-2358.
(26) Jack, Y. L.; Cabrera, B. R.; Wang, R.-J.; Li, J. Inorg. Chem. 1999,
38, 4608-4611.
(27) Na¨ther, C.; Greve, J. Acta Crystallogr. 2001, C57, 377-378.
(28) Na¨ther, C.; Jess, I. Acta Crystallogr. 2002, C58, m190-m192.
(29) Barnett, S. A.; Blake, A. J.; Champness, N. R.; Wilson, C. Chem.
Commun. 2002, 1640-1641.
(30) Na¨ther, C.; Greve, J.; Jess, I. Polyhedron 2001, 20 (9-10), 1017-
1022.
(41) Bernstein, J.; Davey, R.; Henck, O. Angew. Chem. 1999, 111, 3646;
Angew. Chem., Int. Ed. 1999, 38, 3440.
(42) Scho¨del, H.; Na¨ther, C.; Bock, H.; Butenscho¨n, F. Acta Crystallogr.
1996, B52, 842-853.
(43) Na¨ther, C.; Jess, I.; Havlas, Z.; Bolte, M.; Nagel N.; Nick, S. Solid
State Sci. 2002, 4, 859-871.
(31) Na¨ther, C.; Jess, I.; Studzinski. H. Z. Naturforsch. 2001, 56b, 997-
(44) STOE & CIE, X.-Shape, Version 1.03: Program for the crystal
optimisation for numerical absorption correction; STOE & CIE
GmbH: Darmstadt, Germany, 1998.
1002.
(32) Na¨ther, C.; Wriedt, M.; Jess, I. Z. Anorg. Allg. Chem. 2002, 628, 394-
400.
(45) STOE & CIE, X.-Red Version 1.11: Program for data reduction and
absorption correction; STOE & CIE GmbH: Darmstadt, Germany,
1998.
(46) Sheldrick, G. M. SHELXS 97, Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(47) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(33) Na¨ther, C.; Greve, J.; Jess, I. Solid State Sci. 2002, 4 (6), 813-820.
(34) Na¨ther, C.; Jess, I. J. Solid State Chem. 2002, 169, 103-112.
(35) Na¨ther, C.; Jess, I. Z. Naturforsch. 2002, 57b, 1133-1140.
(36) Kromp, T.; Sheldrick, W. S.; Na¨ther, C. Z. Anorg. Allg. Chem. 2003,
629, 45-54.
(37) Na¨ther, C.; Jess, I. Monatsh. Chem. 2001, 132, 897-910.
2392 Inorganic Chemistry, Vol. 42, No. 7, 2003

New CuI Coordination Polymer
Table 1. Crystal Data and Results of the Structure Refinement for
Catena[CuI(2-iodopyrazine-N)] (I) and
Poly[CuI(µ-2-iodopyrazine-N,N′)] (II)
simultaneously using the STA-409CD with Skimmer coupling from
Netzsch, which is equipped with a quadrupole mass spectrometer
QMA 400 (max. 512 amu) from Balzers. The MS measurements
were performed in analogue and trend scan mode, in Al2O3 crucibles
under a dynamic helium (purity 4.6) or nitrogen (purity 5.0)
atmosphere using heating rates of 1, 4, and 8 °C/min. All
measurements were performed with a flow rate of 75 mL/min and
were corrected for buoyancy and current effects. The heating rate
dependent TG measurements were performed under nitrogen
atmosphere (purity 5.0) in Al2O3 crucibles using a STA-409CD
thermobalance from Netzsch. Several measurements with different
heating rates of 0.5, 1, 2, 4, 8, and 16 °C/min were performed.
Differential Scanning Calorimetry. DSC experiments were
performed with the DSC 204/1/F from Netzsch in Al2O3 crucibles.
For all compounds, several measurements were performed using
heating rates between 1 and 10 °C/min. The calorimeter was
calibrated using standard reference substances. All characteristic
temperatures as well as all enthalpies were estimated from different
measurements at 2 °C/min.
I
II
chemical formula
fw
C4H3N2CuI2
396.42
C4H3N2CuI2
396.42
space group
P212121
P21/c
a
b
c
â
V
T
4.1830 (6) Å
10.814 (1) Å
17.961 (4) Å
4.2679 (5) Å
13.942 (2) Å
13.017 (2) Å
92.64 (1)°
773.76 (2) Å3
20 °C
812.5 (2) Å3
20 °C
Z
Dcalcd
4
4
3.241 g‚cm-3
10.20 mm-1
0.71073 Å
0.0214
3.403 g‚cm-3
10.71 mm-1
0.71073 Å
0.0171
µ
λ
R1a [I > 2σ(I)]
wR2a [all data]
0.0477
0.0390
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
.
Optical Microscopy. Microscopy was performed using a hot
stage from Linkam and a microscope BM60 from Olympus
equipped with a video recorder and a video camera.
Table 2. Selected Bond Lengths and Angles for Catena[CuI(2-
iodopyrazine-N)] (I) and Poly[CuI(µ-2-iodopyrazine-N,N′)] (II)a
Elemental Analysis. C, H, N analysis was performed using a
CHN-O-RAPID combustion analyzer from Heraeus, and energy
dispersive analysis of X-rays (EDAX) was performed using a
Philips XL30 environmental scanning electron microscope (ESEM)
with an EDAX system from Philips.
Results
Crystal Structures. Catena[CuI(2-iodoopyrazine-N)] (I)
crystallizes in the orthorhombic, chiral space group P212121
with four formula units in the unit cell and all atoms in
general positions (Table 1). In the crystal structure, the copper
atoms are coordinated by one nitrogen atom of one 2-iodo-
pyrazine ligand and three iodine atoms which are related by
symmetry within a strongly distorted tetrahedron (Figure 1
and Table 2). Corrugated CuI double chains are found as
the CuX substructure which are oriented in the direction of
the crystallographic a-axis (Figure 1, top). The 2-iodo-
pyrazine ligand does not act as a bridging ligand and is
coordinated only with that nitrogen atom to the copper atom
which is not neighbored to the iodine atom. This leads to
the formation of a chain structure. Bond lengths and angles
around the copper atoms are in the range of those found in
other CuI coordination polymers containing CuI double
chains. In the crystal, the chains are closely packed in a way
that each chain is surrounded by six symmetry related chains
(Figure 1, bottom).
Poly[CuI(µ-2-iodopyrazine-N,N′)] (II) crystallizes in the
monoclinic space group P21/c with four formula units in the
unit cell and all atoms in general positions (Table 1). Each
copper atom is surrounded by two nitrogen atoms of two
symmetry related 2-iodopyrazine ligands and two symmetry
related iodine atoms within a distorted tetrahedron (Figure
2 and Table 2). Bond lengths and angles around the copper
atoms are in the range of those found in other CuI
coordination polymers containing a topologically similar
coordination network. The copper atoms are connected via
the iodine atoms to form CuI single chains as the CuX
substructure. These chains are oriented in the direction of
a Displacement ellipsoids are drawn at the 50% probability level.
was determined and is in agreement with the selected setting (Flack
x-parameter: -0.003 (48)). In addition, inversion of the structure
leads to significantly worse reliability factors (R1 for all reflections
with I > 2σ(I) ) 0.0237; wR2 for all data ) 0.0544). Details of
the structure determination and lists with selected bond lengths and
angles are given in Tables 1 and 2.
X-ray Powder Diffraction. Powder diffraction experiments were
performed using a STOE STADI P transmission powder and a
Siemens D-5000 diffractometer in reflection geometry both with
Cu KR radiation (λ ) 1.540598 Å). The temperature-dependent
X-ray powder experiments were performed with a θ-θ-diffrac-
tometer from STOE and CIE in reflection geometry from Bu¨hler-
Otto GmbH with Cu KR radiation which is equipped with a position
sensitive detector and a high-temperature device HDK 2.41. The
measurements were performed every 5 °C under a dynamic nitrogen
atmosphere.
Differential Thermal Analysis, Thermogravimetry, and Mass
Spectrometry. DTA-TG-MS measurements were performed
Inorganic Chemistry, Vol. 42, No. 7, 2003 2393

Na1ther et al.
Figure 3. DTA, TG, DTG, and MS trend scan curves for catena[CuI(2-
iodopyrazine-N)] (I) [simultaneous measurement, crystalline powder, weight
) 22.4 mg, heating rate ) 4 °C/min, dynamic helium atmosphere (flow
rate ) 75 mL/min); Al2O3 crucible; Tp ) peak temperature; Te
extrapolated onset temperature; m/z (206) ) 2-iodopyrazine].
)
Figure 1. Crystal structure of catena[CuI(2-iodopyrazine-N)] (I) with view
along the crystallographic b-axis (top) and a-axis (bottom).
which is accompanied with a mass loss in the TG curve of
50.9% (Figure 3). This experimental mass loss is in good
agreement with that calculated for the complete removal of
the 2-iodo-pyrazine ligands (∆mtheo: -2-iodopyrazine )
52.0%). This is proved by mass spectrometry which shows
that only 2-iodopyrazine (m/z ) 206) is emitted. From the
DTG curve, it is obvious that the reaction occurred in only
one step and that no amine poorer intermediate phase has
formed. The final product of this reaction is CuI as shown
by X-ray powder diffraction.
If form II is heated in a thermobalance, a similar behavior
is found (Figure 4). The decomposition temperature is shifted
to 182 °C, and the experimental mass loss of 51.0% is in
good agreement with that calculated for the complete removal
of the 2-iodopyrazine ligands (∆mtheo: -2-iodopyrazine )
52.0%). These investigations demonstrate that in contrast to
the thermal behavior of most of the 1:1 CuX coordination
polymers no amine poorer phase is obtained. Because the
product formation in such thermal decomposition reactions
can depend on the kinetics of all reactions involved,34
measurements were performed at different heating rates.
However, these investigations gave no evidence for any
additional intermediate compounds.
From the DTA curves presented in Figures 3 and 4, there
is no hint for a phase transition of one form into the other.
To control this and to obtain information on the energy
difference between both forms, investigations using dif-
ferential scanning calorimetry were performed. On heating,
decomposition is observed at peak temperatures of 210 °C
for I and of 214 °C for II (Figure 5).
Figure 2. Crystal structure of poly[CuI(µ-2-iodopyrazine-N,N′)] (II) with
view along the crystallographic b-axis (top) and a-axis (bottom).
the crystallographic a-axis. They are connected via µ-N,N′
coordination by the 2-iodopyrazine ligands to layers parallel
to (010).
Thermoanalytical Investigations. On heating compound
I in a simultaneous DTA-TG-MS experiment, an endo-
thermic event is observed at a peak temperature of 170 °C
The difference between the decomposition energies that
corresponds to the energy difference between both forms is
about 21 kJ/mol. From these measurements, there is no
2394 Inorganic Chemistry, Vol. 42, No. 7, 2003

New CuI Coordination Polymer
Figure 6. Results of the temperature resolved X-ray powder diffraction
experiments on catena[CuI(2-iodopyrazine-N)] (I) (top) and poly[CuI(µ-
2-iodopyrazine-N,N′)] (II) (bottom) measured in reflection geometry (Cu
KR radiation, crystalline powder, dynamic nitrogen atmosphere, powder
patterns were measured every 5 °C).
Figure 4. DTA, TG, DTG, and MS trend scan curves for poly[CuI(µ-2-
iodopyrazine-N,N′)] (II) [simultaneous measurement, crystalline powder,
weight ) 24.5 mg, heating rate ) 4 °C/min, dynamic helium atmosphere
(flow rate ) 75 mL/min), Al2O3 crucible, Tp ) peak temperature, Te )
extrapolated onset temperature, m/z (206) ) 2-iodopyrazine].
The thermal behavior was additionally investigated by
temperature dependent X-ray powder diffraction (Figure 6).
On heating a crystalline powder sample of I under a nitrogen
atmosphere, a dramatic change of the diffraction pattern is
observed at about 135 °C, which corresponds to the
decomposition of the material (Figure 6). The experimental
powder diffraction pattern after this transformation is identi-
cal with that calculated for CuI from single crystal data.
Similar observations were made for II, but the decomposition
temperature is shifted to about 155 °C (Figure 6). From the
temperature resolved X-ray powder diffraction experiments,
there is no evidence for a transformation of one compound
into the other or for the occurrence of a new amine poorer
phase in agreement with the thermoanalytic results already
described.
Crystallization Experiments. When equimolar amounts
of CuI and 2-iodopyrazine were reacted in acetonitrile,
yellow crystals of compound I appeared first. After a few
minutes, red crystals of II started to precipitate and grew on
the cost of the yellow crystals. After about 15 min, the
transformation had finished and the crystals of compound I
had completely disappeared. The same is observed if crystals
of pure I were reacted in acetonitrile. These experiments
clearly prove that compound II is the thermodynamically
most stable form at room temperature. To investigate the
range in which II is the thermodynamic most stable form,
temperature dependent crystallization experiments were
performed in different solvents between -40 and 150 °C
for both modifications. In these experiments, a saturated
solution with an excess of crystal powder of each of the pure
modifications was stirred at different temperatures for about
1 day. Because both modifications are different colors, a
possible transformation could easily be observed. In addition,
all residues obtained at different temperatures were inves-
tigated by X-ray powder diffraction. If solvents were used
Figure 5. DSC curves for catena[CuI(2-iodopyrazine-N)] (I) (top) and
poly[CuI(µ-2-iodopyrazine-N,N′)] (II) (bottom) (crystalline powder, heating
rate ) 2 °C/min, Al pans, Tp ) peak temperature, Te ) extrapolated onset
temperature, given is the difference between the enthalpies for the complete
removal of the 2-iodopyrazine ligand).
evidence for a transformation between both forms or the
occurrence of additional phases. The decomposition tem-
peratures in the DSC measurements are shifted to higher
values compared to those obtained from DTA-TG-MS
investigations because in this case the gaseous reaction
products are not transported out of the crucibles by a
continuous gas flow and, therefore, the reaction takes place
in part under a self-produced atmosphere.
Inorganic Chemistry, Vol. 42, No. 7, 2003 2395

Na1ther et al.
Table 3. Selected Crystal Data for Some 1:1 CuX Coordination Polymersa
CuI
CuCl
CuBr
CuCl
CuBr
CuI
CuI(2-chloro-
pyrazine)
(2-iodo-
pyrazine)
(2-chloro-
pyrazine
(2-chloro-
pyrazine
(2-methyl-
pyrazine)
(2-methyl-
pyrazine)
(2-iodo-
pyrazine)
a/Å
b/Å
c/Å
4.1128 (6)
10.482 (2)
17.9852 (4)
4.1830 (6)
10.814 (1)
17.961 (4)
3.7752(4)
12.859 (1)
12.709 (2)
90.72 (2)
614.1 (1)
monoclinic
P21/c
3.9068 (3)
13.159 (2)
12.765 (2)
90.53 (1)
656.3 (1)
monoclinic
P21/c
3.862 (1)
12.887 (3)
12.585 (3)
95.59 (2)
623.2 (2)
monoclinic
P21/c
4.023 (1)
12.940 (4)
12.801 (4)
96.83 (2)
661.6 (3)
monoclinic
P21/c
4.2679 (5)
13.942 (2)
13.017 (2)
92.64 (1)
773.76 (2)
monoclinic
P21/c
â/deg
V/Å3
775.4 (2)
orthorhombic
P212121
812.5 (2)
orthorhombic
P212121
space group
CuX structure
dimensionality
2:1 on heating
color
double chains
1-D structure
yes
yellow
32
double chains
1-D structure
no
yellow
this work
single chains
2-D structure
no
red
32
single chains
2-D structure
no
red
32
single chains
2-D structure
yes
red
31
single chains
2-D structure
yes
orange-red
31
single chains
2-D structure
no
red
this work
ref
a The phrase “2:1 on heating” means that the 1:1 compound transform into a amine poorer 2:1 compound on heating.
which evaporate below 150 °C, the reaction was performed
in glass ampules. These experiments were performed for both
modifications separately. If form I was stirred at -40 °C,
this compound was stable over several hours before the
transformation into the red form II was observed, whereas
this transformation proceeded faster at elevated temperatures.
However, at all temperatures tested between -40 and 150
°C, form I always transformed into form II, and therefore,
form II is the thermodynamically most stable form in this
temperature range. In agreement with this observations, if
form II is investigated in this temperature range, no phase
transformation is observed.
room temperature. Because the density of II is significantly
higher compared to that of I, form II should also be more
stable at 0 °C.41 However, our thermoanalytical investigations
and temperature dependent crystallization experiments gave
no evidence for a polymorphic phase transition up to the
decomposition point of both forms. In addition, our DSC
experiments show clearly that form II is more stable than
form I. Therefore, it can be assumed that form II is the
thermodynamic most stable form over the whole temperature
range, whereas form I is metastable and both modifications
behave monotropically.41 In addition, none of these modi-
fications form amine poorer phases on heating.
Concerning the structural aspects of the dimorphism, it is
difficult to decide why form II is more stable than form I.
The crystal structure of form I represents one rare example
in which double CuI chains occur in a 1:1 CuX compound
containing organic ligands, which exhibit two possible
coordination sites. It is unlikely that this is due to some steric
crowding of the bulky iodo substituent, because the same
structure is also found if 2-chloropyrazine is used as ligand32
(see later). However, such double chains are mostly found
in 2:1 compounds in which the chains are connected by the
ligands to layers. The corresponding 1:1 compounds usually
exhibit CuX single chains which are connected to layers as
in form II. In the structure of form I compared to that of
form II, the ligands do not bridge different CuX double
chains and only a one-dimensional structure is formed.
Therefore, the connection of the CuX single chains to layers
in form II seems to be responsible for the higher thermo-
dynamic stability.
Discussion
Concerning the correct nomenclature in the case of
coordination polymers, no consistent trend can be found in
the literature.48 Following the nomenclature of Champness
et al., both compounds can be labeled as structural isomers29
whereas according to the notation of Zaworotko et al., they
should correspond to supramolecular isomers.3 There are
arguments for both opinions which predominantly depend
on the point of view. The fact that in the solid no
transformation of one form into the other is observed is also
found for polymorphic systems.49 However, for the present
compounds a transformation is observed in solution. In both
compounds, the coordination sphere and the arrangement of
the building blocks differ which would justify the term
“isomerism”. However, as in the case of polymorphism, this
behavior is coupled to the solid state. In solution, this
arrangement probably does not exist. Therefore, we have
used the older and more general notation “polymorphism”,
which is defined as the ability of a compound to exist in
more than one modification. This should also include the
phenomena of structural and supramolecular isomerism in
the case of coordination polymers.
In this context, it is interesting to compare the structures
and properties of the related CuX coordination polymers with
2-methylpyrazine31 and 2-chloropyrazine32 (Table 3). All
structures can be divided into two different groups (Table
3). The first group contains structures in which CuX double
chains are found and in which the organic part does not act
as the bridging ligand as in I. Therefore, only one-
dimensional structures are found. The second group contains
1:1 CuX compounds in which CuX single chains are found
that are connected by the organic ligands to form two-
dimensional structures as in II. All of them are based on a
similar coordination network (Table 3).
Concerning the thermodynamic aspects of the dimorphism,
the results of the room temperature synthesis show clearly
that form II is the thermodynamically most stable form at
(48) This problem was extensively discussed with Stuart Batten, Neil
Champness, and Mike Zaworotko. Thanks for their help.
(49) Bock, H.; Scho¨del, H.; Na¨ther, C.; Butenscho¨n, F. HelV. Chim. Acta.
1997, 80, 593-605.
2396 Inorganic Chemistry, Vol. 42, No. 7, 2003

New CuI Coordination Polymer
The compounds with CuCl and CuBr as well as 2-chloro-
pyrazine are isotypic to the thermodynamically most stable
form II with 2-iodopyrazine. For these copper(I) halides, only
these modifications can be prepared, and therefore, they
should represent the most stable forms at room temperature.
The same applies to the 2-methylpyrazine compounds.31 They
are not exactly isotypic to the 2-chloro- and 2-iodopyrazine
compounds but contain topologically identical coordination
networks.
Conclusions
In the present contribution, we have shown that polymor-
phism or isomerism plays an important role in the structural
chemistry of coordination polymers and it allows us to
compare structural, thermodynamic, and physical properties
of compounds exhibiting the same chemical composition but
a different arrangement of their building blocks. This is very
important in the area of crystal engineering, in which
compounds with defined structures and desired physical
properties are constructed. However, it also shows how
difficult structure prediction is on the basis of simple con-
sideration of the coordination properties of all building blocks
involved. Therefore, it is justified to label the phenomenon
of polymorphism as the “nemesis” of crystal design.
However, the results of our present work raise some
interesting questions regarding whether it is possible to
influence the structures, the thermodynamic stability, and the
thermal reactivity of these compounds by preparation of
mixed crystals. Investigations in the 2-methylpyrazine/2-
chloropyrazine system reveal that such crystals can be
prepared and that they exhibit a different thermal reactivity.
However, this will be the subject of a further contribution.
If CuI is reacted with 2-chloropyrazine, crystals of the 1:1
compound are obtained which are isotypic with the thermo-
dynamically metastable form I of CuI(2-iodopyrazine). In
contrast to form I, the compound with 2-chloropyrazine
represents the only and therefore thermodynamically most
stable form at room temperature. Unfortunately, no 1:1 CuX
compound with CuI and 2-methylpyrazine can be prepared.
However, from these results it is obvious that no simple
correlation between the structure and the thermodynamic
stability can be found.
The same is true for the thermal reactivity of the
compounds already discussed. The thermodynamically most
stable form II of CuI(2-iodopyrazine) as well as the isotypic
compounds CuX(2-chloropyrazine) (X ) Cl, Br) does not
form an amine poorer phase on heating and decompose
directly to CuX. In contrast, the two compounds CuX(2-
methylpyrazine) (X ) Cl, Br), which are nearly isotypic and
contain an identical coordination network, transform into
amine poorer 2:1 compounds on heating. This is also found
for the thermodynamically most stable 1:1 compound CuI(2-
chloropyrazine), which transforms into the new 2:1 phase
Cu2I2(2-chloropyrazine) on heating.32 In contrast, the meta-
stable modification I of CuI(2-iodopyrazine) which is iso-
typic to the former compound does not form an amine poorer
phase. This shows that in these compounds no simple
property relationships can be found.
Acknowledgment. We gratefully acknowledge the fi-
nancial support by the State of Schleswig-Holstein. We are
very thankful to Professor Dr. Wolfgang Bensch for financial
support and the facility for the use of his experimental
equipment. We also would like to thank Dr. Dirk Hinz-
Hu¨bner for the temperature resolved X-ray powder measure-
ments.
Supporting Information Available: Lists with details of the
structure determination, atomic coordinates, isotropic and aniso-
tropic displacement parameters, as well as CIF files and drawings
of the two structures. These materials are available free of charge
via the Internet at http://pubs.acs.org.
IC026007U
Inorganic Chemistry, Vol. 42, No. 7, 2003 2397