Synthesis, crystal structure and properties of CuBr(2,3-dimethylpyrazine) coordination polymers

Article abstract of DOI:10.1515/znb-2007-0403

Reaction of CuBr with an excess of 2,3-dimethylpyrazine in acetonitrile leads to the formation of the literature known ligand-rich 1:1 compound poly[μ₂-bromo-μ₂-2,3-dimethylpyrazine-N, N′-copper(I)] (1). On heating this compound in a thermobalance a transformation into the new ligand-deficient 2 :1 compound poly[di- μ₃-bromo-μ₂-2,3-dimethylpyrazine-N, N′-dicopper(I)] (3) is observed, which later was also prepared in solution. This compound crystallizes in the monoclinic space group P2 ₁/n with all atoms in general positions. In the crystal structure the Cu atoms are surrounded by three Br atoms and one 2,3-dimethylpyrazine ligand within a distorted tetrahedron. The tetrahedra are connected via common Br edges into CuBr double chains, which are connected by the ligands into layers located in the ab plane. The formation of compounds 1 and 3 was also investigated in solution. The results have shown that compound 1 can only be prepared if an excess of the ligand is used. If CuBr and the ligand are reacted in a ratio of 1 : 1, in the beginning the literature known ligand-deficient 3 : 2 intermediate catena[tribromo-μ₂-bis(μ₂-2,3-dimethylpyrazine-N, N′)-tricopper(I)] (2) is obtained, which transforms within minutes into compound 3. If a crystalline suspension of compound 1 is stirred in acetonitrile a transformation into the most stable compound 3 is also observed. The luminescence properties of compounds 1 and 3 were investigated. The complexes show differences mainly in the emission spectra.

Full text of DOI:10.1515/znb-2007-0403

Synthesis, Crystal Structure and Properties of
CuBr(2,3-dimethylpyrazine) Coordination Polymers
Inke Jeßa, Petr Taborskyb, and Christian Na¨thera
a Institut fu¨r Anorganische Chemie der Christian-Albrechts-Universita¨t zu Kiel, Olshausenstraße 40,
D-24098 Kiel, Germany
b Department of Analytical Chemistry, Faculty of Science, Masaryk University in Brno, Kotlarska 2,
611 37 Brno, Czech Republic
Reprint requests to Dr. Christian Na¨ther. E-mail: cnaether@ac.uni-kiel.de
Z. Naturforsch. 2007, 62b, 501 – 507; received October 23, 2006
Reaction of CuBr with an excess of 2,3-dimethylpyrazine in acetonitrile leads to the forma-
tion of the literature known ligand-rich 1 : 1 compound poly[µ2-bromo-µ2-2,3-dimethylpyrazine-
N,Nꢀ-copper(I)] (1). On heating this compound in a thermobalance a transformation into the new
ligand-deficient 2 : 1 compound poly[di-µ3-bromo-µ2-2,3-dimethylpyrazine-N,Nꢀ-dicopper(I)] (3) is
observed, which later was also prepared in solution. This compound crystallizes in the monoclinic
space group P21/n with all atoms in general positions. In the crystal structure the Cu atoms are
surrounded by three Br atoms and one 2,3-dimethylpyrazine ligand within a distorted tetrahedron.
The tetrahedra are connected via common Br edges into CuBr double chains, which are connected
by the ligands into layers located in the ab plane. The formation of compounds 1 and 3 was also
investigated in solution. The results have shown that compound 1 can only be prepared if an ex-
cess of the ligand is used. If CuBr and the ligand are reacted in a ratio of 1 : 1, in the beginning the
literature known ligand-deficient 3 : 2 intermediate catena[tribromo-µ2-bis(µ2-2,3-dimethylpyrazine-
N,Nꢀ)-tricopper(I)] (2) is obtained, which transforms within minutes into compound 3. If a crystalline
suspension of compound 1 is stirred in acetonitrile a transformation into the most stable compound
3 is also observed. The luminescence properties of compounds 1 and 3 were investigated. The com-
plexes show differences mainly in the emission spectra.
Key words: Coordination Polymers, CuBr(2,3-dimethylpyrazine), Crystal Structures,
Thermal Properties, Luminescence Properties
Introduction
In 2001 we reported on the crystal structure
of the ligand-rich 1 : 1 compound poly[µ2-bromo-
µ2-2,3-dimethylpyrazine-N,Nꢀ-copper(I)] (1) [22]. In
In the last years we were interested in the syn-
thesis, crystal structures and properties of coordina- this structure (CuBr)2 dimers are connected by the
tion polymers based on transition metal (I) halides 2,3-dimethylpyrazine ligands into layers (Fig. 1:
or pseudo halides and N donor ligands, with special
left). Four years later Wells et al. reported on a
attention to CuX compounds (X = Cl, Br, I). Sev- second compound in this system, which was ob-
eral of these compounds are known and were struc- tained by accident as a by-product in the synthe-
turally characterised [1 – 24]. For a certain copper(I) sis of dibromo-2,3-dimethylpyrazine-copper(II) [23].
halide or pseudo halide and a specific nitrogen donor
In the 3 : 2 compound catena[tribromo-µ2-bis(µ2-2,3-
ligand in most cases several compounds of different dimethylpyrazine-N,Nꢀ)-tricopper(I)] (2) (CuBr)3 six-
stoichiometry were reported. We have found that on membered rings are connected by the 2,3-dimethyl-
heating, most of the ligand-rich compounds transform
pyrazine ligands into double chains (Fig. 1: right).
into ligand-deficient intermediate compounds, which For the corresponding CuCl2,3-dimethylpyrazine sys-
can be isolated in quantitative yield [11 – 21]. There- tem we have identified one ligand-rich 4 : 9 compound,
fore, thermal decomposition of suitable CuX precur- two ligand-deficient polymorphic 1 : 1 compounds as
sor compounds is an alternative route for the discov-
ery and preparation of new CuX coordination poly- Therefore, we assumed that for the CuBr(2,3-dimeth-
well as ligand-deficient 3 : 2 and 2 : 1 compounds [24].
mers.
ylpyrazine) system at least one additional ligand-
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I. Jeß et al. · Synthesis, Crystal Structure and Properties of CuBr(2,3-dimethylpyrazine) Coordination Polymers
Fig. 1. Crystal structure of
the literature known 1 : 1 com-
pound 1 with view along the
b axis (left) and of the literature
known 3 : 2 compound 2 with
view along the a axis (right).
deficient intermediate is still missing and investi-
gated the ligand-rich 1 : 1 compound poly[µ2-bromo-
µ2-2,3-dimethylpyrazine-N,Nꢀ-copper(I)] by thermal
measurements. In these investigations we discovered
one additional ligand-deficient compound of compo-
sition (CuBr)2(2,3-dimethylpyrazine) (2), which can
also be prepared in solution. Compounds 1 and 2 show
strong luminescence and therefore, these properties
were also investigated.
Results and Discussion
Thermal properties
The ligand-rich 1 : 1 compound poly[µ2-bromo-µ2-
2,3-dimethylpyrazine-N,Nꢀ-copper(I)] (1) was investi-
gated using differential thermoanalysis and thermo-
gravimetry coupled to mass spectroscopy (DTA-TG-
◦
MS). Up to 330 C two mass steps were observed,
both accompanied by two endothermic events in the
DTA curve (Fig. 2). The final product of this reaction
was identified as CuBr by X-ray powder diffraction.
The DTG curve shows that both events are well re-
solved, and from the MS trend scan curve it is obvi-
ous that in both steps only the 2,3-dimethylpyrazine
ligand is emitted (Fig. 2). The experimental mass loss
of 21.3 % is in good agreement with that calculated for
the removal of half of the 2,3-dimethylpyrazineligands
(21.5%). Therefore, it can be assumed that during ther-
Fig. 2. DTA, TG, DTG and MS trend scan curves for
poly[µ2-bromo-µ2-2,3-dimethylpyrazine-N,Nꢀ-dicopper(I)]
(3) (Tp = peak temperature).
of composition (CuBr)2(2,3-dimethylpyrazine)(3) was
formed.
To prove this assumption a second TG experi-
mal decomposition a new ligand-deficient compound ment was performed and the residue obtained after
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I. Jeß et al. · Synthesis, Crystal Structure and Properties of CuBr(2,3-dimethylpyrazine) Coordination Polymers
503
˚
Table 1. Selected bond lengths (A) and angles (deg) for poly-
[di-µ3-bromo-µ2-2,3-dimethylpyrazine-N,Nꢀ-dicopper(I)]
(3). (Symmetry transformations used to generate equivalent
atoms: A: x − 1, y, z; B: −x + 1/2, y − 1/2, −z + 3/2;
C: x+1, y, z).
Cu(1)–N(1)
Cu(1)–Br(2)
Cu(1)–Br(1)
Cu(1)–Br(2A)
N(1)–Cu(1)–Br(2)
N(1)–Cu(1)–Br(1)
2.030(4) Cu(2)–N(2B)
2.4774(9) Cu(2)–Br(1C)
2.5580(8) Cu(2)–Br(1)
2.5726(8) Cu(2)–Br(2)
113.36(12) N(2B)–Cu(2)–Br(1C) 112.09(11)
117.37(10) N(2B)–Cu(2)–Br(1) 102.80(11)
2.037(4)
2.5175(8)
2.5205(9)
2.5413(8)
Br(2)–Cu(1)–Br(1) 113.28(3) Br(1C)–Cu(2)–Br(1) 107.15(3)
N(1)–Cu(1)–Br(2A) 107.79(11) N(2B)–Cu(2)–Br(2) 123.80(10)
Br(2)–Cu(1)–Br(2A) 106.77(3) Br(1C)–Cu(2)–Br(2) 97.93(3)
Br(1)–Cu(1)–Br(2A) 96.10(3) Br(1)–Cu(2)–Br(2) 112.39(3)
N(1)–Cu(1)–Cu(2) 140.21(12) N(2B)–Cu(2)–Cu(1) 135.55(11)
Br(2)–Cu(1)–Cu(2) 57.29(2) Br(1C)–Cu(2)–Cu(1) 111.84(3)
Br(1)–Cu(1)–Cu(2) 56.00(2) Br(1)–Cu(2)–Cu(1) 57.29(2)
Br(2A)–Cu(1)–Cu(2) 111.90(3) Br(2)–Cu(2)–Cu(1) 55.11(2)
Fig. 3. Experimental X-ray powder pattern of the residue ob-
tained after the first TG step in the thermal decomposition
reaction of the ligand-rich 1 : 1 compound 1 (mid) and cal-
culated pattern for 1 (top) and the ligand-deficient 2 : 1 com-
pound 3 (bottom).
Fig. 4. Part of the crystal structure of poly[di-µ3-bromo-µ2-
2,3-dimethylpyrazine-N,Nꢀ-dicopper(I)] 3 with labelling and
displacement ellipsoids drawn at the 50 % propability level.
the first step was investigated by elemental analysis
(see Experimental Section) and X-ray powder diffrac-
tion. The experimental pattern of the residue is com-
pletely different from that calculated for the ligand-
rich 1 : 1 compound 1 and the results of the elemen-
tal analysis is in good agreement with that calcu-
lated for a ligand-deficient compound of composition
(CuBr)2(2,3-dimethylpyrazine) (Fig. 3). On the basis
of these results we tried to prepare this compound in
Fig. 5. Crystal structure of poly[di-µ3-bromo-µ2-2,3-dimeth-
ylpyrazine-N,Nꢀ-dicopper(I)] (3) with view in the direction
of the c axis.
Crystal structure of poly[di-µ3-bromo-µ2-2,3-
dimethylpyrazine-N,Nꢀ-dicopper(I)] (3)
The ligand-deficient 2 : 1 compound 3 crystallises
solution and succeed in the preparation of single crys- in the monoclinic space group P21/n with Z = 4 and
tals of the new compound 3. If afterwards the experi- all atoms in general positions. In the crystal struc-
mental pattern of the residue obtained in the DTA-TG- ture the copper atoms are surrounded by three bromine
MS investigations is compared with that of compound atoms and one 2,3-dimethylpyrazine ligand in a dis-
3 calculated from single crystal data, it is proven that 3 torted tetrahedral arrangement (Fig. 4 and Table 1).
is an intermediate in the thermal decomposition reac- The Cu–N and Cu–Br distances for both crystallo-
tion of compound 1 (Fig. 3).
graphically independent copper atoms are similar and
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I. Jeß et al. · Synthesis, Crystal Structure and Properties of CuBr(2,3-dimethylpyrazine) Coordination Polymers
Table 2. Results of the crystallisation experiments in different
solvents as a function of the molar ratio between copper(I)
bromide and 2,3-dimethylpyrazine.
CuBr:Ligand 4:1 2:1 3:2 1:1 2:3
1 : 2 1:3
1:4 1:5 1:6
(1)
Water
(3)
(1)/(3)
Acetonitrile
Methanol
(3)
(1)/(3)
(1)
(3)
(1)
comparable to those in other CuBr coordination poly-
mers retrieved from the literature.
The CuBr3N tetrahedra are connected via common
Br edges into CuBr double chains extending in the
direction of the a axis (Fig. 5). These double chains
are connected by the ligands into layers located in
the ab plane. The topology of the coordination net-
work of compound 3 is identical with that in the cor-
responding CuCl compound poly[di-µ3-chloro-µ2-2,3-
dimethylpyrazine-N,Nꢀ-dicopper(I)], but the two com-
pounds are not isotypic [23]. However, this arrange-
ment is frequently found in 2 : 1 CuX coordination
polymers (X = Cl, Br, I).
Investigations on the formation of compounds 1–3 in
solution
The formation of these compounds was also inves-
tigated in solution. In these experiments copper(I) bro-
mide and 2,3-dimethylpyrazine were mixed in differ-
ent stoichiometric ratios in constant amounts of water,
methanol and acetonitrile (Table 2). These crystalline
suspensions were stirred for a week so that the thermo-
dynamically most stable compounds were formed. Af-
terwards the products were identified by X-ray powder
diffraction. These investigations have clearly shown
that the ligand-rich compound 1 can only be prepared
for example in acetonitrile if at least a 3-fold excess of
the 2,3-dimethylpyrazineligand is used, because other-
wise the new ligand-deficient 2 : 1 compound 3 is ob-
tained (Table 2). These findings are further supported
by experiments where a suspension of the ligand-rich
compound 1 is stirred in acetonitrile. After a few days a
complete transformation into compound 3 is observed.
These results also show that the equilibrium strongly
depends on the solvent used in the synthesis (Table 2).
In addition, it is proven that the literature known 3 : 2
compound2 is metastable and cannot be prepared if the
preparation is performed under thermodynamic con-
trol.
Fig. 6. Luminescence emission and excitation spectra
of poly[µ2-bromo-µ2-2,3-dimethylpyrazine-N,Nꢀ-copper(I)]
(1) (top) and poly[di-µ3-bromo-µ2-2,3-dimethylpyrazine-
N,Nꢀ-dicopper(I)] (3) (bottom) in solid state at 298 K. The
spectra are uncorrected and normalised.
precipitate formed was identified as a function of time
using ex situ X-ray powder diffraction. These inves-
tigations have clearly shown, that at the beginning of
this reaction compound 2 is formed, which transforms
within about 1 h into the most stable compound 3.
However, compound 2 cannot be prepared in pure form
in solution, because it is always contaminated with un-
reacted CuBr. Additional heating rate dependent TG
measurements on compound 1 have also shown that
the metastable compound 2 cannot be observed as an
intermediate.
Luminescence measurements
To investigate whether the ligand-deficient 3 : 2 in-
termediate 2 can also be prepared in solution, equimo- sion in the solid state after irradiation by light in the
lar amounts of CuBr and the ligand were mixed and the region between 300 and 550 nm (Fig. 6). Similar lumi-
Compound 1 and 3 show strong luminescent emis-
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I. Jeß et al. · Synthesis, Crystal Structure and Properties of CuBr(2,3-dimethylpyrazine) Coordination Polymers
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Table 3. Luminescence properties of compounds 1 and 3 in Table 4. Selected crystal data and results of the structure
the solid state at 298 K.
refinements for poly[di-µ3-bromo-µ2-2,3-dimethylpyrazine-
N,Nꢀ-dicopper(1)] (3).
Compound
Excitation
maximum (nm)
469
Emission
maximum (nm)
591
Luminescence
lifetime (µs)
25.3
Formula
C6H8Br2Cu2N2
395,04
yellow
1
3
MW [g mol−1
Crystal colour
]
469
577
24.6
Crystal size [mm−1
Crystal system
Space group
]
0.3×0.1×0.1
monoclinic
P21/n
4.0537(2)
17.936(2)
12.911(1)
92.95(1)
937.5(1)
293
nescent properties of Cu(I) halide compounds were de-
scribed by different authors and emission processes are
usually explained as a result of metal centred transi-
tions or halide to copper charge transfer [25, 26]. The
emission maximum of compound 3 is shifted (∆ =
14 nm) to higher wavelength in comparison with the
emission maximum of compound 1 (Table 3). This
observation confirms structural changes observed also
by other techniques. Luminescence lifetime curves for
each complex at r. t. are only single exponential. Dif-
ferences in the lifetimes of compounds 1 and 3 seem to
be insignificant (Table 3).
˚
a [A]
˚
b [A]
˚
c [A]
β [deg]
3
˚
V [A ]
Temperature [K]
Z
4
Dcalc. [g cm−3
]
2.799
2θ-Range [deg]
5 – 56
hkl ranges
−5/5, −23/23, −16/16
µ(MoKα ) [mm−1
Absorption corr.
Transmission min./max.
Refl. measured
Rint.
]
13.001
numerical
0.302/0.364
8397
0.0459
Conclusion
Refl. independent
Refl. with I ≥ 2σ(I)
Parameters
R1 [I ≥ 2σ(I)]
wR2 (all data)
Goof
2225
1769
112
0.0320
0.0786
1.024
0.53/−0.79
In the present work the thermal properties of
a ligand-rich CuBr(2,3-dimethylpyrazine) compound
were investigated, and the formation of a hitherto un-
kown ligand-deficient intermediate has been demon-
strated. These findings clearly prove that thermal de-
composition reactions of suitable ligand-rich precur-
sor compounds offer an alternative route to new co-
ordination polymers, which might be overlooked if the
synthesis is performed in solution. Our investigations
of solutions have also shown that different species are
present and that the composition of the compounds
obtained from solution depends on the nature of the
solvent and the stability of these compounds. There-
fore, a specific coordination polymer cannot be pre-
pared by simple mixing of the reactands in a ratio given
by the formula of the expected composition. To iso-
late specific compounds from solution, a large num-
ber of different experiments must be performed. In
favourable cases, the synthesis can be accomplished
much more easily by thermal decomposition reac-
tions. Starting from the most ligand-rich compound, all
ligand-deficent intermediates may be observed in one
measurement. However, this is sometimes difficult to
achieve if some of the compounds are metastable like
in the present case, but we have shown previously that
very often metastable compounds can also be prepared
by thermal decomposition reactions if heating rate de-
pendent TG measurements are used [13]. Investigation
−3
˚
Residual electron density [e A
]
changes in the emission spectra in which the maximum
of compound 3 is slightly shifted to higher wavelength
compared to compound 1. Small differences are also
found for the luminescence lifetimes. Systematic stud-
ies on this topic will be the subject of further investi-
gations.
Experimental Section
Synthesis of poly[µ2-bromo-µ2-2,3-dimethylpyrazine-N,Nꢀ-
copper(I)] (1)
71.5 mg (0.5 mmol) of CuBr and 162.2 µL (1.5) of 2,3-
dimethylpyrazine were stirred in a glass container at r. t. in
2 mL of acetonitrile for 1 d. The orange precipitate was fil-
tered off and washed with diethyl ether. Yield: 86.5 % based
on CuBr. The homogeneity of the product was confirmed by
comparison of the experimental and calculated X-ray powder
patterns. Elemental analysis: calcd. C 28.5, H 3.2, N 11.0;
found C 28.6, H 3.2, N 11.1.
Synthesis of poly[di-µ3-bromo-µ2-2,3-dimethylpyrazine-
N,Nꢀ-dicopper(I)] (3)
Single crystals of compound 3 were prepared by hy-
of the luminescence properties have shown only small drothermal reaction of 143.4 mg (1 mmol) of CuBr with
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I. Jeß et al. · Synthesis, Crystal Structure and Properties of CuBr(2,3-dimethylpyrazine) Coordination Polymers
Table 5. Atomic coordinates [×104] isotropic displacement
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
2
3
˚
parameters [A ×10 ] for poly[di-µ3-bromo-µ2-2,3-dimeth-
ylpyrazine-N,Nꢀ-dicopper(I)] (3). (All atoms in position 4e;
equivalent isotropic U calculated as a third of the trace of the
orthogonalized Uij tensors).
X-Ray powder diffraction
X-Ray powder diffraction experiments were performed
using a STOE STADI P transmission powder diffractome-
ter equipped with a position sensitive detector using CuKα
x
y
z
Ueq
Cu(1)
Cu(2)
Br(1)
Br(2)
N(1)
C(1)
C(2)
N(2)
C(3)
C(4)
C(5)
C(6)
1238(2)
5239(2)
395(1)
4446(1)
3239(1)
3651(1)
4025(1)
5570(2)
5954(2)
6735(2)
7109(2)
6716(3)
5958(3)
5523(3)
7173(3)
7060(1)
7662(1)
8671(1)
6050(1)
7226(3)
8031(3)
8096(3)
7359(3)
6552(3)
6483(3)
8860(4)
8987(3)
40(1)
39(1)
30(1)
29(1)
27(1)
24(1)
22(1)
27(1)
32(1)
31(1)
34(1)
33(1)
˚
radiation (λ = 1.540598 A).
5932(1)
1022(10)
2406(11)
2015(11)
245(10)
−1083(13)
−713(12)
4276(13)
3513(13)
Differential thermal analysis, thermogravimetry and mass
spectrometry
DTA-TG-MS measurements were performed using the
STA-409CD thermobalance with Skimmer coupling from
Netzsch, equipped with a quadrupole mass spectrometer
from Balzers. The MS measurements were performed in ana-
log and trend scan mode, in Al2O3 crucibles under helium
atmosphere (purity: 4.6) using heating rates of 4 ◦C/min. The
instrument was calibrated using standard reference materials.
54 µL (0.5 mmol) of 2,3-dimethylpyrazine in 3 mL of ace-
tonitrile. The reactands were fil◦led in a glas ampoule which
was sealed and heated to 150 C for 4 d. After cooling to
r. t. light orange single crystals had formed which were suit-
able for single crystal X-ray analysis. The homogeneity of
Elemental analysis
CHN analysis was performed using an EURO EA Ele-
the product was confirmed by comparison of the experimen- mental Analyser, from EURO VECTOR Instruments. Ele-
tal and calculated X-ray powder patterns.
mental analysis of the residue obtained after the first step in
the thermal decomposition reaction of compound 1: calcd.
for the 2 : 1 compound 3: C 18.2, H 2.0, N 7.1; found C 18.3,
H 2.0, N 7.0.
Single crystal structure analysis
Data of 3 were measured using an Imaging Plate Diffrac-
tion System from STOE & CIE. Structure solutions were
performed with Direct Methods using SHELXS-97 [27].
Structure refinement was carried out against F2 using
SHELXL-97 [27]. A numerical absorption correction was
performed using X-RED and X-SHAPE [28]. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. The C–H hydrogen atoms were positioned with
idealised geometry and were refined with isotropic displace-
ment parameters using the riding model. Selected crystal data
and results of the structure refinement are shown in Table 4,
Luminescence measurements
All luminescence measurements including lifetime decay
curves, emission and excitation spectra were performed on a
luminescence spectrometer Aminco-Bowman Series 2 (Ther-
moSpectronic, USA). The instrument is equipped with both
a continuous Xe lamp (150 W) and a Xe flash lamp (7 W).
Acknowledgements
We gratefully acknowledge the financial support by the
atomic coordinates and equivalent isotropic displacement pa- State of Schleswig-Holstein and the Deutsche Forschungsge-
rameters are given in Table 5.
CCDC 628758 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
meinschaft (Projekt No.: NA 720/1-1). We are very thankful
to Professor Dr. Wolfgang Bensch for access to his experi-
mental equipment.
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Unauthenticated
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