Synthesis, crystal structures, and thermal properties of new [ZnX 2(2,5-dimethylpyrazine)] (X = Cl, Br, I) coordination compounds

Article abstract of DOI:10.1002/ejic.200800646

Eight new halidozinc(II) coordination compounds with 2,5-dimethylpyrazine as a ligand were prepared. The reaction of ZnX₂ (X = Cl, Br, I) and 2,5-dimethylpyrazine in methanol results in the formation of catena-(μ₂-2,5-dimethylpyrazin

Full text of DOI:10.1002/ejic.200800646

FULL PAPER
DOI: 10.1002/ejic.200800646
Synthesis, Crystal Structures, and Thermal Properties of New [ZnX2(2,5-
dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds
Mario Wriedt,[a] Inke Jeß,[a] and Christian Näther*[a]
Keywords: Coordination compounds / N ligands / Synthesis design / Structure elucidation / Thermal reactivity
Eight new halidozinc(II) coordination compounds with 2,5-
dimethylpyrazine as a ligand were prepared. The reaction of
ZnX2 (X = Cl, Br, I) and 2,5-dimethylpyrazine in methanol
results in the formation of catena-(µ2-2,5-dimethylpyrazine-
N,NЈ)dichloridozinc(II) (1), catena-(µ2-2,5-dimethylpyrazine-
N,NЈ)dibromidozinc(II) (3I and 3II), catena-(µ2-2,5-dimeth-
ylpyrazine-N,NЈ)diiodidozinc(II) (6), and diiodidobis(2,5-di-
methylpyrazine-N)zinc(II) (7). In addition, in a solvent-free
reaction of ZnX2 (X = Cl, Br) and 2,5-dimethylpyrazine,
the ligand-rich compounds bis(dichlorido)bis(2,5-dimeth-
(X = Cl, Br) unit is coordinated by two halide atoms and one
bridging and one terminal 2,5-dimethylpyrazine ligand. The
structures of the ligand-rich 1:2 diiodido compound 7 and the
1:3 dibromido compound 5 consist of discrete monomeric tet-
rahedral building blocks. In the thermal decomposition reac-
tions of the ligand-rich compounds 2, 5, and 7, different mass
steps are observed. In the first step, the ligand-deficient 1:1
compounds 1 and 6 are formed in quantitative yields from 2
and 7, respectively, and 5 yields a mixture of 3I and 3II, and
the second mass step leads to the formation of new ligand-
deficient 2:1 compounds. On further heating of the com-
pounds, the remaining ligands are discharged. This results
in the formation of the zinc(II) halides, which vaporize on
further heating. Solvent-mediated conversion experiments in
methanol show that, irrespective of the ratio of zinc(II) halide
and ligand, only the 1:1 compounds 1 and 3I are formed and
therefore represent the thermodynamically most stable com-
pounds at room temperature. For ZnI2 the 1:2 compound 7 is
obtained at ratios of zinc(II) iodide to ligand larger than 1:3,
whereas at lower ratios the 1:1 compound 6 is always ob-
tained.
ylpyrazine-N)(µ2-2,5-dimethylpyrazine-N,NЈ)dizinc(II)
(2),
bis(dibromido)bis(2,5-dimethylpyrazine-N)(µ2-2,5-dimethyl-
pyrazine-N,NЈ)dizinc(II) (4), and dibromidobis(2,5-dimeth-
ylpyrazine-N)zinc(II) 2,5-dimethylpyrazine solvate (5) were
obtained. In the crystal structures of all compounds the zinc
atoms are each coordinated by two halide atoms and two 2,5-
dimethylpyrazine ligands within a distorted tetrahedron. The
zinc atoms in the ligand-deficient 1:1 (1:1 = molar ratio of
metal salt to ligand) compounds 1 and 6 and in the two poly-
morphic modifications 3I and 3II are linked by the N-donor
ligands into chains. In contrast, the crystal structure of the
2:3 compounds 2 and 4 are built up of oligomeric discrete
[(ZnX2)2(2,3-dimethylpyrazine)3] units in which each ZnX2
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
can be widely used as a convenient preparative tool for the
facile synthesis of novel coordination compounds.[2] To
check the generality of such reactions for the preparation
of new coordination polymers of other metals we started
systematic investigations on zinc(II) halide compounds and
N-donor ligands.[3,4] In addition to the tetrahedral coordi-
nation shown by copper(I) and zinc(II), they can also adopt
octahedral geometry and hence a rich and diverse structural
chemistry of zinc(II) compounds can be expected. Several
of such compounds are known, but there has been no re-
search on their thermal properties.[5–8] In earlier work we
investigated compounds with pyrazine as ligands.[4] In the
crystal structure of the ligand-rich 1:2 compounds of com-
position [ZnX2(pyrazine)2] (X = Cl,[4] Br[6]), the zinc atoms
are each coordinated by two halide atoms and four pyrazine
ligands within a distorted octahedron. The zinc atoms are
connected by the pyrazine ligands into chains, which are
further joined by ligands into layers. We found that, on
heating, this 1:2 compounds transform quantitatively into
Introduction
Recently, the preparation of new coordination polymers
and metal–organic frameworks has gained increasing inter-
est.[1] In nearly all cases, these compounds were prepared in
solution, which frequently leads to mixtures that have to be
separated by hand. Moreover, in some cases, thermodynam-
ically metastable compounds can easily be overlooked.
Therefore, an alternative route for the discovery and prepa-
ration of new or known pure coordination polymers is es-
sential.
Recently, we showed that thermal decomposition reac-
tions of suitable ligand-rich precursor compounds of cop-
per(I) halides or pseudohalides containing N-donor ligands
[a] Institut für Anorganische Chemie, Universität zu Kiel,
Max-Eyth-Straße 2, 24098 Kiel, Germany
Fax: +49-431-8801520
E-mail: cnaether@ac.uni-kiel.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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M. Wriedt, I. Jeß, C. Näther
FULL PAPER
the ligand-deficient 1:1 compounds. For ZnI2, no ligand-
rich 1:2 compound could be prepared. In the crystal struc-
ture of the 1:1 compounds of composition [ZnX2(pyrazine)]
(X = Cl,[6] Br,[5] I[7]), the zinc atoms are each coordinated
by two halide atoms and two pyrazine ligands within a dis-
torted tetrahedron, in which the ZnX2 units are connected
by the pyrazine ligands into chains. On further heating, the
1:1 compounds transform into the more ligand-deficient 2:1
compounds, which could not be prepared in solution.
In a continuation of this work, we investigated the reac-
tions of zinc(II) halides with 2,5-dimethylpyrazine aimed at
the characterization of new coordination compounds. This
ligand was selected to study the influence of the bulky
methyl substituents attached to the C atom adjacent to the
N-donor atoms, on the structures and thermal reactivity of
the products. Moreover, the N-donor atoms in 2,5-dimeth-
ylpyrazine should be better donors than those in pyrazine
because of the inductive effect of the methyl group, which
can also influence the reactivity. Herein, we report on the
results of these investigations.
Results and Discussion
Crystal Structures
Figure 1. Crystal structure of the 1:1 compound 1 with a view of
the coordination sphere of the zinc cations with labeling and dis-
placement ellipsoids drawn at the 50% probability level (top) and
a view of the structure in the direction of the crystallographic a
axis (bottom). Symmetry codes: A = –x + 2, –y + 1, –z + 1; B =
x, –y + 1/2, z.
The 1:1 dichlorido compound 1 crystallizes in the ortho-
rhombic space group Pnma with four formula units in the
unit cell. The zinc atoms as well as the two crystallographi-
cally independent chlorine atoms are located on a mirror
plane, whereas the 2,5-dimethylpyrazine ligand is situated
on a center of inversion. In the crystal structure, the zinc
Table 1. Selected bond lengths (in Å) and angles (in °) for com-
atoms are each coordinated by two crystallographically in- pound 1 und 2.[a]
dependent chlorine atoms and two symmetry-related 2,5-
Compound 1
dimethylpyrazine ligands within slightly distorted tetra-
Zn1–N1
hedra (Figure 1, top). The Zn–N distances amount to
N1–Zn1–N1B
2.1027(15) Zn1–Cl1
2.2129(6)
2.2312(6)
103.99(4)
97.00(8)
117.83(4)
113.80(2)
Zn1–Cl2
N1–Zn1–Cl2
2.1027(15) Å, and the angles around the zinc atoms range
N1–Zn1–Cl1
between 97.00(8) and 117.83(4)° (Table 1). The ZnCl2 units Cl1–Zn1–Cl2
are connected by the 2,5-dimethylpyrazine ligands by µ-
N,NЈ coordination into chains that elongate in the direction
of the crystallographic b axis (Figure 1, bottom).
Compound 2
Zn1–N1
Zn1–N11
N1–Zn1–N11
N1–Zn1–Cl1
N11–Zn1–Cl1
2.085(4)
2.091(4)
113.05(16) N1–Zn1–Cl2
108.53(12) N11–Zn1–Cl2
105.86(12) Cl1–Zn1–Cl2
Zn1–Cl1
Zn1–Cl2
2.2148(14)
2.2358(13)
104.56(12)
105.64(12)
119.41(5)
The 2:3 dichlorido compound 2 crystallizes in the tri-
¯
clinic space group P1 with one formula unit in the unit cell
and all atoms in general positions. In the crystal structure,
discrete [(ZnCl2)2(2,5-dimethylpyrazine)3] units are found,
in which each of the two zinc atom is coordinated by two
chlorine atoms and two nitrogen atoms of one terminal and
one bridging 2,5-dimethylpyrazine ligand (Figure 2, top).
[a] Symmetry codes: A = –x + 2, –y + 1, –z + 1; B = x, –y + 1/2,
z for 1.
The polymorphic 1:1 dibromido compound 3I crys-
These units are located on centers of inversion. The zinc tallizes in orthorhombic space group Pnma with four for-
atoms are bridged by the 2,5-dimethylpyrazine ligands mula units in the unit cell and is isomorphous to the
through µ-N,N coordination. The Zn–N distances range be- dichlorido compound 1 (Figure 3). The Zn–N distances
tween 2.085(4) and 2.091(4) Å, and the angles around the amount to 2.110(4) Å, and the angles around the zinc
zinc atoms vary from 104.56(12) to 108.53(12)° (Table 1). In atoms range between 97.5(2) and 118.20(11)° (Table 2).
the crystal structure, the discrete complexes are connected
The second modification of the 1:1 dibromido com-
through intermolecular C–H···Cl hydrogen bonds into pound 3II crystallizes in the monoclinic space group C2/c
chains [Є(C2–H2···Cl2) = 169.753(17)°, d(H2···Cl2) = with four formula units in the unit cell. In the asymmetric
2.8391(4) Å]. The terminal 2,5-dimethylpyrazine rings are unit of 3II, the zinc atom is located on a twofold rotation
stacked in the direction of the crystallographic a axis, indi- axis, and the two bromine atoms are related by a twofold
cating π–π interactions (Figure 2, bottom).
rotation axis, whereas the 2,5-dimethylpyrazine ligand is sit-
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[ZnX2(2,5-dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds
Table 2. Selected bond lengths (in Å) and angles (in °) for com-
pounds 3I, 3II, 4, and 5.[a]
Compound 3I
Br1–Zn1
Br2–Zn1
N1B–Zn1–N1
N1–Zn1–Br1
2.3510(10) Zn1–N1
2.3711(10) N1–Zn1–Br2
2.110(4)
105.01(10)
111.05(4)
97.5(2)
Br1–Zn1–Br2
118.20(11)
Compound 3II
Zn1–N1
N1B–Zn1–N1
N1–Zn1–Br1
2.121(2)
93.57(13) N1–Zn1–Br1B
105.32(6) Br1–Zn1–Br1B
Zn1–Br1
2.3560(5)
119.86(7)
112.38(3)
Compound 4
Zn1–N1
Zn1–N11
N1–Zn1–N11
N1–Zn1–Br1
N11–Zn1–Br1
2.082(5)
2.088(5)
113.6(2)
107.88(15) N11–Zn1–Br2
106.79(15) Br1–Zn1–Br2
Zn1–Br1
Zn1–Br2
N1–Zn1–Br2
2.3534(10)
2.3801(10)
105.42(15)
103.98(15)
119.37(4)
Compound 5
Zn1–N2
Zn1–Br1Ј
N2–Zn1–N2A
N2–Zn1–Br1Ј
N2–Zn1–Br1AЈ
2.066(3)
2.323(4)
114.43(15) Br1–Zn1–Br1A
111.9(3) N2–Zn1–Br1
106.33(18) Br1Ј–Zn1–Br1AЈ
Zn1–Br1
N2–Zn1–Br1A
2.391(3)
104.26(11)
124.5(4)
104.95(19)
105.6(9)
Figure 2. Crystal structure of the 2:3 compound 2 with a view of
the coordination sphere of the zinc cations with labeling and dis-
placement ellipsoids drawn at the 50% probability level (top) and
a view of the structure in the direction of the crystallographic a
axis (bottom). Intermolecular C–H···Cl hydrogen bonding is shown
as dashed lines. Symmetry code: A = –x + 1, –y + 2,–z.
[a] Symmetry codes: A = –x, –y + 1, –z + 1; B = x, –y + 1/2, z for
3I; A = –x + 1, –y, –z + 1; B = –x + 1, y, –z + 1/2 for 3II; A =
–x, y, –z + 1/2; B = –x, –y + 2, –z for 5.
Figure 3. Crystal structure of the 1:1 compound 3I with a view
of the coordination sphere of the zinc cations with labeling and
displacement ellipsoids drawn at the 50% probability level. Sym-
metry codes: A = –x, –y + 1, –z + 1; B = x, –y + 1/2, z.
uated on a center of inversion. In the crystal structure, the
zinc atoms are each coordinated by two symmetry-related
bromine atoms and two symmetry-related 2,5-dimethlpyra-
zine ligands within slightly distorted tetrahedra (Figure 4,
top). The Zn–N distances amount to 2.121(2) Å, and the
angles around the zinc atoms range between 93.57(13) and
119.86(7)° (Table 2). The ZnCl2 units are connected by the
2,5-dimethylpyrazine ligands by µ-N,NЈ coordination into
chains that elongate in the direction of the crystallographic
b axis (Figure 4, bottom). Thus, both modifications 3I and
3II are topologically identical.
Figure 4. Crystal structure of the 1:1 compound 3II with a view
of the coordination sphere of the zinc cations with labeling and
displacement ellipsoids drawn at the 50% probability level (top)
and a view of the structure in the direction of the crystallographic
b axis (bottom). Symmetry codes: A = –x + 1, –y, –z + 1; B = –x
The 2:3 dibromido compound 4 crystallizes in the tri-
¯
clinic space group P1 with one formula unit in the unit cell
and is isomorphous to the dichlorido compound 2 (Fig- + 1, y, –z + 1/2.
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ure 5). The Zn–N distances range between 2.082(5) and H2···Br2) = 172.433(15)°, d(H2···Br2) = 2.9427(5) Å], and
2.085(5) Å, and the angles around the zinc atoms range be- the terminal 2,5-dimethylpyrazine rings are stacked in the
tween 106.79(15) and 119.37(4)° (Table 2). As in 2, the dis- direction of the crystallographic c axis (Figure 5).
crete complexes are connected through intermolecular C–
The 1:3 dibromido compound 5 crystallizes in the mono-
H···X (X = Br) hydrogen bonds into chains [Є(C2– clinic space group C2/c with four formula units in the unit
cell. The zinc atoms are located on a twofold rotation axis,
whereas the two bromine atoms are located on general posi-
tions and are related by a twofold rotation axis. The 2,5-
dimethylpyrazine ligand is situated on a center of inversion
and one further ligand occupies a general position. In the
crystal structure, discrete [(ZnBr2)(2,5-dimethylpyrazine)2]·
(2,5-dimethylpyrazine) units are found, in which the zinc
atoms are each coordinated by two symmetry-related bro-
mine atoms and two symmetry-related 2,5-dimethlpyrazine
ligands within slightly distorted tetrahedra (Figure 6, top,
and Table 2). The additional 2,5-dimethylpyrazine ligand is
Figure 5. Crystal structure of the 2:3 compound 4 with a view of
not connected with the zinc atom (Figure 6, bottom).
The 1:2 diiodido compound 6 crystallizes in the mono-
clinic space group C2/c with four formula units in the unit
cell and is isomorphous to the bromine compound 3II (Fig-
ure 7). The Zn–N distances amount to 2.132(2) Å, and the
angles around the zinc atoms range between 93.65(12) and
120.32(7)° (Table 3).
the coordination sphere of the zinc cations with labeling and dis-
placement ellipsoids drawn at the 50% probability. Symmetry code:
A = –x + 1, –y + 2, –z.
Figure 7. Crystal structure of compound 6 with a view of the coor-
dination sphere of the zinc cations with labeling and displacement
ellipsoids drawn at the 50% probability level. Symmetry codes: A
= –x + 1, –y, –z + 1; B = –x + 1, y, –z + 1/2.
Table 3. Selected bond lengths (in Å) and angles (in °) for com-
pounds 6 and 7.[a]
Compound 6
Zn1–N1
N1B–Zn1–N1
N1–Zn1–I1
2.132(2)
93.65(12)
105.91(6)
Zn1–I1
N1–Zn1–I1B
I1–Zn1–I1B
2.5568(4)
120.32(7)
110.60(2)
Compound 7
Zn1–N1
Zn1–N11
N1–Zn1–N11
N1–Zn1–I1
N11–Zn1–I1
2.092(3)
2.100(3)
101.81(12)
110.42(9)
103.69(9)
Zn1–I1
Zn1–I2
N1–Zn1–I2
N11–Zn1–I2
I1–Zn1–I2
2.5600(5)
2.5744(5)
105.85(9)
117.59(9)
116.517(19)
[a] Symmetry codes for 6: A = –x + 1, –y, –z + 1; B = –x + 1, y,
–z + 1/2.
Figure 6. Crystal structure of the 1:3 compound 5 with a view of
the coordination sphere of the zinc cations with labeling and dis-
placement ellipsoids drawn at the 50% probability level (top) and
a view of the structure along [110] (bottom). Symmetry codes: A
= –x, y, –z + 1/2; B = –x, –y + 2, –z.
The ligand-rich 1:2 diiodido compound 7 crystallizes in
the monoclinic space group P21/c with four formula units
in the unit cell and all atoms in general positions. In the
crystal structure, discrete complexes are found, in which the
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[ZnX2(2,5-dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds
zinc atoms are coordinated by two iodine atoms and two curve (Figure 9, left). The MS trend scan curve reveals that
nitrogen atoms of the 2,5-dimethylpyrazine ligands within only the N-donor ligand (m/z = 108) is released during
distorted tetrahedra (Figure 8, top). The Zn–N distances these mass steps. The mass loss of 18.1% in the first step is
vary from 2.092(3) to 2.100(3) Å, and the angles around in perfect agreement with that calculated for the removal of
the zinc atoms range between 101.81(12) and 116.517(19)° one third (1 mol; 18.1%) of the ligands. The mass loss of
(Table 3). In the crystal structure, the discrete complexes are 18.7% in the second step is in good agreement with that
connected by weak intermolecular C–H···I hydrogen bonds calculated for the removal of an additional one third (1 mol;
into a two-dimensional network (Figure 8, bottom, and 18.1%) of the ligands. On the basis of the observed mass
Table 4).
losses, it can be confirmed that in the first step a 1:1 com-
pound and in the second step a ligand-deficient 2:1 com-
pound are formed. Heating the complex further causes the
remaining ligands to be released, leading to the formation
of ZnCl2, which vaporizes on further heating (Figure 9,
left).
For the ligand-rich 1:3 dibromido compound 5, very sim-
ilar thermal behavior as for compound 2 is observed. The
mass loss of 36.8% in the first step is in rough agreement
with that calculated for the removal of two thirds (2 mol;
39.4%) of the ligands. Hence, it can be confirmed that in
this step a ligand-deficient 1:1 compound has been formed.
For the first TG step, two endothermic events in the DTA
curve are observed, which might correspond to the forma-
tion of a 2:3 intermediate compound (Figure 9, middle).
However, heating-rate-dependent measurements do not
show an additional TG step (see the Supporting Infor-
mation). The experimental mass loss of 10.0% observed in
the second TG step is in reasonable agreement with that
expected for the removal of one sixth (0.5 mol; 9.8%) of
the ligands, which may lead to a new ligand-deficient 2:1
compound of composition [(ZnBr2)2(2,5-dimethylpyr-
azine)]. On further heating, the remaining ligands are dis-
charged, and the final product was identified as ZnBr2,
which vaporizes on further heating (Figure 9, middle).
The TG curve of the ligand-rich 1:2 compound 7 exhibits
three well-resolved mass steps (Figure 9, right). The mass
loss of 20.2% in the first step is in perfect agreement with
that calculated for the removal of half of the ligands (1 mol;
20.2%), and the second TG step of 13.5% is in exact agree-
ment with that expected for the removal of one third of the
ligands (0.67 mol; 13.5%). Therefore, it can be inferred that
in the first TG step a 1:1 compound is formed, which de-
composes on further heating into a new ligand-deficient 3:1
compound of composition [(ZnI2)3(2,5-dimethylpyrazine)].
On further heating, the 3:1 compound transforms into
ZnI2.
Figure 8. Crystal structure of the 1:2 compound 7 with a view of
the coordination sphere of the zinc cations with labeling and dis-
placement ellipsoids drawn at the 50% probability level (top) and
a view of the structure in the direction of the crystallographic b
axis (bottom). Intermolecular C–H···I hydrogen bonding is shown
as dashed lines.
Table 4. Distances and angles of the intermolecular hydrogen-
bonding interactions of compound 7.
The DTA/TG/MS measurements indicate that the li-
gand-rich compounds 2, 5, and 7 decompose in the first
step into 1:1 compounds and in the second step into new
ligand-deficient 2:1 or 3:1 intermediate compounds. To ver-
ify the nature of the intermediates formed, additional TG
measurements with heating rates of 4 Kmin–1 were per-
formed and stopped after the first and second TG step,
respectively. Afterward, the residues were investigated by X-
ray powder diffraction (XRPD) and elemental analysis.
The powder pattern of the residue obtained after the first
Distance (H···I) /Å
Angle (C–H···I) /°
C14–H14···I1
C15–H15···I2
C16–H16···I2
3.3873(2)
3.8614(3)
3.5735(2)
139.687(4)
116.753(4)
153.832(5)
Thermoanalytic Investigations
On heating the ligand-rich 2:3 chlorine compound 2 to
500 °C, two well-resolved mass steps are observed in the TG step of the ligand-rich 2:3 chlorine compound 2 is in
thermogravimetric (TG) curve that are accompanied by en- perfect agreement with that calculated for the 1:1 com-
dothermic events in the differential thermal analysis (DTA) pound 1 from single-crystal data (compare A with B in Fig-
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M. Wriedt, I. Jeß, C. Näther
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Figure 9. DTA, TG, DTG, and MS trend scan curves for the ligand-rich compounds 2 (left), 5 (middle), and 7 (right). Heating rate =
4 Kmin–1; m/z = 108 (2,5-dimethylpyrazine); given are the mass changes (%) and the peak temperatures TP (in °C).
ure 10). Hence, this phase is produced in a very pure form
Comparison of the powder X-ray diffraction pattern of
in the thermal decomposition reaction. The powder pattern the residue obtained after the first TG step in the thermal
of the intermediate formed after the second TG step is dif- decomposition reaction of the ligand-rich 1:3 dibromido
ferent from that of the pristine material (compare C with A compound 5 with that calculated for 1:1 compounds 3I and
in Figure 10). A detailed analysis shows that there are still 3II from single-crystal data reveals that these phases were
some reflections of the 1:1 compound 1 and ZnCl2. All our formed as a mixture (compare A with B and C in Fig-
efforts to obtain this intermediate compound in phase-pure ure 11). The powder X-ray diffraction pattern of the residue
form by stopping the TG experiment at different tempera- after the second TG step indicates the formation of a new
tures after the second TG step were not fruitful. Conse- ligand-deficient 2:1 compound. However, there are still
quently elemental analysis of the intermediate is only in rea- some additional reflections of the 1:1 compounds and
sonable agreement with that calculated for a ligand-de- ZnBr2 (compare D with B and C in Figure 11), and there-
ficient 2:1 compound (see the Experimental Section).
fore the elemental analysis is only in reasonable agreement
Figure 10. Experimental XRPD patterns of the residue obtained
after the first (A) and second (C) TG step in the thermal decompo-
sition reaction of compound 2 and calculated XRPD pattern for
the 1:1 compound 1 (B).
Figure 11. Experimental XRPD patterns of the residue obtained
after the first (A) and second (D) TG step in the thermal decompo-
sition reaction of compound 5, and calculated XRPD patterns for
the 1:1 compounds 3I (B) and 3II (C).
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[ZnX2(2,5-dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds
with that calculated (see the Experimental Section). In ad- Table 5. Results of the crystallization experiments in methanol as
a function of the molar ratio between metal and ligand (2,5-di-
methylpyrazine).
dition, the X-ray powder diffraction patterns gave evidence
that the ligand-deficient dichlorido and dibromido interme-
1:6 1:4 1:3 1:2 2:3 1:1 3:2 2:1 4:1
diates are isomorphous (compare C in Figure 10 with D in
Figure 11).
X = Cl
X = Br
X = I
1 (1:1)
The powder X-ray diffraction pattern of the residue ob-
tained after the first TG step in the thermal decomposition
reaction of the ligand-rich 1:2 diiodido compound 7 is in
exact agreement with that calculated for the 1:1 compound
6 from single-crystal data (compare A with B in Figure 12).
Interestingly, the powder X-ray diffraction pattern of the
residue obtained after the second TG step is identical to
that formed in the first step (compare C with B in Fig-
ure 12). Therefore, we stopped the reaction at different tem-
peratures and all residues investigated by X-ray powder dif-
fraction and elemental analysis. In these investigations we
only isolated residues that consist of the 1:1 intermediate or
a compound that is amorphous to X-rays. The IR spectra
of all residues show clearly differences, and the results of
elemental analysis are in good agreement with that calcu-
lated for a 3:1 compound (see the Experimental Section).
Thus, we do not have any explanation for this unusual ob-
servation.
3I (1:1)
7 (1:2)
6 (1:1)
However, it should be noted that compound 7 cannot be
prepared simply by mixing the reactants stoichiometrically
as given by the formula. The reaction of ZnI2 and 2,5-di-
methylpyrazine in 1:2 molar ratio results in the formation
of the 1:1 compound 6, whereas in the case of ZnCl2 and
ZnBr2 only the 1:1 compounds are formed irrespective of
the stoichiometric ratios. Moreover, the 2:3 bromine com-
pound 2 and 1:3 bromine compound 5 can only be prepared
in a solvent-free reaction, whereas compound 4 cannot be
prepared phase-pure either in solution or in a solvent-free
reaction.
To verify which of the two polymorphic modifications 3I
and 3II represents the thermodynamically most stable form
at room temperature, equivalent amounts of both forms
were stirred in methanol solution. After three days, only
modification 3I was present, showing that 3I is the stable
modification, whereas 3II is metastable (Figure 13).
Figure 12. Experimental XRPD patterns of the residue obtained
after the first (A) and second (C) step in the thermal decomposition
reaction of compound 7, and calculated XRPD pattern for the 1:1
compounds 6 (B).
Figure 13. Experimental XRPD patterns of a mixture of the poly-
morphic 1:1 bromine compounds 3I and 3II before (A) and after
(B) stirring for three days in methanol solution, together with cal-
culated XRPD pattern for the 1:1 bromine compound 3I (C). The
arrows indicate the position of the reflections of compound 3II.
Synthetic Aspects
The formation and stability of these compounds was in-
vestigated in solution by solvent-mediated conversion ex-
periments. In these experiments, an excess of metal salt and
N-donor ligand were mixed in different stoichiometric ra-
tios in methanol, resulting in a precipitate, which was
Additional differential scanning calorimetry (DSC) in-
stirred for three days to obtain the thermodynamically most vestigations in combination with X-ray powder diffraction
stable form. Afterwards, the products were investigated by do not show any transition between the two modifications,
X-ray powder diffraction (Table 5).
and the heat flow cannot be precisely extracted from the
These experiments clearly show that only compound 1, DSC curve. Therefore, it cannot be decided if the two forms
3I, 6, and 7 are stable and can also be prepared in solution. behave enantiotropically or monotropically.
Eur. J. Inorg. Chem. 2009, 363–372
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
369

M. Wriedt, I. Jeß, C. Näther
FULL PAPER
Comparison of [ZnX2(pyrazine)] and [ZnX2(2,5-
dimethylpyrazine)] (X = Cl, Br, I)
prepared in solution, like in the present work, such investi-
gations can help to discover new compounds, such as we
have recently shown. Therefore, for the further discovery of
new coordination compounds and their polymorphs, also
typical solid-state reactions such as the thermal decomposi-
tion presented above should be used. In future, we expect
more insight into the chemical reactivity and mechanisms
of the thermal reactions by comparing the thermal behavior
of coordination polymers based on other aromatic N-donor
ligands.
As mentioned above, for the corresponding coordination
compounds with pyrazine as a ligand, a variety of ligand-
rich and ligand-deficient compounds were found.[4–7] In the
ligand-rich 1:2 compounds with ZnCl2 and ZnBr2, each
zinc atom is coordinated by two halide atoms and four pyr-
azine ligands within slightly distorted octahedra. In the
crystal structures, the zinc atoms are connected by the li-
gands into layers. In contrast, in all 2,5-dimethylpyrazine
compounds the metal atoms are only tetrahedrally coordi-
nated by two halide atoms and two N-donor ligands. For
the ligand-rich compounds, this difference leads to oligo-
mers in compounds 2 and 4 and to discrete building blocks
in compound 5 and 7. The presence of the more sterically
demanding methyl groups attached to the carbon atoms ad-
jacent to the N atoms is responsible for this deviating be-
havior. Interestingly, a ligand-rich 1:2 compound with zin-
c(II) iodine was only obtained with 2,5-dimethylpyrazine as
ligand, and there is no simple explanation for this unusual
behavior.
In contrast, in practically all crystal structures of the li-
gand-deficient 1:1 compounds, topologically identical coor-
dination networks are found, in which the ZnX2 units are
connected by the pyrazine or 2,5-dimethylpyrazine ligands
into chains, with the exception of the crystal structure of
the 1:1 dichlorido pyrazine compound,[6] in which the metal
centers are octahedrally coordinated. The zinc(II) atoms are
connected by the chlorine atoms sharing common edges
into chains, which are further linked by the N-donor li-
gands into layers.
Experimental Section
Synthesis: Compounds 1, 3I, 3II, 6, and 7 were prepared by stirring
the reactants in methanol for 3 days at room temperature. The resi-
dues were filtered off and washed with diethyl ether. Compounds
2, 4, and 5 were prepared in a solvent-free reaction by simply add-
ing the liquid ligand to ZnX2. The reaction mixtures were kept
without stirring for a week at room temperature. The powders ob-
tained contained a large number of single crystals and were used
without further purification. Compound 4 could not be prepared
phase-pure as described above. The purity of these compounds was
checked by X-ray powder diffraction and elemental analysis.
Synthesis of Compound 1: The crystalline powder was prepared by
the reaction of ZnCl2 (272.6 mg, 2.0 mmol) and 2,5-dimethylpyr-
azine (216.3 mg, 2.0 mmol) in methanol (1.0 mL). Yield: 411.5 mg
(84.2%). C6H8Cl2N2Zn (244.41): calcd. C 29.48, H 3.30, N 11.46;
found C 29.41, H 3.29, N 10.86. IR (KBr): ν = 3127 (w), 3040 (m),
˜
2929 (w), 1827 (w), 1624 (w), 1508 (s), 1451 (m), 1381 (w), 1346
(s), 1273 (w), 1165 (m), 1078 (s), 1037 (m), 992 (w), 903 (w), 754
(w), 527 (w), 446 (m) cm–1. Single crystals were prepared by the
reaction of ZnCl2 (68.1 mg, 0.5 mmol), 2,5-dimethylpyrazine
(108.1 mg, 1.0 mmol), and methanol (2 mL) in a closed test tube
at 100 °C for 1 day. Colorless crystals were obtained on cooling.
The thermal reactivity of all ligand-rich compounds with
pyrazine or 2,5-dimethylpyrazine is similar. On heating,
they transform in two steps into ligand-deficient 2:1 com-
pounds via the 1:1 compounds as intermediates. Therefore,
these results show clearly that there is no simple relation-
ship between their crystal structure and their thermal be-
havior.
Synthesis of Compound 2: The crystalline powder was prepared by
the solvent-free reaction of ZnCl2 (136.3 mg, 1.0 mmol) and 2,5-
dimethylpyrazine (648.9 mg, 6.0 mmol). C18H24Cl4N6Zn2 (596.97):
calcd. C 36.21, H 4.05, N 14.08; found C 36.50, H 4.00, N 14.13.
IR (KBr): ν = 3105 (w), 3042 (m), 2954 (w), 2918 (w), 2859 (w),
˜
1982 (w), 1830 (w), 1635 (w), 1497 (s), 1445 (s), 1382 (m), 1335 (s),
1268 (w), 1233 (w), 1158 (s), 1077 (s), 1060 (s), 1039 (m), 979 (m),
894 (w), 751 (w), 453 (s), 425 (s) cm–1. Single crystals were prepared
by the solvent-free reaction of ZnCl2 (68.2 mg, 0.5 mmol) and 2,5-
dimethylpyrazine (108.1 mg, 1.0 mmol). Colorless crystals were ob-
tained within about a week.
Conclusion
We have presented eight new coordination compounds
including one pair of polymorphic modifications prepared
by the reaction of zinc(II) halides and 2,5-dimethylpyrazine,
some of which show new [ZnX2] substructures. Comparison
of their crystal structures with those of the corresponding
Synthesis of Compound 3I and 3II: The crystalline powder of com-
pound 3I was prepared by the reaction of ZnBr2 (450.4 mg,
2.0 mmol) and 2,5-dimethylpyrazine (216.3 mg, 2.0 mmol) in meth-
anol (1.0 mL). Yield: 542.7 mg (81.4%). C6H8Br2N2Zn (333.33):
pyrazine compounds reported recently show differences calcd. C 21.62, H 2.42, N 8.40; found C 21.59, H 2.32, N 8.48. IR
(KBr): ν = 3124 (w), 3035 (m), 2926 (w), 1816 (w), 1615 (w), 1506
˜
that originate clearly from the additional methyl groups at-
tached to the neighboring C atoms of the N donor atoms
in 2,5-dimethylpyrazine. Steric and inductive effects of the
methyl group clearly influence the structural diversity. None
of these compounds are isotypic to the corresponding pyr-
azine coordination polymers. In addition, our thermal in-
vestigations show that thermal decomposition of the li-
gand-rich precursor compounds is an adequate alternative
for the preparation of new coordination polymers. Even in
(s), 1445 (m), 1389 (m), 1343 (s), 1272 (m), 1164 (s), 1078 (s), 1035
(m), 987 (m), 897 (w), 753 (w), 525 (w), 425 (m) cm–1. The crystal-
line powder of compound 3II was obtained by fast evaporation
without stirring of the solvent and initial injection of single crystals
of compound 3II. C6H8Br2N2Zn (333.33): calcd. C 21.62, H 2.42,
N 8.40; found C 21.35, H 2.20, N 8.59. IR (KBr): ν = 3126 (w),
˜
3039 (m), 1818 (w), 1616 (w), 1500 (s), 1438 (m), 1384 (m), 1339
(s), 1270 (m), 1163 (s), 1080 (s), 1033 (m), 973 (w), 893 (w), 750
(w), 528 (w), 432 (s) cm–1. Single crystals were prepared by the
such cases where all ligand-deficient 1:1 compounds can be reaction of ZnBr2 (112.6 mg, 0.5 mmol) and 2,5-dimethylpyrazine
370 Eur. J. Inorg. Chem. 2009, 363–372
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[ZnX2(2,5-dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds
(54.1 mg, 0.5 mmol) in methanol (2.0 mL). After slow evaporation
of the solvent at room temperature, colorless crystals of both poly-
morphic modifications were obtained within about a week.
ν = 2911 (w), 1804 (w), 1628 (w), 1501 (s), 1436 (m), 1384 (m),
˜
1333 (s), 1268 (w), 1239 (w), 1162 (m), 1062 (s), 1035 (m), 971 (w),
883 (w), 749 (w), 672 (w), 429 (s) cm–1. Single crystals were pre-
pared by the solvent-free reaction of ZnI2 (75.8 mg, 0.25 mmol)
and 2,5-dimethylpyrazine (108.1 mg, 1.0 mmol). Colorless crystals
were obtained within about a week.
Synthesis of Compound 4: Single crystals were prepared by the sol-
vent-free reaction of ZnBr2 (28.1 mg, 0.125 mmol) and 2,5-dimeth-
ylpyrazine (108.1 mg, 1.0 mmol). Colorless crystals were obtained
within about a week.
Elemental Analysis of the Residues Obtained in the Thermal Decom-
position: (A) Isolated after first heating step for 2:3 chlorine com-
pound 2. For 1:1 compound 1: calcd. C 29.48, H 3.30, N 11.46;
found C 29.74, H 3.19, N 11.63. (B) Isolated after second heating
step for 2:3 chlorine compound 2. For 2:1 compound: calcd. C
18.93, H 2.12, N 7.36; found C 18.43, H 2.25, N 7.26. (C) Isolated
after first heating step for 1:3 bromine compound 5. For 1:1 com-
pound: calcd. C 21.62, H 2.42, N 8.40; found C 21.81, H 2.24, N
8.23. (D) Isolated after second heating step for 1:3 bromine com-
pound 5. For 2:1 compound: calcd. C 12.90, H 1.44, N 5.02; found
C 12.33, H 1.53, N 5.59. (E) Isolated after first heating step for 1:2
iodine compound 7. For 1:1 compound: calcd. C 16.86, H 1.89, N
6.56; found C 16.95, H 1.86, N 6.70. (F) Isolated after second heat-
ing step for 1:2 iodine compound 7. For 3:1 compound: calcd. C
6.76, H 0.76, N 2.63; found C 6.88, H 0.72, N 2.72.
Synthesis of Compound 5: The crystalline powder and single crys-
tals were prepared by the solvent-free reaction of ZnBr2 (450.4 mg,
2.0 mmol) and 2,5-dimethylpyrazine (1297.7 mg, 12.0 mmol). Col-
orless crystals were obtained within about a week. C18H24Br2N6Zn
(549.62): calcd. C 39.34, H 4.40, N 15.29; found C 39.35, H 4.38,
N 15.25. IR (KBr): ν = 3039 (w), 2921 (w), 2363 (w), 1819 (w),
˜
1635 (w), 1499 (s), 1446 (m), 1383 (m), 1337 (s), 1270 (m), 1163 (s),
1081 (s), 1039 (m), 972 (w), 891 (w), 751 (w), 669 (w), 508 (w), 430
(m) cm–1.
Synthesis of Compound 6: The crystalline powder was prepared by
the reaction of ZnI2 (1276.7 mg, 4.0 mmol) and 2,5-dimethylpyr-
azine (432.6 mg, 4.0 mmol) in methanol (1.0 mL). Yield: 542.7 mg
(81.4%) on the basis of ZnI2. C6H8I2N2Zn (427.31): calcd. C 16.86,
H 1.89, N 6.56; found C 16.82, H 1.83, N 6.54. IR (KBr): ν = 3118
˜
Single-Crystal Structure Analysis: All investigations were per-
formed with an imaging plate diffraction system (IPDS-1) with
Mo-Kα-radiation from STOE & CIE. The structure solutions were
performed with direct methods by using SHELXS-97,[9] and struc-
ture refinements were performed against F2 by using SHELXL-
97.[10] For all compounds, numerical absorption correction was ap-
plied with X-Red[11] and X-Shape[12] (1, 3I, 3II, 5, 6, and 7) or
Platon[13] (2 and 4). All non-hydrogen atoms were refined with an-
isotropic displacement parameters. All hydrogen atoms were posi-
tioned with idealized geometry (methyl groups were allowed to ro-
tate but not to tip) and were refined with fixed isotropic displace-
ment parameters [Ueq(H) = –1.5·Ueq(C)] for methyl H atoms and
(w), 3031 (m), 1813 (w), 1497 (s), 1438 (m), 1378 (w), 1337 (m),
1268 (m), 1159 (m), 1077 (s), 1030 (m), 891 (w), 750 (w), 526 (w),
433 (m) cm–1. Single crystals were prepared by the reaction of ZnI2
(159.6 mg, 0.5 mmol) and 2,5-dimethylpyrazine (54.1 mg,
0.5 mmol) in methanol (2.0 mL). After slow evaporation of the sol-
vent, colorless crystals were obtained within about a week.
Synthesis of Compound 7: The crystalline powder was prepared by
the reaction of ZnI2 (1276.7 mg, 4.0 mmol) and 2,5-dimethylpyr-
azine (2595.4 mg, 24.0 mmol) in methanol (1.0 mL). Yield:
542.7 mg (81.4%) based on ZnI2. C12H16I2N4Zn (535.46): calcd. C
26.92, H 3.01, N 10.46; found C 27.04, H 3.04, N 10.90. IR (KBr):
Table 6. Selected crystal data and details of the structure determinations.
Compound
1
2
3I
3II
4
5
6
7
Formula
Ratio ZnX2/L
MW/gmol–1
Crystal system
Space group
C6H8Cl2N2Zn C18H24Cl4N6Zn2 C6H8Br2N2Zn C6H8Br2N2Zn C18H24Br4N6Zn2 C18H24Br2N6Zn C6H8I2N2Zn
C12H16I2N4Zn
1:2
535.46
monoclinic
P21/c
1:1
2:3
1:1
1:1
2:3
1:3
1:1
244.41
orthorhombic triclinic
Pnma
596.97
333.33
333.33
774.81
triclinic
P1
549.62
monoclinic
C2/c
427.31
monoclinic
C2/c
orthorhombic monoclinic
Pnma
¯
P1
¯
C2/c
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å3
6.0621(4)
10.2156(8)
13.9518(8)
7.2149(10)
9.1905(11)
10.1410(13)
79.973(15)°
70.138(15)°
70.264(15)°
593.94(13)
170
6.2698(6)
10.3167(7)
14.3842(9)
16.1453(18)
6.4913(6)
9.8755(10)
7.2840(8)
9.4775(16)
10.1762(11)
81.004(13)
72.489(12)
71.306(12)
633.21(14)
170
8.5956(9)
11.4237(7)
23.1531(16)
16.6286(13)
6.8351(4)
10.0199(8)
12.2908(8)
8.2723(7)
17.2054(13)
115.096(11)
100.174(10)
113.947(9)
101.266(8)
864.01(10)
170
930.42(12)
170
937.29(17)
170
2237.7(3)
170
1040.81(13)
170
1715.6(2)
170
T/K
Z
4
1
4
4
1
4
4
4
Dcalcd./gcm–3
µ /mm–1
1.879
3.393
0.521–0.869
28
1.669
2.487
0.753–0.831
28
2.380
11.166
0.361–0.722
28
2.362
11.084
0.249–0.521
28
2.032
8.222
0.403–0.621
28
1.631
4.683
0.571–0.751
28
2.727
8.237
0.411–0.785
28
2.073
5.024
0.582–0.735
28
Transmission range
θmax/°
Measured reflections
Unique reflections
Reflections [Fo Ͼ 4σ(Fo)]
Parameters
Rint
9309
1092
1026
57
0.0350
0.0241
0.0648
1.072
5213
2740
2050
140
0.0529
0.0496
0.1369
1.074
7814
1164
985
57
0.1052
0.0431
0.0524
1.063
1.006/–1.795
4836
1134
982
53
0.0378
0.0271
0.0707
0.999
0.840/–0.755
5880
2991
2473
140
0.0548
0.0525
0.1526
1.078
7691
2368
1649
137
0.0622
0.0425
0.0992
0.995
0.483/–0.699
5219
1240
1109
53
0.0300
0.0208
0.0524
1.051
12810
3671
3022
173
0.0480
0.0300
0.0773
0.979
R1[a] [Fo Ͼ 4σ(Fo)]
wR2[b] [all data]
GOF
Residual electron density /e Å–3 0.544/–0.562
0.890/–1.195
1.706/–1.335
0.654/–0.864
0.745/–0.846
2
2 2
[a] R1 = ∑||Fo| – |Fc||/∑|Fo|. [b] wR2 = [∑w(Fo – Fc ) /∑w(Fo2)2]1/2
.
Eur. J. Inorg. Chem. 2009, 363–372
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
371

M. Wriedt, I. Jeß, C. Näther
FULL PAPER
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[Ueq(H) = –1.2·Ueq(C)] for aromatic H atoms using a riding model
with dC–H = 0.95 Å for methyl and dC–H = 0.98 Å for aromatic H
atoms. The bromine atom in compound 5 is disordered over two
positions and was refined by using a split model (s.o.f. = 0.5). De-
tails of the structure determination are given in Table 6 and in the
Supporting Information.
CCDC-705375 (1), -705376 (2), -705377 (3I), -705378 (3II), -705379
(4), -705380 (5), -705381 (6), and -705382 (7) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.uk/data_request.cif.
X-ray Powder Diffraction (XRPD): XRPD experiments were per-
formed with a STOE DF4 transmission powder diffractometer with
Cu-Kα radiation (λ = 154.0598 pm) that is equipped with a posi-
tion-sensitive detector (scan range: 5–45°) from STOE & CIE.
Differential Thermal Analysis, Thermogravimetry, and Mass Spec-
troscopy (DTA/TG/MS): The heating-rate-dependent DTA/TG
measurements were performed in a nitrogen atmosphere (purity:
5.0) in Al2O3 crucibles with a STA-409CD instrument from
Netzsch. The DTA/TG/MS measurements were performed with the
same instrument, which was connected to a quadrupole mass spec-
trometer from Balzers by Skimmer coupling from Netzsch. The MS
measurements were performed in analog and trend-scan mode in
Al2O3 crucibles in a dynamic helium atmosphere (purity: 5.0) at
heating rates of 4 Kmin–1. All measurements were performed with
a flow rate of 75 mLmin–1 and were corrected for buoyancy and
current effects. The instrument was calibrated with standard refer-
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Differential Scanning Calorimetry (DSC): DSC investigations were
performed with a DSC 204/1/F device from Netzsch. The measure-
ments were performed in aluminum pans at heating rates of
3 Kmin–1. The instrument was calibrated with standard reference
materials.
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and 7 as well as DSC measurements of the polymorphic modifica-
tions 3I and 3II.
Acknowledgments
We gratefully acknowledge financial support by the State of Schles-
wig-Holstein and the Deutsche Forschungsgemeinschaft (Project
NA 720/1-1). We thank Professor Dr. Wolfgang Bensch for the use
of his experimental facility.
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Received: June 26, 2008
Published Online: December 15, 2008
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Eur. J. Inorg. Chem. 2009, 363–372