Synthesis and structural characterization of four zinc complexes containing 3,5-dimethylpyrazole and carboxylate ligands

Article abstract of DOI:10.1002/zaac.201000380

Four new zinc(II) complexes Zn₂(μ-dmpz)₂(Hdmpz) ₂(L1)₂ (1) (Hdmpz = 3,5-dimethylpyrazole, HL1 = 2-methyl-2-phenoxypropanoic acid), Zn(Hdmpz)₂(L2)₂ (2) [HL2 = 2-hydroxy-5-(phenyldiazenyl)benzoic acid], Zn₂(μ-dmpz) ₂(Hdmpz)₂(L3)₂ (3) [HL3 = 3,4-(methylenedioxy) benzoic acid], and Zn₂(μ-dmpz)₂(Hdmpz) ₂(L4)₂ (4) [HL4 = 3-(4-methoxyphenyl)acrylic acid] were prepared and structurally characterized by different techniques including elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction analysis. The X-ray studies suggested that all these complexes except compound 2 are centrosymmetric dinuclear complexes with a tetrahedral arrangement around each zinc ion, whereas compound 2 is a mononuclear complex. The pyrazole ligand is coordinated in both terminal as well as a bridging fashion in the dinuclear moiety, whereas the pyrazole ligand in compound 2 is coordinated only in monodentate terminal fashion with its neutral nitrogen group. In all four complexes the carboxylate functions behave as monodentate ligands. All complexes show intramolecular hydrogen bonding of N-H...O between N-H of pyrazole and nonbonded oxygen atom of carboxylate. Furthermore, rich intermolecular weak interactions such as classical hydrogen bonds, C-H...O, C-H...N, C-H...π, and CH₃-π interactions exist and complexes 1-4 display a set of 3D superamolecular frameworks. In addition, the four compounds are thermally stable below 150 °C.

Full text of DOI:10.1002/zaac.201000380

ARTICLE
DOI: 10.1002/zaac.201000380
Synthesis and Structural Characterization of Four Zinc Complexes
Containing 3,5-Dimethylpyrazole and Carboxylate Ligands
Shouwen Jin*[a] and Daqi Wang[b]
Keywords: Zinc(II); 3,5-Dimethylpyrazole; Carboxylate ligands; Crystal structure; Supramolecules
Abstract. Four new zinc(II) complexes Zn2(μ-dmpz)2(Hdmpz)2(L1)2 The pyrazole ligand is coordinated in both terminal as well as a bridg-
(1) (Hdmpz = 3,5-dimethylpyrazole, HL1 = 2-methyl-2-phenoxypro- ing fashion in the dinuclear moiety, whereas the pyrazole ligand in
panoic acid), Zn(Hdmpz)2(L2)2 (2) [HL2 = 2-hydroxy-5-(phenyldia- compound 2 is coordinated only in monodentate terminal fashion with
zenyl)benzoic acid], Zn2(μ-dmpz)2(Hdmpz)2(L3)2 (3) [HL3 = 3,4- its neutral nitrogen group. In all four complexes the carboxylate func-
(methylenedioxy)benzoic acid], and Zn2(μ-dmpz)2(Hdmpz)2(L4)2 (4) tions behave as monodentate ligands. All complexes show intramolec-
[HL4 = 3-(4-methoxyphenyl)acrylic acid] were prepared and structur- ular hydrogen bonding of N–H···O between N–H of pyrazole and non-
ally characterized by different techniques including elemental analysis, bonded oxygen atom of carboxylate. Furthermore, rich intermolecular
IR spectroscopy, and single-crystal X-ray diffraction analysis. The X- weak interactions such as classical hydrogen bonds, C–H···O, C–H···N,
ray studies suggested that all these complexes except compound 2 are C–H···π, and CH3–π interactions exist and complexes 1–4 display a set
centrosymmetric dinuclear complexes with a tetrahedral arrangement of 3D superamolecular frameworks. In addition, the four compounds
around each zinc ion, whereas compound 2 is a mononuclear complex. are thermally stable below 150 °C.
The Zn2+ ion with d10 electron configuration is particularly
1 Introduction
Supramolecular assemblies with desirable topological net-
suitable for the construction of coordination polymers. It ex-
hibits a variety of coordination numbers and arrangements
works of selforganized molecular building blocks have re-
varying from tetrahedral, trigonal bipyramidal, and square py-
ceived great attention in recent years.[1–3] This is due to funda-
ramidal to octahedral. Sometimes a severe distortion of the
mental interest in selfassembly processes,[4] supramolecular
ideal polyhedron bonding set is formed. Because of the lability
motifs,[5] and most significantly crystal engineering about their
of zinc complexes, the formation of coordination bonds is re-
intriguing structural topologies. However, the prediction of the
versible, which enables zinc ions and ligands to rearrange dur-
final supramolecular frameworks constructed by a set of li-
ing the assembly process to form highly ordered structures.
gands and metal ions to rationalize the design of compounds
Consequently, zinc can readily form all kinds of architectures
with well defined structures is still a challenge. The final struc-
such as 1D, 2D, and 3D structures.
tures depend on many factors such as the coordination arrange-
ment of the metal ions, the metal-ligand stoichiometric ratio,
The use of pyrazole or pyrazolate derivatives has drawn
strong interest in modeling biological systems.[12–14] Pyrazole
the nature of counterions, weak interactions (hydrogen bond-
has also been widely employed in polypyrazolylborates to sta-
ing, aromatic π–π stacking interactions as well as van der
bilize
a
variety of organometallic and coordination
Waals forces),[6–8] and the experimental conditions.
compounds.[15,16] Up to now, a variety of complexes containing
3,5-dimethylpyrazole (Hdmpz) ligands have been synthesized
and employed in coordination chemistry or organometallic
chemistry.[17–19] Additionally, many complexes with simple
pyrazole ligands both in terminal as well as in bridging mode
are available.[20–22] But in the presence of carboxylic acids and
pyrazole derivatives the complexes are not very common ex-
cept a few recently reported examples.[23,24] Our research
group has been investigating coordination compounds with
mixed ligands of carboxylate and imidazole derivatives for a
long time.[25–27] The pyrazole ligand and the carboxylate ligand
appear to have similar steric requirements and, to a certain
extent, similar bonding capabilities.
Furthermore, coordination polymers with functional proper-
ties, such as magnetism, catalysis, optical, and sorption
properties[9,10] make up rapidly growing field of research. Up
to now, a variety of metal ions or more complex metal-contain-
ing fragments have been employed, and organic ligands con-
taining O- or N-donors have been widely used. These two
markedly different chemical functionalities have also been
widely coupled in the same systems.[11]
* Dr. S. Jin
Fax: +86-571-6374-0010
E-Mail: jinsw@zafu.edu.cn
[a] Faculty of Science
ZheJiang A & F University
Lin'An 311300, P. R. China
[b] Department of Chemistry
Liaocheng University
In order to get a better understanding of the influence of
the carboxylate residue in the formation of new coordination
polymers, we studied the zinc-pyrazolate-carboxylate system,
Liaocheng 252059, P. R. China
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, 618–625

Zinc Complexes with 3,5-Dimethylpyrazole and Carboxylate Ligands
containing Hdmpz (19.2 mg, 0.2 mmol) and 2-hydroxy-5-(phenyldia-
zenyl)benzoic acid (HL2) (96.8 mg, 0.4 mmol), under continuous stir-
ring. The solution was stirred for about 2 h at room temperature and
the solution became turbid. Afterwards, a few drops of conc. ammonia
were added until the solution became clear. The red solution was fil-
tered into a test tube, after several days dark red crystals were formed,
which were filtered off, washed with MeOH and dried under vacuum
to afford 52 mg of the product. Yield 70.26 % (based on Hdmpz).
Elemental analysis performed on crystals exposed to the atmosphere:
C36H34N8O6Zn2: calcd. C 58.37; H 4.59; N 15.13 %; found: C 58.34;
also aiming to investigate the role the weak noncovalent inter-
actions play in forming the final supramolecular frameworks.
In the following we report the synthesis, structural characteri-
zation, and thermal behavior of four stereochemically “stiffer”
zinc complexes using 3,5-dimethylpyrazole (Hdmpz) and dif-
ferent carboxylate ligands (Scheme 1), namely Zn2(μ-
dmpz)2(Hdmpz)2(L1)2 (1) (L1 = 2-methyl-2-phenoxypropion-
ate), Zn(Hdmpz)2(L2)2 (2) [L2 = 2-hydroxy-5-(phenyldia-
zenyl)benzoate]; Zn2(μ-dmpz)2(Hdmpz)2(L3)2 (3) [L3 = 3,4-
(methylenedioxy)benzoate], and Zn2(μ-dmpz)2(Hdmpz)2(L4)2, H 4.56; N 15.10 %. IR (KBr): ν = 3732 w, 3441 w, 3402 w, 3294 w,
˜
3134 m, 3062 m, 3035 m, 2978 m, 2856 m, 2360 s, 2270 m, 1662 m,
1648 m, 1622 s, 1557 s, 1480 s, 1430 s, 1384 s, 1310 m, 1255 s, 1168
m, 1050 m, 1016 m, 907 m, 860 m, 780 m, 688 m, 606 m cm–1.
(4) [L4 = 3-(4-methoxyphenyl)acrylate].
Zn2(μ-dmpz)2(Hdmpz)2(L3)2 (3): A solution of Zn(CH3COO)2·2H2O
(22 mg, 0.1 mmol) in EtOH (5 mL) was added to an EtOH solution
(8 mL) containing Hdmpz (19.2 mg, 0.2 mmol) and 1,3-benzodioxole-
5-carboxylic acid (HL3) (66.4 mg, 0.4 mmol), under continuous stir-
ring. The solution was stirred for about 2 h at room temperature and
the solution became turbid. Afterwards, a few drops of conc. ammonia
were added until the solution became clear. The solution was filtered
into the test tube, after several days colorless crystals were formed,
which were filtered off, washed with EtOH and dried under vacuum
to afford 63 mg of the product. Yield 74.69 % (based on Hdmpz).
Elemental analysis performed on crystals exposed to the atmosphere:
C36H40N8O8Zn2: calcd. C 51.22; H 4.74; N 13.28 %; found: C 51.20;
Scheme 1. Ligands used in this article.
H 4.71; N 13.18 %. IR (KBr): ν = 3405 m, 3240 w, 2920 m, 2660 m,
˜
2540 s, 2520 m, 2320 m, 1725 m, 1638 s, 1580 m, 1540 m, 1460 m,
1442 s, 1408 m, 1380 s, 1300 s, 1260 s, 1140 m, 1096 m, 1020 s, 895
m, 852 m, 816 m, 774 m, 672 m, 620 m cm–1.
2 Experimental Section
2.1 Materials and Physical Measurements
Zn2(dmpz)2(Hdmpz)2(L4)2 (4): A solution of Zn(CH3COO)2·2H2O
(22 mg, 0.1 mmol) in MeOH (5 mL) was added to a MeOH solution
(3 mL) containing Hdmpz (19.2 mg, 0.2 mmol) and 3-(4-methoxy-
phenyl)acrylic acid (HL4) (71.2 mg, 0.4 mmol), under continuous stir-
ring. The solution was stirred for about 2 h at room temperature and
a small amount of precipitate was formed. Afterwards, a few drops of
conc. ammonia were added until the precipitate dissolved completely.
The clear solution was filtered into the test tube, after several days
colorless block crystals formed, which were filtered off, washed with
MeOH and dried under vacuum to afford 63 mg of the product. Yield
72.61 % (based on Hdmpz). Elemental analysis performed on crystals
exposed to the atmosphere: C40H48N8O6Zn2: calcd. C 55.33; H 5.53;
The chemicals and solvents used in this work are of analytical grade
and available commercially and were used without further purification.
The FT-IR spectra were recorded from KBr pellets in range 4000–
400 cm–1 with a Mattson Alpha-Centauri spectrometer. Microanalyti-
cal (C, H, N) data were obtained with a Perkin–Elmer Model 2400II
elemental analyzer. Thermogravimetric analyses (TGA) were studied
by a Delta Series TA-SDT Q600 in a nitrogen atmosphere between
room temperature and 800 °C (heating rate 10 °C·min–1) using Al cru-
cibles.
2.2 Synthesis of Complexes
N 12.91 %; found: C 55.32; H 5.51; N 12.89 %. IR (KBr): ν = 3570
˜
Zn2(μ-dmpz)2(Hdmpz)2(L1)2 (1): A solution of Zn(CH3COO)2·2H2O
(22 mg, 0.1 mmol) in MeOH (5 mL) was added to a MeOH solution
(5 mL) containing Hdmpz (19.2 mg, 0.2 mmol) and 2-methyl-2-phe-
noxypropanoic acid (HL1) (72 mg, 0.4 mmol), under continuous stir-
ring. The solution was stirred for about 2 h at room temperature and
a small amount of precipitate formed. Afterwards, a few drops of conc.
ammonia were added until the precipitate dissolved completely. The
clear solution was filtered into a test tube, after several days colorless
crystals were formed, which were filtered off, washed with MeOH and
dried under vacuum to afford 70 mg of the product. Yield 80.31 %
(based on Hdmpz). Elemental analysis performed on crystals exposed
to the atmosphere: C40H52N8O6Zn2: calcd. C 55.07; H 5.97; N
m, 3429 m, 3237 w, 2880 s, 2782 m, 2360 s, 2320 s, 1950 m, 1880
m, 1680 m, 1645 s, 1624 m, 1598 s, 1550 m, 1520 s, 1448 m, 1408
m, 1360 m, 1296 m, 1260 s, 1220 m, 1180 s, 1056 m, 1014 m, 940
m, 902 m, 828 m, 728 m, 662 m, 628 m cm–1.
2.3 X-ray Crystallography
Suitable crystals were mounted on a glass fiber and were investigated
with a Bruker SMART 1000 CCD diffractometer operating at 50 kV
and 40 mA using Mo-Kα radiation (0.71073 Å). Data collection and
reduction were performed using the SMART and SAINT software.[28]
The structures were solved by direct methods, and the non-hydrogen
atoms were subjected to anisotropic refinement by full-matrix least-
squares on F2 using SHELXTL package.[29] Hydrogen atom positions
for all structures were located in a difference map. Further details of
the structural analysis are summarized in Table 1. Selected bond
12.85 %; found: C 54.91; H 5.91; N 12.74 %. IR (KBr): ν = 3440 m,
˜
3236 w, 3120 s, 2960 s, 2920 s, 2830 m, 2776 m, 1640 s, 1620 s, 1560
s, 1480 s, 1450 m, 1400 s, 1360 m, 1270 m, 1230 m, 1150 s, 1030 m,
980 m, 890 m, 820 m, 780 m, 740 m, 710 m, 680 m, 610 m cm–1.
Zn(Hdmpz)2(L2)2 (2): A solution of Zn(CH3COO)2·2H2O (22 mg, lengths and angles for complexes 1–4 are listed in Table 2, and the
0.1 mmol) in MeOH (5 mL) was added to a MeOH solution (10 mL) relevant hydrogen bond parameters are provided in Table 3.
Z. Anorg. Allg. Chem. 2011, 618–625
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
619

S. Jin, D. Wang
ARTICLE
Table 1. Summary of X-ray crystallographic data for complexes 1, 2, 3, and 4.
1
2
3
4
Formula
C40H52N8O6Zn2
871.64
C36H34N8O6Zn
740.08
C36H40N8O8Zn2
843.50
C40H48N8O6Zn2
867.60
Fw
T /K
298(2)
298(2)
298(2)
298(2)
Wavelength /Å
0.71073
0.71073
triclinic
0.71073
0.71073
Crystal system
triclinic,
triclinic
triclinic
¯
¯
¯
¯
Space group
P1
P1
P1
P1
a /Å
8.7662(11)
9.5918(12)
14.1033(16)
104.370(2)
98.7190(10)
105.345(2)
1077.4(2)
1
11.0730(13)
12.5171(14)
14.7395(15)
106.7940(10)
109.197(2)
93.3510(10)
1820.2(3)
2
9.0247(11)
10.0554(13)
11.1835(17)
95.7440(10)
101.943(2)
102.757(2)
957.0(2)
1
8.3860(10)
8.6601(11)
14.8002(16)
82.9470(10)
89.967(2)
86.2250(10)
1064.4(2)
1
b /Å
c /Å
α /°
β /°
γ /°
V /Å3
Z
Dcalcd /Mg·m3
1.343
1.350
0.730
1.464
1.314
1.354
1.180
Absorption coefficient / 1.166
mm–1
F(000)
456
768
436
452
Crystal size /mm
θ range /deg
0.50 × 0.48 × 0.30
1.53–25.02
–10 ≤ h ≤ 9
–11 ≤ k ≤ 9
–14 ≤ l ≤ 16
5509
0.26 × 0.18 × 0.13
1.72–25.02
–13 ≤ h ≤ 13
–12 ≤ k ≤ 14
–17 ≤ l ≤ 17
9652
0.35 × 0.19 × 0.16
1.88–25.02
–9 ≤ h ≤ 10
–11 ≤ k ≤ 11
–13 ≤ l ≤ 13
5068
0.41 × 0.37 × 0.26
2.37–25.02
–9 ≤ h ≤ 9
–10 ≤ k ≤ 6
–17 ≤ l ≤ 17
5339
Limiting indices
Reflections collected
Reflections independent 3718 (0.0262)
(Rint)
6342 (0.0371)
3319 (0.0364)
3678 (0.0192)
Goodness-of-fit on F2 1.051
0.833
0.950
1.042
R indices [I > 2σI]
R indices (all data)
R1 = 0.0392, wR2 = 0.0900 R1 = 0.0487, wR2 = 0.0896 R1 = 0.0512, wR2 = 0.1173 R1 = 0.0417, wR2 = 0.0933
R1 = 0.0610, wR2 = 0.1033 R1 = 0.1243, wR2 = 0.1095 R1 = 0.0764, wR2 = 0.1285 R1 = 0.0700, wR2 = 0.1100
Largest diff. peak and 0.483, –0.352
0.375, –0.427
0.666, –0.419
0.529, –0.301
hole /e·Å–3
for νas(CO2) and at 1400–1430 cm–1 for νs(CO2).[30] The fre-
quency differences between νas(CO2) and νs(CO2) are 160, 192,
172, and 190 cm–1 for 1, 2, 3, and 4, respectively. This sug-
gests a monodentate coordination mode for the carboxylate li-
gands in all four compounds. Coordinated neutral Hmdpz mol-
ecules are present in all four compounds, which is further
confirmed by the presence of the characteristic NH band in the
region 3400–3000 cm–1.[31] The compositions of these com-
pounds were determined by elemental analysis, and their struc-
tures were fully characterized by X-ray diffraction analysis.
Hydrogen atoms connected to oxygen or nitrogen atoms were
located from the difference electron density map.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained on quoting the depository numbers CCDC-
746309 (1), -746312 (2), -746310 (3), and -746311 (4). (Fax: +44-
1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
3 Results and Discussion
3.1 Preparation and General Characterization
Most compounds based on pyrazole derivatives reported pre-
viously were prepared by conventional solution approach.
Thus, all of the title compounds have already been prepared
by the conventional solution method. The reaction between
Zn(CH3COO)2·2H2O, carboxylic acids, and Hdmpz carried out
in MeOH or EtOH solvents with ratios of 1:2:4 yields pure
Zn2(μ-dmpz)2(Hdmpz)2(L)2 or Zn(Hdmpz)2(L)2 upon addition lizes in triclinic system with space group P1. As illustrated in
of a few drops of conc. ammonium solution. During the proc- Figure 1, the asymmetric unit consists of one ZnII cation, one
ess the acetate was substituted by the corresponding carboxyl- pyrazole molecule, one pyrazolate unit, and one carboxylate
ate ions, and some Hdmpz ligands remain their NH groups, ligand. The pyrazolate unit acts as a N,N'-exobidentate ligand,
and some Hdmpz ligands are deprotonated. Crystal of all com- keeping the two zinc ions at a non-bonding distance of ca.
pounds, suitable for X-ray diffraction, were grown in yields of 3.567 Å. The complex molecule is dinuclear and consists of a
over 70 %. These compounds are insoluble in almost all com- ZnII atom positioned at the center of a planar six-membered
mon solvents. The IR spectra of 1, 2, 3, and 4 display the [Zn(μ-dmpz)2Zn] unit; each single unit has a four-coordinate
characteristic carboxylate bands in the range 1560–1622 cm–1 zinc ion with three Zn–N bonds (one Zn–Npyrazole, two
3.2 Crystal Structure Description
3.2.1 Crystal and Molecular Structure of 1
The structural determination revealed that complex 1 crystal-
¯
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 618–625

Zinc Complexes with 3,5-Dimethylpyrazole and Carboxylate Ligands
Table 2. Selected bond lengths /Å and angles /° for 1, 2, 3, and 4.
103.36(10)°–113.17(10)°. The geometrical parameters of this
species closely match those recently reported for the analogous
[{Zn(μ-C2H5COO)(μ-pz*)(Hpz*)}2][32] (Hpz* = 4-acetyl-3-
1
Zn(1)–O(1)
1.954(2)
1.999(2)
Zn(1)–N(1)
Zn(1)–N(3)
1.974(3)
2.031(3)
amino-5-methylpyrazole)
and
[{Zn(μ-C2H5COO)(μ-pz)-
Zn(1)–N(2)#1
O(1)–Zn(1)–N(1)
(Hpz)}2][24] complexes.
112.92(10) O(1)–Zn(1)–N(2)#1 104.96(10)
N(1)–Zn(1)–N(2)#1 113.17(10) O(1)–Zn(1)–N(3)
N(1)–Zn(1)–N(3)
111.21(10)
110.70(10) N(2)#1–Zn(1)–N(3) 103.36(10)
2
Zn(1)–O(4)
1.914(3)
1.986(3)
1.236(4)
1.447(5)
1.51(3)
Zn(1)–O(1)
Zn(1)–N(5)
N(1)–C(6)
1.921(3)
1.997(3)
1.431(5)
1.24(6)
Zn(1)–N(7)
N(1)–N(2)
N(2)–C(8)
N(3)–N(4)
N(3)–C(19)
N(4)–C(21)
N(3')–C(19)
O(4)–Zn(1)–O(1)
1.46(5)
N(3')–N(4')
1.25(18)
1.53(11)
1.49(11)
106.04(13)
112.22(13)
112.88(13)
N(4')–C(21)
O(4)–Zn(1)–N(7)
O(4)–Zn(1)–N(5)
N(7)–Zn(1)–N(5)
114.58(13) O(1)–Zn(1)–N(7)
102.75(12) O(1)–Zn(1)–N(5)
108.09(13)
3
Zn(1)–O(1)
Zn(1)–N(1)
1.924(3)
1.970(3)
Zn(1)–N(2)#1
Zn(1)–N(3)
1.969(3)
1.997(4)
O(1)–Zn(1)–N(2)#1 108.01(14) O(1)–Zn(1)–N(1)
N(2)#1–Zn(1)–N(1) 112.79(13) O(1)–Zn(1)–N(3)
N(2)#1–Zn(1)–N(3) 111.76(14) N(1)–Zn(1)–N(3)
107.49(14)
110.77(14)
105.97(15)
4
Figure 1. Molecular structure of complex 1 showing the atomic num-
Zn(1)–O(1)
Zn(1)–N(1)
1.923(3)
1.981(2)
Zn(1)–N(2)#1
Zn(1)–N(3)
1.972(2)
bering scheme at 30 % ellipsoid probability level.
2.004(3)
O(1)–Zn(1)–N(2)#1 106.87(11) O(1)–Zn(1)–N(1)
N(2)#1–Zn(1)–N(1) 113.36(10) O(1)–Zn(1)–N(3)
N(2)#1–Zn(1)–N(3) 109.73(11) N(1)–Zn(1)–N(3)
109.22(11)
114.13(12)
103.68(11)
In complex 1, the nonbonded oxygen atom of the carboxylate
forms an intramolecular hydrogen bonding of N(4)–
H(4)···O(2) with N(4)–O(2) and H(4)···O(2) distances of
2.751(3) and 1.97 Å, respectively. The adjacent dinuclear moi-
eties are connected through C–H···π interaction between the 4-
CH on pyrazole rings and the adjacent pyrazolato rings (with
the distance of the carbon atom to the pyrazolato ring of
3.612 Å) to form a one dimensional chain along the a axis.
Additionally, interchain C–H···π interactions (with the distance
of 4-C on pyrazolato rings to the pyrazole ring of 3.761 Å)
exist between the 4-CH on pyrazolato rings and the pyrazole
rings in parallel chains along the b axis. The combined hydro-
gen bonds cause that compound 1 displays a 3D network struc-
ture, as shown in Figure 2.
Symmetry codes for 1: #1 –x + 2, –y, –z. Symmetry code for 3: #1 –
x + 1, –y, –z + 1. Symmetry code for 4: #1 –x + 1, –y + 1, –z + 1.
Table 3. Hydrogen bond lengths and angles in studied structures of 1,
2, 3, and 4.
D–H···A
d(D–H) /Å d(H···A) /Å d(D···A) /Å <(DHA) /°
1
N(4)–H(4)···O(2) 0.86
2
1.97
2.751(3)
150.1
O(6)–H(6A)···O(5) 0.82
O(3)–H(3)···O(2) 0.82
N(8)–H(8)···O(5) 0.86
N(6)–H(6)···O(2) 0.86
3
1.88
1.85
1.99
1.98
2.596(3)
2.570(4)
2.790(4)
2.741(4)
146.0
146.6
153.7
146.2
N(4)–H(4)···O(2) 0.86
4
N(4)–H(4)···O(2) 0.86
1.89
1.93
2.695(5)
2.712(5)
156.0
150.1
Zn–Npyrazolato) and one Zn–Ocarboxylate bond. The bridging
Zn–N distance of 1.976 Å (average) is shorter than the termi-
nal Zn–N distance of 2.031(3) Å, which are similar to the re-
ported values.[24] The carboxylate groups coordinate in monod-
entate manner with a Zn(1)–O(1) distance of 1.954(2) Å. The
distance (3.268 Å) between the nonbonded oxygen atom and
zinc ions is too large for oxygen to coordinate with the metal
ion. The coordination arrangement around zinc in complex 1
Figure 2. Packing diagram of compound 1 showing the three dimen-
sional network structure viewed along the b axis. Dashed lines represent
is a distorted tetrahedron with coordination angles in the range the intramolecular N–H···O, and intermolecular CH–π interactions.
Z. Anorg. Allg. Chem. 2011, 618–625
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S. Jin, D. Wang
ARTICLE
1
3.2.2 Crystal and Molecular Structure of 2
to produce a S1 (6) loop motif, thus the non-bonded oxygen
atoms form two hydrogen bonds in bifurcate mode. In com-
pound 2 the ligand L2 [2-hydroxy-5-(phenyldiazenyl)benzo-
ate] adopts E conformation about the N=N bond. The two pairs
of C–N bond lengths related with the azo groups in the two
anions are in the range of 1.431(5)–1.53(11) Å [(N(1)–C(6),
1.431(5) Å; N(2)–C(8), 1.447(5) Å, (N(3)–C(19), 1.51(3) Å;
N(4)–C(21), 1.46(5) Å; N(3')–C(19), 1.49(11) Å; N(4')–C(21),
1.53(11) Å]. The differences in bond lengths of the pair of C–
N bonds within the two L2 ligands are 0.016(5), 0.05(3), and
0.04(11) Å, respectively For the pair of C–N bonds [N(1)–
C(6), 1.431(5) Å; N(2)–C(8), 1.447(5) Å], the N–C bond con-
cerning the azo group and the salicylate moieties [N(1)–C(6)]
is the shorter in the two C–N bonds. Whereas the correspond-
ing distances between N(3)–C(19) [1.51(3) Å] and N(4)–C(21)
[1.46(5) Å] do not agree with the same results [contrary to the
above results herein the N–C bond concerning the azo group
and the salicylate moieties N(3)–C(19) is the larger in the two
C–N bonds], which may be caused by the fact that N(4) forms
an additional intermolecular C–H···N hydrogen bond whereas
the N(1) atom is not involved in such interaction. These effects
are also reflected in the dihedral angle between the two rings
in the same ligand of L2.
Compound 2 of the formula Zn(Hdmpz)2(L2)2 crystallizes as
triclinic dark red crystals in the centrosymmetric space group
¯
P1 with two formula units in the unit cell. Each zinc ion is
tetrahedrally coordinated by two oxygen atoms of two monod-
entate L2 ligands and by two nitrogen atoms, of two different
monodentate pyrazole ligands (Figure 3). The molecular struc-
ture of 2 resembles the related [Zn(CH3COO)2(ML)2] mono-
mers (ML = a monodentate nitrogen ligand), like the bis-
imidazole[33] and bis-pyridine[34] complexes. The nitrogen at-
oms of the azo group in one L2 molecule are disordered over
two positions with equal occupancies.
The arene rings and the pyrazole rings are almost planar with
the largest deviation from mean plane of 0.009 Å. The dihedral
angles between two phenyl rings in the ligands L2 are 6.5
(containing N3 and N4) and 30.1° (containing N1 and N2),
respectively, which suggest that the ligand L2 containing N3
and N4 is essentially planar but neither is the ligand containing
N1 and N2, whereas the dihedral angles between two phenyl
rings in two distinct ligands L2 are 64.7, 63.2, 80.1, and 75.8°,
respectively. The two pyrazole ligands are almost perpendicu-
lar with each other with a dihedral angle of 88.8°. The mono-
nuclear units are connected through C–H···N interactions (with
a C–N distance of 3.386 Å, and a N–H distance of 2.498 Å)
between 4-CH of the pyrazole molecule and the diazenyl nitro-
gen atom to form 1D zig-zag chain structure along the a axis.
The Zn···Zn distance along the chain is 11.073 Å, whereas the
Zn···Zn distances between two adjacent chains are 15.249 and
14.740 Å, respectively. The adjacent chains were further con-
nected through C–H···O (between the benzene CH and the phe-
nol OH group with C–O distance of 3.502 Å), CH3–O (be-
tween the 5-CH3 on pyrazole ring and the OH group with C–
O distance of 3.376 Å), and C–H···π (with the separation be-
tween the carbon atom in one L2 and the benzene ring in an-
other L2 of 3.614 Å) interactions to form a 3D network struc-
ture, which is shown in Figure 4.
Figure 3. Molecular structure of complex 2 showing the atomic num-
bering scheme at 30 % ellipsoid probability level.
The difference between complexes 1 and 2 lies in the fact
that in 2 bridging pyrazolate ligands do not exist, complex 2
only bears neutral nitrogen ligands. The observance that the
hydrogen atoms H6 and H8 in 2 are bound to nitrogen and
not to oxygen atoms, is in agreement with the different acidic
characters of pyrazole and 2-hydroxy-5-(phenyldiazenyl)ben-
zoic acid,[23] which is also confirmed by the difference electron
density map in which the hydrogen atoms are found. The
ZnN2O2 moiety has coordination distances and angles in the
ranges of 1.914(3)–1.997(3) Å and 102.75(12)–114.58(13)°,
respectively; thus, the overall coordination arrangement resem-
bles that found in compounds [Zn(Hpz)2(Me3NCH2CO2)]-
[24]
(ClO4)2
and {[Zn(CH3COO)2(Hpz)2]·CH3COOH} (Hpz =
pyrazole).[23a] Although the distortion in the ZnN2O2 moiety is
differently oriented in the latter two compounds, the angles
around the zinc atoms are in the range of 98.94(19)–
117.30(18)° and 95.73(7)–118.34(8)°, respectively. Moreover,
complex 2 is not an ionic species, it consists of monocationic
zinc(II) complex and carboxylate counterions. In 2, the non-
bonded oxygen atoms, far away enough from the zinc ions
with distances 3.312 and 3.338 Å, respectively, are involved
3.2.3 Crystal and Molecular Structure of 3
¯
Compound 3 also crystallizes in the triclinic space group P1.
in two intramolecular hydrogen bonds [N(6)–H(6)···O(2), and It contains a dinuclear complex of the formula [Zn2(μ-
N(8)–H(8)···O(5)] with N–O distances in the range from dmpz)2(Hdmpz)2(L3)2], with the zinc atom located at a crystal-
2.741(4)–2.790(4) Å and H–O distances of 1.98–1.99 Å. In lographic inversion center (Figure 5). Each zinc ion is tetrahe-
addition to the intramolecular N–H···O hydrogen bonds, intra- drally coordinated by one oxygen atom of a monodentate L3
molecular O–H···O hydrogen bonds (Table 3) also exist be- ligand and by three nitrogen atoms, of one monodentate pyra-
tween the phenol OH group and the non-bonded oxygen atoms zole molecule and two different bridging pyrazolate ligands.
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Z. Anorg. Allg. Chem. 2011, 618–625

Zinc Complexes with 3,5-Dimethylpyrazole and Carboxylate Ligands
groups are roughly coplanar with the benzene ring of L3 in
a dihedral angle of 4.5°. The adjacent dinuclear moieties are
connected through CH3-π (the distance between 5-CH3 of the
pyrazole molecule and the benzene ring amounts to 3.731 Å),
C–H···π (including two kinds of C–H···π interactions one is
between the 4-CH of pyrazole and the benzene ring with a
C···Cg distance of 3.751 Å, the other is between the 4-CH of
pyrazole and the pyrazolate ring, in which the corresponding
distance is 3.792 Å) to form a 3D network structure when
viewed along the c axis (Figure 6). There also exist CH2–π
interactions between O–CH2–O and the pyrazolate ring with a
C···Cg distance of ca. 3.641 Å. Furthermore, weak CH3···CH3
interactions of van der Waals type are observed between two
methyl groups of two adjacent dinuclear moieties with a C···C
distance of 2.981 Å and a H···H distance of 2.299 Å. Obvi-
ously, these weak interactions contribute to the stabilization of
the 3D framework.
Figure 4. Packing diagram of compound 2 showing the three-dimen-
sional network structure viewed along the c axis. Dashed lines repre-
sent the intramolecular O–H···O, N–H···O, intermolecular CH–N, and
interchain C–H···O, CH3–O, CH–π interactions.
The latter act in the common N,N'-exobidentate mode, to keep
the two zinc ions at a non-bonding distance of ca. 3.544 Å.
The ZnN3O units has coordination distances and angles in the
ranges 1.924(3)–1.997(4) Å and 105.97(15)–112.79(13)°, re-
spectively; the geometrical parameters of this species are simi-
lar to those recently reported for the analogous [{Zn(μ-
C2H5COO)(μ-pz*)(Hpz*)}2][32] (Hpz* = 4-acetyl-3-amino-5-
methylpyrazole, Zn···Zn 3.62 Å) and [{Zn(μ-C2H5COO)(μ-
pz)(Hpz)}2][24] (Hpz = pyrazole, Zn···Zn 3.62 Å) complexes.
However, in our species, where the L3 ligands simply substi-
tute the propionate ligands present in [{Zn(μ-C2H5COO)(μ-
pz*)(Hpz*)}2] or [{Zn(μ-C2H5COO)(μ-pz)(Hpz)}2], the zinc
ions do not have any bonding interaction with the “second”
oxygen atom of each carboxylate, which is found at distance
of 3.364 Å from the zinc ion. In 3, the non-bonded oxygen
atoms, which are far away enough from the zinc ions, form an
intramolecular hydrogen bond [N4–H4···O2 with N4–O2 and
H4–O2 distances of 2.695(5) and 1.89 Å, respectively]. The
dihedral angle between the monodentate coordinated pyrazole
and the terminal pyrazolate ligands is 74.0°. The angle be-
tween the five-membered ring of 1,3-dioxole and the benzene
ring in the same L3 molecule amounts to 4.6°. The carboxylate
Figure 6. Packing diagram of compound 3 showing the three dimen-
sional network structure viewed from the c axis. Dashed lines represent
the intermolecular CH–π, CH2–π, CH3–π, and intramolecular N–H···O
interactions.
3.2.4 Crystal and Molecular Structure of 4
Similar to compounds 2 and 3, complex 4 of the formula
[Zn2(μ-dmpz)2(Hdmpz)2(L4)2] crystallizes in the triclinic
¯
space group P1 with one formula unit in the unit cell. Com-
pound 4 is also a dinuclear complex consisting of a planar
six-membered [Zn(μ-dmpz)2Zn] core and an inversion center
located in the center of the six-membered [Zn(μ-dmpz)2Zn]
ring (Figure 7). Each zinc ion is tetrahedrally coordinated by
two bridging pyrazolate ligands, one monodentate pyrazole
molecule, and one monodentate L4 ligand to complete a bond-
ing set of N3O. The bridging pyrazolate ligands coordinate
with the zinc ions in N,N'-exobidentate fashion, keeping the
two zinc ions at a non-bonding distance of ca. 3.543 Å.
Figure 5. Molecular structure of complex 3 showing the atomic num-
bering scheme at 30 % ellipsoid probability level.
Z. Anorg. Allg. Chem. 2011, 618–625
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623

S. Jin, D. Wang
ARTICLE
tween the 4-CH of pyrazole and the pyrazolate ring) and CH3–
π (between the 3-CH3 of one pyrazole and the other pyrazole
ring) interactions to form a 3D layer network structure, which
is shown in Figure 8.
Figure 8. Packing diagram of compound 4 showing the three dimen-
sional layer structure viewed along the c axis. Dashed lines represent
the intramolecular N–H···O hydrogen bonds and intermolecular C–
H···O, CH3–O, and interchain CH–π, CH3–π interactions.
Figure 7. Molecular structure of complex 4 showing the atomic num-
bering scheme at 30 % ellipsoid probability level.
The ZnN3O unit has coordination distances and angles in the
ranges of 1.923(3)–2.004(3) Å and 103.68(11)–114.13(12)°,
respectively; the geometrical parameters of this species are
similar to the above results presented in this paper. In this case,
the zinc ions do not interact with the non-bonded oxygen atom
of each carboxylate function, which is found at a distance of
3.272 Å from the metal. In 4, the non-bonded oxygen atoms,
far away from the zinc ions, are involved in an intramolecular
hydrogen bond of N4–H4···O2 with N4–O2 and H4–O2 distan-
ces of 2.712(5) Å and 1.93 Å respectively. The dihedral angle
between the monodentately coordinated pyrazole molecule and
the bridging pyrazolate unit is 108.5°. Ligand L4 adopts trans
conformation. The adjacent dinuclear moieties are connected
through C–H···O (between CH from benzene ring and the non-
3.3 Thermogravimetry (TG)
The thermal decomposition process of the complexes 1–4
can be divided into three stages according to the temperature
range, which is listed in Table 4. The first weight loss between
153.2 °C and 255.6 °C corresponds to the release of two neu-
tral pyrazoles. The monodentate carboxylate ligands are lost in
the second stage in the temperature range of 259.3–418.1 °C.
In the third stage the bridging pyrazolates were decomposed
with the temperature of 418.8–613.9 °C.
4 Summary
Four zinc complexes with 3,5-dimethylpyrazole and different
bonded oxygen atom of L4 with a C–O distance of 3.479 Å), carboxylate ligands were synthesized and characterized by X-
CH3–O (between the CH3 of the pyrazole and the methoxyl ray diffraction analysis, IR spectroscopy, and thermal analysis.
group of the ligand L4, in which the C–O distance is 3.390 Å) Crystallographic studies demonstrated that in these complexes,
to form a 1D chain structure viewed along the c axis. The the zinc ions are coordinated by 3,5-dimethylpyrazole. In addi-
adjacent chains are further connected through C–H···π (be- tion, each metal ion is coordinated by carboxylate ligands in a
Table 4. TGA data for complexes 1–4.
Stage Temperature range /°C
Weight loss /%
Probable composition of removed molecules
obsd.
calcd.
1
I
153.2–251.5
259.3–410.4
420.1–608.0
161.5–250.6
278.2–402.0
154.6–248.2
282.6–418.1
430.7–613.9
166.2–255.6
260.4–401.6
418.8–602.4
21.96
40.98
21.72
25.91
65.02
22.67
39.02
22.46
22.05
40.69
21.82
22.03
41.07
21.79
25.94
65.13
22.76
39.12
22.52
22.13
40.80
21.90
two terminal pyrazoles
II
III
I
two 2-methyl-2-phenoxypropionates
two bridging pyrazolates
2
3
two pyrazole molecules
II
I
two 2-hydroxy-5-(phenyldiazenyl)benzoates
two terminal pyrazole molecules
two 3,4-(methylenedioxy)benzoates
two bridging pyrazolates
II
III
I
4
two terminal pyrazole molecules
two 3-(4-methoxyphenyl)acrylates
two bridging pyrazolates
II
III
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Zinc Complexes with 3,5-Dimethylpyrazole and Carboxylate Ligands
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act as monodentate ligands. The strong structural propensity
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We gratefully acknowledge the financial support of the Education Of-
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Received: October 20, 2010
Published Online: March 1, 2011
Z. Anorg. Allg. Chem. 2011, 618–625
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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