Hydrogen binding in coordination compounds of 3(5)-(4-methoxyphenyl)pyrazole

Article abstract of DOI:10.1016/j.ica.2007.10.022

The solid state structures of 3(5)-(4-methoxyphenyl)pyrazole and its coordination compounds with a series of two valent transition metals have been investigated. Since pyrazoles provide not only a nitrogen donor site for the coordination to metal ions, but also an additional N-H function, they are ideal ligands for the formation of hydrogen bound coordination polymers or for the implementation of secondary interactions with other ligands bound to the same central ion, resulting in a rigid ligand environment at the central metal. We chose cobalt, nickel, palladium, copper and zinc as twofold positively charged Lewis acids preferring coordination numbers of four and six to prove the capability of pyrazole to undergo intramolecular hydrogen bonds. In the four-coordinate mode, either tetrahedral (Zn²⁺) or square planar coordination geometries (Pd²⁺) are possible, providing different geometric restrictions for hydrogen bonding.

Full text of DOI:10.1016/j.ica.2007.10.022

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 2059–2069
www.elsevier.com/locate/ica
Hydrogen binding in coordination compounds
of 3(5)-(4-methoxyphenyl)pyrazole
Susann Bergner, Gotthelf Wolmersha¨user, Harald Kelm, Werner R. Thiel *
FB Chemie, Technische Universita¨t Kaiserslautern, Erwin-Schro¨dinger-Strasse, Geb. 54, 67633 Kaiserslautern, Germany
Received 18 May 2007; received in revised form 10 October 2007; accepted 21 October 2007
Available online 26 October 2007
Abstract
The solid state structures of 3(5)-(4-methoxyphenyl)pyrazole and its coordination compounds with a series of two valent transition
metals have been investigated. Since pyrazoles provide not only a nitrogen donor site for the coordination to metal ions, but also an
additional N–H function, they are ideal ligands for the formation of hydrogen bound coordination polymers or for the implementation
of secondary interactions with other ligands bound to the same central ion, resulting in a rigid ligand environment at the central metal.
We chose cobalt, nickel, palladium, copper and zinc as twofold positively charged Lewis acids preferring coordination numbers of four
and six to prove the capability of pyrazole to undergo intramolecular hydrogen bonds. In the four-coordinate mode, either tetrahedral
(Zn2+) or square planar coordination geometries (Pd2+) are possible, providing different geometric restrictions for hydrogen bonding.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Pyrazole ligands; Cobalt; Nickel; Palladium; Zinc; Copper; Hydrogen bonds; Crystal structure
1. Introduction
ally, more complex procedures are required for the synthe-
sis of chelating compared to monodentate ligands.
A different strategy for the suppression of ligand fluxio-
nality without using chelating ligands is to use, in addition
to the metal–ligand interaction, secondary, non-covalent
bonds in the ligand sphere of the complex. This will fix
the ligands in defined positions at the metal site. One way
to do this is to introduce hydrogen bonds in close proxim-
ity to the central ion, which can be realized by applying
ligands possessing highly polar X–H groups in addition
to the donor site. Pyrazole represents a very simple exam-
ple for such a ligand system. Scheme 1 shows the secondary
H–X interaction freezing the rotation around the M–N
bond in comparison to the open system with the isomeric
imidazole ligand.
At a first glance, the geometry of coordination com-
pounds is determined by the nature of the central ion and
the number and the molecular structure of the ligands.
However, this definition results from a static view on the
shape of these systems, disregarding dynamic processes
going on in the ligand sphere. Ligand exchange reactions,
which are of severe importance especially for the coordina-
tion compounds of main group and f-block elements, lead
to a permanent change of the shape. To overcome this and
to create complexes of high stability, a multitude of chelat-
ing ligands have been introduced into coordination chemis-
try. But even chelating ligands may undergo rotations of
side chains or inversions of the chelating ring system.
One further disadvantage of chelating ligands in this con-
text is the reduced solubility of the derived complexes,
which is closely related to the reduced mobility. Addition-
Monodentate pyrazoles provide a plethora of different
substitution patterns, most of them are not realizable in
chelating ligands. Additionally, coordination to a Lewis-
acidic metal ion will increase the polarization of the N–H
bond of the pyrazole and, therefore, increase its capability
to be involved in hydrogen binding.
*
Corresponding author. Tel.: +49 631 2052752; fax: +49 631 2054676.
E-mail address: thiel@chemie.uni-kl.de (W.R. Thiel).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.10.022

2060
S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
IR (KBr, cmꢁ1): 3441 w, 3006 w, 2912 w, 2837 w, 1637s
H
N
N
X
X
H
(mC@O), 1596 s, 1578 s, 1537 s, 1509 m, 1492 m, 1446 m,
1435 s, 1424 s, 1362 s, 1301 m, 1273m, 1260 s, 1240 s,
1203 w, 1182 w, 1165 m, 1121 m, 1059 m, 1028 w, 1010
w, 957 w, 901 m, 839 m, 820 w, 771 s, 737 w, 685 w, 632
s, 599 m, 516 w, 501 w.
N
M
N
M
Scheme 1. Frozen rotation in pyrazole complexes.
Defined and rigid ligand geometries around a metal cen-
tre are of special interest for the generation of the so-called
metal organic frameworks where (large) multidentate but
usually not chelating ligands are used to build up crystal-
line solids possessing pores like zeolithes [1]. Such materials
have attracted the interest due to their special properties,
which can be beneficial for catalysis, gas separation and
more [2].
O
7
1
5
N
6
4
2
O
3
In the present publication, we chose an aryl substituted
monodentate pyrazole ligand to prove the hypothesis that
secondary hydrogen bonds will have a structure-directing
influence on the shape of a series of transition metal com-
plexes. For this, we have systematically changed the coor-
dination number and the geometry as well as the nature
of the additional anionic ligand for a series of transition
metal complexes.
2.3. 3(5)-(4-Methoxyphenyl)pyrazole (2)
6.56 g (32 mmol) of 1 and a sixfold excess of hydrazine
monohydrate were dissolved in 30 ml of EtOH and heated
to reflux for 4 h. After cooling to rt, the volatiles were
removed in vacuum. The remaining yellow oil was redis-
solved in 30 ml of EtOH and the solution was dried over
MgSO4. The solvent was removed in vacuum to obtain a
light yellow solid, which was recrystallized from CH2Cl2
at ꢁ20 °C giving colourless crystals. Yield: 5.02 g (90%).
Mp: 129 °C. Anal. Calc for C10H10N2O (174.20): C,
68.95; H, 5.79; N, 16.08. Found: C, 68.88; H, 5.95; N,
16.07%. 1H NMR (400 MHz, 25 °C, CDCl3): d 3.84 (s,
2. Experimental
2.1. General remarks
3
3H, OCH3), 6.52 (d, 1H, JH2,H3 = 2.2 Hz, H2), 6.92 (d,
All starting materials were obtained from ACROS and
used without further purification. Elemental analyses were
carried out at the Department of Chemistry (TU Kaisers-
lautern). Infrared spectra were recorded with a FASCO
FT-IR 6100 spectrometer. NMR spectra were recorded
with a Bruker Avance 400 or 600 spectrometer. The
assignment of the NMR data is according to the number-
ing schemes given below. The resonances of complexes 3
and 4 are assigned according to the numbering of free
ligand 2.
3
2H, JH6,H5 = 8.8 Hz, H6), 7.59 (d, 1H, H3), 7.67 (d, 2H,
H5), 11.93 (b, 1H, NH). 13C{1H} NMR (100 MHz, 25
°C, CDCl3) d 55.5 (OCH3), 102.3 (C2), 114.4 (C6), 125.0
(C4), 127.3 (C5), 133.7 (C3), 148.8 (C1), 159.8 (C7). IR
(KBr, cmꢁ1): 3394 w, 3125 w, 3114 w, 3048 w, 3000 w,
2965 w, 2915 m, 2839 w, 1612 m, 1573 w, 1550 w, 1527
m, 1509 s, 1472 m, 1455 m, 1439 s, 1354 w, 1306 w, 1276
s, 1252 vs, 1183 s, 1114 m, 1097 m, 1055 w, 1028 s, 954
m, 934 m, 896 w, 854 w, 833 s, 809 w, 794 m, 776 s, 674
w, 612 m, 525 m, 438 w.
2.2. 1-(4-Methoxyphenyl)-3-dimethylaminoprop-2-enone
(1)
H
N
N
1
3
A mixture of 5.10 g (34 mmol) 4-methoxyacetophenone,
4.05 g (34 mmol) N,N-dimethylformamide dimethylacetale
and 1 ml of triethylamine was heated to 110 °C for 72 h.
After cooling to rt 10 ml of Et2O was added resulting in
the precipitation of the product. The yellow solid was fil-
tered off, washed with n-pentane and dried in vacuum.
Yield 5.93 g (85%). Mp: 95 °C; Anal. Calc. for
C12H15NO2 (205.26): C, 70.22; H, 7.37; N, 6.82. Found:
4
2
5
O
7
6
2.4. [(trans-Dichloro)(bis-5-(4-methoxyphenyl)pyrazole)-
palladium(II)] (3)
1
C, 70.34; H, 7.27; N, 6.60%. H NMR (600 MHz, 25 °C,
CDCl3): d 2.89, 3.09 (2 ꢀ s, 2 ꢀ 3H, N(CH3)2), 3.82 (s,
3
174 mg (1 mmol) of 2 was dissolved in 10 ml of CH2Cl2.
To this mixture a solution of 192 mg (0.5 mmol) of
PdCl2(PhCN)2 in 10 ml of CH2Cl2 was added. The result-
ing orange solution was heated to reflux for 1 h. After cool-
ing to rt an orange microcrystalline solid was obtained. 3
3H, OCH3), 5.68 (d, 1H, JH6,H7 = 12.5 Hz, H6), 6.89 (d,
3
2H, JH3,H2 = 8.8 Hz, H3), 7.76 (d, 1H, H7), 7.88 (d, 2H,
H2). 13C{1H} NMR (150 MHz, 25 °C, CDCl3) d 37.3,
45.0 (N(CH3)2), 55.4 (OCH3), 91.8 (C6), 113.3 (C3),
129.5 (C2), 133.2 (C1), 153.8 (C7), 162.0 (C4), 187.4 (C5).

S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
2061
was recrystallised as orange needles by slow diffusion of n-
pentane into a CH2Cl2 solution. Yield: 258 mg (98%). Anal.
Calc. C20H20Cl2N4O2Pd (525.53): C, 45.71; H, 3.84; N,
10.66. Found: C, 45.92; H, 4.03; N, 10.82%. 1H NMR
(400 MHz, 25 °C, CDCl3) d 3.85 (s, 3H, OCH3), 6.35 (t,
2.7. [(trans-Diperchlorato)(tetrakis-5-(4-methoxyphenyl)-
pyrazole)nickel(II)](6)
174 mg (1 mmol) of 2 and 91 mg (0.25 mmol) of
Ni(ClO4)2 ꢂ 6H2O were dissolved in 10 ml of MeOH. The
resulting solution was heated for 2 h. After cooling for rt
the solvent was removed in vacuum and 6 was obtained
as a turquoise solid. Turquoise crystals of 6 were obtained
from slow diffusion of n-pentane into a solution of 6 in
CHCl3. Yield: 200 mg (84%). Anal. Calc. for
C40H40Cl2N8O12Ni (954.40): C, 50.34; H, 4.22; N, 11.74.
Found: C, 49.77; H, 4.16; N, 11.42. IR (KBr, cmꢁ1) 3437
m, 3366 s, 3263 m, 3156 w, 2962 w, 2932 w, 2837 w, 1617
s, 1587 w, 1507 vs, 1475 m, 1441 m, 1404 w, 1351 w,
1308 m, 1278 m, 1256 vs, 1183 s, 1145 s, 1119 vs, 1091
vs, 1031 s, 961 m, 942 w, 918 w, 889 w, 838 m, 788 s, 722
w, 694 w, 663 w, 628 m, 609 w, 526 w.
3
1H, JH2,H3 = 3JH2,NH = 2.2 Hz, H2), 6.97 (d, 2H,
3JH5,H6 = 8.8 Hz, H6), 7.46 (d, 2H, H5), 7.97 (t, 1H,
3JH3,NH = 2.2 Hz, H3), 11.70 (s, 1H, NH). 13C{1H}
NMR (150 MHz, 25 °C, CDCl3) d 55.8 (OCH3), 103.2
(C2), 114.8 (C6), 120.1 (C4), 127.4 (C5), 143.3 (C3),
144.7(C1), 160.9 (C7). IR (KBr, cmꢁ1): 3300 s, 3147 w,
2933 w, 2835 w, 1614 s, 1588 w, 1567 w, 1504 vs, 1477
m, 1458 m, 1439 m, 1403 w, 1363 w, 1303 w, 1276 s,
1253 vs, 1182 s, 1137 m, 1117 m, 1091 s, 1080 m, 1058 w,
1031 m, 968 m, 947 w, 886 w, 828 m, 783 s, 723 w, 685
w, 647 w, 635 m, 613 m, 575 w, 550 w, 526 w, 459 w.
2.5. [(Dichloro)(bis-5-(4-methoxyphenyl)pyrazole)-
zinc(II)] (4)
2.8. [(trans-Diperchlorato)(tetrakis-5-(4-
methoxyphenyl)pyrazole)copper(II)] (7)
1.39 g (8.0 mmol) of 2 was dissolved in 25 ml of hot
EtOH. To this mixture, a solution of 0.55 g (4.0 mmol) of
ZnCl2 in 25 ml of EtOH was added and the resulting clear
solution was heated to reflux for 1 h. After cooling to rt,
the precipitate was filtered off and dried in vacuum. The
product was recrystallized from CHCl3/hexanes. Yield:
1.74 g (90%). Anal. Calc. C20H20Cl2N4O2Zn (484.64): C,
49.56; H, 4.16; N, 11.56. Found: C, 50.04; H, 4.51; N,
11.11%. 1H NMR (400 MHz, 25 °C, CDCl3): d 3.84 (s,
174 mg (1 mmol) of 2 was dissolved in 5 ml of MeOH and
a solution of 93 mg (0.25 mmol) of Cu(ClO4)2 ꢂ 6H2O in hot
water was added. The resulting dark green solution was
heated for 1 h. After cooling to rt a brown precipitate was
obtained. The precipitate was separated, washed with Et2O
and dried in vacuum. Brown crystals of 7 were obtained by
slow diffusion of n-pentane into a solution of 7 in CHCl3.
Yield 189 g (79%). Anal. Calc. for C40H40Cl2N8O12Cu
(959.25): C, 50.08; H, 4.20; N, 11.68. Found: 50.08; H,
4.17; N, 11.64%. IR (KBr, cmꢁ1) 3350 m, 3286 m, 3151 m,
2935 m, 2838 m, 1750 m, 1616 vs, 1510 vs, 1477 s, 1442 s,
1407 w, 1376 w, 1352 w, 1309 m, 1278 s, 1255 vs, 1184 s,
1132 s, 1120 s, 1094 vs, 1029 m, 965 vs, 945 w, 930 w, 893
w, 836 m, 786 s, 728 w, 694 w, 660 w, 624 m, 526 w.
3
3H, OCH3), 6.51 (d, 1H, JH2,H3 = 2.1 Hz, H2), 6.97 (d,
3
2H, JH5,H6 = 8.2 Hz, H6), 7.60 (d, 2H, H5), 7.84 (s, 1H,
H3), 12.69 (b, 1H, NH). 13C{1H} NMR (100 MHz,
25 °C, CDCl3): d 55.5 (OCH3), 103.2 (C2), 114.9 (C6),
120.1 (C4), 127.9 (C5), 141.9 (C3), 145.9 (C1), 161.1 (C7).
IR (KBr, cmꢁ1): 3530 m, 3415 m, 3236 s, 3149 w, 3004
w, 2961 w, 2932 w, 2835 w, 1616 s, 1589 w, 1573 w, 1511
vs, 1482 m, 1453 m, 1437 m, 1410 w, 1352 w, 1317 m,
1310 m, 1280 m, 1255 vs, 1180 s, 1136 m, 1117 m, 1094
s, 1031 m, 969 m, 945 w, 889 w, 835 s, 797 s, 735 w, 669
w, 611 m, 523 m, 484 w, 467 w, 418 m, 404 m.
2.9. [(trans-Dichloro)(tetrakis-5-(4-methoxyphenyl)-
pyrazole)cobalt(II)] (8)
348 mg (2 mmol) of 2 was dissolved in 8 ml of MeOH and
a violet solution of 119 mg (0.5 mmol) of CoCl2 ꢂ 6H2O in
hot MeOH was added. The resulting blue solution was
heated to reflux for 2 h. After cooling to rt the solvent was
removed. After recrystallization from hot MeOH dark violet
crystals of 8 were obtained. Yield 0.39 g (94%). Anal. Calc.
for C40H40Cl2N8O4Co (826.65): C, 58.12; H, 4.88; N,
13.56. Found: C, 58.17; H, 4.90; N, 13.66%. IR (KBr,
cmꢁ1) 3244 s, 3208 s, 3002 m, 2960 w, 2932 w, 2833 m,
2559 w, 2049 w, 1870 w, 1775 w, 1614 vs, 1583 w, 1567 w,
1502 vs, 1468 s, 1438 s, 1402 m, 1346 w, 1306 m, 1269 vs,
1252 vs, 1180 s, 1123 s, 1089 vs, 1033 s, 957 s, 941 s, 892 m,
832 s, 787 s, 725 w, 702 w, 664 m, 611 m, 528 m, 442 w.
2.6. [(trans-Dichloro)(tetrakis-5-(4-methoxyphenyl)-
pyrazole)nickel(II)] (5)
1.39 g (8 mmol) of 2 was dissolved in 25 ml of hot EtOH
and a solution of 0.48 g (2 mmol) of NiCl2 ꢂ (H2O)6 in
25 ml of EtOH was added. The resulting clear solution
was heated to reflux for 1 h. After cooling to rt dark green
crystals of 5 were obtained. They were filtered off, washed
with Et2O and dried in vacuum. Yield: 1.48 g (90%). Anal.
Calc. for C40H40Cl2N8O4Ni (822.38): C, 58.42; H, 4.41; N,
13.63. Found: C, 58.18; H, 4.72; N, 13.63%. IR (KBr,
cmꢁ1): 3265 s, 3228 s, 3001 m, 2960 w, 2928 w, 2832 m,
1613 vs, 1583 w, 1566 w, 1502 vs, 1466 s, 1438 m, 1402
m, 1346 w, 1305 m, 1267 vs, 1250 vs, 1180 s, 1127 s, 1087
vs, 1031 s, 957 vs, 940 s, 889 m, 831 s, 783 vs, 734 w, 724
w, 698 w, 660 m, 635 w, 610 m, 530 m.
2.10. X-ray structure analyses
Single crystals of compounds 2–4 were carefully affixed
with an adhesive on to the tips of glass fibers. The crystals

Table 1
Crystal data and summary of intensity data collection and structure refinement of compounds 2–8
Compound
2
3
4
5
6
7
8
Molecular formula
Formula weight
Crystal size (mm)
Crystal system
Space group
C
174.20
10H10N2O
C20H20Cl2N4O2Pd C20H20Cl2N4O2Zn C40H40Cl2N8NiO4
525.70 484.67 826.41
C40H40Cl2N8NiO12
954.41
C40H40Cl2CuN8O12
959.24
C40H40Cl2CoN8O4
826.63
0.35 ꢀ 0.31 ꢀ 0.26
monoclinic
P21/c
0.48 ꢀ 0.34 ꢀ 0.20 0.50 ꢀ 0.40 ꢀ 0.20 0.35 ꢀ 0.25 ꢀ 0.20 0.35 ꢀ 0.25 ꢀ 0.20
0.42 ꢀ 0.28 ꢀ 0.12
0.24 ꢀ 0.14 ꢀ 0.08
monoclinic
C2/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
triclinic
triclinic
ꢀ
ꢀ
P1
P1
T (K)
a (A)
293(2)
293(2)
293(2)
150(2)
150(2)
150(2)
150(2)
˚
15.7104(10)
14.8836(11)
16.7327(10)
90
106.383(7)
90
14.0458(11)
11.4707(6)
13.5608(10)
90
107.640(8)
90
11.6296(7)
10.0979(5)
19.3239(12)
90
107.102(7)
90
9.9274(2)
18.4292(3)
11.1435(2)
90
103.758(2)
90
12.386(4)
13.969(3)
14.068(3)
71.46(2)
67.07(2)
87.99(2)
2114.4(9)
1.499
12.241(2)
14.076(3)
13.952(2)
72.82(2)
67.28(1)
86.50(1)
2114.8(6)
1.506
9.8997(2)
18.4858(4)
11.1751(2)
90
103.741(2)
90
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
3753.7(4)
1.233
2082.1(2)
1.677
2169.0(2)
1.484
1980.25(6)
1.386
1986.55(7)
1.382
Dcalc (g cmꢁ3
)
Z
16
0.082
4
4
2
2
2
2
lcalc (mmꢁ1
)
1.172
1.402
2.370
0.658
0.716
0.618
Diffractometer
Stoe IPDS
Stoe IPDS
Stoe IPDS
Oxford Diffr. Gemini S Oxford Diffr. Gemini S Oxford Diffr. Gemini S Oxford Diffr. Gemini S
Ultra
Ultra
Ultra
Ultra
Scan type
Radiation
H range (°)
U oscillation
Mo Ka
2.70–26.73
16735
3923
238
0.0779
0.0390
0.0992
0.0904
U rotation
Mo Ka
3.04–26.73
17530
4413
264
0.0318
0.0257
0.0686
0.0667
U oscillation
Mo Ka
2.73–26.73
19076
4597
264
0.0362
0.0260
0.0679
0.0650
X scans
Cu Ka
4.74–62.28
8853
3041
252
0.0403
0.0332
0.1079
0.0898
1.127
X scans
Mo Ka
2.67 -32.19
31044
13355
575
0.1206
0.0421
0.0833
0.0697
0.835
X scans
Mo Ka
2.67–32.00
30.105
12270
X scans
Mo Ka
2.71–29.99
18029
5741
252
0.0497
0.0321
0.0800
0.0742
1.037
Reflexions measured
Independent reflections
Refined parameters
Ra (all data)
575
0.1006
0.0454
0.0926
0.0794
0.936
Ra (I > 2r(I))
b
wR2 (all data)
b
wR2 (I > 2r(I))
Goodness-of-fitnessc
0.809
0.964
0.962
Residual electron density
0.107/ꢁ0.146
0.296/ꢁ0.766
0.249/ꢁ0.336
0.312/ꢁ0.430
0.511/ꢁ0.402
0.402/ꢁ0.471
0.263/ꢁ0.242
ꢁ3
˚
(e/A
)
P
P
a
b
c
R ¼ kF oj ꢁ jF ck= jF oj.
P
P
2
1=2
wR2 ¼ ½ wðF 2o ꢁ F c2Þ = wF 2oꢃ
.
P
2
1=2
Goodness-of-fitness ¼ ½ wðF 2o ꢁ F c2Þ =ðn ꢁ pÞꢃ
.

S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
2063
of complexes 5–8 were immersed in an oil and then attached
to the tips of glass fibers. The glass fibers were subsequently
mounted on goniometer heads in a nitrogen stream at room
temperature or 150 K, respectively. Table 1 summarizes the
cell parameters as well as the experimental details of the
data collection and the structure refinements of all crystals.
In the case of 3, an analytical absorption correction using
ABST from the PLATON program suite was applied [3]. For
the other complexes semi-empirical absorption correction
was applied using the programs MULABS (for compound 4)
and SCALE 3 ABSPACK (for compounds 5–8) from the
CrysAlis Red program suite [4]. The structures were solved
by direct methods (SHELXS-97) and by Fourier difference
maps using the program SHELXL-97. The structural parame-
ters were refined by the full-matrix least squares method
with the program SHELXL-97 [5]. All non-hydrogen atoms
were refined anisotropically. The hydrogen atoms were set
in calculated positions using a riding model with isotropic
thermal parameters.
For the investigation of the coordination behaviour of
aryl substituted pyrazoles, we chose the 4-methoxy substi-
tuted derivative to enhance the solubility of the complexes
in organic solvents, which should also allow a better
spectroscopic characterization of the products and in
addition facilitate crystallization. The two-step synthesis
of 3(5)-(4-methoxyphenyl)pyrazole (2) starts from 4-
methoxyacetophenone (4-acetylanisole) and proceeds via
1-(4-methoxyphenyl)-3-dimethylaminoprop-2-enone (1) to
the target compound (Scheme 2).
Recrystallization of 2 from CH2Cl2 gave crystals of up
to 0.5 mm length. Due to the combination of a proton-
donating and a proton-accepting site in one molecule, it
undergoes intermolecular hydrogen binding resulting in a
tetrameric structure with C2 symmetry (Fig. 1). The unit
cell contains two crystallographically independent
molecules.
The protons in the 12-membered ring, which take part
in the hydrogen bonds, are completely disordered accord-
ing to the N–H tautomerism of pyrazole [7] and its deriv-
atives (see Fig. 1). The observed Nꢂ ꢂ ꢂN distances of
3. Results and discussion
˚
about 2.85–2.89 A are typical for N–Hꢂ ꢂ ꢂN hydrogen
Aryl substituted pyrazoles can be obtained by a conve-
nient and high yield synthesis starting from commercially
available aromatic acetyl derivatives. These compounds
undergo a Claisen-type condensation with N,N-dimethyl-
formamide dimethylacetal to generate 1-aryl-3-dimethyl-
aminoprop-2-enones [6], which can be considered as
masked congeners of 1-aryl-1,3-diketones. The derived
pyrazoles form instantaneously by reacting the aminopro-
penones with hydrazine in ethanolic solution [6a].
bonds of medium strength [8]. Similar tetrameric arrange-
ments have been found in the solid state structures of a
series of pyrazole derivatives [9,10]. All these compounds
exhibit a saddle-like structure as it is the case for 2; how-
ever, the curvature of the saddle can differ from an
almost coplanar orientation of the four pyrazoles to an
arrangement wherein the first and the third as well as
the second and the fourth pyrazoles undergo p-stacking
with each other.
H
N
N
O
O
i
ii
N
O
O
O
1
2
Scheme 2. (i) Me2NCH(OMe)2, reflux, 72 h; (ii) N2H4 ꢂ H2O, EtOH, reflux, 4 h.
0
˚
Fig. 1. Molecular view of the tetrameric arrangement of 2 in the solid state. Selected Nꢂ ꢂ ꢂN distances [A] in N–Hꢂ ꢂ ꢂN hydrogen bonds: N1ꢂ ꢂ ꢂN1
2.8676(18), N2ꢂ ꢂ ꢂN3 2.8587(19), N4ꢂ ꢂ ꢂN40 2.8800(19).

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S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
Cl
Pd
Cl
H
N
O
N
2 + PdCl2(C6H5CN)2
N
H
3
N
O
Scheme 3.
Coordination of 2 to a Lewis-acidic metal ion will occur
at the sterically less hindered nitrogen atom. Thus, if the
pyrazole is not deprotonated, the proton must be shifted
to the nitrogen atom next to the pyrazole–phenyl bond,
resulting in the formation of the 5-(4-methoxyphenyl)pyr-
azole tautomer. This is the case for all coordination com-
pounds we have investigated in this series.
The complexes were obtained by stirring an appropriate
precursor complex with 2 and 4 equiv. of 2. With
PdCl2(C6H5CN)2 the pseudo-square planar complex [(trans-
dichloro)(bis-5-(4-methoxyphenyl)pyrazole)palladium(II)] (3)
was formed in almost quantitative yields (Scheme 3).
The coordination geometry is typical for a central ion
with d8-configuration. Recrystallization of 3 gave pale yel-
low crystals, Fig. 2 shows the molecular structure of 3 in
the solid state.
which are interconnected by additional weak C–Hꢂ ꢂ ꢂO
and C–Hꢂ ꢂ ꢂCl contacts.
To the best of our knowledge, only two solid state struc-
tures of one compound of the type PdCl2L2 (with L = pyr-
azole with intact N–H function) have been published up to
now [11]. In both cases, 3,5-di(tert-butyl)pyrazole was used
as the ligand. This complex crystallizes with the inclusion
of either diethylether or dichloromethane in the solid mate-
rial [12]. In contrast to complex 3, there is no intramolecu-
lar but intermolecular Clꢂ ꢂ ꢂH–N interaction resulting in
hydrogen bound dinuclear complexes, which is quite aston-
t
ishing, since the bulky Bu groups should be unfavourable
for such a dimerization.
Reaction of 2 with ZnCl2 gave [(dichloro)(bis-5-(4-
methoxyphenyl)pyrazole)zinc(II)] (4) (Scheme 4). In the
solid state structure, the zinc cation is coordinated by
two pyrazole and two chloro ligands in a distorted tetrahe-
dral geometry (Fig. 3).
Since the Cl–M–N angles increase from about 90° for
M = Pd to more than 110° for M = Zn, the Clꢂ ꢂ ꢂH dis-
The first remarkable characteristics of this structure are
two relatively strong intramolecular hydrogen bonds
between the two N–H groups at the pyrazole rings and
the two chloro ligands. To optimize the Clꢂ ꢂ ꢂH–N interac-
tion, the plane defined by N2, N4, Cl1 and Cl2 is twisted by
about 22° against the two pyrazole rings. The phenyl and
the pyrazole rings in both 5-(4-methoxyphenyl)pyrazole
ligands are oriented almost coplanar to each other. Com-
plex 3 gains energy in the solid state structure by the forma-
tion of dimers due to weaker secondary intermolecular
O
N
N
Cl
N
H
2 + ZnCl2
Zn
4
H
Cl
N
˚
Pdꢂ ꢂ ꢂN (Pd1ꢂ ꢂ ꢂN: 3.5575 and 3.6122 A) and Clꢂ ꢂ ꢂH inter-
O
˚
actions (Clꢂ ꢂ ꢂH: 2.97 and 2.85 A) between neighboring
molecules. These dimers are arranged in double chains
Scheme 4.
˚
Fig. 2. Molecular structure of 3 in the solid state presenting the dimers formed by secondary Pdꢂ ꢂ ꢂN and Clꢂ ꢂ ꢂH interactions. Selected distances [A], angles
[°] and torsion angles [°]: Pd–Cl1 2.2966(7), Pd–Cl2 2.2987(7), Pd–N2 2.0090(19), Pd–N4 2.0066(19); Cl1–Pd–Cl2 178.68(2), Cl1–Pd–N2 89.11(5), Cl1–Pd–
N4 90.00(5), Cl2–Pd–N2 90.15(5), Cl2–Pd–N4 90.73(5), N2–Pd–N4 178.86(7), Cl1–Pd–N2–N1 ꢁ22.96(16), Cl1–Pd–N4–N3 ꢁ158.95(16). Selected
˚
distances [A] and angles [°] in the hydrogen bonds: N1–H1 0.86, H1ꢂ ꢂ ꢂCl1 2.49, N1ꢂ ꢂ ꢂCl1 2.989(2), N1–H1ꢂ ꢂ ꢂCl1 117, N3–H3 0.86, H3ꢂ ꢂ ꢂCl2 2.60,
N3ꢂ ꢂ ꢂCl2 3.073(2), N1–H1ꢂ ꢂ ꢂCl1 117, N3–H3ꢂ ꢂ ꢂCl2 116.

S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
2065
an intramolecular hydrogen bond with one of the chloro
ligands, while the other N–H group undergoes intermolec-
ular hydrogen binding with the chloro ligand, which is not
yet involved in the intramolecular hydrogen bridge at the
second zinc site [14]. The third example was synthesized
by Wagner and Lerner et al. by the Zn2+-induced cleavage
of [RB(tBu–pz)3]ꢁ, which seems to be a general route to
transition metal pyrazole and pyrazolide complexes [15].
˚
Fig. 3. Molecular structure of 4 in the solid state. Selected distances [A]
and angles [°]: Zn–Cl1 2.2453(5), Zn–Cl2 2.2573(6), Zn–N2 2.0169(15),
Zn–N4 2.0260(16), Cl1–Zn–Cl2 113.30(2), Cl1–Zn–N2 112.57(4), Cl1–Zn–
N4 103.92(5), Cl2–Zn–N2 103.10(5), Cl2–Zn–N4 110.63(5), N2–Zn–N4
˚
113.63(7). Selected distances [A] and angles [°] in the hydrogen bonds: N1–
*
H1 0.86, H1ꢂ ꢂ ꢂCl2* 2.34, N1ꢂ ꢂ ꢂCl2* 3.1708(15), N1–H1ꢂ ꢂ ꢂCl2* 162 (for :
2 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z), N3–H3 0.86, H3ꢂ ꢂ ꢂCl1** 2.38, N3ꢂ ꢂ ꢂCl1**
3.2158(14), N3–H3ꢂ ꢂ ꢂCl1** 164 (for *: 2 ꢁ x, 1/2 + y, 1/2ꢁz).
tances in 4 become too long for intramolecular hydrogen
binding. Therefore, intermolecular hydrogen bonds are
formed resulting in hydrogen bound polymeric chains
along the crystallographic c-axis. These chains are linked
by additional weak C–Hꢂ ꢂ ꢂO interactions. Up to now, only
three solid state structures of compounds of the type
ZnCl2L2 (with L = pyrazole with intact N–H function)
have been published [11]. In one of these compounds, 3-
amino-5-tert-butyl pyrazole was used as the ligand. By
involving the amino function of the ligand, a complex
three-dimensional network of intra- and intermolecular
hydrogen bonds is formed [13]. In the second example,
3,5-di(tert-butyl)pyrazole as the ligand gives rise to a
dimeric structure, wherein one of the N–H groups forms
˚
Fig. 4. Molecular structure of 5 in the solid state. Selected distances [A],
angles [°] and torsion angles [°]: Ni–N2 2.090(2), Ni–N4 2.107(2), Ni–Cl1
2.4658(6), N2–Ni–N4 88.69(7), N20–Ni–N4 91.31(7), N2–Ni–Cl1 89.90(5),
N20–Ni–Cl 90.11(5), N4–Ni–Cl1 90.36(5), N40–Ni–Cl1 89.46(5). Selected
˚
distances [A] and angles [°] in the hydrogen bonds: N1–H1 0.88, H1ꢂ ꢂ ꢂCl1
2.40, N1ꢂ ꢂ ꢂCl1 3.032(2), N1–H1ꢂ ꢂ ꢂCl1 130.00, N3–H3A 0.88, H3Aꢂ ꢂ ꢂCl1
2.33, N3ꢂ ꢂ ꢂCl1 2.992(2), N3–H3Aꢂ ꢂ ꢂCl1 132.00.
O
O
H
H
H
5
6
7
8
N
N
N
X
M
X
N
2 + MX2(H2O)6
M
X
Ni
Ni
Cu Co
N
N
N
Cl ClO4 ClO4 Cl
N
H
5-8
O
O
Scheme 5.

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S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
This paper presents an interesting overview on a series of
such systems although intra-and intermolecular hydrogen
binding is not discussed.
In the 13C NMR spectra of 3 and 4, the resonance of C3,
which is the carbon atom next to the coordinating nitrogen
atom in the pyrazole ring, significantly shifts to lower field
by 8–10 ppm. This effect again can be explained by the
transfer of electron density from the pyrazole to the metal
ions. The resonance of the carbon atom C2 is almost unaf-
fected by the coordination, while the resonance of C1 shifts
is slightly shifted to higher field, probably due to the com-
plete shift of the N–H proton to the neighbouring nitrogen
atom. A similar shift is observed for C4 (at the phenyl
ring). All other carbon resonances are unaffected by the
coordination.
Since ligand 2 and complexes 3 and 4 are diamagnetic
1
species, H and 13C NMR spectroscopy allows to assign
the electronic situation especially of the pyrazole ring.
For the assignment of the NMR data, see Section 2. The
differences in the shifts of H6 of the 4-methoxyphenyl ring
and the methoxy group are not significant. However, a shift
of the resonance of H3 (pyrazole ring) of 0.38 ppm for
compound 3 and 0.25 ppm for compound 4 to lower field
compared to the resonance of H3 of compound 2 is due
to the electron donation from the pyrazole ring to the metal
sites. This effect was already observed for pyrazole-contain-
ing ligands in other coordination compounds [16]. For the
remaining two protons H2 and H5, which are also bound
to carbon, the situation is different: while the resonances
of these protons are almost similar for 2 and the zinc com-
plex 4, they are shifted by about 0.20 ppm to higher field in
the palladium complex 3, indicating an increased shielding
in these positions. This could be caused by intermolecular
p-stacking [17] or N–Hꢂ ꢂ ꢂCl interactions in solution, which
should be possible due to the flat molecular geometry of 3.
Even the resonance of the NH function of 3 is shifted to
higher field (0.23 ppm compared to 2), while it is strongly
shifted to lower field (0.76 ppm) for the zinc complex 4.
For the following investigations, Ni2+, Co2+ and Cu2+
were chosen which preferentially exhibit octahedral coordi-
nation geometries (Scheme 5). This should allow a direct
comparison of the effects of intramolecular N–Hꢂ ꢂ ꢂX
hydrogen bonds on the overall molecular structure as well
as on the crystallographic parameters of the derived coor-
dination compounds.
The complex [(trans-dichloro)(tetrakis-5-(4-methoxy-
phenyl)pyrazole)nickel(II)] (5) is obtained by reacting
NiCl2 ꢂ (H2O)6 with 2 in EtOH. In the solid state structure,
the nickel cation is coordinated by two chloro ligands in
trans position and four pyrazole ligands (Fig. 4).
Due to the octahedral coordination geometry and the
trans orientation of the two chloro ligands, all Cl–M–pz
˚
Fig. 5. Molecular structure of 6 in the solid state. Selected distances [A], angles [°] and torsion angles [°]. Ni1–N2 2.068(2), Ni1–N4 2.060(2), Ni1–O3
2.210(2), Cl1–O3 1.458(2), Cl1–O4 1.432(2), Cl1–O5 1.416(2), Cl1–O6 1.437(2), Ni2–N6 2.074(2), Ni2–N8 2.047(2), Ni2–O9 2.155(2), Cl2–O9 1.439(2),
Cl2–O10 1.431(2), Cl2–O11 1.414(2), Cl2–O12 1.422(2), N2–Ni1–O3 86.90(6), N2–Ni1–N4 88.06(7), N2–Ni1–N40 91.94(7), N20–Ni1–O3 93.10(6), N4–
Ni1–O3 84.67(6), N4–Ni1–O30 95.33(6), O3–Cl1–O4 108.23(9), O3–Cl1–O5 109.52(10), O3–Cl1–O6 108.89(9), O4–Cl1–O5 111.23(11), O4–Cl1–O6
109.73(11), O5–Cl1–O6 109.20(9), Cl1–O3–Ni1 140.20(9), N6–Ni2–N8 89.95(7), N6–Ni2–N80 90.05(7), N6–Ni2–O9 87.04(7), N60–Ni2–O9 92.96(7), N8–
Ni2–O9 84.47(7), N80–Ni2–O9 95.53(7), Cl2–O9–Ni2 155.71(10), O9–Cl2–O10 108.10(11), O9–Cl2–O10 108.81(10), O9–Cl2–O12 107.89(12), O10–Cl2–O1
˚
111.29(10), O10–Cl2–O12 109.14(10), O11–Cl2–O12 111.49(11). Selected distances [A] and angles [°] in the hydrogen bonds: N1–H1 0.88, H1ꢂ ꢂ ꢂO4 2.05,
N1ꢂ ꢂ ꢂO4 2.917(3), N1–H1ꢂ ꢂ ꢂO4 168.0, N3–H3 0.88, H3ꢂ ꢂ ꢂO6 2.10, N3ꢂ ꢂ ꢂO6 2.866(2), N3–H3ꢂ ꢂ ꢂO6 145.0, N5–H5A 0.88, H5Aꢂ ꢂ ꢂO12 2.36, N5ꢂ ꢂ ꢂO12
3.089(3), N5–H5Aꢂ ꢂ ꢂO12 140.00, N7–H7 0.88, H7ꢂ ꢂ ꢂO10 2.16, N7ꢂ ꢂ ꢂO10 2.908(3), N7–H7ꢂ ꢂ ꢂO10 142.00.

S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
2067
angles are found to be about 90°, as it was the case for 3.
This allows intramolecular hydrogen binding as before.
Each of the chloro ligands acts as a proton acceptor for
two pyrazole N–H groups. The nickel(II) ions are the cen-
tres of inversion in the structure, which leads to a linear
arrangement of the complexes along the crystallographic
c-axis. According to an inquiry in the Cambridge Struc-
tural Database [11], only two solid state structures of com-
plexes of the type NiCl2L4 (with L = pyrazole with intact
N–H function) have been published up to now. Pyrazole
and 3,5-diethyl-4-methylpyrazole have been used in these
examples [18]. Similar hydrogen bond patterns as in 5 have
been found in both cases. Due to the reduced steric demand
of the simple pyrazole ligand in [(trans-dichloro)(tetra-
kispyrazole)nickel(II)], additional intermolecular N–
Hꢂ ꢂ ꢂCl bonds are formed resulting in a linear hydrogen
bound polymer.
Formal exchange of the chloro ligands in 5 against per-
chlorate anions elongates the distance between the proton-
accepting site and the central ion without affecting the
coordination geometry of the nickel ion. The correspond-
ing nickel complex 6 was obtained by reacting 2 and
Ni(ClO4)2 ꢂ 6H2O in MeOH. In the solid state structure,
the nickel cation is coordinated by two perchlorato ligands
in trans position and four pyrazole units. There are two
crystallographically different molecules found in the unit
cell (Fig. 5).
Since the metal bound oxygen atom of the perchlorate
anions is a poor proton acceptor, the N–H units undergo
intramolecular hydrogen bonding with two of the non-
coordinating oxygen atoms of the perchlorate anions. This
implies a twist of the pyrazole groups around the N–Ni
bond. While in complex 5 the five-membered rings includ-
ing the N–Hꢂ ꢂ ꢂCl fragments are almost planar, the corre-
sponding seven-membered rings in 6 are strongly bent
out of planarity. There are two molecules in the unit cell
that differ in the direction of the rotation of the pyrazoles
groups: in one of the complexes, the pyrazoles are rotated
in the same direction; (Fig. 5, left) in the second complex,
they are rotated in opposite directions (Fig. 5, right). Com-
pound 6 is the first structurally characterized example of a
complex of the type Ni(ClO4)2L4 (with L = pyrazole with
intact N–H function) [11].
We next moved to the corresponding copper(II) com-
plex. Due to its d9-configuration octahedrally coordinated
copper(II) will show Jahn-Teller distortion, leading to a
slight decrease of the M–N distances but a pronounced
increase of the M–O distances. However, if hydrogen
bonding is a determining factor for the molecular
structures of these complexes, it should have a balancing
˚
Fig. 6. Molecular structure of 7 in the solid state. Selected distances [A], angles [°] and torsion angles [°]. Cu1–N2 2.002(2), Cu1–N4 2.002(2), Cu1–O3
2.520 (2), Cl1–O3 1.443(2), Cl1–O4 1.427(2), Cl1–O5 1.423(2), Cl1–O6 1.439(2), Cu2–N6 2.011(2), Cu2–N8 1.994(2), Cu2–O9 2.426(2), Cl2–O9 1.430(2),
Cl2–O10 1.430(2), Cl2–O11 1.424(2), Cl2–O12 1.429(2), N2–Cu1–N4 88.20(8), N2–Cu1–N40 91.80(8), N2–Cu1–O3 86.06(7), N20–Cu1–O3 93.94(7), N4–
Cu1–O3 85.49(7), N40–Cu1–O3 94.51(7), Cl1–O3–Cu1 135.36(10), O3–Cl1–O4 108.00(12), O3–Cl1–O5 110.94(11), O3–Cl1–O6 109.04(12), O4–Cl1–O5
111.03(12), O4–Cl1–O6 109.63(12), O5–Cl1–O6 109.04(12), N6–Cu2–N8 89.82(8), N60–Cu2–N8 90.18(8), N6–Cu2–O9 87.91(8), N6–Cu2–O90 92.09(8),
N8–Cu2–O9 84.68(8), N8–Cu2–O90 85.32(8), Cl2–O9–Cu2 151.07(13), O9–Cl2–O10 108.41(13), O9–Cl2–O11 110.69(12), O9–Cl2–O12 107.73(13), O10–
˚
Cl2–O11 110.69(12), O10–Cl2–O12 108.38(13), O11–Cl2–O12 111.19(12). Selected distances [A] and angles [°] in the hydrogen bonds: N1–HN1 0.88,
HN1ꢂ ꢂ ꢂO4 2.08, N1ꢂ ꢂ ꢂO4 2.950(3), N1–HN1ꢂ ꢂ ꢂO4 170, N3–H3A 0.88, H3Aꢂ ꢂ ꢂO6 2.08, N3ꢂ ꢂ ꢂO6 2.868(3), N3–H3Aꢂ ꢂ ꢂO6 150.00, N5–H5A 0.88,
H5Aꢂ ꢂ ꢂO11 2.17, N5ꢂ ꢂ ꢂO11 2.935(3), N5–H5Aꢂ ꢂ ꢂO11 146.00, N7–H7 0.88, H7ꢂ ꢂ ꢂO9 2.56, N7ꢂ ꢂ ꢂO9 3.116(3), N7–H7ꢂ ꢂ ꢂO9 122.00, N7–H7 0.8800,
H7ꢂ ꢂ ꢂO10 2.43, N7ꢂ ꢂ ꢂO10 3.166(3), N7–H7ꢂ ꢂ ꢂO10 141.00.

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S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
influence on the overall geometry of the compound. This
can clearly be shown for the couple Ni(ClO4)2(2)4 (6) and
Cu(ClO4)2(2)4 (7). The latter complex was synthesized by
reacting Cu(ClO4)2 ꢂ 6H2O dissolved in hot water with four
equivalents of 2 dissolved in MeOH.
The two complexes 6 and 7 are almost isomorphous
with respect to their macroscopic crystallographic data:
they both crystallize with almost identical cell parameters
(Ni/Cu: a, 12.386(4)/12.241(2); b, 13.969(3)/13.952(2); c,
˚
14.068(3)/14.076(3) A; a, 71.46(2)/72.82(2); b, 67.07(2)/
3
˚
86.50(1); c, 87.99(2)/67.28(1)°; V, 2114.4(9)/2114.8(5) A ;
D, 1.499/1.506 g cmꢁ3). Both structures include two crys-
tallographically different molecules. However, as predicted
by electronic considerations, the bond lengths between the
central ions and the ligands are strongly different (Ni/Cu:
˚
M–N 2.06/2.00 M–O 2.21/2.52 A). These effects are com-
pensated by comparable parameters of the intermolecular
hydrogen bonds (Fig. 6).
There is only one structurally characterized compound
of the type Cu(ClO4)2L4 (with L = pyrazole with intact
N–H function) [11]. In this publication, 3(5)-tert-butyl-
pyrazole was used as the ligand, coordinating to copper(II)
via N1 as the sterically favoured 2H-3-tert-butyl tautomer
[19]. Here both the perchlorate anions and the tert-butyl
groups are disordered, which prevents a structural compar-
ison with complex 7.
˚
Fig. 7. Molecular structure of 8 in the solid state. Selected distances [A],
As a final example, we chose cobalt(II) as the central
ion. Reacting two equiv. of 2 with CoCl2 ꢂ 6H2O in MeOH
gives CoCl2(2)4 (8), which crystallizes in an isomorphic
manner to the corresponding nickel(II) complex 5. As for
the couple, 6 and 7 the cell dimensions of 5 and 8 are
almost identical (Ni/Co: a, 9.9274(2)/9.8997(2); b,
angles [°] and torsion angles [°]: Co1–N2 2.1170(12), Co1–N4 2.1381(11),
Co1–Cl1 2.5258(3), N2–Co1–N4 88.77(4), N20–Co1–N4 91.23(4), N2–
Co1–Cl1 90.67(3), N20–Co1–Cl1 89.33(3), N4–Co1–Cl1 91.21(3), N40–
Co1–Cl1 88.79(3), N20–Co1–Cl10 90.67(3). Selected distances [A] and
˚
angles [°] in the hydrogen bonds: N1–H1 0.88, H1ꢂ ꢂ ꢂCl1 2.39, N1ꢂ ꢂ ꢂCl1
3.0454(13), N1–H1ꢂ ꢂ ꢂCl1 131.0, N3–H3A 0.88, H3Aꢂ ꢂ ꢂCl1 2.33, N3ꢂ ꢂ ꢂCl1
3.0044(12), N3–H3Aꢂ ꢂ ꢂCl1 133.00.
˚
18.4292(3)/18.4858(4); c, 11.1435(2)/11.1751(2) A; b,
3
˚
the data of the perchlorate complexes 6 and 7, where the
asymmetric and the symmetric N–H absorptions are found
at 3366 and 3263 cmꢁ1 for the nickel(II) complex 6 and at
3350 and 3286 cmꢁ1 for the copper(II) complex 7. The pal-
103.758(2)/103.741(2)°; V, 1980.25(6)/1986.55(7) A ; D
1.386/1.382 g cmꢁ3), which was expected, since there is no
big difference in the radii of cobalt(II) and nickel(II) (Ni/
Co: M1–N2, 2.088(2)/2.1168(11); M1–N4, 2.107(2)/
2.1381(11); M1–Cl1, 2.4648(7)/2.5259(3)). Additionally,
the N–Hꢂ ꢂ ꢂCl hydrogen bonds determine the shape of the
molecule (Fig. 7).
ladium complex
3 shows one N–H absorption at
3300 cmꢁ1, indicating a weaker N–H and thus a stronger
Hꢂ ꢂ ꢂCl interaction. Even more weak N–H bonds and thus
stronger Hꢂ ꢂ ꢂX bonds are found for the tetrahedral and
octahedral chloro complexes 4 (Zn, mNH: 3236 cmꢁ1), 5
(Ni, mNH: 3265, 3228 cmꢁ1) and 8 (Co, mNH: 3244,
3208 cmꢁ1).
In the Cambridge Structural Database, there are only
two compounds of the type CoCl2L4 (with L = pyrazole
with intact N–H function) [11]. One is the yet unpublished
complex with 2-hydroxy-5-methyl pyrazole, where the
chloro ligands are acting as proton acceptors for two N–
H and two OH groups. The second is the complex with
unsubstituted pyrazole [20]. This compound is again iso-
morphous to the corresponding nickel(II) complex [18b]
and is thus exhibiting the same pattern of intramolecular
hydrogen bonds as 8.
Infrared spectroscopy can give an idea about the
strength of the intra- and intermolecular hydrogen bonds.
For the free ligand 3(5)-(4-methoxyphenyl)pyrazole (2),
an absorption at 3394 cmꢁ1 was assigned to the N–H
stretching vibration, indicating the presence of strong N–
H bonds and thus relatively weak intermolecular hydrogen
bonds in the tetrameric structure. This corroborates with
4. Conclusion
By the reaction of 3(5)-(4-methoxyphenyl)pyrazole (2)
with two valent transition metal chlorides, a series of pyr-
azole complexes could be synthesized. In all cases, intra-
or intermolecular hydrogen bonding between the pyrazole
N–H group and the anionic ligands coordinated to the cen-
tral ion results in the formation of rigid though well-soluble
structures. We are now going to extend this strategy to sys-
tems which can be used as building blocks for (porous)
coordination polymers. This can be done by attaching a
further anionic group (e.g. a carboxylate or sulfonate func-

S. Bergner et al. / Inorganica Chimica Acta 361 (2008) 2059–2069
2069
Condensed Rings, vol. 22, J. Wiley & Sons, New York, 1967, p. 3ff;
(b) J. Elguero, in: K.T. Potts (Ed.), Comprehensive Heterocyclic
Chemistry, vol. 5, Pergamon Press, Oxford, 1984, p. 169ff.
[8] T. Steiner, Angew. Chem., Int. Ed. 41 (2002) 48.
[9] Reviews on hydrogen bonding in pyrazoles: (a) I. Alkorta, J. Elguero,
C. Foces-Foces, L. Infantes, ARKIVOC (2006) 15;
(b) C. Foces-Foces, I. Alkorta, J. Elguero, Acta Crystallogr., Sect. B
56 (2000) 1018.
tion) to the phenyl ring of the phenyl pyrazole in combina-
tion with kinetically inert pyrazole complexes like 3.
Appendix A. Supplementary material
CCDC 667508, 667509, 667510, 667511, 667512, 667513
and 667514 contain the supplementary crystallographic
data for 2, 3, 4, 5, 6, 7 and 8. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] (a) H.V.R. Dias, T.K.H.H. Goh, Polyhedron 23 (2004) 273;
(b) A. Haghiri, H.-W. Lerner, M. Bolte, Acta Crystallogr., Sect. E 59
(2003) o873;
(c) F. Zhang, M. Wagner, J.W. Bats, Acta Crystallogr., Sect. E 60
(2004) o1804;
(d) C. Foces-Foces, S. Trofimenko, Acta Crystallogr., Sect. E 57
(2001) o32;
References
(e) S. Trofimenko, A.L. Rheingold, L.M. Liable-Sands, R.M.
Claramunt, C. Lopez, M.D. Santa Maria, J. Elguero, New J. Chem.
25 (2001) 819;
(f) J.Q. Puello, B.I. Obando, C. Foces-Foces, L. Infantes, R.M.
Claramunt, P. Cabildo, J.A. Jimenez, J. Elguero, Tetrahedron 53
(1997) 10783;
[1] (a) S.L. James, Chem. Soc. Rev. 32 (2003) 276;
(b) C. Janiak, Angew. Chem., Int. Ed. 36 (1997) 1431;
(c) M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.
O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319;
(d) J.L.C. Rowsell, O.M. Yaghi, Micropor. Mesopor. Mater. 73
(2004) 3;
(e) S. Kaskel, Nachr. Chem. 53 (2005) 394;
(f) W. Lin, J. Solid State Chem. 178 (2005) 2486;
(g) G.K.H. Shimizu, J. Solid State Chem. 178 (2005) 2519.
[2] (a) G. Ferey, Nat. Mater. 2 (2003) 136;
(b) M.D. Ward, Science 300 (2003) 1104;
(g) H.V.R. Dias, X. Wang, Polyhedron 23 (2004) 2533;
(h) R.M. Claramunt, C. Lopez, M. de los A. Garcia, M. Pierrot, M.
Giorgi, J. Elguero, J. Chem. Soc., Perkin Trans. 2 (2000) 2049.
[11] (a) CSD Version 5.28, 2007;
(b) F.H. Allen, Acta Crystallogr., Sect. B 58 (2002) 380.
[12] K. Li, J. Darkwa, I.A. Guzei, S.F. Mapolie, J. Organomet. Chem. 660
(2002) 108.
[13] Z.K. Jacimovic, B.V. Prelesnik, G.A. Bogdanovic, E.Z. Iveges, V.M.
Leovac, Z. Kristallogr.—New Cryst. Struct. 213 (1998) 35.
[14] E. Bouwman, W.L. Driessen, R.A.G. de Graaff, J. Reedijk, Acta
Crystallogr., Sect. C 40 (1984) 1562.
[15] S. Bieller, A. Haghiri, M. Bolte, J.W. Bats, M. Wagner, H.-W.
Lerner, Inorg. Chim. Acta 359 (2006) 1559.
(c) C. Janiak, Dalton Trans. (2003) 2781;
(d) J.L.C. Rowsell, O.M. Yaghi, Angew. Chem., Int. Ed. 44 (2005)
4670;
(e) C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15
(2005) 3559;
(f) U. Muller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J.
¨
Pastre, J. Mater. Chem. 16 (2006) 626;
(g) K.M. Thomas, Catal. Today 120 (2007) 389.
[16] (a) W.R. Thiel, M. Angstl, T. Priermeier, Chem. Ber. 127 (1994)
2373;
(b) W.R. Thiel, J. Eppinger, Chem. Eur. J. 3 (1997) 696.
[17] K. Ro¨ßler, T. Kluge, A. Schubert, Y. Sun, E. Herdtweck, W.R. Thiel,
Z. Naturforsch., Teil B 59 (2004) 1253.
[3] A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht
University, Utrecht, The Netherslands, 2007.
[4] CrysAlis Red, Oxford Diffraction Ltd., Version 1.171.31.5, 2006.
[5] G.M. Sheldrick, SHELXL97-Programs for Structure Refinement, Uni-
versity of Go¨ttingen, Go¨ttingen, Germany, 1988.
[18] (a) R.B. Helmholdt, W. Hinrichs, J. Reedijk, Acta Crystallogr., Sect.
C 43 (1987) 226;
[6] (a) A.-K. Pleier, H. Glas, M. Grosche, P. Sirsch, W.R. Thiel,
Synthesis (2001) 55;
(b) C.W. Reimann, A.D. Mighell, F.A. Mauer, Acta Crystallogr. 23
(1967) 135;
(c) I.A. Krol, V.M. Agre, V.K. Trunov, O.I. Ivanov, Koord. Khim. 5
(1979) 1575.
(b) K. Chanda, M.C. Dutta, E. Karim, J.N. Vishwakarma, J. Ind.
Chem. Soc. 81 (2004) 791;
(c) R. SanMartin, E. Martinez de Marigorta, E. Dominguez,
Tetrahedron 50 (1994) 2255;
(d) J.T. Gupton, C. Colon, C.R. Harrison, M.J. Lizzi, D.E. Polk, J.
Org. Chem. 45 (1980) 4522.
[19] Y.-J. Sun, P. Cheng, S.-P. Yan, D.-Z. Liao, Z.-H. Jiang, P.-W. Shen,
J. Mol. Struct. 597 (2001) 191.
[20] A. Bencini, C. Benelli, D. Gatteschi, C. Zanchini, Inorg. Chem. 19
(1980) 1301.
[7] (a) R. Fusco, in: R.H. Wiley (Ed.), The Chemistry of Heterocyclic
Compunds: Pyrazoles, Pyrazolines, Pyrazolidines, Indazoles and