Solvent induced reactivity of 3,5-dimethylpyrazole towards zinc (II) carboxylates

Article abstract of DOI:10.1039/b905534g

The reactions of 3,5-dimethylpyrazole with zinc(II)acetate dihydrate and varieties of aromatic carboxylic acids led to formation of mono-nuclear zinc complexes of composition [Zn(HDMP)₂(RCO₂)₂] (R = C₆H₅

Full text of DOI:10.1039/b905534g

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Solvent induced reactivity of 3,5-dimethylpyrazole towards zinc (II)
carboxylates†
Rupam Sarma, Dipjyoti Kalita and Jubaraj B. Baruah*
Received 19th March 2009, Accepted 15th June 2009
First published as an Advance Article on the web 23rd July 2009
DOI: 10.1039/b905534g
The reactions of 3,5-dimethylpyrazole with zinc(II)acetate dihydrate and varieties of aromatic
carboxylic acids led to formation of mono-nuclear zinc complexes of composition [Zn(HDMP)2-
(RCO2)2] (R = C6H5, p-CH3-C6H4, p-NO2-C6H4 etc. HDMP = 3,5-dimethylpyrazole) in methanol,
whereas the same reactants in dimethylformamide (DMF) gave binuclear 3,5-dimethylpyrazolato
bridged zinc carboxylate complexes containing monodentate 3,5-dimethylpyraozole ligands with
composition [Zn2(m-DMP)2(HDMP)2(RCO2)2]. The mononuclear complexes can be converted to the
corresponding binuclear complexes by simply dissolving in DMF. The reaction of zinc(II)acetate
dihydrate with p-nitrobenzoic acid and 3,5-dimethylpyrazole in different solvents gave solvated
mononuclear complexes of the corresponding solvent. All these solvated complexes having the core
[Zn(HDMP)2(p-NO2-C6H4CO2)2] contain two structurally independent molecules in the
asymmetric unit (Z¢ = 2).
bidentate 3,5-dimethylpyrazolato (DMP) ligand respectively that
are prepared just by changing the solvent condition.
Introduction
Ligands having pyrazole backbone are of interest in the field
of biology and co-ordination chemistry.1 The zinc complexes
containing pyrazole ligands are shown to mimic the biological
activity of carbonic anhydrase.1k,m The pyrazole as a ligand
shows versatility in co-ordination modes towards metals. While,
neutral monodentate pyrazole ligands are common,2 a number of
literatures are available on pyrazole as anionic monodentate and
bridging bidentate ligands.3 The bidentate bridging pyrazolato
ligand is a precursor for preparation of binuclear or polynuclear
Experimental
Synthesis of the complexes
Bis-3,5-dimethylpyrazole bis-benzoato zinc(2+). [Zn(C6H5-
COO)2(HDMP)2] (1):To a solution of benzoic acid (2 mmol,
0.244 g) in methanol (15 mL) Zn(OAc)2.2H2O (1 mmol, 0.219 g)
was added. After stirring this reaction mixture for about 30 min-
utes 3,5-dimethylpyrazole (2 mmol, 0.190 g) was added followed by
addition of toluene (5 mL) and stirred for another 30 minutes. The
reaction mixture was then kept for crystallization and good quality
colorless crystals of 1 were obtained after 3/4 days. Isolated yield
92%. IR (KBr, cm-1): 2927(m), 2850(m), 1614(s), 1553(s), 1481(m),
1448(m), 1385(s),1295(s), 1191(s), 1049(s), 1017(s), 834(s), 720(s),
679(s), 578(s).1H NMR (DMSO-d6, ppm): 7.9 (d, 4H J = 4 Hz),
7.5 (m, 6H), 5.7 (s, 2H), 2.1 (b, 12H). Elemental anal calcd for
C24H26N4O4Zn; C, 57.66, H, 5.24; found C, 58.08, H, 5.29.
metal complexes. Generally, in pyrazolato bridged complexes the
4
˚
metal-metal separations are in the range of 3.0–4.5 A. On the
other hand binuclear complexes having such a critical metal-
metal separation are of interest from their resemblance to the
various active sites of zinc containing enzymes.5 A good number
of literatures are available on pyrazole based ligands that act as
monodentate ligands under ambient conditions.1,3 Recently we
have shown that different types of zinc carboxylate complexes
can be prepared by varying either the solvent or substituent
on the ligands.6 Such structural changes under neutral reaction
conditions is of interest to understand the reactivity of ligand
and leaves enough scope to understand this simple yet important
chemistry in an organized manner to have variation in reactivity.
The deprotonation of N-H in amine containing ligands of zinc
complexes is of special interest for biological mimics and are
studied with great interest.7 Herein, we report the formation of
a number of mononuclear and binuclear zinc(2+) carboxylate
complexes with monodentate 3,5-dimethylpyrazole (HDMP) and
Bis-3,5-dimethylpyrazole di-l-3,5-dimethylpyrazolato bis-ben-
zoato di-zinc(2+). [Zn2(C6H5COO)2(DMP)2(HDMP)2] (2): To
a solution of benzoic acid (2 mmol, 0.244 g) in dimethylfor-
mamide (15 mL) Zn(OAc)2.2H2O (1 mmol, 0.219 g) was added.
After stirring this reaction mixture for about 30 minutes 3,5-
dimethylpyrazole (2 mmol, 0.190 g) was added and stirred for
another 30 minutes. The solution was then kept for crystallization.
Diffraction quality colorless crystals of 2 were obtained after
a week. Isolated yield 78%. IR (KBr, cm-1): 2923(m), 2853(m),
1608(s), 1552(s), 1529(s), 1483(w), 1390(s), 1345(s), 1298(s),
1191(m), 1048(s), 1014(s), 831(m), 721(s), 676(s), 572(s). 1H NMR
(DMSO-d6, ppm): 7.9 (d, 4H, J = 8 Hz), 7.4 (m, 6H), 5.7 (b,
4H), 2.2 (s, 6H), 2.1 (s, 12H), 1.7 (s, 6H). Elemental anal calcd for
C34H40N8O4Zn2; C, 54.05, H, 5.33; found C, 54.15, H, 5.33.
Department of Chemistry, Indian Institute of Technology Guwahati,
Guwahati, 781 039, Assam, India. E-mail: juba@iitg.ernet.in
† Electronic supplementary information (ESI) available: The PXRD
patterns for the complexes 1 and 2 and the UV-spectra of the complexes
3 and 6 are available. CCDC reference numbers 715018–715024. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b905534g
7428 | Dalton Trans., 2009, 7428–7436
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Aqua and ethanol solvate of bis-3,5-dimethylpyrazole bis-
p-methylbenzoato zinc(2+). [Zn(p-CH3-C6H4COO)2(HDMP)2].
H2O.CH3CH2OH (3): The complex 3 was synthesised by the same
procedure as complex 1 only difference being it’s recrystallization
solvent (ethanol). Isolated yield 87%. IR (KBr, cm-1): 3032(m),
2927(m), 2854(m), 1610(s), 1550(s), 1484(m), 1404(m), 1378(s),
1293(s), 1194(s), 1176(s), 1049(s), 1018(s), 847(s), 766(s), 624(s).
1H NMR (DMSO-d6, ppm): 7.8 (d, 4H, J = 8 Hz), 7.2 (d, 4H, J =
8 Hz), 5.8 (s, 2H), 2.3 (s, 6H), 2.1 (s, 12H). Elemental anal calcd
for C28H38N4O6Zn; C, 56.80, H, 6.47; found C, 57.00, H, 6.45.
527(s). 1H NMR (DMSO-d6, ppm): 8.1 (b, 8H), 5.8 (s, 2H), 3.2 (s,
3H), 2.1 (b, 12H). Elemental anal calcd for C25 H26 N6 O9Zn; C,
48.44, H, 4.22; found C, 48.65, H, 4.34.
The mononuclear complexes 6 and 7 were synthesised by the
same procedure as complex 5 only difference being the solvent. In
case of 6 the solvent is acetone whereas in case of 7 the solvent
used is dimethylformamide.
X-ray crystallography
The X-ray crystallographic data were collected at 296 K with Mo
Ka radiation (l = 0.71073 A◦) using a Bruker Nonius SMART
CCD diffractometer equipped with a graphite monochromator.
The SMART software was used for data collection and also for
indexing the reflections and determining the unit cell parameters;
the collected data were integrated using SAINT software.8 The
structures were solved by direct methods and refined by full-
matrix least-squares calculations using SHELXTL software.8 All
the non-H atoms except C49 and C50 of complex 5 were refined
in the anisotropic approximation against F2 of all reflections.
The C49 and C50 of complex 5 are refined isotropically. The
H-atoms, except those attached to N and O were placed at their
calculated positions and refined in the isotropic approximation;
those attached to hetero-atoms (N, O) were located in the
difference Fourier maps, and refined with isotropic displacement
coefficients. However, some of the hydrogen atoms attached to
solvent molecule could not be located for the structures 5 and 7.
The crystallographic parameters are given in Table 1.
Bis-3,5-dimethylpyrazole di-l-3,5-dimethylpyrazolato bis-p-
mehylbenzoato di-zinc(2+). [Zn2(p-CH3-C6H4COO)2(DMP)2-
(HDMP)2] (4): The complex 4 was synthesised by the same
procedure as complex 2. Isolated yield 84%. IR (KBr, cm-1):
2925(m), 2853(m), 1608(s), 1583(s), 1548(s), 1527(m), 1483(w),
1394(s), 1342(s), 1303(s), 1193(m), 1048(s), 1012(s), 846(m),
1
768(s), 694(m), 622(s). H NMR (DMSO-d6, ppm): 7.8 (b, 4H),
7.2 (b, 6H), 5.7 (b, 4H), 2.3 (s, 6H), 2.18 (s, 6H), 2.11 (s, 12H),
1.7 (s, 6H). Elemental anal calcd for C36H44N8O4Zn2; C, 55.18, H,
5.66; found C, 55.32, H, 5.75.
Methanol solvate of bis-3,5-dimethylpyrozle bis-p-nitrobenzoato
zinc(2+). [Zn(p-NO2-C6H4COO)2(HDMP)2].CH3OH (5): The
complex 5 was also synthesised by the same procedure as complex
1. Isolated yield 82%. IR (KBr, cm-1): 2960(m), 2926(m), 2854(m),
1626(s), 1592(s), 1563(s), 1525(m), 1476(w), 1385(s), 1342(s),
1319(w), 1284(m), 1190(s), 1158(s), 1046(s), 821(s), 726(s), 581(s),
Table 1 The crystallographic parameters of complexes 1–7†
Compound No.
1
2
3
4
5
6
7
CCDC No.
Formulae
715018
715019
715020
715021
715022
715023
715024
C24 H26 N4 O4 Zn C34 H40 N8 O4 Zn2 C28 H38 N4 O6 Zn C36 H44 N8 O4 Zn2 C25 H28N6 O9 Zn C24 H24 N6 O9 Zn C51 H56 N13O18 Zn2
499.86
296 K
Triclinic
P-1
8.7460(3)
12.0489(4)
13.0519(4)
96.598(2)
103.186(2)
108.004(2)
1247.78(7)
2
Formula. wt.
Temperature
Crystal system
Space group
755.48
296 K
591.99
296 K
783.53
296 K
621.90
296 K
Triclinic
P-1
12.5156(11)
13.3426(13)
19.6335(19)
102.918(4)
90.692(4)
113.340(3)
2915.5(5)
4
605.86
296 K
1269.83
296 K
Triclinic
P-1
12.7147(12)
14.2450(14)
17.2331(14)
76.935(4)
82.892(6)
84.316(6)
3008.9(5)
2
Monoclinic
P2(1)/c
8.2047(11)
16.175(2)
14.222(2)
90.00
105.266(7)
90.00
1820.9(4)
2
Monoclinic
P2(1)
8.1158(3)
8.3981(3)
22.7637(8)
90.00
95.686(2)
90.00
1543.88(10)
2
Orthorhombic
Fdd2
16.8246(8)
33.419(2)
14.0874(8)
90.00
90.00
90.00
7920.7(8)
8
1.314
Monoclinic
P2(1)/c
12.1934(6)
19.9578(14)
23.6281(16)
90.00
93.779(2)
90.00
5737.5(6)
8
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g/◦
V/A
Z
3
˚
Density/mg m-3
Abs. Coeff./mm-1
F(000)
1.330
1.020
520
1.378
1.364
784
1.273
0.839
624
1.417
0.902
1288
1.403
0.915
2496
1.402
0.876
1314
1.257
3264
Total no. of reflections 6812
Reflections, I > 2s(I) 4202
21213
4513
28.34
-9 ≤ h ≤ 10
-20 ≤ k ≤ 21
-18 ≤ l ≤ 17
99.4
16951
5176
25.50
-9 ≤ h ≤ 9
-9 ≤ k ≤ 10
-27 ≤ l ≤ 27
98.7
-0.002(12)
5176/1/367
15854
4836
28.43
-22 ≤ h ≤ 21
-28 ≤ k ≤ 44
-18 ≤ l ≤ 18
99.1
0.00
4836/0/233
27296
12913
27.38
-16 ≤ h ≤ 6
-16 ≤ k ≤ 17
-25 ≤ l ≤ 25
97.6
57413
14065
28.27
-6 ≤ h ≤ 16
-26 ≤ k ≤ 26
-31 ≤ l ≤ 31
98.8
11927
3797
17.52
-10 ≤ h ≤ 10
-12 ≤ k ≤ 12
-14 ≤ l ≤ 14
99.3
Max. q/◦
24.99
Ranges (h, k, l)
-10 ≤ h ≤ 10
-14 ≤ k ≤ 6
-14 ≤ l ≤ 15
Complete to 2q (%) 95.4
Flack parameter
Data/Restraints/
Parameters
—
—
—
—
—
4202/0/300
4513/0/221
12913/0/729
14065/0/729
3797/18/767
Goof (F2)
1.000
1.017
1.051
1.023
1.242
0.955
1.081
R indices [I > 2s(I)] 0.0330
0.0337
0.0661
0.0960
0.0345
0.0355
0.0469
0.0957
0.0300
0.0328
0.0475
0.0873
0.0278
0.0785
0.1502
0.2505
0.0475
0.0515
0.1514
0.1607
0.0495
0.0872
0.1025
0.2604
0.0446
R indices (all data)
wR2 (for all data)
R(int)
0.0428
0.0835
0.0150
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7428–7436 | 7429
©

View Article Online
Scheme 1 Formation of mono and dinuclear complexes from two different solvents.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) in 1 and 2
Results and discussion
A
series of zinc(2+) carboxylate complexes with 3,5-
Complex 1
Complex 2
dimethylpyrazole are prepared in one pot reactions using dif-
ferent solvents (Scheme 1). Either dinuclear or mononuclear
complexes are formed depending on the solvent used. The reaction
between zinc (2+) acetate dihydrate, aromatic carboxylic acid
and 3,5-dimethylpyrazole in a mixed solvent of methanol :
toluene (3 : 1) leads to the formation of mononuclear complexes
having composition [Zn(RCOO)2(HDMP)2] (R = C6H5, p-NO2-
C6H4, p-CH3-C6H4 etc. HDMP = 3,5-dimethylpyrazole). The
same reaction when carried out in dimethylformamide solvent
leads to the formation of binuclear complexes having com-
position [Zn2(RCOO)2(DMP)2(HDMP)2] (where DMP = 3,5-
dimethylpyrazolato anion). On the other hand, the mononuclear
complexes get transformed to the binuclear complexes on dis-
solution in dimethylformamide. The reactions are illustrated in
Scheme 1. All the complexes are characterized by various spectro-
scopic techniques and with single crystal X-ray crystallography.
The zinc complex prepared from benzoic acid in presence
of 3,5-dimethylpyrazole from a mixed solvent of methanol :
toluene (3 : 1) is a mononuclear complex having composition
[Zn(C6H5COO)2(HDMP)2] 1. The complex 1 has a near tetrahe-
dral geometry around the metal centre with both the HDMP unit
in cis-orientation to each other. It crystallizes in the triclinic P-1
space group. The crystal structure of 1 is shown in Fig. 1a. The
H-atom of the HDMP remains involved in an intra-molecular
H-bonding with the carboxylato-O of the nearest carboxylate
group. Apart from this interaction, there exist an intermolecular
C-H ◊ ◊ ◊ O interaction between the H12 of phenyl group and the
Zn1-O3
Zn1-O1
Zn1-N1
Zn1-N3
O3-Zn1-O1
O3-Zn1-N1
O1-Zn1-N1
O3-Zn1-N3
O1-Zn1-N3
N1-Zn1-N3
1.9325(17)
1.9380(17)
2.003(2)
2.011(2)
102.45(8)
111.36(8)
114.59(8)
113.87(8)
106.87(8)
107.72(8)
O1-Zn1
N1-Zn1
N3-Zn1
N4-Zn1
O1-Zn1-N4
O1-Zn1-N3
O1-Zn1-N1
N3-Zn1-N1
N4-Zn1-N1
N4-Zn1-N3
1.9474(15)
2.0205(18)
1.9837(17)
1.9768(18)
107.81(8)
107.98(7)
109.60(7)
108.20(7)
111.00(8)
112.18(7)
intra-molecular H-bonding (as in 1) between the N-H atom and
carboxylato-O. The structure of 2 is shown in Fig. 2. Some of the
selected bond lengths and bond angles are tabulated in Table 2.
To check the phase purity of the complexes 1 and 2 we have also
recorded the PXRD patterns which are found to be matched to
a good extent with the simulated patterns (please refer to the
supplementary materials†).
The complex obtained from the reaction of zinc (II) acetate
dihydrate with p-methylbenzoic acid and 3,5-dimethylpyrazole
in methanol : toluene (3 : 1) on crystallization from ethanol
gives a solvated mononuclear complex 3 having the composition
[Zn(p-CH3-C6H4COO)2(HDMP)2].H2O.CH3CH2OH. The com-
plex crystallizes in the monoclinic space group P21. The solvent wa-
ter molecule remains hydrogen bonded to one of the carboxylato-O
atoms of the complex as well as to the ethanol molecule through
O5-H ◊ ◊ ◊ O4, O5-H ◊ ◊ ◊ O6 and O6-H ◊ ◊ ◊ O5 interactions. Similar
to the molecule 1, here also the HDMP molecules are arranged
in a cis orientation relative to each other and the H-atom of
the HDMP remains involved in an intra-molecular H-bonding
with the carboxylato-O of the nearest carboxylate group. Thus
the O4 remains H-bonded through two different interactions, one
intramolecular N4-H ◊ ◊ ◊ O4 and the other an intermolecular O5-
H ◊ ◊ ◊ O4 interaction. This intermolecular interaction along with
˚
O2 carboxylato oxygen (dD-H ◊ ◊ ◊ A (A), C12-H12 ◊ ◊ ◊ O2, 2.59 and
<D-H ◊ ◊ ◊ A (◦), < C12-H12 ◊ ◊ ◊ O2, 129) thereby leading to a one
dimensional H-bonded chain structure. This is shown in Fig. 1b.
The same reaction in dimethylformamide results in the
formation of the binuclear species
2 having composition
[Zn2(C6H5COO)2(DMP)2(HDMP)2]. In this case also the two zinc
centers adopt a near tetrahedral geometry. The pair of HDMP as
well as the pair of benzoato ligands across the metal centers is in
trans geometry to each other. The bridging dimethylpyrazolate
anions and the two-zinc centers adopt a planar arrangement,
which generates a hexagonal geometry at the core. The complex 2
crystallizes in the monoclinic space group P2(1)/c. It exhibits the
˚
the other O-H ◊ ◊ ◊ O interactions (dD-H ◊ ◊ ◊ A (A), O6-H6 ◊ ◊ ◊ O5, 1.92;
O5-H5 ◊ ◊ ◊ O6, 2.05; O5-H5 ◊ ◊ ◊ O4, 1.98 and <D-H ◊ ◊ ◊ A (◦), < O6-
H6 ◊ ◊ ◊ O5, 167; < O5-H5 ◊ ◊ ◊ O6, 176; <O5-H5 ◊ ◊ ◊ O4, 154) imparts
the molecule a one dimensional H-bonded structure with a solvent
channel in between two layers of the assembled mononuclear
7430 | Dalton Trans., 2009, 7428–7436
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 1 (a) The mononuclear complex [Zn(C6H5COO)2(HDMP)2] (1), (b) the 1-D H-bonded chain of 1.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) in 3 and 4
Complex 3
Complex 4
O1-Zn1
O3-Zn1
1.949(3)
1.940(2)
O1-Zn1
N1-Zn1
1.9463(17)
2.013(2)
N1-Zn1
N3-Zn1
2.005(3)
2.015(3)
N3-Zn1
N4-Zn1
1.981(2)
1.986(2)
O3-Zn1-O1
O3-Zn1-N1
O1-Zn1-N1
O3-Zn1-N3
O1-Zn1-N3
N1-Zn1-N3
105.97(12)
107.51(12)
113.10(11)
117.06(11)
104.79(13)
108.55(12)
O1-Zn1-N3
O1-Zn1-N4
O1-Zn1-N1
N3-Zn1-N1
N4-Zn1-N1
N3-Zn1-N4
107.64(13)
107.62(14)
110.46(8)
107.45(11)
111.20(11)
112.43(7)
Fig. 2 The binuclear complex [Zn2(C6H5COO)2(DMP)2(HDMP)2] (2).
of the selected bond lengths and bond angles are tabulated in
Table 3.
moiety. The crystal structure of 3 is shown in Fig. 3a along with
its one-dimensional structure in Fig. 3b.
The formation of the two different types of products viz.
mononuclear and binuclear is confirmed by recording their
The reaction of zinc (II) acetate dihydrate with p-methylbenzoic
acid in presence of 3,5-dimethylpyrazole in dimethylformamide
leads to the binuclear species 4 having composition [Zn2(p-CH3-
C6H4COO)2(DMP)2(HDMP)2]. The complex 4 crystallizes in the
orthorhombic space group Fdd2. Like the structure 2, in this
complex (4) also each of the metal centers have tetrahedral
coordination environment. However, unlike in 2, in the complex 4
the 3,5-dimethylpyrazolato bridged core does not adopt a planar
hexagonal geometry because of the steric congestion between the
methyl groups of the pyrazolato anions and that of the p-toluic
acid of an adjacent binuclear unit. This is shown in Fig. 4b. Some
1
1H-NMR spectra. As a representative case the H-NMR spectra
of the complexes 3 and 4 are shown in Fig. 5. From the figure it is
amply clear that the mononuclear complex 3 has only two peaks
in the aliphatic region of proton NMR. The signals for one set
of methyl groups from 3,5-dimethylpyrazole and as well as from
methyl groups of p-methyl benzoate groups appears together at
2.14 ppm, while the peak at 2.34 ppm appears due to the other
set of methyl protons of 3,5-dimethylpyrazole ligands. The proton
NMR spectrum for the binuclear complex 4 has a total of four
peaks inthealiphaticregion;ofwhich, threedifferentpeaksat1.76,
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7428–7436 | 7431
©

View Article Online
Fig. 3 (a) The solvated mononuclear complex [Zn(p-CH3-C6H4COO)2(HDMP)2].H2O.CH3CH2OH (3), (b) the 1-D H-bonded chain of 3 showing the
solvent channel.
2.11 and 2.33 ppm are due to the three sets of methyl protons of 3,5-
dimethylpyrazole and 3,5-dimethylpyrazolate ligands. Another
peak at 2.18 ppm is due to the methyl groups of the p-methyl
benzoate groups. Moreover, the integration and nature of the
independent peaks of two complexes in 1H-NMR spectra reflects
the purity of both the complexes.
The formation of mononuclear and binuclear complexes from
different solvents clearly indicates a role of solvent in their for-
mation. Based on the observation of conversion of mononuclear
complexes to dinuclear complexes in dimethylformamide it may
be suggested that during the transformation of the mononuclear
complexes to the binuclear one, the solvent molecule gets coor-
dinated to the metal center, thereby leading to a five coordinated
species. We have recently shown that such a penta-coordinated
complex of zinc(II) carboxylate complex can be prepared with
appropriate ancillary ligands.9 Since, the 3,5-dimethylpyrazole
N-H in the mononuclear complexes are intra-molecularly
H-bonded in the mononuclear complexes, the acidity of the
N-H is enhanced and this also facilitates the deprotonation process
during formation of the bridged structures. The formation of
such five coordinated species helps the mononuclear complex to
lose one molecule of carboxylic acid resulting in the formation
7432 | Dalton Trans., 2009, 7428–7436
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 4 (a) The binuclear complex [Zn2(p-CH3-C6H4COO)2(DMP)2(HDMP)2] (4), (b) the steric congestion between methyl group (demarcated by circles)
leading to the loss of planarity of the hexagonal core.
of the binuclear complex. Further support to this result comes
from the fact that the molar conductance of the mononuclear
and the binuclear complex in two different solvents viz. methanol
and dimethylformamide are found to be different. The molar
conductance of complex 1 is 229 S cm2 mol-1 in methanol
and 107 S cm2 mol-1 in dimethylformamide. For complex 2,
the molar conductance in dimethylformamide is found to be
104 S cm2 mol-1. This suggests that on dissolution of complex
1 in dimethylformamide it readily transforms to the binuclear
complex. On the basis of these observations a probable pathway
for the transformation is proposed as shown in Scheme 2.
The role of solvent in the transformation of complex 1 to
2 has been followed spectrophotometrically by observing the
spectral changes that take place upon incremental addition of
dimethylformamide to a methanolic solution (10-3 M) of complex
1 (Fig. 6). It has been observed that the complex 1 has absorption
at around 270 nm with a shoulder at 277 nm due to the
3,5-dimethylpyrazole group. On addition of dimethylformamide
this absorption peak undergoes a hypsochromic shift and the
successive absorption curves pass though an isosbestic point at
264 nm. The presence of this isosbestic point clearly supports
the presence of the two forms 1 and 2 in equilibrium. A similar
observation is found in the case of the transformation of complex
3 to 4 (please refer to the supplementary materials†).
Interestingly the binuclear complex is not formed in the case
of p-nitrobenzoic acid. When zinc (II) acetate is reacted with
p-nitrobenzoic acid and 3,5-dimethylpyrazole in methanol :
toluene (3 : 1) solvent mixture, it leads to the formation of
mononuclear complex 5 co-crystallizing with methanol. The
complex crystallizes in a triclinic P-1 space group and it contains
two structurally independent molecules in the asymmetric unit
(Z¢ = 2). Each of the metal centers in 5 is tetrahedrally coordinated
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7428–7436 | 7433
©

View Article Online
Fig. 5 1H-NMR spectra (DMSO-d6) of the complexes 3 and 4 (signals from solvent are marked with *).
Scheme 2 A possible path for transformation of mono-nuclear zinc complex to di-nuclear zinc complex through ligand dissociation.
to two p-nitrobenzoate and two 3,5-dimethylpyrazole ligands. Like
the structures 1 and 3 here also the HDMP ligands are in cis-
orientation to each other and the H-atom of the HDMP remains
involved in an intra-molecular H-bonding with the carboxylato-
O of the nearest carboxylate group. The C-H ◊ ◊ ◊ O interactions
stabilize the assemblies of the symmetry non-equivalent molecules
in the lattice as illustrated in Fig. 7. These short range interactions
results in a chain like self assembly of the molecule growing along
the ‘a’ crystallographic axis. The structure of 5 is shown in Fig. 7a
below along with the self assembly formation in Fig. 7b.
It is interesting to note here that the symmetry non-equivalence
is independent of the solvent, as in other solvents also the
cocrystals have similar structure with Z¢ = 2. The same reaction
independently carried out in acetone or dimethylformamide
gives the pseudo-polymorphs with inclusion of water [Zn(p-NO2-
C6H4COO)2(HDMP)2].H2O (6) or dimethylformamide [Zn(p-
Fig. 6 Ultraviolet absorption spectra of complex 1 in methanol (10-3 M)
upon addition of dimethylformamide (100 ml per aliquot).
NO2-C6H4COO)2(HDMP)2].3H2O.DMF (7) respectively (Fig. 8).
We have recorded the UV- spectrum of the complex 6 in
7434 | Dalton Trans., 2009, 7428–7436
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Fig. 7 (a) The asymmetric unit of mononuclear complex 5, (b) crystal packing in 5 showing the symmetry non-equivalence Some important we◦ak
˚
interactions are (dD-H ◊ ◊ ◊ A (A), C13-H13 ◊ ◊ ◊ O14, 2.65; C28-H28 ◊ ◊ ◊ O12, 2.56; C35-H35 ◊ ◊ ◊ O10, 2.46; C6-H6 ◊ ◊ ◊ O16, 2.60 and with angle <D-H ◊ ◊ ◊ A ( ),
< C13-H13 ◊ ◊ ◊ O14, 127; < C28-H28 ◊ ◊ ◊ O12, 130.2; < C35-H35 ◊ ◊ ◊ O10, 132; < C6-H6 ◊ ◊ ◊ O16, 132).
Fig. 8 The asymmetric unit of mononuclear complex (a) 6, (b) 7.
methanol (10-3 M) and followed the change in the spectrum
on addition of dimethylformamide. In the case of 6 a distinct
Conclusions
In literature it is reported that the 3,5-dimethylpyrazolato bridged
zinc (2+) complexes can be prepared from metallic zinc using
acetone as solvent;10a however the difference of these complexes
from our system is that those complexes are devoid of carboxylate
ligands. It is worthwhile to note that the carboxylato bridged
zinc (2+) complexes having monodentate 3,5-dimethylpyrazole
are also known.10b In contrast, we have shown examples
of 3,5-dimethylpyrazolato bridged carboxylate complexes. Our
isosbestic point is not found, and in the case of 7 also the
changes are not distinct which can be attributed to the formation
of a solvate in solution. Based on the above observations it
can be inferred that in the case of p-nitrobenzoic acid, it is
the solvent inclusion mononuclear complexes that are favored
probably due to supramolecular interactions that are implicated in
solid state solvated species; which prevents isolation of a dinuclear
complex.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7428–7436 | 7435
©

View Article Online
results add an extra dimension to the existing compounds by
understanding them structurally. Understanding the transforma-
tion of mononuclear to dinuclear complexes through loss of
carboxylate groups provides avenues for exploring them under
neutral conditions. In conclusion a number of mononuclear and
binuclear complexes 1–7 are prepared under similar reaction
condition by varying the solvents. Conversion of mononuclear
to binuclear complexes by solvent is established in zinc(2+) 3,5-
dimethylpyrazole complexes.
N. V. Podberezskaya and L. G. Lavrenova, Zh. Strukt. Khim., 1995,
36, 157; (h) B. Barszcz, T. Głowiak, J. Jezierska and A. Tomkiewicz,
Polyhedron, 2004, 23, 1309; (i) J. L. Shaw, T. B. Cardon, G. A. Lorigan
and C. J. Ziegler, Eur. J. Inorg. Chem., 2004, 1073; (j) N. Zhao and
D. M. Eichhorn, Acta Crystallogr. Sec. E., 2005, 61, m822.
3 (a) G. A. Ardizzoia, S. Brenna, G. LaMonica, A. Maspero, N.
Masciocchi and M. Moret, Inorg. Chem., 2002, 41, 610; (b) A.
Cingolani, S. Galli, N. Masciocchi, L. Pandolfo, C. Pettinari and A.
Sironi, Dalton Trans., 2006, 2479; (c) G. A. Ardizzoia, S. Brenna, G.
La Monica, A. Maspero, N. Masciocchi and M. Moret, Inorg. Chem.,
2002, 41, 610; (d) L. Siegfried, T. A. Kaden, F. Meyer, P. Kircher and
H. Pritzkow, J. Chem. Soc., Dalton Trans., 2001, 2310; (e) Y. Zhou and
W. Chen, Dalton Trans., 2007, 5123; (f) D. J. de Geest, A. Noble, B.
Moubaraki, K. S. Murray, D. S. Larsen and S. Brooker, Dalton Trans.,
2007, 467; (g) E. Spodine, A. M. Atria, J. Valenzuela, J. Jalocha, J.
Manzur, A. M. Garc´ıa, M. T. Garland, O. Pen˜a and J. Y. Saillard,
J. Chem. Soc., Dalton Trans., 1999, 3029; (h) A. X. Zhu, J. P. Zhang,
Y. Y. Lin and X. M. Chen, Inorg. Chem., 2008, 47, 7389; (i) M. Casarin,
C. Corvaja, C. di Nicola, D. Falcomer, L. Franco, M. Monari, L.
Pandolfo, C. Pettinari, F. Piccinelli and P. Tagliatesta, Inorg. Chem.,
2004, 43, 5865; (j) M. Casarin, C. Corvaja, C. Di Nicola, D. Falcomer,
L. Franco, M. Monari, L. Pandolfo, C. Pettinari and F. Piccinelli, Inorg.
Chem., 2005, 44, 6265.
Acknowledgements
The authors thank the Department of Science and Technology,
New-Delhi for financial assistance. The authors R.S. and D.K.
thank the Council of Scientific and Industrial Research, New
Delhi, India for the Junior Research Fellowship.
References
4 H. Mu¨ller, B. Bauer-Siebenlist, E. Csapo, S. Dechert, E. Farkas and F.
Meyer, Inorg. Chem., 2008, 47, 5278.
1 (a) S. Trofimenko, Prog. Inorg. Chem., 1986, 34, 115; (b) G. A. Ardizzoia,
G. La Monica, S. Cenini, M. Moret and N. Masciocchi, J. Chem.
Soc., Dalton Trans., 1996, 1351; (c) R. Mondal, M. K. Bhunia and K.
Dhara, CrystEngComm, 2008, 10, 1167; (d) M. Seredyuk, M. Haukka,
I. O. Fritsky, H. Kozłowski, R. Kra¨mer, V. A. Pavlenko and Philipp
Gu¨tlich, Dalton Trans., 2007, 3183; (e) J. Klingele, A. I. Prikhod’ko,
G. Leibeling, S. Demeshko, S. Dechert and F. Meyer, Dalton Trans.,
2007, 2003; (f) J. X. Zhang, J. He, Y. G. Yin, M. H. Hu, D. Li and X. C.
Huang, Inorg. Chem., 2008, 47, 3471; (g) H.-X. Li, H.-Z. Wu, W.-H.
Zhang, Z.-G. Ren, Y. Zhang and J.-P. Lang, Chem. Commun., 2007,
5052; (h) V. Chandrasekhar;, R. Boomishankara;, P. Sasikumara, L.
Nagarajan; and A. C. Cordes, Z. Anorg. Allg. Chem., 2005, 631, 2727;
(i) M. K. Das and S. Ghosh, Indian J. Chem., 1997, 36A, 324; (j) C.
Ghose and S. N. Poddar, Indian J. Chem., 1977, 15, 248; (k) R. Walz, M.
Ruf and H. Vahrenkamp, Eur. J. Inorg. Chem., 2001, 139; (l) A. Looney,
R. Han, K. McNeill and G. Parkin, J. Am. Chem. Soc., 1993, 115, 4690;
(m) W. N. Lipscomb and N. Strater, Chem. Rev., 1996, 96, 2375; (n) N.
Strater, W. N. Lipscomb, T. Klabunde and B. Crebs, Angew. Chem., Int.
Ed. Engl., 1996, 35, 2024; (o) H. Steinhagen and G. Helmchem, Angew.
Chem., Int. Ed. Engl., 1996, 35, 2339.
2 (a) S. Sharma, N. Barooah and J. B. Baruah, J. Mol. Catal., 2005, 229,
171; (b) N. Barooah, S. Sharma, B. C. Sarma and J. B. Baruah, Appl.
Organomet. Chem., 2004, 18, 440; (c) G. A. van Albada, M. G. van der
Horst, I. Mutikainen, U. Turpeinen and J. Reedijk, Inorg. Chim. Acta,
2008, 361, 3380; (d) X.-Y. Chen, W.-Z. Shen, P. Cheng, S.-P. Yan, Z.-Z.
Liao and Z.-H. Jiang, Z. Anorg. Allg. Chem., 2003, 629, 697; (e) N. V.
Pervukhina, N. V. Podberezskaya, L. G. Lavrenova, S. V. Larionov and
V. V. Bakakin, J. Struct. Chem., 1986, 27, 434; (f) M. Kapon and G. M.
Reisner, Acta Crystallogr. Sec. C., 1990, 46, 349; (g) N. V. Pervukhina,
5 (a) A. Prokofieva, A. I. Prikhod’ko, E. A. Enyedy, E. Farkas, W.
Maringgele, S. Demeshko, S. Dechert and F. Meyer, Inorg. Chem.,
2007, 46, 4298; (b) C. G. Dealwis, L. Chen, C. Brennan, W. Mandecki
and C. AbadZapetero, Protein Eng., 1995, 8, 865; (c) Y. Zhang, J. Y.
Liang, H. Huang, H. Ke and W. N. Lipscomb, Biochemistry, 1993, 32,
7844; (d) S. K. Burley, P. R. David, A. Taylor and W. N. Lipscomb,
Proc. Natl. Acad. Sci. USA, 1990, 87, 6878; (e) S. Roderick and B. W.
Mathews, Biochemistry, 1993, 32, 3907.
6 (a) A. M. Baruah, A. Karmakar and J. B. Baruah, Inorg. Chim. Acta,
2008, 361, 2777; (b) A. Karmakar and J. B. Baruah, Polyhedron, 2008,
27, 3409.
7 (a) B. Dutta, P. Bag, U. Florke and K. Nag, Inorg. Chem., 2005, 44, 147;
(b) J. Alarcon, M. T. Albelda, R. Belda, J. C. FrAlarcon, J. Clares, M. P.
Paz, M., E. Delgado-Pinar, J. C. Frias, E. Gracia-Espana, J. Ganzalez
and C. Soriano, Dalton Trans., 2008, 6538; (c) N. I. Jakab, A. Jancso, T.
Gajda, B. Gyurcsik and A. Rockenbauer, J. Inorg. Biochem., 2008, 102,
1438; (d) K. Osz, K. Varnagy, H. Suli-Vargha, D. Sanna, G. Micera
and I. Sovago, Dalton Trans., 2003, 2009; (e) A. Jancso, I. Torok, L.
Korecz, A. Rockenbauer and T. Gajda, J. Chem. Soc., Dalton Trans.,
2002, 2601; (f) H. R. Jimenez, M. Julve and J. Faus, J. Chem. Soc.,
Dalton Trans., 1991, 1945.
8 G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112.
9 A. M. Baruah, R. Sarma and J. B. Baruah, Inorg. Chem. Commun.,
2008, 11, 121.
10 (a) M. K. Ehlert, S. J. Rettig, A. Storr, R. C. Thompson and J. Trotter,
Can. J. Chem., 1990, 68, 1494; (b) T. O. Denisova, E. V. Amel’chenkova,
I. S. Kislina, N. B. Librovich and S. E. Nefedov, Russ. J. Inorg. Chem.,
2006, 51, 1755.
7436 | Dalton Trans., 2009, 7428–7436
This journal is
The Royal Society of Chemistry 2009
©