Synthesis and characterization of zinc benzoate complexes through combined solid and solution phase reactions

Article abstract of DOI:10.1016/j.poly.2008.07.036

A combined solid and solution phase methodology for the synthesis of a series of mononuclear and polynuclear zinc benzoate complexes is described. The substituent on the aromatic ring and the effect of solvent on deciding the composition of the complexes is presented. From the 4-substituted benzoic acids 4-methylbenzoic acid (ptolH), 4-nitrobenzoic acid (pnitrobenH) and 4-chlorobenzoic acid (pchlorbenH), the mononuclear complexes [Zn(ptol)₂(H₂O)₂], [Zn(pnitroben)₂(H₂O)(DMSO)₂] and [Zn(pchlorben)₂py)₂] (where DMSO = dimethylsulfoxide, py = pyridine) have been synthesized and structurally characterised. Zinc complexes from the reaction of zinc sulfate heptahydrate with 3-methylbenzoic acid (mtolH) and 2-methylbenzoic acid (otolH), the dinuclear complexes [Zn₂(μ²-mtol)₄(py)₂], [Zn₂(μ²-otol)₄(py)₂], pentanuclear complex [Zn₅(μ²-mtol)₆(mtol)₂(μ³-OH)₂ (py)₂] and tetranuclear complex [Zn₄(μ²-otol)₆(μ⁴-O) (DMSO)₂], have been prepared by varying the reaction conditions and the complexes have been structurally characterized.

Full text of DOI:10.1016/j.poly.2008.07.036

Polyhedron 27 (2008) 3409–3416
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of zinc benzoate complexes through combined
solid and solution phase reactions
*
Anirban Karmakar, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781 039 Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A combined solid and solution phase methodology for the synthesis of a series of mononuclear and
polynuclear zinc benzoate complexes is described. The substituent on the aromatic ring and the effect
of solvent on deciding the composition of the complexes is presented. From the 4-substituted benzoic
acids 4-methylbenzoic acid (ptolH), 4-nitrobenzoic acid (pnitrobenH) and 4-chlorobenzoic acid (pchlor-
benH), the mononuclear complexes [Zn(ptol)2(H2O)2], [Zn(pnitroben)2(H2O)(DMSO)2] and [Zn(pchlor-
ben)2py)2] (where DMSO = dimethylsulfoxide, py = pyridine) have been synthesized and structurally
characterised. Zinc complexes from the reaction of zinc sulfate heptahydrate with 3-methylbenzoic acid
Received 24 May 2008
Accepted 25 July 2008
Available online 15 September 2008
Keywords:
Zinc benzoate complexes
Polynuclear complexes
Substituent effect
Solvent effect
(mtolH) and 2-methylbenzoic acid (otolH), the dinuclear complexes [Zn2(
otol)4(py)2], pentanuclear complex [Zn5(
2-mtol)6(mtol)2( 3-OH)2 (py)2] and tetranuclear complex
[Zn4(
2-otol)6( 4-O) (DMSO)2], have been prepared by varying the reaction conditions and the complexes
have been structurally characterized.
l -
2-mtol)4(py)2], [Zn2(l2
l
l
l
l
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
2.1. Preparation of [Zn(ptol)2(H2O)2] (1)
finely ground mixture of 4-methylbenzoic acid (ptolH)
(0.272 g, 2 mmol), zinc sulfate heptahydrate (0.287 g, 1 mmol)
and sodium hydroxide (0.08 g, 2 mmol) was heated at 100 °C
for 15 min. Dimethylsulfoxide: water (4:1) was added to the
mixture and stirred at room temperature. The colourless solution
was left undisturbed to obtain colourless crystals after 14 days.
Yield: 38% (based on Zn). Elemental Anal. Calc. for C16H18O8Zn:
C, 53.08; H, 4.84. Found C, 53.16; H, 4.82%. FT-IR (KBr, cmꢀ1):
3430 (br s), 3036 (w), 1621 (s), 1608 (s), 1566 (s), 1450 (m),
1398 (s), 1374 (s), 1354 (s), 1218 (w), 1164 (w), 1085 (m),
1014 (s), 846 (s), 771 (s), 694 (s), 572 (m), 521 (m). 1H NMR
(DMSO-d6, 400 MHz): 7.84 (2H, d, J = 8 Hz), 7.20 (2H, d,
J = 8 Hz), 2.34 (3H, s).
Zinc carboxylate complexes are important for their relevance to
biological systems [1]. There are a good number of multinuclear
clusters of zinc with large structural variations and their reactivity
also varies [2]. Multinuclear zinc carboxylate complexes are also
effective catalysts for epoxide copolymerization reactions [3].
While studying the synthesis and reactivity of zinc complexes, it
was observed that several multinuclear complexes undergo trans-
formations to mononuclear species and cause catalytic reactions
[3a]. The reactivity of the zinc carboxylate complexes are also
greatly influenced by substituents [3b–d]. We have recently re-
ported that dinuclear and trinuclear zinc benzoate complexes can
be prepared under varying reaction conditions [4]. It is very impor-
tant to understand how subtle effects from the substituents and
reaction conditions can change the structures of zinc complexes.
Thus, there is scope to understand the formation of different zinc
carboxylate complexes with different substituents and under dif-
ferent reaction conditions. In this paper we describe the substitu-
ent and solvent effect on the synthesis of mono, di and
multinuclear zinc carboxylate complexes.
A
2.2. Preparation of [Zn(pnitroben)2(H2O)(DMSO)2] (2)
A similar reaction as described above with 4-nitrobenzoic acid
(pnitrobenH) led to colourless plate-type crystals after 18 days.
Yield: 34% (based on Zn). Elemental Anal. Calc. for
C18H22N2O11S2Zn: C, 37.78; H, 3.85. Found C, 37.80; H, 3.89%. FT-
IR (KBr, cmꢀ1): 3368 (br s), 1617 (m), 1578 (s), 1516 (s), 1407 (s),
1388 (s), 1345 (s), 1319 (m), 1010 (s), 955 (m), 874 (w), 835 (m),
797 (m), 723 (s), 604 (w). 1H NMR (DMSO-d6, 400 MHz): 8.27
(2H, d, J = 8.8 Hz), 8.16 (2H, d, J = 8.8 Hz), 2.54 (6H, s).
* Corresponding author. Tel.: +91 361 2582311; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.07.036

3410
A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
2.3. Preparation of [Zn(pchlorben)2(py)2] (3)
into a round bottom flask and dimethylsulfoxide: water (4:1)
was added to it. The heterogeneous mixture was stirred at room
temp for 5 min followed by addition of pyridine (0.079 g, 1 mmol).
The colourless liquid was filtered and was left undisturbed. Colour-
less crystals were collected after 15 days and dried in air. Yield:
34% (based on Zn). Elemental Anal. Calc. for C74H68N2O18Zn5: C,
55.49; H, 4.25. Found C, 55.52; H, 4.28%. FT-IR (KBr, cmꢀ1): 3432
(br s), 2919 (m), 1623 (s), 1601 (s), 1578 (s), 1531 (s), 1428 (s),
1404 (s), 1289 (w), 1229 (w), 1088 (m), 787 (s), 764 (s), 673 (m),
589 (m), 492 (s). 1H NMR (DMSO-d6, 400 MHz): 7.76 (4H, s), 7.73
(5H, m), 7.30 (6H, m) 2.38 (12H, s).
Complex 3 was prepared by using 4-chlorobenzoic acid (pchlor-
benH) in similar way to that of (1), but was crystallised from a
mixed solvent comprising of DMSO and pyridine. Yield: 46% (based
on Zn). Elemental Anal. Calc. for C24H18Cl2N2O4Zn: C, 53.87; H, 3.37.
Found C, 53.90; H, 3.38%. FT-IR (KBr, cmꢀ1): 3223 (br s), 1925 (w),
1649 (m), 1611 (s), 1593 (s), 1550 (s), 1508 (m), 1383 (s), 1291 (w),
1172 (s), 1139 (w), 1115 (w), 868 (s), 849 (s), 791 (s), 763 (s), 633
(s); 1H NMR (DMSO-d6, 400 MHz): 8.62 (2H, br s), 7.93 (2H, d,
J = 8.4 Hz), 7.83 (1H, t, J = 7.6 Hz), 7.46 (4H, m).
2.4. Preparation of [Zn2(
l
2-mtol)4(py)2] (4)
2.6. Preparation of [Zn2(l
2-otol)4(py)2] (6)
3-Methylbenzoic acid (0.272 g, 2 mmol) and zinc sulfate hepta-
hydrate (0.287 g, 1 mmol) and sodium hydroxide (0.08 g, 2 mmol)
were finely ground in a mortar and heated at 100 °C for 20 min.
The mixture was cooled to room temperature and methanol
(10 ml) was added to it and the heterogeneous mixture was trans-
ferred to a round bottom flask. To this solution pyridine (0.079 g,
1 mmol) was added and stirred at room temperature for 5 min.
Colorless crystals were obtained after 5 days and dried in air. Yield:
48% (based on Zn). Elemental Anal. Calc. for C42H38N2O8Zn2: C,
60.76; H, 4.58. Found: C, 60.79; H, 5.01%. FT-IR (KBr, cmꢀ1): 3431
(br s), 3064 (w), 3030 (w), 2912 (w), 1823 (w), 1644 (s), 1602 (s),
1583 (s), 1403 (br s), 1214 (m), 1082 (m), 1069 (m), 1046 (m),
788 (s), 755 (s), 747 (s), 693 (m), 674 (w), 642 (w), 493 (m), 458
(m). 1H NMR (DMSO-d6, 400 MHz): 8.84 (2H, br s), 8.07 (1H, t,
J = 6.8 Hz), 7.95 (2H, s), 7.92 (2H, t, J = 4 Hz), 7.67 (2H, t,
J = 6.8 Hz), 7.46 (4H, d, J = 5.2 Hz), 2.50 (12H, s).
2-Methylbenzoic acid (0.272 g, 2 mmol), zinc sulfate heptahy-
drate (0.287 g, 1 mmol) and sodium hydroxide (0.08 g, 2 mmol)
were finely ground in a mortar and heated at 100 °C for 20 min.
The mixture was transferred to a round bottom flask and methanol
(10 ml) was added, followed by pyridine (0.079 g, 1 mmol). The
resultant mixture was stirred at room temperature for half an hour
and then was filtered. The colourless filtrate was left undisturbed.
Colourless crystals were obtained after 8 days and dried in air.
Yield: 41% (based on Zn). Elemental Anal. Calc. for C42H38N2O8Zn2:
C, 60.76; H, 4.58. Found C, 60.81; H, 5.01%. FT-IR (KBr, cmꢀ1): 3420
(br s), 3064 (w), 3021 (w), 2929 (w), 1634 (s), 1609 (s), 1568 (s),
1435 (s), 1403 (br s), 1219 (m), 1157 (m), 1103 (m), 1071(m),
1048 (m), 852 (m), 741 (s), 700 (s), 665 (s), 550 (m). 1H NMR
(DMSO-d6, 400 MHz): 8.62 (2 H, br s), 7.87 (1H, t, J = 6.8 Hz), 7.75
(2H, d, J = 7.2 Hz), 7.47 (2H, t, J = 6.8 Hz), 7.27 (2H, t, J = 8.0 Hz),
7.18 (4H, d, J = 7.6 Hz), 2.50 (12H, s).
2.5. Preparation of [Zn5(
l
2-mtol)6(mtol)2(
l
3-OH)2(py)2] (5)
2.7. Preparation of [Zn4(l l
2-otol)6( 4-O)(DMSO)2] (7)
3-Methylbenzoic acid (0.272 g, 2 mmol), zinc sulfate heptahy-
drate (0.287 g, 1 mmol) and sodium hydroxide (0.08 g, 2 mmol)
were finely ground in a mortar and heated at 100 °C for 15 min.
The solid mixture was cooled to room temperature, transferred
2-Methylbenzoic acid (0.272 g, 2 mmol), zinc sulfate heptahy-
drate (0.287 g, 1 mmol) and sodium hydroxide (0.08 g, 2 mmol)
were finely ground in a mortar and heated at 100 °C for 15 min.
The heated mixture was cooled to room temperature, transferred
Table 1
Crystal data and refinement parameters for compound 1–7
1
2
3
4
5
6
7
Formulae
C16H18O6Zn
C18H22N2O11S2Zn
C24 H18 Cl2 N2 O4 Zn
C42H38N2O8Zn2
C74H68N2O18Zn5
C42H38N2O8Zn2
C52H54O15S2Zn4
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
371.67
monoclinic
C2/c
26.8543(12)
5.0620(2)
12.0699(6)
90
106.837(5)
90
1570.40(12)
4
571.87
534.67
monoclinic
C2/c
28.3367(4)
6.23450(10)
15.4315(3)
90
119.9150(10)
90
2362.98(7)
4
1.503
1.299
none
829.48
tetragonal
I-42d
12.4107(2)
12.4107(2)
24.7015(5)
90
1600.2
monoclinic
C2/c
15.9469(8)
22.1141(8)
21.6548(13)
90
107.563(4)
90
7280.6(6)
4
1.46
1.693
none
829.48
monoclinic
P21/c
10.7319(10)
10.9111(10)
18.7835(15)
90
114.387(5)
90
2003.2(3)
2
1.375
1.251
none
1244.6
triclinic
orthorhombic
Pca2(1)
23.7229(8)
11.3117(4)
41.1802(14)
90
ꢀ
P1
6.0159(5)
7.7149(5)
26.7675(17)
84.584(5)
84.464(4)
70.371(3)
1162.13(14)
2
1.634
1.298
none
588
a
(°)
b (°)
90
90
90
90
c
(°)
V (Å3)
3804.66(12)
4
1.448
1.317
none
11050.6(7)
8
1.496
1.855
none
Z
Density (Mg mꢀ3
)
1.572
1.592
none
768
Absorption coefficient (mmꢀ1
Absroption correction
F(000)
)
1088
1712
3280
856
5104
Total number of reflections
9890
1937
14736
5405
17074
4156
23239
2361
39072
8910
26347
4860
125528
26306
Reflections, I > 2
r
(I)
Maximum 2h (°)
28.28
28.2
28.3
28.29
28.46
28.23
28.3
Ranges
ꢀ35 6 h 6 35
ꢀ6 6 k 6 6
ꢀ15 6 l 6 16
99.2
1937/0/106
1.086
ꢀ7 6 h 6 7
ꢀ9 6 k 6 10
ꢀ35 6 l 6 35
94.8
5405/0/316
1.030
ꢀ37 6 h 6 37
ꢀ8 6 k 6 8
ꢀ20 6 l 6 20
98.5
2901/0/150
1.035
ꢀ16 6 h 6 16
ꢀ16 6 k 6 15
ꢀ32 6 l 6 32
99.7
2361/0/124
1.038
ꢀ21 6 h 6 19
ꢀ28 6 k 6 29
ꢀ15 6 l 6 28
96.8
8910/529/455
1.136
ꢀ14 6 h 6 14
ꢀ14 6 k 6 14
ꢀ24 6 l 6 21
98.0
4860/0/246
1.062
ꢀ31 6 h 6 31
ꢀ15 6 k 6 12
ꢀ54 6 l 6 52
99.7
26306/902/1335
0.975
Complete to 2h (%)
Data/restraints/parameters
Goof (F2)
R indices [I > 2
R indices (all data)
r
(I)]
0.0356
0.0495
0.0500
0.0628
0.0375
0.0558
0.0248
0.0365
0.0813
0.2085
0.0272
0.0356
0.0422
0.1005

A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
3411
into a round bottom flask and dimethylsulfoxide: water (4:1) was
added to it. The heterogeneous mixture was stirred at room tem-
perature. The reaction mixture was filtered, the solid precipitate
was discarded and the colourless filtrate was left undisturbed.
Block type colourless crystals were obtained after 10 days and
dried in air. Yield: 24% (based on Zn). Elemental Anal. Calc. for
C52H54O15S2Zn4: C, 50.14; H, 4.34. Found C, 50.19; H, 4.38%. FT-IR
(KBr, cmꢀ1): 3422 (br s), 2968 (m), 2929 (m), 1611 (s), 1591 (s),
1566 (s), 1437 (s), 1399 (s), 1286 (w), 1196 (w), 1160 (m), 1103
(m), 1004 (s), 953 (w), 785 (m), 749 (s), 666 (m), 535 (w). 1H
NMR (DMSO-d6, 400 MHz): 7.75 (6H, d, J = 9.2 Hz), 7.27 (6H, t,
J = 7.2 Hz), 7.18 (10H, d, J = 7.2 Hz), 7.15 (2H, s), 2.52 (40H, m).
anisotropic approximation against F2 of all reflections. The H-
atoms, except those attached to N and O, were placed at their cal-
culated positions and refined in the isotropic approximation; those
attached to the heteroatoms (N, O) were located in the difference
Fourier maps, and refined with isotropic displacement coefficients.
The crystal data for the complexes are listed in Table 1.
4. Results and discussion
A series of combined solid and solution phase reactions of the
sodium salt of substituted benzoic acids with zinc sulfate heptahy-
drate, followed by dissolution of the reactant in methanol or
dimethylsulfoxide in the presence or absence of pyridine were car-
ried out (Scheme 1). A variety of complexes, as listed in Scheme 1,
was obtained and each of the complexes was characterized by X-
ray crystallography. This combined phase reaction methodology
is in accordance with our earlier observation on the variation of
product formation as compared to solution chemistry [5]. From
the para-substituted benzoic acids, 4-methylbenzoic acid, 4-nitro-
benzoic acid and 4-chlorobenzoic acid, on reaction with zinc sul-
fate and pyridine we could prepare mononuclear zinc complexes.
However, the compositions and coordination geometries of the
products were different in each case and depend on the solvent
3. Structure determination
The X-ray data were collected at 296 K with Mo K
a radiation
(k = 0.71073 Å) using a Bruker Nonius SMART CCD diffractometer
equipped with a graphite monochromator. SMART software was
used for data collection and also for indexing the reflections and
determining the unit cell parameters; the collected data were inte-
grated using SAINT software. The structures were solved by direct
methods and refined by full-matrix least-squares calculations
using SHELXTL software. All the non-H atoms were refined in the
[Zn(pclben)2(py)2]
3
DMSO/py
O
ONa
[Zn (pnitroben)2 (Η2Ο) (DMSO)2]
[Zn(ptol)2(H2O)2]
1
DMSO
2
NaO
O
DMSO
O
Cl
NaO
O2N
ONa
ONa
MeOH/py
[Zn2(μ−otol)4(py)2]
O
O
solid state
ZnSO4.7H2O
[Zn2 (μ−mtol)4(py)2]
solid state
NaO
MeOH/py
6
4
O
ONa
O
DMSO
DMSO/py
[Zn4 (μ−otol)6(μ4−O) (DMSO)2]
[Zn5(μ−mtol)6 (mtol)2(μ3−OH)2 (py)2]
7
5
ONa
O
ONa
O
ONa
O
Where
= ptol
=
mtol
= otol
N
ONa
O
ONa
O
= py
= pnitroben
= pclben
O2N
Scheme 1.

3412
A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
of crystallization. For example, complex 1 has the composition
[Zn(ptol)2(H2O)2], having two water ligands, complex has
dimethylsulfoxide ligands with the composition [Zn(pnitro-
water molecule occupying an axial position. The complex shows
sharp IR signals at 3368 cmꢀ1 for the aqua ligand and 1010 cmꢀ1
for DMSO. The monodentate carboxylate stretching appears at
1617 cmꢀ1 and 1578 cmꢀ1 (sym) and the anti-symmetric stretch-
ing appears at 1345 cmꢀ1. The monodentate nature of the carbox-
ylate group is reflected in the Zn–O distances in the crystal
structure; the distances between the two carbonyl oxygen atoms
that are not participating in the metal ligand bond and zinc,
namely Zn1–O6 and Zn–O1, are 3.166 Å and 2.584, whereas the
other metal ligand bond distances, Zn–O2, Zn1–O5, Zn1–O9,
2
ben)2(H2O)(DMSO)2] and 3 has pyridine ligands [5a] with the com-
position
[Zn(pchlorben)2(py)2].
These
complexes
were
characterized by various spectroscopic techniques and also by X-
ray crystallography. The structures of the three complexes are
shown in Fig. 1. Complex 2 has a penta-coordinated structure close
to a square pyramidal geometry with two DMSO and two carbox-
ylate groups opposite to each other in the basal plane, and with a
Fig. 1. The solid state structures of (a) [Zn(ptol)2(H2O)2] (1), (b) [Zn (pnitroben)2(H2O) (DMSO)2] (2) and (c) [Zn(pchlorben)2(py)2] (3).

A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
3413
Zn1–O10 and Zn1–O11, are 2.045, 2.011, 2.120, 2.109 and 2.004 Å,
respectively. The lone pairs of electrons on the two sulfur atoms of
the dimethylsulfoxide ligands in the complex project in opposite
directions and it becomes one of the factors for the five coordina-
tion geometry around the zinc centre. However, there are other
very weak interactions such as C–HꢁꢁꢁO and SꢁꢁꢁN(pi) interactions
[9] that also contribute to this geometry. These lone pairs in this
complex also push the two carbonyl oxygens of the carboxylate
away from each other and thereby making space for a water ligand
to approach the metal center. In the case of complexes 1 and 3 such
effects are not present; complex 1 can be considered to have a dis-
torted square planar structure with two symmetric halves, having
two types of Zn–O bonds (Zn1–O2, 2.009 Å and Zn1–O3, 1.990 Å).
Complex 1 has an IR absorption at 3430 cmꢀ1 and has two closely
spaced carboxylate stretching frequencies at 1621 and 1608 cmꢀ1
.
Whereas complex 3 has the coordination number four with a dis-
torted tetrahedral geometry in which the two carboxylate groups
are monodentate. The molecule has a mirror plane and the Zn1–
N1 and Zn1–O2 bond distances are 2.043 and 1.961 Å, respectively.
The distance of separation between Zn1 and O1 is 2.787 Å, suggest-
ing no bond formation between them and making the carboxylate
group bind in a monodentate fashion. Mononuclear complexes of
2,6-dichlorobenzoate and 2,6-difluorobenzoate are characterized
in the literature and have six and five coordination geometries,
respectively [3a]. However our reactions are done under thermo-
dynamically favorable conditions and the product composition
varies from substituent, as observed in earlier studies. In our ear-
lier study we have shown that the number of ancillary ligands in
a carboxylate complex is decided by the substituent attached to
the aromatic ring [6].
The reaction of zinc(II) sulfate heptahydrate with sodium salts
of 3-methylbenzoic acid or 2-methylbenzoic acids in a methanolic
solution containing pyridine gave dinuclear zinc complexes having
a paddlewheel type structure with two pyridine ligands at axial
positions (Fig. 2a and 2b). The metal ligand bond distances and an-
gles are listed in Tables 2 and 3. In each case there are four carbox-
ylate ligands bridging the two zinc centers. On a careful look at the
geometry of the complexes it is observed that in the case of com-
plex 4 the orientation of the two planes of the pyridine rings are
perpendicular to each other, whereas in the case of 6 the two rings
Table 2
Selected internuclear distances (in Å) and angles (in °) in 4
Bond distances
Bond angles
Zn(1)–O(1)
Zn(1)–N(1)
Zn(1)–O(2)
Zn(1)–Zn(10)
C(1)–O(1)
2.0346(19)
2.0361(19)
2.0557(19)
2.9799(5)
1.250(3)
O(1)–Zn(1)–O(10)
O(1)–Zn(1)–N(1)
O(1)–Zn(1)–O(2)
N(1)–Zn(1)–O(2)
O(1)–Zn(1)–O(20)
O(2)–Zn(1)–O(20)
152.25(10)
103.87(5)
88.32(8)
97.45(5)
88.12(7)
C(1)–O(2)
1.255(3)
165.10(10)
Table 3
Selected internuclear distances (in Å) and angles (in °) in 6
Bond distances
Bond angles
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–O(3)
Zn(1)–O(4)
Zn(1)–N(1)
Zn(1)–Zn(10)
C(1)–O(1)
C(1)–O(2)
C(9)–O(3)
C(9)–O(4)
2.0340 (12)
2.0544 (12)
2.0340 (12)
2.0465 (12)
2.0287 (13)
2.9341 (4)
1.247(2)
1.251(2)
1.246(2)
1.252(2)
N1–Zn1–O3
O1–Zn1–O3
N1–Zn1–O4
O1–Zn1–O4
O3–Zn1–O4
N1–Zn1–O2
O1–Zn1–O2
O3–Zn1–O2
O4–Zn1–O2
101.81(5)
89.23(6)
98.27(5)
88.97(6)
159.72(5)
96.92(6)
159.88(5)
87.41(6)
87.36(6)
Fig. 2. The solid state structure of (a) [Zn2(
otol)4(py)2] (6) and (c) the packing pattern in (4).
l-mtol)4(py)2] (4), (b) [Zn2(l-

3414
A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
lies in one plane. The orientation of the rings is attributed to the
steric effect of the methyl groups on the crystal packing. The pack-
ing pattern of 4 is shown in Fig. 2c, which has a grid-like structure.
Dinuclear paddlewheel structure benzoate complexes have been
reported [7], but these have not been studied in detail. It has also
been observed that non-symmetric carboxylato bridged complexes
of zinc can be prepared with pyridine as an ancillary ligand [3a].
We have also recently shown that binuclear or co-crystals of bi
and trinclear zinc benzoate complexes can be prepared by varying
the reaction conditions [4].
and 1583 cmꢀ1 along with a ms COO stretching at 1403 cmꢀ1. It has
an absorption at 1644 cmꢀ1 due to a C@C absorption of the pyri-
dine ring. In a similar manner, complex 6 has a C@C absorption
from pyridine at 1634 cmꢀ1 ma COOꢀ at 1602 cmꢀ1 and 1583 cmꢀ1
,
and ms COO stretching at 1403 cmꢀ1. However, complex 7 has all
bridging carboxylate ligands and IR absorptions at 1611, 1591
(m
s COO) and 1399 cmꢀ1
(m
a COO) are observed. The S@O stretching
of coordinated DMSO appears at 1004 cmꢀ1
.
Complex 7 is a tetranuclear complex having one oxy-ligand
shared by four zinc centers. The oxy anion acts as a central point
in a tetrahedral geometry formed by four zinc ions and one oxide
anion. Among the four zinc centers, three zinc centers are equiva-
lent, are anchored by three carboxylate bridges and are attached to
the central oxygen, whilst the other zinc center is bridged by three
carboxylate, one oxy ligand and two dimethylsulfoxide ligands.
Complex 5 has two symmetry non-equivalent molecules per unit
cell. This is a rare example of a polynuclear complex having sym-
metry non- equivalent molecules in the unit cell. The bond dis-
tances and bond angles of zinc with the different ligands, as
illustrated in Fig. 4c and d, are listed in Tables 4 and 5. Both the
complexes have a combination of six and four coordination geom-
etry around zinc. Complex 7 is symmetric and has a mirror plane
that bisects two halves containing the two tetrahedral units an-
chored by a six coordinated zinc. The complex has two hydroxyl li-
gands bridging Zn1, Zn2 and Zn3 centers, in which the OH are
intramolecularly hydrogen bonded with two free carbonyl groups
of the monodentate carboxylate hydrogen. The donor acceptor
bond distances in this intramolecular hydrogen bonding O9–
The reaction of zinc sulfate with sodium salts of 3-methylben-
zoic acid and 2-methylbenzoic acid in dimethylsulfoxide gave the
tetranuclear and pentanuclear zinc complexes 5 and 7, respec-
tively. From the observation of the formation of two paddle wheel
type structures in the case of 4 and 6, it is clear that methanol has
no role in the formation of these dinuclear structures, whereas
DMSO, being a more coordinating solvent, leads to an expansion
of the nuclearity. The IR spectrum of the dinuclear complex 4, pre-
pared in methanol solution, shows clear distinctions as compared
to the tetranuclear complex 6, as shown in Fig. 3. Distinctions be-
tween the dinuclear complex 5 and the pentanuclear complex 7
are also observed, again shown in Fig. 3. In the case of 5 all the car-
boxylate groups are bridging, whereas in complex 7 there are
bridging as well as monodentate carboxylate ligands. The mono-
dentate carboxylate of 5 shows an IR absorption at 1623 cmꢀ1
,
whereas for the bridging carboxylate absorptions of the complex,
ma are at 1601 cmꢀ1 and 1578 cmꢀ1 and ms COO stretchings appear
at 1428 cmꢀ1 and 1404 cmꢀ1. Complex 4 has ma at 1602 cmꢀ1
Fig. 3. The solid state IR spectra (KBr) of complexes 4–7.

A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
3415
Fig. 4. The crystal structures of (a) [Zn5(
l
2-mtol)6(mtol)2(
l
3-OH)2(py)2] (5) (b) [Zn4( 2-otol)6( 4-O)(DMSO)2] (7) (the hydrogen atoms are omitted for clarity). The ligand
l l
environments around (c) 5 and (d) 7.
H9ꢁꢁꢁO2 are dD–H 0.77 Å, dD–A,intra 2.73 Å; hD–HꢁꢁꢁA 170.93°. On the
six coordinated zinc center the two bulky groups occupy cis posi-
tions, which also comply with the strong intramolecular hydrogen
bonding. The presence of two uncoordinated carbonyl oxygens at
this position could be responsible for further abstraction of protons
from the hydroxyl groups. Complex 5, however, has a central oxy
ligand which coordinates to four independent zinc centers to com-
plete three tetrahedral and an octahedral geometry. Complex 5 is
similar to the tetranuclear symmetric zinc core reported by Strau-
ghan et al. [8], but with the difference that one of the zinc centers
gets coordinated to solvent making the complex less symmetric.
Additionally such a change of coordination environment by a li-
gand with the retention of four uniformly coordinated geometries
in tetranuclear zinc clusters is known in the literature [3a]. The
presence of hexa and tetra coordinated zinc makes our clusters
structurally unique. Thus, our clusters possess a new motif con-
taining four coordinated zinc centers along with six coordinated
zinc centers. Recently a tetranuclear oxo-cluster of zinc has been
reported and it was observed that molecular oxygen is responsible
for the formation of such clusters from organo-zinc reactions [2d].
In organo-zinc complexes there is a Zn–C bond, leaving scope for
reactions with molecular oxygen, whilst in our reactions the oxo
or hydroxo species are generated from water molecules in the sol-
Table 4
Selected internuclear distances (in Å) and angles (in °) in 5
Bond distances
Bond angles
Zn(1)–O(4)
Zn(1)–O(5)
Zn(1)–O(9)
Zn(2)–N(1)
Zn(2)–O(6)
Zn(2)–O(9)
Zn(2)–O(8)
Zn(2)–O(1)
Zn(3)–O(3)
Zn(3)–O(9)
Zn(3)–O(7)
C(1)–O(1)
C(1)–O(2)
C(9)–O(3)
C(9)–O(4)
C(17)–O(5)
C(17)–O(6)
C(25)–O(7)
C(25)–O(8)
2.061(5)
2.080(5)
2.096(4)
2.016(6)
1.938(5)
1.954(4)
1.974(5)
1.920(5)
1.936(6)
1.956(4)
1.978(5)
1.285(10)
1.216(10)
1.264(14)
1.259(14)
1.276(13)
1.221(13)
1.232(10)
1.249(10)
O(4)–Zn(1)–O(40)
O(4)–Zn(1)–O(5)
O(40)–Zn(1)–O(5)
O(5)–Zn(1)–O(50)
O(4)–Zn(1)–O(9)
O(40)–Zn(1)–O(9)
O(5)–Zn(1)–O(9)
O(50)–Zn(1)–O(9)
O(9)–Zn(1)–O(90)
O(6)–Zn(2)–O(9)
O(6)–Zn(2)–O(8)
O(9)–Zn(2)–O(8)
O(6)–Zn(2)–N(1)
O(8)–Zn(2)–N(1)
O(21)–Zn(2)–N(1)
O(1)–Zn(3)–O(3)
O(1)–Zn(3)–O(9)
O(3)–Zn(3)–O(9)
O(1)–Zn(3)–O(7)
O(3)–Zn(3)–O(7)
O(9)–Zn(3)–O(7)
175.2(3)
85.9(2)
91.0(3)
96.8(4)
98.7(2)
84.64(18)
86.8(2)
169.9(2)
91.2(2)
122.9(2)
103.3(2)
105.59(18)
108.7(2)
111.3(2)
102.8(2)
117.3(2)
112.0(2)
114.8(2)
104.3(2)
101.2(3)
105.02(19)

3416
A. Karmakar, J.B. Baruah / Polyhedron 27 (2008) 3409–3416
Table 5
vent and hydrated salts, as these reactions proceed under nitrogen
Selected internuclear distances (in Å) and angles (in °) in 7
atmosphere and the use of dry solvents led to poor yields of the
clusters. DMSO, being an aprotic coordinating solvent, allows the
deprotonation of water in basic medium whereas in methanol
the labile hydrogen is in a large excess; this thus facilitates the for-
mation of neutral dinuclear species. The formation of the mononu-
clear aqua complex in the case of 1 suggests that not only the
reaction conditions but also the substituent controls the formation
of different zinc complexes.
Bond distances
Bond angles
Zn(1)–O(1)
Zn(1)–O(8)
Zn(1)–O(10)
Zn(1)–O(13)
Zn(2)–O(1)
Zn(2)–O(3)
Zn(2)–O(4)
Zn(2)–O(9)
Zn(3)–O(1)
Zn(3)–O(5)
Zn(3)–O(6)
Zn(3)–O(11)
Zn(4)–O(1)
Zn(4)–O(2)
Zn(4)–O(7)
Zn(4)–O(12)
Zn(4)–O(14)
Zn(4)–O(15)
Zn(5)–O(16)
Zn(5)–O(24)
Zn(5)–O(25)
Zn(5)–O(27)
Zn(6)–O(16)
Zn(6)–O(20)
Zn(6)–O(21)
Zn(6)–O(28)
Zn(7)–O(16)
Zn(7)–O(17)
Zn(7)–O(19)
Zn(7)–O(26)
Zn(8)–O(16)
Zn(8)–O(18)
Zn(8)–O(22)
Zn(8)–O(23)
Zn(8)–O(29)
Zn(8)–O(30)
C(1)–O(2)
1.909(3)
1.956(4)
1.944(4)
1.930(4)
1.924(3)
1.912(4)
1.966(4)
1.983(4)
1.935(3)
1.958(4)
1.928(4)
2.046(4)
2.007(3)
2.104(4)
2.257(4)
2.104(3)
2.195(3)
2.077(3)
1.912(3)
1.936(4)
1.946(4)
1.968(4)
1.919(3)
1.949(4)
1.912(4)
1.982(4)
1.936(3)
1.946(4)
1.979(4)
2.036(4)
2.004(3)
2.224(4)
2.096(4)
2.082(4)
2.216(4)
2.068(3)
1.252(6)
1.270(6)
1.258(6)
1.269(6)
1.268(6)
1.237(6)
1.239(6)
1.257(6)
1.248(6)
1.226(6)
1.283(7)
1.243(7)
1.245(7)
1.240(7)
1.276(6)
1.238(6)
1.244(6)
1.234(6)
1.236(6)
1.246(6)
O(1)–Zn(1)–O(13)
O(1)–Zn(1)–O(10)
O(13)–Zn(1)–O(10)
O(1)–Zn(1)–O(8)
O(13)–Zn(1)–O(8)
O(10)–Zn(1)–O(8)
O(3)–Zn(2)–O(1)
O(3)–Zn(2)–O(4)
O(1)–Zn(2)–O(4)
O(3)–Zn(2)–O(9)
O(1)–Zn(2)–O(9)
118.10(15)
112.00(16)
107.05(19)
111.60(15)
104.38(18)
102.3(2)
120.01(14)
106.50(17)
112.91(15)
106.89(17)
110.36(15)
97.73(17)
121.81(14)
112.24(17)
117.11(16)
97.16(16)
105.70(15)
96.53(17)
170.65(14)
96.22(14)
87.90(14)
99.72(13)
88.36(13)
93.54(15)
79.68(12)
92.02(13)
89.15(14)
177.29(15)
92.12(14)
83.06(14)
170.02(15)
90.45(14)
86.94(14)
117.06(15)
112.12(16)
107.84(18)
111.51(16)
104.48(18)
102.60(19)
121.73(14)
105.38(18)
110.93(16)
106.45(17)
110.71(15)
99.16(17)
123.03(15)
115.12(15)
112.66(18)
104.59(15)
98.54(17)
96.94(16)
167.77(14)
100.61(13)
90.80(14)
96.77(14)
86.66(15)
94.31(16)
80.32(13)
88.00(14)
175.93(15)
89.51(15)
90.48(15)
85.28(15)
89.01(16)
171.32(16)
87.01(15)
5. Supplementary data
Crystallographic details of the compounds in the form of CIF
files are available and they have the CCDC numbers 668242,
668243, 676945–676948 and 612955. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html or from the Cambridge Crystallographic Data Centre, 12 Un-
ion Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk.
O(4)–Zn(2)–O(9)
O(6)–Zn(3)–O(1)
O(6)–Zn(3)–O(5)
O(1)–Zn(3)–O(5)
O(6)–Zn(3)–O(11)
O(1)–Zn(3)–O(11)
O(5)–Zn(3)–O(11)
O(1)–Zn(4)–O(15)
O(1)–Zn(4)–O(2)
O(15)–Zn(4)–O(2)
O(1)–Zn(4)–O(12)
O(15)–Zn(4)–O(12)
O(2)–Zn(4)–O(12)
O(1)–Zn(4)–O(14)
O(15)–Zn(4)–O(14)
O(2)–Zn(4)–O(14)
O(12)–Zn(4)–O(14)
O(1)– Zn(4)–O(7)
O(15)–Zn(4)–O(7)
O(2)–Zn(4)–O(7)
O(12)–Zn(4)–O(7)
O(14)–Zn(4)–O(7)
O(16)–Zn(5)–O(24)
O(16)–Zn(5)–O(25)
O(24)–Zn(5)–O(25)
O(16)–Zn(5)–O(27)
O(24)–Zn(5)–O(27)
O(25)–Zn(5)–O(27)
O(21)–Zn(6)–O(16)
O(21)–Zn(6)–O(20)
O(16)–Zn(6)–O(20)
O(21)–Zn(6)–O(28)
O(16)–Zn(6)–O(28)
O(20)–Zn(6)–O(28)
O(16)–Zn(7)–O(17)
O(16)–Zn(7)–O(19)
O(17)–Zn(7)–O(19)
O(16)–Zn(7)–O(26)
O(17)–Zn(7)–O(26)
O(19)–Zn(7)–O(26)
O(16)–Zn(8)–O(30)
O(16)–Zn(8)–O(23)
O(30)–Zn(8)–O(23)
O(16)–Zn(8)–O(22)
O(30)–Zn(8)–O(22)
O(23)–Zn(8)–O(22)
O(16)–Zn(8)–O(29)
O(30)–Zn(8)–O(29)
O(23)–Zn(8)–O(29)
O(22)–Zn(8)–O(29)
O(16)–Zn(8)–O(18)
O(30)–Zn(8)–O(18)
O(23)–Zn(8)–O(18)
O(22)–Zn(8)–O(18)
O(29)–Zn(8)–O(18)
Acknowledgement
The authors thank the Department of Science and Technology,
New Delhi, India, for financial assistance.
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C(17)–O(6)
C(17)–O(7)
C(25)–O(12)
C(25)–O(13)
C(41)–O(10)
C(41)–O(11)
C(53)–O(17)
C(53)–O(18)
C(61)–O(19)
C(61)–O(20)
C(69)–O(21)
C(69)–O(22)
C(85)–O(25)
C(85)–O(26)
C(93)–O(27)
C(93)–O(28)