Self-assembly of neutral dinuclear and trinuclear zinc-benzoate complexes

Article abstract of DOI:10.1016/j.inoche.2006.07.013

A structurally important self-assembly of dinuclear and trinuclear zinc-benzoate complexes having formula [Zn₃(μ-OBz)₆(py)₂] · [Zn₂(OBz)₄(py)₂] (A) (where OBz is benzoate and py is pyridine ligand) is reported. The dinuclear and trinuclear complexes present in the crystal lattice of the self-assembly are held together by weak C-H?π and C-H?O interactions.

Full text of DOI:10.1016/j.inoche.2006.07.013

Inorganic Chemistry Communications 9 (2006) 1169–1172
www.elsevier.com/locate/inoche
Self-assembly of neutral dinuclear and trinuclear
zinc-benzoate complexes
*
Anirban Karmakar, Rupam J. Sarma, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781 039, India
Received 14 June 2006; accepted 12 July 2006
Available online 22 July 2006
Abstract
A structurally important self-assembly of dinuclear and trinuclear zinc-benzoate complexes having formula [Zn3(l-OBz)6(py)2] Æ
[Zn2(OBz)4(py)2] (A) (where OBz is benzoate and py is pyridine ligand) is reported. The dinuclear and trinuclear complexes present
in the crystal lattice of the self-assembly are held together by weak C–Hꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀO interactions.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Zinc benzoate; Polynuclear complex; Self assembly; Crystal structure
Mononuclear as well as dinuclear zinc carboxylate com-
plexes are of great importance to study them as biological
model compounds [1]. Due to the ambidentate nature of
carboxylate ligands, they can bind in bridging or monoden-
tate fashion to a metal ion [2]. The paddle-wheel type of
structure from zinc-benzoate is already reported in litera-
ture [3]; however, it is not very clear on the condition at
which such complexes will be formed. In the present study
we have synthesized and crystallographically characterized
an unusual self-assembly of a trinuclear and dinuclear zinc-
benzoate complexes. Such self assemblies of neutral com-
plexes are expected to behave in collective manner of the
independent complexes.
Solid state reaction of zinc sulphate heptahydrate [4]
and benzoic acid with potassium hydroxide followed by
addition of pyridine resulted in an self-assembly which
has a composition [Zn3(l-OBz)6(py)2] Æ [Zn2(OBz)4(py)2]
(A) where OBz is benzoate and py is pyridine ligand (Eq.
(1)). The complex is unusual for the following reasons as
it is an self-assembly of two neutral complexes of which
dinuclear complex [Zn2(py)2(OBz)4] (B) can also be inde-
pendently prepared [5]. The dinuclear complex B could
be prepared by a similar solid state reaction of anhydrous
zinc chloride, benzoic acid and potassium hydroxide fol-
lowed by reaction with pyridine.
ZnðSOÞ4 ꢀ 7H2O þ BzOH þ KOH
Py
!½Zn3ðOBzÞ6ðpyÞ2ꢁ½Zn2ðOBzÞ4ðpyÞ2ꢁ
ð1aÞ
ð1bÞ
py
ZnCl2 þ BzOH þ KOH !½Zn2ðOBzÞ4ðpyÞ2ꢁ
where BzOH = C6H5COOH, Py = C5H5N.
The crystal structure of A and B are determined to con-
firm their identity. The structure of the self-assembly A is
shown in Fig. 1a. The two neutral complexes leading to
the self-assembly is shown in (Fig. 1b and c). The self-
assembly A has an independent trinuclear unit constructed
by six bridging benzoate group and two terminal pyridine
ligands. Out of the three zinc centers the central zinc is
attached to six benzoate group to have all O-donors
around it to complete a hexa-coordinated metal center;
whereas the other two zinc centers are symmetric to each
other has one terminal pyridine ligand each and there are
three oxygen atoms co-coordinating each of the zinc ions.
Thus, the later two zinc centers each has NO4 co-ordina-
tion environment with distorted trigonal bipyramid geom-
etry. In the case of the central zinc it has a distorted
octahedral structure. The bond angles around the central
zinc are <O8–Zn1–O6 is 87.37(11)°, <O10–Zn1–O6 is
92.63(11)° and <O10–Zn1–O is 93.32(10)°. In the case of
*
Corresponding author. Tel.: +91 361 2582311; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.07.013

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A. Karmakar et al. / Inorganic Chemistry Communications 9 (2006) 1169–1172
Fig. 1. (a) The crystal structure of A drawn with 30% thermal ellipsoid (hydrogen atoms are omitted for clarity); (b) the C–Hꢀ ꢀ ꢀp interactions in the self-
˚
˚
assembly; (c) a view of the self-assembly to show the C–Hꢀ ꢀ ꢀO interactions; C(28)–Hꢀ ꢀ ꢀO(3) [3.51 A, 160(7)°] and C(16)–Hꢀ ꢀ ꢀp (aromatic) [3.59 A,
148(6)°].
the two zinc centers at the two ends; the bond angles are
<O7–Zn2–O6, 87.30(9)°; <O9–Zn2–O6, 107.08(9)°; <O9–
Zn2–O7 117.78(9)°; <O7–Zn2–N2 96.82(9)°; <O9–Zn2–
N2 101.99(9)°; <O5–Zn2–N2 96.72(10)°; with Zn2–N2
To understand their behavior of the complexes in solu-
tion we have carried out reactions of pyridine with com-
plex A as well as B in NMR tube and examined the
changes in the chemical shifts of the parent compound.
1
˚
bond length 2.039(2) A. The Zn2–O6 distance is
The H NMR spectra of compound A has corresponding
˚
2.329(3) A whereas bond-distances Zn2–O7, Zn2–O9 and
peaks from the pyridine ring (marked as x in Fig. 2) as
well as from the benzoate group (marked as y in Fig. 2)
in the aromatic region. The peak positions of a solution
of A are responsive to the addition of externally added
˚
Zn2–O5 are 1.9558(19), 1.949(2), 2.120(3) A, respectively.
The dinuclear part of the assembly has resemblance with
the already reported structure in the literature [3]. We have
also determined the crystal structure of dinuclear com-
pound B that is prepared through solid state reaction and
a found to tally with the reported structure [3]. From
Fig. 1, it is clear that the two neutral molecules are held
by C–Hꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀO interactions between the two
complexes. We have looked examined the X-ray powder
pattern of compound (please see supplementary material)
and observed that both have different powder X-ray dif-
fraction pattern. The solid state IR of the compound A
and B have close similarities for the obvious reason of com-
position. Both A and B have characteristic peaks that are
present in the backbone of benzoic acid. A clear demarca-
tion for identifying the two counterparts of self-assemblies
from their respective IR absorptions could not be made.
The pure product of A and B could be obtained in low yield
as these were only obtained from crystallization process
and synthetic route involves multiple components.
1
pyridine. The compound B has similar H NMR spectra
to that of A (Fig. 3) but with different integration ratios
among the benzoate and pyridine parts. In this case, the
addition of one equivalent of pyridine causes changes to
the 1H NMR signals of the parent compound and the
1
spectra obtained could be matched to the H NMR spec-
tra that is obtained from the solution of A to which two
equivalent of pyridine was added. These results suggests
that in solid state the A and B are two independent com-
pounds; however, we could not obtain distinguishable fea-
1
tures in terms of chemical shift in H NMR from these
two compounds. This indicates that in solution the
exchange of ligands of A takes place. This is further sup-
ported by the fact that addition of pyridine in both the
complexes results in the formation of a common species,
however, the structure of this common species that is
formed by reaction with pyridine is not clear from solu-

A. Karmakar et al. / Inorganic Chemistry Communications 9 (2006) 1169–1172
1171
Fig. 3. 1H NMR (400 MHz, CDCl3) spectra of (a) B; (b) B with one mole
equivalent of pyridine; (peaks x are from pyridine and y are from benzoic
acid).
It is interesting to note that the solid state reaction of zinc
chloride with p-chlorobenzoic acid, potassium hydroxide
followed by treatment with pyridine gave a mononuclear
complex bis-pyridine zinc(II) p-chlorobenzoate (C). The
complex has a distorted tetrahedral geometry [7] around
the zinc (Fig. 4a). The bond angle \O1–Zn1–O1 is
98.25(8)°, \N1–Zn1–N1 is 106.46(9)° and \O1–Zn1–N1
Fig. 2. 1H NMR (400 MHz, CDCl3) spectra of (a) A; (b) A with one mole
equivalent of pyridine; (c) A with two mole equivalent of pyridine (peaks x
are from pyridine and y are from benzoic acid).
tion study. The zinc carboxylate complexes can have dif-
ferent types of structures ranging from dinuclear [3], tetra-
nuclear [6a], heptanuclear [6b], etc.
Fig. 4. (a) The ORTEP diagram (30% thermal ellipsoid) for bis-pyridine zinc(II) p-chlorobenzoate (C) (b) self-assembly of (C).

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A. Karmakar et al. / Inorganic Chemistry Communications 9 (2006) 1169–1172
[3] T. Ohmura, W. Mori, T. Takei, T. Ikeda, A. Maeda, Mater. Sci.-
Poland 23 (2005) 729–736.
is 122.84(7)°. This complex self assembles through weak C–
˚
Hꢀ ꢀ ꢀO interactions [3.242(3) A, 145°; (x, ꢂy, ꢂ1/2 + z)]
[4] Preparation of complex A:
A mixture of benzoic acid (0.24 g,
arising from the interactions between carbonyl oxygen
atom of p-chlorobenzoic acid and an para-hydrogen atom
of a pyridine ring as shown in Fig. 4b.
In conclusion we have described self-assembly represent-
ing a new class of compounds formed by a dinuclear and
trinuclear zinc benzoate complex. The dinuclear and trinu-
clear units independently present in the crystal lattice of
the self-assembly are held together by weak C–Hꢀ ꢀ ꢀp and
C–Hꢀ ꢀ ꢀO interactions.
2 mmol), potassium hydroxide (0.11 g, 2 mmol) and zinc sulphate
heptahydrate (0.287 g, 1 mmol) was finely ground in a pastel mortar
and heated at 100 °C for 15 min. The mixture was cooled to room
temperature transferred into a round bottom flask and toluene
(20 ml) was added to it and the heterogeneous mixture was stirred at
room temp for 5 min followed by the addition of pyridine (0.158 g,
2 mmol). The white residue was filtered off and the colorless filtrate
was left undisturbed. Colorless crystals were collected after 3days and
dried in air. Yield of crystalline compound 17% (based on zinc).
Crystallographic details: The reflection data were collected using
SMART software (Bruker) which was also used for indexing and cell
refinement. The collected data were integrated using SAINT software,
and the structure was solved by direct methods. The non-hydrogen
atoms were refined against F2 in the anisotropic approximation. The
hydrogen atoms were placed at calculated positions and refined
Acknowledgement
The author thanks Department of Science and Technol-
ogy (New Delhi) India, for financial support.
ꢀ
isotropically. Unit cell parameters: triclinic, space group P1, a =
˚
˚
˚
10.7105(4) A, b = 10.9616(4) A; c = 18.8659(8) A, a = 101.771(2)°,
b = 101.908(2)°, c = 97.202(2)°, Z = 1, density (g/cm3), 1.474, abs.
coeff./mmꢂ1, 1.489; total no. of reflections, 18366; refinement method,
full-matrix least-squares on F2; data/restraints/parameters, 6986/0/
538; GoF (F2), 0.981; R indices [I > 2r(I)], 0.0396; R indices (all
data), 0.0682.
Appendix A. Supplementary data
The CCDC number of the complexes A, B and C are
610569, 610568 and 612955, respectively. The X-ray pow-
der pattern of the two complexes A and B; the IR spectra
of A and B as compared with benzoic acid are available
as supplementary material. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.inoche.2006.07.013.
[5] Preparation of complex B: A mixture of benzoic acid (0.24 g, 2 mmol),
potassium hydroxide (0.11 g, 2 mmol) and anhydrous zinc chloride
(0.136 g, 1 mmol) was finely ground in pastel-mortar and heated at
100 °C for 10 min. The mixture was cooled to room temperature
transferred into a round bottom flask and toluene (20 ml) was added to
it and the heterogeneous mixture was stirred at room temp for 5 min
followed by the addition of pyridine (0.158 g, 2 mmol). The white
residue was filtered off and the colorless filtrate was left undisturbed.
Colorless crystals were collected after 3 days and dried in air. Yield of
crystalline compound 15% (based on zinc). Unit cell parameters:
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