Role of nitro-substituent in pseudo-polymorphism and in synthesis of metal carboxylato complexes of copper, zinc and manganese

Article abstract of DOI:10.1016/j.inoche.2007.04.026

The consequence of nitro-group attached to aromatic ring of 2-nitrobenzoato complexes of copper(II) on pseudo-polymorphism and in synthesis of metal carboxylato complexes is demonstrated and the results are compared with analogous first row transition met

Full text of DOI:10.1016/j.inoche.2007.04.026

Inorganic Chemistry Communications 10 (2007) 959–964
www.elsevier.com/locate/inoche
Role of nitro-substituent in pseudo-polymorphism and in synthesis
of metal carboxylato complexes of copper, zinc and manganese
*
Anirban Karmakar, Kusum Bania, Abhilasha M. Baruah, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
Received 21 March 2007; accepted 1 April 2007
Available online 10 May 2007
Abstract
The consequence of nitro-group attached to aromatic ring of 2-nitrobenzoato complexes of copper(II) on pseudo-polymorphism and
in synthesis of metal carboxylato complexes is demonstrated and the results are compared with analogous first row transition metal carb-
oxylato complexes.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: 3d-Metal carboxylates; 2-Nitrobenzoato ligand; Polymorphism; Crystal structure; Aqua-bridged complexes; Paddle-wheel structure
Reticular synthesis of metallo-organic framework of
carboxylato complexes needs thorough understanding of
co-ordination chemistry of simple synthons [1]. Prepara-
tion of metallo-organic motif having carboxylato ligands
has become increasingly important for storage materials
and for molecular recognition [2]. In a recent article it is
shown in zinc metallo-organic framework, that multiple
number of solvents gets incorporated [3]. In a series of arti-
cles we have reported synthetic methodologies to provide
varieties in structures of metal carboxylato complexes [4].
In this regard, we had observed a new class of polymorphic
compounds from the orientation of nitro-groups in aqua-
bridged 2-nitrobenzoato complexes of cobalt(II) [4a]. It is
also observed that aqua-bridged dinuclear 2-nitrobenzoato
complexes or mononuclear 2-nitrobenzoato complex of
nickel(II) and cobalt(II) can be prepared [4b] by variation
of reaction conditions. In the cases of nickel [4b] and cobalt
[4i] complexes we could obtain pseudo-polymorphs(poly-
morphs due to solvent of crystallization). Thus, to delin-
eate polymorphism in other transition metal complexes is
a challenge and the understanding at basic level will lead
to reticular synthesis of different polymorphs in MOF
which is emerging as a new area of research. Accordingly,
we have taken up structural study and synthesis through
solid and solution phase to understand the polymorphism
and structural consequences in a series of 2-nitrobenzoato
complexes of manganese(II), copper(II) and zinc(II). Preli-
minary reports are presented in this manuscript.
We have studied various reactions of copper(II) acetate
with 2-nitrobenzoic acid in the presence of nitrogen donor
ligands [5] as listed in Scheme 1. Copper(II) acetate reacts
with 2-nitrobenzoic acid and 3,5-dimethylpyrazole to give
a mono-nuclear copper(II) complex (I). In this complex
the copper has six-cordination geometry with elongated
˚
Cu–O bond along the z-axis. The bond distances (A) are
Cu1–N, 1.973; Cu1–O1, 1.974 and Cu1–O2, 2.685, respec-
tively. The structure of the complex is shown in Fig. 1a. In
this case we could not observe dinuclear compound either
through solid state or through solution phase reactions.
The two nitro-groups of the rings are oriented trans to each
other with mirror inversion. When a similar reaction of
copper(II) acetate with 2-nitrobenzoic acid was carried
out in solid state followed by treatment with pyridine we
obtained dinuclear complex having paddle-wheel structure
(Fig. 1c). This compound can be obtained in the solvated
or unsolvated form. The solvated form or pseudo-polymor-
phs can be easily crystallized by dissolving III in methanol
*
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 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.04.026

960
A. Karmakar et al. / Inorganic Chemistry Communications 10 (2007) 959–964
Cu(OAc)2.H2O
MeOH
C6H5CH3
L2 + L3
MeOH/H2O
L2 + L3
L1 + L2
L2 + L3
MeOH/C6H5NO2
L2 + L3
MeOH
MeOH
[Cu2(L3)4(L2)2]
[Cu(L1)2(L2)2]
[Cu(L3)2(L2)2.H2O]
[Cu2(L3)4(L2)2].C6H5NO2
[Cu2(L3)4(L2)2].C6H5CH3
III
I
II
V
IV
MeOH/H2O
O-
O
H
N
N
NO2
L1
L3
L2
N
Scheme 1.
Fig. 1. a–e The crystal structures of I–V (drawn with 20% thermal ellipsoid).

A. Karmakar et al. / Inorganic Chemistry Communications 10 (2007) 959–964
961
Fig. 2. Representation of torsion angles in complexes III–V. A1 is the torsion angle between NA1–CA111–CA11–CA1 in complex III, analogous definition
holds good for torsion angles B1, C1, D1 in IV; A2, B2, C2, D2, in V and A3, B3, C3, D3 in V.
solution with toluene, nitrobenzene etc. The unsolvated
form III is not stable in aqueous methanol it disproportion-
ate to give mononuclear aqua (bis-2-nitrobenzoato) (bis-
pyridine) copper(II) complex (II). The structure of complex
II is determined by X-ray diffraction and is shown in
Fig. 1b. The structure of the toluene and nitrobenzene sol-
vated complexes IV and V and also the structure of unsol-
vated dinuclear copper(II) complex III are determined. The
copper–copper distances in the complexes III–V are 2.68,
at particular orientation. The three molecules are then
viewed along the z-axis passing through the copper(II)
atoms. For each pseudo-polymorph such angles are defined
as illustrated in Fig. 2. The torsion angles for the three
complexes III–V are listed in Table 1. From the torsion
angles it is clear that in each case the orientation of
nitro-groups are similar to each other and they are
arranged such that the alternate nitro-groups are symmetry
related with slight variations of angles for each other. The
˚
2.71, 2.71 A, respectively, it suggests that the packing effect
of solvent of crystallization plays a role on the structures.
˚
The CuÁ Á ÁCu distance in copper(II) acetate [6] is 2.61 A,
which suggests that the presence of nitro-group at 2-posi-
tion makes the CuÁ Á ÁCu contact to slightly elongate due
to obvious reason of steric congestion.
Nitro-groups of the complexes are symmetry related to
each other through a C2 axis and their orientations are ana-
lysed by looking at the torsion angles between the nitro-
plane with that of aromatic ring. For torsion angle analysis
we have demarcated the torsion angles by A1, B1, C1, D1
for each nitro-group in a compound by fixing the molecules
Table 1
Different torsion angles in III–V indicating the orientation of the nitro-
group
Complex Torsion angle Torsion angle Torsion angle Torsion angle
(°)
(°)
(°)
(°)
III
IV
V
A1 = 4.04
A2 = 8.46
A3 = À11.0
B1 = À9.29
B2 = À9.07
B3 = À9.08
C1 = À4.04
C2 = À8.46
C3 = 11.06
D1 = 9.29
D2 = 9.07
D3 = 9.08

962
A. Karmakar et al. / Inorganic Chemistry Communications 10 (2007) 959–964
˚
highest non-planarity among nitro and aromatic ring is
obtained in the complex V where nitrobenzene is being
trapped in the lattice. Thus, the variation of the orientation
of the nitro-group is attributed to the packing effect result-
ing from the co-crystallisation process.
The compound II and III can be distinguished by IR spec-
troscopy and the solid state IR spectra of the two compounds
are shown in Fig. 3. The complex II has an aqua ligand which
has the mOH at 3415 cmÀ1. In the case of dinuclear the IR has
the carboxylato group absorption frequency at 1633 cmÀ1
whereas the mononuclear carboxylato complex has strong
absorptions at 1606 and 1568 cmÀ1. The complex IV hav-
ing toluene as solvent of crystallization has similar IR fea-
tures as that of III, but additional peak for aromatic C@C
bond frequency at 1609 cmÀ1 is observed.
We also studied similar complexation reaction with zinc
acetate and observed that paddle-wheel type structure can
be easily constructed from solid state reactions of zinc(II)
acetate with 2-nitrobenzoic acid followed by treatment
with pyridine (Scheme 2). Crystal structure of the com-
pound is shown in Fig. 3a. The ZnÁ Á ÁZn separation in this
complex is found to be 3.03 A. This distance in similar
dimeric unsubstituted paddle-wheel benzoate complex of
zinc [7,4d] is 2.96 A. In the 2-nitrobenzoate complexes the
˚
nitro-groups are symmetry related and the two sets of tor-
sion angle of nitro-group with the aromatic ring are 5.63°
and 0.94° suggesting that the nitro-group containing
O–N–O plane is nearly perpendicular to the phenyl ring
attached to it. This is not the case with copper(II) com-
plexes, suggesting that the size of the metal and longer
ZnÁ Á ÁZn separation reduces the steric constrains for
nitro-group to twist from most favorable geometry
observed in molecules where free C–N bond rotation takes
place. Since two axial positions of the paddle-wheel struc-
ture are occupied by a mono-dentate pyridine ligand, we
felt that use of a chelating ligand would prevent the pad-
dle-wheel structure and give mono-nuclear structure that
would serve the basic frame for polymeric structures. In
fact we have succeeded in getting five co-ordinated zinc
complex from the reaction of zinc acetate with 2-nitroben-
zoic acid in the presence of 1,10-phenanthroline. The com-
plex VII obtained from this reaction is characterized by
Fig. 3. a and b The crystal structures of VI and VII, respectively (drawn with 20% thermal ellipsoid).
Zn(OAc)2.2H2O
L2 + L3
L4 + L2
MeOH
MeOH
[Zn2(L3)4(L2)2]
[Zn(L4)(L2)2.H2O]
VI
VII
O
O-
N
N
NO2
L4
L2
L3
N
Scheme 2.

A. Karmakar et al. / Inorganic Chemistry Communications 10 (2007) 959–964
963
X-ray crystallography and the structure is given in Fig. 3b.
The compound has a distorted square pyramidal structure
in which two carboxylate groups occupy cis-positions. The
two nitrogen and the two oxygen atoms complete the
dative sites of the basal plane. Selective bond distances in
the complex are as follows: Zn1–O5, 1.99; Zn1–O2, 2.08
of Mn2+ > Co2+ > Ni2+, but the M–M separation in the
aqua-bridged 2-nitrobenzoato dinuclear complexes are of
the order of Mn2+ ꢀ Co2+ > Ni2+; in addition to the steric
factor other factors such as magnetic interactions has a role
in deciding these M–M separations, however this needs fur-
ther experimental evidences.
In conclusion we have shown here the diverse scope for
formation of pseudo-polymorphs in 2-nitrobenzoato com-
plexes of copper(II) and also shown the complexity
involved in understanding the basic frameworks that could
serve as the guideline for building metallo-organic frame-
works. Works are in progress to use these synthons for
reticular synthesis of polymorphs of metallo-organic
framework.
˚
Zn1–O9(aqua), 2.04 Zn1–N1, 2.17; Zn1–N2, 2.08 A.
While carrying out reaction of manganese acetate with
2-nitrobenzoic acid in the presence of pyridine we have
observed aqua and carboxylato bridged manganese dinu-
clear complex having two pyridines per manganese
(Scheme 3). This is similar to our observation on reactions
of cobalt(II) [4i] and nickel(II) [4b] with 2-nitrobenzoate
under solid phase followed by treatment with pyridine; in
which we obtained such aqua and carboxylato bridged
complexes. The structure of the complex is shown in
Fig. 4. In this complex there are both bridging and
mono-dentate carboxylato groups are present. The com-
pound is not symmetric and the dihedral angle of the
nitro-groups to the phenyl ring attached to it are À0.79°
and À5.53° for the bridging pair, whereas for the mono-
dentate pair are 1.56° and 10.75°, respectively. The manga-
Acknowledgements
The authors thank Department of Science and Technol-
ogy, New Delhi, India, for financial assistance and X-ray
diffraction facility.
Appendix A. Supplementary materials
˚
nese and manganese separation is 3.65 A. The analogous
trifluoroacetato complex [8] compound has MnÁ Á ÁMn sep-
CCDC 638265, 638266, 638267, 638268, 639910,
639911, 639912, and 640151 contain the supplementary
crystallographic data for I, II, III, IV, V, VI, VII, and VIII.
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 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version,
at doi:10.1016/j.inoche.2007.04.002.
˚
aration 3.74 A. However more close structures are the com-
pound of cobalt and nickel with identical environment
˚
which shows Co–Co separation [4i] distance is 3.65 A
˚
Ni–Ni [4b] distance is 3.56 A, respectively. Since ionic radii
MeOH
[Mn2(L3)4(L2)4(H2O)]
Mn(OAc)2.4H2O
L2 + L3
VIII
O
O-
N
NO2
L3
L2
References
[1] (a) O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M.
Eddaoudi, J. Kim, Nature 423 (2003) 705;
Scheme 3.
(b) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc.
Chem. Res. 31 (1998) 474.
[2] (a) F.A. Cotton, C. Lin, C.A. Murillo, Acc. Chem. Res. 34 (2001) 759;
(b) M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.
Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319;
(c) B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629;
(d) S.L. James, Chem. Soc. Rev. 32 (2003) 276;
(e) R. Murugavel, M.G. Walawalkar, M. Dan, H.W. Roesky, C.N.R.
Rao, Acc. Chem. Res. 37 (2004) 763;
(f) S. Noro, S. Kitagawa, M. Kondo, K. Seki, Angew. Chem., Int. Ed.
39 (2000) 2081.
[3] Q.-R. Fang, G.-S. Zhu, M. Xue, Q.-L. Zhang, J.-Y. Sun, X.-D. Guo,
S.-L. Qiu, S.-T. Xu, P. Wang, D.-J. Wang, Y. Wei, Chem. Eur. J. 12
(2006) 3754.
[4] (a) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2007)
643;
(b) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2006)
4673;
(c) N. Barooah, A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem.
Commun. 9 (2006) 1251;
(d) A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem. Commun. 9
(2006) 1169;
Fig. 4. The crystal structures of VIII (drawn with 20% thermal ellipsoid)
The select bond distances Mn2–O9, 2.19; Mn2–O13, 2.15; Mn2–N8, 2.27;
Mn2–O6 , 2.19; Mn2–O17(aqua), 2.22; Mn1–O14, 2.17; Mn1–O5, 2.17;
˚
Mn1–O17(aqua), 2.23; Mn1–N5, 2.26; Mn1–N6, 2.27 A.

964
A. Karmakar et al. / Inorganic Chemistry Communications 10 (2007) 959–964
(e) K. Deka, R.J. Sarma, J.B. Baruah, Inorg. Chem. Commun. 9 (2006)
931;
[5] For the solid phase synthesis as well as solution phase synthesis; the
synthetic procedures were identical to the ones reported in Ref. [4], but
with appropriate metal salts and ligands are used in each cases.
[6] E.C. Constable, Chem. Ind. (1994) 56.
[7] T. Ohmura, W. Mori, T. Takai, T. Ikeda, A. Maeda, Mater. Sci.
Poland 23 (2005) 729.
(f)N.Barooah, R.J. Sarma, J.B. Baruah, Eur.J.Inorg. Chem.(2006)2942;
(g) A. Karmakar, K. Deka, R.J. Sarma, J.B. Baruah, Inorg. Chem.Com-
mun. 9 (2006) 836;
(h) K. Deka, N. Barooah, R.J. Sarma, J.B. Baruah, J. Mole. Struct. 827
(2007) 44;
[8] S.-B. Yu, S.J. Lippard, I. Shweky, A. Bino, Inorg. Chem. 31 (1992)
3502.
(i) A. Karmakar, R.J. Sarma, J.B. Baruah, Polyhedron 26 (2007) 1347.