Two novel Zn(II) helical chain polymers containing both bridging and terminal carboxylato groups

Article abstract of DOI:10.1016/S1387-7003(03)00108-4

Two novel carboxylato-bridged Zn(II) polymeric complexes [Zn(L)(CCl₃COO)]_n and [Zn(L)(CF₃COO)]_n where [L=2-N-(2′pyridylimine)benzoic acid] have been synthesized and structurally characterized. The structures of

Full text of DOI:10.1016/S1387-7003(03)00108-4

Inorganic Chemistry Communications 6 (2003) 790–793
www.elsevier.com/locate/inoche
Two novel Zn(II) helical chain polymers containing both
bridging and terminal carboxylato groups
a
Chirantan Roy Choudhury , Amitabha Datta , Volker Gramlich , G.M. Golzar
a
b
c
c,
a,1
Hossain , K.M. Abdul Malik *, Samiran Mitra
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Department of Chemistry, Laboratium f€ur Kristallographie ETH, Eidgen€ossische Technische Hochschule Z€urich, Z€urich CH-8092, Switzerland
Department of Chemistry, Cardiff University, Cardiff, Park Place, P.O. Box 912, Cardiff CF10 3TB,UK
b
c
Received 7 February 2003; accepted 27 February 2003
Abstract
Two novel carboxylato-bridged Zn(II) polymeric complexes ½ZnðLÞðCCl3COOފ and ½ZnðLÞðCF3COOފn, where [L ¼ 2-N-ð20-
n
pyridylimine)benzoic acid] have been synthesized and structurally characterized. The structures of the two complexes are similar
with the ZnðLÞðCl3CCO2Þ or ZnðLÞðF3CCO2Þ units being repeated to form infinite helical chains. In each structure, the neigh-
bouring Zn atoms are bridged sequentially by syn–anti carboxylate groups of the Schiff base.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Zinc; Carboxylate; Crystal structure; Helical chain; Schiff base
The crystal engineering of solid materials is of ever
increasing interest to chemists and material scientists
alike. It has become apparent that coordination poly-
mers of specific network topology may be prepared by
the appropriate selection of metal and multifunctional
ligands [1–3]. The different network derived so far in-
clude one-dimensional chains, which can be helical, two-
dimensional sheets with a variety of connectivities and
various three-dimensional structures [4]. In this regard
studies on metal carboxylato complexes as models for
metalloproteins have long been of interest [5–7]. More-
over, because in metalloproteins, carboxylate ligands are
provided by side amino acid chains, polydentate che-
lating ligands containing a carboxylate groups are of
biological relevance [8].
bridges prefer the formation of chain or layer structures
[9,10]. Carboxylate groups are known to assume differ-
ent types of bridging conformations: triatomic syn–syn,
syn–anti, anti–anti and monoatomic [10]. Carboxylato-
bridged zinc complexes are of continuous interest be-
cause of their potential relevance as enzyme model [11].
In recent years, there has been a growing interest in
enzymes with more than one zinc atom in the active site
[12–15]. Another reason for the importance of zinc in
enzyme chemistry is that it can adopt various coordi-
nation numbers, i.e., 4, 5, or 6 relatively easily [16,17].
Recently Colacio et al. [18] has reported some new
carboxylato-bridged copper(II) complexes with triden-
tate Schiff-base ligands containing one carboxylate
group, where the carboxylato group undergoes self-
assembly process promoted by metal ions leading to
syn–anti polymeric network.
As a general trend on the basis of their representative
geometries, the syn–syn conformations prefer to form
dinuclear complexes while the syn–anti or anti–anti
In our efforts to investigate the bonding nature of
carboxylato-bridged zinc(II) complexes, we have now
synthesized and structurally characterized two helical
* Corresponding author. Tel.: +44-29-2087-4000x7244; fax: +44-29-
2087-4030.
chain polymeric network ½Znðl-LÞðCCl3COOފ (1) and
n
½Znðl-LÞðCF3COOފ (2) ðL ¼ C13H10N2O2Þ. To our
n
E-mail addresses: malikka@cardiff.ac.uk (K.M. Abdul Malik),
smitra_2002@yahoo.com (S. Mitra).
knowledge, these complexes are the first examples of
carboxylato-bridged zinc(II) polymers, containing both
1
Also corresponding author.
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00108-4

C.R. Choudhury et al. / Inorganic Chemistry Communications 6 (2003) 790–793
791
bridging and terminal unidentate carboxylate groups
derived from zinc-trichloro or trifluoroacetate and Schiff
base. The synthesis 2 and structural characterization 3 of
these interesting complexes are reported herein. Re-
cently, it has been reported that in a new oxalato-
bridged complex, the same Schiff-base ligand acts as a
tridentate [19], but in the present case it behaves as a
tetradentate ligand. It should be mentioned that the
helical chain structural motif is of current interest in
supramolecular chemistry owing to its involvement in
biological systems and enantioselective catalysis [20].
The structures of the complexes 1 and 2 are similar in
the sense that they are both polymeric, in which the
ZnðLÞðCl3CCO2Þ orZnðLÞðF3CCO2ÞðL ¼ C13H9N2OÀ2 Þ
units are repeated to form infinite helical chains. In each
structure, the neighbouring Zn atoms are bridged se-
quentially by syn–anti carboxylate groups of the Schiff
base. The immediate environments of the Zn centre and a
perspective view of the helical chain of 1 are given in Figs.
1 and 2, respectively; the corresponding views of 2 are
given in Figs. 3 and 4, respectively.
Fig. 1. A general view of the structure of 1 showing the immediate
environments of the Zn atom and atom labeling. Atom numbers
marked with #1 and #2 are generated by symmetry (1=2 À x,
À1=2 þ y, 1=2 À z) and (1=2 À x, 1=2 þ y, 1=2 À z), respectively. Se-
In the structure of 1, the Schiff-base anion acts as a
tridentate ligand involving its two nitrogen [N(1), N(2)]
and one carboxylate oxygen [O(1)] atoms towards a
zinc(II) ion, while it utilizes its other carboxylate oxygen
[O(2)] atom to form a bridge with another zinc(II) ion.
The Cl3CCOÀ2 anion is bonded to the Zn ion through
one of its oxygen [O(3)] atoms only. The metal ion ex-
ꢀ
lected distances (A) and angles (°): Zn(1)–N(1) 2.161(2), Zn(1)–N(2)
2.163(2), Zn(1)–O(1) 2.018(2), Zn(1)–O(2)#1 1.985(2), Zn(1)–O(3)
1.993(2), O(2)#1–Zn(1)–O(3) 108.71(8), O(2)#1–Zn(1)–O(1), 93.37(8),
O(3)–Zn(1)–O(1) 108.78(8), O(2)#1–Zn(1)–N(1) 160.39(9), O(3)–
Zn(1)–N(1) 90.63(8), O(1)–Zn(1)–N(1) 83.05(8), O(2)#1–Zn(1)–N(2)
92.84(9), O(3)–Zn(1)–N(2) 116.12(9), O(1)–Zn(1)–N(2) 129.69(9),
N(1)–Zn(1)–N(2) 75.17(9), C(1)–O(1)–Zn(1) 130.0(2), C(1)–O(2)–
Zn(1)#2 124.6(2), C(14)–O(3)–Zn(1) 119.4(2).
2
The Schiff-base ligand (HL) [L ¼ 2-N-ð20-pyridylimine)benzoic
acid] was prepared by refluxing pyridine 2-carboxaldehyde (1 mmol)
with anthranilic acid (1 mmol) in 20 ml methanol for half an hour. The
resultant brownish-red liquid was used as ligand without further
purification. The ½Znðl-LÞðCCl3COOފ (1) and ½Znðl-LÞðCF3COOފ
n
n
(2) were prepared by drop-wise addition of the ligand solution (1
mmol) to a solution of zinc-trichloroacetate (0.5 g, 1 mmol) or
trifluoroacetate (0.48 g, 1 mmol) in methanol. The resulting brownish-
red solutions for both 1 and 2 were kept at room temperature for two
days. This yielded light-brownish rectangular crystals for both
complexes, which were isolated by filtration and air-dried. Yield:
46% for 1 and 50% for 2. Anal. Complex 1: Found: C, 39.5; H 1.9; N
6.0; Zn, 14.5. Calcd. for C15H9Cl3N2O4Zn: C, 39.7; H, 2.0; N, 6.2; Zn,
14.4. Complex 2: Found. C, 44.4; H, 2.0; N 6.8; Zn, 16.3. Calc. for
C15H9F3N2O4Zn: C, 44.6; H, 2.2; N, 6.9; Zn, 16.2.
3
Crystal data for 1, empirical formula C15H9Cl3N2O4Zn, FW
452.96, monoclinic, P21=n, a ¼ 11:6198ð12Þ, b ¼ 9:088ð2Þ, c ¼
ꢀ
ꢀ3
ꢀ
17:0098ð8Þ A, b ¼ 109:372ð6Þ°, V ¼ 1694:6ð3Þ A , Z ¼ 4,
D
ðcalculatedÞ ¼ 1:775 mg=m3, kðMo-KaÞ ¼ 0:71073 A, lðMo-KaÞ ¼
1:945 mmÀ1, T ¼ 293ð2Þ K, Nonius CAD4 diffractometer, crystal size
0:40 Â 0:35 Â 0:30 mm, 3243 reflections collected, 3082 unique
(Rint ¼ 0:0137), R1 ¼ 0:0295 [data with I > 2rðIފ, wR2 ¼ 0:0808 (all
data). Crystal data for 2, C15H9F3N2O4Zn, Mr ¼ 403:61, monoclinic,
ꢀ
space group P21=c, a ¼ 10:727ð8Þ, b ¼ 16:382ð9Þ, c ¼ 9:122ð7Þ A,
ꢀ3
b ¼ 106:80ð6Þ°,
V ¼ 1535ð2Þ A ,
Z ¼ 4,
DðcalculatedÞ ¼
1:747 mg=m3, kðCu-KaÞ ¼ 1:54178 A, lðCu-KaÞ ¼ 2:789 mmÀ1
,
4 circle Stoe diffractometer, crystal size
ꢀ
T ¼ 293ð2Þ K, Picker
0:10 Â 0:07 Â 0:05 mm, 1740 reflections collected, 1578 unique
ðRint ¼ 0:0278, R1 ¼ 0:0667 [data with I > 2rðIފ, wR2 ¼ 0:1707 (all
data). Programs: structure solution (SHELX-S) [22] and refinement
(SHELXL-96) [23] for both compounds.
Fig. 2. A view of the structure of 1 showing the formation of the helical
chain along the b-axis.

792
C.R. Choudhury et al. / Inorganic Chemistry Communications 6 (2003) 790–793
and the axial sites are occupied by the N(1) atom of the
Schiff base and O(2)#1 atom from a symmetry-related
Schiff base. The two Zn–N distances 2.161(1) and
ꢀ
2.163(2) A are identical. The two Zn–O distances in-
volving the bridging carboxylate group 2.018(2),
ꢀ
1.985(2) A are also similar and comparable with the Zn–
O distance involving the trichloroacetate group 1.993(2)
ꢀ
A. It is to be noted that the C–O distance involving the
ꢀ
O(4) atom not bonded to the metal 1.211(4) A is con-
siderably shorter than those involving the O(1), O(2)
ꢀ
and O(3) atoms bonded to zinc 1.250(4)–1.264(3) A. The
ꢀ
intrachain Zn ꢀ ꢀ ꢀ Zn separation is 5.080 A. The orien-
tation of the carboxylate groups leads to an infinite
helical chain structure generated by the 2-fold screw axis
parallel to the crystallographic b-axis.
The structure of 2 is similar to that of 1 in many re-
spects. The Schiff-base anion is again tridentate towards
a zinc(II) ion, and simultaneously it also forms a bridge
with a neighbouring zinc(II) centre related by a c-glide
plane. The Schiff-base ligand thus utilizes both of its
carboxylato-oxygen atoms, while the F3CCOÀ2 anion is
monodentate. The Zn atom in this structure also ex-
hibits ZnN2O3 coordination environments, but here the
geometry may be best described in terms of a distorted
square pyramid. The pyramid base is formed by the
N(1), N(8), O(16) and O(1f) atoms, while the apical
position is occupied by the bridging O(17)#1 atom. The
N(1), N(8), O(16) and O(1f) atoms show deviations
Fig. 3. A general view of the structure of 2 showing the immediate
environments of the Zn atom and atom labeling. Atom numbers
marked with #1 and #2 are generated by symmetry (x, 1=2 À y,
ꢀ
1=2 þ z) and (x, 1=2 À y, 1=2 À z) respectively. Selected distances (A)
and angles (°): Zn–N(1) 2.124(5), Zn–N(8) 2.165(5), Zn–O(16)
2.015(4), Zn–O(17)#1 1.962(5), Zn–O(1F) 1.986(5), O(17)#1–Zn–
O(1F) 112.4(2), O(17)#1–Zn–O(16), 99.5(2), O(1F)–Zn–O(16) 96.3(2),
O(17)#1–Zn–N(1) 114.7(2), O(1F)–Zn–N(1) 88.8(2), O(16)–Zn–N(1)
140.4(2), O(17)#1–Zn–N(8) 92.8(2), O(1F)–Zn–N(8) 154.5(2), O(16)–
Zn–N(8) 82.7(2), N(1)–Zn–N(8) 76.6(2), C(2F)–O(1F)–Zn 122.8(4),
C(15)–O(16)–Zn 132.1(4), C(15)–O(17)–Zn#2 128.5(4).
ꢀ
of )0.114(8), 0.118(8), )0.105(8) and 0.101(8) A, re-
spectively, from the mean plane of these atoms, showing
a tetrahedral distortion from planarity. The Zn atom is
ꢀ
displaced from this plane by 0.546(5) A towards the
apical O(17)#1 atom. The carboxylate bridges are
forced to orient in a mutually basal–apical positions.
ꢀ
The intrachain Zn ꢀ ꢀ ꢀ Zn distance of 5.169 A is only
marginally longer than that in 1. The Zn–N and Zn–O
ꢀ
distances [2.124(5), 2.165(5) and 1.962(5)–2.015(4) A,
respectively], and the dimensions of the Schiff-base li-
gand are similar to the corresponding values in 1. In-
terestingly, the infinite helical chain in 2 propagates
along the crystallographic c-axis.
In the IR spectrum, complex 1 shows bands at 1695
and 1400 cmÀ1 and complex 2 at 1705 and 1395 cmÀ1
assigned as masðCOOÀÞ and msymðCOOÀÞ, respectively.
The differences (Dm) 295 cmÀ1 for 1 and 310 cmÀ1 for 2
are consistent with the bridging coordination modes of
the carboxylate group [21]. The complexes also show
two bands masðCOOÀÞ and msymðCOOÀÞ at 1636 and
1448 cmÀ1 for 1 and at 1638 and 1445 cmÀ1 for 2, re-
spectively, suggesting
a monodentate coordination
Fig. 4. A view of the structure of 2 showing the formation of the helical
chain along the c-axis.
mode of the carboxylate group [21]. These findings are
confirmed by the X-ray structure analyses, which clearly
reveal that the carboxylate groups belonging to the
Schiff base and trihaloacetate groups, respectively, are
indeed bridging and monodentate in nature. The bands
at 1586, 1473 and 1430 cmÀ1 for 1 and at 1593, 1491
hibits a ZnN2O3 coordination mode, showing a highly
distorted trigonal bipyramidal geometry. The basal po-
sitions are occupied by the N(2) and O(1) atoms of the
Schiff base and the O(3) atom of the Cl3CCOÀ2 group

C.R. Choudhury et al. / Inorganic Chemistry Communications 6 (2003) 790–793
793
and 1428 cmÀ1 for 2 may be assigned for the pyridine
skeleton.
The complex 1 is stable up to 130 °C and complex 2 is
stable up to 137 °C. The mass loss in the TGA curve
corresponds to the release of one coordinated trichloro-
acetate or trifluoroacetate group per ½Znðl-LÞ
ðCCl3COOފ or ½Znðl-LÞðCF3COOފ unit between 131
and 210 °C for 1 and 137 and 219 °C for 2, followed by the
loss of one molecule of the Schiff-base ligand between 211
and 580 °C for 1 and 219 and 592 °C for 2.
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Full crystallographic details including atomic coordi-
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the Cambridge Crystallographic Data Centre, CCDC
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Financial assistance from CSIR, UGC, DST and
AICTE, New Delhi is gratefully acknowledged.
K.M.A.M. is thankful to EPSRC for support of the X-
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