Synthesis, spectral characterization and structural investigation on some 4-aminoantipyrine containing Schiff base Cu(II) complexes and their molecular association

Article abstract of DOI:10.1016/j.poly.2006.09.004

Compounds [Cu(L₁)₂] (1) and [Cu(L₂)₂] (2), where L₁ and L₂ are Schiff base ligands of 4-aminoantipyrine and substituted salicylaldehydes, were synthesized and characterized using various spectroscopic techniques such as elemental analysis, UV-Vis, IR, and NMR. The single crystal X-ray structures for L₁, L₂, and their corresponding Cu(II) complexes assembled in a 1:2 metal to ligand ratio were analyzed for their various weak H-bonding and dimeric association. The structural analysis of compounds 1 and 2, being the first crystal structures in this series, deserves special attention to help further the understanding in this area of structure-reactivity correlation studies. Further these compounds, composed of very similar chemical composition with a small difference in the substituent on the salicylaldehyde moiety, influenced through various weak inter- and intramolecular H-bonding and C-H?π interactions, rearrange the geometry around Cu(II) from a tetrahedrally distorted square planar geometry in [Cu(L₁)₂] (1) to square planar in [Cu(L₂)₂] (2). Steric strain imposed by the methyl substitution on the 4-aminoantipyrine moiety of the Schiff base ligand, causing this small change of the Cu(II) geometry, along with various weak interactions is analyzed in detail.

Full text of DOI:10.1016/j.poly.2006.09.004

Polyhedron 26 (2007) 749–756
www.elsevier.com/locate/poly
Synthesis, spectral characterization and structural investigation on
some 4-aminoantipyrine containing Schiff base Cu(II) complexes
and their molecular association
*
*
P. Mosae Selvakumar, E. Suresh , P.S. Subramanian
Analytical Science Division, Central Salt and Marine Chemicals Research Institute (CSIR), Gijubhai Badhegha Marg, Bhavnagar, Gujarat 364 002, India
Received 1 August 2006; accepted 4 September 2006
Available online 22 September 2006
Abstract
Compounds [Cu(L1)2] (1) and [Cu(L2)2] (2), where L1 and L2 are Schiff base ligands of 4-aminoantipyrine and substituted salicylal-
dehydes, were synthesized and characterized using various spectroscopic techniques such as elemental analysis, UV–Vis, IR, and NMR.
The single crystal X-ray structures for L1, L2, and their corresponding Cu(II) complexes assembled in a 1:2 metal to ligand ratio were
analyzed for their various weak H-bonding and dimeric association. The structural analysis of compounds 1 and 2, being the first crystal
structures in this series, deserves special attention to help further the understanding in this area of structure–reactivity correlation studies.
Further these compounds, composed of very similar chemical composition with a small difference in the substituent on the salicylalde-
hyde moiety, influenced through various weak inter- and intramolecular H-bonding and C–Hꢀ ꢀ ꢀp interactions, rearrange the geometry
around Cu(II) from a tetrahedrally distorted square planar geometry in [Cu(L1)2] (1) to square planar in [Cu(L2)2] (2). Steric strain
imposed by the methyl substitution on the 4-aminoantipyrine moiety of the Schiff base ligand, causing this small change of the Cu(II)
geometry, along with various weak interactions is analyzed in detail.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Copper (II) complex; Crystal structure; Schiff base; Antipyrine
1. Introduction
taining different 2-hydroxy aromatic moieties such as
furan-2-aldehyde, thiophene-2-aldehyde [4], and pyrrole-
2-carboxaldehyde [5], etc. Thus there are a number of
reports on compounds based on aminopyridine Schiff
bases, though accounts of their crystal structures are lim-
ited. The salicylaldehyde based SAAP [6] and pyrrole based
HPAP [5] Schiff base ligands and their Cu(II) [7] and lan-
thanum (Pr, Nd, Gd, Dy, Y, U) complexes [8,9] have been
reported already. Although there are many compounds in
the literature, our search on the CCDC shows no crystal
structure has been reported so far and the structures of
those entire complexes are elucidated only with spectro-
scopic evidence. With this in view, the present report
accounts for the synthesis of two antipyrine Schiff base
ligands L1 and L2 (Fig. 1) and their Cu(II) complexes
[Cu(L1)2] (1), and [Cu(L2)2] (2), with a special impetus on
their structural investigations. Further, the structural
Schiff bases of 4-aminoantipyrine and its complexes are
known for their variety of applications in the area of catal-
ysis [1,2], clinical applications [3], and pharmacology [4].
New kinds of chemo-therapeutic agents containing Schiff
bases have gained significant attention among biochemists
[1] and of those aminopyrines are commonly administered
intravenously to detect liver disease [3] in clinical treat-
ment. Hence a systematic understanding on the struc-
ture–reactivity correlation has warranted synthesis and
characterization of a variety of Schiff base complexes con-
*
Corresponding authors. Tel.: +91 278 2567760; fax: +91 278 2566970
(P.S. Subramanian).
E-mail addresses: esuresh@csmcri.org (E. Suresh), siva140@yahoo.
co.in (P.S. Subramanian).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.09.004

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P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
CH3
s, OH, 1H), 9.75 (s, CH@N, 1H), 7.55-7.26 (m, Ar 7H),
CH3
H3C
H3C
3.23 (s, N-CH3,3H), 2.45 (s, C–CH3,3H). IR data (KBr,
cmꢁ1) 3639–2593 (br), 1665 (sharp, st), 1589 (sharp, st).
UV–Vis [CH3CN, kmax, nm, (e, Mꢁ1, cmꢁ1)], 345 (3000),
380 (2800). ES [MSI] Calc for C18H15Cl2N3O2 (m/z) =
375.05; Found = (m/z+H+) = 376.25; (m/z+Na+)= 398.24.
Anal. Calc. for C18H15Cl2N3O2; C, 57.46; H, 4.02; N,
11.17. Found: C, 57.5; H, 3.95; N, 11.0%.
N
L1
N
L2
N
N
N
N
O
O
HO
OH
Cl
Cl
HO
Fig. 1. Schiff base ligand system L1 and L2.
2.2.3. Preparation of complexes [Cu(L1)2] (1), and
[Cu(L2)2] (2)
The Schiff base ligand L1 (0.071 gm, 0.002 mmol), dis-
solved in chloroform, was mixed with an ethanolic solution
of [Cu(CH3COO)2] Æ H2O (0.199 gm, 0.001 mmol) and
allowed to stir continuously. The immediate color change
from green to dark red indicates complexation. Upon slow
evaporation at room temperature, the dark red crystals
were obtained in 48 h. [Cu(L1)2] (1): UV–Vis [CH3CN,
analysis in the present report explores that the weak inter-
actions introduced by varying the substitution on the sali-
cylaldehyde unit, modify the Cu(II) geometry from a
tetrahedrally distorted square planar to a distorted square
planar geometry.
k
max, nm, (e, Mꢁ1, cmꢁ1)], 364 (1980), 509 (80). ES [MSI]
Calc for CuC36H30N6O6 (m/z+H+) = 708.18. Found =
708.30. Anal. Calc. for CuC36H30N6O6: C, 61.05; H, 4.55;
N, 11.88. Found: C, 61.5; H, 4.48; N, 11.36%. [Cu(L2)2]
(2): The above procedure was repeated for 2 adapted for
L2 with a 1:2 Cu(CH3COO) Æ H2O: L2 ratio. UV–Vis
[CH3CN, kmax, nm, (e, Mꢁ1, cmꢁ1)], 359 (6970), 427
(1300), 531 (300). ES [MSI] Calc for CuC36H28Cl4N6O4
(m/z+H+) = 814.02. Found = 814.02. Anal. Calc. for
CuC36H28Cl4N6O4: C, 53.11; H, 3.46; N, 10.32. Found:
C, 52.99; H, 3.97; N, 10.20%.
2. Experimental
2.1. Materials
Cu(CH3COO)2 Æ H2O, 4-aminoantipyrine, 2,4-dihydr-
oxybenzaldehyde, and 3,5-dichlorosalicylaldehydes were
purchased from Aldrich & Co. All these chemicals were
used as received without any further purification.
2.2. Synthesis
2.2.1. Preparation of the ligands: L1 Æ MeOH
2.3. Physical measurements
A 1:1 equimolar methanolic solution of 4-aminoantipy-
rine (0.406 gm, 0.001 mol) and 2,4-dihydroxybenzaldehyde
(0.276 gm, 0.001 mol) were mixed and gently heated for 2 h
with constant stirring. The characteristic yellow precipitate
obtained by Schiff base condensation was filtered out and
kept for crystallization, dissolving in methanol. Fine yellow
crystals obtained upon slow evaporation at room tempera-
ture were characterized, including single crystal X-ray
diffraction. L1: 4-{[(1Z)-(2,4-dihydroxyphenyl)methylene]-
amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one.
NMR data (d, Methanol-d4): 13.55 (br s, –OH, 1H), 9.63 (s,
CH@N, 1H), 7.56–7.20 (m, Ar phenyl, 5H), 6.40–6.31 (m,
Ar, 3H), 3.14 (s, N–CH3, 3H), 2.35 (s, C–CH3, 3H). IR
data (KBr, cmꢁ1): 3500–2900 (weak, br), 1615 (st, intense),
1586 (s, br). UV–Vis [CH3CN, kmax, nm, (e, Mꢁ1, cmꢁ1)],
336 (3030), 422 (2840). ES [MSI] Calc for C19H21N3O4
(m/z+H+) = 323.34. Found = 324.26. Anal. Calc. for
C19H21N3O4: C, 64.21; H, 5.95; N, 11.82. Found: C,
63.86; H, 5.56; N, 12.25%.
Microanalysis of the complexes was done using a
Perkin–Elmer PE 2400 series II CHNS/O elemental ana-
lyzer. IR spectra were recorded using KBr pellets (1% w/
w) on a Perkin–Elmer Spectrum GX FT-IR spectropho-
tometer. Electronic spectra were recorded on a Schimadzu
UV 3101PC spectrophotometer. Mass analyses were per-
formed using electron spray ionization (ESI) technique
1
on a Waters Q Tof-micro mass spectrometer. H NMR
spectra were recorded on a Bruker Avance DPX 200 FT-
NMR spectrometer. Chemical shifts for proton resonances
are reported in ppm (d) relative to tetramethylesilane.
2.4. X-ray crystallography
A summary of the crystallographic data and details of
data collection for L1, L2 and 1, 2 are given in Table 1.
Selected bond distances and bond angles for compounds
1 and 2 are given in Table 2. In each case, a crystal of suit-
able size was selected from the mother liquor and immersed
in partone oil, then mounted on the tip of a glass fiber and
cemented using epoxy resin. Intensity data for all three
2.2.2. Synthesis of L2
4-{[(1Z)-(3,5-dichloro-2-hydroxyphenyl)methylene]amino}-
1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one. The
above synthetic procedure was repeated for L2 appropri-
ately adapting 3,5-dichlorosalicylaldehyde in place of 2,4-
dihydroxybenzaldehyde. NMR data (d, CDCl3): 14.3 (br
˚
crystals were collected using MoKa (k = 0.71073 A) radia-
tion on a Bruker SMART APEX diffractometer equipped
with a CCD area detector. The data integration and reduc-
tion were processed with SAINT [10] software. An empirical

P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
751
Table 1
Summary of crystallographic data for the compounds
Code
L1
L2
1
2
Chemical formula
Formula weight
C
19H21N3O4
C18H15Cl2N3O2
376.23
7.0141(8)
8.0449(9)
30.487(3)
90
90.972(2)
90
4
1720.1(3)
monoclinic
P21/n
0.71073
1.453
C36H32Cu1N6O6
708.22
C18H14Cl2Cu0.50N3O2
406.99
23.197(2)
6.7034(6)
22.443(2)
90
90.918(3)
90
8
3489.4(6)
monoclinic
C2/c
0.71073
1.549
355.39
13.9443(16)
6.8950(8)
19.390(2)
90.0
109.953(2)
90
4
1752.4(3)
monoclinic
P21/c
0.71073
1.347
˚
a (A)
10.2073(16)
12.1721(19)
13.776(2)
74.189(3)
74.273(3)
80.569(3)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
Z
3
˚
V (A )
1577.7(4)
triclinic
Crystal system
Space group
ꢀ
P1
˚
Radiation used, k (A)
0.71073
1.491
0.751
293
qcalc. (g cmꢁ3
)
Absolute coefficient, l (mmꢁ1
)
0.096
100
0.0534
0.394
293
0.0482
0.983
293
0.0382
Temperature (K)
a
R1 on ½ðF 2oÞꢂ
0.0621
b
wR2 on ðF 2oÞ
0.1285
0.1237
0.1526
0.0954
P
P
a
R ¼ kF oj ꢁ jF ck= jF oj.
Rw ¼ ½SwðF 2o ꢁ F c2Þ ꢂ=S½wðF o2Þ2ꢂ1=2; w ¼ 1=rðF oÞ .
2
2
b
Table 2
3. Results and discussion
3.1. Spectral investigation
˚
Selected bond distances (A) and angles (°) for complexes 1 and 2 with
estimated standard deviations in the parenthesis
1
2
˚
Bond length (A)
The Schiff base ligands L1 and L2 are yellow in color and
are soluble in almost all organic solvents, such as metha-
nol, acetonitrile, chloroform, acetone, DMF and DMSO,
and the corresponding metal complexes [Cu(L1)2] (1) and
[Cu(L2)2] (2) obtained with a 1:2 metal ligand ratio are
found to be dark red in color. The electronic spectra for
both these ligands behave very similarly and show two
characteristic bands at 336–360 nm and 420 nm represent-
ing intra ligand charge transfer transitions. The corre-
sponding metal complexes 1 and 2 exhibit a characteristic
d-d band at 500–530 nm with a broad spectral feature.
While the position of kmax is attributable to the square pla-
nar geometry, the broadness of the spectral pattern can be
attributed to the degree of distortion in the geometry. The
three characteristic IR signals for L1 and L2, corresponding
to (i) OH; (ii) C@O, C@N; (iii) phenolate oxygen Cꢀ ꢀ ꢀO,
were analyzed. The broad IR signal centered at
3430 cmꢁ1 obtained for both the free ligands L1 and L2
can be ascribed to the OH functional groups. The com-
pounds 1 and 2 show sharp and intense peaks in the range
of 1572–1589 cmꢁ1 and 1616–1665 cmꢁ1 which can be
attributed to mCH@N and mC@O stretching modes respec-
tively. The characteristic peak at 9.6–9.75 d in the 1H
NMR spectrum further confirms the azomethine CH@N
condensation. The disappearance of the OH peak and the
significant shift in the mC@N stretching frequency in 1 and
2, with respect to the corresponding free ligands, indicate
the complexation with the Cu(II) center. The shift in the
azomethine peak by 10–15 cmꢁ1 in the IR spectra indicates
its involvement in coordination. The IR signal observed at
Cu(1)–O(3)
Cu(1)–O(2)
Cu(1)–O(2)#1
Cu(1)–O(6)
Cu(1)–N(6)
Cu(1)–N(3)
Cu(1)–N(3)#1
1.887(3)
1.8878(14)
1.8878(14)
1.924(3)
1.953(3)
1.967(3)
1.9567(17)
1.9567(17)
Bond angle (°)
O(3)–Cu(1)–O(6)
O(3)–Cu(1)–N(6)
O(6)–Cu(1)–N(6)
O(3)–Cu(1)–N(3)
O(6)–Cu(1)–N(3)
N(6)–Cu(1)–N(3)
O(2)#1–Cu(1)–O(2)
O(2)#1–Cu(1)–N(3)#1
O(2)–Cu(1)–N(3)#1
O(2)#1–Cu(1)–N(3)
O(2)–Cu(1)–N(3)
N(3)#1–Cu(1)–N(3)
91.78(12)
145.30(13)
94.07(13)
95.95(13)
143.74(13)
99.23(13)
154.31(9)
90.75(6)
93.63(6)
93.63(6)
90.75(6)
160.18(10)
absorption correction was applied to the collected reflec-
tions using SADABS [11]. The structures were solved by direct
methods using SHELXTL [11b] and were refined on F2 by the
full-matrix least-squares technique using the SHELXL-97 [12]
package. Graphics are generated using ORTEP. Packing
and H-bonding diagrams are generated by PLATON [13]
and MERCURY [116]. In all the four compounds, non-hydro-
gen atoms were refined anisotropically until convergence
was reached and hydrogen atoms attached to all carbon
atoms were geometrically fixed using the program SHELXTL.

752
P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
1616–1665 cmꢁ1, representing the C@O group attached to
the antiypyrine five-membered ring remains unchanged in
compounds 1 and 2 with respect to the free ligands, sym-
bolizing that it is not involved in the coordination.
[15b]. In addition to the C–Hꢀ ꢀ ꢀN interaction, the lattice
methanolic OH acts as both donor and acceptor through
O–Hꢀ ꢀ ꢀO hydrogen bonding with the molecular pairs
(Fig. 2b) of the L1 moiety from either side creating a lay-
ered hydrogen bonded motif. The O–Hꢀ ꢀ ꢀO hydrogen
bonding parameters, with the symmetry code, between
the lattice methanol with ketonic oxygen O1 of the antipy-
rine ring and the phenolic hydrogen H2A of the ligand
moiety are as follows: O(4)–H(4A)ꢀ ꢀ ꢀO(1); H(4A)ꢀ ꢀ ꢀ
3.2. Crystal structure and molecular association
3.2.1. Structure of L1
The Schiff base ligand L1 crystallizes in the monoclinic
space group P21/c, along with methanol as a solvent of crys-
tallization. Fig. 2a–c represents the ORTEP diagram for L1, its
dimeric association and the intermolecular C–Hꢀ ꢀ ꢀN inter-
action through molecular pairs along the c-axis, respec-
tively. The meta-dihydroxy substituted salicylyl phenyl
ring C11–C16 is almost in the same plane (mean plane devi-
ation 12.89°), while the phenyl ring C1–C6 of the antipyrine
moiety is tilted by 49.67° with respect to the central five-
membered ring. The mode of molecular packing influenced
through the mode of H-bonding is dictated by variation in
the substitutions on the salicyl phenyl ring.
˚
˚
O(1) = 1.86 A; O(4)ꢀ ꢀ ꢀO(1) = 2.671(2) A, \O(4)–H(4A)ꢀ ꢀ ꢀ
O = 171°, symmetry code: 1 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z and
˚
O(2)–H(2A)ꢀ ꢀ ꢀO(4):H(2A)ꢀ ꢀ ꢀO(4) = 1.85 A, O(2)ꢀ ꢀ ꢀO(4) =
˚
2.665(2) A, \O(2)–H(2A)ꢀ ꢀ ꢀO(4) = 171°, symmetry code:
x, 5/2 ꢁ y, ꢁ1/2 + z. In addition to this intermolecular
hydrogen bonding, intramolecular O–Hꢀ ꢀ ꢀN and C–
Hꢀ ꢀ ꢀO interactions also exist for the stabilization of the
ligand moiety, incorporating the methanol molecule in
the crystal lattice (Fig. 2c).
3.2.2. Structure of L2
As depicted in Fig. 2b, the molecules are oriented in
such a way that the C18 methyl group of the antipyrine
ring is aligned towards the azomethine N3, making strong
intermolecular C–Hꢀ ꢀ ꢀN interactions forming molecular
pairs along the c-axis [C(18)–H(18A)ꢀ ꢀ ꢀN(3); H(18A)ꢀ ꢀ ꢀ
Ligand L2 crystallizes in monoclinic space group P21/n
with four molecules in the unit cell. Fig. 3a and b represents
the ORTEP diagram and their dimeric association of L2. In
L2, the ortho and para positions of the phenyl ring, with
respect to the OH group, are substituted by Cl1 and Cl2
respectively. The dichloro substituted ‘‘sal’’ phenyl ring
(C11–C16) is in the same plane, while the phenyl ring
C1–C6 of the antipyrine moiety is tilted by 36.64° with
respect to the central five-membered ring. The tilt of the
phenyl group (C1–C6) with respect to central five-
membered ring can be attributed to various molecular
interactions involved in the packing pattern.
˚
˚
N(3) = 2.58 A
C(18)ꢀ ꢀ ꢀN(3) = 3.536(2) A,
\C(18)–
H(18A)ꢀ ꢀ ꢀN(3) = 171°, symmetry code: 1 ꢁ x, ꢁ1/2 + y,
1/2 ꢁ z]. H18C from the same methyl group is involved
in an intermolecular C–Hꢀ ꢀ ꢀO interaction with the exocy-
clic ketonic oxygen of the antipyrine ring along the b-axis
˚
[C(18)–H(18C)ꢀ ꢀ ꢀO(1); H(18C)ꢀ ꢀ ꢀO(1) = 2.41 A, C(18)ꢀ ꢀ ꢀ
˚
O(1) = 3.358(2) A, \C(18)–H(18C)ꢀ ꢀ ꢀO(1) = 167° symme-
try code: x, ꢁ1 + y, z] which is well within the range in
The molecules are packed along the ac-plane with a
slight offset in adjacent layers to make an effective dimeric
the review articles reported by Steiner [15a] and Nangia
c
a
b
Fig. 2. (a) ORTEP diagram with the atom numbering for L1, (b) intermolecular CHꢀ ꢀ ꢀN interaction mediated molecular pairs and (c) dimeric association
through methanol.

P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
753
Fig. 3. (a) ORTEP diagram with the atom numbering for L2 and (b) dimeric association of L2.
association via a C@Oꢀ ꢀ ꢀCl interaction. The close up view
of the dimeric association via the C=Oꢀ ꢀ ꢀCl interaction
between a centro-symmetric pair is shown in Fig. 3b. Thus,
Cl1 attached to the ‘‘sal’’ phenyl ring makes contact with
the exocyclic ketonic oxygen O1 of the five-membered anti-
˚
pyrine ring with an Oꢀ ꢀ ꢀCl distance of 3.010 A and angle
\C@Oꢀ ꢀ ꢀCl of 158.12°, which is well within the range
reported earlier [15c]. The observed Oꢀ ꢀ ꢀCl distance
˚
(3.010 A) is shorter than the sum of the Van der Waals
˚
radii for Cl and O (3.2 A) and this short contact is due to
the partial positive and negative charge residing on Cl1
and O1 respectively. Further, the Oꢀ ꢀ ꢀCl short contacts
observed in pentachloronitrobenzene by Tanaka et al.
[15d] and 2-chloro-4,6-dinitrophenol by Andersen et al.
[15e] also support a similar mode of interaction with partial
negative and positive charges between the O and Cl atoms
Fig. 4. ORTEP diagram of 1 with the atom numbering scheme.
˚
˚
at a distance 3.01 A and 3.054 A, respectively. Intramolec-
ular O–Hꢀ ꢀ ꢀN H-bonding between the phenolic hydrogen
˚
H2 and N3 (O(2)–H(2)ꢀ ꢀ ꢀN(3):H(2)ꢀ ꢀ ꢀN(3) = 1.87 A;
the antipyrine exocyclic ketonic oxygen O4 and O1 free.
The steric strain imposed by the methyl substitution on
the antipyrine moiety restricts the ligand towards bidentate
coordination from its tridentate nature. The Cu–N
˚
O(2)ꢀ ꢀ ꢀN(3) = 2.597(2) A; \O(2)–H(2)ꢀ ꢀ ꢀN(3) = 148°) and
a C–Hꢀ ꢀ ꢀO interaction (C(10)–H(10)ꢀ ꢀ ꢀO(1):H(10)ꢀ ꢀ ꢀO(1) =
˚
˚
2.28 A; C(10)ꢀ ꢀ ꢀO(1) = 2.972(3) A and C(10)–H(10)ꢀ ꢀ ꢀ
O(1) = 130°) between H10 and the exocyclic ketonic oxy-
gen O1 of the five-membered ring are also observed, and
these intramolecular H-bonding interactions have an effect
on the conformation of the molecule, including the tilt of
the phenyl ring with respect to the five-membered ring.
˚
˚
distances (Cu1–N3 = 1.967(3) A; Cu1–N6 = 1.953(3) A)
˚
and Cu–O distances (Cu1–O3 = 1.887(3) A; Cu1–O6 =
˚
1.924(3) A) are well within the range reported for related
Schiff base complexes of Cu(salen) [16,17]. The cis angles
subtended by the six-membered chelate ring with Cu(II)
from each ligand [O3–Cu1–N3 = 95.95(13)°, O6–Cu1–
N6 = 94.07(13)°, N6–Cu1–N3 = 99.23(13)°, O3–Cu1–
O6 = 91.78(12)°] deviate only marginally from the ideal
square planar value. However, the trans angles [N6–Cu1–
O3 = 145.30(13)° and O6–Cu1–N3 = 143.74(13)°] at the
Cu(II) center deviate significantly from an ideal square pla-
nar geometry, indicating a tetrahedral bias to the Cu(II)
coordination. This may be due to the steric constraints
imposed by the methyl groups attached to the five-
membered antipyrine ring towards the right approach of
3.2.3. Structure of [Cu(L1)2] (1)
An ORTEP view of the neutral Cu(II) complex, 1 with the
atom numbering scheme is shown in Fig. 4. The metal cen-
ter possesses a tetrahedrally distorted square planar geom-
etry with N2O2 donor atoms coordinating from two
different Schiff base ligands L1. Even though the ligand is
tridentate in nature, it binds the Cu(II) ion, in a bidentate
fashion through the deprotonated phenolic oxygen of the
phenyl ring and nitrogen of the azomethine group, leaving

754
P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
the ligand moiety for metal coordination. This is further
strengthened by the mean plane involving the ‘‘sal’’ units
from different ligands, which are twisted relative to each
other by approximately 58.09° and make angles of 28.86°
(C11–C16 and O3) and 37.89° (C29–C34 and O6) with
respect to the N2O2 mean plane. The N and O atoms show
considerable deviation from the mean plane involving
N2O2; trans N and O from a different ligand moiety (N3
and O6) are displaced in the downward direction
O4 and O1 respectively (C(18)–H(18A)ꢀ ꢀ ꢀO(4):H(18A)ꢀ ꢀ ꢀ
˚
O(4) = 2.49 A; C18ꢀ ꢀ ꢀO(4) = 3.364(6), \C(18)–H(18A)ꢀ ꢀ ꢀ
O(4) = 151° and C(35)–H(35A)ꢀ ꢀ ꢀO(1):H(35A)ꢀ ꢀ ꢀO(1) =
˚
˚
2.31 A, C(35)ꢀ ꢀ ꢀO(1) = 3.242(6) A and \C(35)–H(35A)ꢀ ꢀ ꢀ
O(1) = 162°). Thus, the ketonic oxygen O1 and O4 from
the two different ligand moieties may be orienting the pref-
erential hydrogen bonding mode rather than coordination
with the metal center to impose a tetrahedrally distorted
square pyramidal geometry around the Cu(II) center.
˚
˚
(ꢁ0.540 A and ꢁ0.625 A) while N6 and O3 are in the
˚
˚
opposite direction (0.494 A and 0.540 A) with Cu(II) con-
3.2.4. Structure of [Cu(L2)2] (2)
˚
tained well within the coordination plane (0.015 A). It is
Compound 2 crystallizes in the centrosymmetric space
group C2/c with Cu(II) sitting on a twofold axis. An ORTEP
view of the neutral Cu(II) complex with the atom number-
ing scheme is shown in Fig. 6. The metal center possesses a
distorted square planar geometry with N2O2 donor atoms
coordinating from two different Schiff base ligands. The
Cu(II) ion occupies a special position and coordinates with
two symmetry related ligand moieties in a bidentate fash-
ion through the deprotonated phenolic oxygen and nitro-
gen of the azomethine group, leaving the antipyrine
exocyclic ketonic oxygen free. The Cu–N distance (Cu1–
pertinent to note that such distortions in the Cu(II) geom-
etry of blue copper proteins [18–20] and Cu(salen) [21,22]
complexes with tetrahedrally distorted square planar geom-
etry are reported to deserve importance in view of catalytic
activity.
The packing diagram viewed down the a-axis with vari-
ous H-bonding interactions in 1 is depicted in Fig. 5. The
molecules are aligned as layers along the b-axis by strong
intermolecular O–Hꢀ ꢀ ꢀO interactions, between adjacent
molecules via the phenolic hydrogen H5 (from O5) of each
coordinated ligand moiety with the ketonic oxygen O1 of
the five-membered antipyrine moiety. These hydrogen
bonded layers are further cross linked along the c-axis gen-
erating the bilayer by another intermolecular O–Hꢀ ꢀ ꢀO H-
bonding between the phenolic hydrogen H2 of the second
ligand with the coordinated ‘‘sal’’ oxygen O2 [O(2)–
˚
N3 = 1.9567(17) A) and Cu–O distance (Cu1–O2 =
˚
1.8878(14) A) are well within the range reported for related
Schiff base complexes [16,17]. The cis angles subtended by
the six-membered chelate ring with Cu(II) from each ligand
[O2–Cu1–N3 = 90.75(6)°, O(2)a–Cu(1)–N(3) = 93.63(6)°]
are within the close range of an ideal square planar value,
while the trans angles [O(2)a–Cu(1)–O(2) = 154.31(9)° and
N(3)a–Cu(1)–N(3) = 160.18(10)°] at the Cu(II) center devi-
ate from square planar geometry, indicating a distorted
square planar geometry for the Cu(II) coordination. This
can be attributed to the six-membered metal-chelate ring
formation by the ligand moiety around Cu(II) and the
ligand steric constraints imposed by the methyl groups
attached to the five-membered antipyrine ring towards
the closer approach for metal coordination. The mean
˚
H(2)ꢀ ꢀ ꢀO(6):H(2)ꢀ ꢀ ꢀO(6) = 1.97 A, O(2)ꢀ ꢀ ꢀO(6) = 2.701(4)
˚
A, \O(2)–H(2)ꢀ ꢀ ꢀO(6) = 147°]. Intermolecular C–Hꢀ ꢀ ꢀO
interactions between the bilayers creates a two dimensional
H-bonded network in the bc-plane [C(17)–H(17A)ꢀ ꢀ ꢀ
˚
˚
O(3):H(17A)ꢀ ꢀ ꢀO(3) = 2.51 A, C(17)ꢀ ꢀ ꢀO(3) = 3.323(5) A,
\C(17)–H(17A)ꢀ ꢀ ꢀO(3) = 142° and C(22)–H(22)ꢀ ꢀ ꢀO(2):
˚
˚
H(22)ꢀ ꢀ ꢀO(2) = 2.43 A, C(22)ꢀ ꢀ ꢀO(2) = 3.340(6) A,\C(22)–
H(22)ꢀ ꢀ ꢀO(2) = 167°]. In addition to the above mentioned
intermolecular H-bonding, intramolecular C–Hꢀ ꢀ ꢀO H-
bonding also exist between the methyl hydrogens H18A
and H35A from different ligands with the ketonic oxygens
Fig. 5. Packing diagram of 1 viewed down the a-axis with various
Fig. 6. ORTEP diagram of 2 with the atom numbering scheme. (Hydrogen
H-bonding interactions.
atoms are omitted for clarity.)

P.M. Selvakumar et al. / Polyhedron 26 (2007) 749–756
755
Fig. 7. Packing diagram of 2 viewed down the b-axis showing the hydrogen bonding interactions.
plane involving the ‘‘sal’’ units of symmetry related ligands
are twisted relative to each other by 34.43° and make a
17.15° angle (angle of C11–C16 and O2) with respect to
the N2O2 mean plane, which is comparatively less in 2
indicating much less distortion from the square planar
geometry. The Cu(II) atom is contained well in the square
base, the trans oxygens and trans nitrogens coordinated to
it from the twofold related ligands are displaced by 0.729°
and 1.789° respectively in opposite directions.
geometry in 1 and 2, rearranging from tetrahedrally dis-
torted square planar geometry into distorted square planar.
Acknowledgements
The authors greatly acknowledge Dr. P.K. Ghosh
Director of the Central Salt and Marine Chemicals Re-
search Institute for his keen interest and encouragements.
The authors P.S. and M.S. acknowledge DST, New Delhi,
for the financial support
A packing diagram of compound 2 viewed down the b-
axis with hydrogen bonding interactions is shown in Fig. 7.
As depicted in the packing, the molecules are arranged in
layers along the c-axis. The aligned adjacent molecules
within the layers are bridged via intermolecular C–Hꢀ ꢀ ꢀO
hydrogen bonding between the methyl hydrogens H17C
and H18C of the five-membered antipyrine ring with the
exocyclic ketonic oxygen O1 from either side of the symme-
try allied ligand moiety. Details pertinent to the hydrogen
bonding interactions and symmetry code are: C(17)–
Appendix A. Supplementary material
CCDC 614053, 614054, 614055 and 614056 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.ca-
m.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 associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2006.09.004.
˚
H(17C)ꢀ ꢀ ꢀO(1):H(17C)ꢀ ꢀ ꢀO(1) = 2.38 A; C(17)ꢀ ꢀ ꢀO(1) =
˚
3.303(3) A; \C(17)–H(17C)ꢀ ꢀ ꢀO(1) = 160°, symmetry code:
ꢁx, 1 ꢁ y, ꢁz and C(18)–H(18C)ꢀ ꢀ ꢀO(1):H(18C)ꢀ ꢀ ꢀO(1) =
˚
2.57 A; C(18)ꢀ ꢀ ꢀO(1) = 3.502(3); \C(18)–H(18C)ꢀ ꢀ ꢀO(1) =
163°, symmetry code; x, 1 + y, z. The layers along the c-
axis are also involved in weak C–Hꢀ ꢀ ꢀp interactions [23]
between the antipyrine phenyl ring and azomethane hydro-
gen H10, showing the effective closer approach and orien-
tation of the neighboring molecules. Pertinent C–Hꢀ ꢀ ꢀp
interactions with the symmetry code are C10–
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