Copper(II) complexes of symmetrical and unsymmetrical tetradentate Schiff base ligands incorporating 1-benzoylacetone: Synthesis, crystal structures and electrochemical behavior

Article abstract of DOI:10.1016/j.poly.2007.10.021

Three new copper(II) complexes [CuL¹]₂(ClO₄)₂ (1), [CuL²]ClO₄ (2) and [CuL³] (3) with three Schiff base ligands [HL¹ = 1-phenyl-3-{3-[(pyridin-2-ylmethylene)-amino]-pr

Full text of DOI:10.1016/j.poly.2007.10.021

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 693–700
www.elsevier.com/locate/poly
Copper(II) complexes of symmetrical and unsymmetrical tetradentate
Schiff base ligands incorporating 1-benzoylacetone: Synthesis,
crystal structures and electrochemical behavior
c
c
a,
*
Biswarup Sarkar a,b, Gabriele Bocelli , Andrea Cantoni , Ashutosh Ghosh
a
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b
Bolpur College, Bolpur 731 204, Birbhum, West Bengal, India
IMEM-CNR, Parceo Area delle scienze 37a, I-43100 Fontanini-Parma, Italy
c
Received 31 July 2007; accepted 29 October 2007
Abstract
Three new copper(II) complexes [CuL1]2(ClO4)2 (1), [CuL2]ClO4 (2) and [CuL3] (3) with three Schiff base ligands [HL1 = 1-phenyl-3-
{3-[(pyridin-2-ylmethylene)-amino]-propylimino}-butan-1-one,
HL2 = 1-phenyl-3-[3-(1-pyridin-2-yl-ethylideneamino)-propylimino]-
butan-1-one and H2L3 = 3-[3-(1-methyl-3-oxo-3-phenyl-propylideneamino)-propylimino]-1-phenyl-butan-1-one] have been synthesized
and structurally characterized by X-ray crystallography. The mono-negative tetradentate asymmetric Schiff base ligands (L1)ꢀ and
(L2)ꢀ are chelated in complexes 1 and 2 to form square planar copper(II) complexes. In complex 1, the two units are associated weakly
through ketonic oxygen of benzoylacetone fragment to form the dimeric entity. The square planar geometry of complex 3 is unusually
distorted towards tetrahedral one. All three complexes exhibit reversible cyclic voltammetric responses in acetonitrile solution corre-
sponding to the CuII/CuI redox process. The E1/2 (ꢀ0.47 V versus SCE) of 3 shows significant anodic shift due to the tetrahedral dis-
tortion around Cu(II) compare to that of 1 and 2 (ꢀ0.82 and ꢀ0.87 V versus SCE, respectively).
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Copper(II); Unsymmetrical Schiff base; 1-Benzoylacetone; Crystal structure; Electrochemical behavior
1. Introduction
7-amino-4-methyl-5-azahept-3-en-2-one (AMAH) [3] and
8-amino-4-methyl-5-azaoct-3-en-2-one (AMAO) [4], the
monocondensation products of 2,4-pentanedione with 1,2-
ethanediamine and or 1,3-propanediamine, respectively,
these ‘half units’ have been used extensively as precursors
for the preparation of unsymmetrical quadridentate Schiff
base ligands [2,5]. In most of the ligands reported so far,
the acetylacetoneiminato side is usually kept unchanged
and the other side of the diamine is condensed with various
aromatic carbonyl compounds to study the variation and
fine tuning of stability, reactivity and electronic properties
of such complexes [5,6]. The apparent reason for keeping
the acetylacetoneiminato fixed, seems to be the facile
synthesis of the ‘half unit’ by high dilution method [3,4].
Recently, we successfully synthesized the analogous triden-
tate Schiff base by monocondensation of 1-benzoylacetone
The interest in the design, synthesis and characterization
of the transition metal complexes of unsymmetrical Schiff
base ligands has come from the realization that coordinated
ligands around central metal ions in natural systems are
unsymmetrical [1]. Recently, this class of compounds has
also attracted much attention in the field of optoelectronic
technologies for their large nonlinear responses [2]. Com-
pare to their symmetrical counterpart, syntheses of such
ligands are rather difficult because simple condensation
methodology with three components is no longer applied.
However, since the first reports on the synthesis of
*
Corresponding author.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.10.021

694
B. Sarkar et al. / Polyhedron 27 (2008) 693–700
to a methanolic solution (25 cm3) of HAMPA (10 mmol).
The mixture was warmed at 50 °C for 1 h and cooled to
room temperature. HL2 was prepared in the same way as
HL1 using 2-acetyl pyridine (10 mmol, 1.12 cm3) instead
of pyridine-2-carboxaldehyde. The tetradentate ligands
HL1 and HL2 were not isolated and the methanol solutions
were used for the synthesis of the complexes.
N
N
O
N
O
N
O
N
O
N
N
N
H2L3
HL2
HL1
Scheme 1.
2.3. Synthesis of complex [CuL1ClO4]2 (1), [CuL2ClO4]
(2)
with 1,2-ethanediamine or 1,2-propanediamine under the
similar conditions and used those ligands to synthesize tri-
nuclear Cu(II) complexes with a l3-OH core [7]. On the con-
trary, the corresponding ‘half unit’ of 1,3-propanediamine
did not result the similar trinuclear complex, instead under-
went hydrolysis during complex formation indicating its
lower stability [7]. This encourages us to synthesize
tetradentate Schiff base using the precursor 7-amino-3-
methyl-1-phenyl-4-azahept-1,3-dien-1-ol (HAMPA, the
monocondensation product of 1,3-diaminopropane and 1-
benzoylacetone) and to explore if the double condensed
ligands are stable enough to result in the formation of com-
plexes with copper(II).
A solution of Cu(ClO4)2 Æ 6H2O (10 mmol, 3.7 g) in
methanol (20 cm3) was added to the resulting methanol
solution of each tetradentate ligands HL1 and HL2 with
continuous stirring. Brown colored solid of complex 1 sep-
arated immediately. The dark brown single crystal of 1,
suitable for X- ray diffraction was obtained on dissolving
it in acetonitrile. In case of complex 2, separation of brown
mass took place on keeping the mixture overnight in a
refrigerator. The reddish-brown single crystal for suitable
for X- ray diffraction was obtained on dissolving the mass
in a mixture of acetonitrile and methanol (1:2).
In this paper, we report synthesis, spectral characteriza-
tion, crystal structures and electrochemical behavior of
three Cu(II) complexes. Two of them are derived from
the unsymmetrical tetradentate Schiff bases HL1 and HL2
formed by the condensation of free amine group of
HAMPA with pyridine-2-carboxaldehyde and 2-acetylpyr-
idine, respectively, and the other one from symmetrical
Schiff base H2L3 containing 1-benzoylacetone at the both
ends of 1,3-diaminopropane (Scheme 1). The significant
structural differences of these complexes from their acetyl-
acetone analogs are also discussed here.
Complex 1: Yield: 1.9 g (57%). Anal. Calc. for
C38H40Cl2Cu2N6O10: C, 48.62; H, 4.29; N, 8.95; Cu,
13.54. Found: C, 48.74; H, 4.12; N, 8.87; Cu, 13.46%.
k
max/nm (emax/dm3 molꢀ1 cmꢀ1) (acetonitrile), 631 (144);
IR: m(C@N), 1525, 1572 cmꢀ1, m(ClO4ꢀ), 1086 cmꢀ1
.
Complex 2: Yield: 2.0 g (55%). Anal. Calc. for
C20H22ClCuN3O5: C, 49.69; H, 4.59; N, 8.69; Cu, 13.15.
Found: C, 49.86; H, 4.61; N, 8.53; Cu, 13.19%. kmax/nm
(emax/dm3 molꢀ1 cmꢀ1
)
(methanol), 607 (152); IR:
m(C@N), 1522 and 1557 cmꢀ1, m(ClO4ꢀ), 1091 cmꢀ1
.
2.4. Synthesis of complex [CuL3] (3)
2. Experimental
The symmetrical Schiff base in complex 3 was formed by
the condensation of 1-benzoylacetone at both end of 1,3-
propanediamine. To synthesize H2L3, a methanolic solu-
tion (15 cm3) of 1-benzoylacetone (10 mmol, 1.6 g) was
added to a methanolic solution (20 cm3) of HAMPA
(10 mmol). The mixture was warmed at 50 °C for 1 h and
All chemicals were of reagent grade and used without
further purification.
2.1. Synthesis of the precursor tridentate ligand, HAMPA
The precursor mono-condensed Schiff base, HAMPA
was synthesized by high dilution technique [7,8]. 1-Benzo-
ylacetone (10 mmol, 1.6 g) in chloroform (50 cm3) was
added drop wise to a solution of 1,3-propanediamine
(10 mmol, 0.76 cm3) in chloroform (50 cm3). After comple-
tion of the addition, the solution was stirred for an addi-
tional 3 h and then chloroform was evaporated under
reduced pressure, yielding HAMPA as a viscous liquid.
cooled to room temperature.
A
solution of
Cu(ClO4)2 Æ 6H2O (10 mmol, 3.7 g) in methanol (20 cm3)
was added to the resulting solution of the tetradentate
ligand with continuous stirring. The resulting solution
was left overnight when greenish blue crystalline com-
pound suitable for X-ray analysis were separated out.
Complex 3: Yield: 1.7 g (45%). Anal. Calc. for
C12H15CuN3O3: C, 65.15; H, 5.71; N, 6.61; Cu, 14.99.
Found: C, 65.32; H, 5.66; N, 6.57; Cu, 14.87%. kmax/nm
(emax/dm3 molꢀ1 cmꢀ1) (methanol), 568(159); IR: m(C@
2.2. Synthesis of the ligands, 1-phenyl-3-{3-[(pyridin-2-
ylmethylene)-amino]-propylimino}-butan-1-one (HL1) and
1-phenyl-3-[3-(1-pyridin-2-yl-ethylideneamino)-
propylimino]-butan-1-one ( HL2)
N), 1520 cmꢀ1
.
2.5. Physical measurements
To synthesize HL1, a methanolic solution (15 cm3) of
pyridine-2-carboxaldehyde (10 mmol, 0.95 cm3) was added
Elemental analyses (carbon, hydrogen and nitrogen)
were performed using a Perkin–Elmer 240C elemental ana-

B. Sarkar et al. / Polyhedron 27 (2008) 693–700
695
lyzer and the copper contents in all the complexes were esti-
mated spectrophotometrically. The IR spectra in KBr
(4500–500 cmꢀ1) were recorded using a Perkin–Elmer
RXI FT-IR spectrophotometer. The electronic spectra in
methanol (1200–350 nm) were recorded in a Hitachi U-
3501 spectrophotometer. Cyclic voltammetry was carried
out using Sycopel model AEW2 1820F/S instrument. The
measurements were performed at 300 K in acetonitrile
solutions containing 0.2 M TEAP and 10ꢀ3 M Cu(II) com-
plex deoxygenated by bubbling with nitrogen. The work-
ing, counter, and reference electrodes used were a
platinum wire, a platinum coil, and an SCE.
The crystal structures of the three complexes were solved
by direct methods using SHELXS-97 [13] program and refined
by using SHELXL-97 [13]. The nonhydrogen atoms were
refined anisotropically while the hydrogen atoms were
located from difference Fourier map. The positional and
thermal parameters were kept fixed during refinement.
Neutral atom scattering factors were taken from Cromer
and Weber [14] and anomalous dispersion effects were
included in Fcalc [15]. In complex 2 there are disordered car-
bon atoms in the diamine fragments. The site occupation
factors of the disordered pairs of atoms (C3A and C3B;
C4A and C4B) were constrained to be unity and were tied
to FVAR. The crystallographic illustrations were prepared
using ORTEP-3 [16]. Significant crystallographic data are
summarized in Table 1.
2.6. Crystallographic studies
The data for complex 1 were collected at room temper-
ature with Mo Ka radiation on Bruker AXS Smart single
crystal diffractometer with CCD (area detector).The
absorption correction was performed with the method
inserted in SHELXTL-NT V5.1 [9]. The structure was solved
by direct methods with the software SHELXTL-NT V5.1
inserted in the Bruker AXS software [9].
The data for complexes 2 and 3 were collected at room
temperature with Cu Ka radiation on Siemens AED single
crystal diffractometer with a local program [10]. The preli-
minary cell parameters were obtained from least squares of
the (h, v, /) angular values of 3897 and 1948 reflections (h
range = 4–70° and 5.5–70°) respectively for complexes 2
and 3, accurately well centered on the diffractometer. The
intensity of one standard reflection, recorded for every
100 reflections, showed no significant changes. The
recorded data were corrected for polarization and Lorentz
effects. The absorption correction was performed with the
method of Walker and Stuart [11] with a program written
by Gluzinski [12].
3. Results and discussion
3.1. IR and electronic spectra
All these compounds contain no significant peak in the
region of 3100–3250 cmꢀ1, which clearly indicates that
there is no free –NH2 group i.e. all are the quadridentate
ligands that form the complexes. The bands corresponding
to azomethylene group (C@N) are distinct in all three com-
plexes and appear within 1572–1520 cmꢀ1. The sharp,
strong, single peak at 1086 and 1091 cmꢀ1 for complexes
1 and 2 clearly indicates the presence of anionic perchlorate
group. As expected, complex 3 does not show any such
peak.
The electronic spectral data in acetonitrile solution sug-
gest basically square planar geometry for 1 and 2. In gen-
eral square-planar complexes are known to exhibit one or
two bands [17]. Complexes 1 and 2 show a broad band at
631 and 607 nm, respectively, as was also observed for
Table 1
Crystal data and structure refinement of complexes 1–3
1
2
3
Formula
M
C
938.76
38H40Cl2Cu2N6O10
C20H22ClCuN3O5
483.41
C46H48Cu2N4O4
847.96
Crystal System
Space group
triclinic
monoclinic
P21/c
7.525(2)
12.372(3)
21.807(2)
90
96.070(2)
90
2018.8(7)
4
monoclinic
Cc
ꢀ
P1
˚
a (A)
7.627(3)
11.847(3)
12.263(3)
67.11(2)
75.75(2)
85.14(2)
989.3(5)
1
12.188(2)
20.503(2)
9.154(2)
90.00
116.97(3)
90.00
2038.7(6)
4
1.381
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
1.576
1.276 [Mo Ka]
0.026
1.590
3.083 [Cu Ka]
0.019
l (mmꢀ1
Rint
)
1.669 [Cu Ka]
0.218
Number of unique data
Number of data with I > 2r(I)
R1, wR2
Goodness-of-fit on F2
4561
3412
0.0562, 0.1872
0.930
3806
3231
0.0463, 0.1475
1.000
1948
1842
0.0512, 0.1750
1.155

696
B. Sarkar et al. / Polyhedron 27 (2008) 693–700
the complexes of similar asymmetric ligands [5,18,19]. The
appearance of similar d-d transition in both 1 and 2 con-
firms the semi-coordinated character of the ketonic oxygen
of the ligand in 1, which does not significantly affect the
square planar geometry around the Cu(II) ions of the com-
plex. At the higher energy region, the ligand to metal
charge transfer bands were located for both the complexes
(319 and 281 nm for complex 1 and 345 and 284 nm for
complex 2). Complex 3 also shows a broad band centering
at 568 nm for the d-d transition and 341 and 301 nm for
ligand to metal charge transfer transitions.
3.2. Description of structures of complexes 1, 2 and 3
3.2.1. Complexes 1 and 2
The structures of the complexes 1 and 2 are shown in
Figs. 1 and 2, respectively, with common atom numbering
scheme. For both compounds, the structures consist of
[CuL]+ cation together with perchlorate anion. In complex
1, two symmetry related (1 ꢀ x, ꢀy, 1 ꢀ z) cations are
joined together through very weak interaction from the
oxygen atom of the benzoylacetone moiety to the axial
position of copper atom at a distance of 2.767(3) (Fig. 3).
Although the distance is very long but is less than the
sum of the van der Waals radii for copper(II) and oxygen
Fig. 2. ORTEP plot of 2 with atom-numbering scheme; ellipsoids at 50%
probability.
˚
(2.92 A) [20]. Therefore, a weak bond is considered to be
formed between the two. Comparable distances have been
regarded as bond in several other reported systems [21]. In
2, the complex cations are discrete; the closest copper–
oxygen distance between the two neighboring units is
1
˚
3.029 A. In both complexes, the deprotonated ligands L
and L2 are quadridentate forming two adjacent six-mem-
bered and one five-membered chelate rings (6-6-5). The
copper(II) ions have square-planar co-ordination with a
slight tetrahedral distortion in both the complexes.
Fig. 3. A view (PLUTO) of the dimer of 1. Perchlorate ions are not shown
0
for clarity. Symmetry transformation: = (1 ꢀ x, ꢀy, 1 ꢀ z). Dashed
bonds represent the weak axial bond.
Deviations of donor atoms O(1), N(1), N(2) and N(3) from
the mean plane passing through them are ꢀ0.057(2),
˚
0.054(4), ꢀ0.059(3) and 0.062(3) A, respectively, for 1
˚
and ꢀ0.047(2), 0.043(3), ꢀ0.047(3) and 0.051(2) A, respec-
tively, for 2. The central Cu(II) atom deviates negligibly
˚
(0.0037(3) A) in complex 1 and slightly more (0.0274(4)
˚
A) in complex 2 from the same plane. The tetrahedral dis-
tortion is apparent since one pair of diagonal donor atoms
clearly lies below the plane while the other pair is above the
plane with the metal ion in the mean plane. The dihedral
angles between the two planes [N(l)–Cu–O(1) and N(2)–
Cu–N(3)] is 6.09° for complex 1 and 4.82° for complex 2
Fig. 1. ORTEP plot of 1 with atom-numbering scheme; ellipsoids at 50%
probability.

B. Sarkar et al. / Polyhedron 27 (2008) 693–700
697
compares with 0° for a perfectly square-planar arrange-
ment and 90° for a perfect tetrahedral arrangement. The
two diagonal N–Cu–O and N–Cu–N angles of complex 1
[169.3(1) and 172.8(1)°] and 2 [168.0(1) and 174.7(1)°] are
less than 180° as a result of this tetrahedral distortion.
The metal ion is not perfectly centered, and the four bond
distances between donor atoms to metal ion are signifi-
cantly different as shown in Table 2. In both the complexes,
with no atoms deviating from the least square plane by
0.021 and 0.026 A for complexes 1 and 2, respectively. It
˚
is interesting to note that the mean plane of this ring is
not coplanar with the benzene plane, the angles between
the two planes are 17.27° and 17.91° for complexes 1 and
2, respectively. The other six-membered ring incorporating
the diamine fragment is not planar as all the carbon atoms
are sp3 hybridized. Two of the carbon atoms (C3 and C4)
in 2 are disordered (occupancy of C3b is 0.59(7) and that
for C4b is 0.60(1)). Therefore, we perform the conforma-
tional analysis with C3b and C4b. The conformation of
the chelate ring is intermediate between chair and half-
˚
the mean Cu–N distances (1.999 and 1.988 A for 1 and 2,
respectively) are slightly longer than that of Cu-O (1.916
˚
and 1.906 A for 1 and 2, respectively). This elongated
Cu–N bond than the Cu–O is also observed for similar
Schiff bases [5,19]. It is worth mentioning that, the Cu–
O(1) bonds in complexes 1 and 2 are slightly greater than
the similar known copper systems having acetylacetone
residue [5].
˚
chair conformation for both complexes 1 (Q = 0.258 A,
˚
h = 47.7° and / = 147.1°) and 2 (Q = 0.273 A, h = 36.7°
and / = 17°) [22]. The bond distances and bond angles of
this part of Schiff base are comparable with those of related
Cu(II) complexes with other nonsymmetrical Schiff base
ligands [5].
The six-membered chelate ring incorporating the benzo-
ylacetone moiety is delocalized by the influence of coordi-
nation to the metal. In complex 1, this ring containing
Cu(1), N(1), C(14), C(16), C(17) and O(1); the C(16)–
The dimerization of 1 through the ketonic oxygen bridge
seems to be a noteworthy phenomenon for this type of
complexes as the acetylacetone analogue of complex 1 is
a discrete monomer [5]. The similar bridging property of
phenoxo group is well documented in the literature [23]
but the ketonic oxygen of acetylacetone or its derivatives
is considered as nonbridging in asymmetrical or symmetri-
cal tetradentate ligands. Till date, the bridging property of
such ketonic oxygen is illustrated only in conjunction with
other stronger bridging group e.g. hydroxo, oximato, etc.
[7,8a,8d,24]. Therefore, to our knowledge, complex 1 is
the first example of dimerization in Cu(II) complexes con-
taining tetradentate Schiff base ligand via solely ketonic
oxygen bridges.
˚
C(17) distance of 1.379 A is much shorter than the normal
2
2
˚
sp –sp C–C single bond (1.51 A), the C(14)–N(1) distance
˚
of 1.288 A is a little longer than normal imine C@N double
bond (1.28 A) and the C(17)–O(1) bond length of 1.299 A
is shorter than normal sp C–O single bond (1.34 A). All
˚
˚
2
˚
these changes in bond length indicate delocalization of
the chelating ring containing the benzoylacetone fragment.
The same trend in bond lengths is also found in the ring of
the benzoylacetone fragment in complex 2 with the C(16)–
C(17), C(18)–N(1) and C(16)–O(1) distances of 1.370, 1.301
˚
and 1.302 A, respectively. It is clear that the replacement of
acetylacetone residue by
a benzoylacetone, slightly
increases the C–O (ketonic) and C(16)–C(17) bond dis-
tances (1.280 and 1.342 A, respectively, in the acetylacetone
derived complex [5]). However, other bond distances (C–C
and C@N) do not show significant changes.
The chelating ring tends to remain planar as a result of
this delocalization to form a stable conjugation structure
In both complexes the intermolecular packing is con-
trolled by p-stacking interaction between the aromatic
rings. In 1, there is a strong p–p interaction between the
phenyl ring and the pyridyl ring of a neighboring (2 ꢀ x,
ꢀy, 1 ꢀ z) dimer with a slip angle of 28.87°, dihedral angle
˚
˚
of 5.93° and centroids separation of 3.863(3) A to form a
one-dimensional chain along the a axis as illustrated in
Fig. 4. In 2 also there is similar p–p interaction between
the phenyl ring and the pyridyl ring of a neighboring
(2 ꢀ x, ꢀy, 1 ꢀ z) units ( slip angle = 31.71°, dihedral
Table 2
˚
Selected bond lengths (A) and bond angles (°) for complexes 1 and 2
1
2
˚
angle = 4.65° and centroid separation = 4.145(2) A) result-
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–O(1)0
1.916(2)
1.948(4)
2.017(3)
2.032(4)
2.767(3)
1.906(2)
1.945(2)
2.007(3)
2.012(2)
ing the stack of molecule along the a-axis (Fig S1).
3.2.2. Complex 3
The structure of complex 3 with atomic numbering
scheme is shown in Fig. 5. The structure consists of discrete
four coordinate copper(II) complex having quadridentate
symmetrical Schiff base ligand. Selected bond distances
and bond angles given in Table 3. The chelated deproto-
nated ligand, (L3)2ꢀ forms three adjacent six-membered
rings (6-6-6); the two terminal rings are consist of benzoyl-
acetone fragments and the one at the middle by the starting
diamine. The central Cu(II) atom has a distorted square
planer coordination with a significant tetrahedral distor-
tion. The coordinating atoms N1, N2, O1 and O2 deviates
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(3)
O(1)–Cu(1)–O(1)0
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(1)0
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(1)0
N(3)–Cu(1)–O(1)0
94.50(13)
94.21(9)
167.95(10)
88.38(8)
169.29(14)
89.49(12)
85.34(9)
96.21(15)
172.83(14)
100.59(13)
79.88(14)
92.53(11)
85.65(11)
97.85(11)
174.65(10)
79.63(10)
0
Symmetry transformations: = 1 ꢀ x, ꢀy,1 ꢀ z.

698
B. Sarkar et al. / Polyhedron 27 (2008) 693–700
Fig. 4. One-dimensional p–p interactions shown in the dotted lines in complex 1. Hydrogen atoms of the diamine fragments are not shown for clarity.
plex (3.73°) [25], [Cu(acactn)H2O]2 (7.77°) and [Cu2-
(acactn)2KClO4]2 (10.85° and 4.6°) where H2acactn is
bis(acetylacetone)-trimethylenediimine [6a] and also than
the two asymmetric complexes 1 (6.09°) and 2 (4.82°)
reported here. The only known tetradentate copper(II)
complex having much distorted system is Cu(sa12tmput)
[26] (H2sa12tmput is 2,5-bis(salicylaldimino)-2,5-dimethyl-
hexane (48.5°)) where the diamine (2,5-dimethylhexane-
2,5-diamine) fragment produces a more flexible seven
membered chelate ring.
These six-membered chelate rings derived from benzoyl-
acetone are not identical with respect to bond distances and
bond angles though both the chelating rings are delocalized
by the influence of the metal, which is comparable to com-
plexes 1 and 2. In complex 3, the first ring containing
Cu(1), O(1), C(3), C(4), C(5) and N(1), the C(3)–C(4) dis-
˚
˚
tance of 1.379 A, the C(5)–N(1) distance of 1.294 A and
˚
the C (17)-O(1) bond length of 1.283 A. Whereas in the sec-
Fig. 5. ORTEP plot of 3 with atom-numbering scheme; ellipsoids at 50%
probability.
ond, the chelate ring containing Cu(1), O(2), C(10), C(9),
C(8) and N(2), the C(9)–C(10), C(8)–N(2) and C(10)–
O(2) distances are 1.427, 1.294 and 1.291 A, respectively.
˚
The angles between the mean plane of these rings with
the benzene plane are 24.57° and 24.77° indicating greater
nonplanarity compare to that in complexes 1 and 2. The
conformation of six-membered chelate ring incorporating
the diamine is an intermediate between boat and twist-boat
Table 3
˚
Selected bond lengths (A) and bond angles (°) for complex 3
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–N(2)
1.898(9)
1.924(9)
1.928(11)
1.955(12)
˚
confirmation [22] (Q = 0.858 A, h = 88.7° and / = 265.4°).
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
90.7(4)
95.4(4)
151.5(4)
156.3(4)
93.5(5)
92.0(5)
Unlike 1 and 2 there is no significant p–p interaction
between the molecules in complex 3. A comparison of the
structure of 3 with its acetylacetone analogue [6a] reveal
that although the bond angles and distances do not show
any significant change, the geometry around the copper
atom shows unusually high tetrahedral distortion in 3.
from the least-square mean plane through them by
3.3. Electrochemical study
˚
– 0.442(3), 0.427(3), ꢀ0.427(3), 0.442(3) A. The Cu atom
˚
is located 0.039 A from the same plane. The dihedral angles
The cyclic voltammograms of the complexes are
recorded in acetonitrile solvent and the diagrams are
shown in Fig. 6. As is evident from Fig. 6, all three com-
plexes show reversible reductive responses. The reduction
potentials for 1 and 2 are observed at ꢀ0.86 and ꢀ0.90 V
between the two planes [N(l)–Cu–O(1) and N(2)–Cu–O(2)]
is 35.98°, which is much greater than that in the
similar tetracoordinated copper(II) complexes like bis-
(benzoylacetone)-ethylenediiminato-N,N0copper(II) com-

B. Sarkar et al. / Polyhedron 27 (2008) 693–700
699
their potential use in understanding the intricate differences
that occur in NLO responses or in natural systems with
slight variations in the ligand fragments.
Acknowledgements
We like to thank Dr. G. Mostafa, Jadavpur University,
Kolkata, India, for his help and valuable discussions
regarding the crystal structures of the compounds.
Appendix A. Supplementary material
CCDC 655614, 655615 and 655616 contain the supple-
mentary crystallographic data for 1, 2 and 3. These data
can be obtained free of charge via www.ccdc.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.
2007.10.021.
Fig. 6. The cyclic voltammogram of 1, 2 and 3 in acetonitrile solution
(scan rate 100 mV/s).
(Epc). The corresponding Epa values are observed at ꢀ0.77
and ꢀ0.83 V. The E1/2 values of 1 and 2 are ꢀ0.82 and
ꢀ0.87 V (versus SCE), respectively. Under identical exper-
imental conditions the potential values for 3 are ꢀ0.52 V
(Epc), ꢀ0.42 V (Epa) and ꢀ0.47 V (E1/2) (versus SCE). It
is well known that in four-coordinate systems, Cu(II) and
Cu(I) have very distinct preference for square-based and
tetrahedral geometry, respectively [27]. This preference is
well reflected in the redox behavior of these systems. More
is the distortion in the coordination geometry around the
metal towards tetrahedral arrangement, higher is the ano-
dic shift of the CuII/CuI redox potentials [27]. Thus the
anodic shift of the potential of 3 compared to 1 and 2
and also to other similar systems corroborates the severe
tetrahedral distortion of the copper atom from square pla-
nar geometry as observed in the X-ray structural analysis.
A similar shift of the potential value and distortion were
also observed earlier [26,28].
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