Synthesis and characterization of some novel copper(II) complexes with an optically active Betti base

Article abstract of DOI:10.1016/j.poly.2011.12.034

Novel Cu(II) complexes with racemic as well as optically pure 1-(α-amino benzyl)-2-napthol, commonly known as Betti base, and its derivative 1-(α-pyrrolidinyl benzyl)-2-napthol have been synthesized. The general composition of all the isolated complexes, as determined by elemental analysis, thermal analysis and FAB mass spectra, is found to be Cu(L-H) ₂. The coordination of the ligand with metal has been confirmed by IR and ESR spectra of the complexes. The CD spectra of complexes with the S-form of the ligands exhibit a mirror image relationship with the CD spectra of the complexes with the corresponding R-form of the ligands. The Cu(II) complex with racemic Betti base has been characterized structurally. It crystallizes in the monoclinic space group P2₁/c. The complexes display quasi reversible redox behavior due to the Cu(II)/Cu(I) reduction process.

Full text of DOI:10.1016/j.poly.2011.12.034

Polyhedron 35 (2012) 15–22
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of some novel copper(II) complexes
with an optically active Betti base
⇑
Shardul Bhatt, Bhavna Trivedi
Department of Chemistry, Faculty of Science, The M.S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel Cu(II) complexes with racemic as well as optically pure 1-(a-amino benzyl)-2-napthol, commonly
known as Betti base, and its derivative 1-(a-pyrrolidinyl benzyl)-2-napthol have been synthesized. The
Received 9 August 2011
Accepted 18 December 2011
Available online 13 January 2012
general composition of all the isolated complexes, as determined by elemental analysis, thermal analysis
and FAB mass spectra, is found to be Cu(L-H)2. The coordination of the ligand with metal has been con-
firmed by IR and ESR spectra of the complexes. The CD spectra of complexes with the S-form of the
ligands exhibit a mirror image relationship with the CD spectra of the complexes with the corresponding
R-form of the ligands. The Cu(II) complex with racemic Betti base has been characterized structurally. It
crystallizes in the monoclinic space group P21/c. The complexes display quasi reversible redox behavior
due to the Cu(II)/Cu(I) reduction process.
Keywords:
Betti base
Amino naphthol
Circular Dichroism
Copper complexes
Redox chemistry
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
accelerated reactions in the presence of a metal salt, isolation
and characterization of the metal complex is essential. The Betti
The multi component reaction between 2-naphthol, aryl alde-
hydes and ammonia or amines yields aminobenzylnaphthols, in a
process known as the Betti reaction, was first uncovered at the begin-
ning of the 20th century by Mario Betti, an Italian chemist [1]. The
aminobenzylnaphthols could be easily resolved into their enantio-
mers [2] (Fig. 1). In recent years, attention has been paid to the Betti
reaction performed by either using other naphthols [3] or quinolinols
[4,5], or by replacing ammonia with alkyl amines [6–9].
Betti base derivatives have provided convenient access to many
useful synthetic building blocks via the amino and phenolic hydro-
xy functional groups [10,11]. Optically active Betti bases com-
plexed to dialkyl zinc have been used for enantioselective
addition to aryl aldehydes [12–14]. Various applications of enan-
tiopure amino naphthols as catalysts in stereoselective reactions
have been reviewed by Fulop and co-worker [15] and Palmieri
and co-worker [16].
Recently, a large library of aminonapthylphenols obtained by
the Betti reaction and their applications have been reviewed cate-
gorically by Naso and co-workers under the title of ‘‘The Betti base:
the awakening of a sleeping beauty’’ [17]. In this review it is appro-
priately mentioned that, ‘‘in spite of the number of structures re-
ported, that are important from the stereochemical point of view,
and of the work performed so far, the synthetic potential of the
Betti base family has been uncovered only to a small extent’’. It
is evident that to understand the role of the Betti base in ligand
base, with –NH2 and –OH groups at 1 and 3 positions, respectively,
is expected to act as an excellent ligand for coordination with tran-
sition metal ions, however, so far no metal complex of the Betti
base or its derivatives has been reported. A survey of the literature
for the coordination chemistry of transition metal complexes
formed by O-aminophenols reveals unsuccessful attempts to iso-
late complexes of this kind [18]. Thus, the need to understand
the role of the non-racemic Betti base as an auxiliary ligand in
asymmetric synthesis along with our continuous interest in the
synthesis of transition metal complexes with optically active li-
gands [19,20] prompted us to investigate the peculiarities of com-
plexation of the optically active Betti base and its derivative with
copper(II). In the present paper, the synthesis and characterization
of copper(II) complexes with racemic as well as optically pure 1-
(a
-amino benzyl)-2-napthol [Cu1, complex of racemic Betti base
(L1); Cu2, complex of S-Betti base (L2); Cu3, complex of R-Betti base
(L3)] and its derivative 1-( -pyrrolidinyl benzyl)-2-napthol [Cu4,
complex of racemic 1-( -pyrrolidinyl benzyl)-2-napthol (L4); Cu5,
complex of S-1-( -pyrrolidinyl benzyl)-2-napthol (L5); Cu6, com-
plex of R-1-( -pyrrolidinyl benzyl)-2-napthol (L6)] are reported.
a
a
a
a
2. Experimental
2.1. Materials
Freshly distilled solvents were employed for all synthetic pur-
poses. Spectroscopic grade solvents were employed for spectral
work. All other chemicals were of AR grade.
⇑
Corresponding author. Tel.: +91 265 2795552; fax: +91 265 2795392.
E-mail address: trivedibhavna@yahoo.com (B. Trivedi).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.034

16
S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
a
b
CHO
OH
+ 2
NH2
+
NH3
OH
Betti base L1
H
N
O
NH2
H
N
0.75 eq L(+)tartaric acid
acid catalyzed
acetone, 0.5eq H2O, RT 3Hr
R(-) oxazine
N,O-ketalization
OH
O
enantioselective
O
L1
NH2
HO
OH
OH
+
OH
HO
O
L2
c
H
N
HO
1.5 eq D(-)tartaric acid
acetone, 1.0 eq water
rt, 3hr
NH2
NH2
aq Na2CO3 DCM
0o C , 30 min
OH
OH
O
+
OH
99%
OH
HO
R(-) oxazine
R(-) Betti base
L3
Fig. 1. (a) synthesis of the Betti base. (b and c) Resolution of the Betti base into its enantiomers.
2.2. Physical measurements
counter electrode. Cyclic voltammetric studies were carried out
using a dichloromethane solution of the complexes (1.0 mM) using
tetra-n-butylammoniumhexafluorophosphate (0.1 M) as the sup-
porting electrolyte. Solutions were deoxygenated by bubbling dry
nitrogen through prior to the potential sweep. X-ray diffraction
data of the Cu1 complex were collected on a Bruker SMART APEX
diffractometer equipped with a CCD detector, using graphite-
Elemental analysis (C, H, N) of the complexes were done on a
VARIO EL-III elemental analyzer. ICP-OES analysis for the estima-
tion of metal was done using a Thermo scientific iCAP 6200 ICP-
OES. Thermal analysis of the complexes was done on a SII TG/
DTA 6300. Fast atomic bombardment mass spectra (FAB MASS)
were recorded on a Jeol SX 102/Da-600 mass spectrometer using
m-nitro benzyl alcohol as the matrix. FTIR spectra were recorded
on a Perkin Elmer RX1 spectrometer. UV–Vis spectra were re-
corded on a Perkin Elmer Lambda 35 UV–Vis spectrometer. NMR
spectra were collected with a 400 MHz spectrometer (Bruker AV
400). X-band EPR spectra were obtained using a Varian E-112
ESR spectrometer. The optical rotation of the ligands was measured
on a Jasco P-2000 Polarimeter. Circular Dichroism spectra were
measured using a Jasco J-810 Spectro-polarimeter in the range
250–800 nm. Electrochemical measurements were performed on
a CH Instruments 600C potentiostat, using a Pt disk as the working
electrode, Ag/AgCl as the reference electrode and a Pt wire as the
monochromatized Mo Ka radiation (k = 0.71073 Å) at 298 °C. The
structure was solved by direct methods using SHELXTL [21] and
refined on F2 by the full-matrix least-squares technique using the
SHELXL-97 [22] program package.
2.3. Preparation of the ligands
The Betti base and its derivatives were synthesized and resolved
by the reported method [23–25] as shown in Fig. 1. Formation of
the ligands was confirmed by elemental analysis, FTIR, 1H NMR
and 13C NMR spectroscopy.

S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
17
2.3.1. L1
M.P. 122–124 °C; Reported M.P. 124 °C.
a
0.4
0.3
0.2
0.1
0.0
-0.1
L3
L6
Anal. Calc. for C17H15NO (249.31): C, 81.8; H, 6.01; N, 5.6. Found:
C, 81.43; H, 6.02; N, 5.55%.
1H NMR (CDCl3) d: 7.67–7.70 (m, 3H), 7.13–7.42 (m, 8H), 6.10 (s,
1H), 2.39 (br s, 2H).
13C NMR (CDCl3) d: 156.9, 142.4, 132.0, 129.7, 129.0, 128.7(2C),
128.5, 127.9, 127.3(2C), 126.4, 122.4, 121.2, 120.5, 115.1, 55.9.
IR (KBr, cmꢀ1): 3384, 3297, 3029, 1622, 1599, 1469, 1452.
UV (nm): 292 (
e
ꢁ 3500), b 337 ( ꢁ 2900).
e
2.3.2. L2
M.P. 130 °C; Reported M.P. 133–135 °C.
Specific rotation: Found [aD] = +93.9 (c 1.0, CHCl3), Reported
[
aD] = +94.1 (c 1.0, CHCl3).
280
300
320
340
360
380
400
CD: positive Cotton effect at 290 and 340 nm (Fig. 2a).
λ , nm
2.3.3. L3
M.P. 131 °C; Reported M.P. 133–135 °C.
Specific rotation: Found [
D] = ꢀ88.3 (c 1.0, CHCl3), Reported
D] = ꢀ94.3 (c 1.0, CHCl3).
CD: negative Cotton effect at 290 and 340 nm (Fig. 2a).
b
Cu3
Cu6
a
0.8
0.6
0.4
0.2
0.0
[
a
2.3.4. L4
M.P. 176–177 °C; Reported M.P. 176–177 °C.
IR (KBr, cmꢀ1): 2971, 1620, 1238, 750.
1H NMR (CDCl3) d: 13.8 (br s, 1H), 7.88–7.65 (m, 4H), 7.64–7.21
(m, 6H), 5.12 (s, 1H), 3.27 (br s 1H), 2.60–2.27 (br s, 3H), 1.85 (br s,
4H).
UV (nm): 293 (
e
ꢁ 3500), b 338 ( ꢁ 2900).
e
2.3.5. L5
M.P. 176–178 °C.
Specific rotation [
CD, positive Cotton effect at 290 and 340 nm (Fig. 2c).
280
300
320
340
360
380
400
λ , nm
aD] = +185 (c 1.0, CHCl3).
c
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Cu3
Cu6
2.3.6. L6
M.P. 176–178 °C.
Specific rotation [
CD, negative Cotton effect at 290 and 340 nm (Fig. 2c).
aD] = ꢀ184.5 (c 1.0, CHCl3).
2.4. Preparation of the complexes CuL2 where L = L1, L2, L3, L4, L5 or L6
The copper(II) complexes were prepared by mixing a 2 mM
solution of the ligand in methanol with a 1 mM solution of
CuCl2ꢂ2H2O. The mixture was stirred for 30 min at room tempera-
ture. The solid that separated from the solution was collected on a
Whatman filter paper, washed with small amount of methanol and
dried under vacuum. The purity of the isolated complex was con-
firmed by TLC (Hexane:ethyl acetate; 5:1). No unreacted ligand
was found.
400 450 500 550 600 650 700 750 800 850
λ , nm
Fig. 2. UV–Vis spectra. (a) UV spectra of L3 and L6. (b) UV spectra of Cu3 and Cu6. (c)
Visible spectra of Cu3 and Cu6.
2.4.1. Cu1 [C34H28CuN2O2]
Brick red solid, 0.77 g, 68.7% (Calc. MWt for Cu1) = 560.11; (FAB
Mass): 561.00; Anal. Calc.: Cu, 11.3; C, 72.8; H, 4.99; N, 4.99. Found:
Cu, 11.29; C, 72.33; H, 5.01; N, 4.85%. IR (KBr, cmꢀ1): 3439, 3341 (–
2.4.3. Cu3 [R(ꢀ)-C34H28CuN2O2]
NH2); X-band EPR: g = 2.261, g\ = 2.049, a
2 = 0.869. UV (nm): 292
k
Dark green solid, 0.61 g, 55%; Anal. Calc.: C, 72.8; H, 4.99; N,
4.99. Found: C, 72.16; H, 4.89; N, 4.87%. CD: bands at 294, 340
and 388 nm with a negative Cotton effect and a band at 670 nm
with a positive Cotton effect.
(e
ꢁ 8580), b 334 (
e
ꢁ 6300).
2.4.2. Cu2 [S(+)-C34H28CuN2O2]
Dark green solid, 0.66 g, 59%; Anal. Calc.: C, 72.8; H, 4.99; N,
4.99. Found: C, 72.50; H, 4.89; N, 4.99%. CD: bands at 294, 340
and 388 nm with a positive Cotton effect and a band at 670 nm
with a negative Cotton effect.
2.4.4. Cu4 [C42H40CuN2O2]
Light brown solid, 0.86 g, 64.9%; (Calc. MWt for Cu4) 668.33;
(FAB Mass): 669; Anal. Calc.: C, 75.5; H, 5.99; N, 4.19. Found: C,

18
S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
(c)120
80
60
40
20
0
(a)
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
-140
-20
-40
-60
300
400
500
600
700
800
300
400
500
600
λ, nm
700
800
λ, nm
200
150
100
50
(d)
(b)
150
100
50
0
0
-50
-50
-100
-150
-100
-150
-200
400
500
600
700
800
400
500
600
700
800
λ, nm
λ, nm
Fig. 3. Circular Dichroism spectra (0.5 mM) in 10 mL chloroform. (a) CD spectra of L2 and L3. (b) CD spectra of Cu2 and Cu3. (c) CD spectra of L5 and L6. (d) CD spectra of Cu5
and Cu6.
75.08; H, 5.83; N, 4.02%. X-band EPR: g = 2.213, g\ = 2.043,
molecular ion peaks corresponding to the [Cu(L-H)2]+ ion, confirm-
ing the formation of the Cu(L-H)2 complexes.
a
2 = 0.793. UV (nm): 294 (
e
ꢁ 8500), b 337 (
e
kꢁ 6000).
The IR spectra of the ligands L1, L2 and L3 exhibit a relatively
broad and strong peak at ꢁ3447 cmꢀ1 and a weak, sharp doublet
at 3380 and 3285 cmꢀ1, corresponding to –OH stretching and asym-
metric and symmetric –NH2 stretching vibrations, respectively.
These are shifted by 15–20 cmꢀ1 towards lower wavenumber
values due to intra molecular hydrogen bonds of the type –O–H–
NH2–R. In the IR spectra of the complexes Cu1, Cu2 and Cu3 the band
due to –OH stretching disappears, whereas the two bands of
the –NH2 stretching are shifted to slightly higher wavenumbers, at
3439 and 3341 cmꢀ1. This shift indicates the absence of H-bonding
and a very weak coordination bond between –NH2 and the cop-
per(II) ion [26]. In the IR spectra of the ligands L4, L5 and L6, apart
from bands due to –OH and –C–N of the pyrrolidine group at 3437
and 1238 cmꢀ1, respectively, a sharp band at 2977 cmꢀ1 corre-
sponding to sp3 hybridized –C–H stretching is observed. The band
due to –OH stretching is not seen in the IR spectra of the complexes
Cu4, Cu5 and Cu6 and the bands of –C–H and –C–N are shifted
8–10 cmꢀ1 to higher wavenumbers. These small shifts can be attrib-
uted to small changes on coordination with the metal ion.
2.4.5. Cu5 [S(+)-C42H40CuN2O2]
Dark brown solid, 0.81 g, 61.2%; Anal. Calc.: C, 75.5; H, 5.99; N,
4.19. Found: C, 75.18; H, 5.73; N, 3.99%. CD: bands at 294, 340, 408
and 550 nm with a positive Cotton effect and a band at 760 nm
with a negative Cotton effect.
2.4.6. Cu6 [R(ꢀ)-C42H40CuN2O2]
Dark brown solid, 0.79 g, 59.7%; Anal. Calc.: C, 75.5; H, 5.99; N,
4.19. Found: C, 75.28; H, 6.43; N, 4.22%. CD: bands at 294, 340, 408
and 550 nm with a negative Cotton effect and a band at 760 nm
with a positive Cotton effect.
3. Results and discussion
The complexes Cu1–Cu6 were isolated in moderate to good
yields (50–75%) by mixing methanolic solutions of the ligands
L1–L6 with Cu(II) chloride. Attempts to isolate complexes of the
type CuL by reacting equimolar solutions of the metal salt and li-
gand were unsuccessful, as it always resulted in the formation of
CuL2, and the excess metal salt remained unreacted. Elemental
analysis of the complexes correspond to a 1:2 metal:ligand stoichi-
ometry. FAB mass spectra of the complexes Cu1 and Cu4 display
Thermal analysis of the complexes Cu1 and Cu4 in N2 flow indi-
cates the complexes to be anhydrous and unsolvated. No weight
loss was observed until decomposition, which began at about
220 °C for Cu1 and 232 °C for Cu4. The endothermic peak at the
decomposition temperature indicates a change of phase from solid

S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
19
respectively, for Cu1 and Cu4). The agreement between the exper-
imental and theoretical weight losses confirms the general formula
of the complexes as Cu(L-H)2.
a
The electronic spectra (Fig. 2a) of a 0.1 mM solution of the
ligands L3 and L6 in chloroform show two relatively intense in-
tra-ligand transition bands at ꢁ292 nm (
e
ꢁ 3500) and ꢁ337 nm
(e
ꢁ 2900). The electronic spectra (Fig. 2b, 0.1 mM in chloroform)
of the complexes Cu3 and Cu6 exhibit slightly broad, more intense,
intra-ligand transition bands at ꢁ290 nm with an extinction coef-
ficient of 8580 and at ꢁ337 nm with an extinction coefficient of
6300, as expected for a coordinated phenoxo ligand. Apart from
these intense bands, the spectra of complexes Cu1, Cu2 and Cu3 ex-
hibit a band at ꢁ360 nm with an extinction coefficient of 1800,
which can be attributed to a phenoxo ligand to metal charge trans-
fer. A broad band appears at ꢁ650 nm with an extinction coeffi-
cient of 164 for the complexes with optically active ligands, Cu2
and Cu3, (Fig. 2c, 5 mM in chloroform), which can be assigned to
a d–d transition, as expected for a square planar complex, and
b
can be attributed to a combination of three transitions B1g ? A1g
,
B1g ? B2g and B1g ? Eg Due to poor solubility of the complexes
Cu1 and Cu4 in chloroform, the spectra in the visible region exhibit
no significant absorptions. Similarly, a broad band at ꢁ510 nm
with an extinction coefficient ꢁ450, for Cu5 and Cu6 (Fig. 2c) can
be assigned to a d–d transition. The shift of the d–d transition to-
wards lower wavelength values compared with that of the com-
Cu2
plexes Cu2 and Cu3 indicates
a more distorted tetragonal
geometry around the central metal ion. This is expected for ligands
bearing a bulkier group like pyrrolidine. A similar shift has been
observed by Elias et al. [27].
The CD spectra of 0.5 mM solution of the ligands L2, L3, L5 and L6
in chloroform are given in Fig. 3. The spectra of the enantiomeric
pairs L2 and L3, and L5 and L6 bear a mirror image relationship,
as expected. The S-ligands (L2 and L5) exhibit positive Cotton ef-
fects for the intra-ligand electronic transitions at 290 and
340 nm, whereas the R-ligands (L3 and L6) exhibit negative Cotton
effects for the intra-ligand electronic transitions at 290 and
340 nm. The CD spectra of the corresponding complexes Cu2 and
Cu3, and Cu5 and Cu6 are like the ligands, and are mirror images
of each other. However, the bands corresponding to the intra-li-
gand transitions in the region 280–305 nm cannot be reported
due to high HT values. The band at 340 nm is observed with a Cot-
ton effect similar to that of the corresponding ligands. Incorpora-
tion of a metal ion during complex formation, as in Cu2 and Cu3,
also results in the exhibition of a charge transfer at ꢁ388 nm with
a Cotton effect similar to that of the corresponding ligand and a d–
d transition band at ꢁ670 nm of lower intensity, but with a rever-
sal of the Cotton effect. The reversal of the Cotton effect for the d–d
transition band could be explained considering the coupling of the
c
Cu
Cu54
2850 2900 2950 3000 3050 3100 3150 3200 3250 3300 3350 3400 3450
Magnetic Field (Gauss)
d–d transitions with one of the
p ?
p⁄ transitions. Similar types of
Fig. 4. EPR spectra of Cu(II) complexes prepared in DCM (10 mM). Spectra were
collected at room temperature, m0 = 9.1 GHz. (a) Cu1, (b) Cu2, (c) Cu4 and Cu5.
opposite Cotton effects resulting from exciton coupling have been
reported [28,29]. The CD spectra of Cu5 and Cu6 are different from
that of Cu2 and Cu3. In the case of Cu5 and Cu6, charge transfer
bands appear at lower energies ꢁ408 nm with a Cotton effect sim-
ilar to that of the corresponding ligands, In contrast to the absorp-
tion spectra, the CD spectra of the complexes exhibit two bands
with opposite Cotton effects in the 400–800 nm range. The band
at 550 nm can be assigned to a combination of B1g ? B2g and
Table 1
EPR spectral data of the complexes.
Copper complexes
g -2.0023
g\-2.0023
A (cmꢀ1
)
a2
k
Cu1
Cu2
Cu3
Cu4
Cu5
Cu6
0.259
0.117
0.117
0.141
0.129
0.129
0.047
0.013
0.013
0.057
0.047
0.047
0.01980
0.01985
0.01985
0.02112
0.02035
0.02035
0.869
0.715
0.715
0.792
0.754
0.754
B1g ? Eg d–d transitions and the one at 760 nm can be assigned
to B1g ? A1g. This observation indicates a different geometry
around copper in Cu5 and Cu6 than in Cu2 and Cu3. Either strong
ligand field or bulkiness of the substituent causes splitting of the
three merged bands into two bands. However, the B1g ? A1g tran-
sition, being very weak, is not observed in the absorption spectra.
The EPR spectra of all the complexes exhibit good hyperfine
splitting (Fig. 4) and the corresponding g||, g\ and A|| values are
tabulated in Table 1. A distorted tetragonal geometry around the
to liquid. The maximal weight loss (75% and 79%, respectively, for
Cu1 and Cu4) corresponds to the loss of two ligand molecules,
ultimately leaving Cu2O as the residue (Calc. 74.5% and 78.6%,

20
S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
a
20.0µ
10.0µ
0.0
-10.0µ
-20.0µ
-30.0µ
-40.0µ
Cu
Cu14
CuCl2
0.5
0.4
0.3
0.2
0.1
0.0
V,Volts vs. Ag/AgCl
b
c
13.0µ
12.0µ
11.0µ
10.0µ
26.0µ
R2 = 0.987
R2 = 0.9837
24.0µ
22.0µ
20.0µ
0.10
0.12
0.14
0.16
0.18
0.20
0.10
0.12
0.14
0.16
0.18
ν
1/2/V1/2s-1/2
ν
1/2/V1/2s-1/2
Fig. 5. (a) Cyclic voltammograms of a 1 mM solution of complexes Cu1 and Cu4 in dichloromethane containing 0.1 M tetra-n-butylammonium hexafluorophosphate as the
supporting electrolyte. (b) Graph of the anodic peak current for Cu1 as a function of square root of scan rate in the range 10 mV to 50 mV/s. (c) Graph of the anodic peak
current for Cu4 as a function of square root of scan rate in the range 10 mV to 50 mV/s.
Table 2
Electrochemical dataa for the copper(II) complexes at 25 2 °C in DCM solution.
Complex
Epc (V)
Epa (V)
E1/2 (V)
DEp (mV)
Cu1
Cu2
Cu3
Cu4
Cu5
Cu6
0.178
0.180
0.177
0.106
0.104
0.105
0.360
0.387
0.372
0.350
0.356
0.353
0.087
0.077
0.079
ꢀ0.016
ꢀ0.022
ꢀ0.019
0.182
0.207
0.195
0.244
0.252
0.248
a
Potential measured vs. Ag/AgCl reference electrode; scan rate: 50 mV/s; sup-
porting electrolyte: tetra-n-butylammonium hexafluorophosphate (0.1 M); com-
plex concentration: 0.001 M.
values of a2 greater than 0.5 are reported to indicate a more ionic
character of the bond between the metal and the coordinating
atom of the ligand [32]. In the present study, the a2 values of all
the complexes are larger than 0.5, indicating some ionic character
of the Cu(II)–N and Cu(II)–O bonds. A close look at the magnitude
of the a2 values reveals significant differences, like it is 0.715 for
Cu2 and Cu3 and 0.86 for Cu1, suggesting a lesser ionic character
of the metal ligand bond in Cu2 and Cu3 compared to that in Cu1.
Fig. 6. ORTEP diagram of Cu1.
copper ion is evident from g > g\ > 2 with a d
ground state, as
x2ꢀy2
k
expected for a square planar copper(II) complex [30]. For a cop-
per(II) complex, the g parameter is sensitive enough to indicate
k
covalance. It is known that [31] for a covalent Cu(II) complex,
g < 2.3, and for an ionic environment, normally g = 2.3 or more.
From the values of g (Table 1), a more covalent character is pre-
k
k
Thus, both g and a2 values point to a different environment
k
k
dicted for Cu2, Cu3, Cu5 and Cu6, a less covalent environment
around the Cu(II) ion in Cu1 and Cu2 compared to Cu3, despite
being coordinated to chemically identical ligands. This signifies a
more distorted arrangement of the optically active ligands around
the metal ion.
around metal in Cu4 and ionic character of the bond around Cu1.
The values of a2
,
calculated using the equation
a
2 = ꢀ(A/
0.036) + (g ꢀ 2) + (3/7)(g\ ꢀ 2) + 0.04, are given in Table 1. The
k

S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
21
Table 3
Table 4
Crystal data for the complex Cu1.
Selected bond lengths (Å) and angles (°) for the complex Cu1.
Compound
Complex
C34H28CuN2O2
560.11
293(2)
0.71073
monoclinic
P21/c
14.964(16)
5.763(6)
20.524(16)
90
132.92(5)
1296(2)
2
1.430
0.878
Cu–N1
Cu–N1o
Cu–O
Cu–Oo
N1–Cu–O1
N1o–Cu–O1o
N1–Cu–N1o
O1–Cu–O1o
Cu–O1–C1
Cu–O1–C1o
C10–C11–C12
N1–C11–C12
1.90(1)
1.90(1)
1.925(7)
1.925(7)
89.6(4)
Empirical formula
Formula mass
T (K)
k (Å)
89.6(4)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
180.0(4)
180.0(3)
129.2(7)
129.2(7)
122.0(1)
115.6(1)
a
(°)
b (°)
V (Å3)
The sign ’o’ suggest the mirror image of the element in the
single crystal structure.
Z
Dcalc (mg mꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
578
Crystal size (mm)
h-range (°)
Index ranges
0.28 ꢃ 0.14 ꢃ 0.05
1.98–24.00
ꢀ17 6 h 6 15
ꢀ6 6 k 6 4
ꢀ23 6 l 6 22
5546/2043
Based on F2
2043/0/190
1.233
H2
N
O
Cu
N
O
H2
Reflection collected/unique
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.1188
wR2 = 0.2227
R1 = 0.1596
wR2 = 0.2414
0.693/ꢀ0.741
Fig. 7. Proposed structure of Cu2.
Largest difference in peak/hole
(e Åꢀ3
)
Analysis of the EPR data for Cu2 and Cu3 stipulated us to antic-
ipate a structure with more distortion around the metal ion. The
proposed structure is depicted in Fig. 7. The distortion around
the metal ion may be due to both phenyl rings lying on the same
side of the molecular plane.
The electrochemical behavior of the complexes in CH2Cl2 using
tetra-n-butylammonium hexafluorophosphate as supporting
a
electrolyte was investigated. The ligands were first examined for
their electrochemical stability in the 0.6 to ꢀ0.8 V range, and were
found to be stable. Fig. 5a exhibits the voltammograms of Cu1 and
Cu4 (along with that of CuCl2 for the purpose of comparison) and
the corresponding values of the cathodic peak potentials Epc and
anodic peak potentials Epa for all the complexes are given in Table
2. The Epc values for Cu1, Cu2 and Cu3 are about 178 mV, which are
less positive by 100 mV compared to the Cu(II) ion in the absence
of ligand. The shift in reduction potential is expected for a coordi-
nated metal ion [33]. A linear relationship is observed (Fig. 5b and
c) between the anodic current and the square root of the sweep
rate, which indicates a diffusion controlled electrochemical pro-
cess. The separation between the anodic and cathodic peaks,
4. Conclusion
The reaction of amino naphthol ligands with Cu(II) in a 1:2 mole
ratio resulted in the formation of Cu(L-H)2 complexes. The geome-
try of the Cu(II) complex with a racemic composition of the Betti
base is square planar, with phenyl rings lying on opposite sides
of the molecular plane, as confirmed by single crystal X-ray analy-
sis. The geometry of the other complexes is speculated to be more
distorted. The structure of the complexes with optically active li-
gands has been proposed, with two phenyl rings lying on the same
side of the molecular plane, though in absence of XRD data, this
could not be confirmed.
D
E = Epa ꢀ Epc, is found to be in the range 185–250 mV at
50 mV/s scan rate, and is larger than E = 59/n mV, and the ratio
D
of the current intensity of the cathodic and anodic peaks are also
different from unity, indicating a quasi-reversible process [33],
probably due to rearrangement of the coordination sphere on
going from the Cu(II) to the Cu(I) oxidation state [34]. The cathodic
peak potential for Cu4 is shifted to a more negative value by 72 mV
compared to Cu1, this can be attributed to a distortion from planar-
ity [35–37].
Acknowledgments
Mr. Shardul Bhatt is thankful to University Grant Commission
(UGC, F.33-254/2007 28-2-2008(SR)), New Delhi for financial sup-
port. We are thankful to CDRI Lucknow and IIT Mumbai for spectral
analysis and CSMCRI Bhavnagar for the single crystal analysis
study.
The ORTEP diagram of Cu1 is shown in Fig. 6. The crystal data for
Cu1 are given in Table 3 and selected bond lengths and angles are
presented in Table 4. The copper atom is bonded to two nitrogen
and two oxygen atoms; both pairs are in trans positions with a per-
fect square planar geometry around copper. It is evident from the
crystal structure that the two phenyl rings are positioned at oppo-
site sides of the molecular plane. Moreover the naphthyl rings are
not strictly planar, they are slightly bent away from the phenyl
rings with 174° instead of 180°, as shown in Fig. 5. Our efforts to
crystallize a Cu(II) complex with an optically active ligand were
unsuccessful.
Appendix A. Supplementary data
CCDC 824514 contains the supplementary crystallographic data
for Cu1. 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 associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.12.034.

22
S. Bhatt, B. Trivedi / Polyhedron 35 (2012) 15–22
[20] P.S. Umare, R. Anthony, K. Gopalkrishnan, G.L. Tembe, B.M. Trivedi, J. Mol.
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