Synthesis, X-ray crystal structure, and electrochemistry of copper(II) complexes of a new tridentate unsymmetrical Schiff base ligand and its hydrolytically rearranged isomer

Article abstract of DOI:10.1016/j.ica.2011.12.025

A new unsymmetrical Schiff base ligand HL¹, HBacabza, and its copper(II) complexes [Cu₂L¹₂(OAc)₂] (1) and [Cu₂L²₂(N₃) ₂]·2H₂O (2) with HBacabza = 3-(2-aminobenzylimino)- 1-phenylbutan-1-one as HL¹ and its hydrolytically rearranged isomer 3-(2-aminomethylphenyleneimino)-1-phenylbutan-1-one as HL², have been synthesized and characterized by elemental analyses and spectroscopic methods. The rearrangement of HL¹ to HL² occurs in a hydrolysis-recondensation process in the reaction of HL¹ with Cu(ClO₄)₂·6H₂O and NaN₃. The crystal structures of the ligand and its complexes have been determined by single crystal X-ray diffraction. The deprotonated Bacabza^- coordinates to the metal center as a tridentate ligand. The acetate anion coordinates through one oxygen atom in complex 1 leading to a mono-atomic acetate oxygen-bridging dimeric copper(II) complex. Similarly, the azide anion coordinates through one nitrogen atom in complex 2 leading to a mono-atomic azide nitrogen-bridging dimeric copper(II) complex. The copper(II) ions adopt a distorted square pyramidal (4 + 1) coordination in these two complexes. The cyclic voltammetric studies of these complexes in N,N-dimethylformamide indicate that the reduction process corresponding to Cu^II/Cu^I is electrochemically irreversible in complex 1, presumably due to the structural changes during the course of redox reaction, and quasi-reversible in complex 2.

Full text of DOI:10.1016/j.ica.2011.12.025

Inorganica Chimica Acta 385 (2012) 31–38
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Synthesis, X-ray crystal structure, and electrochemistry of copper(II) complexes of
a new tridentate unsymmetrical Schiff base ligand and its hydrolytically
rearranged isomer
Soraia Meghdadi a, Kurt Mereiter b, Vratislav Langer c, Ahmad Amiri a, Roghayeh Sadeghi Erami a,
Alshima’a A. Massoud c, Mehdi Amirnasr a,
⇑
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna, Austria
c Environmental Inorganic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
a r t i c l e i n f o
a b s t r a c t
A new unsymmetrical Schiff base ligand HL1, HBacabza, and its copper(II) complexes [Cu2L12(OAc)2] (1) and
[Cu2L22(N3)2]ꢀ2H2O (2) with HBacabza = 3-(2-aminobenzylimino)-1-phenylbutan-1-one as HL1 and its
hydrolytically rearranged isomer 3-(2-aminomethylphenyleneimino)-1-phenylbutan-1-one as HL2, have
been synthesized and characterized by elemental analyses and spectroscopic methods. The rearrangement
of HL1 to HL2 occurs in a hydrolysis-recondensation process in the reaction of HL1 with Cu(ClO4)2ꢀ6H2O and
NaN3. The crystal structures of the ligand and its complexes have been determined by single crystal X-ray
diffraction. The deprotonated Bacabzaꢁ coordinates to the metal center as a tridentate ligand. The acetate
anion coordinates through one oxygen atom in complex 1 leading to a mono-atomic acetate oxygen-bridg-
ing dimeric copper(II) complex. Similarly, the azide anion coordinates through one nitrogen atom in com-
plex 2 leading to a mono-atomic azide nitrogen-bridging dimeric copper(II) complex. The copper(II) ions
adopt a distorted square pyramidal (4 + 1) coordination in these two complexes. The cyclic voltammetric
studies of these complexes in N,N-dimethylformamide indicate that the reduction process corresponding
to CuII/CuI is electrochemically irreversible in complex 1, presumably due to the structural changes during
the course of redox reaction, and quasi-reversible in complex 2.
Article history:
Received 14 August 2011
Received in revised form 14 November 2011
Accepted 15 December 2011
Available online 24 December 2011
Keywords:
Unsymmetrical Schiff base
Copper(II)
Crystal structure
Cu(II) catalyzed hydrolysis
Cyclic voltammetry
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
monodentate, chelating, bidentate bridging, monodentate
bridging, and chelating bridging [13]. The azide anion can act as
The coordination chemistry of transition metal complexes of
unsymmetrical Schiff base ligands has attracted much attention
in recent years due the fact that the ligands around central metal
ions in natural systems are unsymmetrical [1–3]. The unsymmetri-
cal Schiff bases are obtained by the mono-condensation of dia-
mines with aldehydes/ketones, or by stepwise condensation of
the appropriate diamine with two different carbonyl compounds
[1,3–6]. The electronic properties and steric demands of these
Schiff base ligands can be tuned selectively via appropriate tailor-
ing and modification of the ligand structure. The Schiff bases de-
rived from the 1:1 condensation of diamines are a set of mono-
anionic N2O donor ligands that react readily with copper(II) ion
to form complexes of variable nuclearity depending upon the co-li-
gand present [7–12]. The acetate and azide anions are among the
most versatile co-ligands that exhibit diverse coordination modes.
The acetate group is known to coordinate to the metal ions as
a monodentate ligand as well as a bridging ligand adopting the
-1,1 (end-on, EO) and -1,3 (end-to-end, EE) modes [14–16].
l
l
The acetato- and azido-bridged copper complexes are among the
most interesting compounds that have been investigated by biolo-
gists and bioinorganic chemists for elucidating the structure and
role of active sites in copper proteins and by physical inorganic
chemists for seeking to design new magnetic materials [17–21].
An interesting feature that has recently been documented by the
elegant work of Ghosh and coworkers is the anion-directed hydro-
lysis and rearrangement of mono-condensed Schiff bases during
complex formation [5]. In the course of our recent studies on
unsymmetrical Schiff base and carboxamide ligands and their me-
tal complexes [22,23] we have synthesized and characterized the
unsymmetrical Schiff base HBacabza. This ligand reacts with cop-
per acetate to give a bis-(l2-1,1-OAc) bridged dinuclear complex,
but with copper perchlorate, the ligand undergoes hydrolysis and
recondensation, and in the presence of sodium azide, forms the
dinuclear bis-(
l2-1,1-N3) bridged copper(II) complex of its hydro-
⇑
Corresponding author. Tel.: +98 311 391 2351; fax: +98 311 391 2350.
E-mail address: amirnasr@cc.iut.ac.ir (M. Amirnasr).
lytically rearranged isomer (Scheme 1). Herein we report the syn-
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.025

32
S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
Scheme 1. Synthesis of the unsymmetric HBacabza ligand, its copper(II) acetate complex and the copper(II) azide complex of its hydrolytically rearranged isomer.
thesis and X-ray crystal structures of HBacabza, its dinuclear cop-
per acetate complex, and the dinuclear copper azide complex of its
isomer. The spectroscopic and electrochemical properties of the
two copper complexes are also presented and discussed.
electrochemical potentials were calibrated versus internal
Fc+/o (E0 = 0.45 V versus SCE) couple under the same conditions [24].
2.2. Synthesis of the tridentate ligand, HL1 (HBacabza)
To a solution of 1.62 g (10 mmol) benzoylacetone in 20 mL eth-
anol was added a solution of 1.22 g (10 mmol) 2-aminobenzyl-
amine in 10 mL ethanol. The reaction mixture was refluxed at
70 °C for 90 min. Milky white crystals of HBacabza suitable for X-
ray crystallography were obtained after one day. These crystals
were filtered off and washed with cold ethanol and dried under
vacuum. Yield 66%. Anal. Calc. for C17H18N2O: C, 76.66; H, 6.81;
2. Experimental
2.1. Materials and methods
All chemicals were purchased from commercial sources and
used as received. Elemental analyses were obtained on a Perkin–El-
mer 2400II CHNS-O elemental analyzer. Infrared spectra (KBr pel-
let) were recorded on a FT-IR JASCO 680 PLUS instrument. 1H
NMR spectra were measured with a Bruker AVANCE DR X500 spec-
trometer (500 MHz). Proton chemical shifts are reported in ppm rel-
ative to Me4Si as internal standard. The redox properties of the
complexes were studied by cyclic voltammetry. Cyclic voltammo-
grams were recorded using a SAMA Research Analyzer M-500.
Three electrodes were utilized in this system, a glassy carbon work-
ing electrode, a platinum disk auxiliary electrode and Ag wire as ref-
erence electrode. The glassy carbon working electrode (Metrohm
N, 10.52. Found: C, 76.64; H, 6.81; N, 10.59%. IR: (KBr, cmꢁ1
) mmax:
3369 and 3462 (NH2), 1597 (C@N). 1H NMR (CDCl3, 500 MHz):
d = 2.12 (s, 3H, CH3), 3.78 (s, 2H, NH2), 4.42–4.41 (d, 2Hbenzylic),
5.77 (s, 1H, CH), 7.88–6.72 (m, 9H, ArH), 11.53 (s, 1H, OH).
2.3. Synthesis of [Cu2L12(OAc)2] (1)
To a stirring solution of Cu(OAc)2ꢀH2O (91 mg, 0.05 mmol) in
10 mL methanol, was added a solution of 13.3 mg (0.05 mmol) of
the ligand in 15 mL CH2Cl2 at room temperature. The solution
turned green immediately and after 1 h, it was filtered. The filtrate
was left undisturbed at room temperature for two days. The result-
ing dark green crystals of (1) suitable for X-ray crystallography
6.1204.110) with 2.0 0.1 mm diameter was cleaned with 1 lm
alumina polish prior to each scan. Tetrabutylammonium hexa-
fluorophosphate (TBAH) was used as supporting electrolyte. The
solutions were deoxygenated by purging with Ar for 5 min. All

S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
33
were filtered off and washed with cold methanol and dried under
of H2O and methanol disordered in continuous channels parallel
to [1 0 1] of this structure and combined with a partial orienta-
tion disorder of the azide group in 0.757(3)/0.243(3) proportion.
This disorder was taken into account as described in the depos-
ited CIF. For simplicity the solid was given the idealized formula
[Cu2L22(N3)2]ꢀ2H2O. The crystallographic and refinement data of
all three compounds are summarized in Table 1.
vacuum.Yield 64%. Anal. Calc. for C19H20N2O3Cu: C, 58.8; H, 5.20; N,
7.22. Found: C, 58.0; H, 5.0; N, 7.16%. IR: (KBr, cmꢁ1
) mmax: 1593
(C@N), 1608 (COas), 1383 (COs).
2.4. Synthesis of complex [Cu2L22(N3)2]ꢀ2H2O (2)
To a stirring solution of Cu(ClO4)2ꢀ6H2O (37.1 mg, 0.1 mmol) in
methanol 15 mL was added a solution of 26.6 mg (0.1 mmol) of the
ligand in 15 mL methanol. The solution turned green immediately.
After 15 min, a solution of sodium azide 0.013 g (0.2 mmol), dis-
solved in minimum of methanol, was added to the above solution.
Dark green crystals of (2) suitable for X-ray crystallography were
obtained after three days. The crystals were collected by filtration
and washed with cold methanol. Yield 51%. Anal. Calc. for
3. Result and discussion
A prerequisite of our study is the determination of the coordina-
tion modes of acetate, and azide, and the possibility of hydrolysis
and recondensation of the new mono-condensed Schiff base HBa-
cabza. While Schiff base ligands react with transition metal ions
and form stable complexes that are of significant biological appli-
cations [6,30–33] these ligands are at risk of hydrolytic attack in
reaction mixtures that contain transition metal ions. In the process
of hydrolytic cleavage of –C@N– that occurs on metal sites, the sol-
vent system, co-ligands and reaction conditions play a significant
role [3,9]. Interesting cases of hydrolysis and recondensation of
unsymmetrical Schiff base ligands have been investigated by pio-
neer research groups in the field. The anion-directed hydrolysis
of mono-condensed Schiff base ligands in Ni and Cu complexes
[5] and copper catalyzed rearrangement of unsymmetrical to sym-
metrical complexes [34] are noticeable examples.
The mono-condensed ligand 3-(2-aminobenzylimino)-1-phe-
nylbutan-1-one (HL) is prepared by the reaction of benzoylacetone
and 2-aminobenzylamine in 1:1 M ratio in ethanol. As expected,
condensation occurs at the benzyl amine arm and the X-ray crystal
structure of the ligand confirms the structure (vide infra). By mix-
ing a solution of HL1 in dichloromethane with a methanolic solu-
tion of copper acetate monohydrate, a mono-atomic acetate
oxygen-bridging dimeric copper(II) complex, 1, is obtained. The
C
17H17N5OCuꢀH2O: C, 52.50; H, 4.92; N, 18.01. Found: C, 52.9; H,
4.62; N, 18.1%. The discrepancy may stem from methanol inclusion
in the crystal lattice of this complex (vide infa). IR: (KBr, cmꢁ1
max: 1583 (C@N), 2037 (N3ꢁ).
)
m
2.5. X-ray crystallography
Single crystal X-ray diffraction data were collected at low
temperature with graphite-monochromated Mo radiation
K
a
(k = 0.71073 Å) utilizing a STOE IPDS-II image plate instrument
for the ligand HL1 (HBacabza), a Siemens SMART CCD diffractom-
eter for 1, and a Bruker Kappa APEX-II CCD diffractometer for 2.
Data were processed with STOE X-area software for HL1, and
programs SAINT and SADABS for 1 and 2 [25–29]. The crystal struc-
tures were solved with direct methods and program SHELXL97,
and were refined on F2 with program SHELXL97 [29]. All non-H
atoms were refined anisotropically. The H atoms were con-
strained to idealized geometries and were refined isotropically.
Complex 2 was found to be a solvate in solid state with a mix
Table 1
Crystal structure and refinement data for HL1 [Cu2L12(OAc)2] (1) and [Cu2L22(N3)2]ꢀ2H2O (2).
Chemical formula
C17H18N2O (HL1)
C38H40Cu2N4O6 (1)
C34H34Cu2N10O2ꢀ2H2O (2)
Formula weight
T (K)
Crystal system, space group
266.33
120(2)
triclinic P1
775.84
173(2)
tetragonal P42/n
777.82
100(2)
monoclinic, P21/n
ꢀ
a (Å)
b (Å)
c (Å)
9.4069(5)
11.3617(7)
13.8749(8)
21.232(2)
21.232(2)
7.8356(13)
8.7969(3)
20.5683(5)
9.8550(3)
a
(°)
72.380(4)
90
90
b (°)
80.514(5)
90
107.484(2)
c
(°)
80.808(5)
1384.48(14)
4, 1.278
90
3532.2(8)
4, 1.459
90
V (Å3)
1700.76(9)
2, 1.519
Z, Dcalc (Mgcm3)
Crystal size (mm)
l
0.35 ꢂ 0.30 ꢂ 25
024 ꢂ 0.04 ꢂ 0.04
0.40 ꢂ 0.38 ꢂ 0.22
(mmꢁ1
)
0.080
1.256
1.305
F(000)
568
1608
804
h Range (°)
2.21–29.21
2.71–25.12
2.62–30.00
Index ranges
Reflections collected
/unique/Rint
ꢁ12 6 h 6 12, ꢁ15 6 k 6 15, ꢁ18 6 l 6 18
ꢁ25 6 h 6 25, ꢁ23 6 k 6 25, ꢁ9 6 l 6 9
ꢁ12 6 h 6 12, ꢁ28 6 k 6 28, ꢁ13 6 l 6 13
15,247/7426/0.086
20,965/3155/0.160
40,925/4930/0.028
Absorption correction
Minimum and
maximum
none
multi-scan
0.75, 0.95
multi-scan
0.63, 0.76
transmission
Data/restraints/parameters
Goodness-of-fit (GOF)
on F2
7426/0/385
1.101
3155/0/237
0.981
4177/0/248
1.207
Final R indices
R1 = 0.0877, wR2 = 0.1784
R1 = 0.0465, wR2 = 0.0810
R1 = 0.0356, wR2 = 0.0790
[I > 2r(I)]
R indices (all data)
R1 = 0.1407, wR2 = 0.2027
0.32 and ꢁ0.37
R1 = 0.1150, wR2 = 0.0977
0.40 and ꢁ0.34
R1 = 0.0378, wR2 = 0.0801
0.79 and ꢁ0.63
Maximum/minimum
Dq )
(eÅꢁ3

34
S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
ligand retains its structure in this complex. However, the reaction
of HL1 with copper perchlorate hexahydrate and NaN3 leads to the
formation of the end-on azido-bridged dimeric complex 2, with the
copper atom binding to the rearranged Schiff base ligand L2
(Scheme 1). Since the amine used in this reaction is unsymmetrical,
a unique and clear cut picture of the hydrolytic cleavage of –C@N–
in L1 and recondensation to its isomer L2 is obtained. The hydroly-
sis of closely related ligands to the corresponding diamine and
diketone and subsequent recondensation has been documented
experimentally [3,5,34].
[5]. These bands are also observed at lower frequencies
(ꢃ100 cmꢁ1) in the IR spectra of complexes 1 and 2. The characteris-
tic azomethine group (C@N) band of the free ligand appears at
1597 cmꢁ1. The
m(C@N) is generally shifted to lower frequencies in
the copper complexes relative to the free ligand, indicating a de-
crease in the C@N bond order due to the coordination of the imine
nitrogen to the metal and back bonding from theCu(II) to the p⁄ orbi-
tal of azomethine group. This band appears at 1593 cmꢁ1 and
1587 cmꢁ1 in the spectra of 1 and 2, respectively and is in agreement
with that observed in the related Schiff base complexes [5,11].
The IR spectrum of complex 1 revealed two intense bands at
Alternatively, the possibility of equilibrium between HL1 and
HL2 can be envisaged under the reaction conditions (Scheme 1).
The dominant form may be influenced by solvent and anion effects,
and the relative solubility of the products may also manipulate the
reaction pathway.
1608 and 1384 cmꢁ1 for
Comparison of the value of the acetate complex with the
lue of sodium acetate can be used as a guideline in assigning the
acetate coordination mode [35,36]. The
= 224 cmꢁ1 value of
the acetate complex is larger than that reported for sodium acetate
= 164 cmꢁ1) and indicates a monoatomic bridging mode of
masym(OCO) and
m
sym(OCO), respectively.
va-
D
m
Dm
D
m
(Dm
3.1. Spectroscopic studies
acetate coordination that conforms to the structural data obtained
by X-ray crystallography (vide infra). The appearance of a sharp
band at 2037 cmꢁ1 in the IR spectrum of complex 2 is characteristiꢁc
of the end-on asymmetric bridging mode of the coordinated N3
ion in the structure [5,8].
The FT-IR data of the ligand and copper complexes are listed in
Section 2. The IR spectrum of the free ligand exhibits two bands at
3462 and 3369 cmꢁ1 characteristic of the asymmetric and symmet-
ric stretching vibrations of the NH2 substituent on the phenyl ring
Fig. 1. Structural diagram of ligand HL1 with the atom numbering scheme and hydrogen bonds. Displacement ellipsoids are drawn at 50% probability.
Table 2
Selected bond lengths (Å) and angles (°) for the ligand, (1) and (2).
Ligand (Molecule 1)
Ligand (Molecule 2)
Complex (1)
Complex (2)
Bond length
C6–C7
C7–N2
C8–N2
C8–C9
C8–C10
C10–C11
C11–O1
1.505(4)
1.454(3)
1.335(3)
1.503(4)
1.390(3)
1.414(4)
1.266(3)
C23–C24
C24–N4
C25–N4
C25–C26
C25–C27
C27–C28
C28–O2
1.507(4)
1.459(3)
1.331(3)
1.502(4)
1.390(3)
1.416(4)
1.260(3)
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–O2
Cu1–O2i
O1–C11
O2–C18
O3–C19
2.017(3)
1.943(3)
1.937(2)
1.976(3)
2.479(2)
1.292(5)
1.294(4)
1.240(4)
Cu1–N1
Cu1–N2
Cu1–O1
Cu1–N3
Cu1–N3i
O1–C11
N3–N4
1.9599(15)
2.0063(19)
1.9333(13)
2.0124(16)
2.5501(17)
1.295(2)
1.254(3)
1.133(4)
N4–N5
Bond angles
C5–C6–C7
C1–C6–C7
N2–C7–C6
C8–N2–C7
N2–C8–C10
C8–C10–C11
C10–C11–C12
O1–C11 –C10
O1–C11–C12
C13–C12–C11
C17–C12–C11
122.7(2)
118.3(2)
114.5(2)
126.2(2)
121.3(2)
123.7(2)
119.0(2)
123.4(2)
117.6(2)
118.9(2)
122.5(2)
C22–C23–C24
C18–C23–C24
N4–C24–C23
C25–N4–C24
N4–C25–C27
C25–C27–C28
C27–C28–C29
O2–C28 –C27
O2–C28–C29
C30–C29–C28
C34–C29–C28
122.3(2)
119.1(2)
113.4(2)
127.0(2)
121.1(2)
124.1(2)
119.6(2)
122.6(2)
117.8(2)
118.3(2)
122.8(2)
O1–Cu1–N2
N2–Cu1–N1
N1–Cu1–O2
O2–Cu1–O1
O1–Cu1–N1
N2–Cu1–O2
O1–Cu1–O2i
N2–Cu1–O2i
N1–Cu1–O2i
O2–Cu1–O2i
C11–O1–Cu1
C1–N1–Cu1
94.60(12)
92.22(14)
90.85(14)
88.80(11)
162.90(12)
157.62(11)
87.35(9)
123.22(11)
75.81(11)
79.00(9)
O1–Cu1–N1
N1–Cu1–N2
N2–Cu1–N3
N3–Cu1–O1
O1–Cu1–N2
N1–Cu1–N3
O1–Cu1–N3i
N1–Cu1–N3i
N2–Cu1–N3i
N3–Cu1–N3i
N3–N4–N5
91.87(6)
92.23(6)
89.05(6)
88.26(6)
173.08(6)
166.58(7)
86.06(5)
104.01(6)
87.54(6)
89.39(6)
177.9(3)
124.3(2)
114.7(3)
Symmetry codes: for (1): i ꢄ 1 ꢁ x, 1 ꢁ y, ꢁz; for (2): i ꢄ ꢁx, ꢁy, ꢁz.

S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
35
The 1H NMR spectral data of HL1 is given in the experimental
section. The enolic proton appears at 11.53 ppm as a broad peak
due to extensive intramolecular hydrogen bonding. The signals
corresponding to the protons of the phenyl rings resolve in the
range 6.72–7.88 ppm, and the signal appearing at 5.77 ppm is as-
signed to the @CH proton. The benzylic (CH2) protons of the ligand
appear at 4.41–4.42 ppm and a broad singlet at 3.78 ppm can be
attributed to NH2 group. These two signals correlate well with
those reported for related ligands [37,38]. The singlet at
2.12 ppm is assigned to the methyl group linked to C@N.
3.2. Description of structures of the ligand and complexes 1 and 2
3.2.1. Crystal structure of the ligand (HL1)
The X-ray structure analysis confirms that the ligand is a mono-
condensation product of the benzyl amine group with benzoylace-
tone. The molecular structure of ligand, HL1, with atomic number-
ing scheme is presented in Fig. 1 and selected bond distances and
angles are listed in Table 2. The structure is centrosymmetric,
ꢀ
space group P1, contains two independent molecules of similar
dimensions and conformation (Fig. 1) and exhibits also a pseudo-
translation of c/2 relating the two independent molecules. After fit-
ting the two molecules to each other, the r.m.s. deviation of
corresponding non-hydrogen atom positions is 0.336 Å, with a
maximum deviation between C16 and C33 of 0.71 Å. This differ-
ence between the two molecules is largely due to the differing rel-
ative orientations of the unsubstituted phenyl rings. The ligand is
not planar and there is a twist of C6–C7–N2–C8 = ꢁ86.6(3)° for
molecule 1 (+86.6(3)° for its enantiomer), and of C23–C24–N4–
C25 = 93.9(3)° for molecule 2. The angle between the mean planes
passing through the two phenyl rings of molecule 1 is 75.67° and
80.42° for molecule 2. The bond distances C6–C7 = 1.505(4) Å,
Fig. 2. Molecular structure of [Cu2L12(OAc)2] (1). The Cu1–O2 distance is 1.976(3) Å
and Cu1–O20 is 2.479(2) Å. Symmetry code for primed atoms: 1 ꢁ x, 1 ꢁ y, ꢁz.
Hydrogen bond distances are N1ꢀꢀꢀO10 = 3.055(5) and N1ꢀꢀꢀO300 = 2.881(4) Å.
C11–C12 = 1.505(3) Å, and C8–C9 = 1.503(4) Å of molecule
1
(1.507(4), 1.503(3), 1.502(4) Å for molecule 2) are within the range
of single bonds. N2 = C8 imine bond length of 1.335(3) Å
(1.331(3) Å for molecule 2) is typical of a double bond. The
C10–C8 and C10–C11 bond lengths are 1.390(3) and 1.414(4) Å,
respectively (1.390(3) and 1.416(4) Å for molecule 2), which are
intermediate between common values for single and double bonds
due to the resonance. Both molecules are involved in strong intra-
and weak to very weak inter-molecular N–HꢀꢀꢀO hydrogen bonding
interactions (Table 3 and Fig. 1).
(Cu1–N2 = 1.943(3) Å), from the enolic oxygen, O1 (Cu1–
O1 = 1.937(2) Å), of the tridentate Schiff base ligand, and from
one of the two oxygen atoms, O2 (Cu1–O2 = 1.976(3) Å), of an ace-
tate anion. The copper atom, Cu1, of this fragment is linked via a
much longer bond to the in-plain coordinated acetate oxygen
atom, O2i (1 ꢁ x, 1 ꢁ y, ꢁz), of the other copper (Cu1–
O2i = 2.479(2) Å). The basal plane formed by the four donor atoms
N1, N2, O1, and O2, has a distinct saddle-like deformation (Fig. 3).
These atoms have therefore a considerable r.m.s. deviation from a
common least squares plane of 0.334 Å and the individual dis-
tances from this plane are ꢁ0.33, 0.33, ꢁ0.34 and 0.34 Å,
3.2.2. Crystal structure of [Cu2L12(OAc)2] (1)
The molecular structure of (1) with atomic numbering scheme
is presented in Fig. 2 and selected bond distances and angles are
listed in Table 2. The complex crystallizes as a centrosymmetric
l-1,1-OAc-bridged dinuclear molecule. Each copper atom shows
a distorted (4 + 1) square-pyramidal coordination geometry. The
Cu1–O2–Cu1i bridging angle is 101.00(9)° and the four-membered
Cu2O2 bridging unit is strictly planar owing to inversion symmetry.
The Cu1ꢀꢀꢀꢀCu1i distance is 3.4524(8) Å and comparable with those
reported for related complexes [11,12]. The four short bonds to Cu
are from the two nitrogens, N1 (Cu1–N1 = 2.017(3) Å) and N2
Table 3
Hydrogen bonds for HL1 (Å and °).
D–Hꢀ ꢀ ꢀA
d(D–H)
0.88(4)
0.88(4)
0.84(4)
0.83(4)
0.88(4)
0.85(4)
d(Hꢀ ꢀ ꢀA)
2.88(4)
2.34(4)
2.00(4)
2.80(4)
2.64(4)
1.99(3)
d(Dꢀ ꢀ ꢀA)
3.523(3)
3.115(3)
2.677(3)
3.579(3)
3.371(3)
2.661(3)
\(DHA)
161(4)
147(4)
138(4)
156(4)
141(4)
136(3)
N1–H1Bꢀ ꢀ ꢀO2i
N1–H1Aꢀ ꢀ ꢀO2ii
N2–H2Bꢀ ꢀ ꢀO1
N3–H3Bꢀ ꢀ ꢀO1iii
N3–H3Cꢀ ꢀ ꢀO1iv
N4–H4Bꢀ ꢀ ꢀO2
Symmetry code: (i) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z; (ii) ꢁ1 + x, ꢁ1 + y, 1 + z ; (iii) 1 ꢁ x, 1 ꢁ y, ꢁz;
(iv) 1 + x, 1 + y, ꢁ1 + z.
Fig. 3. Comparison of the edge-sharing square-pyramidal (4 + 1) coordination
figures of Cu in [Cu2L12(OAc)2] (1) and [Cu2L22(N3)2]ꢀ2H2O (2).

36
S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
Table 4
Table 5
Hydrogen bonds for 1 (Å and °).
Hydrogen bonds for 2 (Å and °).
D–Hꢀ ꢀ ꢀA
d(D–H)
0.85(4)
0.81(3)
0.81(3)
d(Hꢀ ꢀ ꢀA)
2.22(4)
2.17(3)
2.48(4)
d(Dꢀ ꢀ ꢀA)
3.055(5)
2.881(4)
2.945(5)
\(DHA)
172(4)
146(4)
117(3)
D–Hꢀ ꢀ ꢀA
d(D–H)
0.92
0.92
d(Hꢀ ꢀ ꢀA)
2.18
2.33
d(Dꢀ ꢀ ꢀA)
3.039(2)
2.999(4)
2.852(5)
\(DHA)
155
130
N1–H12ꢀ ꢀ ꢀO1i
N1–H11ꢀ ꢀ ꢀO3ii
N1–H11ꢀ ꢀ ꢀO3
N2–H2NAꢀ ꢀ ꢀO1i
N1–H2NBꢀ ꢀ ꢀO2wii
O2wꢀ ꢀ ꢀN3
Symmetry code: (i): 1 ꢁ x, 1 ꢁ y, ꢁz; (ii): 1 ꢁ x, 1 ꢁ y, 1 ꢁ z.
Symmetry code: (i): 1 ꢁ x, 1 ꢁ y, ꢁz; (ii): 1 ꢁ x, 1 ꢁ y, 1 ꢁ z.
respectively. The deviation of copper atom from this plane is
0.047(2) Å toward O2i which is 2.323(3) Å above the plane. The
N,O-Cu-O2i bond angles between the basal atoms and the apical
oxygen vary between 75.81(11)° for N1–Cu1–O2i and
123.22(11)° for N2–Cu1–O2i showing that the coordination figure
about Cu1 is very distorted (Fig. 3). The structural data of complex
1 are in agreement with those reported for related copper com-
plexes [11,12], for which it should be noted that the deformation
of the Cu < O2 > Cui rhombus and of the Cu (4 + 1) square-pyramid
varies considerably.
Complex 1 shows extensive hydrogen bonding. Both the hydro-
gen atoms of the coordinated amine group (H11 and H12) are in-
volved in hydrogen bonding interactions (Fig. 2, Table 4). The
hydrogen bond of H11 shows a very asymmetric bifurcation, the
main acceptor is O3i of a neighboring complex, whereas the minor
acceptor is the intramolecular O3. For clarity, this interaction was
omitted in Fig. 2.
from the two nitrogens, N1 (Cu1–N1 = 1.9599(15) Å) and N2 (Cu1–
N2 = 2.0063(16) Å), from the enolic oxygen, O1 (Cu1–
O1 = 1.9333(13) Å), of the tridentate Schiff base ligand and from
one of the three nitrogen atoms, N3 (Cu1–N3 = 2.0124(16) Å), of an
azide anion. The copper atom, Cu1, of this fragment is linked via a
much longer bond to the in-plain coordinated azide nitrogen atom,
N3i (ꢁx, ꢁy, ꢁz) (Cu1–N3i = 2.5501(17) Å). The four donor atoms,
N1, N2, O1, N3, forming the basal plane of the square pyramid are
approximately coplanar with a r.m.s. deviation from this plane of
0.168 Å and with O1, N2, and Cu1 displaced by 0.172, 0.164, and
0.064(1) Å to the apical nitrogen N3i, whereas N1 and N3 are dis-
placed by ꢁ0.165 and ꢁ0.170(1) Å in opposite direction. The bond
angles between the basal ligands and the apical N3i vary between
86.06(5)° and 104.01(6)° proving that the Cu(4 + 1)-square pyramid
in complex 2 is more regular than that in complex 1 (Fig. 3). The
hydrogen bonds in 2 are included in Fig. 4 and listed in Table 5.
The hydrogen bond N2–H2NAꢀꢀꢀO1i, NꢀꢀꢀO = 3.039(2) Å, and its cen-
trosymmetric equivalent link the two halves of the dinuclear com-
plex analogous to complex 1, The second hydrogen of the amino
group makes a rather bent H-bond to O2w and has, in case that this
siteis notoccupieddue to disorder(seebelow), alternative acceptors
at somewhat larger distances, e.g. N5a not shown in Fig. 4. In general
terms the structural data of complex 2 are in reasonable agreement
with a larger group of related azide bridged dinuclear Cu-complexes
[14–16]. Most of these complexes have the two bridging azide an-
ions in trans-disposition combined with inversion symmetry like
shown in Fig. 4 and only few in cis-disposition (both N3 groups bent
up or bent down). However, the angle of inclination of the azide
groupsrelative to the plane of the Cu1–N3–Cu1i–N3i rhombusvaries
widely from ca. 0° to 60°, compared to the 52.9° in 2. As stated in the
experimental section, the crystal structure of 2 contains, according
to X-ray analysis, water and some methanol as solvent yielding the
idealized formula of [Cu2L22(N3)2]ꢀ2H2O. It also shows a concerted
orientation disorder of the azide group in 0.76/0.24 proportion. This
disorder can well be visualized by Fig. 4: In case of the aberrant min-
or orientation the azide group N3–N4a–N5a terminates approxi-
mately where O2w is located and simultaneously the vicinity of
the now vacant N4 and N5 site is then occupied by methanol, accord-
ing to X-ray evidence. This is possible owing to the fact that the azide
group terminations (N5, N5a) and the solvent water and methanol
molecules reside in continuous channels of the structure extending
parallelto[101]ofthemonoclinicunitcell. ByadditionalX-raymea-
surements on a crystal of another batch of (2) we obtained evidence
that the degree of the azide disorder varies with the H2O/CH3OH
ratio.
3.2.3. Crystal structure of [Cu2L22(N3)2]ꢀ2H2O (2)
The molecular structure of (2) with atomic numbering scheme is
presented in Fig. 4 and selected bond distances and angles are listed
in Table 2. The complex crystallizes as a centrosymmetric l-1,1-N3-
bridged dinuclear molecule. Each copper atom shows a distorted
(4 + 1) square-pyramidal geometry. The Cu1–N3–Cu1i bridging an-
gle is 90.61(6)° and the four-membered Cu2N2 bridging unit is
strictly planar owing to inversion symmetry. The Cu1ꢀꢀꢀꢀCu1i dis-
tance is 3.2653(4) Å and compares well with those reported for re-
lated complexes [5,8,14–16,39]. The four short bonds to Cu are
The bond distance between copper and the bridging donor
atom, Cu–D, the planarity of the Cu2D2 ring, and the Cu–D–Cu an-
gle are among the key factors determining the magnetic coupling
between two copper ions in binuclear copper(II) complexes with
bridging ligands. The nature of magnetic coupling is sensitive to
subtle changes in the structural parameters of the bridging unit
and can be either ferromagnetic or antiferromagnetic [12]. Though
the similarity between the structural parameters of complex 1 and
2 with those reported for closely related complexes [12] may lead
to similar magnetic properties, this subject remains to be fully
investigated in the future.
Fig. 4. Molecular structure of [Cu2L22(N3)2]ꢀ2H2O (2). The distance Cu1–N3 is
2.0124(16) Å and Cu1–N30 is 2.5501(17) Å. Symmetry code for primed atoms: -x, -y,
-z. Hydrogen bond distances are N2ꢀꢀꢀO10 = 3.039(2) and N2ꢀꢀꢀO2w00 = 2.999(4) Å.

S. Meghdadi et al. / Inorganica Chimica Acta 385 (2012) 31–38
37
The cyclic voltammogram of [Cu2L22(N3)2] (2) in DMF solution is
shown in Fig. 6. The voltammogram shows a cathodic peak (Epc
at ꢁ0.69 V with an anodic counterpart (Epa at ꢁ0.52 V
E = 170 mV),
)
)
[E1/2 = ꢁ0.61 V] and a large peak-to-peak separation (
D
indicating a quasi-reversible nature for the CuII/CuI reductive re-
sponse. The electrochemical behavior and the data are in agreement
with those reported for related complexes [5].
4. Conclusion
In this investigation, we have reported the synthesis and char-
acterization an unsymmetrical N2O Schiff base ligand, its copper
acetate complex, and the copper azide complex of its hydrolytically
rearranged isomer. Both complexes are dinuclear with two l2-(1,1)
O or N bridges and the copper(II) ion adopts in each a distorted
square pyramidal (4 + 1) coordination geometry. The effect of the
counteranions of the metal salt on the hydrolysis of the Schiff base
ligand and recondensation to its isomer is clearly demonstrated.
The cyclic voltammetry studies in DMF show that the acetate com-
plex is most probably involved in a solvolysis reaction leading to
two complex species in equilibrium.
Fig. 5. Cyclic voltammogram of [Cu2L12(OAc)2] (1) in DMF at 298 K,
c = 2.3 ꢂ 10ꢁ3 M, scan rate = 100 mVsꢁ1. Potential range: (A) 0.3 to ꢁ1.10 V and
(B) 0.3 to ꢁ1.35 V.
Acknowledgment
3.3. Electrochemical studies
Partial support of this work by the Isfahan University of Tech-
nology Research Council is gratefully acknowledged.
The electrochemical behavior of the two [Cu2L12(OAc)2] (1) and
[Cu2L22(N3)2]ꢀ2H2O (2) complexes have been studied in DMF solu-
tion. Fig. 5 shows the cyclic voltammograms of [Cu2L12(OAc)2] (1)
complex in DMF solution in the potential ranges of 0.3 to
ꢁ1.10 V (A, solid line), and 0.3 to ꢁ1.35 V (B, dashed line). The elec-
trochemically irreversible reduction peak observed at ꢁ0.72 V is
due to the reduction of CuII/CuI center. This value is in agreement
with those observed in the related Cu(II) Schiff base complexes
[40]. The irreversibility of the redox processes can be attributed
to the instability of the reduced species in DMF solvent. By expand-
ing the CV potential range and monitoring the cyclic voltammo-
gram of (1) from 0.3 to ꢁ1.35 V, two reduction waves at ꢁ0.72
and ꢁ1.1 V (B, dashed line) are obtained. These two responses
are presumably due to the two copper complex species [Cu-
L1(OAc)] (lower potential) and [CuL1(OAc)(DMF)] (higher poten-
tial) resulting from partial solvation of complex (1).
Appendix A. Supplementary data
CCDC 838267, 838268 and 838269 contain the supplementary
crystallographic data for HL1 and complexes 1 and 2, respec-
tively. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2011.12.025.
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