Effect of ligand substituents on supramolecular self-assembly and electrochemical properties of copper(II) complexes with benzoylhydrazones: X-ray crystal structures and cyclic voltammetry

Article abstract of DOI:10.1016/j.poly.2012.02.007

Complexation of copper(II) with a series of hetero-donor chelating Schiff bases (HLL^R) of para-substituted benzhydrazides (with R = OH, NO ₂, CH₃O, Cl and tert-butyl substituents) and acetone affords mononuclear [Cu(LL^R)₂] molecules: 1 [Cu(anbhz)₂] (R = NO₂); 2 [Cu(ahbhz)₂] (R = OH) 3 [Cu(ambhz)₂] (R = CH₃O); 4 [Cu(acbhz)₂] (R = Cl); and 5 [Cu(atbhz)₂] (R = tert-butyl). Single-crystal X-ray diffraction results for 2-5 reveal their various supramolecular architectures including 1D and 2D dimensionalities. A detailed analysis of crystal structures allows to gain insight into intermolecular interactions accountable for self-assembly into different networks as well as for various physicochemical properties of the compounds. The major interactions include O-H...N hydrogen bonds (2) as well as CH...N and axial Cu...O (3); π...π stacking (4) and van der Waals CH...C interactions (5). All compounds are characterized by elemental analyses; IR and UV-Vis spectroscopy as well as magnetic susceptibility and cyclic voltammetry measurements. Electrochemical studies reveal the dependence of [Cu(LL ^R)₂]/[Cu(LL^R)₂]^- reduction potentials on substituents of benzoylhydrazonoate ligands.

Full text of DOI:10.1016/j.poly.2012.02.007

Polyhedron 36 (2012) 120–126
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Effect of ligand substituents on supramolecular self-assembly and
electrochemical properties of copper(II) complexes with benzoylhydrazones:
X-ray crystal structures and cyclic voltammetry
⇑
Dariusz Matoga , Janusz Szklarzewicz, Wojciech Nitek
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Complexation of copper(II) with a series of hetero-donor chelating Schiff bases (HLLR) of para-substituted
benzhydrazides (with R = OH, NO2, CH3O, Cl and tert-butyl substituents) and acetone affords mononuclear
[Cu(LLR)2] molecules: 1 [Cu(anbhz)2] (R = NO2); 2 [Cu(ahbhz)2] (R = OH) 3 [Cu(ambhz)2] (R = CH3O); 4
[Cu(acbhz)2] (R = Cl); and 5 [Cu(atbhz)2] (R = tert-butyl). Single-crystal X-ray diffraction results for 2–5
reveal their various supramolecular architectures including 1D and 2D dimensionalities. A detailed anal-
ysis of crystal structures allows to gain insight into intermolecular interactions accountable for self-assem-
bly into different networks as well as for various physicochemical properties of the compounds. The major
Article history:
Received 16 December 2011
Accepted 5 February 2012
Available online 11 February 2012
Keywords:
Copper
Complex
Hydrazone
Supramolecular
Substituent
Voltammetry
interactions include O–Hꢀ ꢀ ꢀN hydrogen bonds (2) as well as CHꢀ ꢀ ꢀN and axial Cuꢀ ꢀ ꢀO (3);
pꢀ ꢀ ꢀp stacking (4)
and van der Waals CHꢀ ꢀ ꢀC interactions (5). All compounds are characterized by elemental analyses; IR and
UV–Vis spectroscopy as well as magnetic susceptibility and cyclic voltammetry measurements. Electro-
chemical studies reveal the dependence of [Cu(LLR)2]/[Cu(LLR)2]ꢁ reduction potentials on substituents of
benzoylhydrazonoate ligands.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
[5–7,11–14]. The ability of prediction of the self-assembly of small
molecules in the crystalline state is still one of the most challenging
The construction of coordination polymers through crystal
engineering is an important field of chemistry with tremendous
potential in the development of functional materials. The applica-
tions where such polymers may be used include, for instance
electronic, magnetic and optoelectronic devices, advanced nano-
composites or biological sensors [1–7]. In this wide class of materi-
als, porous metal–organic frameworks (MOFs), incorporating
metals and organic ligands, give rise to a considerable and con-
stantly increasing interest among researchers mostly due to their
further promising applications in gas storage and catalysis [5–10].
MOFs are compounds that extend ‘infinitely’ into one, two or three
dimensions (1D, 2D or 3D, respectively) via both covalent metal–li-
gand coordination as well as weaker non-covalent interactions,
topics in crystal engineering.
In the field of metal–organic frameworks, copper(II) complexes,
exhibiting Jahn–Teller distortion, have recently emerged as attrac-
tive building blocks offering weak axial binding and coordination
versatility [15,16]. Among various organic ligands used in the
construction of porous copper coordination polymers, incorpora-
tion of Schiff-base metal complexes into channels’ walls of a well-
defined porous framework still belongs to a rarity [17–19]. The pio-
neering synthesis of such materials has been described in the group
of Kitagawa who used a ‘‘metalloligand’’ concept with carboxylate
substituted N,N0-phenylenebis(salicylideneimine) [18]. Recently
also a supramolecular lamellar structure of copper(II) Schiff-base
complex was reported with relevance to enantioselective recogni-
tion and separation [19]. Schiff bases themselves and their
complexes are also known to self-assemble only through supramo-
lecular interactions, forming defined pores in the crystalline state as
well as showing high surface areas and selective gas uptake [20,21].
We recently became interested in coordination polymers based
on copper(II) Schiff-base complexes that may exhibit porous and/
or conducting properties. As a line of this study we were investigat-
ing a series of ortho-hydroxybenzoyl hydrazones that have been
shown to act as bridging ligands and to form interesting unusual
single-chain electropolymerizable copper–organic backbones [22].
For comparison, it appeared attractive to study analogous copper
such as hydrogen bonding or
p p stacking. These non-covalent
ꢀ ꢀ ꢀ
interactions are also important in biological systems and generally
govern the physicochemical properties of molecular systems in
the solid state. Undoubtedly, exploration and understanding how
weak interactions work in the supramolecular systems is essential
for crystal engineers who design structures, in particular those of
materials based on coordination compounds with organic ligands
⇑
Corresponding author. Tel.: +48 12 6632223; fax: +48 12 6340515.
E-mail address: matoga@chemia.uj.edu.pl (D. Matoga).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2012.02.007

D. Matoga et al. / Polyhedron 36 (2012) 120–126
121
benzoylhydrazone systems with a series of para-substituents
(including hydroxy group), potentially capable of forming supramo-
lecular structures through various intermolecular interactions. The
position of substituents in benzoylhydazonate ligands has been
chosen intentionally in this study as favourable for perpendicular
arrangements of complexes that potentially could result in porous
supramolecular networks. These complexes could also potentially
become useful building blocks either as metalloligands (linkers) or
as nodes (connectors) in the crystal engineering of polymeric frame-
works. In spite of their attractiveness as building blocks for the con-
struction of advanced materials, hydrazone ligands and their
copper(II) complexes are also interesting as potential chemothera-
peutic agents since analogous systems were found to exhibit antitu-
mor and antibacterial activities [23–28].
In this work we report on the synthesis, X-ray crystal structures
as well as spectroscopic and electrochemical properties of a series of
copper(II) complexes with para-substituted hydrazone ligands. A
detailed analysis of crystal structures is presented and the intermo-
lecular interactions accountable for the self-assembly into different
supramolecular networks as well as for various physicochemical
properties of the compounds, are discussed.
Fig. 1. Keto-enol tautomerism and reversible deprotonation of substituted ben-
zoylhydrazones utilized as templates in the syntheses.
amount of cold EtOH and dried in air at room temperature. Yield:
0.435 g; 86.3%. Anal. Calc. for C20H20N6O6Cu: C, 47.67; H, 4.00; N,
16.68. Found: C, 47.04; H, 3.98; N, 16.53%. IR (KBr, cmꢁ1): vCN(imine)
1596s, 1618s, vCO(enolate) 1269 m, vsym(nitro) 1335vs, vasym(nitro)
1563vs. UV–Vis (solid state) k, nm: 500–700broad, 420sh, 371,
272. Magnetic moment: lef = 1.7 lB.
2. Materials and methods
Copper(II) acetate monohydrate was synthesized according to
published method [29]. All other chemicals and solvents were of
analytical grade (Aldrich, Lach-ner, POCh, Polmos) and were used
as supplied. Carbon, hydrogen and nitrogen were determined using
an Elementar Vario MICRO Cube elemental analyzer. Solid samples
for IR spectroscopy were compressed as KBr pellets and the IR spec-
tra were recorded on a Bruker EQUINOX 55 FT-IR spectrophotome-
ter. Electronic absorption spectra were measured with a Shimadzu
UV-3600 UV–Vis–NIR spectrophotometer. Diffuse reflectance spec-
tra were measured in BaSO4 pellets with BaSO4 as a reference using
Shimadzu 2101PC equipped with ISR-260 attachment. Magnetic
susceptibility measurements were carried out at room temperature
on a Sherwood Scientific Magway MSB MK1 balance. In magnetic
moments calculations corrections for diamagnetism were not used.
Cyclic voltammetry measurements were carried out in DMSO with
[Bu4N]PF6 (0.10 M) as the supporting electrolyte using Pt working
and counter and Ag/AgCl reference electrodes on an AUTOLAB/
PGSTAT 128N Potentiostat/Galvanostat. Cyclic voltammograms
were obtained under argon at room temperature. E1/2 values were
calculated from the average anodic and cathodic peak potentials,
E1/2 = 0.5(Ea + Ec). The redox potentials were calibrated versus ferro-
cene, which was used as an internal potential standard for measure-
ments to avoid the influence of liquid junction potential; the final
values are reported versus ferrocenium/ferrocene couple.
2.1.2. Synthesis of [Cu(ahbhz)2]ꢀ0.5H2O (2ꢀ0.5H2O)
The synthetic procedure was analoguous to that of 1 except that
4-hydroxybenzhydrazide (304 mg, 2.00 mmol) was used instead of
4-nitrobenzhydrazide. Yield: 0.272 g; 59.8%. Anal. Calc. for
C
20H23N4O4.5Cu: C, 52.80; H, 5.10; N, 12.31. Found: C, 52.81; H,
4.90; N, 12.13%. IR (KBr, cmꢁ1):
CN(imine) 1598s, 1610vs, vCO(enolate)
1243s, CO(phenolic) 1279s. UV–Vis (solid state) k, nm: 500–700broad,
428, 283. UV–Vis [DMSO solution: k, nm (
, dm3 molꢁ1 cmꢁ1)]:
730broad (60), 420sh, 290sh. Magnetic moment: ef = 1.7 B. Gold-
v
v
e
l
l
en-green crystals of 2 suitable for single-crystal X-ray diffraction
were obtained after recrystallization from DMSO.
2.1.3. Synthesis of [Cu(ambhz)2]ꢀ0.5H2O (3ꢀ0.5H2O)
The synthetic procedure was analoguous to that of 1 except that
4-methoxybenzhydrazide (332 mg, 2.00 mmol) was used instead
of 4-nitrobenzhydrazide. Yield: 0.359 g; 74.3%. Anal. Calc. for
C
22H27N4O4.5Cu: C, 54.70; H, 5.63; N, 11.60. Found: C, 54.84; H,
5.42; N, 11.60%. IR (KBr, cmꢁ1): vCN(imine) 1590s, 1609s, vCO(enolate)
1265m, vCO(methoxy) 1245s, 1309 m. UV–vis (solid state) k, nm:
500–700broad, 420sh, 280. Magnetic moment:
lef = 1.5 lB. Dark
green crystals of 3 suitable for single-crystal X-ray diffraction were
obtained after recrystallization from THF/DMF mixture.
2.1.4. Synthesis of [Cu(acbhz)2]ꢀ0.5H2O (4ꢀ0.5H2O)
The synthetic procedure was analoguous to that of 1 except that
4-chlorobenzhydrazide (341 mg, 2.00 mmol) was used instead of
4-nitrobenzhydrazide. Yield: 0.460 g; 93.5%. Anal. Calc. for
2.1. Syntheses
The approach to synthesize copper benzoylhydrazone com-
plexes utilizes a condensation reaction between acetone and a ser-
ies of para-substituted benzoylhydrazides. The list of formed
hydrazones HLLR that were used as templates in the reaction with
copper(II) acetate to give [Cu(LLR)2] complexes 1–5, is presented in
Fig. 1.
C
20H21Cl2N4O2.5Cu: C, 48.84; H, 4.30; N, 11.39. Found: C, 48.96;
H, 4.14; N, 11.33%. IR (KBr, cmꢁ1):
CN(imine) 1588s, 1615 m vCO(eno-
late) 1267 m. UV–Vis (solid state) k, nm: 500–700broad, 420sh, 274.
UV–Vis [DMSO solution: k, nm (
, dm3 molꢁ1 cmꢁ1)]: 688broad
(60), 280sh. Magnetic moment: ef = 1.8 B. Dark green crystals
v
e
l
l
of 4 suitable for single-crystal X-ray diffraction were obtained after
recrystallization from DMSO.
2.1.1. Synthesis of [Cu(anbhz)2] (1)
4-Nitrobenzhydrazide (362 mg, 2.00 mmol) and acetone
2.1.5. Synthesis of [Cu(atbhz)2] (5)
(140
l
L, 10.0 mmol) were dissolved in EtOH (40 mL) and heated
The synthetic procedure was analoguous to that of 1 except that
4-tert-butylbenzhydrazide (384 mg, 2.00 mmol) was used instead
of 4-nitrobenzhydrazide. Yield: 0.180 g; 34.2%. Anal. Calc. for
under reflux for approx. 10–15 min. [Cu(O2CCH3)2]ꢀH2O (200 mg,
1.00 mmol) was then added and the heating under reflux of the
resultant suspension was continued for approx. 15 min. Green-
ish-grey precipitate of 1 was filtered off, washed thrice with small
C
28H38N4O2Cu: C, 63.91; H, 7.28; N, 10.65. Found: C, 63.39; H,
7.18; N, 10.50%. IR (KBr, cmꢁ1):
v
CN(imine) 1584s, 1625 m, vCO(enolate)

122
D. Matoga et al. / Polyhedron 36 (2012) 120–126
1267s, vCH(tertbutyl) 2964s, 2903 m, 2867 m. UV–Vis (solid state) k,
nm: 710sh, 604, 420sh, 281. UV–Vis [DMSO solution: k, nm
80.7–80.9°) whereas compound 5 exhibits an intermediate
geometry between square planar and tetrahedral (Fig. 2). The dihe-
dral angle between the two coordination planes defined by O(8)–
Cu(1)–N(10) and O(28)–Cu(1)–N(30) is 28.5°. Crystal structures of
analogous mononuclear trans-CuN2O2 four-coordinate complexes
with Schiff bases derived from salicyl- or hydroxynaphthalenecarb-
oxaldehydes and bulky amines (1-ethylpropylamine, 1-naphthyl-
ethylamine, 1-phenylethylamine or cycloamines) also exhibited
mostly geometries that are intermediate between square planar
and tetrahedral; dependent on steric effects of substituents at the
nitrogen atom [32–37]. Bond angles in complexes 2–5 support the
square planar (2–4) and intermediate (5) geometries (Table 2), with
O–Cu–Oi ranging from 180 (2–4) to 156.2° (5) and N–Cu–Ni angles
from 180 (2–4) to 168.2° (5). The Cu–O and Cu–N bonds in 2, 3, 4
and 5 have similar lengths and are in the same range as those
reported for salicyloylhydrazone copper(II) complexes, i.e. Cu–O
distances are in the range of 1.88–1.91 Å, and Cu–N distances have
values between 1.99 and 2.05 Å [22]. The values of other bond
lengths are also comparable to those found for salicyloylhydrazone
copper(II) complexes.
The various packing patterns of compounds 2–4 in the solid state
are illustrated in Figs. 3 and 4. The supramolecular structure of
complex 2 is based on strong O–Hꢀ ꢀ ꢀN hydrogen bonds between
hydroxo substituents and nitrogen atoms of hydrazonate ligands
from adjacent complexes (Fig. 3). The observed hydrogen-bond
distances are: O7ꢀ ꢀ ꢀN11 2.804 Å, H7ꢀ ꢀ ꢀN11 2.044 Å, O7–H7
0.763 Å; and the hydrogen-bond angle is 174° (O7–H7ꢀ ꢀ ꢀN11). The
complexes are arranged interchangeably perpendicular to each
other forming herringbone layers, thus validating our assumption
on favourable perpendicular arrangements introduced with para-
substituents of the ligands. Interestingly, the supramolecular layers
formed are 2D rectangular grids with approximately 8 ꢂ 11 Å cavi-
ties. Unfortunately however, these layers do not assemble into
porous network since the lack of proper interlayer separator makes
adjacent layers decrease the cavity dimension, as is shown on the
example of two layers in Fig. 3.
(e
, dm3 molꢁ1 cmꢁ1)]: 674broad (60), 280sh. Magnetic moment:
lef = 1.5 lB. Dark green crystals of 5 suitable for single-crystal
X-ray diffraction were obtained after recrystallization from DMF.
2.2. Crystallographic data collection and structure refinement
The crystals of 2–5 suitable for X-ray analysis were selected
from the materials prepared as described in Section 2. Intensity
data for 5 were collected on Oxford Diffraction SuperNova dual
source diffractometer with an Atlas electronic CCD area detector
using Mova microfocus Mo K
Intensity data for 2–4 were collected on a Nonius Kappa CCD
diffractometer using graphite monochromated Mo K radiation
a radiation source (k = 0.71073 Å).
a
(k = 0.71073 Å). The crystal data, details of data collection and
structure refinement parameters are summarized in Table 1. The
positions of most atoms for all structures were determined by
direct methods, other non-hydrogen atoms were located on differ-
ence Fourier maps. All non-hydrogen atoms were refined aniso-
tropically using weighted full-matrix least-squares on F2.
Hydrogen H7 (2) was identified on difference Fourier maps and
refined with geometrical restraints. All other hydrogen atoms
bonded to carbons were included in the structure factor calcula-
tions at idealized positions. In the structure of 5 there are electron
density peaks which are not arranged in any chemically meaning-
ful molecule. It seems that they could be associated with the
strongly disordered DMF molecule. Unfortunately all attempts to
model this molecule and its disorder have not given satisfactory
results. The structures were solved using SIR-97 and refined by SHEL-
XL program [30,31].
3. Results and discussion
3.1. X-ray crystal structures
The solid state structure of complex 3 can be described as
supramolecular 2D layers parallel to the bc plane that are stacked
along the a axis, and includes a combination of CHꢀ ꢀ ꢀN and Cuꢀ ꢀ ꢀO
interactions, the latter involving hydrazonate methoxy substitu-
ents whose oxygen atoms can be treated as occupying axial
Compounds 2–5 possess four-coordinate copper centers that are
surrounded by two bidentate hydrazones bound in the trans-N,O
mode. The centers in 2–4 adopt square planar geometry (with
deviations from an ideal square induced by chelate bite angles of
Table 1
Crystal data and structure refinement parameters for 2–5.
2
3
4
5
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
C
20H22N4O4Cu
C22H26N4O4Cu
474.01
0.14 ꢂ 0.14 ꢂ 0.03
monoclinic
P21/c
7.8720(4)
9.3920(5)
15.7670(8)
90.00
112.788(3)
90.00
C20H20Cl2N4O2Cu
482.84
0.60 ꢂ 0.25 ꢂ 0.10
monoclinic
P21/c
15.3700(4)
4.66400(10)
16.8490(4)
90.00
122.097(2)
90.00
C31.5H38N4O2Cu
568.20
445.97
0.25 ꢂ 0.20 ꢂ 0.06
monoclinic
P21/c
6.7090(3)
14.6870(7)
11.9480(5)
90.00
123.369(3)
90.00
983.21(8)
2
0.51 ꢂ 0.28 ꢂ 0.08
triclinic
ꢀ
P1
7.777(5)
11.610(5)
17.410(5)
85.957(5)
80.031(5)
77.191(5)
1508.9(12)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
1074.72(10)
2
1023.21(4)
2
Z
T (K)
293(2)
1.506
1.146
293(2)
1.465
1.053
293(2)
1.567
1.353
293(2)
1.251
0.757
Dcalc (mg/m3)
l
(mmꢁ1
)
Reflections measured
16,607
8679
14,840
9734
Reflections unique
2241
2450
2311
6243
Reflections observed [I > 2
r
(I)]
1930
1589
1955
4242
R indices [I > 2r(I)]
R
wR2
S
0.0628
0.1145
1.303
0.0417
0.0905
0.978
0.0347
0.0916
1.091
0.0488
0.1207
0.980

D. Matoga et al. / Polyhedron 36 (2012) 120–126
123
Fig. 2. Single-crystal X-ray structures of [Cu(ahbhz)2] (2), [Cu(ambhz)2] (3), [Cu(acbhz)2] (4) and [Cu(atbhz)2] (5) showing the atom labeling scheme and 50% displacement
ellipsoids.
Table 2
Tetrahedrally distorted complexes 5 form chains along the
[100] direction that alternate along the b axis (Fig. 5). The com-
Selected bond lengths (Å) and angles (°) for [Cu(ahbhz)2] (2), [Cu(ambhz)2] (3),
[Cu(acbhz)2] (4) and [Cu(atbhz)2] (5) with estimated standard deviations in paren-
plexes are stabilized by weak van der Waals CHꢀ ꢀ ꢀC interactions.
theses. Unless otherwise stated, all bond lengths and angles involve atoms within
five-membered rings containing copper.
The lack of specific intermolecular interactions such as observed
for complexes 2, 3 and 4 seems to be accountable for the tetrahe-
dral distortion of compound 5. Chains of complexes, related by a
translation along the c axis, are separated by layers of strongly dis-
ordered molecules.
2
3
4
5
Bond lengths
Cu–O
1.880(3)
2.044(2)
1.291(4)
1.306(4)
1.419(5)
1.289(5)
1.9006(18)
2.037(2)
1.301(3)
1.298(3)
1.421(3)
1.298(3)
1.8916(14)
2.0396(15)
1.292(3)
1.301(2)
1.895(2)
1.904(2)
1.996(2)
2.004(2)
1.311(3)
1.299(3)
1.310(3)
1.300(3)
1.403(3)
1.408(3)
1.283(3)
1.295(3)
Cu–N
C–O
3.2. Cyclic voltammetry
The redox properties of all copper(II) complexes 1–5 have been
studied by cyclic voltammetry (CV). The cyclic voltammograms
were recorded at various scan speeds (100–1000 mVsꢁ1) over the
range from ꢁ1500 to 1000 mV (versus Ag/AgCl) in DMSO. All re-
corded voltammograms were performed with the cell-off mode
(zero current flow) before and after measurements so as not to
generate any new species near the working electrode. We have
found, that independently on single or multiscan mode or else
starting potential there were no significant changes in the number
of peaks observed. All narrow ranges were scanned in the multi-
scan mode in order to verify appearing signals as reproducible
and stable ones as well as to obtain good quality curves. At the
end of analysis for each compound, ferrocene was added as an
internal potential standard.
C–N
N–N
1.410(2)
N–C(outside
1.296(2)
the ring)
Bond angles
O–Cu–Oi
N–Cu–Ni
O–Cu–N
180.00(16) 180
180.0(2)
180
180
80.92(6)
156.25(10)
168.18(9)
81.90(9)
81.29(9)
100.62(9)
101.12(9)
180
80.89(8)
80.68(10)
O–Cu–Ni
99.32(10)
99.11(8)
99.08(6)
positions of a Jahn–Teller elongated octahedral complex (Fig. 4).
The adjacent stacks of complexes are held together by pairs of
weak CHꢀ ꢀ ꢀN interactions along the [010] direction (Hꢀ ꢀ ꢀN distance
is 2.660 Å) as well as the axial coordination with the Cu1–O14 dis-
tance 3.050 Å that links complexes along the [001] direction and is
most probably responsible for the poor solubility of the compound.
In the crystal structure of 4 planar copper complexes are stabi-
The electrochemical data are summarized in Table 3 and the
representative voltammograms for all complexes are presented
in Fig. 6. One distinct quasireversible or reversible electrochemical
process is observed for each compound within the DMSO window.
These redox waves appear at relatively low potentials and are
attributable to a ligand-centered process involving the imino group
of the enol form of the coordinated ligands. The reduction of the
imine group to the radical anion has been previously reported for
Schiff-base complexes [38]. Here, the noteworthy feature is the
reversibility or quasireversibility of the reduction which is due to
Cu(II) stabilized anion radicals. Apart from the distinct signals, pre-
sented for all complexes in Fig. 6, we have also observed some
lized by
pꢀ ꢀ ꢀp face-to-face stacking interactions along the [010]
direction forming infinite 1D supramolecular chains. The com-
plexes are arranged along the [001] lattice vector with alternating
stacking planes that are perpendicular to each other (Fig. 4). The
closest observed intermetallic Cuꢀ ꢀ ꢀCu distance is 4.664 Å.

124
D. Matoga et al. / Polyhedron 36 (2012) 120–126
Fig. 3. Single-crystal X-ray crystal structure of [Cu(ahbhz)2] (2), illustrating the packing pattern with 2D supramolecular layers containing rectangular cavities, and viewed in
a spacefill mode (a) along a⁄ and (b) along b and (c) perspectively along a. Separate layers are indicated in blue and green and hydrogen atoms are omitted for clarity.
Hydrogen bonds (pale blue lines) are additionally shown and viewed along a (c). (For interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
Fig. 4. Single-crystal X-ray crystal structures of (a) [Cu(ambhz)2] (3) and (b) [Cu(acbhz)2] (4) illustrating the packing patterns and viewed along a (a) and along a⁄ (b).
Hydrogen atoms are omitted for clarity. Selected intermolecular separations are indicated as red dashed lines. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Table 3
Selected electrode potentials E1/2 (V vs. Fc+/Fc) and peak-to-peak separations
DE (V)
for the anodic and cathodic processes exhibited by the complexes 1–5 in DMSO
solutions (measurements carried out at 200 mV/s).
Complex
E1/2
D
E
Comments
1
2
3
4
5
ꢁ1.52
ꢁ1.17
ꢁ1.23
ꢁ1.02
ꢁ1.11
0.10
0.26
0.13
0.30
0.38
Reversible
Reversible
Reversible
Quasireversible
Quasireversible
The reduction potentials become considerably less negative in
the following order of benzoylhydrazonate para-substituents: nitro
(1) < methoxy (3) < hydroxy (2) < tert-butyl (5) < chloro (4). Taking
into account the electro-donating and -withdrawing character of
the substituents expressed by Hammett rp parameters, the order
of decreasing electro-withdrawing character (decreasing
nitro ( p = 0.78; 1) < chloro ( p = 0.23; 4) < tert-butyl ( p = -0.20;
5) < methoxy (
p = ꢁ0.37; 2) [39]. Clearly
rp) is:
Fig. 5. Single-crystal X-ray crystal structure of [Cu(atbhz)2] (5), illustrating the
packing pattern, and viewed (a) along a, (b) along the [1–10] direction. Hydrogen
atoms and disordered molecules are omitted for clarity.
r
r
r
r
p = ꢁ0.27; 3) < hydroxy (
r
the two orders differ from each other; compounds with tert-butyl
(5) and chloro substituents (4) are the easiest to reduce in the whole
series whereas they both are characterized by relatively high Ham-
peaks at a more positive potential but these have been found to be
ill-defined, of low intensity, and often elusive when narrow-range
scans have been performed. The only reliable signal, attributed to a
copper-centered process, has been observed as a reversible wave at
E1/2 = ꢁ0.12 V (versus Fc+/Fc) for compound 1. Analogous waves
have not been observed for the rest of complexes. Most probably
copper-centered reductions of these complexes lead to their in-
stant decomposition.
mett rp parameter. The redox potentials for the rest of compounds
(with oxygen containing substituents) are in agreement with sub-
stituent electro-withdrawing character. Most probably the break-
down of this trend can be explained by: (a) bulky substituents of
complex 5 leading to its tetrahedrally distorted geometry, different
from the rest of the complexes with a square planar geometry; as
well as (b) by different solvation pattern for complexes 4 and 5 in
comparison with the other compounds. The latter effect may be

D. Matoga et al. / Polyhedron 36 (2012) 120–126
125
Fig. 6. Cyclic voltammograms recorded for complexes 1–5 in DMSO solutions. Experimental potential values are measured vs. Ag/AgCl at 200 mV sꢁ1
.
associated with a coordination of DMSO that is an aprotic solvent
with Lewis base character. Such coordination in complexes 1–3 is
hindered by the presence of substituents with oxygen donors. This
explanation is also supported by the fact that complexes 4 and 5
are freely soluble in DMSO whereas compounds 1–3 are poorly sol-
uble. Another feature that distinguishes complexes 4 and 5 from the
rest of the copper compounds is the quasireversible character of the
electrochemical reduction. It indicates a relatively slow electron
transfer in comparison with applied scan rates, that may be the re-
sult of the presence of a steric hindrance in the proximity of imine
groups.
Appendix A. Supplementary data
CCDC 858982, 858983, 858984 and 858985 contain the supple-
mentary crystallographic data for 2, 3, 4 and 5, respectively. 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.
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