Redox-active metal(II) complexes of sterically hindered phenolic ligands: Antibacterial activity and reduction of cytochrome c. Part III. Copper(II) complexes of cycloaminomethyl derivatives of o-diphenols

Article abstract of DOI:10.1016/j.poly.2013.04.015

Redox-active copper(II) complexes of sterically hindered phenolic ligands have been synthesized using 5-tert-butyl-3-(pyrrolidinomethyl)-1,2- dihydroxybenzene (HL^I), 5-tert-butyl-3-(piperidinomethyl)-1,2- dihydroxybenzene (HL^II), 5-t

Full text of DOI:10.1016/j.poly.2013.04.015

Polyhedron 57 (2013) 39–46
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Polyhedron
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Redox-active metal(II) complexes of sterically hindered phenolic ligands:
Antibacterial activity and reduction of cytochrome c. Part III. Copper(II)
complexes of cycloaminomethyl derivatives of o-diphenols
a
a
a
b
c
N.V. Loginova a, , A.T. Gres , G.I. Polozov , T.V. Koval’chuk , N.P. Osipovich , R.A. Zheldakova ,
⇑
Y.V. Faletrov b, I.S. Strakha d, I.I. Azarko e
a Department of Chemistry, Belarusian State University, Leningradskaya str. 14, 220030 Minsk, Belarus
b Research Institute for Physico-Chemical Problems of the Belarusian State University, Leningradskaya str. 14, 220030 Minsk, Belarus
c Department of Biology, Belarusian State University, Nezavisimosti Ave. 4, 220030 Minsk, Belarus
d Centre of Expertise and Testing in Healthcare, Tovarishchesky al. 2a, 220037 Minsk, Belarus
e Department of Physics, Belarusian State University, Bobruiskaya str. 5, 220030 Minsk, Belarus
a r t i c l e i n f o
a b s t r a c t
Article history:
Redox-active copper(II) complexes of sterically hindered phenolic ligands have been synthesized using
5-tert-butyl-3-(pyrrolidinomethyl)-1,2-dihydroxybenzene (HLI), 5-tert-butyl-3-(piperidinomethyl)-1,2-
dihydroxybenzene (HLII), 5-tert-butyl-3-(azepanylmethyl)-1,2-dihydroxybenzene (HLIII), 5-tert-butyl-3-
(morpholinomethyl)-1,2-dihydroxybenzene (HLIV), and 5-tert-butyl-3-(methylpiperazinomethyl)-1,2-
dihydroxybenzene (HLV). The novel compounds have been characterized by means of chemical and
physico-chemical methods. The coordination core of these complexes is a square planar chromophore,
[CuO2N2], and thephenolic ligandscoordinate intheir monoanionic forms. The ligands and Cu(II)complexes
Received 11 November 2012
Accepted 4 April 2013
Available online 13 April 2013
Keywords:
Copper(II) complexes
Phenolic ligands
Cyclic voltammetry
Cytochrome c
have been screened for their antibacterial activity. The lowest MIC value (0.020 l
mol mlÀ1) has been found
for Cu(LIII)2 and Cu(LV)2 against Mycobacterium smegmatis, Sarcina lutea and Staphylococcus aureus, and this
is comparable to thevalue for chloramphenicol. Their antibacterial activities werefound tofollow theorder:
Antibacterial activity
(1) HLI > HLV P HLIII ꢀ HLII > HLIV
;
(2) Cu(LIII
)
2 > Cu(LII)2 ꢀ Cu(LI)2 ꢀ Cu(LV)2 > Cu(LIV)2; their reducing
ability (determined electrochemically) followed the same order. The most bioactive complex, Cu(LIII)2,
has the highest lipophilicity. A spectrophotometric investigation was carried out in order to estimate the
rate of the reduction of bovine heart cytochrome c with the ligands and their Cu(II) complexes. HLI and
the complex Cu(LIII)2 have the highest reducing abilities (determined electrochemically), which are charac-
terized by the highest Cyt c reduction rates respectively amongst the ligands and Cu(II) complexes.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
reaction because it is one of the main tools for the development
of bioactive compounds [5]. The aim of modifying phenolic ligands
In earlier investigations, using the method of cyclic voltamme-
try, we have shown some mono- and di-substituted derivatives of
sterically hindered phenolic ligands as well as their metal com-
plexes to have a pronounced reducing ability, correlating with
the antimicrobial activity and rate of reduction, of bovine heart
cytochrome c (Cyt c) in a limited series of these compounds
[1–4]. These results prompted us to extend our research to other
sterically hindered phenolic ligands and their metal complexes in
order to gain greater insight into what structural modifications of
these compounds play an important role in their reducing ability
and biological activity. For this purpose we chose the Mannich
was to produce novel redox-active ligands for complexation with
transition metals and to further evaluate the changes in antibacte-
rial activity and the rate of the reduction of Cyt c for both the phe-
nolic ligands and their metal complexes.
Copper is known as an essential element, participating in many
biological processes, and the role of Cu(II) complexation in enhanc-
ing the pharmacological profile of the antimicrobial activity of
some drugs and bioactive compounds is known [6,7]. In this con-
nection it is of interest to study various aspects of the coordination
chemistry of Cu(II) ions interacting with phenolic ligands, and to
investigate the physico-chemical and biological properties of the
complexes synthesized.
In the present work, the reduction of Cyt c was investigated
spectrophotometrically with five cycloaminomethyl derivatives
of sterically hindered o-diphenols, namely 5-tert-butyl-3-(pyrro-
⇑
Corresponding author. Tel.: +375 172 09 55 16; fax: +375 172 26 46 96/26 55
67.
E-mail addresses: loginonv@gmail.com, loginonv@yahoo.com (N.V. Loginova).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.015

40
N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
lidinomethyl)-1,2-dihydroxybenzene (HLI), 5-tert-butyl-3-(pipe-
ridinomethyl)-1,2-dihydroxybenzene (HLII), 5-tert-butyl-3-(azepa-
HL (Sigma) (10 mmol), paraformaldehyde (10 mmol), a secondary
amine (pyrrolidine, piperidine, hexamethylene amine, morpholine
or methylpiperazine) (11 mmol) and isopropyl alcohol (20 ml)
were mixed, and the mixture was refluxed for 2–4 h. Then the solu-
tion was boiled dry under vacuum, and the residue was recrystal-
lized from petroleum ether (or ethyl acetate) and dried under
vacuum.
nylmethyl)-1,2-dihydroxybenzene
(HLIII),
5-tert-butyl-3-
(morpholinomethyl)-1,2-dihydroxybenzene (HLIV) and 5-tert-bu-
tyl-3-(methylpiperazinomethyl)-1,2-dihydroxybenzene (HLV) (see
Section 2.2), as well as with their Cu(II) complexes. For the first
time, the redox properties of the above-mentioned ligands and
their Cu(II) complexes were determined electrochemically. As be-
fore [3,4], the results obtained are discussed in view of the pre-
sumed correlation between the capability of the compounds
under study for reducing Cyt c, their antibacterial activity, redox
properties, determined electrochemically, and lipophilicity.
2.2.1. 5-Tert-butyl-3-(pyrrolidinomethyl)-1,2-dihydroxybenzene (HLI)
Yield: 72%; M.p.: 132–134 °C. M.wt.: 249.35. Anal. Calc. for
C
15H23O2N: C, 72.25; H, 9.30; N, 5.62. Found: C, 72.17; H, 9.18;
N, 5.57%. Mass spectrum (m/z, I%): 249.25 (M+, 27), 234.20 (M–
CH3, 14). Prominent IR absorption bands (
, cmÀ1): 3409w (O–H),
m
1608w and 1490m (C@C arom), 1320m, 1301m and 1189s (C–O),
1120m and 1102m (C–N). 1H NMR dH(100 MHz, CDCl3): 7.70 [s,
2H, 2OH], 6.89 [dd, 1H, aromatic H, J 0.4, 2.3 Hz], 6.52 [m, 1H, aro-
matic H], 3.82 [s, 2H, CH2], 2.67 [m, 4H, N(CH2)2], 1.87 [m, 4H,
(CH2)2], 1.28 [s, 9H, Me3C].
2. Experimental
2.1. Materials and methods
All the reagents used for the synthesis of the cycloaminomethyl
derivatives of sterically hindered o-diphenols HLI–HLV and their
complexes are commercially available and were used without fur-
ther purification. The synthesis of the above-mentioned ligands
and their Cu(II) complexes was performed as described in Sections
2.2 and 2.3. The purity of the o-diphenol derivatives HLI–HLV was
checked by Thin Layer Chromatography (TLC). Elemental analyses
were carried out with a Vario EL (CHNS mode) instrument. Copper
was determined using an atomic emission spectrometer with an
inductively coupled plasma excitation source (Spectroflame Mod-
ula). Infrared spectra of solids were recorded with a Nicolet 380
spectrometer in the wavelength range 4000–400 cmÀ1 at room
temperature, using «Smart Performer». Thermal analysis was per-
formed with a Simultaneous Thermal Analyzer STA 449 C. X-ray
Diffraction (XRD) analysis was carried out with an HZG 4A diffrac-
2.2.2. 5-Tert-butyl-3-(piperidinomethyl)-1,2-dihydroxybenzene (HLII)
Yield: 87%; M.p.: 164–166 °C. M.wt.: 263.38. Anal. Calc. for
C
16H25O2N: C, 72.96; H, 9.57; N, 5.32. Found: C, 73.09; H, 9.79;
N, 5.29%. Mass spectrum (m/z, I%): 263.25 (M+, 24), 248.20 (M–
CH3, 16). Prominent IR absorption bands (
, cmÀ1): 3403 m (O–
m
H), 1609w and 1490m (C@C arom), 1326w, 1307s, 1197s and
1188s (C–O), 1098m and 1029w (C–N). 1H NMR dH(100 MHz,
CDCl3): 7.10 [br s, 2H, 2OH], 6.61 [m, 1H, aromatic H], 6.45 [m,
1H, aromatic H], 3.75 [s, 2H, CH2], 2.60 [m, 4H, N(CH2)2], 1.67 [m,
6H, (CH2)3], 1.35 [s, 9H, Me3C].
2.2.3. 5-Tert-butyl-3-(azepanylmethyl)-1,2-dihydroxybenzene (HLIII)
Yield: 88%; M.p.: 160–162 °C. M.wt.: 277.41. Anal. Calc. for
tometer (Co Ka radiation, MnO2-filter). ESR (Electron Spin Reso-
C
17H27O2N: C, 73.60; H, 9.81; N, 5.05. Found: C, 73.52; H, 9.64;
N, 4.97%. Mass spectrum (m/z, I%): 277.40 (M+, 30), 262.35 (M–
CH3, 14). Prominent IR absorption bands (
, cmÀ1): 3403 m (O–
nance) spectra of polycrystalline samples were measured with an
ERS-220 X-band spectrometer (9.45 GHz) at room temperature
and at 77 K, using 100-kHz field modulation; g factors were quoted
relative to the standard marker 2,2-diphenyl-1-picrylhydrazyl
(DPPH). 1H NMR spectra were recorded with a Varian Unit Plus
300 MHz spectrometer in CDCl3; chemical shifts were reported in
parts per million (ppm) relative to an internal standard of Me4Si.
Mass spectra (EI) were recorded on Shimadzu GCMS-QP 2010 Plus
spectrometer. Ultraviolet–Visible (UV–Vis) absorption spectra
were recorded with a SPECORD M500 spectrophotometer. The mo-
lar conductance of 10À3 mol lÀ1 solutions of the Cu(II) complexes in
acetonitrile was measured at 20 °C using a TESLA BMS91 conduc-
tometer (cell constant 1.0). The lipophilicity test was made by
determining the n-octanol/water partition coefficient (Pow) [8].
Electrochemical measurements were performed under dry nitro-
gen in a three-electrode two-compartment electrochemical cell
using a glassy-carbon (GC) working electrode, Pt auxiliary elec-
trode and Ag|AgCl|0.1 mol lÀ1 (C2H5)4NCl reference electrode. The
supporting electrolyte was 0.1 mol lÀ1 (C2H5)4NClO4. The
Ag|AgCl|0.1 mol lÀ1 (C2H5)4NCl reference electrode was calibrated
m
H), 1606w and 1489 m (C@C arom), 1331w, 1307m, 1224w and
1188s (C–O), 1119w, 1102m and 1063m (C–N). 1H NMR dH(100 -
MHz, CDCl3): 7.20 [br s, 2H, 2OH], 6.77 [d, 1H, aromatic H, J
2.3 Hz], 6.55 [d, 1H, aromatic H, J 2.3 Hz], 3.78 [s, 2H, CH2], 2.53
[m, 4H, N(CH2)2], 1.67 [m, 8H, (CH2)4], 1.27 [s, 9H, Me3C].
2.2.4. 5-Tert-butyl-3-(morpholinomethyl)-1,2-dihydroxybenzene
(HLIV)
Yield: 85%; M.p.: 150–152 °C. M.wt.: 265.35. Anal. Calc. for
C
15H23O3N: C, 67.89; H, 8.74; N, 5.28. Found: C, 68.03; H, 8.92;
N, 5.17%. Mass spectrum (m/z, I%): 265.15 (M+, 49), 250.20 (M–
CH3, 12). Prominent IR absorption bands (
, cmÀ1): 3375m (O–H),
m
1605w and 1493m (C@C arom), 1312m, 1300s, 1228m, 1192m
and 1107s (C–O), 1127w, 1071m and 1031m (C–N). 1H NMR
dH(100 MHz, CDCl3): 6.77 [d, 1H, J 2.3 Hz, aromatic H], 6.56 [d,
1H, J 2.3 Hz, aromatic H], 3.71 [m, 4H, O(CH2)2], 3.66 [c, 2H, CH2],
2.55 [m, 4H, N(CH2)2], 1.23 [s, 9H, Me3C].
with the ferrocenium|ferrocene redox couple located at E1/2
=
+0.54 V. Anhydrous acetonitrile was used as the solvent. The (C2-
H5)4NClO4 and (C2H5)4NCl used to prepare solutions were dried
respectively at 80 and 100 °C under vacuum for 3 h. Preparation
of solutions and the subsequent filling of the electrochemical cells
were carried out in a glove box under dry nitrogen.
2.2.5. 5-Tert-butyl-3-(methylpiperazinomethyl)-1,2-
dihydroxybenzene (HLV)
Yield: 84%; M.p.: 157–158 °C. M.wt.: 278.39. Anal. Calc. for
C
16H26O2N2: C, 69.03; H, 9.41; N, 10.06. Found: C, 68.94; H, 9.27;
N, 9.84%. Mass spectrum (m/z, I%): 278.35 (M+, 20), 263.30 (M–
CH3, 4). Prominent IR absorption bands (
, cmÀ1): 3038w (O–H),
m
2.2. Synthesis of the ligands – cycloaminomethyl derivatives of o-
diphenols (HLI–HLV)
1594w and 1497w (C@C arom), 1326m, 1299w, 1168m and
1135m (C–O), 1203w, 1082w, 1050w and 1036w (C–N). 1H NMR
dH(100 MHz, CDCl3): 6.89 [d, 1H, aromatic H, J 2.4 Hz,], 6.82 [br s,
2H, 2OH], 6.52 [d, 1H, aromatic H, J 2.4 Hz,], 3.70 [s, 2H, CH2],
2.55 [m, 8H], 2.32 [s, 3H, NMe], 1.26 [s, 9H, Me3C].
The compounds HLI–HLV were synthesized according to the
methodology described in the literature [9]. 4-Tert-butylcatechol

N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
41
2.3. Synthesis of the Cu(II) complexes with cycloaminomethyl
derivatives of o-diphenols
OH
OH
OH
OH
HN
X
(CH2O)n,
i-PrOH,
Based on the data of potentiometric titrations in a water–etha-
nol (1:1) solution [10], the conditions were created to purposefully
provide the preferential formation of complexes with a Cu(II):L ra-
tio of 1:2. A solution of 0.05 mmol of Cu(CH3COO)2 in 6 ml of water
was added dropwise to a colorless solution of 0.1 mmol of the com-
pounds HL, HLI–HLV dissolved in 14 ml of ethanol (molar ratio
Cu(II):L = 1:2). The reaction mixture was stirred for 1.5 h, after
which the metal complex solution was left for several days to pre-
cipitate. The solid phase that formed was collected on membrane
X
N
HL
HLI: without X
HLII: X = CH2
HLIII: X = (CH2)2
HLIV: X = O
HLV: X = NCH3
Fig. 1. Reagents and conditions for the synthesis of compounds HLI–HLV.
filters (JG 0.2 lm), washed with ethanol and water, and dried in va-
cuo (yield > 70%). We tried the following methods of growing sin-
gle crystals: (i) slow evaporation of acetonitrile, chloroform, 70%
aqueous ethanol or 70% aqueous acetone solutions of metal com-
plex at room temperature in the dark; (ii) recrystallization of the
powdery material from acetonitrile, 2-propanol or chloroform;
(iii) a small portion (0.02 g) of the complex was anaerobically
redissolved in 2.5 ml of acetonitrile (closed system vapor diffusion
method), and carefully overlaid with 25 ml of anhydrous ether and
left in a refrigerator. Unfortunately, none of these methods gave
crystals suitable for X-ray diffraction studies.
parameters (77 K): g|| = 2.290, g\ = 2.062, A|| = 113 Gs. UV–Vis data
(acetonitrile) (kmax, nm (log )): 475 (2.87), 340sh (3.51), 305sh
(3.77), 282 (3.85), 224 (4.13).
e
2.4.2. Cu(LII)2
Molar conductivity (in acetonitrile): Kmol = 28.2
X
À1 cm2 -
molÀ1. Thermogravimetric/differential thermal analysis (TG/DTA)
data: no weight loss was observed until decomposition, which be-
gan at about 125 °C, with four endothermic peaks at 143, 238, 283
and 493 °C, ultimately leaving CuO as the residue. The maximal
weight loss of 86.99% corresponds to the loss of two ligand mole-
cules in the Cu(LII)2 complex (Calc. 86.48%). Prominent IR absorp-
2.3.1. Cu(LI)2
Beige. Yield: 73–75%. M.wt.: 559.70. Anal. Calc. for C30H44N2O4
Cu: C, 64.32; H, 7.92; N, 5.00; Cu, 11.34. Found: C, 64.26; H, 8.01;
N, 4.87; Cu, 11.41%.
tion bands (m
, cmÀ1): 3385w, br (O–H), 1553m, sh and 1483m
(C@C arom), 1316m, 1283m, 1237m, sh and 1154w (C–O),
1103m and 1037m, br (C–N), 525m, sh (Cu–O), 501w (Cu–N).
ESR parameters (77 K): g|| = 2.290, g\ = 2.062, A|| = 120 Gs. UV–Vis
data (acetonitrile) (kmax, nm (loge)): 505 (2.57), 410sh (3.07),
340sh (3.51), 282 (3.87), 225 (4.12).
2.3.2. Cu(LII)2. Deep brown
Yield: 70–73%. M.wt.: 587.79. Anal. Calc. for C32H48N2O4Cu: C,
65.33; H, 8.22; N, 4.76; Cu, 10.80. Found: C, 65.16; H, 8.37; N,
4.65; Cu, 10.87%.
2.3.3. Cu(LIII)2. Greyish-yellow
Yield: 70–73%. M.wt.: 612.76. Anal. Calc. for C34H52N2O4Cu: C,
66.26; H, 8.50; N, 4.55; Cu, 10.31. Found: C, 66.37; H, 8.41; N,
4.61; Cu, 10.44%.
2.4.3. Cu(LIII)2
Molar conductivity (in acetonitrile): Kmol = 29.9
X
À1 cm2 -
molÀ1. Thermogravimetric/differential thermal analysis (TG/DTA)
data: no weight loss was observed until decomposition, which be-
gan at about 110 °C, with four endothermic peaks at 120, 206, 311
and 470 °C, ultimately leaving CuO as the residue. The maximal
weight loss of 86.76% corresponds to the loss of two ligand mole-
cules in the Cu(LIII)2 complex (Calc. 87.09%). Prominent IR absorp-
2.3.4. Cu(LIV)2. Beige
Yield: 70–72%. M.wt.: 591.72. Anal. Calc. for C30H44N2O6Cu: C,
60.84; H, 7.49; N, 4.73; Cu, 10.73. Found: C, 60.99; H, 7.60; N,
4.66; Cu, 10.81%.
tion bands (m
, cmÀ1): 3245w, br (O–H), 1560s, br and 1476s (C@C
arom), 1304w, 1286s, sh, 1224s and 1174w (C–O), 1104w,
1032w, 1016w and 1004w, sh (C–N), 538m and 519m, sh (Cu–O),
506w (Cu–N). ESR parameters (77 K): g|| = 2.300, g\ = 2.050,
A|| = 124 Gs. UV–Vis data (acetonitrile) (kmax, nm (loge)): 470
(2.93), 355sh (3.89), 302 (3.85), 292 (3.85), 224 (4.15).
2.3.5. Cu(LV)2. Deep brown
Yield: 78–80%. M.wt.: 617.76. Anal. Calc. for C32H50N4O4Cu: C,
62.16; H, 8.15; N, 9.06; Cu, 10.28. Found: C, 62.25; H, 8.09; N,
8.99; Cu, 10.17%.
2.3.6. Cu(L)2
Analytical data for this Cu(II) complex with the ligand HL
(Fig. 1) are given in Ref. [11].
2.4.4. Cu(LIV)2
Molar conductivity (in acetonitrile): Kmol = 25.1
X
À1 cm2 -
2.4. Physico-chemical characterization of the Cu(II) complexes
2.4.1. Cu(LI)2
molÀ1. Thermogravimetric/differential thermal analysis (TG/DTA)
data: no weight loss was observed until decomposition, which be-
gan at about 115 °C, with two exothermic peaks at 148 and 254 °C,
and two endothermic peaks at 360 and 465 °C, ultimately leaving
CuO as the residue. The maximal weight loss of 87.18% corresponds
to the loss of two ligand molecules in the Cu(LIV)2 complex (Calc.
Molar conductivity (in acetonitrile): Kmol = 22.1
X
À1 cm2 -
molÀ1. Thermogravimetric/differential thermal analysis (TG/DTA)
data: no weight loss was observed until decomposition, which be-
gan at about 110 °C, with five endothermic peaks at 145, 196, 233,
270 and 495 °C, ultimately leaving CuO as the residue. The maxi-
mal weight loss of 86.49% corresponds to the loss of two ligand
molecules in the Cu(LI)2 complex (Calc. 85.80%). Prominent IR
86.57%). Prominent IR absorption bands (m
, cmÀ1): 3360m, br (O–
H), 1584s, br and 1483m, sh (C@C arom), 1305m, 1216m, 1163w
and 1115s, sh (C–O), 1069w, 1032w and 1003m (C–N), 540m and
522m (Cu–O), 506w (Cu–N). ESR parameters (77 K): g|| = 2.276,
absorption bands (
m
, cmÀ1): 3391w, br (O–H), 1567w and 1481s,
g\ = 2.062, A|| = 128 Gs. UV–Vis data (acetonitrile) (kmax
(log )): 510 (2.55), 340sh (3.47), 305sh (3.61), 278 (3.84), 224
(4.12).
, nm
sh (C@C arom), 1286s, sh, 1261s, 1201w and 1164w (C–O),
1102w and 1062m, sh (C–N), 527m (Cu–O), 506m (Cu–N). ESR
e

42
N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
2.4.5. Cu(LV)2
pounds. Tests using dimethyl sulfoxide (or acetonitrile) as a nega-
tive control were carried out in parallel. The results were always
verified in three separate experiments.
Bovine heart cytochrome c (Sigma) was used. Spectrophotomet-
ric experiments were performed with a Shimadzu UV-1202 spec-
trophotometer using a quartz cuvette with a 1 cm optical path.
The Cyt c concentration was determined by its interaction with ex-
Molar conductivity (in acetonitrile): Kmol = 26.9
X
À1 cm2 -
molÀ1. Thermogravimetric/differential thermal analysis (TG/DTA)
data: no weight loss was observed until decomposition which be-
gan at about 110 °C, with three exothermic peaks at 146, 174 and
224 °C and three endothermic peaks at 254, 297 and 470 °C, ulti-
mately leaving CuO as the residue. The maximal weight loss of
86.31% corresponds to the loss of two ligand molecules in the
Cu(LV)2 complex (Calc. 87.14%). Prominent IR absorption bands
cess sodium dithionite, using the absorption coefficient e550
-
= 21 mmolÀ1 l cmÀ1 [15]. Ar-saturated acetonitrile solutions of
(m
, cmÀ1): 3350m, br (O–H), 1555m, br and 1469m (C@C arom),
the ligands and the Cu(II) complexes under study and Cyt c
1307m, 1279m, br and 1218m (C–O), 1194m, 1148w and 1029m,
sh (C–N), 530m and 515m (Cu–O), 501w (Cu–N). ESR parameters
(77 K): g|| = 2.280, g\ = 2.056, A|| = 118 Gs. UV–Vis data (acetoni-
trile) (kmax, nm (log
277 (3.82), 224 (4.07).
(7 l
mol lÀ1) were used. The experiments were performed in
10 mmol lÀ1 sodium phosphate buffer (pH 7.4) at 20 °C. Aliquots
of the compounds under study were added to the Cyt c solution
e
)): 500 (2.71), 405sh (3.01), 340sh (3.28),
up to a final concentration of 35
reduction ( ) was evaluated by the slope of the kinetic curve A550
l
mol lÀ1. The initial rate of Cyt c
m
vs. time according to Ref. [16]. The characteristic absorption bands
at 550 nm, appearing when the ligands or their metal complexes
are added to the solution of the oxidized Cyt c, bear witness to
the ability of the phenolic compounds to reduce Cyt c in vitro
[17,18]. The results were confirmed in three independent
experiments.
2.4.6. Cu(L)2
For physico-chemical characteristics of this Cu(II) complex with
the ligand HL (Fig. 1) see Ref. [11].
2.5. Biological assays
The following test microorganisms (collection of the Depart-
ment of Microbiology, Belarusian State University) were used:
Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, Sal-
monella typhimurium, Bacillus subtilis, Sarcina lutea, Staphylococcus
saprophyticus, Staphylococcus aureu and Mycobacterium smegmatis.
The antibacterial (bacteriostatic) activity of the compounds was
tested in vitro using the twofold serial dilutions in liquid broth
method for determination of a minimum inhibitory concentration
(MIC) [12,13]. The microorganisms were subcultured for testing in
Mueller–Hinton broth (Merck) that contains 300 g of beef infusion,
17.5 g of acid hydrolysate of casein, 1.5 g of starch and 1 l of dis-
tilled water (pH 7.4 0.2). The composition of this medium pro-
vides favorable conditions for growing bacteria cultures and does
not contain any substances that can destroy the ligands synthe-
sized and their metal complexes during testing in the incubation
medium, as was shown spectrophotometrically in the preliminary
examination. The cells were suspended, according to the McFar-
land protocol, in saline solution to produce a suspension of about
106 CFU/ml (colony-forming units per ml). Because all the com-
pounds tested are insoluble in water, they were dissolved, accord-
ing to standard recommendations [13,14], in a small volume of an
organic solvent (dimethyl sulfoxide or acetonitrile), which is well
miscible with the aqueous nutrient medium on dilution and does
not react with the compounds under test. This solvent was used
for making stock solutions (3.2 mg mlÀ1) of the test compounds.
Stock solutions can be further diluted with broth. Serial dilutions
of stock solutions were carried out in test tubes to certain concen-
trations. The concentrations of the compounds under test, evalu-
3. Results and discussion
3.1. Physico-chemical characterization
Cu(II) complexes with the ligands HLI–HLV were synthesized in
the amorphous state as judged from the reproducible results of dif-
fuse X-ray diffraction patterns. As described in Section 2.3, re-
peated attempts to produce crystalline specimens failed. The
general formula CuL2 has been established on the basis of the ele-
mental analysis data for the Cu(II) complexes (Sections 2.3.1–
2.3.5).
The Cu(II) complexes are insoluble in water, diethyl ether and
chloroform, but soluble in ethanol, acetone, acetonitrile, tetrahy-
drofuran, dimethyl sulfoxide and dimethylformamide. The values
of the molar conductivity in acetonitrile for all these complexes
(
Kmol = 22.1–29.9 X
À1 cm2 molÀ1) suggest that the cycloaminom-
ethyl derivatives of the o-diphenols HLI–HLV are coordinated to
the Cu(II) ion as monoanionic species of the ligands, since the
above-mentioned data indicate their being essentially non-electro-
lytes [19].
Thermal analysis in a nitrogen flow, with identification of the fi-
nal products by X-ray powder diffraction, has shown all the com-
plexes to be anhydrous and unsolvated, because their DTA curves
lack any endothermic peaks over the range from 60 to 110 °C. A
summary of the thermoanalytical results is given in Sections
2.4.1–2.4.5. The complexes behaved similarly on thermal analysis
and showed four stages of decomposition. The agreement between
the experimental and theoretical weight losses for the above pro-
cesses confirms the formulas of the Cu(II) complexes, that is the
TG/DTA data are consistent with the results of the elemental
analyses.
Because of the amorphous state of the complexes synthesized,
we used several spectroscopic methods to specify the coordination
modes in the Cu(II) complexes. The IR parameters of the Cu(II)
complexes are presented in Sections 2.4.1–2.4.5. In the spectra of
HLI–HLV (Sections 2.2.1–2.2.5) there is a single band in the range
3409–3375 cmÀ1, indicating the presence of intermolecular hydro-
gen bonds involving phenolic hydroxyls [20]. The spectrum of HLV
differs from the other spectra in that it demonstrates a broad band
at 3038 cmÀ1, which corresponds to an intermolecular hydrogen
bond involving hydroxyl groups [20,21]. The shift of this band to
the 3365–3290 cmÀ1 region in the spectra of Cu(II) complexes sug-
gests that the phenolic hydroxyl groups participate in metal ion
ated in Mueller–Hinton broth, ranged from 200 to 3.1 l
g mlÀ1. A
specified volume of a 24 h old inoculum was added to each tube.
The organic solvent (dimethyl sulfoxide or acetonitrile) at the final
concentration (1%) in the nutrient medium did not affect the
growth of the test microorganisms. The incubation was carried
out at 37 °C for 24 h, and the optical density (OD 600) was deter-
mined for bacterial cultures in the presence and absence of the
compounds tested. The contents of the test tubes in which the con-
centration of these compounds was sufficient to suppress the
microbial growth remained clear, while their turbidity was evi-
dence for the presence of bacteria. The MIC, defined as the lowest
concentration of the test compound which inhibits the visible
microbial growth, was determined after an incubation period.
The MIC values are given in
ence [14], as well as in
mol mlÀ1 to reveal a correlation between
the antibacterial activity and the reducing ability of the com-
l
g mlÀ1, according to the CLSI refer-
l

N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
43
coordination. Besides, the frequencies of the aromatic ring vibra-
tions (1609–1594 and 1497–1489 cmÀ1) were found to be shifted
to 1584–1553 and 1483–1469 cmÀ1 due to the complexation.
The change in the band intensity of the C–O stretching vibrations
and their shift to the lower frequency region in the spectra of the
Cu(II) complexes are also evidence in favor of the ligands HLI–
HLV being coordinated to the Cu(II) ions via the oxygen atoms of
the phenolic hydroxyl groups. The shift of the bands at 1220–
1020 cmÀ1, assigned to C–N bond vibrations, to the lower fre-
quency region in the spectra of these complexes suggests that
the nitrogen atom is involved in the complexation. It should be
noted that in the spectra of all these complexes there are bands
in the region 540–501 cmÀ1, which may be assigned to the stretch-
ing vibrations of Cu–O and Cu–N bonds [20].
gands, which agrees with the data obtained by other physico-
chemical methods.
The geometry of coordination cores of the Cu(II) complexes
with HLI–HLV was determined on the basis of analysis of the elec-
tronic absorption spectra. The electronic absorption spectra of the
Cu(II) complexes, the main characteristics of which are summa-
rized in Sections 2.3.1–2.3.5, generally include crystal field transi-
tions (d–d) and charge transfer transitions involving orbitals of a
ligand and the metal (LMCT), as well as intraligand absorptions
(ILA) [28]. In the spectra of the Cu(II) complexes there are absorp-
tion maxima in the UV region (225–285 nm) which belong to ILA. A
band in the region 440–570 nm in the spectra of all the Cu(II) com-
plexes under study may be indicative of the square planar shape of
their chromophores [CuN2O2] [28,29]. The maxima in the regions
300–370 and 330–440 nm, appearing in the spectra as a result of
complexation with the Cu(II) ions, suggest the presence of LMCT
The solid state ESR spectra of all the complexes were recorded
both at room temperature and at 77 K, and the ESR parameters
are presented in Sections 2.4.1–2.4.5. A representative spectrum
for one of the Cu(II) complexes in solid state at 77 K is shown in
Fig. 2.
transitions: N(r
) ? CuII and Ophen ? CuII [28,29].
In the light of the physico-chemical characterization, the gen-
eral mode of the ligating atoms in the Cu(II) complexes can be rep-
resented as shown in Fig. 3.
At room temperature the spectra look virtually the same as at
liquid nitrogen temperature (77 K), and it is just the signal inten-
sity that is significantly lower. A hyperfine structure is observed
for all the complexes. The solid state ESR spectra of all these com-
plexes at 77 K are quite similar and exhibit axially symmetric g-
tensor parameters, with g|| > g\ > 2.0023. The g||, g\ and A|| values
were calculated from the spectra using the DPPH free radical as
the ‘g’ marker. The spin Hamiltonian parameters of the complexes
do not differ much (Sections 2.4.1–2.4.5), thus their coordination
cores are similar. Because of the steric effects produced by bulky
groups of the cycloaminomethyl derivatives of the sterically hin-
dered o-diphenols, in their Cu(II) complexes there is a significant
distortion of their square-planar coordination core. According to
[22], g|| values less than 2.3 indicate considerably covalent charac-
ter of M–L bonds, and those greater than 2.3 are indicative of ionic
character. The g|| values of the Cu(II) complexes were found to be
less than 2.3. Applying this criterion, a covalent character of the
M–L bonds in the Cu(II) complexes under study can be predicted.
The geometric parameter G = (g|| À 2)/(g\ À 2), which is a measure
of the exchange interaction between copper centers in solid com-
plexes [23], appears to be greater than 4 for all the compounds:
4.8 (for Cu(LI)2), 4.8 (for Cu(LII)2), 6.2 (for Cu(LIII)2), 4.6 (for Cu(LIV)2)
and 5.2 (for Cu(LV)2). The calculated G values show that the ex-
change interaction between the copper centers is negligible [23].
Thus, all the Cu(II) complexes, with the trend g|| > g\ > ge and a G
3.2. Electrochemical studies
On the basis of our previous data, it is safe to assume that the
redox properties of the phenolic derivatives under study and their
metal complexes can significantly affect their bioactivity. This pro-
vided a reason to carry out an electrochemical investigation of the
redox properties of the compounds HLIÀHLV and their Cu(II) com-
plexes; the experimental data of which are presented in Table 1
and Fig. 4. These ligands belong to o-diphenol derivatives and read-
ily undergo electrochemical oxidation reactions. Differing from
each other only by the substituent in position 3, these compounds
have similar electrochemical properties: the potential values of the
peaks are in a narrow range (Table 1 and Fig. 4).
In voltammograms of the compounds HLI–HLV on an anodic po-
tential scan there are two peaks in the ranges 0.37–0.56 and 0.90–
1.32 V, with cathodic counterparts on the reverse scan correspond-
ing to reduction of oxidation products in the ranges 0.14–0.32 and
0.71–1.11 V.
Controlled electrolysis of solutions of these compounds demon-
strated that at potentials corresponding to the peaks at 0.37–0.56
and 0.90–1.32 V the compounds HLI–HLV undergo successive
one-electron oxidation processes to form o-benzosemiquinones
and o-benzoquinones, as confirmed by their absorption spectra
(kmax = 380–400 nm). Cathodic peaks in the ranges 0.14–0.32 and
0.71–1.11 V correspond to their reduction on the reverse scan. As
we might expect, no reduction of these compounds is observed un-
der cathodic polarization (down to À2.0 V).
value falling within the above-mentioned range, are consistent
2
with a dx 2 ground state [24–27], and the Cu(II) ion being in a dis-
Ày
torted square planar environment formed by N,O-coordinating li-
Depending on the peculiarities of their redox properties, the
Cu(II) complexes of o-diphenol derivatives with cycloaminomethyl
substituents can be arbitrarily divided into two groups: (i) Cu(LI)2
and Cu(LII)2; (ii) Cu(LIII)2–Cu(LV)2. Cu(LI)2 and Cu(LII)2 show a lower
reducing ability as compared with that of respective ligands
(Table 1).
X
OH
I
Cu(L)2
without X
X = CH2
O
N
II
Cu(L )2
III
Cu
Cu(L )2
(CH )
X
=
2
2
IV
O
N
Cu(L )2
X = O
V
OH
Cu(L )2
NCH
X =
3
X
Fig. 2. ESR spectrum of the complex Cu(LV)2 in the solid state at 77 K.
Fig. 3. The plausible coordination mode of the Cu(II) complexes.

44
N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
Table 1
Cyclic voltammetry data (anodic scan) for HLI–HLV and their Cu(II) complexes.
Compound
Processes under anodic polarization
Processes under cathodic polarization
E1pa, V
E1pc, V
E11/2, V*
E2pa, V
E2pc, V
E21/2, V
E1 pc, V
E1 pa, V
E2pc, V
E2pa, V
HLI
0.37
1.09
0.45
1.20
0.43
0.53
0.56
1.38
0.45
1.10
1.33
0.19
0.13
0.24
0.16
0.26
0.20
0.32
0.18
0.14
0.17
1.11
0.28
0.61
0.35
0.59
0.35
0.37
0.44
0.78
0.30
0.64
1.22
1.22
1.50
1.30
1.55
1.32
1.30
1.28
À
0.78
1.12
0.71
1.10
0.78
1.13
1.08
À
1.00
1.31
1.01
1.33
1.05
1.22
1.18
À
À
À
À
À
Cu(LI)2
HLII
À0.71
À
À0.12
À
À
À
À
À
Cu(LII)2
HLIII
À0.71
À
À0.14
À
À1.06
À
À0.07
À
Cu(LIII
HLIV
)
À
À
À1.55
À
À0.13
À
2
À
À
Cu(LIV
HLV
)
À0.82
À
À0.15
À
À1.83
À
À0.20
À
2
0.91
1.64**
À
0.44
0.68
Cu(LV)2
Cu(L)2
À0.75
À
À1.23
À0.11
À
À
À0.27
0.07
À
À
*
The formal potential of the redox system E11/2, used as a criterion of reducing ability (according to [30]), was calculated as the average potential of the peaks found by the
cyclic voltammetry method: E11/2 = (Epa1 + Epc1)/2).
**
Irreversible process.
Upon cathodic polarization one (for Cu(LIII)2) or two (for Cu(LIV)2
and Cu(LV)2) peaks related to copper reduction are observed in vol-
tammograms. As noted above, they may be related to a two-step
Cu(II) reduction. An anodic peak in the potential range À0.10 to
À0.20 V on reverse scan corresponds to oxidation of the reduction
products of Cu(LIII)2, Cu(LIV)2 and Cu(LV)2, which is similar to the
behavior of the first group of complexes.
Thus, the electrochemical investigation substantiated the re-
sults of the spectroscopic one, according to which the coordination
cores of Cu(LI)2, Cu(LII)2, Cu(LIII)2, Cu(LIV)2 and Cu(LV)2 are formed
by Cu(II) ions and the ligands in the monoanionic form. Further-
more, it was found that the complexes rank below the ligands in
reducing ability, with the E11/2 values being close both for the phe-
nolic ligands (from 0.59 to 0.78 V) and for the Cu(II) complexes
(from 0.28 to 0.44 V), but with Cu(LIII)2 showing a reducing ability
comparable to that of the ligands.
3.3. Biological evaluation
3.3.1. Antibacterial activity
Continuing our previous biological evaluation of sterically hin-
dered o-diphenol derivatives and their metal complexes, we have
carried out a microbiological investigation of o-diphenol deriva-
tives with cycloaminomethyl substituents and their Cu(II) com-
plexes to assess how modifying the composition and structure of
the phenolic ligands affects the antibacterial activity of these new-
ly synthesized compounds. MIC values of HLI-HLV and their Cu(II)
complexes are listed in Table 2. Commonly used antibiotics (strep-
tomycin, tetracycline and chloramphenicol) were tested as positive
controls. These compounds were found to have various antibacte-
rial activities (MIC ꢀ12.5–100
l
g mlÀ1) against the Gram-negative
bacteria Pseudomonas aeruginosa, Serratia marcescens, Salmonella
Fig. 4. Cyclic voltammograms (50 mV/s) of HLI–HLV (1.36 mmol lÀ1) in 0.1 mol lÀ1
(C2H5)4NClO4 acetonitrile solution on glassy-carbon electrode and background
cyclic voltammogram of a glassy-carbon electrode (dotted line).
typhimurium and Escherichia coli. The lowest MIC value
(12.5
aeruginosa, is comparable with that of chloramphenicol
(12.5
g mlÀ1 or 0.039 mol mlÀ1) and is significantly lower than
that of streptomycin (100
g mlÀ1 or 0.172 mol mlÀ1). Gram-po-
l l
g mlÀ1 or 0.021 mol mlÀ1) of Cu(LII)2 for Pseudomonas
l
l
On cathodic polarization, beginning from about À0.5 V for
Cu(LI)2 and Cu(LII)2, copper is reduced, its oxidation peak on the re-
verse scan being observed at about À0.12 V. It should be noted that
on cathodic polarization two current waves are characteristic of
Cu(LII)2, which may be related to the realization of a two-step
reduction of Cu(II) to Cu(I) and then to Cu(0). For Cu(LI)2 the second
reduction step possibly lies beyond the accessible potential range.
In contrast to Cu(LI)2 and Cu(LII)2, the complexes Cu(LIII)2,
Cu(LIV)2 and Cu(LV)2 are readily oxidized electrochemically and
demonstrate pronounced reducing properties (Table 1). Neverthe-
less, they are also weaker reductants than their respective ligands.
l
l
sitive bacteria (Staphylococcus saprophiticus, Bacillus subtilis, Sarcina
lutea, Staphylococcus aureus and Mycobacterium smegmatis) are
more sensitive to these compounds than Gram-negative ones (Ta-
ble 2). On the whole, the Cu(II) complexes are characterized by a
comparable inhibiting action against the Gram-positive bacteria
tested (Table 2).The lowest MIC value (12.5
l
g mlÀ1 or
0.020
l
mol mlÀ1), found for Cu(LIII)2 and Cu(LV)2 against Mycobac-
terium smegmatis, Sarcina lutea and Staphylococcus aureus, is com-
parable respectively to those of chloramphenicol and
streptomycin (for Sarcina lutea).

N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
45
Table 2
Antibacterial activity of HLI–HLV and their Cu(II) complexes, evaluated by their minimum inhibitory concentration.*
Compound
Pseudomonas
aeruginosa
Serratia
marcescens
Salmonella
typhimurium
Escherichia
coli
Bacillus
subtilis
Sarcina lutea
Staphylococcus
saprophyticus
Staphylococcus
aureus
Mycobacterium
smegmatis
HLI
100 (0.401)
100 (0.179)
100 (0.380)
12.5 (0.021)
100 (0.363)
100 (0.163)
200 (0.754)
25 (0.042)
100 (0.359)
100 (0.162)
100 (0.337)
>100 (0.172)
–
100(0.401)
100 (0.179)
100 (0.380)
100 (0.170)
100 (0.363)
100 (0.163)
200 (0.754)
100 (0.169)
100 (0.359)
100 (0.162)
100 (0.337)
6.2 (0.011)
–
100 (0.401)
100 (0.179)
100 (0.380)
50 (0.085)
100 (0.401)
100 (0.179)
100 (0.380)
100 (0.170)
100 (0.363)
100 (0.163)
200 (0.754)
25 (0.042)
100 (0.359)
100 (0.162)
100 (0.337)
3.1 (0.005)
3.1 (0.007)
6.2 (0.019)
100 (0.401)
25 (0.045)
100 (0.380)
25 (0.043)
100 (0.363)
25 (0.041)
200 (0.754)
50 (0.084)
100 (0.359)
50 (0.081)
25 (0.169)
6.2 (0.011)
6.2 (0.014)
3.1 (0.009)
50 (0.201)
25 (0.045)
100 (0.380)
25 (0.043)
100 (0.363)
12.5 (0.020)
200 (0.754)
50 (0.084)
100 (0.359)
12.5 (0.020)
12.5 (0.084)
12.5 (0.021)
6.2 (0.014)
–
50 (0.201)
25 (0.045)
100 (0.380)
25 (0.043)
100 (0.363)
25 (0.041)
200 (0.754)
50 (0.084)
100 (0.359)
25 (0.041)
12.5 (0.084)
6.2 (0.011)
6.2 (0.014)
6.2 (0.019)
50 (0.201)
25 (0.045)
50 (0.190)
25 (0.043)
100 (0.363)
12.5 (0.020)
200 (0.754)
50 (0.084)
100 (0.359)
50 (0.082)
25 (0.169)
6.2 (0.011)
3.1 (0.007)
6.2 (0.019)
50 (0.201)
25 (0.045)
100 (0.380)
25 (0.043)
100 (0.363)
12.5 (0.020)
200 (0.754)
50 (0.084)
100 (0.359)
12.5 (0.020)
12.5 (0.084)
3.1 (0.005)
–
Cu(LI)2
HLII
Cu(LII)2
HLIII
100 (0.363)
100 (0.163)
200 (0.754)
100 (0.169)
100 (0.359)
100 (0.162)
100 (0.337)
12.5 (0.021)
6.2 (0.014)
6.2 (0.019)
Cu(LIII
HLIV
)
2
Cu(LIV
HLV
)
2
Cu(LV)2
Cu(L)2
Streptomycin
Tetracycline
Chloramphenicol
12.5 (0.039)
–
12.5 (0.039)
*
MIC values are given in
l
g mlÀ1 (in parentheses À MIC, mol mlÀ1).
l
Table 3
Octanol/water partition coefficients (logPow) of HLI–HLV and their Cu(II) complexes.
Table 4
Rates of reduction of Cyt c (m
) with HLI–HLV and their Cu(II) complexes.
Compound
logPow
Compound
HLIV
logPow
Compound
m* (nmol minÀ1
)
Compound
m )
(nmol minÀ1
HLI
2.00 0.14
2.40 0.11
2.61 0.10
3.10 0.09
3.58 0.08
4.40 0.07
1.65 0.16
2.76 0.10
1.15 0.18
2.15 0.13
1.82 0.15
2.30 0.12
HLI
6.5
1.1
3.3
1.0
3.5
Cu(LIII
HLIV
)
2.3
3.3
0.9
3.5
1.1
2
Cu(LI)2
HLII
Cu(LIV
HLV
)
2
Cu(LI)2
HLII
Cu(LIV
HLV
)
2
Cu(LII)2
HLIII
Cu(LV)2
HL
Cu(L)2
Cu(LII)2
HLIII
Cu(LV)2
Cu(LIII
)
2
The absolute error does not exceed 0.1 nmol minÀ1
.
*
Microbiological tests show that the majority of the Cu(II) com-
plexes are more active than the respective o-diphenol derivatives
with cycloaminomethyl substituents, being another testimony to
complexation being able to enhance the action of bioactive organic
compounds [7]. These effects can be due to the higher lipophilicity
of the complexes (Table 3). Changing the lipophilicity is likely to
result in bringing down the solubility and permeability of cell bar-
riers, which in turn enhances the bioavailability of biocides.
It has been revealed that the starting Cu(II) acetate does not act
3.3.2. Reduction of cytochrome c
The results of the spectrophotometric investigation of the redox
interaction of the oxidized form of Cyt c with HLI–HLV and their
Cu(II) complexes are given in Table 4. As in our previous publica-
tions [1–4], when discussing experimental evidence, we take into
account the findings presented in [17,18,31] as well as the results
of the electrochemical investigation (see Section 3.2), which allow
us to suggest a possible route of oxidation for the compounds un-
der study: different phenolic derivatives transfer electrons in an
outer-sphere process involving the exposed heme edge surrounded
by positively charged amino acid residues; aromatic redox active
amino acid residues also can be potential participants of the elec-
tron transfer. Oxidation of o-diphenol derivatives with cycloami-
nomethyl substituents in vitro under anaerobic conditions can
include two successive one-electron steps of oxidation of their io-
nic forms to yield o-benzoquinones on interaction with Cyt c via
intermediate o-benzosemiquinone formation (Fig. 5).
against bacteria, up to a dose of 200 l
g mlÀ1, that is the antibacte-
rial activity of the complexes synthesized does not correlate with
the toxicity of the Cu(II) ions to the bacteria.
The antibacterial activity of the novel o-diphenol derivatives
with cycloaminomethyl substituents and their Cu(II) complexes
may be characterized as follows: (i) HLI–HLV show virtually the
same low inhibiting action; (ii) the majority of the Cu(II) com-
plexes also almost do not differ in their bioactivity, but the com-
plex Cu(LIII)2, with the highest reducing ability and the highest
lipophilicity among these complexes, is the most active one against
bacteria (Tables 2 and 3). This result could be expected considering
our previous data [1–4] about the correlation between redox prop-
erties of o-diphenol derivatives and their antibacterial activity. The
It was found that the rate of Cyt c reduction with HLII–HLV is
lower as compared to compound HLI (Table 4), which may be
due to its higher reducing ability, determined electrochemically
(Table 1). Comparable rates of Cyt c reduction with HLII–HLV can
be due first of all to similar reducing abilities of these compounds:
MIC (
l
mol mlÀ1) values of the compounds were found to follow
E1 is found to be in the range 0.3–0.4 V (Table 1).
1/2
the order (Table 2): (i) HLI > HLV P HLIII ꢀ HLII > HLIV; (ii)
Cu(LIII)2 > Cu(LII)2 ꢀ Cu(LI)2 ꢀ Cu(LV)2 > Cu(LIV)2; their reducing
ability (determined electrochemically) followed the same order
(Table 1). Thus, introduction of cycloaminomethyl substituents
into the molecule of the sterically hindered o-diphenol HL resulted
All the Cu(II) complexes reduce Cyt c at a rate several times low-
er than that of the redox process involving HLI–HLV (Table 4).
Among the Cu(II) complexes it is the complex Cu(LIII)2 that shows
the highest rate of Cyt c reduction. According to the electrochemi-
cal data, this complex is the most active reducing agent in the ser-
ies of Cu(II) complexes (Table 1). It is known [17] that metal
complexes can interact with Cyt c both in their molecular form
and via the phenolate ligand, formed upon their dissociation. The
rate of Cyt c reduction with the rest of the Cu(II) complexes varies
in a very narrow range (0.9–1.1 nmol minÀ1) and correlates with
their reducing ability determined electrochemically, which also is
characterized by a narrow range of E11/2 values (0.60–0.78 V). Fur-
thermore, the comparable antibacterial activity of these Cu(II)
in the level of antibacterial activity (see MIC, l
mol mlÀ1) becoming
slightly higher as compared to that of the complex Cu(L)2 with 4-
tert-butylcatechol HL (Fig. 1, Table 2). This change in activity
may be related to the fact that the ligand modification had a pro-
nounced effect on some physico-chemical characteristics of the no-
vel Cu(II) complexes: their reducing ability increased (Table 1) and
the lipophilicity of the most active complexes, Cu(LIII)2 and Cu(LII)2,
also became higher (Table 3).

46
N.V. Loginova et al. / Polyhedron 57 (2013) 39–46
Fig. 5. Scheme of the reduction of Cyt c with the anion form of the phenolic ligands; Fe(III)-Cyt c and Fe(II)-Cyt c are respectively the oxidized and reduced forms of Cyt c
(disproportionation of the ligand is depicted by a dashed line) [17].
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complexes should be noted. In summary, the sequences character-
izing the decrease of the rates of Cyt c reduction with the ligands
Kucherov, A.A. Chernyavskaya, V.L. Sorokin, O.I. Shadyro, Eur. J. Med. Chem. 43
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and their Cu(II) complexes are the same as those characterizing
their antibacterial activity and reducing ability:
(1) HLI > HLV ꢀ HLIII P HLII ꢀ HLIV; (2) Cu(LIII)2 > Cu(LII)2 ꢀ
Cu(LI)2 ꢀ Cu(LV)2 P Cu(LIV)2.
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O.I. Shadyro, Polyhedron 30 (2011) 2581.
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4. Conclusions
Novel redox-active Cu(II) complexes with cycloaminomethyl
derivatives of o-diphenols were synthesized and isolated in the
amorphous state. These complexes were characterized by means
of analytical, thermochemical, spectral and electrochemical meth-
ods. According to the data obtained, the ligands coordinate in their
monoanionic forms in an O,N-bidentate fashion. The Cu(II) com-
plexes are characterized by a distorted square planar geometry of
their CuO2N2 coordination cores. The cycloaminomethyl deriva-
tives of o-diphenols and their Cu(II) complexes have been screened
for their antimicrobial activity against different species of patho-
genic bacteria. All the Cu(II) complexes are more lipophilic and
more bioactive than the respective ligands. The complexes were
found to have a low inhibition activity against Gram-negative bac-
teria, while Gram-positive ones are more sensitive to these com-
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g mlÀ1 or 0.020 mol mlÀ1),
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chloramphenicol (12.5
mycin for Sarcina lutea (12.5
l
g mlÀ1 or 0.039
l
mol mlÀ1) and strepto-
l
g mlÀ1 or 0.021
l
mol mlÀ1). The
antibacterial activity of the compounds was found to follow the or-
der: (1) HLI > HLV P HLIII ꢀ HLII > HLIV; (2) Cu(LIII)2 > Cu(LII)2 -
ꢀ Cu(LI)2 ꢀ Cu(LV)2 > Cu(LIV)2; their reducing ability (determined
electrochemically) followed the same order. Moreover, the most
bioactive complex, Cu(LIII)2, has the highest lipophilicity. It is the
ligand HLI and the complex Cu(LIII)2 with the highest reducing abil-
ity (determined electrochemically) that are characterized by the
highest Cyt c reduction rate respectively among the ligands and
Cu(II) complexes. Correspondingly the rate of Cyt c reduction for
the rest of the Cu(II) complexes as well as their antibacterial activ-
ity, lipophilicity and reducing ability (determined electrochemi-
cally) vary in a very narrow range.
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