Synthesis, characterization, theoretical studies and biological activity of coordination compounds with essential metals containing N4-donor ligand 2,9-di(ethylaminomethyl)-1,10-phenanthroline

Article abstract of DOI:10.1016/j.ica.2017.06.040

The work shows the synthesis and characterization of N4-donor L: 2,9-di(ethylaminomethyl)-1,10-phenanthroline and its coordination compounds with some divalent essential metals (Mn, Fe, Co, Ni Cu and Zn). L was obtained as the hydrochloride derivative H₃LCl₃. The complexes were synthesized by two routes: 1) using L as hydrochloride for Mn, Fe, Co and Zn complexes and 2) from L as a free base for Cu and Ni ones. [Co(HL)Cl₂]NO₃ and [Zn(HL)Cl₂]Cl were crystallized and reveal that L is bound to metallic nuclei by only three nitrogen atoms. Cyclic voltammetry studies showed that L alone is capable of being reduced and in complexes, the potential of the coordinated compound process is lower. DFT calculations explain the observable redox behavior regarding HOMO-LUMO orbital energies. Also, the modeling of protonated species of the ligand L suggests that the tetradentate behavior depends on pH conditions of the synthesis. The antiproliferative activity observed on Entamoeba Histolytica and HeLa human tumor cell cultures confirmed that even if complexes are less active than the first choice drugs (metronidazole and cisplatin respectively), it exists a correlation between the observed cytotoxicity and the compounds redox potential.

Full text of DOI:10.1016/j.ica.2017.06.040

Accepted Manuscript
Research paper
Synthesis, Characterization, Theoretical Studies and Biological Activity of Co-
ordination Compounds with Essential Metals Containing N4-donor Ligand 2,9-
di(ethylaminomethyl)-1,10-phenanthroline
Luis Felipe Hernández-Ayala, Marcos Flores-Álamo, Sigfrido Escalante-Tovar,
Rodrigo Galindo-Murillo, Juan Carlos García-Ramos, Jesús García-Valdés,
Virginia Gómez-Vidales, Karen Reséndiz-Acevedo, Yanis Toledano-Magaña,
Lena Ruiz-Azuara
PII:
S0020-1693(17)30315-8
http://dx.doi.org/10.1016/j.ica.2017.06.040
ICA 17689
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 March 2017
12 June 2017
14 June 2017
Please cite this article as: L.F. Hernández-Ayala, M. Flores-Álamo, S. Escalante-Tovar, R. Galindo-Murillo, J.C.
García-Ramos, J. García-Valdés, V. Gómez-Vidales, K. Reséndiz-Acevedo, Y. Toledano-Magaña, L. Ruiz-Azuara,
Synthesis, Characterization, Theoretical Studies and Biological Activity of Coordination Compounds with Essential
Metals Containing N4-donor Ligand 2,9-di(ethylaminomethyl)-1,10-phenanthroline, Inorganica Chimica Acta
(2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.040
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Synthesis, Characterization, Theoretical Studies and Biological Activity of
Coordination Compounds with Essential Metals Containing N4-donor Ligand 2,9-
di(ethylaminomethyl)-1,10-phenanthroline
Luis Felipe Hernández-Ayalaa, Marcos Flores-Álamoa, Sigfrido Escalante-Tovara, Rodrigo
Galindo-Murillob, Juan Carlos García-Ramose,c, Jesús García-Valdésd, Virginia Gómez-
Vidalese, Karen Reséndiz-Acevedoa, Yanis Toledano-Magañaa, c and Lena Ruiz-Azuaraa*.
a
Departamento de Química Inorgánica y Nuclear, Facultad de Química, Universidad
Nacional Autónoma de México. Avenida Universidad 3000, 04510, Mexico City, Mexico.
Fax: (+) (5255) 56223529. E-mail: lenar701@gmail.com
b
Department of Medicinal Chemistry, College of Pharmacy, Skaggs Hall 201, University
of Utah, Salt Lake City, Utah 84112, United States.
c
At present CONACYT - UNAM - Centro de Nanociencias y Nanotecnología - Km 107
Carretera Tijuana-Ensenada, 22860, Ensenada, B.C. Mexico.
d
Departamento de Química Analítica, Facultad de Química, Universidad Nacional
Autónoma de México. Avenida Universidad 3000, 04510, Mexico City, Mexico.
e
Instituto de Química, Universidad Nacional Autónoma de México. Circuito Exterior s/n,
Ciudad Universitaria., 04510, Mexico City, Mexico.
Dedicatory - This paper is dedicated to Carlo Mealli, an outstanding scientist, and very good
friend,
1

Abstract
The work shows the synthesis and characterization of N4-donor L: 2,9-
di(ethylaminomethyl)-1,10-phenanthroline and its coordination compounds with some
divalent essential metals (Mn, Fe, Co, Ni Cu and Zn). L was obtained as the hydrochloride
derivative H3LCl3. The complexes were synthesized by two routes: 1) using L as
hydrochloride for Mn, Fe, Co and Zn complexes and 2) from L as a free base for Cu and Ni
ones. [Co(HL)Cl2]NO3 and [Zn(HL)Cl2]Cl were crystallized and reveal that L is bound to
metallic nuclei by only three nitrogen atoms. Cyclic voltammetry studies showed that L
alone is capable of being reduced and in complexes, the potential of the coordinated
compound process is lower. DFT calculations explain the observable redox behavior
regarding HOMO-LUMO orbital energies. Also, the modeling of protonated species of the
ligand L suggests that the tetradentate behavior depends on pH conditions of the synthesis.
The antiproliferative activity observed on Entamoeba Histolytica and HeLa human tumor
cell cultures confirmed that even if complexes are less active than the first choice drugs
(metronidazole and cisplatin respectively), it exists a correlation between the observed
cytotoxicity and the compounds redox potential.
Keywords: 1,10-phenanthroline, essential, synthesis, characterization.
1. Introduction
Most of the metal ions in the first transition row are essential trace elements for living
beings. They are found in the active sites of many enzymes and are involved in several
metabolic processes [1]. To regulate the amount of these elements within itself, the human
2

body has different mechanisms (homeostatic processes), contrary to the other biological
systems that do not have regulatory mechanisms and may present toxic effects when
exposed to metallic compounds. The above, is one of the main factors that promote the
chemists encourage their efforts to create and study coordination compounds with essential
metals that help the mitigate diseases such as cancer [2-5], diabetes [6-8], parasitic diseases
[9-12], among others.
A consequence of the planar nature of 1,10-phenanthroline is its ability to participate as a
DNA intercalator [13], this same effect is observed in some of its metal complexes [14-16].
At the same time, some of this kind of complexes or natural products incorporating similar
heterocyclic core possess interesting anticancer properties [17-19]. In particular, the
coordination compounds of 2,9-di(ethylaminomethyl)-1,10-phenanthroline have shown
great versatility in applications ranging from the recognition and catalysis of the hydrolysis
in different molecules of biological interest [20-22] to the ability to interact with DNA [23].
These properties were used to design and to synthesize compounds with antitumor activity
[24].
On the other hand, our group [25] revealed that coordination compounds with a nitrogen
based hexadentate ligand, 2,9-bis(3,5-diazahexenyl)-1,10-phenanthroline and metals of the
first transition series exhibit antitumor activity in vitro comparable to that shown by cis-Pt
in human tumor cell lines CHP-212 (neuroblastoma) and HeLa (cervix). These compounds
also presented unusual antiparasitic activity against Entamoeba histolytica cultures, some
of them with a 100-fold potency greater than that observed for metronizadole.
3

The phenanthroline-based ligands and its coordination compounds own the ability to
recognize and interact with biomolecules such as phosphates and DNA in a specific way.
Besides, some of them possess redox and hydrolytic catalytic capacity. These properties
could trigger different cellular mechanisms that produce the growth inhibition of several
cultures such as tumor cells or protozoa.
In the present work, we reported the synthesis and characterization of 2,9-
di(ethylaminomethyl)-1,10-phenanthroline trihydrochloride and their coordination
compounds with essential metals. The structural characterization of all compounds carried
out by different spectroscopic and analytical techniques, also, the X-ray crystal structures of
[Co(HL)Cl2]NO3 and [Zn(HL)Cl2]Cl.H2O are presented. The redox behavior was analyzed
by cyclic voltammetry and the electron density distribution through DFT calculations.
Antiproliferative assays in tumor cells and amoebic cultures to investigate its potential
biological activity are reported.
2. Experimental
2.1. Materials
2,9-dimethyl-1,10-phenanthroline (neocuproine) was purchased from (Sigma-Aldrich,
Mexico) and used as received. 2,9-diformyl-1,10-phenanthroline was prepared according to
the literature method [26-28]. Other reagents and solvents were RA quality. The solvents
employed were dried by standard techniques before use.
4

2.2. Physicochemical measurements
A Fisons Instruments analyzer model EA 1108 was used for elemental analysis
determination, using a sulfanilamide standard. Nicolet Avatar 320 FT-IR spectrometer was
used to record the infrared spectra from the KBr pellet in the region of 400-4000cm-1. Mass
spectra were obtained with a JEOL SX102A mass spectrometer with a matrix of
nitrobenzyl alcohol; the samples were dissolved in CH3OH. A VARIAN Unity Inova
spectrometer was used to record all NMR spectra, 1H (300 MHz) and 13C (75.5 MHz);
TMS was the reference and D2O to dissolve compounds H3LCl3 and [Zn(HL)Cl2]Cl.H2O. A
dual beam spectrophotometer (ThermoFisher Scientific, Genesys 10S) was used to acquire
UV-vis spectra in the range of 200-1200 nm using DMSO as a solvent. The solid diffuse
reflectance spectra were acquired (Agilent, Cary 5000 UV-Vis-NIR spectrophotometer) in
the range of 175-33000cm-1. The EPR spectra were obtained in DMSO solution (Jeol, TES-
300 spectrophotometer) operating at the X-band, at a modulation frequency of 100 kHz and
a cylindrical cavity in the mode TE011 and were measured at T= -195.8°C. Magnetic
susceptibility data were collected with a Magnetic Susceptibility Balance (Sherwood
Scientific, MK-1). The conductivity data were obtained at sample concentrations of ca. 1 ×
10−3 M in DMSO solutions at 25.0 °C (JENWAY 4330 conductivity-pH meter). All
electrochemical measurements were performed in DMSO (HPLC grade) using an EDAQ
EA161 or PAR 273A potentiostats/galvanostats, both controlled by PC software. X-ray
diffraction measurements were acquired on an Oxford Diffraction Gemini-Atlas
diffractometer.
5

2.3. Synthesis of 2,9-di(ethylaminomethyl)-1,10-phenanthroline trihydrochloride
(H3LCl3)
2,9-diformyl-1,10-phenanthroline (2 mmol, 0.54 g) was suspended in 50 ml of dry EtOH,
and 4 mmol of ethylamine (0.120 ml) was added, the formed solution was kept under reflux
and stirred for 4h. Solid NaBH4 (8 mmol, 0.30 g) was added to the reaction mixture in
small portions. The above solution had been stirred for 12 h, and it was concentrated until a
pale yellow solid appeared. The residue was dissolved in 25 ml water and extracted three
times with 15 ml CHCl3. The organic extract was concentrated to yield a yellow oil. The oil
was dissolved in 5 ml EtOH and conc. hydrochloric acid was added (2 mmol, 0.165 ml).
The resulting yellow solid was filtered and washed with a small portion of EtOH. Final
recrystallization of the product was carried out from MeOH and filtered through an
activated charcoal bed resulting in a white powder.
H3LCl3 white powder: Yield (75%); Anal. Calc. H3C18H22N4Cl3•2H2O, (439.81 g/mol):
1
C(49.38), H(6.21), N( 12.79). Found: C(49.42), H(6.45), N(12.70)%. NMR H (for
reference about NMR assignation see Figure 3): ꢀ= 1.29 (t, 3H, CH3-, J= 5.1 Hz), 3.19 (q,
2H, CH3-CH2- J= 5.4 Hz), 4.51 (s, 2H, -CH2-Phen), 7.50 (s, 1H, Phen H-C7,8), 7.58 (d, 1H,
Phen H-C5,12, J= 6.3 Hz), 8.14 (d, 1H, Phen H-C11,6, J= 6.3Hz). NMR 13C): ꢀ= 10.63 (CH3-
), 43.05 (CH3-CH2-), 51.10 (NH-CH2-Phen), 122.95 (Phen C7,8), 126.66 (Phen C5,12),
128.38 (Phen C1,9), 138.52 (Phen C6,11), 143.45 (Phen C4,13), 151.11 (Phen C2,10). MS: m/z
(%) 295 (100)[HL]+, 331 (10) [H2LCl]+; IR, (KBr): ꢁ= 3419, 2946, 2788, 1631, 1610, 1542,
1457, 1436, 1351, 881cm-1. UV-Vis (DMSO): ꢂmax (ꢃ), nm (M-1cm-1); 326 (869), 276
(22559). UV-vis (DR): no bands appear (limit 250nm). ꢄ(DMSO)(ꢅS): 53.8. E1/2 (V vs
Fc+/Fc): -0.793 (L + e- ꢆ L-).
6

2.4. Synthesis of coordination compounds of L
The coordination compounds were synthetized by two different routes:
Method 1) A solution with 1 mmol (0.440 g) of H3LCl3 in 10 ml of water was added to
another with 1 mmol of the corresponding metal salt (MnCl2·4H2O, Co(NO3)2·6H2O,
FeCl2·4H2O, and ZnCl2·4H2O) in the same amount of solvent. The resultant solution was
adjusted to pH= 6 by additions of NaOH 0.1 M. After 2 hours of stirring at room
temperature, solvent was evaporated slowly at vacuum. To precipitate the iron compound 1
mmol (0.340 g) of solid ammonium tetraphenylborate was added.
[Mn(HL)Cl]Cl2 Orange needles: Yield (81%); Anal. Calc. [Mn(HC18H22N4)Cl]Cl2•2H2O,
(492.72 g/mol): C(43.9), H(5.5), N(11.4). Found: C(44.0), H(5.7), N(10.9)%. MS: m/z (%)
384 (100) [Mn(L)Cl]+, 420 (10) [Mn(HL)Cl2]+; IR, (KBr): ꢁ= 3446, 2977, 2939, 2792,
1621, 1591, 1565, 1454, 1430, 1377, 866 cm-1. UV-Vis(DMSO): ꢂmax (ꢃ), nm (M-1cm-1);
493 (197), 443 (226), 440(s), 326 (1790), 279 (22390). UV-Vis (DR), nm: 349, 442, 472,
504 ꢄ (DMSO) (ꢅS): 52.9. ꢅeff (BM): 5.62; EPR (DMSO, -198.5°C) : g= 2.0085 , A= 82.79
x 10-4 cm-1. E1/2 (V vs Fc+/Fc): Epc = -1.341 (MnIIL + e- ꢆ MnIIL-), E1/2 = -1.942 (MnIIL- +
e- ꢆ MnIIL2-).
[Fe(HL)Cl2]B(Ph)4
brown
powder:
Yield:
71%.
Anal.
Calc.
[Fe(HC18H22N4)Cl2]B(C6H5)4•H2O, (759.39 g/mol): C(66.4), H(6.0), N(7.4). Found:
C(66.6), H(6.1), N(7.5)%. MS: m/z (%) 385 (10) [Fe(HL)Cl]+, 136 (100) [(CH3)2-Ph-
(NH4)]+. IR, (KBr): ꢁ= 3475, 3218, 3051, 2983, 2788, 1579, 1504, 1475, 1430, 1374, 860,
737, 706 cm-1. UV-Vis (DMSO): ꢂmax (ꢃ), nm (M-1cm-1); 320 (s), 281 (20855). UV-Vis
7

(DR), nm: 411. ꢄ (DMSO)(ꢅS): 35.4. ꢅeff (BM)= 3.67. E1/2 (V vs Fc+/Fc): -1.055 (FeIIL +
e- ꢆ FeIIL-), E1/2 = -1.272 (FeIIL- + e- ꢆ FeIIL2-), Epa = +0.442 (FeIIL ꢆ FeIIIL + e-).
[Co(HL)Cl2]NO3 Orange needles: Yield (85%). Anal. Calc. [Co(HC18H22N4)Cl2]NO3ꢇH2O
(487.24 g/mol): C(44.0), H(5.1), N(11.4). Found: C(44.4), H(4.8), N(11.8)%. MS: m/z (%)
388 (100) [Co(L)Cl]+, 424 (70) [Co(HL)Cl2]+. IR, (KBr): ꢁ= 3479, 3185, 2967, 2941, 2790,
1619, 1571, 1448, 1429, 1382, 867 cm-1. UV-Vis (DMSO): ꢂmax (ꢃ), nm (M-1cm-1); 676
(70), 612 (77), 548 (55), 276 (22407), 296 (s). UV-vis (DR), nm: 542, 600, 1200 ꢄ
(DMSO) (ꢅS): 31.3. ꢅeff (BM): 2.07. EPR (solid, -198.5 °C): g= 2.29. E1/2 (V vs Fc+/Fc)= -
1.471 (CoIIL + e- ꢆ CoIIL-), E1/2= -1.645 (CoIIL- + e- ꢆ CoIIL2-), E1/2 = +0.226 (CoIIIL + e-
ꢆ CoIIL). Suitable crystals were obtained by recrystallization with MeCN/water 10:1
mixture.
[Zn(HL)Cl2]Cl White needles: Yield (82%); Anal. Calc. [Zn(HC18H22N4)Cl2]Clꢇ5H2O,
(520.78 g/mol): C(41.5), H(6.2), N(10.7). Found: C(41.8), H(6.3), N(10.8)%. NMR 1H (for
reference about NMR assignation see Figure 3): ꢀ= 1.27 (t, 3H, CH3-, J= 5.4 Hz), 3.10 (q,
2H, CH3-CH2- J= 5.4 Hz), 4.50 (s, 2H, -CH2-Phen), 7.58 (s, 1H, Phen H-C7,8), 7.75 (d, 1H,
Phen H-C5,12, J= 6.3 Hz), 8.34 (d, 1H, Phen H-C6,11, J= 6.6 Hz). NMR 13C: ꢀ= 12.44 (CH3-
), 44.48 (CH3-CH2-), 51.05 (NH-CH2-Phen), 124.91 (Phen C7,8), 126.88 (Phen C5,12),
128.38 (Phen C1,9), 139.46. (Phen C4.13), 140.95 (Phen C6,11), 154.93 (Phen C2,10). MS: m/z
(%) 431 (100) [Zn(HL)Cl2]Cl+, 395 (90) [Zn(HL)Cl2]+. IR, (KBr): ꢁ= 3407, 2969, 2935,
2792, 1619, 1592, 1569, 1467, 1423, 1382, 864 cm-1. UV-Vis (DMSO): ꢂmax (ꢃ), nm (M-
1cm-1); 327(s), 300(s), 278 (21719). UV-vis (DR): No bands appear; ꢄ (DMSO) (ꢅS): 37.5.
Epc (V vs Fc+|Fc): -1.514 (ZnIIL + e- ꢆ ZnIIL-), E1/2=-1.882 (ZnIIL- + e- ꢆ ZnIIL2-). Suitable
crystals were obtained by recrystallization with MeCN/water 10:1 mixture.
8

Method 2) 3.3 mmol of solid NaOH (0.130 g) was added to an aqueous solution (10 mL)
containing 1.1 mmol of H3LCl3 (0.485 g), after 0.5 hours of strong stirring, the free base L
was extracted with CH2Cl2 (3x10 mL). The organic extracts were mixed and the solvent
was evaporated. The oily residue was dissolved in 10 ml of MeCN. This solution was
added to another previously prepared containing 1 mmol of the metallic salt
Cu(NO3)2·2.5H2O and Ni(NO3)2·6H2O) in 10 ml of the same solvent. After two hours of
stirring, powders of different shades of green begin to precipitate. The powders were
washed three times with cold MeCN.
[Cu(L)(H2O)](NO3)2
dark
green
needles:
Yield
(50%);
Anal.
Calc.
[Cu(C18H22N4)(H2O)](NO3)2, (499.96 g/mol): C(43.2), H(4.8), N(16.4); Found: C(43.7),
H(4.5), N(16.8)%. MS: m/z (%) 357 (100) [Cu(H-1L)]+, 419 (33) [Cu(L)NO3]+. IR, (KBr):
ꢁ= 3421, 3207, 2973, 2937, 1623, 1592, 1581, 1506, 1467, 1384, 868 cm-1. UV-Vis
(DMSO): ꢂmax (ꢃ), nm (M-1cm-1); 455 (326), 281 (22727). UV-Vis (DR), nm: 733, 1383.
ꢄ (DMSO) (ꢅS): 88.0. ꢅeff (BM): 1.98. EPR (DMSO, -198.5°C): g = 2.064, gꢈ= 2.27, Aꢈ=
150x10-4 cm-1, A = 14.48x10-4cm-1. Epa (V vs Fc+/Fc): -1.310 (CuIIL + e- ꢆ CuIIL-), E1/2= -
0.348 (CuIIL+ e- ꢆ CuIL), Epc = -0.722 (CuIL+ e- ꢆ Cu0 + L).
[Ni(L)(H2O)(NO3)]NO3
pale green
powder:
Yield
(64%).
Anal.
Calc.
[Ni(C18H22N4)(H2O)(NO3)]NO3ꢇH2O, (513.12 g/mol): C(42.1), H(5.1), N(16.4). Found:
C(42.4), H(4.6), N(16.1)%. MS: m/z (%) 414 (20)[Ni(L)NO3]+, 351 (100) [Ni(H-1L)]+. IR,
(KBr): ꢁ= 3299, 3228, 2971, 1623, 1592, 1573, 1504, 1467, 1390, 1384, 867 cm-1. UV-Vis
(DMSO): ꢂmax (ꢃ), nm (M-1cm-1): 350 (368) 305 (s), 279 (23104). UV-vis (DR): 612, 836,
1044 nm. ꢄ (DMSO) (ꢅS): 74.6. ꢅeff (BM): 2.86. Epc (V vs Fc+/Fc): -1.110 (NiIIL + e- ꢆ
NiIIL-), E1/2=- 1.782 (NiIIL- + e- ꢆ NiIIL2-).
9

2.5. Electrochemical studies
Electrochemical measurements were performed at sample concentrations of 2x10-3 M in
DMSO solution containing 0.1
M
of TBAPF6 (Tetra-N-butylammonium
hexafluorophospate). The potentiostats/galvanostats (EDAQ EA161 or PAR 273A) were
used with the typical three-electrode array. A platinum disk (ꢉ = 1 mm) was used as
working electrode. A platinum wire served as a counter-electrode, and a silver wire was
used as a pseudo-reference electrode. Prior each measurement, the solutions were bubbled
with nitrogen. The electrochemical behavior was explored using cyclic voltammetry
initiated from open circuit potential. All measurements were recorded using a scan rate of
100 mV s-1. Potentials were reported versus the couple Fc-Fc+ according to IUPAC [29].
The current interrupt method was used for ohmic (iR) compensation in all the experiments.
2.6. X-ray crystal structure determination
Single crystals of [Co(HL)Cl2]NO3 or [Zn(HL)Cl2]Cl.H2O compounds were mounted on
glass fibre to be studied with the diffractometer, equipped with a CCD area detector, a
sealed tube X-ray source (λMoKα = 0.71073 Å) and a monochromator of graphite. The
method of scanning was the double pass to avoid noise. Collected frames were integrated
using an orientation matrix determined from the narrow frame scans. CrysAlis Pro and
CrysAlis RED software packages [30] were employed to collect and integrate data.
Integrated data analysis did not reveal any decay. Final cell parameters were determined by
a global refinement of reflections. Collected data were rectified for absorbance by
analytical numeric absorption correction [31], using a multifaceted crystal model based on
expressions upon the Laue symmetry using equivalent reflections.
10

Structure solution and refinement were carried with the programs SHELXS-2014 and
SHELXL-2014 respectively [32]; for molecular graphics: ORTEP-3 for Windows [33]; and
the software used to prepare material for publication: WinGX [34]. Full-matrix least-
squares refinement was carried out by minimizing (Fo2 - Fc2)2. All non-hydrogen atoms
were refined anisotropically.
For H atom of the water group (H–O) and amine group (H-N) were located in a difference
map and refined isotropically with Uiso(H) = 1.5 for H–O, and Uiso(H) = 1.2 for H–N. H
atoms attached to C atoms were placed in geometrically idealized positions and refined as
riding on their parent atoms, with C—H = 0.95 and 0.99 Å with Uiso (H) of 1.2Ueq(C) for
methylene and aromatic groups and 1.5Ueq(C) methyl groups. Crystal data and
experimental details of the structure determinations are listed in Table 1.
Crystallographic data have been deposited at the Cambridge Crystallographic Data Center
as supplementary material number CCDC 1535013-1535014. Copies of the data could be
obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK. E-mail: deposit@ccdc.cam.ac.uk.
Table 1. Crystal data and structure refinement for [Co(HL)Cl2]NO3 and
[Zn(HL)Cl2]Cl.H2O
[Co(HL)Cl2]NO3
C18 H23 Cl2 Co N5 O3
487.24
Identification code
Empirical formula
Formula weight
Temperature
[Zn(HL)Cl2]Cl.H2O
C18 H25 Cl3 N4 O Zn
485.14
130(2) K
130(2) K
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Monoclinic
P 21/c
Monoclinic
C 2/c
a = 16.4250(9) Å
b = 8.8552(4) Å
c = 14.9998(7) Å
β= 103.809(5)°.
2118.61(19) Å3
4
1.528 Mg/m3
1.092 mm-1
1004
a = 28.658(2) Å
b = 10.8432(9) Å
c = 13.4876(9) Å
β= 99.694(7)°.
4131.4(5) Å3
8
1.560 Mg/m3
1.593 mm-1
2000
Unit cell dimensions
Volume
-Z
Density (calculated)
Absorption coefficient
F(000)
11

Crystal size
Theta range for data collection
0.483 x 0.330 x 0.161 mm3
3.438 to 29.443°.
0.580 x 0.140 x 0.080 mm3
3.564 to 29.537°.
-20<=h<=21, -11<=k<=12, -
19<=l<=20
-32<=h<=38, -13<=k<=14, -
16<=l<=17
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 25.242°
Refinement method
14609
11851
5070 [R(int) = 0.0489]
99.7 %
Full-matrix least-squares on F2
5070 / 3 / 273
4963 [R(int) = 0.0434]
99.7 %
Full-matrix least-squares on F2
4963 / 5 / 261
Data / restraints / parameters
Goodness-of-fit on F2
1.064
1.060
Final R indices [I>2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0465, wR2 = 0.0850
R1 = 0.0859, wR2 = 0.1051
0.407 and -0.445 e.Å-3
R1 = 0.0444, wR2 = 0.0702
R1 = 0.0797, wR2 = 0.0851
0.426 and -0.501 e.Å-3
2.7 Computational Details
Electronic properties and calculations of all the complexes were calculated using Gaussian
09 (ver. D01) [35] using the m06-2x [36, 37] level of theory with a 6-31G(d,p) basis set.
The ligand and [Zn(L)Cl2].H2O calculations were performed using the Kohn-Sham
formalism as implemented in deMon2k [38], version 4.3.4. The basis set was DZVP [39]
with the PBE [40] exchange-correlation functional. Harmonic frequency calculations were
performed to obtain the corresponding thermochemical corrections and to confirm that the
structures obtained correspond to a minimum energy structure. A full geometric
optimization, starting from the available crystallographic structures, was performed for
each transition metal complex presented on this work. The DFT functional employed has
been proven in previous work to successfully represent the type of compounds studied here
[41-43]. For each optimization, frequency analysis showed no negative results, ensuring
full structural convergence. All calculations were performed in the gas phase.
12

2.8. Antiproliferative activity assays
2.8.1 Parasite culture and amoebicidal activity
Entamoeba histolytica HM1: IMSS trophozoites were axenically grown in TYI-S33
medium. Trophozoites (1 × 105) were placed in tubes with 3 ml of TYI-S33 medium
supplemented with Adult Bovine Serum (10%), Diamond vitamins (5%) and antibiotic
(1%) incubating at 37°C and 5% CO2. Each compound was added to reach the final
concentrations as follows: 1000, 100, 10, 1, 0.1 and 0.01 ꢅM. Amoebic trophozoites
viability was assessed employing optical microscope with vital marker trypan blue and
confirmed with fluorescent staining with carboxyfluorescein diacetate (CFDA) and
propidium iodide (PI) as markers. Trophozoite viability determinations were done at 24, 48
and 72 hours. In brief, 100 ꢅl samples containing around 1×104 treated parasites were
added with trypan blue (5 ꢅL, 0.4%) or CFDA (1 ꢅL) or PI (1ꢅL). Cell viability was
determined with an hemocytometer in an optical or fluorescent microscope.
2.8.2 Human tumor cell culture and antiproliferative activity
2 × 104 HeLa cells (ATCC) per well were plated in a 96-well microplate (Costar®) with
Dulbecco’s modified Eagle medium (D-MEM) supplemented with 10% fetal bovine serum
(FBS) and allowed to attach incubating at 37 °C and CO2 (5%) for 24 h. At the end of
incubation time, the medium was aspirated, and cells were exposed to 10 ꢅL of test solution
at different concentrations (1, 3, 10, 30, 100 and 300 ꢅM) for 24 h under the conditions
mentioned above. Cell growth was determined according to the sulforhodamine B assay
[44, 45]. Absorbance was measured at 564 nm in a Microplate reader BIORAD 550 and
percentage of cell growth at each concentration of drug was calculated as growth% = 100 ×
13

[T/C], where T is the absorbance of treated wells and C is the absorbance of untreated
wells. This equation was used to determine the concentration needed to inhibit the culture
growth in a 50%. Reported IC50 values are the average of at least three independent
experiments. Details of measuring cell growth inhibition are described elsewhere [44].
3. Results and Discussion
3.1 Synthesis and structural characterization of L and its coordination compounds
with essential metals ions
Figure 1. Synthesis of ligand hydrochloride, H3LCl3. (2 columns)
The ligand L was synthesized through the route outlined in Figure 1. Since it is an oily
product, we purify it by precipitation adding conc. HCl to get the hydrochloride salt. The
14

elemental analysis showed this salt was L in its trihydrochloride form. In DMSO solution,
+
H3LCl3 is partially dissociated, and it behaves like a 1:1 electrolyte (H3LCl2 Cl-) according
to the measured conductivity value (53.8 ꢅS).
Initially, we tried to prepare [M(L)]2+, in aqueous media, by the reaction of MX2 (X= Cl-,
-
NO3 ) with an equimolar amount of H3LCl3 and adjusting the solution pH by adding NaOH
as described by H. Sun and coworkers [46]. Although this method was satisfactory for
manganese, cobalt and zinc complexes, products precipitated as needles, resulted
unsatisfactory for the iron, copper and nickel ones since products obtained were oils. To
overcome this obstacle, we change the counter-ion in the iron complex while the copper
and nickel compounds were synthesized using MeCN as a solvent and L in its free base
form.
Structures of Co(II)L and Zn(II)L complexes were unambiguously established by single
crystal X-ray diffraction analysis. Structures of Mn(II), Fe(II), Ni(II) and Cu(II) were
proposed through elemental analysis, IR, EPR, UV-vis, mass spectrometry, conductivity
and magnetic susceptibility measurements. The IR spectroscopy showed that the bands,
associated with the vibration of the aromatic C = N (1506 and 1610cm-1) and aliphatic C-N
(3200cm-1) bonds of L, after coordinating the metal ion shifted at high frequencies. Also, a
robust band of L close to 2788 cm-1, assigned to aliphatic protonated amine ꢁ(N+-H), was
slightly shifted to higher frequencies due to coordination and only appears in Mn(II), Fe(II),
Co(II) and Zn(II) complexes. That fact infers L is protonated in these compounds and
agrees with the results obtained from X-ray diffraction analysis (see below).
15

1
The signal assignation of NMR H spectra for Zn(II) complex acquired in D2O, was done
by direct comparison with those signals identified in the spectra of the protonated ligand
H3LCl3 (Figure S1, S2 and Table S1). The protons of the 1,10-phenanthroline moiety of the
ligand and their corresponding signals in the coordination compound were observed
between 7.50 and 8.40 ppm. A small shift (0.1 to 0.2 ppm) to lower field in these signals
was observed due to the coordination to the metal ion. Also, a slight displacement to high
field, compared with those found for L, is observed for the signals associated with the
hydrogen atoms attached to the aliphatic nitrogen atoms of Zn(II) complex. This fact
suggests that in D2O solution, the ligand can bind strongly to Zn via aromatic nitrogen
atoms, but the bond with the nitrogen of the secondary amines is weak. That information
indicates a transition between the four-coordinated distorted tetrahedral to distorted
octahedral geometry passing by a penta-coordination due to the weak aliphatic nitrogen
coordination described above.
[Mn(HL)Cl]Cl2 EPR spectra, in
DMSO frozen solution at -198°C, exhibit a
pseudoisotropical signal centered at g= 2.0085, with six hyperfine lines as expected for
55Mn(II) complex, due the central transition (+1/2 → -1/2) with a hyperfine coupling
constant A= 82.79 x 10-4 cm-1. The EPR data and the 5.62 BM of µeff are in agreement with
an S = 5/2 Mn(II) center [47]. Three bands observed in both electronic spectra, solid state,
in DMSO solution and electrolytic conductivity of 52.9 µS suggest a tetrahedral geometry
with three nitrogen atoms from L and a chlorido linked directly to the Mn(II) center to get
[Mn(HL)Cl]Cl2. In the case of [Fe(HL)Cl2]B(Ph)4 complex, the µeff = 3.67 BM and ꢄ =35.4
µS (DMSO) indicate a penta-coordination with a trigonal bipyramid arrangement around
the metal. Cu(II), and Ni(II) compounds give the same type of information, so the same
16

conformation is proposed. The ꢁ(N+-H) signal, in these complexes, was not observed
indicating that L is totally deprotonated. The shape and values of EPR signals for
[Cu(L)(H2O)](NO3)2 (gꢊ= 2.069, gꢈ = 2.27, Aꢈ = 150x10-4 cm-1, Aꢊ= 14.48 cm-1, S = 1/2)
let us to consider an axial spectrum that corresponds to a square-base pyramidal geometry
in solid state and, with a coordination of a solvent molecule, octahedral in frozen DMSO
solution, respectively. The [Ni(L)(H2O)(NO3)]NO3 IR spectrum showed a double band in
the region of 1380-1390 cm-1 which is indicative of two types of nitrate ion [48], the first
one linked directly to the metal and the second one as a counter ion. The above fact, the
three bands in UV-vis-NIR spectrum (acquired in the solid state) and the µeff measurement,
are indicative of an octahedral geometry in the solid state for the Ni(II) complex. In DMSO
solution, the information was insufficient to conclude about the geometry.
After acetonitrile-water recrystallization, analysis of single crystals by XRD revealed that
[Co(HL)Cl2]NO3 and [Zn(HL)Cl2]Cl crystallize in the monoclinic space groups P21/c and
C2/c respectively. Table 1 summarizes crystals data collection and the refinement
parameters. Selected bond lengths and angles are listed in Table 2. An ORTEP diagram of
cationic part of the complexes is shown in Figure 2. In the cobalt complex the anion is
nitrate, and for the zinc one is chloride. In both cases, the metal ion shows a five-
coordinated bonding by three nitrogen atoms of the ligand L and two chloride ions. They
show rather unsaturated coordination spheres. In penta-coordinated compounds, the angular
distortion value ꢋ [49], is a key parameter to describe the geometry. Since the Co(II) and
the Z(II) complexes have ꢋ values of 0.61 and 0.52, respectively, their geometry is an
intermediate between square pyramidal and trigonal bipyramid. Cobalt shows short (2.01
and 2.07 ꢌ) and zinc long (2.36 - 2.39 ꢌ) M-N bond lengths with the aromatic nitrogen.
17

The bond length of metal-aliphatic nitrogen is quite similar in both cases (Co= 2.167 ꢌ and
Zn= 2.172 ꢌ) both bond lengths indicate that the metal is not centered on L. On the other
hand, the uncoordinated nitrogen atom (N3) of the aliphatic residue from L is protonated.
One of the hydrogen atoms linked to N3 participates in an intramolecular hydrogen bond
with a coordinated chloride, N-HꢍꢍꢍCl= 2.23(1) Å. In the crystal array, there are
intermolecular hydrogen bond interactions of the type N-HꢍꢍꢍO and N-HꢍꢍꢍCl. Especially for
the cationic complex [Co(HL)Cl2]+, the N3-H3E···O1 and N3-H3EꢍꢍꢍO2 produce an R1 (4)
2
motif. Similar interactions were observed in the Zn(II) compound, where the hydrogen
bond interactions O1w-H1DꢍꢍꢍCl1 and O1W-H1EꢍꢍꢍCl3 show the D2 (5) motif.
2
Figure 2. ORTEP diagram of [Zn(HL)Cl2]Cl and [Co(HL)Cl2]NO3 at 50% probability. The
anions and water molecules was omitted for clarity (2 column)
Finally, notice that the ligand rigidity, imposed by the large heteroaromatic moiety, does
not allow the simultaneous coordination of both aliphatic nitrogen atoms and the
heteroaromatic ones to the same metal ion. At least in this geometry, in axial configurations
(octahedral, square or pyramidal), the possibility of binding L completely is increased.
18

Table 2. Selected bond lengths (ꢌ) and angles (°) for [Co(HL)Cl2]NO3 and [Zn(HL)Cl2]Cl
[Co(HL)Cl2]+ [Zn(HL)Cl2]+
Bond Lengths (Å )
Co-N1
Co-N2
Co-N4
2.362(2)
2.012(2)
2.172(2)
Co-Cl1
Co-Cl2
2.2512(9)
2.3219(9)
Zn-N1
Zn-N2
Zn-N4
2.075(2)
2.395(2)
2.167(3)
Zn-Cl1
Zn-Cl2
2.2606(7)
2.2585(8)
Bond Angles (°)
N2-Co-Cl2 116.92(7) N4-Co-Cl1 104.17(8) N2-Zn-Cl2
N2-Co-Cl1 125.42(7) N4-Co-N2 79.31(10) N2-Zn-Cl1
90.86(6)
94.78(5)
114.65(3)
77.83(10)
100.63(7)
N4-Zn-Cl1
N4-Zn-N2
N1-Zn-Cl2
N1-Zn-Cl1
N1-Zn-N2
105.01(7)
150.09(9)
125.03(6)
118.73(6)
73.00(9)
Cl1-Co-Cl2 116.58(4) N1-Co-Cl2
N1-Co-N4 153.82(9) N1-Co-Cl1
91.21(6) Cl1-Zn-Cl2
94.14(6)
74.81(9)
N1-Zn-N4
N4-Zn-Cl2
N4-Co-Cl2
97.17(7)
N1-Co-N2
3.3. Electrochemistry
Redox potentials of the ligands and complexes were obtained by cyclic voltammetry, and
they are summarized in Table 3.
Figure 3 (left) shows the voltammogram obtained for H3LCl3 from open circuit potential
Eoc to a positive direction. One oxidation signal Ia (Epa= 1.124 V), without its corresponding
reduction signal was assigned, by the anion exchange test, to an irreversible chloride
-
oxidation (2Cl-(aq) = Cl2(g) + 2e-). The chloride ion was exchanged for nitrate (NO3 ) with the
stoichiometric amount of AgCl, and new voltammogram (inset Figure. 3 left) was recorded,
the result was the disappearance of the signal Ia. When the scan reversed from the inversion
potential Eꢂ to the negative direction, the signal II was detected. The reduction signal IIc,
with its corresponding oxidation signal IIa, was associated with ligand reduction. It has
been reported that the redox processes of aromatic diimines and those carried out on the
aromatic moiety in their complexes are given at negative potential values [50-52]. The
signal II is a quasi-reversible process (E1/2= 0.793, ꢎEp= 254 mV and ipa/ipc= 0.76) and
according to the signal, shape looks like a coupled electrochemical-chemical process [53].
19

The shape of the same signal in the anion exchange test suggests that the process becomes
reversible (ipa/ipc= 0.97).
Figure 3 (right) shows the voltammogram, from the open circuit potential to negative
potentials, for [Fe(HL)Cl2]Bꢏ4; it was possible to observe four signals. Two subsequent
reduction processes of the ligand with their corresponding oxidations (Ia,c and IIa,c) ; this
behavior has been reported previously in complex of Co, Ru and Ni with 1,10-
phenanthroline and other aromatic diimines [52]. An oxidation process with its
corresponding reduction (IVa,c) was detected, and it was assigned, by association with
similar Fe(II) complexes [54], to a quasi-reversible iron oxidation. Moreover, an adsorption
process (III) was also detected, but the study of these signals is beyond our interest.
Analogous redox behavior was found for L in the other coordination compounds, but no
oxidation process for the metal ions was observed. The Co(II) shows a reversible redox
process Co(III)|Co(II) at 0.226 V vs. Fc+|Fc, but no oxidation process for the metal ions
Mn(II), Ni(II) and Zn(II) was observed. That could be due to their high redox potential
values compared with the solvent oxidation under the experimental conditions.
Particularly for [Cu(L)(H2O)](NO3), the ligand reduces in one step (Epc= -1.310 V). Two
additional electrochemical processes were associated with the transformations of
Cu(II)/Cu(I) (E1/2= -0.348 V) and Cu(I)/Cu(0) (E1/2= -0.550 V). Signals were assigned
according to reports of similar Cu(II) complex [55,56].
In general, the reduction of L requires more energy in the coordinated state, however, its ꢐ-
acceptor character and its capacity to participate in back-bonding interactions to redistribute
20

the electron density is evident due to the two subsequent reduction processes on the
aromatic moiety of the ligand.
Figure 3. Cyclic voltammograms of 20 mM H3LCl3 (left) and 20 mM of
[Fe(HL)Cl2)]B(Ph)4 in DMSO containing 0.1 M TBAH. The working electrode was Pt.
Scan rate 0.1 V/s.
Table 3. Redox potentials for H3LCl3 and complexes
Compound
Ligand
MIIL ꢆ
MIIIL + e-
n. o.
MIIL + e- ꢆ
MIL + e- ꢆ
M0 + L
n. o.
MIL
H3LCl3
- 0.793
n. o.
[Mn(HL)Cl]Cl2
[Fe(HL)Cl2]B(Ph)4
[Co(HL)Cl2]Cl
- 1.341*, -1.942
- 1.055, -1.272
- 1.471, 1.645
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
n. o.
0.442*
0.226
n. o.
[Ni(L)(NO3)(H2O)]NO3 - 1.110*, -1.782
[Cu(L)(H2O)](NO3)2
[Zn(HL)Cl2]Cl2
- 1.310*
- 1.514, -1.882*
n. o.
n. o.
-0.348
n. o.
- 0.550
n. o.
*quasi-reversible/irreversible process, n. o., not observable. Redox potentials are reported
in (V) vs. Fc+|Fc in 0.1M de TBAH-DMSO.
3.5 Theoretical Studies
Computational studies were performed to explore the electron density distribution of the
ligand, and also in the coordination complexes, The protonation equilibria studies of L
21

suggest that all species, L, HL+, H2L2+, and H3L3+, have similar energy. At pH= 6,
employed in the reaction media, the predominant species is H2L2+, stabilizing this
protonated form the hydrogen bonds formed between the nitrogen atoms of the
phenanthroline. This experimental result agrees with those reported by Guo and co-workers
[22], who informed about the penta-coordination for the Zn(II) derivative. The detailed
information regarding the energies and geometries of the most stable species could be
consulted in the supplementary information.
Calculations for the coordination compounds were initialized from the crystallographic
structures. Two protonated forms of L (HL+ and H2L2+) coordinated to the metal ion were
studied. The angular distortion calculated for Co(II) and Zn(II) derivatives is 0.99 and 0.64
respectively. The disagreement with the experimental results could be explained by the fact
that the calculations were made in the gas phase, so, the intra and intermolecular
interactions were not considered.
The main differences were found at the LUMO orbital energies as the key element to
distinguish between two possible states of the ligand (L or HL interacting with the metal
ion) and observed in all compounds due to the redox processes. Table 4 shows that HL
derivatives present a subtle higher stabilization. Exploration of LUMO orbitals of both
systems indicates a similar distribution of the orbitals in the atoms of L. However; the
energy behavior is quite different as can be seen in Figure 4.
22

Table 4. Stabilization energies obtained from the calculated coordination compounds
involving various protonated forms of L.
[M(L)Cl2]+
[M(HL)Cl2]+
Energy
kcal/mol
Energy
kcal/mol
Derivative HOMO LUMO
ꢎ
HOMO LUMO
ꢎ
Mn(II)
Fe(II)
Co(II)
Ni(II)
Cu(II)
Zn(II)
-0.268 -0.0557 -0.212 -1875.6 -0.3810 -0.169 -0.212 -1875.9
-0.2649 -0.0519 -0.213 -1946.3 -0.3919 -0.168 -0.224 -1946.6
-0.2717 -0.0541 -0.218 -2021.0 -0.3962 -0.172 -0.224 -2021.3
-0.2640 -0.0604 -0.204 -2099.8 -0.3889 -0.176 -0.213 -2100.0
-0.2721 -0.0559 -0.216 -2182.7 -0.4099 -0.170 -0.240 -2183.0
-0.2729 -0.0557 -0.217 -2269.9 -0.4078 -0.162 -0.246 -2270.1
Figure 4. Grid representations of HOMO and LUMO orbitals for neutral (left) and +1
(right) compounds using an isodensity value of 0.2 a. u.
23

As expected, by electrochemical results, the ꢎHOMO-LUMO value, as well as the energy of the
LUMO orbitals of coordination compounds, is highest as the redox potential becomes more
negative as observed in Figure 5 for Mn, Ni, Cu, and Zn. The Fe(II) and Co(II) are outliers,
and this could be explained by its capacity to redistribute the electron density. On the other
hand, HOMO orbitals of coordination compounds principally involve the metal ion and
some atoms of the ligand. The Zn(II) derivative is the main example of electron
delocalization in both frontier orbitals, where the participation of the phenanthroline moiety
atoms is essential. This result helps to explain why so much energy is needed to
electroreduce the ligand.
-1.0
Fe
-1.1
Ni
-1.2
Cu
-1.3
Mn
-1.4
Co
-1.5
Zn
-1.6
-0.178 -0.176 -0.174 -0.172 -0.170 -0.168 -0.166 -0.164 -0.162
LUMO energy (a.u.)
Figure 5. Redox potential modification as a function of the energy change in the LUMO of
coordination compounds. (1 column)
24

3.6 Antiproliferative Activity
The in vitro antiproliferative activity of synthesized compounds was evaluated in HeLa
(ATCC) cells after 24 h of drug exposure using the sulforhodamine B (SRB) assay [44, 45].
In vitro amoebicidal activity was evaluated on cell cultures of Entamoeba histolytica
trophozoites. The percentage of viable trophozoites was determined with a
hematocytometer at three different times: 24, 48 and 72 h using the trypan blue vital
exclusion technique. H2O:DMSO (95:5 v/v) mixture was used to dissolve all compounds.
The concentration of DMSO was not toxic neither for tumor cells nor trophozoites. Table 5
show the IC50 values. Positive controls: cisplatin and metronidazole (antitumor and
amoebicidal drugs respectively) [25] and negative controls with PBS 1X were used.
Table 5. Antiproliferativea and amoebicideb activity
(IC50, ꢅM) in human tumor cell lines HeLa and cell
cultures of Entamoeba histolytica trophozoites of
H3LCl3 and its coordination compounds.
IC50(ꢅM)a
HeLa
N. A.
N. A.
N. A.
35.49
N. A.
N. A.
N. A.
---
IC50(ꢅM)b
E. histolityca
4300
Compound
H3LCl3
[Mn(HL)Cl]Cl2
[Co(HL)Cl2]NO3
[Fe(HL)Cl2]Bꢏ4
[Ni(L)(H2O)(NO3)]NO3
[Cu(L)(H2O)](NO3)2
[Zn(HL)Cl2]Cl
Metronidazole
2600
162
1300
1700
3200
2400
6.8
cis-Pt
5.5
---
N.A.: IC50>300ꢅM
In HeLa cells, the maximum concentration tested was 300 ꢅM, under these conditions only
[Fe(HL)Cl2]B(Ph)4 exhibited a moderate activity with and IC50= 35.49 ꢅM, that is about
five times greater than that of cisplatin (5.5 ꢅM). The activity of this compound, with the
25

lowest redox potential in this work, could be linked to the production of reactive oxygen
species (ROS; such as hydroxyl, superoxide and peroxide radicals) associated with cellular
damage. On the other hand, although with moderate activity, [Fe(HL)Cl2]B(Ph)4 contains
an essential metal which would make it less toxic to healthy cells and may contribute in the
transport of the molecule throughout the cellular environment.
It is important to notice that compound of Fe(II) behaves as an anti-proliferative effector.
Meanwhile, coordination compounds (same transition metal ions) with the ligand L’: 2,9-
bis-(2ꢑ,5ꢑ-diazahexanyl)-1,10-phenanthroline have been active against the same human
tumor cells, being all of them even more potent to inhibit cellular growth than cisplatin,
except the Zn derivative [25]. This fact could be associated with the lower stability
provided to the compounds by the coordination of L compared with that gained with the
coordination of L’. The comparison of the potential redox values (-1.055 and -1.779 V vs.
Fc+|Fc) and the antiproliferative activities of Fe(II) coordination compounds at 35.49 µM
and 2.4 µM for L and L’, respectively, became evident the influence of the stability of the
compounds on their redox behavior and the biological activity.
On the other hand, H3LCl3 and the coordination complexes exhibited slight activity against
Entamoeba histolytica trophozoites cultures. The Co(II) complex (IC50= 162 ꢅM) was the
most active, however, is about twenty times lower than the metronidazole (6.8 µM). An
interesting fact is that the little amoebicidal activity of trihydrochloride ligand increases
with the presence of the metal ions, in the case of the Co(II) complex almost thirty times.
Su [20], Guo [21, 22] and coworkers concluded that polyamine compounds increased the
catalytic efficiency when they bound to metals. Similarly, our group demonstrated that
antiproliferative activity against trophozoites is due to the coordination compounds instead
26

than the ligand itself [57,58]. In a general way, the antiproliferative activity of the
compounds increases as the energy employed for the reduction decreases.
It is notable that the compounds with the highest antiproliferative activity, Fe(II) complex
in HeLa cells and Co(II) complex in E. histolytica, are the only ones where the metal
oxidizes with moderate energy, and those where only L is reduced does not have or present
marginal activity. With these results, we can rule out that the ligand reduction is the only
process to produce cytotoxicity and that the redox process of the metal ion also contributes
in this task [25]. Also, these results suggest that the mechanisms by which these
compounds produce cellular damage may be related to an imbalance in the redox system of
the cell thus, causing its death.
Conclusions
The N4-donor ligand L and its coordination compounds with first-row transition divalent
metals Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) were synthesized and fully
characterized. Analysis showed that the coordination compounds have different structures,
but these almost are the same in the solid state and DMSO solution. In addition, XRD
analysis indicated that complexes have a similar pentacoordinate structure for the Co (II)
and Zn (II); both are compounds with an intermediate conformation between the ideal
geometries, and L is a tridentate chelate. Like the geometry, the redox behavior is variable.
The ligand hydrochloride can be reduced; the potential of this process becomes more
negative when it is bound to a metal exposing its character of acceptor ꢐ, and the theoretical
calculations explain this fact regarding LUMO orbital energies. Calculations also
27

demonstrate that the synthetic conditions determinate the complex configuration. Ligand
activity against E. histolytica is enhanced with the presence of the metal in some cases
thirty times. In general, the synthesized compounds had a moderate antitumor and
amoebicidal activity; the highest are those that present a process of oxidation centered in
the metal, this suggests a mechanism of cell damage by redox imbalance. Although the
efficacy is lower than that of traditional drugs, it is necessary to continue studying their
properties since containing essential metals may be less toxic and more selective.
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