Co(II) complexes derived from (1-methyl-1H-imidazol-2-yl)methanol: Synthesis, characterization, spectroscopic study, DFT/TD-DFT calculations and biological evaluation

Article abstract of DOI:10.1016/j.ica.2019.119073

We report in this paper, a combined experimental and theoretical study of two new Co(II) complexes [Co(OAc)₂(L)₂ (1) and CoCl₂(L)₂ (2)] based on (1-methyl-1H-imidazol-2-yl)methanol (L). The synthetized complexes were characterized by single-crystal X-ray diffraction, FT-IR and UV–Vis. spectroscopy. In complex 1, two molecules of L are coordinated to the Co(II) ion in bidentate mode through the nitrogen and oxygen atoms, displaying a hexa-coordinated compound in a distorted octahedral geometry, while in complex 2, L is ligated to the metal ion in monodentate fashion through the nitrogen atom exhibiting a distorted tetrahedron. The molecular geometries, electronic transitions and vibrational frequencies of the two complexes and L in the ground state were calculated using the global hybrid (B3LYP) and the range separated hybrid (CAM-B3LYP) density functional. The theoretical calculations are in good agreement with the experimental values. The ligand and its Co(II) complexes have been screened and evaluated for their potential as DPPH radical scavengers as well as antimicrobial agents against five pathogenic microorganisms such as Gram-positive and Gram-negative bacteria, a yeast and a fungus. The SAR study revealed that complex 1 exhibited the best antimicrobial effects and higher antioxidant activity than complex 2 and L.

Full text of DOI:10.1016/j.ica.2019.119073

Journal Pre-Proof
Co(II) complexes derived from (1-methyl-1H-imidazol-2-yl)methanol: Synthe-
sis, characterization, spectroscopic study, DFT/TD-DFT calculations and bio-
logical evaluation
Anfel Benhassine, Houssem Boulebd, Barkahem Anak, Mounira Kara Ali,
Abdelmalek Bouraiou, Hocine Merazig, Noureddine Kacem-Chaouche, Ali
Belfaitah
PII:
S0020-1693(19)30798-4
https://doi.org/10.1016/j.ica.2019.119073
ICA 119073
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 June 2019
8 August 2019
11 August 2019
Please cite this article as: A. Benhassine, H. Boulebd, B. Anak, M.K. Ali, A. Bouraiou, H. Merazig, N. Kacem-
Chaouche, A. Belfaitah, Co(II) complexes derived from (1-methyl-1H-imidazol-2-yl)methanol: Synthesis,
characterization, spectroscopic study, DFT/TD-DFT calculations and biological evaluation, Inorganica Chimica
Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119073
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Co(II) complexes derived from (1-methyl-1H-imidazol-2-yl)methanol:
Synthesis, characterization, spectroscopic study, DFT/TD-DFT calculations
and biological evaluation.
Anfel Benhassine1, Houssem Boulebd1, Barkahem Anak2, Mounira Kara Ali3, Abdelmalek Bouraiou4,
Hocine Merazig4, Noureddine Kacem-Chaouche3, and Ali Belfaitah1*
1 Laboratoire des Produits Naturels d’Origine Végétale et de Synthèse Organique. Faculté des Sciences Exactes,
Campus de Chaabat Ersas, Université des frères Mentouri-Constantine. Constantine 25000, Algeria.
2 Laboratoire chimie des matériaux de Constantine. Faculté des Sciences Exactes, Campus de Chaabat Ersas,
Université des frères Mentouri-Constantine. Constantine 25000, Algeria.
3
Laboratoire de Mycologie, de Biotechnologie et de l’Activité Microbienne, Faculté des Sciences de la vie,
Université des Frères Mentouri, Constantine 25000, Algeria.
4 Unité de Recherche de Chimie de l’Environnement et Moléculaire Structurale, Faculté des Sciences Exactes,
Campus de Chaabat Ersas, Université des Frères Mentouri, Constantine 25000, Algeria.
Abstract: We report in this paper, a combined experimental and theoretical study of two new Co(II)
complexes [Co(OAc)2(L)2 (1) and CoCl2(L)2 (2)] based on (1-methyl-1H-imidazol-2-yl) methanol (L).
The synthetized complexes were characterized by single-crystal X-ray diffraction, FT-IR and UV-Vis.
spectroscopy. In complex 1, two molecules of L are coordinated to the Co(II) ion in bidentate mode
through the nitrogen and oxygen atoms, displaying a hexa-coordinated compound in a distorted
octahedral geometry, while in complex 2, L is ligated to the metal ion in monodentate fashion through
the nitrogen atom exhibiting a distorted tetrahedron. The molecular geometries, electronic transitions
and vibrational frequencies of the two complexes and L in the ground state were calculated using the
global hybrid (B3LYP) and the range separated hybrid (CAM-B3LYP) density functional. The
theoretical calculations are in good agreement with the experimental values. The ligand and its Co(II)
complexes have been screened and evaluated for their potential as DPPH radical scavengers as well as
antimicrobial agents against five pathogenic microorganisms such as Gram-positive and Gram-negative
bacteria, a yeast and a fungus. The SAR study revealed that complex 1 exhibited the best antimicrobial
effects and higher antioxidant activity than complex 2 and L.
Keywords: Imidazole; Cobalt (II) complex; X-ray crystal structure; IR and UV-Vis. spectroscopy;
DPPH; Antimicrobial activity; DFT calculations.
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1. Introduction
Compounds containing imidazole ring system play important roles in biochemical processes
[1,2]. Numerous imidazole derivatives have proved their usefulness as catalysts [3,4], ionic
liquids [5,6], supra-molecular entities [7], anti-corrosives [8], as well as in therapeutic field as
antitumor agents [9,10], antifungal [11], antibacterial [12], antiprotozoal [13], anti-
inflammatory [14], antiviral [15], anti-hypertensive [16], etc…
In recent years, imidazole and related compounds have attracted much attention in coordination
chemistry, because of their ability to bind metals in various coordination modes [17]. In metal
transition coordination compounds, the selection of the anion is important because the
architecture of the complex is often related to the nature of the anion even for structures
containing the same ligand and metal ion. [18]. In addition, metallo-drugs containing ligands
derived from biomolecules have gained remarkable importance due to their anti-infective
[19,20] and interesting antioxidant activities [21].
In continuation of our ongoing program devoted to the synthesis of new metal-coordination
compounds containing imidazole and related compounds as privileged scaffolds [22-25], we
describe herein the synthesis and characterization of two Co(II) complexes derived from (1-
methyl-1H-imidazol-2-yl)methanol. This study includes the structural elucidation by
conventional techniques such as single-crystal X-ray diffraction, IR and UV-Vis spectroscopy,
as well as DFT calculations and biological evaluation.
2. Material and methods
IR spectra were recorded by means of a Shimadzu FT IR-8201 PC spectrophotometer with KBr
pellets. UV-Visible spectra were recorded in methanol on a UV–Visible Jeway 6300
spectrophotometer. NMR spectra were recorded in CDCl3 on a Bruker Avance DPX250.
Chemical shifts () are given in ppm and J values in Hertz (Hz). All high-resolution mass
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spectra (HRMS) was recorded on a Bruker Maxis 4G at the CRMPO (Centre Régional de
Mesures de Physiques l’Ouest, Rennes, France) using positive ion electrospray. Elemental
analyzes for C, H and N were recorded on a Carlo-Erba CHNS apparatus / O. EA1108 at Faculté
des Sciences (UMR 7565), Université de Lorraine (France), and onThermo Finnigan Flash EA-
1112 series at the CRMPO (Centre Régional de Mesures de Physiques l’Ouest, Rennes, France).
Melting points were determined on a Köfler melting point apparatus and are uncorrected.
Anhydrous 1,2-dimethoxyethane (DME) was freshly distilled under argon from
sodium/benzophenone. Commercial grade reagents were used as supplied: 1-methyl-1H-
imidazole (Alfa Aesar), HCHO 35% aq. (Alfa Aesar), Co(OAc)2 (Sigma-Aldrich) and
CoCl2.6H2O (Sigma-Aldrich). The fungus Trichoderm sp (Ts) was furnished by Laboratoire de
Mycologie, Biotechnologie et de l'Activité Microbienne, Faculté des sciences de la vie life,
Université des frères Mentouri- Constantine 1, Algeria.
3. Experimental
3.1. Synthesis
3.1.1. (1-Methyl-1H-imidazol-2-yl) methanol (L)
(1-Methyl-1H-imidazol-2-yl) methanol (L) was prepared from 1-methyl-1H-imidazole
according to the reported procedure [26].
Colorless crystals; 83%; M.p. 112-114°C; 1H NMR (250 MHz, CDCl3): δ 7.24 (d, J= 2.0 Hz,
13
1H, H-4), 7.23 (d, J= 2.0 Hz, 1H, H-5), 4.77 (s, 2H, CH2-O), 3.66 (s, 3H, CH3-N); C NMR
(62.9 MHz, CDCl3): δ 145.3, 123.6, 117.9, 53.3, 33.8. FT-IR (4000-500 cm-1): 3371, 3124,
3016, 2943, 2827, 1600, 1496, 1438, 1357, 1284, 1242, 1188, 1141, 1018, 960, 744, 640; UV-
Vis (MeOH) (λ max, nm (log, M-1cm-1): 276 (5.272).
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3.1.2. Synthesis of complexes 1 and 2
General procedure: To a stirred solution of L (0.4 g) dissolved in anhydrous DME (0.15M)
(e.g. 0.2 g in 10 mL), 0.5 equiv. of Co(OAc)2 (0.44 g) or CoCl2.6H2O (0.424 g) was added. The
solution was stirred at room temperature for 48h. The resulting solid was filtered, washed with
cold DME (3×5 mL) and dried on air. Suitable crystals for X-ray diffraction were collected by
slow evaporation at room temperature from concentrated CH3CN solution.
3.1.2.1. Diacetatobis(1-methyl-1H-imidazol-2-yl)methanol cobalt (II) [Co(OAc)2(L)2] (1)
Pink crystals, yield: 51 % (0.73 g), M.p. 144-146 °C; Elemental analysis: calcd. for
[C14H20N4O6Co]: C, 42.12; H, 5.05; N, 14.03 Found: C, 42.06; H, 5.48; N, 13.75; ESI(+)-
HRMS: calcd. for [M-2OAc+H]+(C10H15N4O2Co) m/z = 282.05215. Found: 282.0519; FT-IR
(4000-500 cm-1): 3417, 3124, 3008, 2970, 2920, 1951, 1859, 1519, 1419, 1346, 1284, 1234,
1157, 1064, 960, 848, 752, 659, 505 cm-1; UV-Vis (MeOH): λ max, nm (log, M-1 cm-1): 218
(5.397), 527.8 (1.29).
3.1.2.2. Dichlorurobis(1-methyl-1H-imidazol-2yl)methanol) cobalt (II) [Co Cl2(L)2] (2)
Blue crystals, yield: 70 % (0.424 g), M.p. 140-142 °C; Elemental analysis: calcd. for
[C10H16Cl2N4O2Co]: C, 33.92; H, 4.55; N, 15.82. Found: C, 33.91; H, 4.60; N, 15.93. FT-IR
(4000-500 cm-1): 3317, 3147, 3105, 2943, 2823, 1650, 1550, 1504, 1415, 1373, 1284, 1222,
1145, 1091, 1006, 964, 852, 786, 736, 655, 509; UV-Vis (MeOH). λ max, nm (log, M-1 cm-1) :
218 (5.334).
3.2. Electronic spectra
In the UV-vis experiment, the absorbance of L as reference sample and metal complexes were
measured at rt in methanolic solution at c = 10-5 M and 10-3 M.  (molar extinction coefficient)
was determined as follows:
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Where A is the absorbance, L is the thickness of cell, and c the molar concentration
3.3. Crystallographic analysis
Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker APEX
II diffractometer equipped with a CCD area detector at 150 K with a graphite monochromatized
MoKα radiation, λ = 0.71073 Å. The reported structures were solved by direct methods with
SIR2002 [27]. to locate all the non-H atoms which were refined anisotropically with
SHELXL97 [28] using full-matrix least-squares on F2 procedure from within the WinGX [29]
suite of software used to prepare material for publication. All absorption corrections were
performed with the SADABS program [30]. The DIAMOND package and ORTEP-3 for
Windows programs were used for generating figures of structures [31-32]. The data collection
and refinement details of cobalt complexes and pivotal geometric parameters related to
coordination of metal ions together with intra and intermolecular interactions hydrogen bonds,
C-H… and - stacking for complexes 1 and 2 are summarized in Tables 5-8. Molecular
structures of 1 and 2 are illustrated in Figs. 5 and 8 respectively.
3.4. Computational methods
Molecular structure modeling, vibrational spectra calculations, and natural bond orbital analysis
of the ligand as well as of the Co(II) complexes were performed at the DFT/TD-DFT level to
explain the available IR and UV–Vis spectroscopic data and to confirm the ligand coordination
to the metal ion. The theoretical study of the structure, electronic, and optical properties were
carried out in the framework of the density-functional theory (DFT) using the second-
generation type of generalized gradient approximation (GGA), the global hybrid (B3LYP) [33]
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and the range-separated hybrid (CAM-B3LYP) functional [34]. The 6–31+G(d) basis sets were
used for non-metal atoms (C, H, N, O) [35]. The Co atom was described though relativistic core
potential ECPs designated Lanl2dz [36]. Harmonic frequency analysis was performed to
guarantee that optimized structures are local minima and the calculations of the UV-Visible
absorption spectra in framework of Time Dependent DFT (TD-DFT) [37, 38]. All calculations
were performed using the Gaussian 09 version E.01 suite of programs [39].
3.5. Biological essay
3.5.1. In vitro antioxidant activity
The in vitro antioxidant activity of the free ligand and its Co(II) complexes was evaluated using
DPPH (2,2-diphenyl-1-picrylhydrazil) free radical scavenging method, as described in previous
papers [22,40]. To 3 mL of a DPPH free radical solution in MeOH (0.1 mM), was added 1 mL
methanolic solution of each test sample and standard (Asc: ascorbic acid) at a concentration of
100 g/mL. The absorbance was measured at t = 0 min, 30 min, 3h, and 24h of incubation in
dark at room temperature with UV–Vis. spectrophotometer operating at = 517 nm. All
samples tests were performed in triplicate and the results are expressed as mean ± standard
deviation (SD). Percentage inhibition of DPPH free radical (I %) was calculated using the
following equation:
Abs Control − Abs Sample
I(%) =
× 100
Abs Control
Where Abs Control is the absorbance of DPPH radical + methanol, Abs sample is the
absorbance of DPPH radical + test sample/standard Asc.
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3.5.2.. In vitro antimicrobial activity
The prepared complexes have a very weak solubility in DMSO commonly used as negative
standard in antimicrobial evaluation, we therefore anticipated that their diffusion on the agar
plate will not occur or could be very slow. Consequently, the use of standard disc-diffusion
method by dissolution of compounds on paper disc is not appropriate. We axed our efforts on
the evaluation of the antimicrobial activity of the ligand and its cobalt complexes 1 and 2 using
a process based on the adsorption of the sample on water-soluble solid pellets [25].
The adsorbent agents chosen must verify the criteria of solubility in water, neutrality (no interaction
with both, microorganisms and tested compounds) and maneuverability (can be compacted under high
pressure to form cylindrical discs). For this purpose, two appropriate adsorbing agents were used:
KBr pellets, as suitable adsorbent for Pseudomonas syringae (Ps: ATCC-27853) and Pichia
caribbica (Pc: Kc 977491), and peptone pellets, as a fitting adsorbing agent for Staphylococcus
aureus (Sa: ATCC-25923), Klebsiela pneumoniae (Kp: ATCC-700303) and Trichoderm sp
(Ts). Each tested compound (2.5 mg) was thoroughly mixed with a dry adsorbing agent
(approximately 100 mg of peptone), then finely grinded in a smooth agate mortar. Microbial
cells were swabbed onto nutrient medium (Mueller–Hinton Broth for bacteria, Yeast Extract-
Peptone-Glycerol (YPG) Medium for yeast and Potato Dextrose Agar (PDA) for fungi) in Petri
plates, then the solid pellets containing different compounds were taken up using sterile forceps
and placed onto the agar. The inoculated plates were incubated for 24 h at 37 °C and 30 °C for
bacterial and yeast strains, respectively. The diameter of the inhibition zone was measured in
millimeter. For comparison, antibiotics adsorbed on peptone such as benzylpenicillin sodium
(Penicillin G), Amoxicillin and Tinidazol were used as positive controls. The zone of inhibition
value less than 6 mm was considered as resistant against the corresponding microorganism
(inactive).
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4. Results and discussion
4.1. Synthesis and characterizationsꢀ
[Co(OAc)2(k2-L)2] (1) and [CoCl2(k1-L)2] (2) were prepared at room temperature by addition
of Co(OAc)2 or CoCl2.6H2O (1 equiv.) to 2.0 eq. of L dissolved in anhydrous DME (Scheme
1). The complexes were air stable with high melting point, and were insoluble in water and
most common solvents, and had a very weak solubility in MeOH and DMSO. The molecular
structures of complexes 1 and 2 were characterized by IR and UV-Vis spectroscopy and by
single crystal X-ray diffraction analysis.
Scheme 1. Synthesis of Co(II) complexes 1 and 2
4.1.1. Infrared spectra
FTIR is an important tool for identifying the coordination nature the ligand with metal ions.
The synthesized complexes were characterized based on significant absorption bands such as
υ(O-H), υ(C=N), υ(C-O), υ(C=O) υ(Co-N) with respect to that of the free ligand. The
vibrational spectra calculations were carried out at the DFT levels. The spectrum of L shows
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broad medium bands at 3371, 3124/3016 and 2943/2827 cm-1 attributable to υ(O-H), υ(C-H)
aromatic and υ(CH3/CH2) stretching frequencies, respectively [41]. Upon coordination to the
cobalt ion, a strong broad band was observed at 3317 cm-1 in the spectrum of 2 and was assigned
to υ(O-H) stretching vibration. The broad band centered at 3417 cm-1 in the spectrum of 1 was
attributed to υ(O-H) stretching vibration. The peaks attributed to υ(C-H) aromatic remain
practically intact (3124/3008 cm-1) for 1 and slightly increased for 2 (3147/3105 cm-1), while
those attributed to υ(CH3/CH2) are slightly shifted to 2970/2920 cm-1 for 1 and are unchanged
for 2. The calculated C–H bands show similar results with B3LYP and CAMB3LYP and were
observed in the ranges: 3120–3020 cm−1 (CH arom) and 3097-3069 cm−1 (CH alkyl) for 1 and
2. The strong band at 1600 cm-1 attributed to υ(C=N) stretching vibration underwent a
bathochromic shift to 1519 cm-1 for 1 and 1550 cm-1 for 2, indicating the coordination of the
imidazole nitrogen atom to the metal ion [42]. This band was calculated at 1505 and 1551 cm-
1 for 1 and 2 respectively, with CAM-B3LYP level. The sharp band at 1018 cm-1 attributable to
υ(C-O) stretching vibration appears at 1006 cm-1 for 2 and was shifted to higher wavenumber
at 1064 cm-1 for 1 proving the coordination of the oxygen atom to the metal center [43]. This
band was depicted at 1057 and 1054 cm-1 with CAM-B3LYP and B3LYP levels respectively.
For the remaining alkyl groups, the C-H asym./sym. bending vibrations assigned to L at
1438/1361 cm-1, were slightly shifted to 1419/1346 cm-1 for 1 and 1415/1373 cm-1 for 2. The
calculated frequencies were observed at 1414/1337 and 1409/1349 cm-1 with CAMB3LYP and
at 1423/1341 and 1409/1368 cm-1 with B3LYP level, for 1 and 2 respectively. The sharp bands
at 960 and 748 cm-1 attributable to the in-plane and out-of-plane bending vibrations of C-H
aromatic, respectively, remain practically intact at 960/752 cm-1 for 1 and at 964/736 cm-1 for
2, and calculations show these latter in the range [928-918] and [721-777] cm-1. In the spectrum
of 1, the new weak band appeared at 1859 cm-1 was assigned to υ(C=O) asymmetric stretching
vibrations, confirming the coordination of carboxylate group through the oxygen atom to the
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metal center. It was calculated at 1636/1681 cm−1 with B3LYP/CAM-B3LYP levels. Finally,
the appearance in the low-frequency region of new absorption bands at 505 and 509 cm−1 for 1
and 2, respectively, were attributed to υ(Co-N) confirming the coordination of L to Cobalt ions
[43]. The theoretical calculations corroborate the experimental results, and absorption bands
are depicted at 491/493 and 512/527 cm-1 for 1 and 2 respectively, with B3LYP/CAM-B3LYP
levels. The calculated vibrational frequencies are in reasonable agreement with experimental
data. The IR spectral information thus supports the suggestion of the coordination of N and O
atoms of L and carboxylate oxygen atoms to the transition metal ion. Fig. 1 shows the
overlapped experimental FTIR spectra of L, 1 and 2, and Table 1 summarizes major selected
lines together with experimental values. Theoretical overlapped IR spectra are pictured in Fig.
S1 in SI.
Fig. 1. Overlapped IR spectra of L, [Co(OAc)2(L)2] (1) and [CoCl2(L)2] (2)
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Table 1.
Selected experimental and calculated vibrational frequencies (cm-1) for L, 1 and 2.
Assignment
L
1
2
Exp.
Exp.
Calculated
Exp.
Calculated
B3LYP CAM-B3LYP
B3LYP CAM-B3LYP
stret.)
3371
3124
2943
-
3417
3124
2970
1859
1519
1419
1064
752
-
-
3317
3147
2943
-
3336
3020
3082
-
3477
3339
3095
-
3120
3069
1636
1526
1423
1057
748
3097
3097
1681
1505
1414
1054
777
(C-H)ar. (stret)
CH3/CH2 (stret
C=stret.)
1600
1438
1018
744
-
1550
1415
1006
736
1554
1409
1012
721
1551
1409
1014
742
C=N) (stret.)
C/Cbend
(C-O) (stret.)
(C-H)ar. (bend)
(Co-N)
505 491
493
509
512
527
4.1.2. UV-Vis study
Overlapped electronic spectra of the ligand (L) and complexes 1 and 2 recorded at 10-5 M and
10-3 M in MeOH solution are shown in Fig. 2. The spectrum of L displays a broad absorption
band at λ max = 276 nm attributable to intra-ligand π-π* transition. Upon complexation, it was
shifted to lower wavelength at λmax = 221 and 217 nm for 1 and 2, respectively. The calculated
excited energies at the TD-DFT with CAM-B3LYP (Table 2) show that the ligand has a well
resolved experimental absorption at 276 nm that originates in the first excited state transition
and attributed to HOMO → LUMO transition state with * character (Fig. 3), this band is
theoretically obtained at 289 nm. For 1, the sharp absorption band observed at 221 nm (band I)
was calculated at 220 nm mainly involved HOMO-4 → LUMO+1 transition state (Fig. 3) and
was attributed to a ligand-metal charge transfer ((L) → dz2 orbital on Co atom). The expected
metal-originated low intensity d-d transitions for octahedral complex 1 was observed solely in
10-3 M solution at 527.8 nm (band II) as depicted in the insets (Fig. 2) [29]. This band calculated
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at 530 nm correspond to HOMO-1→ LUMO transition state (Fig. 3) and was attributed to a
ligand-metal charge transfer (LMCT). For 2, the calculated wavelength correlates nicely with
the experimental value. The broad band obtained at 217 nm, was calculated at 218 nm
corresponding to HOMO-3→LUMO+2 electronic transition state and was attributed to metal-
ligand charge transfer (MLCT) from M(dCo)+(L) to *(L), as clearly represented in Fig. 3.
None of metal-originated transition was observed for 2 in 10-3 M solution. In addition,
significant variations of the constant  (expressed in log) with respect to free ligand (5.272 vs
5.397 and 5.334 for 1 and 2, respectively), confirmed the formation of complexes [43].
Theoretical overlapped UV-Vis. spectra are shown in Fig. S2 in SI.
Fig. 2. Overlapped electronic spectra of L, [Co(OAc)2(L)2] (1) and [Co Cl2(L)2] (2) (10-5 M) together
with the insets (10-3 M).
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Fig. 3. Isodensity plots of selected molecular orbitals of L, 1 and 2.
The transitions associated with the excited states were described by a substantial list of main
transition states with major contributions. For orbital contribution, the analysis showed a spin
allowed single–singlet electronic transition (Fig. 3). An almost pure one natural orbital to one
natural orbital transition was detected. As seen in Table 2, the contributions to the excitation
are 74% at 289 nm for L, 86% at 220 nm and 78% at 530 nm for 1 and 27% for 2. Experimental
and calculated absorption wavelength λ (nm), excitation energy E (eV), oscillator strength (f),
and main transition states with their major contribution (in percentage) of L and complexes 1
and 2 are collected in Table 2.
Table 2.
TD-DFT calculations for L and complexes 1 and 2 using CAM-B3LYP
Oscillator
Strength (f)
Transition state and
major contribution (%)
E(eV)
4.29
xp.(nm)

calcd. (nm)
289
L
Band I
276
0.006
H → L (73.78%)
[Co(OAc)2(L)2] (1)
Band I
221
527
220
530
5.62
2.33
0.022
0.002
H-4 → L+1 (85.37%)
H-1 → L (77.61%)
Band II
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[CoCl2(L)2] (2)
Band I
217
218
5.73
0.017
H-3 → L+2 (26.41%)
4.1.3. Atomic charges analysis
Atomic charge distributions on atoms of the complexes and L were calculated in gas phase
using B3LYP and CAM-B3LYP functional density, and evaluated by natural population
analysis (NPA). The calculated charge distribution on Co(II) ions are lower than their formal
charge of +2, while the electron density of N and O atoms as near-neighboring atoms slightly
increased with respect to that of L (Table 3). Electron transfer of N and O as donor atoms of L
(four oxygen and two nitrogen atoms for 1 or two nitrogen atoms for 2) reduces the positive
charge on the cobalt atom, from (+2) to +1.341/+1.557 and +1.204/+1.420 in 1 and 2,
respectively, using functional hybrids B3LYP/CAM-B3LYP. Thus, cobalt (II) ions are
coordinated to L across the nitrogen and/or oxygen atoms.
Table 3.
NPA atomic charge distribution for [Co(OAc)2(L)2] (1), [Co(Cl2L)2] (2) and L.
Atom
1
2
L
B3LYP
+1.341
CAM-B3LYP
B3LYP CAM-B3LYP B3LYP CAM-B3LYP
Co
N
+1.557
-0.408
-0.589
-0.950
-
+1.204
-0.510
-0.646
-
+1.420
-0.564
-0.673
-
-
-
-0.378
-0.664
-0.870
-
-0.414
-0.440
O
-0.650
-0.668
OOCO
Cl
-
-
-
-
-0.702
-0.805
4.1.4. Frontier molecular orbitals (FMOs)
The FMOS theory is a powerful practical model for describing chemical reactivity, and the most
important aspect is focused on the HOMO and LUMO states. The eight important orbitals
energies of complexes 1 and 2, including LUMO+3 to HOMO-3 were calculated in gas phase
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(Table S1 in SI). The contour plots and energy of selected frontier orbitals for 1 and 2 are
illustrated in fig.4. As seen in Table S1, 2 has a lower HOMO energy than 1 with CAM-B3LYP
((-8.15) vs (-7.33 eV) and B3LYP levels ((-6.58) vs (-5.73 eV)). The HOMO in 1 is made up of
d(Co) and  (L) contributions, while 2 presents d(Co) and pronounced (Cl2) contributions.
The LUMO energy is made up of d(Co)+*(L) for 1, while it is made up of d(Co)+*(Cl) for
2. Analysis on electronic structures shows that the originating transitions were assigned to
LMCT for 1 and MLCT for 2. The M.O diagrams are pictured bellow.
Fig. 4. Contour plots and energy of selected frontier molecular orbitals for complexes 1 and 2.
4.2. Determination of molecular structures
4.2.1. Crystal structure descriptions
X-ray quality crystals of [Co(OAc)2(k2-L)2] (1) and [CoCl2(k1-L)2] (2) were obtained by slow
evaporation at rt from concentrated CH3CN solution, as pink crystals in the monoclinic space
group C2/c for complex 1, and colorless crystals in the monoclinic space group P21/n for
complex 2 (Table 4).
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In [Co(OAc)2(k2-L)2] (1), two molecules of L act as bidentate chelating agents through the
nitrogen and oxygen atoms, and the Co(II) ion is also bonded to two carboxylate oxygen atoms
displaying a six-membered ring around the divalent metal center (Fig.5). The distances between
the Co(II) ion and the same type of donor atoms (N, O, OOCO) are identical. The Co—N bond
lengths [Co-N = 2.0928(15) Å] and Co—OOCO [Co — O11= 2.1054(14) Å] are very close and
slightly shorter than Co—O distances [Co—O1 = 2.1778(14) Å)]. The difference of bond
lengths is due to the Jahn–Teller distortion effect as observed in octahedral geometry of hexa-
coordinated transition metal complex [45]. The OOCO and N atoms define the equatorial plane
and methanolic oxygen atoms occupy opposite vertices of the octahedron. The bond angles
[O1—Co—O11= 90.41(5)° and 89.59 (5)°, N1—Co—O1 = 92.66(6)° and 87.34(6)°], are very
close to the ideal 90°. In addition, the bond angles [N1—Co—N3 = O11—Co—O11a = O1—
Co—O1a = 180°], define three crystallographic C2 axis whose inversion center is located at
the position of the metal center (Table 5). The hexa-coordinated compound exhibited a quasi-
regular square bipyramidal shape. When comparing the bonds lengths reported previously for
[Co(OAc)2(Hmbm)2] [22] to the values in [Co(OAc)2(L)2], in which the Co(II) displayed an
octahedral geometry, the Co—OOCO values are very close (2.1045(3) Å and 2.1053 Å,
respectively. The Co—Nimidazole distance (2.0928(15) Å) is longer than Co—Nbenzimidazole
(2.0787(17) Å), while the bond length Co—O is shorter (2.1778 Å vs 2.1911(3) Å). The crystal
packing can be described as alternating layers parallel to (100) plane along the b axis (Fig. 6).
These layers are connected with strong intra-molecular hydrogen bonding (C-H….O) (Table 6).
The packing is reinforced by C(1)-H(1C)….Cg3 (N(1)-C(3)-C(2)-N(2)-C(4)) interactions with
a distance of C-H…Cg is 3.827(4) and the C-H…Cg angle of 165° (Fig.7a; Table 7). In
addition,  stacking is also observed between the imidazole rings of independent molecules
[Cg1…Cg1i (N(1)-C(2)-C(3)-N(2)-C(4)) for symmetry operation: (-x, -y, 1-z) with centroïd-
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centroïd distance = 3.857(4) Å, and a slippage of 1.628 Å (Fig.7b; Table 7). These interactions
combine to build up a three-dimensional network.
Fig. 5. Oak ridge thermal ellipsoid plots (ORTEP) plot of the X-ray crystal structure of 1. Displacement
ellipsoids are drawn at the 50% probability level (H atoms with arbitrary radii; blue: nitrogen, red:
oxygen).
Fig. 6. View of the crystal structure of 1 showing layers in a=1/4 and a=3/4 parallel to (100)
plane. Intramolecular CH···O hydrogen bonds are shown as dashed lines.
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Fig. 7. C-H…. (a) and … (b) Interactions operating in the crystal structure of 1.
In [CoCl2(k1-L)2] (2), four donor atoms surround the Co(II) ion displaying a tetrahedral
environment. The crystal structure indicates that the metal ion ligated two L through the
nitrogen atom, and two chlorine atoms (Fig 8). The bond lengths between the metal center and
nitrogen atoms are [Co—N = 2.0335(18) and 2.0169 (18) Å], and distances Co—Cl are longer
at 2.2681(7) and 2.3506(7) Å, respectively. The bond angles are N1—Co—N3 = 116.32(7)°,
and Cl1—Co—Cl2 = 105.97(3)°. The N—Co—Cl bond angles are in range [99.50(6)°-
120.93(6)°] (Table 5). The crystal packing can be described as layers parallel to (100) plane
along the b axis (Fig. 9). In this structure, strong intra-molecular hydrogen bonding between
….
…
.
remaining OH groups of the ligand and the chlorine atom (O-H Cl), as well as (H-O H) are
observed (Table 6). The packing is also reinforced by weak interactions Co(1)-Cl(1)....Cg2
(N(2)-C(8)-C(7)-N(4)-C(9)) with a distance Co-Cl….Cg of 6.058(4) Å and Co-Cl-Cg angle of
161° (Fig. 10b; Table 7). In addition, a -stacking is observed between the imidazole rings of
independent molecules [Cg1…Cg1i (N(1)-C(2)-C(3)-N(2)-C(4)), for symmetry operation: (-x,
-y, -z), with a centroïd-centroïd distance of 3.973(5) Å, and a slippage of 2.131 Å] (Fig. 10a;
Table 7). These interactions combine to build up a three-dimensional network.
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Fig. 8. Oak ridge thermal ellipsoid plots (ORTEP) of the X-ray crystal structure of 2.
Displacement ellipsoids are drawn at the 50% probability level (H atoms with arbitrary radii;
blue: nitroge, red: oxygen, green: chlorine).
Fig. 9. The packing of 2 viewed along the b axis, showing layers at a= 1/4, a=3/4 parallel to
(010) plane. Intra- and intermolecular OH···Cl and Cl…H-C hydrogen bonds are shown as
dashed lines.
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Fig. 10. … (a) and C-H… (b) Interactions operating in the crystal structure of 2.
Table 4.
Crystal data and structure refinement parameters for 1 and 2
[Co(OAc)2(L)2] (1)
C14H20CoN4O6
399.24
[Co Cl2(L)2 ] (2)
Empirical formula
Formula weight
T(K)
C10H16CoCl2N4O2
354.10
150(2)
295(2)
Wavelenght
Crystal system
Space group
a(Å)
0.71073 Mo K
monoclinic
C2/c
monoclinic
P21/n
16.1251(15)
7.4337(7)
14.9903(15)
106.751(3)
1720.6(3)
4
8.2361(7)
13.7982(16)
13.3219(16)
102.739(4)
1476.4(3)
4
b(Å)
c (Å)
(°)
V (Å3)
Z
Absorption coefficient (mm-1)
D cak (Mg.m-3)
θ (°)
1.036
1.526
1.541
1.595
0.992
0.997
Measured reflections
Independent reflections
Absorption correction
Refinement method
Rint
R[F2> 2(F2)]
wR(F2)
7481
11761
1917
2968
Tmin = 0.666, Tmax= 0.745
Full-matrix least-square on F2
0.034
0.023
0.033
0.088
0.030
0.070
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Goodness-of-fit on F2
maxeÅ-3)
min(eÅ-3)
1.078
1.022
0.248
-0.259
0.624, -0.241
0.023
Table 5.
Selected experimental and calculated bond lengths (Å) and angles (°) for 1 and 2.
Complex 1
calculated
Complex 2
Experimental
Experimental
calculated
B3LYP
CAM-B3LYP
B3LYP CAM-B3LYP
Bond length (Å)
Co-N1
2.092 (15)
2.092 (15)
2.105 (14)
2.177 (14)
1.903
1.903
1.839
1.932
1.884
1.884
1.808
1.903
2.033 (18)
2.016 (18)
2.019
1.996
2.041
2.031
Co-N3
Co-OOH
Co-OOCO
Co-Cl1
2.268 (7)
2.350 (7)
2.352
2.364
2.421
2.333
Co-Cl2
Angles (°)
N1- Co-O11
92.66 (6)
93.63
86.36
118.10
92.40
93.35
86.84
92.28
87.71
N1a-Co-O11a 87.34 (6)
O1 -Co-O11 90.41 (5)
O1a- Co-O11a 89.59 (5)
N1-Co-Cl1
110.14 (6)
99.50 (6)
105.97 (3)
116.32 (7)
97.35
96.76
108.00
94.46
N3-Co-Cl2
Cl1-Co-Cl2
145.44
137.58
107.87
118.32
N1-Co-N3
Table 6.
Hydrogen bonding distances (Å) and angles (°) of 1 and 2.
D–H….A
d(C–H)
d(H…A)
d(D–A)
D–H–A
Symmetry
[Co(OAc)2(L)2] (1)
C1-H1A…O12
C5-H5B…O12
[Co Cl2(L)2] (2)
O1-H1…Cl2
0.960
0.970
2.590
2.490
3.539 (3)
3.406 (3)
171.00
157.00
x+1/2, y+3/2, z+1
x+1/2, y+3/2, z+1
0.820
0.820
2.400
2.000
3.160 (18)
2.765 (2)
154.00
154.00
x, y, z
O2-H2… O1
x-1, y, z
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Table 7.
D–X··· distances (Å) and angles (°) and … interaction in 1 and 2
d(D–X)
d(X…Cg)
d(D…Cg)
3.827 (4)
3.857 (4)
D–X–Cg
Symmetry
-x, y, 1/2-z
-x, –y, 1-z
[Co(OAc)2(L)2] (1)
C(1)-H(1C)…Cg3 (N(1)-
C(3)-C(2)-N(2)-C(4)
0.96
2.89
165
Cg1…….Cg1 (N(1)-
C(3)-C(2)-N(2)-C(4))
Slippage = 1.628 Å
[Co Cl2(L)2] (2)
Co(1)-Cl(1)....Cg2 (N(2)-
C(8)-C(7)-N(4)-C(9))
Cg1…….Cg1 (N(1)-
C(3)-C(2)-N(2)-C(4))
Slippage = 2.131 Å
0.93
3.867
6.058 (4)
3.973 (5)
161
1/2+x, 1/2-y, 1/2+z
-x, -y, -z
4.2.2. Optimized structures
The optimized structures of [Co(OAc)2(L)2] (1) and [CoCl2(L)2] (2) are shown in Fig. S4 in SI,
and selected bond lengths and angles together with experimental values are summarized in
Table 5. The selected geometry parameters of the complexes in the ground state were calculated
in gas phase by DFT method using B3LYP and CAM-B3LYP functional densities. The
calculated bond lengths are similar in magnitude with the experimental values, with deviations
varying between 0.189-0.266 and 0.014-0.084 Å for 1 and 2, respectively, using B3LYP (Table
5). The theoretical calculations of bond angles are in good agreement with the experimental
values with slight deviations within 0.70º-1.88º for 1 and 1.90º-5.04º for 2 using CAM-B3LYP
levels. Detailed comparison of calculated bond lengths and angles with experimental values
shows that the best results are obtained using B3LYP as functional density for bond lengths
calculations and CAM-B3LYP level for angles. The calculated structural parameters for 1 and
2 are in good agreement with the experimental values. Optimized Cartesian coordinates are
collected in Tables S2-S3 in SI.
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4.3. Biological essay
4.3.1. In vitro DPPH radical scavenging essay
The in vitro antioxidant activity of L and complexes 1 and 2 was carried out at low
concentration (100 μg/mL), using DPPH (1,1-diphenyl-2-picryl hydrazyl) free radical
scavenging assay [22,46]. Ascorbic acid (vitamin C) was used as positive control. The results
are collected in Table S4 in SI, and the DPPH activity profile is shown in Fig. 11.
The instantaneous antioxidant activity of the free ligand was found interesting compared with
control (33.673.82 vs 83.331.22) and was similar in magnitude for both complexes
(32.471.71% and 32.972.11%, respectively). After 24h of incubation, 1 showed a higher
antioxidant activity than 2 and L (30.393.39% vs 26.000.63 and 26.955.33%, respectively).
The tested compounds exhibited antioxidant activities with a percentage of inhibition of DPPH
free radical, higher than 25%, whatever the incubation time. No significant antioxidant activity
was observed for Co(OAc)2 and CoCl2.6H2O whatever the incubation time (inhibition less than
5%). The DPPH radical scavenging ability can be ranked in the order vitamin C > 1 > L ≈ 2 >>
Co(OAc)2 ≈ CoCl2.6H2O, after 24h of incubation.
90
80
70
60
50
40
30
20
10
0
Complex 1 Complex 2
0 min
L
Co(OAc)2 CoCl2.6H2O
3h 24h
Asc
30 min
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Fig. 11. DPPH radical scavenging activity profile of L, [Co(OAc)2(L)2] (1), [CoCl2(L)2] (2), Co(OAc)2
and CoCl2.6H2O
4.3.2. In vitro antimicrobial activity
The free ligand L and its complexes 1 and 2 were screened for their antimicrobial potential
against five pathogenic microorganisms such as Gram (+) bacteria viz. S. aureus Sa and P.
syringae Ps, Gram (-) bacteria viz. K. pneumonia Kp, a yeast viz. P. caribbica Pc, and a fungus
viz. Trichoderm sp Ts, using the disk-diffusion method. To draw meaningful comparison,
Penicillin G (benzylpenicillin sodium), Amoxicillin and Tinidazol (antifungal) were used as
standards. The antimicrobial screening of L and its two Co(II) complexes (Table S5 in SI),
revealed significant difference in their inhibition activity profile (Fig. 12).
No inhibitory activity of L and 2 against the microorganisms selected in this study was
observed, except a moderate activity against Pc compared to Penicillin G (13 and 10 mm vs 60
mm). 1 has shown a remarkable activity against Sa compared to Penicillin G and Amoxillin (32
mm vs 49-50 mm), and a moderate activity against Ps and Pc with respect to the same controls
(10 mm vs 60 mm). All the tested compounds were inactive against the fungus strain used in
this study.
Generally, the complexation of the ligand with dipositive metal ions leads to an increasing in
lipophilicity. The enhanced growth inhibition activity of 1 when compared to 2 might be due to
its chelating ability, which reduces polarity of metal ion due to partial sharing of its positive
charge with donor groups in complexes. This process increased the lipophilicity of the metal
complex and there by facilitated its permeation through the membrane cell [47].
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Antimicrobial activity profil
60
50
40
30
20
10
0
L
Complex 1 Complex 2 Penicillin G Amoxicillin Tinidazole
Sa Kp Ps Pc T s
Fig. 12. Antimicrobial activity profile of L, [Co(OAc)2(L)2] (1) and [Co Cl2(L)2] (2)
5. Conclusions
Two new Co(II) complexes derived from (1-methyl-1H-imidazol-2-yl)methanol (L) have been
successfully synthesized and characterized by single-crystal X ray analysis, IR and UV-Vis.
spectroscopy. The complex 1 exhibits an octahedral environment in which two molecules of L
are ligated to the metal center through its two basic sites (N and O), the Cobalt ion is also
coordinated by two carboxylate oxygen atoms forming a six-membered ring. The complex 2
the Co(II) ion is bounded to two L via the nitrogen atom, and two chlorine atoms displaying a
distorted tetrahedral geometry. The geometry parameters, FT-IR as well as electronic spectral
data of the ligand and its Co(II) complexes supported by DFT and TD-DFT calculations using
B3LYP and CAM-B3LYP as functional density, confirmed the structural assignment. The in
vitro evaluation of the DPPH free radical scavenging ability for L and its Cobalt complexes at
c = 100 g/mL, showed moderate antioxidant activities. The antimicrobial tests revealed that 1
was more active than 2 and L, exhibiting a remarkable activity against S. aureus as Gram-
positive bacteria, and a moderate activity against P. syringae and P. carribica, compared to
Penicillin G and Amoxillin as controls. All the tested compounds were inactive against the
25

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selected fungus strain. Similar compounds are currently under investigations and will be
reported in due course.
Acknowledgments
We would like to thank the National Scientific Research and Technological Development Fund
(FNRSDT), Algeria, for financial support. The authors are grateful to Pr A. Zertal at Laboratoire
TIPE (Techniques Innovantes de Préservation de l'Environnement), Université des frères
Mentouri-Constantine 1, Algeria, for the UV-Vis recording, and Pr. S. Bouacida at Unité de
Recherche CHEMS, Université des Frères Mentouri-Constantine 1, Algeria, for collecting
single crystal X-ray diffraction data. A.B thanks Dr L. Ismaili at Laboratoire de Chimie
Organique et Thérapeutique, Neurosciences intégratives et cliniques EA 481, Université
Bourgogne Franche-Comté, France, for elemental analysis, and Dr. F. Berrée at Institut de
Sciences Chimiques de Rennes (ISCR)-UMR CNRS 6226, Chimie Organique et interfaces
(COrint), Université de Rennes 1, France for HRMS and elemental analysis.
Supporting Information Available
Crystallographic data (excluding structure factors) have been deposited as supplementary
publication number CCDC 1451820 for complex 1 and 1451816 for complex 2. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary material includes Figures S1-S3 and
Tables S1-S5 associated with this article can be found, in the online version,
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