Dinuclear Hg(II) complex of new benzimidazole-based Schiff base: one-pot synthesis, crystal structure, spectroscopy, and theoretical investigations

Article abstract of DOI:10.1080/00958972.2019.1680833

A new dinuclear complex, [HgCl₂L]₂, was prepared using a Schiff base derived from benzimidazole (L = [1-(1H-benzo[d]imidazol-2-yl)-N-(4-methoxyphenyl)ethan-1-imine]. The mercury complex was obtained in good yield by one-pot reaction

Full text of DOI:10.1080/00958972.2019.1680833

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Dinuclear Hg(II) complex of new benzimidazole-
based Schiff base: one-pot synthesis, crystal
structure, spectroscopy, and theoretical
investigations
Youssra Doria Lahneche, Houssem Boulebd, Meriem Benslimane, Mustapha
Bencharif & Ali Belfaitah
To cite this article: Youssra Doria Lahneche, Houssem Boulebd, Meriem Benslimane, Mustapha
Bencharif & Ali Belfaitah (2019): Dinuclear Hg(II) complex of new benzimidazole-based Schiff
base: one-pot synthesis, crystal structure, spectroscopy, and theoretical investigations, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2019.1680833
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1680833
Dinuclear Hg(II) complex of new benzimidazole-based
Schiff base: one-pot synthesis, crystal structure,
spectroscopy, and theoretical investigations
Youssra Doria Lahnechea,b, Houssem Boulebda, Meriem Benslimaneb,
Mustapha Bencharifc and Ali Belfaitaha
a
ꢀ
ꢀ ꢀ
ꢁ
Faculte des Sciences Exactes, Laboratoire Des Produits Naturels D’Origine Vegetale et de Synthese
b
ꢀ
ꢁ
ꢀ
Organique, Universite des Freres Mentouri-Constantine 1, Constantine, Algeria; Unite de Recherche
de Chimie de L’Environnement et Moleculaire Structurale, Universite des Freres Mentouri-
Constantine 1, Constantine, Algeria; Faculte des Sciences Exactes, Laboratoire Des Materiaux,
Universite des Freres Mentouri-Constantine 1, Constantine, Algeria
ꢀ
ꢀ
ꢁ
c
ꢀ
ꢀ
ꢀ
ꢁ
ABSTRACT
ARTICLE HISTORY
Received 3 June 2019
Accepted 23 September 2019
A new dinuclear complex, [HgCl2L]2, was prepared using a Schiff
base derived from benzimidazole (L ¼ [1-(1H-benzo[d]imidazol-2-
yl)-N-(4-methoxyphenyl)ethan-1-imine]. The mercury complex was
obtained in good yield by one-pot reaction under microwave
irradiation from 2-acetylbenzimidazole and p-anisidine, followed
by addition of HgCl2. The Hg(II) complex has been characterized
by single-crystal X-ray diffraction analysis, IR and UV-vis spectros-
copy. The mercury complex is built up from a dinuclear unit
related by an inversion center through two bridging chlorine
atoms. Each mercury atom is coordinated by two N atoms of the
Schiff base L, one terminal Cl atom and two bridging Cl atoms in
a square pyramidal geometry. To support experimental data, the-
oretical calculations including molecular geometry, electronic tran-
sitions and vibration frequencies of the ligand and its complex in
the ground state were carried out using the global hybrid (B3LYP)
density functional. In addition, a qualitative description of excited
states and charge transfer character of electronic transitions states
were carried out by plotting the Natural Transition Orbitals (NTOs)
for main states. Theoretical calculations are in good agreement
with experimental values.
KEYWORDS
Benzimidazole; Schiff base;
Hg(II) complex; crystal
structure; IR spectroscopy;
UV-vis; spectroscopy; DFT
calculations
CONTACT Mustapha Bencharif
mustapha.bencharif@umc.edu.dz; Ali Belfaitah
abelbelfaitah@yahoo.fr;
ali.belfaitah@umc.edu.dz
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2019.1680833.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
Y. D. LAHNECHE ET AL.
1. Introduction
In recent years, the chemistry of azole-based ligand has known a considerable growth
as proved by the increasing number of reports that highlight their properties as
versatile N-donors in coordination chemistry [1–3], their structural novelty and their
potential as new materials with biological importance [4]. The benzimidazole is an
important nucleus containing two nitrogen atoms that can be found in some alkaloids
such as Granulatimide [5]. Although rare in its natural state, this structural motif is also
present inside our body as cobalt ligand in vitamin B12. In contrast, synthetic benzimi-
dazole derivatives exhibited wide applications in various fields such as supramolecular
ligands [6], medicinal drugs [7], biomimetic catalysts [8], agrochemicals [9], etc. In
coordination chemistry, benzimidazole derivatives exhibited various coordination
modes upon their ligation to metal ions [10, 11], and numerous corresponding com-
plexes have been the subject of various biological applications such as antibacterial
[12], anticancer [13], antioxidant [14], antifungal [15], anti-inflammatory [16], and
enzyme inhibitor [17]. Benzimidazole complexes have also found some applications as
catalysts [18].
Schiff bases derived from benzimidazole and their transition metal complexes were
widely investigated as potential metallo-drugs because of their very easy synthesis
with high yields and their strong ability to form stable coordination complexes with
almost all metal ions [19]. These compounds exhibited remarkable biological activities
[20, 21] and possess a flexibility to change and adapt their structure to a particular
application in biological fields [22, 23].
The coordination chemistry of mercury was investigated by several research teams
[24] due to the significant toxicological effects on living organisms and on the ecosys-
tem we live in [25]. However, the mercury(II) compounds are used in various areas
such as in paper industry, dyes, cosmetics, preservatives, fluorescent light bulbs, poly-
mers, and to a lower extent, in mercury batteries [26]. Divalent mercury ion has a fully
filled 5d orbital energy level (no unpaired electrons, 5d10), hence it possesses a strong
tendency to form complexes easily. Metal complexes containing Hg(II) showed various
structural arrangements, in which the metal ion exhibited various coordination num-
bers [26]. HgCl2L-type complexes, where L is a Schiff base containing two N donor
atoms, invariably dimerize [27].

JOURNAL OF COORDINATION CHEMISTRY
3
In line with our ongoing program devoted to the synthesis of new heterocyclic
compounds containing imidazole derivatives or related compounds as main scaffold
[28–31], we describe herein the synthesis and characterization of a new binuclear mer-
cury complex containing a Schiff base derived from benzimidazole. This study includes
an accurate and detailed description of structures, significant absorption bands assign-
ments of IR and UV-vis spectra, molecular binding and other properties using the
DFT method.
2. Experimental
2.1. Measurements and materials
IR spectra were recorded in the range of 4000–600 cmꢀ1 by means of a Jasco FT-IR
6300 type A spectrometer with ATR sampling accessory. UV-visible spectra were
recorded in CH3CN (10ꢀ5 M and 10ꢀ3 M) on a UV–visible Jenway 6300 spectrophotom-
eter. NMR spectra were recorded in CDCl3 on a Bruker Avance DPX250. Chemical shifts
(d) were given in ppm and J values in Hertz (Hz). The melting points were determined
€
on a Kofler melting point apparatus and are uncorrected. All high-resolution mass
ꢀ
spectra (HRMS) were recorded on a Bruker Maxis 4 G at the CRMPO (Centre Regional
de Mesures de Physiques l’ Ouest, Rennes, France) using positive ion electrospray.
Microwave irradiation experiments were performed with
a microwave oven
(P ¼ 300 W). 2-Acetylbenzimidazole was prepared according to a literature procedure
[32]. Commercial grade reagents were used as supplied: o-phenylenediamine (Alfa
Aesar), lactic acid (Alfa Aesar), p-anisidine (Fluka), HgCl2 (Sigma-Aldrich). Acetonitrile
was freshly distilled before use.
2.2. Synthesis
2.2.1. Synthesis of N-(4-methoxyphenyl)-1-(1H-benzo[d]imidazol-2-yl)ethanimine (L)
0.5 mmol (80 mg) of 2-acetylbenzimidazole and 0.5 mmol (62 mg) of p-anisidine were
placed into a sealed Pyrex-glass vessel and subjected to microwave irradiation with
magnetic stirring for 4 min at 90 ꢁC [33]. After cooling to room temperature, the result-
ing residue was purified by recrystallization from ethanol. Green crystals; 90%; M.p.
1
172–173 ꢁC; H NMR (250 MHz, CDCl3), d (ppm): 10.68 (s, 1 H, NH), 7.89 (d, J ¼ 7.0 Hz,
1 H, H7), 7.47 (d, J ¼ 7.0 Hz, 1 H, H4), 7.41–7.31 (m, 2 H, H5, H6), 6.96 (dd, J ¼ 6.9, 2.2 Hz,
2 H, H20, H60), 6.89 (dd, J ¼ 6.9, 2.0 Hz, 2 H, H30, H50), 3.81 (s, 3 H, CH3-O), 2.46 (s, 3 H,
CH3); 13C NMR (62.9 MHz, CDCl3), d (ppm): 159.0, 157.0, 151.3, 143.9, 142.3, 133.7,
124.9, 122.8, 121.7, 120.8, 114.4, 111.4, 55.6, 16.5. ESI(þ)-HRMS: Calcd. for
[C16H15N3NaO]þ [M þ Na]þ: m/z ¼ 288.11073. Found: 288.1110; FT-IR (cmꢀ1): 3347,
3147, 3086, 2924, 2820, 1659, 1617, 1583, 1504, 1416, 1372, 1243, 1123, 1029, 960,
752; UV-vis (CH3CN), kmax (nm): 236.5, 294, 339.5.
2.2.2. One-pot synthesis of di-m-chlorido-bis-(chlorido-f(1H-benzo[d]imidazole-2-yl)
ethanimino)-4-methoxy phenyl)gmercury(II), [HgCl2L]2
Without isolation, the Schiff base prepared following the procedure above was dis-
solved in CH3CN and treated with 0.5 mmol (137.7 mg) of HgCl2 salt in the same pot.

4
Y. D. LAHNECHE ET AL.
The mixture was stirred at room temperature for 5 min and the resulting precipitate
was filtered off, washed with cold CH3CN and dried in air to give the pure complex in
92% (245 mg) of yield. The structure determination of the dinuclear mercury complex
[HgCl2L]2 is based on an X-ray study of a suitable crystal obtained by slow evaporation
at room temperature from a concentrated EtOH/DMF (2:1) solution. Yellow crystals,
M.p. > 260 ꢁC, FT-IR (cmꢀ1): 3347, 3147, 3086, 2949, 2828, 1631, 1583, 1504, 1416,
1374, 1258, 1149, 1024, 960, 833, 752; UV-vis (CH3CN), kmax (nm): 238.5, 296.5, 339.5.
2.3. Crystallographic analyses
A single-crystal of mercury complex was mounted under inert perfluoropolyether at
the tip of a glass fiber and cooled under power nitrogen. The measurement was
recorded on a Nonius Bruker APEXII diffractometer at 150 K with molybdenum Ka radi-
ation (k ¼ 0.71073 Å).
The structure was solved using direct methods SIR92 [34] and refined by the least-
squares method on structural factors F2 using SHELXL-2014 [35]. All hydrogens
attached to carbons were placed in geometrically idealized positions, with C-H
distances of 0.93 Å (aromatic), 0.96 Å (methyl) and refined using a riding model, with
Uiso(H) ¼ 1.5 Ueq(C) for the methyl group and 1.2 Ueq(C) for the aromatic.
The design of the molecules was realized with the help of the software ORTEP-3
[36]. The crystal data and the refinement parameters of Hg(II) complex and pivotal
geometric parameters related to coordination of metal ion together with intermolecu-
lar interactions hydrogen bonds, C-H … p and p-p stacking are summarized in Tables
1–3. Molecular structure of the complex is illustrated in Figure 1.
2.4. Computational methods
The theoretical study of the structure, electronic and optical properties was carried
out in the framework of the density-functional theory (DFT) in gas phase, using the
second-generation type of generalized gradient approximation (GGA), the global
hybrid (B3LYP) [37]. The Hg atom was described through Stuttgart-Dresden effective
core potential ECPs, designated as SDD. These energy-consistent ECPs are constructed
to reproduce experimental observables of a single atom, such as ionization potentials
and excitation energies, within the quasi-relativistic Wood and Boring theory [38]. The
MWB60 pseudo-potential type implemented in Gaussian 09 was used for mercury
atom. The 6-31 þ G(d) basis sets were used for nonmetal atoms (C, H, N, O) [39].
Further analyses of electronic and optical properties were performed by calculations of
both vertical and adiabatic ionization potentials, the natural population analysis [40],
and by the calculation of the UV-vis absorption in the framework of Time-Dependent
DFT (TD-DFT) [41]. Excited states and charge transfer character of electronic transitions
were characterized by plotting the electron density between the excited and ground
states for the main peaks of natural transition orbitals (NTOs) [42]. All calculations
have been performed using the Gaussian 09 version E.01 suite of programs [43].

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystallographic data and structure refinement parameters for [HgCl2L]2.
Empirical formula
Formula weight
Crystal size (mm)
Crystal morphology
Temperature (K)
Crystal system
Space group
a (Å)
C32H28Cl4Hg2N6O2
1071.58
0.15 ꢂ 0.1 ꢂ 0.05
Prism
293
Triclinic
P-1
8.5055(2)
9.8773(3)
11.2851(3)
79.279(1)
68.938(1)
71.366(1)
835.64(4)
1
b (Å)
c (Å)
a (ꢁ)
b (ꢁ)
c (ꢁ)
V (Å3)
Z
Dx (g cmꢀ3
)
2.129
9.54
Abs. coefficient (mmꢀ1
)
Transmission factors (min, max)
0.538, 0.746
2.7 to 27.5
15768
3780; 0.018
3486
h Range (ꢁ)
Reflections measured
Independent reflections; Rint
Reflections with I > 2r(I)
Number of parameters
R(F) [I > 2r(I)]
210
0.027
0.085
1.18
wR(F2) (all data)
Goodness-of-fit on F2
Dqmax,min (e Åꢀ3
)
1.30, ꢀ0.98
Table 2. Selected experimental and calculated bond lengths (Å) and angles (ꢁ) for [HgCl2L]2.
Experimental
Calculated
Bond lengths (Å)
Hg-N1
Hg-N2
2.511(3)
2.205(3)
2.434(13)
2.474(11)
3.084(12)
4.336(4)
2.725
2.380
2.431
2.606
2.897
4.054
Hg-Cl1
Hg-Cl2
Hg-Cl2i
Hg–Hgi
Angles (ꢁ)
N1-Hg1-Cl1
N1-Hg1-Cl2
N2-Hg1-Cl1
N2-Hg1-Cl2
Cl1-Hg1-Cl2
N2-Hg1-N1
N2-Hg1-Cl2i
N1-Hg1-Cl2i
Cl1-Hg1-Cl2i
Cl2-Hg1-Cl2i
114.57(9)
96.71(9)
97.08
86.31
122.04
136.78
99.01
66.96
96.38
159.84
101.60
85.24
121.72(10)
136.96(9)
101.12(5)
70.63(12)
83.59(9)
135.81(8)
109.46(4)
78.03(4)
Symmetry code: (i) –x þ 2, –y þ 1, –z.
ꢁ
Table 3. Intermolecular interactions (Å, ) operating in the crystal structure for [HgCl2L]2.
D-H
H … A
D … A
D-H … A
Symmetry
C5-H5—Cl2ii
0.93
0.96
0.96
0.96
2.93
2.92
2.98
2.78
3.652(5)
3.798(6)
3.8932
3.6625
3.5462
135
153
159
154
–x þ 1, –y þ 1, –z
–x þ 2, –y, –z þ 1
1 – x, 1 – y, 1 – z
1 – x, 1 – y, 1 – z
1 – x, 1 – y, –z
C17-H17B—Cl2iii
C17-H17C—Cg1iv
iv
C17-H17C—Cg2
Cg1—Cg1v
(Cg1: N2C2N3C4C9).
(Cg2: C4-C9).

6
Y. D. LAHNECHE ET AL.
Figure 1. View of the molecular structure of [HgCl2L]2, with atom labeling. Displacement ellipsoids
are drawn at the 40% probability level (hydrogens with arbitrary radii; blue: nitrogen, red: oxygen,
green: chloride). Symmetry code: (i) 2 – x, 1 – y, –z.
3. Results and discussion
3.1. Synthesis
The 1-(1 H-benzo[d]imidazol-2-yl)ethan-1-one (2) was prepared in two steps according to
the protocol described by Mathew et al. [32]. The addition of 1.5 equiv. of lactic acid to o-
phenylenediamine furnishes the corresponding 1-(1 H-benzo[d]imidazol-2-yl)) ethanol
and subsequent oxidation reaction gives the 2-acetylbenzimidazole (Scheme 1).
The Hg(II) complex was prepared in two distinct steps: the synthesis of the Schiff
base was followed by a complexation reaction. The benzimidazole-based-Schiff base
was prepared by a reaction condensation of 2-acetylbenzimidazole with p-anisidine
according to the procedure reported by Mermer et al. [33] (the NMR spectra of the
Schiff base are illustrated Supplemental Figure S2). Subsequent addition of mercury
chloride conducted in CH3CN at room temperature gave the expected complex
[HgCl2L]2. To optimize the yield, the sequence condensation/complexation was per-
formed in a one-pot reaction (without isolation of the Schiff base) to give the mercury
complex in excellent yield (92%). The dimeric compound is air stable with a high melt-
ing point. The molecular structure of the complex was characterized by IR and UV-vis
spectroscopy, and by single crystal X-ray diffraction analysis. The synthetic pathway is
outlined in Scheme 1.

JOURNAL OF COORDINATION CHEMISTRY
7
Scheme 1. Synthesis of [HgCl2L]2.
3.2. Crystal structure description
ꢂ
The mercury complex crystallizes in the triclinic space group Pi (Table 1) with one half
crystallographically independent molecule per asymmetric unit. The crystal structure is
built up of a binuclear unit related around an inversion center through two bridging
Cl atoms. The bridge is unsymmetrical with distances Hg1-Cl2 2.474(11) Å and Hg1-
Cl2i 3.084(12) Å, (i): 2 – x, 1 – y, –z. The Hg1 … Hg1 separation is 4.336(4) Å (Table 2).
We observe in the crystal structure that the Schiff base L acts as bidentate chelating
agent through its two nitrogen atoms with the Hg-Nimino bond length (2.511(3) Å) lon-
ger than Hg-Nimidazol bond (2.205(3) Å) and the distance Hg-Cl1 (2.434(13) Å) shorter
than Hg-Cl2 (2.474(11) Å). These results are in good agreement with those reported
for similar compounds [27].
The Hg2þ ion is linked to the N atoms of the chelating ligand and to three Cl atoms
resulting in the metal coordinated to five atoms. The coordination around the metal
could be regarded as a quasi-regular square pyramidal geometry as quantified by the
value of s5 ¼ 0.019 with Cl2, Cl2i, N1, and N2 in the equatorial plane and Cl1 at the
apical. The Hg1 atom is 0.929 Å out of the basal plane (Figure 1). The selected geom-
etry parameters of [HgCl2L]2 in the ground states were calculated in gas phase using
B3LYP functional density and are in agreement with the experiments (Table 2). The
calculated bond lengths are in good accordance with the experimental values within
0.002–0.214 Å. The calculated bond angles for the complex exhibit some differences
compared to the experimental values with deviations within 0.18–24.03ꢁ. This result is
can be explained by the fact that calculations have been carried out in gas phase.
According to this observation, the calculations are in accordance with experiments.
The optimized structure of [HgCl2L]2 with atom numbering is shown in Supplemental
Figure S1 in the Supporting Information, and optimized Cartesian coordinates are
collected in Supplemental Table S1.
The crystal packing can be described as alternating layers parallel to the ab plane along
the b axis, connected with weak intermolecular hydrogen bonding C-H… Cl (Figure 2;
Table 3). The packing is also reinforced by two C-H… p interactions between the hydrogen

8
Y. D. LAHNECHE ET AL.
Figure 2. Hydrogen bonding network operating in the crystal packing of [HgCl2L]2 viewed down
the c axis.
Figure 3. (a) p … p stacking and (b) C-H … p interactions operating in [HgCl2L]2.
atoms of methoxy group H17C and the p-system of the imidazole ring (H17C … Cg1
2.98Å, C17-H17C … Cg1 159ꢁ) and with the p-system of the phenyl ring (H17C … Cg2
2.78Å, C17-H17C … Cg2 154ꢁ) (Figure 3(b); Table 3). A p-p stacking interaction Cg1 … Cg1v
(symmetry operation: (v) 1 – x, 1 – y, –z) is also observed with centroid-centroid distance of
3.546 Å with a slippage of 0.429 Å and a dihedral angle of 0ꢁ (Figure 3(a); Table 3). These
interactions link the layers together to build up a 3-D network.
3.3. Spectroscopic studies
3.3.1. Infrared spectra
FTIR spectroscopy is an important tool for identifying the coordination nature of
ligands with metal ions. To support experimental spectral data, significant absorption
bands of IR spectra of L and [HgCl2L]2 were calculated using the DFT/TD-DFT with
B3LYP level as functional density. The spectrum of the free ligand L shows a broad
weak band centered at 3347 cmꢀ1 attributable to t(N-H) stretching vibration, very
weak bands in the range 3127–3086 cmꢀ1 assignable to t(C-H) aromatic, and two
broad weak bands at 2924 and 2820 cmꢀ1 attributable to t(CH3/OCH3) asymmetric

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 4. Overlapped experimental and theoretical IR spectra of the ligand (L) and [HgCl2L]2.
Table 4. Selected experimental and calculated vibrational frequencies (cmꢀ1) using B3LYP func-
tional for L and [HgCl2L]2.
L
Complex
Assignment
Exp.
Calculated
Exp.
Calculated
t(N-H) (stret.)
t(C-H) arom (stret.)
t(CH3) (stret.)
t(C ¼ N) (stret.)
t(C ¼ C) (stret.)
t(CH3) (bend.)
t(C-O) (stret.)
3347
3147-3086
2924/2820
1659/1617
1583
3660
3223-3182
3157/3084
1697/1677
1543
3347
3147-3086
2949/2828
3696
3244/3204
3038/3043
1641-1561
1618
1631/[1604-1553]
1583
1416/1372
1029
1525-1491
1070
1416/1374
1024
1538/1453
1023
t(C-H) arom (bend.)
960/752
848/754
960/752
877/788
stretching vibrations. Calculated frequencies occur at 3660, 3223–3182, and 3157/
3084 cmꢀ1 for t(N-H), t(C-H) aromatic and t(CH3/OCH3) stretching vibrations, respect-
ively [44]. Upon complexation to mercury ion, the peaks attributed to t(N-H) and t(C-
H) aromatic remain practically intact, while t(CH3/OCH3) are slightly shifted at 2949/
2828 cmꢀ1, these bands are calculated at 3696, 3244–3204, and 3184/3167 cmꢀ1 for
t(N-H), t(C-H) aromatic and t(CH3/OCH3) stretching vibrations, respectively. In the
experimental spectra (Figure 4), the two broad medium bands of L at 1659 and
1617 cmꢀ1 attributable to the open-chain imino t(-C ¼ N-) and t(C ¼ N) stretching
vibrations, respectively, underwent a bathochromic shift to 1631 cmꢀ1 and merged
with t(C ¼ C) stretching vibrations in the range 1604–1553 cmꢀ1, indicating the coord-
ination of the imino- and imidazole-nitrogen atoms to the metal ion [45, 46]. The cal-
culated frequencies with B3LYP occur at 1697 and 1677 cmꢀ1 for t(-C ¼ N-), and
1641 cmꢀ1 for t(C ¼ N) for the complex (Figure 4). In the spectrum of the ligand, the
strong broad band observed at 1583 cmꢀ1 was attributed to t(C ¼ C) stretching vibra-
tions, and those observed at 1416/1372 cmꢀ1 were assigned to tasym/sym(CH3) bending
vibrations. These bands remain practically intact for the complex and were calculated

10
Y. D. LAHNECHE ET AL.
Figure 5. Overlapped experimental and theoretical UV-vis spectra of the ligand (L) and [HgCl2L]2.
at 1543 cmꢀ1 for t(C ¼ C) stretching vibrations, and 1525/1491 cmꢀ1 for tasym/sym(CH3)
bending vibrations for L. The complex [HgCl2L]2 calculations gave peaks at 1618 and
1538/1453 cmꢀ1 attributable to t(C ¼ C) and t(CH3), respectively. The sharp band
observed at 1029 cmꢀ1 was slightly shifted at 1024 cmꢀ1 for the complex and was
assigned to t(C-O) stretching vibrations. It was calculated at 1070 cmꢀ1 for L and at
1023 cmꢀ1 for the complex. Finally, the sharp bands occurring at 960 and 752 cmꢀ1
are attributed to the in-plane and out-of-plane bending vibrations of C-H aromatic,
respectively, for L, these bands remain unchanged for [HgCl2L]2. The calculated vibra-
tion frequencies are in good agreement with experimental data. The experimental and
theoretical data are slightly different because calculations were carried out in gas
phase. The IR spectral information thus supports the suggestion of the coordination of
N atoms of L to the mercury ion. Overlapped experimental and theoretical IR spectra
of the ligand (L) and the complex are pictured in Figure 4, and major bands are sum-
marized in Table 4 together with the experimental values.
3.3.2. UV-vis study
Overlapped electronic experimental and theoretical spectra of the free ligand L and its
corresponding complex [HgCl2L]2 recorded at 10ꢀ5 M concentration in CH3CN solution
are shown in Figure 5. The spectrum of L displays three broad absorption bands at
k
¼ 236.5 (I), 296.5 (II) and 339.5 nm (III). Bands I, II, and III were attributed to
max

JOURNAL OF COORDINATION CHEMISTRY
11
Table 5. Electronic transition orbitals of L and [HgCl2L]2.
Oscillator
strength (f)
Main transition states with
major contribution (%)
kexp. (nm)
kcalcd. (nm)
E (eV)
L
Band I
Band II
Band III
[HgCl2L]2
Band I
236.5
296.5
339.5
204.4
312.2
405.5
0.3528
3.9715
3.0578
0.3495
0.5103
0.5892
HOMO-1 !LUMO þ 4 (49.51%)
HOMO-1 ! LUMO (81.83%)
HOMO ! LUMO (96.15%)
238.5
296.5
339.5
262.45
318.34
429.36
4.7241
3.8948
2.8876
0.2070
0.9483
1.1889
HOMO-10 ! LUMO þ 2 (54.94%)
HOMO-6 ! LUMO þ 2 (18.99%)
HOMO-4 ! LUMO (36.28%)
Band II
Band III
HOMO-5 ! LUMO þ 1 (37.64%)
HOMO ! LUMO þ 1 (49.49%)
HOMO-1 ! LUMO (46.74%)
ꢃ
intraligand p-p transitions [47]. Upon complexation, no change was perceived for
absorption bands II and III of the Schiff base, while absorption band I was shifted to a
slightly higher wavelength (236.5 nm ! 238.5 nm). The calculated excited energies at
the TD-DFT with B3LYP (Table 5) show that the ligand’s well-resolved experimental
absorption at 236.5 nm originating in the first excited transition state was attributed to
ꢃ
HOMO-1 ! LUMO þ 4 with p-p character; this band was calculated at 204.4 nm. The
broad absorption band observed at 296.5 nm (band II) was calculated at 312.2 nm,
mainly involved HOMO-1 ! LUMO transition state and was attributed to intraligand
ꢃ
transition with p–p character. The weak absorption band centered at 339.5 nm (band
III) was calculated at 405.5 nm and assigned to HOMO ! LUMO transition state. For
the complex two main transition states were involved for each absorption band with
variable contributions (Table 5). Bands I, II, and III were calculated at 262.4, 318.3, and
429.3 nm, respectively. These bands correspond to: HOMO-10 !LUMO þ 2/HOMO-6
! LUMO þ 2 transitions attributed to a charge transfer from the ligand to the metal
ion (LMCT) for band I, HOMO-4 ! LUMO/HOMO-5 ! LUMO þ 1 transition states
ꢃ
assigned to a metal-ligand charge transfer Hg(II) ! p ligand (MLCT) for band II.
Finally, band III mainly involved HOMO ! LUMO þ 1/HOMO-1 ! LUMO transition
ꢀ3
ꢃ
states as intraligand charge transfer p-p . In 10 M solution of the complex, none of
metal-originated transition was observed. A schematic illustration of some selected
molecular orbital transitions with the Kohn-Sham orbitals of [HgCl2L]2 is expressed in
Figure 6 and Supplemental Figure S3 and shows the contour plots of some selected
molecular orbital transitions of L.
A qualitative description of electronic excitation states was carried out using the
Natural transition orbitals (NTOs) [43]. The transitions associated with excited states
were described by a substantial list of main transition states with major contributions.
The NTOs analysis (Figure 6) showed a spin-allowed single-singlet electronic transition.
An almost pure one natural orbital to one natural orbital transition was detected. As
seen in Table 5, the natural orbital pairs contribute to the excitations with 49.51% at
236.5 nm, 80.83% at 296.5 nm and 96.15% at 339.5 nm for L. For the complex, the
highest contribution is 54.94% at 262.4 nm, 37.54% at 318.3 nm and 49.49% at
429.3 nm. Experimental and calculated absorption wavelength k (nm), excitation
energy E (eV), oscillator strength (f), and main transition states with their major contri-
bution (in percentage) of L and the complex are collected in Table 5.

12
Y. D. LAHNECHE ET AL.
Figure 6. Schematic illustration of selected molecular orbital transitions of [HgCl2L]2 with the
ꢃ
Kohn-Sham orbitals corresponding to (a) LMCT, (b) MLCT and (c) intraligand charge-transfer p-p .
Table 6. NPA atomic charge distribution for [HgCl2L]2 and L.
Atom
Complex
L
Hg
N1 (imino)
N2
Cl1
Cl2
þ0.467
–0.484
–0.587
–0.383
–0.389
–
–0.009
–0.336
–
–
3.3.3. Frontier molecular orbitals
The HOMO and LUMO levels, called frontier molecular orbitals (FMOs), play an import-
ant role in the optical and electric properties, and in UV-vis spectra. The energy and
electron density are an indicator of the stability, the chemical reactivity and the excita-
tion properties of a molecule. The calculations were carried out in PCM solvent model.
The HOMO and LUMO pictures with the contour plots and energy of selected frontier
orbital energies of the complex and L are reproduced in Supplemental Figure S4. As
shown in Supplemental Figure S4, the HOMO-LUMO energy gaps are 3.77 and 3.53 eV

JOURNAL OF COORDINATION CHEMISTRY
13
for L and the complex, respectively. This intramolecular charge transfer belongs mainly
ꢃ
to the local excitations p-p .
3.3.4. Other properties of the complex
Atomic charge distributions on atoms of the complex and L were calculated using
B3LYP functional density and evaluated by natural population analysis (NPA) in gas
phase. The calculated charge distribution on Hg(II) ion are lower than its formal charge
of þ2, while the electron density of N atoms as near-neighboring atoms slightly
increased with respect to that of free ligand (Table 6). Electron transfer of N as donor
atoms of the ligand (two nitrogen atoms) reduces the positive charge on the mercury
atom from (þ2) to (þ0.464), indicating charge transfer between L and the Hg ion
(Table 6). These results confirm that Hg(II) ion is coordinated to L across the nitro-
gen atoms.
To confirm some properties observed in the experiments such as stability and solu-
bility, the total energy and dipole moment for the complex were calculated. The low
value of the total energy of the mercury complex (–3863.964 Hartree) shows that it is
stable with a high melting point, and the value of the dipole moment (0.0002 debye)
indicates that it is not soluble in water and in polar solvent, as noted previously.
4. Conclusion
A new dinuclear Hg(II) complex derived from benzimidazole-based-Schiff-base has
been synthesized and characterized by single-crystal X-ray analysis, IR and UV-vis spec-
troscopy. [HgCl2L]2 is a dimeric complex in which each mercury atom is coordinated
by two N atoms of the ligand, one terminal Cl atom and two bridging Cl atoms in a
square pyramidal geometry. The geometry parameters, ground state structure, frontier
molecular orbitals, IR as well as electronic spectral data of the ligand and the mercury
complex were described by DFT and TD-DFT calculations using B3LYP functional lev-
els, and are in good accordance with the experimental data.
Acknowledgments
ꢀ
The authors thank Pr. A. Zertal at Laboratoire TIPE (Techniques Innovantes de Preservation de
ꢀ
ꢁ
l’Environnement), Universite des freres Mentouri-Constantine 1, Algeria, for the UV-vis recording,
ꢀ
and Pr. B. Boudine at Laboratoire de cristallographie, department de physique, Universite des
ꢁ
freres Mentouri-Constantine 1, Algeria, for the IR recording. The authors are grateful to Dr. F.
ꢀ
Berree at Institut de Sciences Chimiques de Rennes (ISCR)-UMR CNRS 6226, Chimie Organique et
ꢀ
interfaces (COrint), Universite de Rennes 1, France for elemental analysis.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
ꢁ
ꢀ
This work was supported by MESRS (Ministere de l’Enseignement Superieur et de la Recherche
Scientifique), Algeria.

14
Y. D. LAHNECHE ET AL.
ORCID
Ali Belfaitah
http://orcid.org/0000-0003-0448-5810
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