Palladium(II) and platinum(II) complexes containing benzimidazole ligands: Molecular structures, vibrational frequencies and cytotoxicity

Article abstract of DOI:10.1016/j.molstruc.2011.02.014

(1H-benzimidazol-2-ylmethyl)-(4-methoxyl-phenyl)-amine (L¹), (1H-benzimidazol-2-ylmethyl)-(4-methyl-phenyl)-amine (L²) and their Pd(II) and Pt(II) complexes have been synthesized as potential anticancer compounds and their structures were elucidated using a variety of physico-chemical techniques. Theoretical calculations invoking geometry optimization, vibrational assignments, ¹H NMR, charge distribution and molecular orbital description HOMO and LUMO were done using density functional theory. Natural bond orbital analysis (NBO) method was performed to provide details about the type of hybridization and the nature of bonding in the studied complexes. Strong coordination bonds (LP(1)N11 → σ(MCl22)) and (LP(1)N21 → σ(MCl23)) (M = Pd or Pt) result from donation of electron density from a lone pair orbital on the nitrogen atoms to the acceptor metal molecular orbitals. The experimental results and the calculated molecular parameters revealed square-planar geometries around the metallic centre through the pyridine-type nitrogen of the benzimidazole ring and secondary amino group and two chlorine atoms. The activation thermodynamic parameters were calculated using non-isothermal methods. The synthesized ligands, in comparison to their metal complexes were screened for their antibacterial activity. In addition, the studied complexes showed activity against three cell lines of different origin, breast cancer (MCF-7), Colon Carcinoma (HCT) and human heptacellular carcinoma (Hep-G2) comparable to cis-platin.

Full text of DOI:10.1016/j.molstruc.2011.02.014

Journal of Molecular Structure 991 (2011) 108–126
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Palladium(II) and platinum(II) complexes containing benzimidazole ligands:
Molecular structures, vibrational frequencies and cytotoxicity
⇑
Nour T. Abdel Ghani , Ahmed M. Mansour
Chemistry Department, Faculty of Science, Cairo University, Gamaa Street, Giza 12613, Egypt
a r t i c l e i n f o
a b s t r a c t
(1H-benzimidazol-2-ylmethyl)-(4-methoxyl-phenyl)-amine (L1), (1H-benzimidazol-2-ylmethyl)-(4-
methyl-phenyl)-amine (L2) and their Pd(II) and Pt(II) complexes have been synthesized as potential anti-
cancer compounds and their structures were elucidated using a variety of physico-chemical techniques.
Theoretical calculations invoking geometry optimization, vibrational assignments, 1H NMR, charge
distribution and molecular orbital description HOMO and LUMO were done using density functional
theory. Natural bond orbital analysis (NBO) method was performed to provide details about the type
of hybridization and the nature of bonding in the studied complexes. Strong coordination bonds
(LP(1)N11 ? rꢀ(MACl22)) and (LP(1)N21 ? rꢀ(MACl23)) (M = Pd or Pt) result from donation of electron
density from a lone pair orbital on the nitrogen atoms to the acceptor metal molecular orbitals. The
experimental results and the calculated molecular parameters revealed square-planar geometries around
the metallic centre through the pyridine-type nitrogen of the benzimidazole ring and secondary amino
group and two chlorine atoms. The activation thermodynamic parameters were calculated using non-iso-
thermal methods. The synthesized ligands, in comparison to their metal complexes were screened for
their antibacterial activity. In addition, the studied complexes showed activity against three cell lines
of different origin, breast cancer (MCF-7), Colon Carcinoma (HCT) and human heptacellular carcinoma
(Hep-G2) comparable to cis-platin.
Article history:
Received 3 November 2010
Received in revised form 12 January 2011
Accepted 7 February 2011
Available online 12 February 2011
Keywords:
Benzimidazole
Antitumor
NBO
Palladium complexes
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
azole scaffold is a useful structural motif for displaying chemical
functionality in biologically active molecules. Some of its deriva-
Despite the widespread use of cis-diaminedichloroplatinum(II)
(cis-platin) as an anticancer drug there is still scope for improve-
ment, with respect to reduced toxicity [1], increased clinical effec-
tiveness, broader spectrum of action, elimination of side effects
(e.g., nausea, hearing loss, vomiting, etc.), increased solubility,
and the ability to use them in combination with other drugs. The
generation of new platinum complexes for this application was
based on an early structure–activity relationship, which stated that
potentially active complexes should be neutral and contain two in-
ert amine ligands (with at least one NAH bond) in the cis orienta-
tion and two semilabile leaving groups [2] (one of the reasons for
choosing the ligands under study). The NH group is responsible
for important hydrogen-bond donor properties, either in the ap-
proach of the biological target or the final structure. Recently,
interest was directed toward the developing cis-platin analogs
which have heterocyclic amine ligands coordinated to the
cyctotoxic platinum(II) moiety. Several platinum complexes with
N-heterocyclic ligands such as imidazole, thiazole, benzimidazole,
benzothiazole, and benzoxazole were reported [3]. The benzimid-
tives have potent biological activities as antitumor [4], anti-HIV
[5], anti-Parkinson [6], and antimicrobial [7] agents. At the same
time, because of the coordination chemistry of azoles acting as li-
gands in transition metal compounds, the chelating ligands incor-
porating benzimidazole groups have been extensively studied in
the context of modeling biological systems in recent years [8].
Our aim was to take into account all the previously mentioned
properties of anticancer drugs and synthesize new platinum(II)
and palladium(II) complexes of the benzimidazole ligands (L1,2
)
(Fig. 1) that could be proved to be potent antitumor agents through
characterization and elucidation of their structures using spectro-
scopic, thermal and conductance measurements. Density functional
theory (DFT) calculations were done in order to correlate between
the theoretical and experimental results.
2. Experimental
2.1. Synthesis of ligands (L1,2) and their complexes
All chemicals used in the preparation and investigation of the
present study were of reagent grade (Sigma, Merk). The precursor
compound 2-chloromethylbenzimidazole was prepared and
⇑
Corresponding author. Fax: +20 2 35728843.
E-mail address: noureta2002@yahoo.com (N.T. Abdel Ghani).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.014

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
109
Fig. 1. The optimized structures of the benzimidazoles L1 and L2.
recrystallized according to the previously reported methods [9].
The ligands L1,2 were prepared by condensation of equimolar quan-
tities of 2-chloromethylbenzimidazole with 4-methoxylaniline (L1)
and 4-methylaniline (L2). The reaction mixture was refluxed in eth-
anol in presence of small amount of sodium iodide for about 18–
24 h. At the end of the reaction period, the mixture was neutralized
and the solid was separated by dilution with de-ionized water, and
recrystallized from xylene and ethanol, respectively.
One millimole K2PdCl4 was prepared by dissolving 0.177 g of
PdCl2 in 0.149 g/50 mL aqueous KCl solution. The solid metal com-
plexes of the benzimidazole compounds (L1,2) with Pd(II), and Pt(II)
metal ions were prepared by adding a hot ethanolic solution
(60 °C) of the ligand (1 mmol) to a hot aqueous solution (60 °C)
of the metal ions (1 mmol; K2PdCl4, or K2PtCl4). The resulting mix-
tures were stirred under reflux for about 1–2 h, whereupon the
complexes, Pd–L1 (1), Pt–L1 (2), Pd–L2 (3), and Pt–L2 (4) were
precipitated.
evaluated at the optimized geometry [13] using the same basis
sets. Vibrational modes were analyzed using GAUSSVIEW software
[14]. The main reason in choosing the LANL2DZ basis set is its
inclusion of relativistic effect that is essential for heavy elements
e.g. Pd(II) and Pt(II), in order to compare between the optimized
structures of the ligands and their complexes. The 1H NMR chem-
ical shifts of the benzimidazoles (L1,2) were computed at the
B3LYP/6-311 + G(2d,p) and B3LYP/LANL2DZ levels of theory in
the gaseous state by applying the (GIAO) approach [15] and the
values for the 1H-isotropic were referenced to TMS, which was cal-
culated at the same level of theory. The optimized structures,
vibrational frequencies, 1H-chemical shifts, and the natural bond
(NBO) analysis of the metal complexes were obtained at B3LYP/
LANL2DZ level of theory.
2.4. Biological activity
2.4.1. Antimicrobial activity
2.2. Instruments
The antimicrobial activities of the test samples were deter-
mined using a modified Kirby-Bauer disc diffusion method [16] un-
der standard conditions using Mueller–Hinton agar medium, as
described by NCCLS [17]. The antimicrobial activities were carried
out using culture of Bacillus subtilis, Staphylococcus aureus,
Streptococcus faecalis as Gram-positive bacteria and Escherichia coil,
Pseudomonas aeruginosa, Neisseria gonorrhea as Gram-negative
Infrared spectra of the ligands and their complexes were re-
corded as potassium bromide disc using FTIR-460 plus, JASCO, Ja-
pan, in 4000–200 cmꢁ1 region. The 1H NMR spectra were run at
300 MHz using deuterated dimethylsulphoxide (DMSO-d6) as a
solvent and tetramethylsilane (TMS) as a reference using Varian-
Oxford Mercury VX-300 NMR. Deuterium oxide (D2O) was used to
confirm the presence of ionizable protons. The mass spectra mea-
surements were recorded with the aid of SHIMADZU QP-2010 plus
mass spectrometer at 70 eV. The thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) were carried out by
DTG-60H SIMULTANEOUS DTA-TG APPARATUS-SHIMADZU instru-
ment in a dynamic nitrogen atmosphere (20 mL minꢁ1), with a
heating rate of 10 °C minꢁ1 using platinum crucible. Digital Jenway
4330 Conductivity-pH meter with (1.02) cell constant was used for
pH and molar conductance measurements. Spectrophotometric
measurements were carried out using automated spectrophotom-
eter UV/vis. SHIMADZU Lambda 4B using 1 cm matched quartz cells.
The X-ray powder diffraction patterns of the benzimidazole L2 and
its Pd(II) and Pt(II) complexes were recorded over 2h = 5–60° range
using Philips X-ray diffractometer model PW 1840. Radiation was
bacteria. Briefly, 100
of fresh media until they reached a count of approximately
108 cells/mL. A 100 L of microbial suspension was spread onto
lL of the test bacteria were grown in 10 mL
l
agar plates corresponding to the broth in which they were main-
tained. DMSO (0.1 mL) alone was used as control under the same
conditions for each microorganism, subtracting the diameter of
inhibition zone resulting with DMSO, from that obtained in each
case. The results were compared with a similar run of Tetracycline
as an antibacterial. The antimicrobial activities could be calculated
as a mean of three replicates.
2.4.2. Cell culture and cytotoxicity determination
Three human cancer cell lines were used for in vitro screening
experiments; breast cancer (MCF7),Colon Carcinoma (HCT) and human
heptacellular carcinoma (Hep-G2). They were obtained frozen in
liquid nitrogen (ꢁ180 °C) from the American Type Culture
Collection. The tumor cell lines were maintained in the National
Cancer Institute, Cairo, Egypt, by serial sub-culturing. Cell culture
cytotoxicity assays were carried out as described previously [18].
RPMI-1640 medium was used for culturing and maintenance of
the human tumor cell lines [18]. Cells were seeded in 96-well
provided by copper anode (Ka, k = 1.54056 Å) operated at 40 kV
and 25 MA. Divergence slit and the receiving slit were 1 and
0.05, respectively.
2.3. Theoretical calculations
The molecular structures of the benzimidazoles (L1,2) in the
ground state were optimized by a DFT method using B3LYP func-
tional [10,11] combined with 6-31G(d) and LANL2DZ basis sets.
Calculations were carried out by GAUSSIAN 03 [12] suite of pro-
grams run on a PC workstation equipped with Pentium IV 3 GHz
processor and 1 GB RAM memory. The vibrational frequencies of
the benzimidazoles and the corresponding normal modes were
plates at
a
concentration of 5 ꢂ 104–105 cell/well in a fresh
medium and left to attach to the plates for 24 h. Growth inhibition
of cells was calculated spectrophotometrically using a standard
method with the protein-binding dye sulforhodamine B (SRB)
[19]. The results were compared with a similar run of cis-platin
as an antitumor compound.

110
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Table 1
Band assignment of experimental and theoretical FT-IR spectra of benzimidazole L1.
No
Exp. freq
Calculated un-scaled
frequency (B3LYP)
Scaled frequency
uniform scaling
Scaled frequency
linear regression scaling
Vibrational assignments
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
19
20
21
22
3649
3554
3222
3218
3213
3208
3197
3196
3187
3178
3150
3069
3015
3001
2934
1687
1683
1643
1638
1594
1573
3681
3616
3231
3238
3226
3224
3206
3209
3196
3188
3177
3098
3018
2975
2957
1682
1675
1635
1626
1581
1561
3514
3423
3103
3099
3095
3089
3079
3078
3069
3060
3033
2956
2903
2890
2826
1625
1621
1582
1577
1535
1515
3570
3507
3134
3140
3129
3127
3109
3112
3100
3092
3081
3005
2927
2885
2868
1631
1624
1585
1577
1533
1514
3538
3446
3124
3120
3115
3111
3100
3099
3090
3081
3054
2976
2923
2910
2845
1635
1631
1593
1588
1545
1525
3504
3442
3075
3082
3071
3069
3052
3054
3042
3034
3024
2949
2873
2832
2814
1600
1594
1556
1547
1504
1485
m
m
m
m
m
m
m
m
m
m
m
m
m
NHss/Bz
NHss/An
CHss/An
CHss/Bz
CHss/An
CHass/Bz
CHass/An
CHass/Bz
CHass/Bz
CHass/An
CHa3ss
3428
3048
2999
2937
CHa3ss
CHs3s
CHa2ss
CHs2s
2832
1680
1617
1592
m
m
m
m
m
CC/An
CC/Bz +
CC/Bz
m C@N/Bz
CC/An + b NHsc/An
C@N/Bz + b NHsc/An + b NHsc/Bz + dsCH2
CC=An þ CHo3pb þ dsCH2
1518
m
23
24
27
1548
1544
1537
1530
1538
1521
1491
1487
1480
1484
1491
1475
1500
1497
1490
1456
1463
1447
b NHsc/An + dsCH2
dsCH2
CHo3pb
CHo3pb þ dsCH2
þ m CC=Bz
28
29
30
31
1535
1520
1501
1513
1512
1485
1480
1478
1464
1446
1442
1467
1466
1440
1435
1488
1473
1455
1439
1438
1413
1408
CHi3pb
1456
1418
CHs3b
1497
1046
1474
1456
1426
1352
1349
1334
1317
1301
1281
1259
1247
1218
1451
1014
1429
1411
1382
1310
1307
1293
1276
1261
1242
1220
1209
1180
m CC/Bz (boat shape)
32
36
1437
1431
1419
1402
1373
1303
1299
1285
1268
1253
1234
1213
1201
1173
1393
1388
1367
1361
b CC/Bz + b CC/An +
x
CH2
m
CC/Bz + x
CH2 + b NHsc/An + b NHsc/Bz + b CH/Bz + b CH/An
37
40
41
42
44
45
46
47
48
1308
1340
1375
1352
1329
1257
1333
1311
1289
1219
1308
1286
1264
1196
m
CC/An + b CH/Bz
b CH/An
b CH/Bz + b CH/An + b NHsc/An
1268
1232
m
CAO/An + b CH/An +
b CH/Bz +
CH2 + b NHsc/An
CAO/An + bCH/An + b NHsc/An
x
CH2 + b CH/Bz + b NHsc/An
s
m
1226
1248
1193
1189
1210
1157
1166
1155
1135
s
s
CH2 + b NH/Bz + b CH/Bz
CH2
CHo3pr þ bCH=Bz
1179
49
50
51
1207
1212
1187
1162
1167
1143
1170
1175
1150
b NH/Bz + b CH/Bz + b CH/An
b CHss/An
CHi3pr
1150
1167
1115
1131
1094
1110
52
53
1183
1162
1137
1086
1038
675
1139
1120
1095
1046
1000
1146
1126
1102
1052
1006
653
b CH/Bz
1124
1096
1031
m
CH2ANH/An + b CH/An
54
55
m
CAOACH3
1053
1021
1001
qCH2
318
1025
676
1017
657
1035
1009
908
982
896
799
786
655
986
307
993
642
967
56
57
987
985
Rtorsion/An
Rtorsion/Bz
Rtrigd/Bz
1023
1004
978
880
952
869
775
762
991
984
959
863
934
852
760
747
58
59
912
1143
973
930
861
776
757
589
929
917
828
807
877
1101
937
896
830
748
729
567
896
883
798
777
883
1108
942
901
834
752
733
570
900
888
802
782
938
820
749
c
CH/Bz
716
900
60
980
960
849
832
739
761
871
950
931
823
807
716
738
844
932
913
807
791
702
723
828
c
CH/An
61
62
787
658
758
634
762
637
Para deformation of aniline ring
Rtrigd/An

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
111
Table 1 (continued)
No
Exp. freq
Calculated un-scaled
frequency (B3LYP)
Scaled frequency
uniform scaling
Scaled frequency
linear regression scaling
Vibrational assignments
Rtorsion/Bz
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
63
64
65
66
617
432
632
615
437
258
634
514
592
243
608
593
421
249
614
498
574
235
612
595
422
249
602
488
562
230
c
c
NHanilino
NHBz
s
CH3
For 6-31G(d) basis set, the slope is equal 1.0310 and the linear coefficient is 0.9982.
For LANL2DZ basis set, the slope is equal 1.0500 and the linear coefficient is 0.9990.
R: ring; ss: symmetric stretching; ass: asymmetric stretching;
m
, stretching; b, in-plane bending;
c
, out-of-plane bending;
q
, rocking;
x
, wagging;
s
, torsion; trig: trigonal; trigd:
trigonal deformation. ops: out-of-plane stretching; sb: symmetric bending; ipb: in-plane-bending; opb: out-of-plane bending; ipr: in-plane-rocking; opr: out-of-plane rocking.
The stretching vibrational mode of C@N bond in benzimidazole
L1 is in agreement with those of Mohan and Sundaraganesan [26]
and Sundaraganesan et al. [27]. The band at 1680 cmꢁ1 is assigned
3. Results and discussion
3.1. IR spectral studies
to t(C@N) and is in a good agreement with the un-scaled calcu-
lated mode, 1687 cmꢁ1. It is possible to notice that in the latter
range, the scaling is not necessary, as already pointed by Agatha-
bay et al. [28] and Miranda et al. [29]. However, in benzimidazole
L2, this band was overlapped with the aromatic C@C bands in the
same region under the effect of the intermolecular hydrogen bond.
The theoretical IR spectra of the benzimidazole derivatives un-
der study (L1,2) were obtained at DFT/B3LYP level of theory using
the 6-31G(d) and LANL2DZ basis sets. All the band assignments
are presented in Tables 1 and 2. At this level, the calculated har-
monic force constants and frequencies are higher than the corre-
sponding experimental values, due to basis set truncation and
neglecting of electron correlation and mechanical anharmonicity
[20]. To compensate these shortcomings, scale factors were intro-
duced and an explanation of this approach was discussed [21].
Two different methods were applied: (i) uniform scaling [21], the
scaling factors are 0.963 for B3LYP/6-31G(d) and 0.970 for
B3LYP/LANL2DZ methods (ii) linear regression method [22], in this
method, the plot of the calculated frequencies versus their experi-
mental values resulted in a straight line, whose equation was used
The theoretical value at 1684 cmꢁ1 is assigned to
t
(C@N). In Pt–L1
(C@N) is shifted to lower frequency in compara-
complex (2), the
t
ble with that found in the uncoordinated ligand (Table 3). For Pt–L2
complex (4), the C@N group is liberated from the effect of hydro-
gen bonding with increasing of double bond character and was
observed at 1665 cmꢁ1. For Pd–L1,2 complexes, this band is over-
lapped with the aromatic C@C bands in the same region and it is
difficult to assign this vibration mode experimentally. The decrease
of m(NH) and m(C@N) vibration modes in the imidazole ring indi-
cated the participation of benzimidazole moiety in coordination
sphere through the pyridine-type nitrogen.
to correct the calculated frequencies (mcalc).
Benzimidazole derivatives (L1,2) have a strong intermolecular
hydrogen bond [3]; between the pyridine-type nitrogen, (N3) and
the benzimidazolic NH group; in the solid state, which makes the
IR spectra show strong and broad absorption band in the region
3500–2200 cmꢁ1. It is not surprising to find this effect since benz-
imidazole and imidazole derivatives, possessing free imino hydro-
gen, are known to be capable of associating through hydrogen bond
formation [23]. By using B3LYP/6-31G(d) method, the scaled calcu-
The low-frequency vibrational data of the prepared complexes
give an insight into the structure and bonding in the solid state.
Unfortunately, the metal-nitrogen stretching bands could not be
distinguished from other ring skeleton vibrations present in the
same region [3]. However, the theoretical assignments of these
bands are easily assigned by the visualization of the normal mode
displacement vectors utilizing the GAUSSVIEW program as tabu-
lated in Table 3. The far-IR spectra of Pd–L1,2 complexes showed
two medium bands at 369 and 363 cmꢁ1; which were absent in
lated value at 3514 cmꢁ1 is assigned to the benzimidazolic NH (NHBz
stretching vibration in the benzimidazoles (L1,2). For Pt–L1, Pd–L2 and
)
Pt–L2 complexes (2–4), the
t(NH)Bz bands are observed at 3225, 3227
the free ligands; due to the m(PdACl) in a cis-square planar struc-
and 3243 cmꢁ1, respectively, and are sharper than found in the unco-
ordinated ligand [3,24] due to the breaking of the intermolecular
ture [24]. Similarly, the platinum complexes showed two bands
at 372 and 360 cmꢁ1 assign to the PtACl bonds in cis-square-
planar geometry [24]. The asymmetric and symmetric stretching
modes as well as the scissoring bending mode of MACl bonds are
presented in Table 3. The IR spectra of complexes (1,2,4) showed
a very broad band centered at 3400 cmꢁ1 associated by librational
modes of water [30] as presented in Table 3.
hydrogen bond. In Pd–L1 complex (1), the
t(NH)Bz vibration mode
is embedded by the hydrogen bond effect generated by water of
hydration. The theoretically scaled stretching mode of NHBz group
in the benzimidazoles (L1,2) and their complexes is found in the same
position near 3570 cmꢁ1 (Table 3). This reveals that the NHBz group
remains intact in the complexes as found experimentally.
Theoretically, the benzimidazoles L1,2 give rise to eight CAH
aromatic stretching mode of vibration corresponding to the pres-
ence of eight aromatic CAH bonds as shown in Tables 1 and 2.
For assignments of CH3 group frequencies theoretically, nine fun-
damentals can be associated to each CH3 group [31], namely,
On the other hand, the presence of sharp bands at 3428 and
3434 cmꢁ1, respectively, in the spectra of the free ligands (L1,2
indicated the existence of free secondary amino group (NHsec
)
)
and these values coincide with the theoretically scaled modes,
3423 and 3428 cmꢁ1, respectively. These bands are still observed
in the metal complexes, but they become broad and/or slightly
shifted to lower frequency as shown in Table 3. This offers a proof
that the NHsec proton is not deprotonated [25] in the complex for-
mation, whereas such shift and broadness reflect its involvement
in coordination to the metal ions. This is confirmed theoretically
by observing the computed stretching mode for the NHsec group
at lower wavenumbers than that of the free ligand.
CHa3ss
:
asymmetric stretch (i.e. in-plane hydrogen stretching
modes); CHs3s: symmetric stretch; CHi3pb: in-plane-bending (i.e.
hydrogen deformation modes); CHs3b: symmetric bending; CHi3pr
:
in-plane rocking; CH3opr: out-of-plane rocking and
sCH3: twisting
hydrogen bending modes. In addition to that, CHo3ps: out-of-plane
stretch and CHo3pb: out-of-plane bending modes of the CH3 group
would be expected to be depolarized for A00 symmetry species.
For L1 derivative, the bands observed at 3033, 2956, and

112
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Table 2
Band assignment of experimental and theoretical FT-IR spectra of benzimidazole L2.
No
Exp. freq.
Calculated un-scaled
frequency (B3LYP)
Scaled frequency
uniform scaling
Scaled frequency
linear regression scaling
Vibrational assignments
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
3649
3560
3218
3208
3203
3197
3187
3184
3172
3166
3116
3083
3034
3000
2939
1684
3682
3615
3238
3224
3214
3209
3196
3198
3180
3172
3127
3095
3027
2979
2960
1675
1678
1635
1624
1583
1568
3514
3428
3099
3089
3084
3079
3069
3066
3055
3049
3001
2969
2922
2889
2830
1622
3571
3506
3140
3127
3117
3112
3100
3102
3084
3076
3033
3002
2936
2889
2871
1624
1627
1585
1575
1535
1520
3499
3414
3086
3076
3071
3066
3056
3053
3042
3036
2988
2956
2909
2877
2818
1614
3506
3442
3083
3070
3060
3056
3043
3045
3028
3020
2977
2947
2882
2837
2818
1594
1597
1556
1546
1507
1492
m
m
m
m
m
m
m
m
m
m
m
m
m
NHss/Bz
NHss/An
CHss/Bz
CHass/Bz
CHss/An
CHass/Bz
CHass/Bz
CHss/An
CHass/An
CHass/An
CHa3ss
3434
3051
2908
1614
CHa3ss
CHs3s
CHa2ss
CHs2s
m
m
m
m
m
m
CC/An +
CC/An
m CC/Bz + m
CN/Bz + b NHsc/Bz
1642
1635
1595
1576
1547
1581
1575
1536
1518
1581
1574
1567
1529
1511
1479
CC/Bz + b NHsc/Bz
CC/An + b NHsc/An
CC/Bz + b NH/Bz + b NH/An + dsCH2
CC/An + b NH/An + dsCH2
+ m C@N/Bz
1519
bNH=An þ CHo3pb
23
24
25
1543
1535
1523
1538
1533
1486
1478
1467
1491
1487
1479
1471
1460
1464
1459
dsCH2
dsCH2 + m CAN/Bz + m CC/An
bNH=An þ CHo3pb
1517
1516
1471
1470
1444
1443
CHi3pb
26
1515
1459
1452
27
28
29
1497
1471
1453
1480
1442
1417
1399
1435
1435
1410
1393
1408
m
m
CHs3b
CHs3b
m
m
CC/Bz (boat shape)
CC/An + b CH/An + b CH/Bz +
1421
m CANH/Bz + xCH2
1454
1446
1410
1402
1384
1376
þ xCH2 þ bCH=An
30
1445
1425
1392
1372
1385
1366
31
32
33
34
35
36
37
38
39
40
41
42
1434
1382
1360
1325
1290
1250
1390
1340
1319
1285
1251
1212
1365
1315
1294
1261
1227
1189
CC/Bz +
xCH2 + b NH/Bz + b NH/An + b CH/Bz
CC/An + b NH/An
1306
1264
1226
1355
1332
1305
1283
1299
1277
b CH/An
b CHss,sc/Bz + b CHss,sc/An + b NH/An +
b CH/Bz + b NH/An
xCH2
m
CCH3/An
1259
1247
1244
1221
1207
1212
1201
1198
1176
1162
1207
1195
1192
1170
1157
sCH2 + b CH/Bz + b NH/Bz
1248
1210
1187
sCH2
m
CCH3/An + b CH/An
b CHss,sc/An
1224
1229
1223
1193
1170
1169
1145
1139
1084
1187
1192
1186
1157
1134
1133
1110
1104
1051
1165
1169
1164
1135
1113
1112
1089
1084
1031
b CH/An + b CH/Bz + b NH/Bz
b NH/Bz + bCHss,sc/An
43
44
45
1183
1166
1144
1139
1123
1102
1134
1118
1096
b CHss,sc/Bz
m
m
1121
CH2ANH/An + bCHss,sc/An
CH2ANH/An + b CHss,sc/An + b CHss,sc/Bz
46
47
b CHss,sc/Bz
CHo3pr
1076
1036
1031
48
49
50
1035
1031
1050
675
1008
1015
1003
1000
1018
654
977
984
985
981
999
642
959
966
Rtorsion/Bz
Rtorsion/An + qCH2
1032
1039
674
994
1001
649
989
996
646
qCH2
51
52
Rtorsion/Bz
CHi3pr
1016
978
974
53
742
974
930
861
776
757
589
946
932
827
814
912
885
789
659
1024
982
896
786
938
896
829
747
729
567
911
897
796
784
878
852
760
635
993
952
869
762
933
891
825
743
725
564
906
893
792
780
874
848
756
631
974
934
852
747
c
c
CH/Bz
CH/An
899
806
997
852
840
967
826
814
948
810
799
54
55
56
57
58
908
873
782
660
880
846.
758
640
864
830
744
627
Rtrigd/Bz
Rtorsion/An
Para deformation of aniline ring
Rtrigd/An

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
113
Table 2 (continued)
No
Exp. freq.
Calculated un-scaled
frequency (B3LYP)
Scaled frequency
uniform scaling
Scaled frequency
linear regression scaling
Vibrational assignments
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
6-31G(d)
LANL2DZ
59
60
61
62
631
597
589
323
635
522
595
337
608
575
567
311
615
506
577
326
604
572
564
309
603
496
565
320
Rtrigd/Bz
NH/An
NH/Bz
586
s
CH3
For 6-31G(d) basis set, the slope is equal 1.0425 and the linear coefficient is 0.9997.
For LANL2DZ basis set, the slope is equal 1.0497 and the linear coefficient is 0.998.
Table 3
IR band assignment of Pd(II) and Pt(II) complexes with benzimidazoles L1,2 calculated at the B3LYP/LANL2DZ.
Calculated/(observed)
Band assignment
[PdL1Cl2]ꢃ3H2O
[PtL1Cl2]ꢃH2O
3572 (3225)
3361 (3427)
1595 (1449)
1588
1526 (1636)
1440 (1332)
1183
[PdL2Cl2]
[PtL2Cl2]ꢃ2H2O
3573 (3243)
3357 (3430)
1597 (1451)
1588
1526 (1665)
1441 (1384)
1183 (1284)
631
3571
3375 (3433)
1593 (1444)
1587
1524
1430
1249
629
586
478
333
324
3572 (3227)
3376 (3411)
1595 (1446)
1588
1524
1429
1183 (1281)
631
586
466
274
m
m
m
m
m
m
m
m
c
m
m
m
m
m
NHss/Bz
NHss/An
CC/An + b NHsc/An
CC/Bz + b NHsc/Bz
C@N/Bz
CAN/Bz
CAN/An
MANpy
NH/Bz
MANHsec
631
568
536
570
531
MACl23 trans to NHsec
MACl22 trans to Npy
ClAMAClss
ClAMAClass
323
320
314
113
276
331
322
127
304
285
126
323
317
114
b ClAMAClsc
(658w
q
w(OH2), 527w
qt(OH2))
(533w
q
t(OH2))
(697w
q
w(OH2), 525w
qt(OH2))
Librational modes of water
Bz: benzimidazole, sec: secondary amino group, py: pyridine, others below Table 1.
2903 cmꢁ1 are ascribed to CH3 asymmetric and symmetric stretch-
ing vibration as calculated using the basis set 6-31G(d). Infrared
bands established at 1480, 1464, 1446, 1173, 1143 and 249 cmꢁ1
behavior can be avoided by assigning a local gauge origin to each
basis function, which is known as a ‘‘gauge-independent atomic
orbital’’ (GIAO) [15]. In the present study, the 1H NMR calculations
were performed using GIAO method using 6-311 + G(2d,p) and
LANL2DZ basis sets. The 1H shielding was converted into the pre-
dicted chemical shifts using dH-TMS values 31.88 and 32.77 ppm
for 6-311 + G(2d,p) and LANL2DZ basis sets, respectively. The
experimental and theoretical 1H-chemical shifts d(ppm) of the
benzimidazoles and their complexes are presented in Table 4.
The spectral assignments of the complexes were based on com-
parison with the shapes and positions in the corresponding spec-
trum of the uncoordinated ligand. A comparison indicated a
positive coordination-induced shift (c.i.s. = dcomplex ꢁ dligand),
implying that decomposition has not occurred in solution [32]
and there is a considerable deshielding effect upon coordination.
Almost all signals are shifted upon complexation, as a result of
are attributed to CHo3pb; CH3ipb; CHs3b, CHo3pr; CH3ipr, and
sCH3 vibra-
tion mode, respectively. For benzimidazole (L2), the IR bands ob-
served at 3001, 2969; 2922, 1467, 1459, 1392, 1036, 978 and
311 cmꢁ1 are unambiguously assigned to CH3ass; CH3ss; CHo3pb
,
CHi3pb; CHs3b; CHo3pr; CH3ipr, and
Other vibration modes are shown in Tables 1 and 2.
sCH3 vibration mode, respectively.
Any discrepancy noted between the observed and the calcu-
lated frequencies may be justified on the basis that the theoretical
calculations were carried out on a single molecule in the gaseous
state contrary to the experimental values recorded in the presence
of intermolecular interactions. The difference in vibrational fre-
quencies calculated by the two different basis sets may be partially
explained by the electron-correlation effect. The RMS error of the
frequencies between the un-scaled and experimentally observed
in case of benzimidazole L2 for example was found to be
73 cmꢁ1. After scaling, the RMS error are found to be 8 and
25 cmꢁ1 for 6-31G(d) and LANL2DZ basis sets, respectively, sug-
gesting that the 6-31G(d) basis set gives more accurate results
which is in agreement with the previously reported work [28,29].
the electric field effect caused by complexation,
p-bonding and
temperature-independent paramagnetism of the palladium and
platinum metal ion [32].
The ligands L1,2 showed broad singlet signals at 12.23 and
12.21 ppm [33], respectively, due to the benzimidazolic NH proton.
The large downfield shift of this proton may be attributed to the
presence of intermolecular hydrogen bond as previously men-
tioned. In complexes, this proton is observed at 13.14 ppm for
Pd–L1,2 and 13.29 and 13.35 ppm for Pt–L1,2, respectively, which
can be related to the charge density change in the benzimidazole
ring supporting that coordination occurs via the pyridine-type
nitrogen and causing inhibition of the fluxional behavior of the
imine ring [34]. The protons of the CH2 groups in benzimidazoles
L1,2 are deshielded by the presence of the secondary amino group
3.2. 1H NMR studies
The theoretical prediction of NMR shieldings is important in the
characterization of molecular electronic structure. Calculations of
magnetic properties inherently suffer from a ‘‘gauge origin’’ prob-
lem, which simply means that the results depend on the choice
of the origin of the coordinate system. This clearly non-physical

114
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Table 4
The experimental and theoretical 1H NMR chemical shifts d (ppm) from TMS for the studied compounds.
L1
Pd–L1
Exp.
Pt–L1
Exp.
L2
Pd–L2
Exp.
Pt–L2
Exp.
Assignment
Exp.
Calcd
(a)
Calcd
(b)
Calcd
(b)
Exp.
Calcd
(a)
Calcd (b)
Calcd
(b)
(b)
(b)
12.23 8.13 7.50 13.14
6.89 7.08 7.34
6.86 7.06 6.96
6.57 6.98 6.69
6.55 6.49 6.47 6.28–
8.80
11.17
6.83
6.75
5.23
5.22
13.29
9.86
6.38
5.71
5.66
5.56
12.21 8.13 7.50 13.14
6.88 7.48 7.48
6.85 7.19 7.25
6.56 7.14 7.08
6.53 6.25 6.37 6.44–
8.60
11.16
6.62
6.08
5.17
5.16
13.35
9.87
6.76
5.90
5.66
5.56
(1H), Benzimidazolic NH
(4H), aniline ring protons
6.54–
8.57
6.34–
8.09
7.52 8.13 8.11
8.26
8.72
7.51 8.14 8.13
8.17
7.98
8.72
(4H), Benzimidazole ring
protons
7.47 7.61 7.66
7.12 7.57 7.59
7.10 7.55 7.40
5.80 5.16 5.29
8.25
7.59
6.86
3.79
8.02
7.31
7.14
6.86
7.46 7.61 7.67
7.13 7.58 7.60
7.09 7.56 7.40
6.00 5.36 5.42 7.46
8.02
7.31
7.13
6.85
7.66 7.42
4.00
7.07
(1H), secondary amino
proton
4.41 4.40 4.12 5.00
4.29
3.77
1.93
4.75
5.06
3.72
4.61
3.12
2.45
4.42 4.41 4.07 4.93
5.27
2.10 2.32 2.29 2.14
4.27
3.84
0.87
4.70
4.92
2.13
4.63
3.10
0.47
(2H), CH2
3.61 3.78 3.55 3.75
(3H), CH3
(a) 6-311+(2d,p) (b) LANL2DZ.
Table 5
Kinetic parameters (Eꢀ,
D D D
Hꢀ, Gꢀ; kJ/mol), A (sꢁ1) and Sꢀ (J/K mol); determined using Coats–Redfern method and Horowitz–Metzger methods of the metal complexes under
study PtL1 and PdL2 (selected).
Compound
Decomp. range
Coats–Redfern
Horowitz–Metzger
Eꢀ
A
D
Sꢀ
D
Hꢀ
D
Gꢀ
Eꢀ
A
D
Sꢀ
D
Hꢀ
D
Gꢀ
[PtL1Cl2]ꢃH2O
31–97
97–394
394–500
25
19
119
1 ꢂ 105
2 ꢂ 105
4 ꢂ 105
ꢁ148
ꢁ149
ꢁ145
ꢁ150
ꢁ106
22
15
113
73
96
213
25
22
112
1 ꢂ 105
1 ꢂ 106
2 ꢂ 105
ꢁ148
ꢁ132
ꢁ151
ꢁ151
ꢁ95
22
17
106
73
91
210
[PdL2Cl2]
225–340
340–507
92
230
2 ꢂ 105
87
224
169
295
90
247
2 ꢂ 105
85
241
169
305
4 ꢂ 107
1 ꢂ 108
with appearance of doublet signal at 4.41 ppm. This signal shifts to
higher frequencies in the studied complexes, indicates that the sec-
ondary amino group is also coordinated to the metal centre. As
shown in Table 4, the methylene resonance appears as a pair of
quartet at 4.70–5.27 ppm for all the studied complexes. A quartet
would be expected due to that the methylene CH2 geminal protons
are no longer isochronous. This can be attributed to differences in
polarization of the CAH bonds between the axial and equatorial
CH2 protons in the complexes. This polarization; enhanced in
DMSO; results in a deshielding of the equatorial protons, which
point away from the metal ion [35]. Benzimidazoles (L1,2) dis-
played additional singlet signals at d 3.60 and 2.10 ppm, with inte-
gration corresponding to three protons of the substituents,
methoxyl and methyl groups in the aromatic ring, respectively.
Ligands L1,2 exhibited signals for the secondary amino NH pro-
tons at 5.80 and 6.00 ppm, respectively, that obscured by the aro-
matic signals and disappear on deuteration. This proton is
deshielded due to the anisotropy of the ring and the resonance that
removes electron density from nitrogen and changes its hybridiza-
tion. In complexes, this proton moves downfield with respect to its
position in the free ligands as a result of the participation of the
NHsec group in the coordination sphere and still obscured by the
aromatic protons (Fig. 2). The calculations from the integration
curves of aromatic signals indicated the presence of eight protons
ascribes to the aromatic protons and one proton is suggested to the
proton of NHsec group.
6.89 ppm are assigned to aromatic protons in the ortho-position
with respect to methoxyl group. In addition, the benzimidazole
protons are much broader due to the expected cross-ring cou-
plings, which are characteristic of such systems [36] at 7.10,
7.12, 7.47 and 7.52 ppm. Similar, L2 has the same assignments for
the aromatic protons as L1 at 6.53, 6.56, 6.85, 6.88 and 7.09, 7.13,
7.46, 7.51, respectively. In the metal complexes, the aromatic pro-
tons nearest the pyridine-type nitrogen and the secondary amino
group were found to suffer maximum downfield shifts in compar-
ison with the other aromatic protons confirming that ligands L1,2
interact with metal ions through the pyridine like nitrogen and
secondary amino group.
The calculated chemical shifts of the methylene, methyl, and
methoxyl groups in the studied benzimidazoles (L1,2) using 6-
311 + G(2d,p) and LANL2DZ basis sets are in a good agreement
with the experimental values as shown in Table 4. However, the
experimental chemical shifts of the other protons in the benzimi-
dazoles are slightly smaller than the calculated values and the larg-
est deviations are observed in the low field of the spectra. The
experimental chemical shift of the benzimidazolic NH proton is
shifted towards higher magnetic field than the calculated ones by
about 4 ppm, as previously reported by other authors [37]. This
may be due to neglect of the non-specific solute–solvent interac-
tions (in the gas phase), and the intermolecular hydrogen bond
in our calculations as compared with the experimental chemical
shifts that are obtained from the DMSO solutions (hydrogen-
bonded solvent). The NH protons form hydrogen bond with DMSO
molecules and move downfield. GIAO method showed significant
difference in chemical shifts between the hydrogen-bonding pro-
tons and non-hydrogen-bonding protons [37]. These differences
were 3.02 in the gaseous phase, 1.17 in DMSO, and 1.22 ppm in
The multi-signals of the aromatic protons appear as a complex
signal at d = 6.55–7.52 ppm in benzimidazoles L1,2. For benzimid-
azole L1, the protons of the aniline ring give rise to four-line pattern
at 6.55 and 6.57 ppm for protons in the ortho-position with respect
to secondary amino group whereas the protons at 6.86 and

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
115
Fig. 2. 1H NMR spectra of Pd–L2 complex in DMSO.
the aqueous phase as discussed by Miranda et al. [29]. The calcu-
lated chemical shifts for non inter- or intramolecular hydrogen-
bonding protons are far from the experimental values and the
inclusion of solvent is necessary in order to increase the chemical
shift by 1.50 ppm (in case of DMSO); as discussed by several
authors [29]; which shows the importance of the use of solvation
in shielding calculations.
by the removal of methoxyl group, two chlorine atoms and two
acetylene groups from the benzimidazole ring to give a fragment
at m/z 365. Thus, the participation of the pyridine-type nitrogen
in the coordination sphere withdraws the electron density from
the benzimidazole ring and introduces a second weakness point
through which the fragmentation can proceed with degradation
of benzimidazole ring to imidazole moiety. The latter fragment
eliminates the imidazolic NH group to offer a fragment at m/z 350.
The fragmentation pattern of Pd–L2 complex resembles that of
Pd–L1 with two decomposition routes. The first step results in
the formation of fragments at m/z 365 and 350 as found in Pd–L1
complex. The 2nd route involves either the loss of one chlorine
atom and p-toluidine moiety to give a peak at m/z 258, [Pd(benz-
imidazole)Cl]+ or benzimidazole ring to give a fragment at m/z
295. These two fragments confirmed the bidentate nature of this
ligand. For Pt–L2 complex, the fragment at m/z 450 is due to the
dissociation of methyl group and one chlorine atom from M+. In
addition, this complex shows the same fragments at m/z 365 and
350 as previously found in Pt–L1.
3.3. Mass spectrometry
The electron impact mass spectra of the ligands L1,2 and their
complexes were recorded and investigated at 70 eV. The benzimi-
dazoles (L1,2) have strong molecular ion peaks (M+) at m/z 253
and 237, respectively. These compounds undergo fragmentation
through cleavage of CH2ANH bond with the appearance of a frag-
ment at m/z 131; which assign to 2-methylene benzimidazole frag-
ment; and fragments at m/z 123 and 107 due to the formation of
para-anisidine and para-toluidine fragments, respectively. The
fragment at m/z 118 is assigned to the benzimidazole ring. For
benzimidazole L2, further fragmentation occurs by the elimination
of NHsec group from the most predominant fragment (para-tolui-
dine) forming a benzyl cation, which spontaneously rearranges to
form a tropylium ion. However, L1 derivative shows a fragment at
m/z 238 due to the loss of methyl group from M+, followed by the
removal of CO and one ionizable proton to give a peak at m/z 209.
The mass spectrum of Pd(II)–L1 shows three fragmentation
routes as shown in Scheme 1. The first route represents the loss
of methoxyl group to provide a peak at m/z 399; [Pd(L1AOCH3)Cl2];
followed by the removal of one chlorine atom to give the most
abundant peak at m/z 365; [Pd(L1AOCH3)Cl]. The latter fragment
eliminates the imidazolic NH group to offer a fragment at m/z
350;[Pd(L1AOCH3ANH)Cl]+. The 2nd dissociation process involves
the loss of CH3 group and one chlorine atom to supply a peak at
m/z 381, [Pd(L1ACH3)Cl]. The 3rd stage represents the elimination
of benzimidazole ring, and two chlorine atoms to give peak at m/z
243, followed by the dissociation of methoxyl group to afford a
peak at m/z 211. Several observed fragments confirmed the biden-
tate nature of the ligand L1 were observed.
3.4. Electronic absorption
The electronic spectra of L1,2 displayed five absorption bands in
ethanol. The first band at 204 nm may be assigned to the medium
energy
benzimidazole moieties [38]; (1LaA1A). While the second band at
223 (L1) and 222 (L2) nm is attributed to the low energy pꢀ elec-
pA
pꢀ transition within the phenyl rings of the aniline and
pA
tronic transition of the phenyl rings of both the aniline and benz-
imidazole moieties (1LbA1A) [38]. In the benzimidazole ring, three
kinds of transitions are possible: (i) nApꢀ, (ii)
pA
pꢀ, and (iii)
charge-transfer (CT). The nature of the transition depends upon
the type of solvent used and the nature of the substituents [39]. It
is well established that the nApꢀ transition is not observed in the
benzimidazole compounds, although the system has a lone pair of
electrons on the tertiary nitrogen atom [39,40]. Distinction be-
tween pA
pꢀ and charge-transfer bands can be made from the study
of the effects of solvents on the absorption spectra and changing of
the substituent (nature and position) on the benzene ring [38].
Therefore, the remaining bands at 242, 273 (L1) or 274 (L2) and
The mass spectrum of Pt–L1 complex has a weak molecular ion
peak at m/z 516 corresponding to M-2; (M = [Pt(L1)Cl2]+); followed
280 nm may be assigned to
pA
pꢀ transitions in the delocalized

116
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Cl
Cl
B
Pd
A
A
N
B
C
C
NH
O
CH3
N
H
Cl
Cl
Pd
m/z = 430 ( 0 %)
Pd
N
H
O
CH3
N
B
H2C
NH
Cl
N
m/z = 243 ( 14%)
H
Pd
N
NH
O
m/z = 399 ( 14 %)
N
H
Pd
Cl
N
m/z = 381 ( 11 %)
H2C
H
Pd
N
NH
m/z = 211 ( 12%)
Cl
Cl
N
H
HN
O
CH3
Pd
N
m/z = 365 ( 100 %)
H2C
NH
m/z = 122 ( 14%)
m/z = 190 ( 12 %)
Cl
Pd
Cl
Cl
N
NH
Pd
Pd
m/z = 105 ( 33 %)
N
m/z = 141 ( 12 %)
m/z = 155 ( 11 %)
m/z = 350 ( 58 %)
Scheme 1. Fragmentation pattern of Pd–L1 complex.
1.81
p
-electron system and in the heteroatomic groups inside the benz-
Pd-L1
Pt-L1
Pd-L2
Pt-L2
imidazole molecule [41]. The latter two bands; 273 (or 274) and
280 nm appear doubled like benzoic acid due to probable existence
of a tautomeric structure [42]. This phenomenon is supported by
comparing our spectra with the spectrum of 1-methyl-2-phenyl-
benzimidazole [39], where this fine structure is lost. On comparing
the absorption spectra of L with the parent 2-methylbenzimidazole
(280, 274 nm, Lb; 245 nm, La) [39], it can be concluded that the two
normal band systems are kept nearly intact except that La transition
is slightly blue shifted.
1.61
1.41
1.21
1.01
0.81
0.61
0.41
0.21
0.01
The electronic spectra of 10ꢁ4 M of the studied complexes were
scanned in DMF as shown in Fig. 3. The two bands between 270
and 280 nm (35714–37000 cmꢁ1) in Pd–L1,2 and Pt–L1,2 complexes
are assigned to the internal ligand transitions (
the benzimidazole ring). Both, the
pA
pꢀ transitions in
210
260
310
360
410
460
510
560
pꢀ transitions in the ligand
Wavelength, nm
pA
and the cutoff of the solvent prevent the observation of LMCT in
the UV region. By assuming a value of F2 = 10F4 = 600 cmꢁ1 for a
Slater–Condon interelectronic repulsion parameters [43], it is pos-
Fig. 3. Electronic absorption spectra of 10ꢁ4 M solutions of Pd–L1,2 and Pt–L1,2
complexes in DMF.
sible to derive the values of D1
allowed d-d transition. The first low energy spin allowed bands at
22,831 log
D1 = 24,931 cmꢁ1 max = 3.66) and 23,696 cmꢁ1
D1 = 25,796 cmꢁ1, log max = 364) in Pt–L1,2 complexes, respec-
; D1 = m1 + 3.5F2; from the first spin
located at 27,700 (log
emax = 3.48), 27,472 (log emax = 3.59) and
27,247 cmꢁ1 (log max = 3.25) in Pt–L2 and Pd–L1,2 complexes,
e
(
,
e
respectively, are assigned to this combination. Therefore, the elec-
tronic spectra of Pd–L1,2 and Pt–L1,2 metal complexes are indicative
of square-planar geometry [44].
(
e
tively, have been assigned to the transition b2gðdxyÞ ꢁ d1gðdx2ꢁy2 Þ,
i.e. A1g
!
1A2g 1). While, the band at 26,385 cmꢁ1 (log emax
(m =
1
3.55) in Pt–L1 complex is spin-allowed transition; a1gðdx2ꢁy2 Þꢁ
1
1
b1gðdx2ꢁy2 Þ, i.e. A1g ! B1g
(m2). Strong charge-transfer transitions
3.5. Molar conductance measurements
may interfere and prevent observation of all the expected bands
[44]. However, strong bands between 350 and 380 nm (28,000–
The molar conductance values of 10ꢁ3 M for Pd–L1,2 and Pt–L1,2
complexes in DMF revealed low conductance values 9.68, 17.42,
8.02 and 8.63 ohmꢁ1 cm2 moleꢁ1, respectively, this may be taken
26,500 cmꢁ1
)
are assignable to
a
combination of MLCT and
3). Thus, the bands
1
1
0
egðdyz ; Þ ꢁ b1gðd
Þ i.e. A1g ! Eg bands (
m
x2ꢁy2
dzx

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
117
180
160
140
120
100
80
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.0005
0.0000
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.002
0.000
250
200
150
100
50
TGA
DTG
DTG
-0.0005
-0.0010
-0.0015
-0.0020
-0.0025
-0.0030
-0.0035
-0.002
-0.004
-0.006
-0.008
-0.010
-0.012
2
Pd-L
60
DTA
40
DTA
20
TGA
0
0
1
Pt-L
-20
-50
200
400
600
Temp. oC
800
1000
1200
100
200
300
400
500
Temp. oC
Fig. 4. TGA, DTG and DTA curves of Pt–L1, and Pd–L2.
as an evidence for the presence of chlorine atoms inside the coor-
dination sphere of these complexes, indicating the non electrolytic
nature of these complexes.
ceeds with the formation of Pt metal, as final residue as no other
platinum compounds are present over 1200 ° C. This behavior
[47] reflects the high stability of this complex in comparable with
Pt–L1 complex. The rate of decomposition of most complexes is fast
in air than that in nitrogen [48]. Although the masses losses in air
in most metal complexes, [49] start at comparatively high temper-
atures, complete thermal degradation is achieved at lower temper-
atures than in nitrogen. This change in temperature seems to be
due to the oxidative nature of air, which facilitates the oxidative
decomposition [48,49]. In air, this complex shows two degradation
steps with overall mass loss 63.61%.
3.6. Thermal analyses and kinetics studies
The simultaneous TGA-DTA curves for Pd–L1complex (1) exhibit
three distinct decomposition stages maximized at 330, 395 and
817 °C. The first pronounced peak is accompanied by a mass loss,
which amounts to 25.94%. This value finds a parallelism with the
calculated value (25.80%) responsible for desorption of three water
molecules of hydration and chlorine gas. The 2nd and 3rd thermal
stages, bring the total mass loss up to 77.94% (calcd. 78.02%) of the
parent complex with degradation of one ligand molecule leaving
Pd metal as a final residue. It is clear from DTA curve that these
mass losses are accompanied by two endothermic peaks at 330
and 395 °C and one exothermic peak at 817 °C. The powder X-ray
diffraction pattern of the residue of Pd–L1 complex was recorded
directly after TGA and the results were compared with those that
are available at the international center for the diffraction data.
The XRD pattern shows major diffraction peaks at 2h = 40.1
(1 1 1) and 46.7 (2 0 0) as previously reported for the palladium
metal. In addition, the residue of the Pd–L1 complex exhibited
metallic luster confirming its metallic nature.
There has been increasing interest in determining rate-depen-
dent parameters of solid-state non-isothermal decomposition
reactions by analysis of TGA curves. Several equations [50–52]
have been proposed to analyze TGA curves and obtain values for
kinetic parameters. The kinetics parameters are calculated by using
Coats–Redfern [50] and Horowitz–Metzger methods [51] (Table 5).
According to the thermodynamic data obtained from the TGA
curves, it was found that the metal complexes have negative entro-
py, which indicates that the complexes are formed spontaneously
and are highly ordered in their activated states. For [PdL2Cl2] com-
plex, the activation energy values increases on going from one
decomposition stage to another for a given complex, indicating
that the rate of decomposition decreases in the same order. The
values of the free activation energy
for the subsequent decomposition stages of a given complex. This
is due to increasing the values of T
Sꢀ significantly from one-step
to another which override the values of
Hꢀ. This may be attrib-
D
Gꢀ increases significantly
For Pd–L2 complex (3), the degradation starts at 225 °C, with
two endothermic peaks maximized at 280 and 399 °C as shown
in Fig. 4. The 1st stage reveals the elimination of one chlorine atom
with mass loss amounts to 8.50% (calcd. 8.45%), while the 2nd
decomposition step is corresponding to the loss of one ligand mol-
ecule and another chlorine atom leaving Pd + C as a final residue.
This statement is in accordance with the results obtained for other
Pd(II) and Pt(II) complexes [45]. This behavior confirms that the
two chlorine atoms are not isoenergetically bound and thus their
elimination takes place via two different overlapping steps [46].
Examining the simultaneous TGA-DTA curves constructed for
Pt–L1 complex (2), Fig. 4, one can observe three mass loss stages
maximized at 73, 280 and 414 °C. The first two endothermic
decomposition steps are responsible for desorption of one
physically adsorbed water molecule, one chlorine atom and benz-
imidazole ring with a mass loss 30.89% (calcd. 30.81%). The 3rd
endo-thermic stage is accompanied by a mass loss of 29.46% (calcd.
29.46%) responsible for degradation of the rest of the organic part
and another chlorine atom leaving Pt + C as a final residue.
D
D
uted to the structural rigidity of the remaining part of the complex
after the expulsion of one of the coordinated anions or water mol-
ecules, as compared with the precedent complex, which require
more energy, T
structural change. The positive
D
Sꢀ for its rearrangement before undergoing any
D
Hꢀ values mean that the decom-
position processes are endothermic.
3.7. X-ray powder diffraction
Single crystals of the studied complexes could not be obtained,
because the studied complexes are amorphous in their nature, in
addition to their insolubility in most organic solvents except
DMF and DMSO. The X-ray powder diffraction patterns of the benz-
imidazole L2 and its Pd(II) and Pt(II) complexes as representative
examples were recorded over 2h = 5–60° in order to obtain an idea
about the lattice dynamics of these compounds. The comparison
between the obtained XRD patterns of L2 and its complexes
(Fig. 5), throw light on the fact that each complex represents a def-
inite compound with a distinct structure. This identification of the
Degradation of Pt–L2 complex (4) is incomplete up to 1200 °C
with two endothermic peaks at 280 and 414 °C; like Pt–L1; in nitro-
gen atmosphere. The overall mass loss amounts to 52.75% (calcd.
63.82%) assuming that the thermal decomposition process pro-

118
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Fig. 5. X-ray diffraction pattern of (a) benzimidazole L2, (b) Pd–L2 and (c) Pt–L2.
benzimidazole ring of compounds L1,2 are 1.310 and 1.377 Å for
Table 6
X-ray diffraction data of benzimidazole L2.
B3LYP/6-31G(d) as shown in Tables 7 and 8. This discrepancy be-
tween the C@N and CANH bond lengths confirm that the hydrogen
atom is fixed at one of the two nitrogen atoms through the previ-
ously mentioned intermolecular hydrogen bond. These values are
coincided with those found from the optimization of benzimid-
azole and 2-methylbenzimidazole [55] under the same level of the-
ory. In addition, these latter bonds have a shorter distance than any
amine compound, which is due to the participation of the lone
electron pair of nitrogen atom in resonance of benzimidazole rings
[55]. The most optimized geometry in L2 derivative reveals that,
the methyl group is in antiform, i.e. the hydrogen atoms in the
plane are in the anti-position with respect to the aniline ring. Be-
cause of the increased conjugation effect, there is strengthening
(and shortening) of the aryl-O bond C31AO30. These benzimid-
azole compounds show accumulation of the negative charge den-
sity on the pyridine-type nitrogen in the imidazole ring as shown
in Tables 7 and 8 and this negative charge facilitates intermolecu-
lar hydrogen bonding, which is a very important structural feature
related directly to the ability to bind the metal ions. Several calcu-
lated thermodynamic parameters are presented in Tables 7 and 8.
The dipole moment increases dramatically from a value of 2.296 D
(L1 derivative) to 3.386 D (L2 derivative), where the high electro-
negativity of CH3 group induces strong polarization both in
Angle (2h)
d-Value (Å)
Relative intensity (I/I°)%
7.21
8.71
12.25
10.14
8.42
5.13
4.87
4.45
4.19
4.11
3.89
3.69
3.56
3.44
3.16
2.97
2.82
2.46
2.06
55.6
56.9
100
51.6
96.4
54.2
50.3
86.0
87.7
73
92.9
76.2
59.7
38.2
46.5
20.7
4.4
10.49
17.27
18.16
19.92
21.15
21.59
22.80
24.08
24.93
25.84
28.20
30.05
31.69
36.36
44.01
complexes was done by the known method [53]. Such facts suggest
that the prepared complexes are amorphous. The X-ray powder
diffraction pattern of benzimidazole (L2) showed a higher degree
of crystallinity as the parent benzimidazole [54]. The values of
2h, interplanar spacing d (Å) and the relative intensities (I/I°) of
benzimidazole L2 were tabulated in Table 6. The data from this pat-
tern show three low-angle diffraction peaks (below 10.5°) that are
not observed in the parent benzimidazole with acceptable inten-
sity. Several peaks characterized to the benzimidazole moiety are
also observed at 17.27°, 18.17°, 19.92°, 31.69°, and 36.36°.
r
- and
p-frameworks of the aniline moiety than the OCH3 group.
The fully optimized geometries of cis-PdL1,2Cl2 and cis-PtL1,2Cl2
and numbering of atoms are shown in Fig. 6. Some selected geo-
metric parameters as calculated by B3LYP/LANL2DZ are listed in
Tables 9 and 10. According to the theoretical calculations, the low-
est energy structures of these complexes are of C1 symmetry. The
Pd(II) and Pt(II) ions are in square-planar geometry, in which the
metal ion is coordinated to a bidentate ligand through the pyri-
dine-type nitrogen of imidazole ring and the secondary amino
group, providing a five-membered chelate ring as well as two
chlorine atoms. The optimized structures of cis-PdL2Cl2 (2) and
3.8. Theoretical calculations
Full geometry optimizations were performed at the DFT level of
theory [13]. The optimized bond lengths of C@N and CANH in the

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
119
Fig. 6. The optimized structures of (a) Pd–L1 (b) Pt–L1 (c) Pd–L2 (d) Pt–L2 complexes.
cis-PtL2Cl2 (4) will be discussed in details as examples for the stud-
ied complexes. As shown in Fig. 6, the four donor atoms (N, NH,
2Cl) are coplanar, while the phenyl group of the aniline ring is bent
out of the plane by angle 89.349° and 158.500° for cis-PdL2Cl2 and
cis-PtL2Cl2 complexes, respectively. The calculated geometrical
parameters (bond lengths and angles) are comparable to the exper-
imental data of cis-platin.
The M-nitrogen bonds (M = Pd or Pt) are of comparable lengths,
the MANHsec (NHsec = secondary amino nitrogen) distance is only
about 4.43% longer than that of MANpy (Npy = pyridine-like nitro-
gen) bond distance (Tables 9 and 10) [56]. It is seen that the opti-
mized MANHsec and MANpy bond lengths in Pd–L2 complex are
slightly larger than the corresponding PtANH3 in cis-platin by
0.16 and 0.06 Å, respectively, while the MACl bond lengths are lar-
ger by 0.06 and 0.04 Å for MACl22 and MACl23, respectively, ow-
ing to the trans-effect. For cis-PtL2Cl2 (4) complex, the MANpy and
MANHsec bond distances are shorter than the corresponding one in
cis-PdL2Cl2 (2) complex by 0.017 Å.
Cl22APdACl23 angle is larger than the experimental one in cis-pla-
tin molecule by about 3.19° [57]. This indicates that in a bare pal-
ladium complex (in gas phase), the intramolecular hydrogen
bonding N21AH27ꢃ ꢃ ꢃCl22 (2.730 Å) opens up Cl22APdACl23 an-
gle. On the other hand, the optimized N11APtAN21 angle in case
of cis-PtL2Cl2 is close to that found in cis-PdL2Cl2 complex, while
the Cl22PtC23 angle is slight smaller than that exist in cis-platin
by about 1.50°. This indicates that there is weak or no intramolec-
ular hydrogen bonding as found in the PdL2Cl2 and this may be
attributed to the significant difference in the bending angle of ani-
line ring as previously mentioned.
It was found that the C13N11 and C13N12 bond distances of the
benzimidazole ring in the cis-PdL2Cl2 (2) and cis-PtL2Cl2 (4) com-
plexes were increased upon the interaction of the pyridine-type
nitrogen with the metal centre as compared with the L2 ligand (Ta-
ble 10). In addition, the C14N28 and C15N28 bond distances were
enlarged upon the coordination of the secondary amino group to
the metal centre. The benzimidazole nitrogenAPd bond distance
is 2.071 Å which coincide with that observed in the crystal
structures of some benzimidazole complexes [58]. The PdAN21
bond distance to the secondary nitrogen donor is 2.167 Å which
is in a good agreement with the average value of 2.165 Å observed
for sec-NH donor groups accompanied in some benzimidazole
complexes e.g. nickel complex of N,N-bis(benzimid- azol-2-yl-
For cis-PdL2Cl2 complex, the optimized N11APdAN21 and
Cl22APdACl23 angles are 80.529° and 95.090°, respectively. It
was found that the N11APdAN21 is smaller than that found in
cis-platin (NH3APtANH3) by 6.471° and this can be interpreted
in terms of CH2 group, which connects the two coordination sites
(N11 and N21) and prevent opening of this angle. The calculated

120
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
(a)
(b)
HOMO (-0.219 ev)
LUMO (-0.088 ev)
HOMO (-0.204 ev)
LUMO (-0.063 ev)
(c)
(d)
HOMO (-0.219 ev)
LUMO (-0.089 ev)
HOMO (-0.204 ev)
LUMO (-0.063 ev)
Fig. 7. Molecular orbital surfaces and energy levels of (a) Pd–L1 (b) Pt–L1 (c) Pd–L2 (d) Pt–L2.
methyl)amine [59]. The dipole moment increases spectacularly
from a value of 3.969 D (L2 derivative) to 14.979 D (Pd–L2) and
15.347 (Pt–L2), where the electropositive metal ions induce strong
on two nitrogen atoms, N11 and N21, respectively. According
to the NBO, the electron configuration of Pt is: [cor-
e]6s0.565d8.786p0.036d0.01. Thus, 68 core electrons, 9.34 valence
electrons (on 6s and 5d atomic orbitals) and 0.04 Rydberg electrons
(mainly on 6p and 6d orbitals) give the 77.380 electrons. This is
consistent with the calculated natural charge on Pt atom (+0.620)
in Pt–L2 complex, which corresponds to the difference between
77.380e and the total number of electrons in the isolated Pt atom
(78e). In addition, the two chlorine atoms (Cl22 and Cl23) coordi-
nated to platinum atom have larger negative charge ꢁ0.552e and
ꢁ0.508e, respectively. Thus, the positive atomic charge upon the
Pt(II) was substantially reduced as a consequence of electron den-
sity donation from the pyridine-type nitrogen, secondary amino
group and two chlorine atoms. The charges on N11 and N21 atoms
polarization in the
r- and p-frameworks of the ligand molecule.
In addition, the energies of the Pd–L2 and Pt–L2 become more
negative indicate the high stability of these complexes with respect
to the free ligands.
The natural bond orbital (NBO) analysis of PtL2Cl2 complex for
example was performed and could be used to estimate the delocal-
ization of electron density between occupied Lewis-type orbitals
and formally unoccupied non-Lewis NBOs (antibonding or Ryd-
berg), which corresponds to a stabilizing donor–acceptor interac-
tion [60]. Tables 9 and 10 collect the natural charges on atoms.
The largest negative charges (ꢁ0.560e and ꢁ0.603e) are located

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
121
are not the same. It confirms that the electron density delocalized
from the two nitrogen atoms to the metal ion is not the same,
which is in agreement with the different bond lengths of PtN11
and PtN21. The atomic charges of the remaining atoms of the
framework are reasonable according to electronegativity consider-
ations. While in case of PdL2Cl2, the electron configuration of Pd is:
[core]5s0.354d8.935p0.025d0.016p0.01, with 36 core electrons, 9.28 va-
lence electrons (on 5s and 4d atomic orbitals) and 0.037 Rydberg
electrons (mainly on 5d and 6p orbitals) giving total 45.312 elec-
trons. Similar, the charges on N11 and N21 atoms are not the same;
with the different bond lengths of PdN11 and PdN21; as found in
Pt–L2 complex.
Table 11 lists the calculated occupancies of natural orbitals in
Pt–L2. Three classes of NBOs are included, the Lewis-type (s and
p bonding or lone pair) orbitals, the valence non-Lewis (acceptors,
formally unfilled) orbitals and the Rydberg NBOs, which originate
from orbitals outside the atomic valence shell. The calculated nat-
ural hybrids on atoms are also given in Table 11. According to cal-
culations, the platinum atom forms two sigma bonds with two
chlorine atoms, while the two bonds between platinum and the
nitrogen atoms can be described as donation of electron density
from a lone pair (LP) orbital on each nitrogen atom to platinum
35
30
25
20
15
10
5
Tetracycline
L2
L1
Pt-L2
Pt-L1
Pd-L2
0
G1
G2
Pd-L1
G3
G4
G5
G6
Fig. 8. Antibacterial activities of ligands L1,2 and their complexes; Pd–L1,2 and Pt–
L1,2 against Bacillus subtilis (G1), Staphylococcus aureus (G2), Streptococcus faecalis
(G3) as Gram-positive, (G4) Pseudomonas aeruginosa (G4), Escherichia coli (G5),
Neisseria gonorrhoeae (G6) as Gram-negative bacteria.
molecular orbitals. As follows from Table 10, the
r(PtACl22) bond
Table 7
Geometrical parameters optimized in L1 derivative: bond length (Å), bond angles (°) and charges.
Bond lengths (Å)
B3LYP/6-31G(d)
B3LYP/LANL2DZ
Bond angles (°)
B3LYP/6-31G(d)
B3LYP/LANL2DZ
Charge B3LYP/6-31G(d)
C1C2
C2C3
C3C4
C4C5
C5C6
C1C6
C13C14
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C13N11
C13N12
C14N28
C15N28
C18O30
C31O30
C3H7
1.415
1.399
1.392
1.409
1.393
1.395
1.502
1.404
1.394
1.400
1.398
1.392
1.409
1.310
1.377
1.445
1.397
1.373
1.415
1.086
1.086
1.086
1.086
1.107
1.102
1.088
1.084
1.086
1.086
1.099
1.092
1.099
1.009
1.015
1.428
1.404
1.406
1.421
1.404
1.407
1.506
1.417
1.405
1.410
1.407
1.402
1.421
1.330
1.395
1.448
1.395
1.407
1.452
1.086
1.087
1.087
1.087
1.107
1.107
1.089
1.086
1.086
1.086
1.100
1.092
1.100
1.011
1.014
C2C1C6
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
122.584
119.808
118.006
121.409
121.524
116.669
112.976
105.274
107.164
109.331
120.610
122.363
121.523
120.124
118.896
120.851
117.739
117.801
106.053
115.373
111.599
126.191
121.712
119.077
119.167
122.026
119.213
121.041
120.591
120.323
122.379
120.125
117.893
121.346
121.516
116.740
112.271
105.790
107.440
109.079
123.538
122.029
121.339
120.009
119.331
120.656
120.790
118.004
106.151
120.804
115.656
126.042
121.684
119.074
119.306
122.253
119.369
121.133
118.536
118.734
C1 = 0.353
C2 = 0.238
C3 = ꢁ0.179
C4 = ꢁ0.143
C5 = ꢁ0.148
C6 = ꢁ0.174
C13 = 0.514
C14 = ꢁ0.173
C15 = 0.356
C16 = ꢁ0.203
C17 = ꢁ0.198
C18 = 0.369
C19 = ꢁ0.184
C20 = ꢁ0.183
H7 = 0.140
N11C13N12
C2N11C13
C1N12C13
C13C14N28
C14N28C15
C20C15N28
C15C16C17
C16C17C18
C17C18C19
C19C20C15
C16C15C20
C18O30C31
H22C14H23
C15N28H29
C14N28H29
C13N12H21
H7C3C4
H8C4C5
H9C5C6
H10C6C1
H24C16C17
H25C17C18
H26C19C20
H27C20C15
H8 = 0.128
H9 = 0.129
H10 = 0.128
H21 = 0.332
H22 = 0.161
H23 = 0.153
H24 = 0.123
H25 = 0.128
H26 = 0.132
H27 = 0.119
H32 = 0.146
H33 = 0.164
H34 = 0.147
N11 = ꢁ0.574
N12 = ꢁ0.745
N28 = ꢁ0.676
O30 = ꢁ0.515
C4H8
C5H9
C6H10
C14H22
C14H23
C16H24
C17H25
C19H26
C20H27
C31H32
C31H33
C31H34
N12H21
N28H29
B3LYP/6-31G(d)
B3LYP/LANL2DZ
E (a.u.)
ꢁ820.104
ꢁ819.975
Zero-point E (kcal molꢁ1
)
174.226
174.587
Rotational constants (GHz)
1.779, 0.116, 0.109
1.832, 0.112, 0.106
Entropy (cal molꢁ1 Kꢁ1
Translational
Rotational
)
42.486
33.924
54.552
2.296
42.486
33.953
54.544
2.956
Vibrational
Total dipole moment (D)

122
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Table 8
Geometrical parameters optimized in L2 derivative: bond length (Å), bond angles (°) and charges.
Bond lengths (Å)
B3LYP/6-31G(d)
B3LYP/LANL2DZ
Bond angles (°)
B3LYP/6-31G(d)
B3LYP/LANL2DZ
Charge B3LYP/6-31G(d)
C1C2
C2C3
C3C4
C4C5
C5C6
C1C6
C13C14
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C13N11
C13N12
C14N28
C15N28
C3H7
1.415
1.399
1.391
1.408
1.393
1.395
1.501
1.409
1.387
1.404
1.396
1.397
1.404
1.309
1.376
1.443
1.390
1.085
1.086
1.086
1.086
1.106
1.101
1.088
1.088
1.088
1.085
1.097
1.095
1.098
1.009
1.014
1.511
1.428
1.403
1.405
1.420
1.404
1.406
1.505
1.421
1.399
1.416
1.408
1.408
1.417
1.329
1.394
1.448
1.391
1.085
1.087
1.087
1.086
1.106
1.106
1.088
1.089
1.089
1.086
1.099
1.096
1.099
1.010
1.014
1.519
C2C1C6
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
122.579
119.820
117.996
121.409
121.528
116.664
112.987
105.276
107.149
109.306
121.275
122.544
120.806
121.735
117.116
120.421
117.897
106.050
116.223
112.264
126.222
121.716
119.078
119.162
122.030
120.000
119.412
118.671
120.395
120.132
122.375
116.735
121.522
121.342
117.891
112.270
105.795
107.437
109.077
123.553
122.279
120.716
121.759
117.204
121.942
120.427
106.170
120.798
115.646
126.041
121.683
119.075
119.300
122.246
120.038
119.322
118.761
120.501
C1 = 0.353
C2 = 0.238
C3 = ꢁ0.178
C4 = ꢁ0.143
C5 = ꢁ0.148
C6 = ꢁ0.174
C13 = 0.514
C14 = ꢁ0.170
C15 = 0.359
C16 = ꢁ0.188
C17 = ꢁ0.184
C18 = 0.178
C19 = ꢁ0.196
C20 = ꢁ0.180
H7 = 0.140
N11C13N12
C2N11C13
C1N12C13
C13C14N28
C14N28C15
C20C15N28
C15C16C17
C16C17C18
C17C18C19
C19C20C15
C16C15C20
H22C14H23
C15N28H33
C14N28H33
C13N12N21
H7C3C4
H8C4C5
H9C5C6
H10C6C1
H24C16C17
H25C17C18
H26C19C20
H27C20C15
H8 = 0.128
H9 = 0.128
H10 = 0.128
H21 = 0.331
H22 = 0.162
H23 = 0.154
H24 = 0.121
H25 = 0.117
H26 = 0.114
H27 = 0.114
H30 = 0.154
H31 = 0.149
H32 = 0.156
H33 = 0.348
N11 = ꢁ0.574
N12 = ꢁ0.745
N28 = ꢁ0.677
C29 = ꢁ0.677
C4H8
C5H9
C6H10
C14H22
C14H23
C16H24
C17H25
C19H26
C20H27
C29H30
C29H31
C29H32
N12H21
N28H33
C18C29
B3LYP/6-31G(d)
B3LYP/LANL2DZ
E (a.u.)
ꢁ744.902
ꢁ744.781
Zero-point E (kcal molꢁ1
)
170.955
171.763
Rotational constants (GHz)
1.934, 0.140, 0.131
1.962, 0.137, 0.128
Entropy (cal molꢁ1 Kꢁ1
Translational
Rotational
)
42.292
33.470
52.801
3.386
42.292
33.505
51.401
3.969
Vibrational
Total dipole moment (D)
is formed from an sp0.01d1.19 hybrid on platinum atom (which is
the mixture of 45.58s, 0.30p and 54.12%d atomic orbitals) and
sp8.89 hybrid on the chlorine atom (89.89%p contribution). The re-
B3LYP/LANL2DZ level for Pd–L1,2 and Pt–L1,2 complexes. The value
of the energy separation between the HOMO and LUMO is 0.131,
0.141, 0.130, and 0.141 eV for cis-PdL1Cl2, cis-PtL1Cl2, cis-PdL2Cl2,
and cis-PtL2Cl2, respectively.
sults from NBO analysis show that the
r(PtACl22) bond is strongly
polarized towards the chloride, with about 75.28% of electron
density concentrated on the chlorine atom, i.e. about 75.28% of
electron density is accommodated on the Cl atom. The strength
of this interaction can be estimated by the second order perturba-
tion theory.
Table 12 lists the selected values of the calculated second
order interaction energy (E2) between donor–acceptor orbitals in
Pt–L2 complex. The strongest interactions are the electron
3.9. Biological activity
3.9.1. Antimicrobial activity
The data showed that the ligands L1,2 have the capacity of inhib-
iting the metabolic growth of the investigated bacteria; B. subtilis,
S. aureus and S. faecalis as Gram-positive bacteria and P. aeruginosa,
E. coli and N. gonorrhoeae as Gram-negative bacteria; to different
extents as shown in Fig. 8. The size of the inhibition zone depends
upon the culture medium, incubation conditions, rate of diffusion
and the concentration of the antibacterial agent (the activity
increases as the concentration increases). In classifying the anti-
bacterial activity as Gram-positive or Gram-negative, it would
generally be expected that a much greater number of drugs would
be active against Gram-positive than Gram-negative bacteria.
However, in this study, L1,2 are active against both types of the bac-
teria, which may indicate broad-spectrum properties. The remark-
able activity of these compounds may be arising from the
benzimidazole ring, which may play an important role in the anti-
bacterial activity. The mode of action may involve the formation of
a hydrogen bond through the tertiary nitrogen of the imidazole
donations from
a lone pair orbital on the nitrogen atoms,
LP(1)N11 to the antibonding acceptor rꢀ(PtACl22) orbitals, i.e.
LP(1)N11 ? rꢀ(PtACl22). As shown in Table 10, the LP(1)N11 orbi-
tal has 70.59% p-character and is occupied by 1.698 electrons (this
is consistent with a delocalization of electron density from the ide-
alized occupancy of 2.0e). The donation of electron density from
the coordination sites in the ligand to the Pt molecular orbitals
has a clear correspondence to a chemical picture of the coordina-
tion bonds.
The frontier molecular orbitals play also an important role in
the electric and optical properties, as well as in UV/vis. spectra
and chemical reactions [61]. Fig. 7 show the distributions and en-
ergy levels of the HOMO, and LUMO orbitals computed at the

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
123
Table 9
Selected bond lengths (Å), angles (°) and charge for Pd–L1 and Pt–L1.
Pd–L1
Pt–L1
Bond lengths (Å)
Bond angles (°)
Charge
Bond lengths (Å)
Bond angles (°)
Charge
C1C2
C2C3
C3C4
C4C5
1.425
1.406
1.402
1.422
1.404
1.409
1.410
1.337
1.378
1.505
1.502
1.468
1.406
1.404
1.411
1.410
1.401
1.411
1.393
2.071
2.163
2.391
2.371
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
C6C1C2
C2N11C13
C1N12C13
N11C13N12
C13C14N21
C14N21C15
N21C15C16
C15C16C17
C16C17C18
C17C18C19
C18C19C20
C19C20C15
C20C15C16
N11PdN21
Cl22PdCl23
121.017
116.987
121.736
121.652
116.492
122.116
107.554
107.666
111.183
108.252
116.648
118.460
120.408
119.858
119.717
120.253
120.105
119.653
80.569
C1 = 0.151
C2 = 0.149
C1C2
C2C3
C3C4
C4C5
1.425
1.406
1.403
1.421
1.404
1.415
1.408
1.338
1.377
1.496
1.521
1.478
1.409
1.396
1.414
1.409
1.409
1.399
1.392
1.458
2.054
2.150
2.400
2.398
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
C6C1C2
120.728
117.091
121.847
121.483
116.527
122.323
107.330
107.701
111.378
109.059
111.061
119.807
119.716
120.161
120.111
119.311
120.323
120.365
118.786
81.287
C1 = 0.153
C2 = 0.149
C3 = ꢁ0.187
C4 = ꢁ0.214
C5 = ꢁ0.199
C6 = ꢁ0.250
N11 = ꢁ0.560
N12 = ꢁ0.603
C13 = 0.473
C14 = ꢁ0.252
C15 = 0.139
C16 = ꢁ0.203
C17 = ꢁ0.281
C18 = 0.331
C19 = ꢁ0.226
C20 = ꢁ0.247
N21 = ꢁ0.688
Cl22 = ꢁ0.555
Cl23 = ꢁ0.511
Pd = 0.687
C3 = ꢁ0.186
C4 = ꢁ0.214
C5 = ꢁ0.198
C6 = ꢁ0.252
C5C6
C5C6
C2N11
C1N12
C13N11
C13N12
C13C14
C14N21
C15N21
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C18O33
N11Pd
N21Pd
Cl22Pd
Cl23Pd
C2N11
C1N12
C13N11
C13N12
C13C14
C14N21
C15N21
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C18O33
O33C34
N11Pt
C2N11C13
C1N12C13
N11C13N12
C13C14N21
C14N21C15
N21C15C16
C15C16C17
C16C17C18
C17C18C19
C18C19C20
C19C20C15
C20C15C16
C18O33C34
N11PtN21
Cl22PtCl23
N11 = ꢁ0.561
N12 = ꢁ0.599
C13 = 0.473
C14 = ꢁ0.243
C15 = 0.142
C16 = ꢁ0.214
C17 = ꢁ0.219
C18 = 0.338
C19 = ꢁ0.289
C20 = ꢁ0.217
N21 = ꢁ0.695
Cl22 = ꢁ0.518
Cl23 = ꢁ0.498
Pt = 0.619
95.215
O33 = ꢁ0.566
90.444
O33 = ꢁ0.563
C34 = ꢁ0.262
N21Pt
Cl22Pt
Cl23Pt
C34 = ꢁ0.263
E (a.u.)
ꢁ976.728
ꢁ969.141
178.603
0.405, 0.107, 0.089
Zero-point E (kcal molꢁ1
)
178.647
Rotational constants (GHz)
0.261, 0.140, 0.107
Entropy (cal molꢁ1 Kꢁ1
Translational
Rotational
)
44.059
35.660
74.764
13.865
44.621
35.676
75.732
15.714
Vibrational
Total dipole moment (D)
Table 10
Selected bond lengths (Å), angles (°) and charge for Pd–L2 and Pt–L2.
Pd–L2
Pt–L2
Bond lengths (Å)
Bond angles (°)
Charge
Bond lengths (Å)
Bond angles (°)
Charge
C1C2
C2C3
C3C4
C4C5
C5C6
C2N11
C1N12
C13N11
C13N12
C13C14
C14N21
C15N21
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C18C32
N11Pd
1.425
1.406
1.402
1.422
1.404
1.409
1.410
1.336
1.378
1.505
1.502
1.467
1.410
1.401
1.415
1.409
1.408
1.406
1.517
2.071
2.167
2.389
2.370
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
C6C1C2
C2N11C13
C1N12C13
N11C13N12
C13C14N21
C14N21C15
N21C15C16
C15C16C17
C16C17C18
C17C18C19
C18C19C20
C19C20C15
C20C15C16
N11PdN21
Cl22PdCl23
121.037
116.983
121.728
121.661
116.492
122.098
107.572
107.668
111.171
108.290
116.773
118.255
119.728
121.548
117.764
121.445
119.749
119.764
80.529
C1 = 0.151
C2 = 0.149
C1C2
C2C3
C3C4
C4C5
1.424
1.407
1.402
1.421
1.403
1.415
1.409
1.338
1.377
1.496
1.522
1.480
1.402
1.409
1.410
1.415
1.401
1.405
1.517
2.054
2.150
2.399
2.398
ꢁ893.946
175.699
0.413, 0.130, 0.104
C1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6C1
C6C1C2
C2N11C13
C1N12C13
N11C13N12
C13C14N21
C14N21C15
N21C15C16
C15C16C17
C16C17C18
C17C18C19
C18C19C20
C19C20C15
C20C15C16
N11PtN21
Cl22PtCl23
120.721
117.09
C1 = 0.153
C2 = 0.149
C3 = ꢁ0.187
C4 = ꢁ0.214
C5 = ꢁ0.199
C6 = ꢁ0.251
N11 = ꢁ0.560
N12 = ꢁ0.603
C13 = 0.474
C14 = ꢁ0.252
C15 = 0.155
C16 = ꢁ0.215
C17 = ꢁ0.196
C18 = ꢁ0.004
C19 = ꢁ0.204
C20 = ꢁ0.253
N21 = ꢁ0.690
Cl22 = ꢁ0.552
Cl23 = ꢁ0.508
Pd = 0.688
121.844
121.489
116.526
122.326
107.335
107.694
111.343
108.843
110.589
119.575
119.626
120.962
118.172
121.384
119.327
120.522
81.128
C3 = ꢁ0.186
C4 = ꢁ0.214
C5 = ꢁ0.198
C6 = ꢁ0.252
N11 = ꢁ0.561
N12 = ꢁ0.599
C13 = 0.473
C14 = ꢁ0.244
C15 = 0.156
C16 = ꢁ0.232
C17 = ꢁ0.207
C18 = 0.004
C19 = ꢁ0.196
C20 = ꢁ0.214
N21 = ꢁ0.696
Cl22 = ꢁ0.516
Cl23 = ꢁ0.497
Pt = 0.61962
C33 = ꢁ0.651
C5C6
C2N11
C1N12
C13N11
C13N12
C13C14
C14N21
C15N21
C15C16
C16C17
C17C18
C18C19
C19C20
C15C20
C18C33
N11Pt
95.090
90.398
N21Pd
C32 = ꢁ0.650
N21Pt
Cl22Pt
Cl23Pt
Cl22Pd
Cl23Pd
E (a.u.)
Zero-point E (kcal molꢁ1
ꢁ901.532
)
175.724
Rotational constants (GHz)
0.276, 0.171, 0.125
Entropy (cal molꢁ1 Kꢁ1
Translational
Rotational
)
43.945
35.255
71.919
14.979
44.528
35.303
72.644
15.347
Vibrational
Total dipole moment (D)

124
N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
Table 11
Occupancy of natural orbitals (NBOs) and hybrids calculated for Pt–L2 (selected).
Donora Lewis-type NBOs (A-B)
Occupancy
1.978
Hybridb
AO (%)c
Acceptord non–Lewis NBOs
NBOs
0.036
r
(C2AN11)
sp1.86 (N11)
sp2.59 (C2)
s(34.96)p(65.04)
s(27.85)p(72.15)
rꢀ(C2AN11)
r
(C13AN11)
1.978
1.899
1.975
1.983
1.966
1.968
sp1.80 (N11)
sp2.14 (C13)
s(35.65)p(64.35)
s(31.88)p(68.12)
rꢀ(C13AN11)
pꢀ(C13AN11)
rꢀ(Cl4AN21)
rꢀ(C15AN21)
rꢀ(PtACl22)
rꢀ(PtACl23)
0.026
0.441
0.028
0.041
0.313
0.286
p(C13AN11)
sp (N11)
sp (C13)
p(100.00)
s(0.01)p(99.99)
r
r
r
r
r
(C14AN21)
(C15AN21)
(PtACl22)
(PtACl23)
(N21AH27)
sp2.94 (N21)
sp3.52(C14)
sp2.01 (N21)
sp2.91 (C15)
s(25.40)p(74.60)
s(22.15)p(77.85)
s(33.22)p(66.78)
s(25.56)p(74.44)
sp0.01d1.19 (Pt)
sp8.89(C122)
sp0.01d1.12 (Pt)
sp9.12 (C123)
s(45.58)p(0.30)d(54.12)
s(10.11)p(89.89)
s(46.97)p(0.36)d(52.86)
s(9.79)p(90.12)
1.981
1.698
1.718
1.993
1.993
1.993
1.986
1.980
sp3.07 (N21)
sp2.40
s(24.57)p(75.43)
s(29.41)p(70.59)
s(16.80)p(83.20)
s(79.40)p(20.60)
LP(1)N11
LP(1)N21
LP(1)Cl22
LP(1)Cl23
LP(1)Pt
RYꢀ(1)N11
RYꢀ(1)N21
RYꢀ(1)Cl22
RYꢀ(1)Cl23
RYꢀ(1)Pt
0.004
0.009
0.0003
0.0004
0.020
0.004
0.002
sp4.95
sp0.26
sp0.26
s(79.65)p(20.35)
sp0.06d49.45
sp0.01d19.99
sp1.00d99.99
s(1.98)p(0.13)d(97.89)
s(4.76)p(0.06)d(95.18)
s(0.01)p(0.06)d(99.94)
LP(2)Pt
LP(3)Pt
RYꢀ(2)Pt
RYꢀ(3)Pt
a
b
c
LP(n)A is a valence lone pair orbital (n) on A atom.
Hybrid on A atom in the AAB bond or otherwise, as indicated.
Percentage contribution of atomic orbitals in NBO hybrid.
d
Starred label (ꢀ) denotes antibonding, and Ry corresponds to the Rydberg NBO orbital.
ring with the active centers of the cell constituents, resulting in
interference with the normal cell process [62].
3.9.2. Antitumor activity
To evaluate the potential usefulness of Pd–L1,2 and Pt–L1,2 com-
plexes synthesized as antitumor agents, three cell lines of different
origin; breast cancer (MCF-7), Colon carcinoma (HCT) and human
heptacellular carcinoma (Hep-G2) were treated at the concentration
It is known that the chelation process facilitates the ability of a
complex to cross a cell membrane and can be explained by Twee-
dy’s chelation theory [63]. However, the Pt–L2 complex shows
activity comparable to that found in the uncoordinated ligand
(L2) suggesting that the platinum atom has no responsibility in
the activity of this compound. In addition, the Pt–L1 complex was
inactive against all the tested bacteria except the Gram-positive
bacterium Bacillus subtilis with activity lower than found in the
uncoordinated ligand. Comparison between the two platinum
complexes confirms the role of the methyl group in the inhibition
process. On the other hand, the Pd–L1,2 complexes are inactive
against all the tested organisms and the possible explanation for
these results is their inability to chelate metals essential for the
metabolism of microorganisms and/or to form hydrogen bonds
with the active centers of cell structures, resulting in an interfer-
ence with the normal cell cycle. Furthermore, the low activity of
these complexes may be also due to their low lipophilicity, because
of which penetration of the complex through the lipid membrane
was decreased and hence, they could neither block nor inhibit
the growth of the microorganism.
of 100 lM. All the compounds showed activity against all the stud-
ied cell lines. On screening against HCT cells, it was found that all
the complexes show similar activity; irrespective of changing the
types of metals and ligands; but lower than found in cis-platin.
The remarkable activity of these complexes may be arising from
the benzimidazole ring itself. In case of MCF-7 cells, the Pd–L1
and Pt–L1,2 complexes are more toxic than cis-platin with the order
Pt–L1 > Pd–L1 > Pt–L2 suggesting that the methoxyl group plays a
role in the cytotoxicity of these complexes through the important
hydrogen-bond donor properties, either in the approach of the bio-
logical target or the final structure. For Hep-G2 cells, it was found
that all the complexes show higher activity than cis-platin with the
order Pt–L1 > Pd–L2 > Pd–L1 > Pt–L2. The high toxicity of Pd–L1,2
complex than Pt–L2 happens because the ligand-exchange behav-
ior of platinum compound is quite slow, which gives them a high
kinetic stability and results in ligand-exchange reactions within
minutes to days, rather than microseconds to seconds for many
other coordination compounds. In addition, another unusual phe-
nomenon deals with the preferred ligands for platinum ions is
that Pt(II) has a strong thermodynamic preference for binding to
S-donor ligands and for this reason, one would predict that platinum
compounds would perhaps never reach DNA, with many cellular
platinophiles (S-donor ligands, such as glutathione, methionine)
as competing ligands in the cytosol [65]. In addition, Pt–L1 complex
shows the same cyctotoxic activity against the MCF-7 and Hep-G2
cell lines and twice that found against HCT cells.
Therefore, we confirm that the toxicity of the complexes can be
related to the strengths of the metal–ligand bond, besides other
factors such as size of the cation, receptor sites, diffusion and a
combined effect of the metal and the ligands for inactivation of
the biomolecules, as previously reported by other authors [64].
Table 12
Second-order interaction energy (E2, kcal/mol) between donor and acceptor orbitals
in Pt–L2 complex calculated at B3LYP/LANL2DZ level of theory (selected).
Donor ? Acceptor
E2
Donor ? Acceptor
E2
4. Conclusion
LP(1)N11 ? rꢀ(PtACl22)
98.31
76.30
4.70
r
r
r
(N11AC13) ? rꢀ(PtACl22)
(N21AH27) ? rꢀ(PtACl23)
(N21AC14) ? rꢀ(PtACl23)
6.08
5.95
2.89
LP(1)N21 ? rꢀ(PtACl23)
Owing to the antibacterial activity of the benzimidazole ring, it
was found that the studied benzimidazole ligands have the capacity
r
(PtACl23) ? rꢀ(PtACl22)

N.T. Abdel Ghani, A.M. Mansour / Journal of Molecular Structure 991 (2011) 108–126
125
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of inhibiting the metabolic growth of the investigated bacteria to
different extent and these benzimidazoles L1,2 are more toxic than
their metal complexes. This may be attributed to the inability of
the complexes to chelate metals essential for the metabolism of
microorganisms and/or to form hydrogen bonds with the active
centers of cell structures, resulting in an interference with the nor-
mal cell cycle. All the studied complexes showed activity against
three cell lines of different origin, breast cancer (MCF-7), Colon car-
cinoma (HCT) and human heptacellular carcinoma (Hep-G2) and
represent an interesting class of new compounds from the view-
point of their physicochemical and structural properties. The re-
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the metal complexes; one can conclude that the studied ligands
behave as neutral bidentate ligands coordinated to the metal ions
via he pyridine-type nitrogen of the benzimidazole ring and sec-
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for all the studied complexes; [PdL1Cl2]ꢃ3H2O, [PtL1Cl2]ꢃH2O,
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