Sterically modulated palladium(II)-N-heterocyclic carbene complexes for the catalytic oxidation of olefins: Synthesis, crystal structure, characterization and DFT studies

Article abstract of DOI:10.1016/j.poly.2014.06.054

The synthesis of a series of sterically modulated palladium(II)-NHC complexes (9-11) of the general formula [Pd(NHC)₂Cl₂] (NHC = 1-benzyl-3(2′-methyl)-propylbenzimidazol-2-ylidine, 1-benzyl-3(2′-methyl)-butylbenzimidazol-2-ylidine and 1-benzyl-3- hexylbenzimidazol-2-ylidine) from their respective silver(I) counterparts (6-8) is presented. Two novel triazine-tethered Zwitterionic (benz)imidazolium salts, 4 and 5, were prepared and tested as NHC precursors for the preparation of silver(I) complexes. However, all our attempts to prepare silver(I) derivatives from salts 4 and 5 ended with negative results due to the low acidity of the C2 protons. The Zwitterionic derivative 4 was additionally characterized by the single crystal X-ray diffraction technique. The molecular structure of 4 evidenced π-π stacking interactions between the triazine rings of two crystallographically independent units. The palladium complexes 9-11 showed good catalytic activities in the epoxidation of 1-octene and styrene under homogeneous conditions with aqueous hydrogen peroxide as an oxidant. The effect of temperature and solvent on the epoxidation of the aforementioned olefins using complexes 9-11 was also explored. Density functional theory (DFT) was used to model the structures of the isomers of the palladium complexes. Geometry parameters, electronic energy, molecular orbital energies, band gap, vibrational frequencies and the cis-trans energy barrier were calculated.

Full text of DOI:10.1016/j.poly.2014.06.054

Polyhedron 81 (2014) 499–510
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Sterically modulated palladium(II)ꢀN-heterocyclic carbene complexes
for the catalytic oxidation of olefins: Synthesis, crystal structure,
characterization and DFT studies
d
Abbas Washeel Salman a,c, Ghani Ur Rehman b, Norbani Abdullah c, , Srinivasa Budagumpi ,
⇑
Salasiah Endud b, , Hassan Hadi Abdallah e, Wen Yee Wong b
⇑
a Department of Chemistry, College of Science, Wasit University, Kut, Wasit, Iraq
b Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
c Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
d Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Bangalore 562 112, Karnataka, India
e Department of Chemistry, Education College, Salahaddin University, Erbil, Iraq
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a series of sterically modulated palladium(II)–NHC complexes (9–11) of the general
formula
[Pd(NHC)2Cl2]
(NHC = 1-benzyl-3(20-methyl)-propylbenzimidazol-2-ylidine,
1-benzyl-3
Received 30 April 2014
Accepted 18 June 2014
Available online 5 July 2014
(20-methyl)-butylbenzimidazol-2-ylidine and 1-benzyl-3-hexylbenzimidazol-2-ylidine) from their
respective silver(I) counterparts (6–8) is presented. Two novel triazine-tethered Zwitterionic (benz)
imidazolium salts, 4 and 5, were prepared and tested as NHC precursors for the preparation of silver(I)
complexes. However, all our attempts to prepare silver(I) derivatives from salts 4 and 5 ended with
negative results due to the low acidity of the C2 protons. The Zwitterionic derivative 4 was additionally
characterized by the single crystal X-ray diffraction technique. The molecular structure of 4 evidenced
Keywords:
1-Octene
DFT study
Olefin epoxidation
Palladium(II)-carbene complex
X-ray diffraction
p–p stacking interactions between the triazine rings of two crystallographically independent units.
The palladium complexes 9–11 showed good catalytic activities in the epoxidation of 1-octene and
styrene under homogeneous conditions with aqueous hydrogen peroxide as an oxidant. The effect of
temperature and solvent on the epoxidation of the aforementioned olefins using complexes 9–11 was
also explored. Density functional theory (DFT) was used to model the structures of the isomers of the
palladium complexes. Geometry parameters, electronic energy, molecular orbital energies, band gap,
vibrational frequencies and the cis–trans energy barrier were calculated.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
bioorganometallic chemistry and catalysis from isostructural com-
plexes having NHC ligands. Silver(I)–NHC complexes have been
The field of N-heterocyclic carbene (NHC) chemistry has under-
gone an incredible expansion ever since its discovery more than
half a century ago. Recent investigations have focused on under-
standing the mode of interactions of NHC ligands with both transi-
tion and inner transition metals in various oxidation states [1].
Interestingly, these outcomes shed light on the complexation
behavior of multidentate NHC systems containing three or more
coordination donor sites to address key factors influencing
complex formation. The second row congeners of group 10 and
11, palladium and silver, present a unique combination for both
active as anticancer [2] and antibacterial agents due to their favor-
able biocompatible properties [3], while palladium(II)–NHC com-
plexes have been studied for numerous C–C, C–N coupling and
cross-coupling [4] and C–H activation [5] reactions (Chart 1). In par-
ticular, organometallic palladium(II)–NHC complexes, popularized
by many research groups, have gained considerable attention in
recent years due to their facile preparation through NHC transfer
from silver(I) counterparts via a transmetallation technique [6]. A
number of recent studies have targeted the polymerization of olefins
with these molecules [7] and their heterogeneous counterparts [8].
Mono- and bidentate NHC systems derived from imidazolium
and benzimidazolium derivatives provide a square-planar coordi-
nation sphere for palladium(II)–NHC complexes and generally they
posses increased stability towards air and moisture. A plethora of
⇑
Corresponding authors. Tel.: +60
97311635; fax: +60 7 5566162 (S. Endud).
3 7967 4264 (N. Abdullah). Tel.: +60 1
E-mail addresses: norbania@um.edu.my (N. Abdullah), salasiah@kimia.fs.utm.
my (S. Endud).
http://dx.doi.org/10.1016/j.poly.2014.06.054
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

500
A.W. Salman et al. / Polyhedron 81 (2014) 499–510
Chart 1. Benzimidazole-derived Pd(II)–NHC complexes.
these complexes are used for various C–C and C–N bond formation
reactions, however their potential in olefin epoxidations is very
rarely reported. On the other hand, the use of hydrogen peroxide,
one of the greenest terminal oxidants, for the epoxidation of olefins
in connection with palladium(II)–NHC complexes is of great
interest due to the importance of epoxides as well as other oxi-
dized products in the manufacture of both bulk and fine chemicals
[9]. A large portion of these catalyses have involved group 7 to 9
metals with numerous multidentate N-donor and N,O-donor
ligands [10]. A key point in the production of epoxides or other
desired oxidized products from olefins is the activation of catalyst
species in the absence or at very low concentrations of oxidants.
With this perspective, new oxidative processes based on the
activation of hydrogen peroxide by stable, proficient and highly
selective catalysts were developed [11].
In view of ever increasing environmental awareness, imidazole-
based acetamide/group 7 systems and their counterparts have
been reported as homogeneous and heterogeneous catalysts for
olefin epoxidation with hydrogen peroxide as a terminal oxidant.
Special attention was recently devoted to the use of group 10 metal
complexes for this application, which are more specific and robust
compared to group 7 systems [12]. These types of complex systems
show considerable catalytic activities under different reaction con-
ditions [13]. The efficiency of these homogeneous catalysts and
their heterogeneous counterparts in olefin epoxidations led us to
design the present NHC complexes. Starting from these results
and the efficient catalytic applications of palladium(II)–NHC com-
plexes, an investigation of the steric influence on olefin epoxida-
tion is presented in the present manuscript. We investigate here
the synthesis, characterization and DFT studies of a series of new
palladium(II)–NHC complexes prepared via the transmetallation
method from silver(I)–NHC complexes. Further, we described the
efficient and selective epoxidation of 1-octene and styrene with
30% aqueous hydrogen peroxide in the presence of palladium(II)–
NHC complexes as homogenous solid catalysts.
DMSO-d6 or D2O solvent using TMS as an internal reference. The
FTIR spectra of all the compounds were recorded in potassium bro-
mide disks using a Perkin Elmer 2000 system spectrometer in the
range 4000–400 cmꢀ1. The melting points were assessed using a
Stuart Scientific SMP-1 (UK) instrument. All reported compounds
were analyzed for carbon, hydrogen and nitrogen by CHN microa-
nalyses using a Perkin Elmer 2400 LS Series CHN/S analyzer. For
single crystal X-ray diffraction analysis, a Bruker SMART APEX2-
2009 CCD area detector diffractometer was used for the data
collection. SAINT Bruker-2009 was used for the cell refinement,
SAINT was used for the data reduction and SHELXTL was used to solve
the structure. Calculations, structure refinement, molecular graph-
ics and the material for publication were obtained using the SHELXTL
and PLATON software packages. The structures were solved by direct
methods and refined by full-matrix least-squares against F2. All
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were fixed at calculated positions and refined using a riding
mode. The oxidized products of the olefins were analyzed by a
Hewlett–Packard 6890N gas chromatograph using an Ultra-1
column and a Hewlett–Packard GC-MSD instrument using an
HP5 column.
2.2. Syntheses
2.2.1. Synthesis of 1-benzyl-3-(20-methyl)propylbenzimidazolium-
bromide (1)
In
a 100 mL round-bottom flask, 1-benzylbenzimidazole
(0.208 g, 1.0 mmol) was dissolved in acetonitrile (20 mL) and
2-methylpropyl bromide (0.164 g, 1.2 mmol) was added while stir-
ring. The reaction mixture was brought to reflux and stirred for
48 h. The solvent was removed in vacuo, leaving a beige solid
which was washed with diethyl ether (3 ꢁ 5 mL) and dried. Yield:
78.4%. M.P.: 143–143.5 °C. 1H NMR (500 MHz, d6-DMSO) d: 1.12
(6H,
d J = 7.0 Hz, NCH2CH(CH3)2), 2.16–2.25 (1H, m, NCH2-
CH(CH3)2), 4.67 (2H, t J = 6.5 Hz,NCH2CH(CH3)2), 5.64 (2H, s, ben-
zylic CH2), 7.23–7.31 (5H, m, benzyl-H), 7.76–7.85 (2H, m,
benzimidazole-H), 8.11–8.18 (2H, m, benzimidazole-H), 10.12 (H,
s, benzimidazole-H20). 13C{1H} NMR (125 MHz, d6-DMSO) d: 13.7
(NCH2CH(CH3)2), 21.7 (NCH2CH(CH3)2), 50.4 (NCH2CH(CH3)2),
52.8 (benzylicCH2), 118.9, 122.5, 125.3, 133.2 (Ar-C benzyl),
117.3, 121.4, 125.6, 133.7, 134.1, (Ar-C benzimidazole), 140.2
2. Experimental
2.1. Reagents and instruments
All the chemicals used were of reagent grade, and the solvents
were dried and distilled before use according to the standard pro-
cedures. Benzyl bromide, benzimidazole, imidazole, cyanuric chlo-
ride, 2-methylpropyl bromide, 2-methylbutyl bromide, n-hexyl
bromide, silver(I) oxide, palladium(II) chloride, 1-octene, styrene
and 30% aqueous hydrogen peroxide were purchased from
Sigma–Aldrich and used as received. 1-Benzylbenzimidazole, 1-
butylimidazole and 1-butylbenzimidazole were synthesized
according to a literature procedure with minor modifications
[14]. 1H and 13C NMR spectra of all the compounds were recorded
at room temperature on a Bruker 500 MHz Ascend spectrometer in
(benzimidazolium C20). FT-IR (KBr disc) cmꢀ1
(C–H), 1612 (C@N, benzimidazole), 1101 (C–N, benzimidazole).
:
2923, ꢂ2860
m
m
m
Anal. Calc. for C18H21N2Br: C, 62.6; H, 6.1; N, 8.1. Found: C, 62.9; H,
6.0; 8.3%.
2.2.2. Synthesis of 1-benzyl-3-(20-methyl)butylbenzimidazolium
bromide (2)
This compound was prepared in a manner analogous to that for
1, only with 2-methylbutyl bromide (0.18 g, 1.2 mmol) instead of
2-methylpropyl bromide. Yield: 91.1%. M.P.: 121–122 °C. 1H NMR

A.W. Salman et al. / Polyhedron 81 (2014) 499–510
501
(500 MHz, d6-DMSO) d: 0.91 (6H, d J = 6.5 Hz, NCH2CH2CH(CH3)2),
2.01–2.10 (1H, m, NCH2CH2CH(CH3)2), 3.35 (2H, quart J = 5.5 Hz,
NCH2CH2CH(CH3)2), 4.72 (2H, t J = 6.0 Hz,NCH2CH2CH(CH3)2), 5.91
(2H, s, benzylic CH2), 7.11–7.20 (5H, m, benzyl-H), 7.65–7.73 (2H,
m, benzimidazole-H), 8.0–8.09 (2H, m, benzimidazole-H), 9.92
(H, s, benzimidazole-H20). 13C{1H} NMR (125 MHz, d6-DMSO) d:
12.2 (NCH2CH2CH(CH3)2), 21.7 (NCH2CH2CH(CH3)2), 28.4
(NCH2CH2CH(CH3)2), 51.2 (NCH2CH2CH(CH3)2), 52.7 (benzylicCH2),
116.6, 118.3, 124.2, 134.7 (Ar-C benzyl), 117.5, 122.7, 124.0, 133.5,
135.0, (Ar-C benzimidazole), 139.4 (benzimidazolium C20). FT-IR
(2H, m, NCH2CH2CH2CH3), 1.85–1.92 (2H, m, NCH2CH2CH2CH3),
4.76 (4H, t, J = 7.2 Hz, NCH2CH2CH2CH3), 7.32 (3H, t J = 5.5 Hz,
benzimidazolium-H), 7.73 (2H, d J = 6.0 Hz, benzimidazolium-H),
8.23–8.30 (5H, m, benzyl-H), 9.01 (2H, s, benzimidazolium H20).
13C{1H} NMR (125 MHz, D2O) d: 11.8 (NCH2CH2CH2CH3), 21.0
(NCH2CH2CH2CH3), 33.5 (NCH2CH2CH2CH3), 50.6 (NCH2CH2CH2
CH3), 116.3, 123.2, 127.0, 133.0 (Ar-C benzimidazole), 137.3 (benz-
imidazolium C20), 162.0 (cyanuric NCN), 164.4, 164.9 (cyanuric
2 ꢁ C-O). FT-IR (KBr disc) cmꢀ1: ꢂ2950, ꢂ2830 (C–H), 1613
(C@Nbenzimid), 1621 (C@Ntriazine), 1034, ꢂ1080 (Carom–Nbenzimid).
Anal. Calc. for C14H15N5O2: C, 58.9; H, 5.3; N, 24.5. Found: C,
59.3; H, 5.6; N, 24.1%.
(KBr disc) cmꢀ1: ꢂ2930, 2867
m(C–H), 1622 m(C@N, benzimid-
azole), ꢂ1050
m(C–N, benzimidazole). Anal. Calc. for C19H23N2Br:
C, 63.5; H, 6.4; N, 7.8. Found: C, 63.3; H, 6.0; 7.3%.
2.2.6. Synthesis of bis-[1-benzyl-3-(20-methyl)propylbenzimidazolium]
silver(I) bromide (6)
2.2.3. Synthesis of 1-benzyl-3-hexylbenzimidazolium bromide (3)
This compound was prepared in a manner analogous to that for
1, only with n-hexyl bromide (0.198 g, 1.2 mmol) instead of
2-methylpropyl bromide. Yield: 83.1%. M.P.: 144–145 °C. 1H NMR
(500 MHz, d6-DMSO) d: 0.85 (3H, t J = 7.2 Hz, N(CH2)5CH3), 1.30–
1.40 (2H, m, N(CH2)4CH2CH3), 2.11–2.28 (2H, quart, N(CH2)3
CH2CH2CH3), 2.87–2.95 (2H, quart, N(CH2)2CH2(CH2)2CH3), 3.60–
3.67 (2H, quart, NCH2CH2(CH2)3CH3), 4.74 (2H, t J = 6.4 Hz, NCH2
(CH2)4CH3), 5.77 (2H, s, benzylic CH2), 7.16–7.23 (3H, m, benzyl-
H), 7.30–7.36 (2H, m, benzyl-H), 7.65 (2H, t J = 7.2 Hz, benzimid-
azole-H), 8.21–8.30 (2H, m, benzimidazole-H), 10.05 (H, s,
benzimidazole-H20). 13C{1H} NMR (125 MHz, d6-DMSO) d: 11.6
(N(CH2)5CH3), 19.3 (N(CH2)4CH2CH3), 23.7 (N(CH2)3CH2CH2CH3),
27.8 (N(CH2)2CH2(CH2)2CH3), 35.2 (NCH2CH2(CH2)3CH3), 51.6
(NCH2(CH2)4CH3), 52.8 (benzylicCH2), 117.6, 119.1, 125.5, 133.4
(Ar-C benzyl), 118.7, 123.4, 132.6, 135.3, (Ar-C benzimidazole),
137.7 (benzimidazolium C20). FT-IR (KBr disc) cmꢀ1: 2939, ꢂ2870
To a stirred solution of the benzimidazolium salt 1 (0.347 g,
1 mmol) in methanol (25 mL) was added silver(I) oxide (0.126 g,
0.055 mmol) in a 50 mL round-bottom flask. The reaction mixture
was allowed to stir at room temperature for 48 h in the dark, after
which the reaction mixture was filtered through a bed of Celite to
remove unreacted silver(I) oxide. The solvent was then removed
in vacuo, leaving a colorless solid which was washed with diethyl
ether (3 ꢁ 5 mL) and recrystallized from methanol/diethyl ether.
Yield: 52.1%. M.P.: 223–224 °C. 1H NMR (500 MHz, d6-DMSO) d:
0.97 (12H, d J = 7.2 Hz, 2 ꢁ NCH2CH(CH3)2), 1.96–2.05 (2H, m,
2 ꢁ NCH2CH(CH3)2), 4.43 (4H, t J = 6.8 Hz,2 ꢁ NCH2CH(CH3)2), 5.38
(4H, s, benzylic CH2), 7.20–7.28 (10H, m, benzyl-H), 7.50–7.58
(4H, m, benzimidazole-H), 7.91–8.01 (4H, m, benzimidazole-H).
13C{1H} NMR (125 MHz, d6-DMSO) d: 12.2 (NCH2CH(CH3)2), 20.7
(NCH2CH(CH3)2), 51.1 (NCH2CH(CH3)2), 52.3 (benzylic CH2), 117.3,
122.3, 124.9, 133.8 (Ar-C benzyl), 116.1, 120.8, 123.6, 134.3,
m
(C–H), 1628
m(C@N, benzimidazole), 1064
m
(C–N, benzimidazole).
135.3,
(Ar-C
benzimidazole),
179.7
(benzimidazolium
(C–H), 1594 (C@N,
Anal. Calc. for C20H25N2Br: C, 64.3; H, 6.7; N, 7.5. Found: C, 64.7; H,
6.3; 7.8%.
C20-Ag). FT-IR (KBr disc) cmꢀ1: ꢂ2925, 2838
m
m
benzimidazole), 1012 m(C–N, benzimidazole). Anal. Calc. for
C36H40N4AgBr: C, 60.3; H, 5.6; N, 7.8. Found: C, 60.5; H, 6.0; 7.9%.
2.2.4. Synthesis of 4-(3-butyl-2,3-dihydro-imidazol-1-yl)-
[1,3,5]triazin-2,4-diol anion (4)
2.2.7. Synthesis of bis-[1-benzyl-3-(20-methyl)butylbenzimidazolium]
silver(I) bromide (7)
1-Butylimidazole (0.124 g, 1 mmol) in 20 mL of dry acetone was
placed in a 150 mL round bottom flask connected to N2 gas and
equipped with a stirrer. The flask was immersed in an ice bath
and cyanuric chloride (0.183 g, 1 mmol) in 20 mL of dry acetone
was added dropwise over 30 min. An off-white precipitate formed
after the complete addition. The stirring was continued for another
2 h under the same reaction conditions and then for 4 h at room
temperature. The precipitate that formed was collected by filtra-
tion, washed with fresh acetone (2 ꢁ 5 mL) and dried in a vacuum
desiccator. Yield: 68.5%. M.P.: >300 °C. 1H NMR (500 MHz, D2O) d:
0.83 (3H, t, J = 6.2 Hz, NCH2CH2CH2CH3), 1.28–1.37 (2H, m,
NCH2CH2CH2CH3), 1.83–1.91 (2H, m, NCH2CH2CH2CH3), 4.33 (4H,
t, J = 6.5 Hz, NCH2CH2CH2CH3), 7.58 (2H, d, J = 6.0 Hz, imidazolium
H50), 7.97 (2H, d, J = 6.0 Hz, imidazolium H40), 8.91 (2H, s, imidazo-
lium H20). 13C{1H} NMR (125 MHz, D2O) d: 13.3 (NCH2CH2CH2CH3),
20.2 (NCH2CH2CH2CH3), 30.8 (NCH2CH2CH2CH3), 51.8 (NCH2
CH2CH2CH3), 122.2, 124.7 (imidazolium C50 and C40), 136.1
(imidazolium C20), 160.3 (cyanuric NCN), 163.1, 164.7 (cyanuric
2 ꢁ C-O). FT-IR (KBr disc) cmꢀ1: 2933, ꢂ2875 (C–H), ꢂ1615
(C@Nimid), 1628 (C@Ntriazine), ꢂ1045, 1083 (Carom–Nimid). Anal. Calc.
for C10H13N5O2: C, 51.1; H, 5.6; N, 29.8. Found: C, 50.7; H, 5.6; N,
30.1%.
This complex was prepared in a manner analogous to that for
complex 6, only with the benzimidazolium salt
2 (0.359 g,
1.0 mmol) instead of salt 1. Further work up and purification of this
complex were done in a methanol/n-hexane mixture. Yield: 48.4%.
M.P.: >300 °C. 1H NMR (500 MHz, d6-DMSO) d: 1.01 (12H, d
J = 6.5 Hz, 2 ꢁ NCH2CH2CH(CH3)2), 2.01–2.10 (2H, m, 2 ꢁ NCH2CH2
CH(CH3)2), 3.41 (4H, quart J = 5.5 Hz, 2 ꢁ NCH2CH2CH(CH3)2), 4.79
(4H, t J = 6.2 Hz,2 ꢁ NCH2CH2CH(CH3)2), 5.96 (4H, s, 2 ꢁ benzylic
CH2), 7.24–7.32 (10H, m, benzyl-H), 7.72–7.81 (2H, m, benzimid-
azole-H), 8.11–8.19 (2H, m, benzimidazole-H). 13C{1H} NMR
(125 MHz, d6-DMSO) d: 11.0 (NCH2CH2CH(CH3)2), 22.2 (NCH2CH2
CH(CH3)2), 30.1 (NCH2CH2CH(CH3)2), 51.5 (NCH2CH2CH(CH3)2),
52.7 (benzylic CH2), 118.4, 119.8, 123.3, 133.0 (Ar-C benzyl),
116.6, 123.9, 125.1, 132.0, 134.2 (Ar-C benzimidazole), 181.5
(benzimidazolium C20-Ag). FT-IR (KBr disc) cmꢀ1: 2933, ꢂ2875
m
(C–H), 1582
m
(C@N, benzimidazole), ꢂ1010
m(C–N, benzimid-
azole). Anal. Calc. for C38H44N2AgBr: C, 61.3; H, 6.0; N, 7.5. Found:
C, 61.0; H, 6.3; 7.2%.
2.2.8. Synthesis of bis-[1-benzyl-3-hexylbenzimidazolium]silver(I)
bromide (8)
This complex was prepared in a manner analogous to that for
2.2.5. Synthesis of 4-(3-butyl-2,3-dihydro-benzimidazol-1-yl)-
[1,3,5]triazin-2,4-diol anion (5)
This compound was prepared in a manner analogous to that for
4, only with 1-butybenzlimidazole (0.175 g, 1 mmol) instead of
1-butylimidazole. Yield: 72.7%. M.P.: >300 °C. 1H NMR (500 MHz,
D2O) d: 0.91 (3H, t, J = 6.6 Hz, NCH2CH2CH2CH3), 1.32–1.39
complex 6, only with the benzimidazolium salt 3 (0.373 g,
1.0 mmol) instead of salt 1. Further work up and purification of this
complex were done in a methanol/n-hexane mixture. Yield: 57.7%.
M.P.: >300 °C. 1H NMR (500 MHz, d6-DMSO) d: 0.92 (6H,
t
J = 6.8 Hz, 2 ꢁ N(CH2)5CH3), 1.26–1.35 (4H, m, 2 ꢁ N(CH2)4
CH2CH3), 2.17–2.24 (4H, quart, 2 ꢁ N(CH2)3CH2CH2CH3),

502
A.W. Salman et al. / Polyhedron 81 (2014) 499–510
2.89–2.96 (4H, quart, 2 ꢁ N(CH2)2CH2(CH2)2CH3), 3.65–3.71 (4H,
quart, 2 ꢁ NCH2CH2(CH2)3CH3), 4.83 (4H, t J = 6.0 Hz, 2 ꢁ NCH2
(CH2)4CH3), 5.89 (4H, s, 2 ꢁ benzylic CH2), 7.33–7.40 (6H, m,
benzyl-H), 7.52–7.60 (4H, m, benzyl-H), 7.95 (4H, t J = 7.0 Hz, benz-
imidazole-H), 8.20–8.28 (4H, m, benzimidazole-H). 13C{1H} NMR
(125 MHz, d6-DMSO) d: 10.8 (N(CH2)5CH3), 18.0 (N(CH2)4CH2CH3),
22.2 (N(CH2)3CH2CH2CH3), 29.3 (N(CH2)2CH2(CH2)2CH3), 36.1
(NCH2CH2(CH2)3CH3), 51.8 (NCH2(CH2)4CH3), 52.9 (benzylic CH2),
116.1, 118.4, 123.6, 134.1 (Ar-C benzyl), 117.1, 124.5, 133.0,
135.1 (Ar-C benzimidazole), 188.3 (benzimidazolium C20-Ag).
(4H, t J = 6.2 Hz, 2 ꢁ NCH2(CH2)4CH3), 6.03 (4H, s, 2 ꢁ benzylic
CH2), 7.20–7.28 (6H, m, benzyl-H), 7.61–7.68 (4H, m, benzyl-H),
7.90 (4H,
t J = 6.0 Hz, benzimidazole-H), 8.23–8.29 (4H, m,
benzimidazole-H). 13C{1H} NMR (125 MHz, d6-DMSO) d: 13.1
(N(CH2)5CH3), 19.4 (N(CH2)4CH2CH3), 24.2 (N(CH2)3CH2CH2CH3),
27.9 (N(CH2)2CH2(CH2)2CH3), 38.0 (NCH2CH2(CH2)3CH3), 50.3
(NCH2(CH2)4CH3), 51.8 (benzylic CH2), 117.3, 118.8, 124.3, 133.3
(Ar-C benzyl), 122.2, 123.1, 134.2, 135.6 (Ar-C benzimidazole),
173.2 (benzimidazolium C20-Pd). FT-IR (KBr disc) cmꢀ1: 2933,
ꢂ2880
m(C–H), 1591 m(C@N, benzimidazole), 1008 m(C–N, benz-
imidazole). Anal. Calc. for C40H48N4PdCl2: C, 63.0; H, 6.4; N, 7.4.
FT-IR (KBr disc) cmꢀ1: 2928, ꢂ2855
m
(C–H), 1598
m(C@N, benz-
imidazole), 1017
m(C–N, benzimidazole). Anal. Calc. for
Found: C, 63.1; H, 6.7; 7.0%.
C40H48N4AgBr: C, 62.2; H, 6.3; N, 7.2. Found: C, 62.7; H, 6.5; 7.5%.
2.3. DFT calculations
2.2.9. Synthesis of bis-[1-benzyl-3-(20-methyl)propylbenzimidazolium]
palladium(II)chloride (9)
GAUSSIAN 09 [15] was used to perform all the theoretical calcula-
tions. The structures of the isomers were built based on the crystal
structure of the similar complex (trans-bis[1-benzyl-3-(2,3,4,5,6-
pentafluorobenzyl)benzimidazol-2-ylidene]dibromopalladium(II)),
a palladium complex with a substituted benzimidazole ligand [16].
The 6-31G(d,p) basis set was used for the atoms C, H, N and Cl,
while Pd was described with LANL2DZ basis set. The optimisation
was carried out without any restriction in symmetry or internal
coordinates. Frequencies calculations have shown no imaginary
frequencies, hence the optimized structures are in their optimum
minima. The optimized structures were used to calculate the full
set of molecular orbitals, however we are going to focus on the
orbitals from HOMOꢀ2 to LUMO+2 and the band gap energy
(LUMOꢀHOMO).
To a solution of bis-NHC silver(I) bromide complex 6 (0.358 g,
0.05 mmol) in dichloromethane (25 mL) was added [PdCl2(CH3-
CN)2] (0.130 g, 0.05 mmol). The mixture was stirred for 12 h at
room temperature in the dark. The resultant suspension was fil-
tered through a bed of Celite and the volume of the filtrate was
reduced on a rotary evaporator to 5 mL. Further addition of diethyl
ether to the filtrate afforded the bis-NHC palladium(II) complex 9
as a pale brown powder which was filtered, washed with fresh
diethyl ether (2 ꢁ 3 mL) and recrystallized from methanol/diethyl
ether. Yield: 76.4%. M.P.: >300 °C. 1H NMR (500 MHz, d6-DMSO)
d: 0.82 (12H, d J = 7.0 Hz, 2 ꢁ NCH2CH(CH3)2), 1.81–1.89 (2H, m,
2 ꢁ NCH2CH(CH3)2), 4.30 (4H, t J = 7.0 Hz, 2 ꢁ NCH2CH(CH3)2),
5.77 (4H, s, benzylic CH2), 7.38–7.46 (10H, m, benzyl-H), 7.61–
7.68 (4H, m, benzimidazole-H), 7.82–7.90 (4H, m, benzimidazole-
H). 13C{1H} NMR (125 MHz, d6-DMSO) d: 11.0 (NCH2CH(CH3)2),
18.1 (NCH2CH(CH3)2), 50.8 (NCH2CH(CH3)2), 52.0 (benzylic CH2),
116.0, 118.6, 122.7, 134.0 (Ar-C benzyl), 117.5, 123.8, 124.4,
135.0, 135.7, (Ar-C benzimidazole), 172.0 (benzimidazolium
2.4. Catalytic olefin oxidation studies
The epoxidation of the olefins 1-octene and styrene with 30%
aqueous hydrogen peroxide was performed in a 25 mL round bot-
tom flask equipped with a condenser. In a typical epoxidation
experiment, the flask was charged with 10 mL of acetone (or aceto-
nitrile/dichloromethane), 1.0 mmol olefin as the substrate,
1.0 mmol hydrogen peroxide as the oxidant and 1.0 mmol of com-
plex 9–11 as the catalyst. The reaction was carried out in an oil
bath under stirring at a constant temperature of 70 °C for 4 h. After
every hour, aliquots were removed and analyzed immediately with
a GC instrument. Before analysis, these aliquots were treated with
diethyl ether and centrifuged to remove unreacted hydrogen per-
oxide, water and catalyst. The organic part was subsequently sep-
arated and analyzed by the GC instrument. Olefin conversion (%)
and the reaction products were quantified by GC and identified
by comparison with the retention time to those of authentic stan-
dard samples.
C20-Pd). FT-IR (KBr disc) cmꢀ1: ꢂ2950, 2822
m(C–H), 1581 m(C@N,
benzimidazole), 1048
m(C–N, benzimidazole). Anal. Calc. for
C36H40N4PdCl2: C, 61.2; H, 5.7; N, 7.9. Found: C, 61.5; H, 6.1; 8.0%.
2.2.10. Synthesis of bis-[1-benzyl-3-(20-methyl)butylbenzimidazolium]
palladium(II)chloride (10)
This complex was prepared in a manner analogous to that for
complex 9, only with the bis-NHC silver(I) complex 7 (0.37 g,
0.05 mmol) instead of complex 6. Yield: 61.6%. M.P.: >300 °C. 1H
NMR (500 MHz, d6-DMSO) d: 0.88 (12H, d J = 6.2 Hz, 2 ꢁ NCH2CH2-
CH(CH3)2), 1.91–2.00 (2H, m, 2 ꢁ NCH2CH2CH(CH3)2), 3.21–3.26
(4H, quart J = 5.2 Hz, 2 ꢁ NCH2CH2CH(CH3)2), 4.69 (4H,
t
J = 6.2 Hz,2 ꢁ NCH2CH2CH(CH3)2), 5.91 (4H, s, 2 ꢁ benzylic CH2),
7.18–7.27(10H, m, benzyl-H), 7.60–7.68 (2H, m, benzimidazole-
H), 7.95–8.03 (2H, m, benzimidazole-H). 13C{1H} NMR (125 MHz,
d6-DMSO) d: 13.1 (NCH2CH2CH(CH3)2), 19.8 (NCH2CH2CH(CH3)2),
33.7 (NCH2CH2CH(CH3)2), 51.0 (NCH2CH2CH(CH3)2), 52.76 (ben-
zylicCH2), 116.9, 118.0, 122.3, 134.1 (Ar-C benzyl), 117.5, 123.6,
124.7, 134.0, 135.1, (Ar-C benzimidazole), 175.7 (benzimidazolium
3. Results and discussion
In the present work a series of new palladium(II)–NHC-com-
plexes of 1,3-dialkyl-aryl substituted benzimidazol-2-lyidenes is
presented. All the synthesized compounds were stable towards
air and moisture, while the silver complexes were found to be light
sensitive. All the compounds are insoluble in water (except 4 and
5), but readily soluble in common organic solvents, like DMF,
DMSO, methanol, acetone, dichloromethane and acetonitrile. The
bis-NHC palladium(II) complexes were used as homogeneous cat-
alysts for olefin epoxidation reactions as the focus of the present
work. A quantitative correlation is drawn between the changes in
the steric bulk around the metal center and their activity towards
olefin oxidations. Microanalysis data of all the synthesized com-
pounds agree well with their proposed structures. Although it
was demonstrated that the palladium(0/II)–NHC complexes could
C20-Pd). FT-IR (KBr disc) cmꢀ1: 2948, ꢂ2850
m(C–H), 1597 m(C@N,
m(C–N, benzimidazole). Anal. Calc. for
benzimidazole), 1027
C38H44N2PdCl2: C, 62.2; H, 6.0; N, 7.6. Found: C, 62.6; H, 6.5; 7.9%.
2.2.11. Synthesis of bis-[1-benzyl-3-hexylbenzimidazolium]
palladium(II) chloride (11)
This complex was prepared in a manner analogous to that for
complex 9, only with the bis-NHC silver(I) complex 8 (0.372 g,
0.05 mmol) instead of complex 6. Yield: 68.7%. M.P.: >300 °C.1H
NMR (500 MHz, d6-DMSO) d: 1.01 (6H, t J = 7.0 Hz, 2 ꢁ N(CH2)5
CH3), 1.20–1.28 (4H, m, 2 ꢁ N(CH2)4CH2CH3), 2.06–2.14 (4H, quart,
2 ꢁ N(CH2)3CH2CH2CH3), 2.66–2.71 (4H, quart, 2 ꢁ N(CH2)2CH2
(CH2)2CH3), 3.80–3.87 (4H, quart, 2 ꢁ NCH2CH2(CH2)3CH3), 4.98

A.W. Salman et al. / Polyhedron 81 (2014) 499–510
503
easily be obtained via transmetallation from their silver(I) counter-
parts and isolated in almost quantitative yields, it seems that these
complexes have been largely ignored until recently for application
as olefin oxidation catalysts.
3.1. Syntheses
The reaction of 1H-benzimidazole and benzyl bromide in the
presence of a strong base resulted in the formation of 1-benzyl-
benzimidazole [14], which was exploited for connecting N-alkyl
groups such as 2-methylpropyl, 2-methylbutyl and hexyl to afford
a series of 1-benzyl-3-alkylbenzimidazolium bromide salts, 1–3, in
good yield. Analogously, using standard Schlenk techniques,
1-butylimidazole and 1-butylbenzimidazole were prepared and
treated with an equimolar amount of cyanuric chloride at ice
temperature in dry acetone under nitrogen atmosphere to afford
2,4,6-triazine-functionalized 1-butylimidazolium and benzimi-
dazolium salts 4 and 5, respectively, in good yield as shown in
Scheme 1. This new ligand set was fully characterized using differ-
ent spectral and analytical tools, and the molecular structure of the
imidazolium salt 4 was confirmed by the single-crystal X-ray
diffraction technique.
With a mono carbene carbon coordination site potentially avail-
able for hosting a palladium ion, the benzimidazolium salts 1–5
were reacted with [PdCl2(CH3CN)2] at different stoichiometric
ratios, reaction conditions and solvents over different reaction
periods, but this did not result in any complex formation. There-
fore, we followed the carbene transfer method [17] to prepare
the desired sterically modulated palladium(II)–NHC complexes.
With use of the in situ deprotonation method [18], the benzimi-
dazolium salts 1–3 were treated with half an equivalent of silver
oxide in stirring methanol at room temperature for 48 h in the
dark, yielding the colorless silver(I)–NHC complexes, 6–8, respec-
tively. All of these silver derivatives were fully characterized by
elemental analysis, FT-IR and 1H and 13C NMR spectroscopy. These
complexes were formed in good yield. The reactions involved in
the preparation of the silver(I) complexes 6–8 are shown in
Scheme 2. Conversely, the reactions of imidazolium and
benzimidazolium salts 4 and 5 under the same and many other
Scheme 2. Synthesis of the sterically modulated silver(I)–NHC complexes.
reaction conditions resulted in no silver(I) complex formation
due to the low acidity of the C2 proton. This inactivity of com-
pounds 4 and 5 towards complex formation could be attributed
to the insertion of the triazine moiety, which perhaps made the
C2 proton less acidic, that in turn did not allow the deprotonation
of the C2 proton.
The synthesis of palladium(II)–NHC complexes 9–11 was
accomplished via the transmetallation method [19] by treating
equimolar amounts of the silver(I)–NHC complexes 6–8 with
[PdCl2(CH3CN)2] in dichloromethane at room temperature, over-
night under the exclusion of light. The desired complexes 9–11
were purified by repeated precipitation in methanol/diethyl ether.
The palladium complexation reactions proceeded smoothly to
afford neutral palladium(II) complexes 9–11 in around 61–76% iso-
lated yields. These title complexes were obtained as pale green to
brown powders and were fully characterized by elemental analy-
sis, FT-IR and 1H and 13C NMR spectral techniques. The reactions
involved in the formation of these complexes are outlined in
Scheme 3.
3.2. Spectral characterization
Successful formation of the NHC precursor salts 1–5 and their
carbene complexes 6–11 in each step can be nicely traced by spec-
troscopic methods, followed by microanalysis data. In the FTIR
spectra of the silver(I) and palladium(II) complexes 6–11, the mC@N
stretching frequencies upon coordination shifted to lower values
(1582–1598 cmꢀ1) in comparison to those of the benzimidazolium
salts (1612–1628 cmꢀ1) [20]. Notably, the corresponding mC–N
stretching frequencies followed the same trend. Upon coordination
to the metal center, the mC–N vibrations shifted to lower values
(1008–1048 cmꢀ1) in comparison to those of the benzimidazolium
salts (1050–1101 cmꢀ1). These observations are collectively
attributed to the successful complex formation. In the case of the
Scheme 1. Synthesis of the sterically modulated (benz)imidazol-2-ylidenes.

504
A.W. Salman et al. / Polyhedron 81 (2014) 499–510
Scheme 3. Synthesis of the sterically modulated Pd–NHC complexes via the transmetallation technique.
triazine-tethered (benz)imidazolium salts, 4 and 5, an additional
medium intensity band at 1628 and 1621 cmꢀ1 was observed
respectively, due to the mC@N stretching frequencies originating
from the triazine core. The spectra also evidenced the presence
of two medium intensity sharp bands at around 2860 and
2930 cmꢀ1, attributed to the stretching vibrations of both aliphatic
and aromatic C–H bonds.
technique. Slow diffusion of diethyl ether into a methanolic solu-
tion of salt 4 yielded crystals suitable for single crystal X-ray dif-
fraction analysis. The crystal data, structure refinement details,
selected bond distances and angles of salt 4 are tabulated in Tables
1–3, respectively. The molecular structure of salt 4 in the solid
state is depicted in Figs. 1 and 2.
The imidazolium salt 4 crystallizes in the monoclinic space
group P21/C, with two crystallographically independent units (A
and B) in the dibasic Zwitterionic form. Two crystallographically
inequivalent dibasic salts and two water molecules occupy the
asymmetric unit of salt 4 with a set of disordered butyl carbon
atoms. In the formation of salt 4, both the chloride atoms at the
3- and 5-positions of the triazine ring have undergone hydrolysis
to form hydroxyl units, which in turn form the dibasic salt by
ionization to balance the positive charges originating from
quarternization at the imidazolium group and N2 of the triazine.
It is interesting that both the triazine and imidazolium rings show
The NMR spectroscopic characterization of both the NHC precur-
sors and their carbene complexes was assisted by a combination of
1H and 13C NMR spectroscopic experiments. Data from room tem-
perature 1H and 13C NMR spectroscopic investigation for com-
pounds 1–11 was collected using d6-dimethylsulfoxide/D2O
solution. The successful formation of benzimidazolium salts 1–3
was confirmed by the downfield benzimidazolium-C2-ptoton signal
at d10.12, 9.92 and 10.05 respectively, followed by relatively less
low field aromatic proton resonances in the range d 7.01–8.11. Sig-
nals for the protons bound to the 2-methylpropyl, 2-methylbutyl
and n-hexyl chains appear in the range d 0.88–4.8, which are in good
agreement with the literature [21]. The 13C NMR spectra of salts 1–3
exhibit characteristic downfield resonances at d 140.2, 139.4 and
137.7 respectively for the benzimidazolium-C2 carbon nuclei. The
spectra also show a set of upfield resonance signals in the range d
11–52 and 116–134, assignable to the aliphatic and aromatic carbon
nuclei, respectively. The successful formation of the silver(I) and
palladium(II)–NHC complexes was confirmed by the absence of
the resonance for the most downfield acidic benzimidazolium-C2
proton signals in the 1H NMR spectra of the complexes. Further,
the 13C NMR spectra of the carbene complexes 6–11 reveal a signif-
icant downfield shift of the benzimidazolium-C2 signal at d 179.7,
181.5, 188.3, 172.0, 175.7 and 173.2, respectively, confirming coor-
dination of the carbene carbon atom to the metal center. These
observations are in good agreement with the data reported for car-
bene complexes having similar ligand architectures [22].
Siebert and co-workers reported [23] a series of imidazole-2-
yldine based macrocycles and their inability to undergo carbene
complexation when treated with NiCl2, Ni(OAc)2, Pd(OAc)2 and
[(Ph3P)2PtCl2]. Further, they demonstrated the cause for the inac-
tivity of the macrocycles towards complex formation is the low
acidity of the imidazole C2 protons (d 6.64–7.5 in CD2Cl2). In com-
parison with Siebert’s work, we concluded that the inability of our
Zwitterionic triazine-tethered (benz)imidazolium salts 4 and 5 to
form silver(I)–NHC complexes was based, in part, on the low acid-
ity of their C2 proton, whose resonance was found at d 8.91 and
9.01 in D2O, respectively. These chemical shift values of the C2 pro-
tons are in the upfield region compared to the C2 proton resonance
of salts 1–3 by ca d 1.
p–p stacking interactions perpendicular to the ab-plane with tri-
azine and imidazole rings of other molecules, with interaction bond
distances of 3.372 and 3.372 Å, respectively. The triazine rings are
slightly tilted from the associated imidazolium rings by a dihedral
angle of 5.19(18)° and 15.04(18)° for the A and B units, respectively.
The internal ring angles, N5A–C4A–N4A and N5B–C4B–N4B, at the
carbene centers are found to be 107.64(14)° and 108.48(14)°,
Table 1
Crystal data and structure refinement details of the Zwitterionic imidazole salt 4.
4
Formula
C20H30N10O6
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
506.54
monoclinic
P2(1)/c
13.7996(19)
9.9497(15)
20.955(2)
90.00
b (Å)
c (Å)
a
(°)
b (°)
125.785(6)
90.00
2334.0(5)
4
1.442
0.110
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
Absorption coefficient (mmꢀ1
F(000)
)
1072
Crystal size (mm)
T (K)
0.68 ꢁ 0.17 ꢁ 0.14
100
Radiation (Å)
h Min, max (°)
Dataset
Total Unique data
Rint
Mo Ka 0.71073
1.82, 30.07
ꢀ19:19; ꢀ14:13; ꢀ29:29
18868
3.3. X-ray structural description of salt 4
0.0406
Nref, Npar
R, wR2, S
6782, 392
0.0501, 0.1235, 1.043
The solid state structure of the Zwitterionic imidazolium salt 4
has been identified by the single crystal X-ray diffraction

A.W. Salman et al. / Polyhedron 81 (2014) 499–510
505
Table 2
Selected bond distances (Å) of the crystallographically independent units, A and B, of
the Zwitterionic salt 4.
Unit A of 4
C7A–N5A
N5A–C4A
N5A–C6A
C4A–N4A
C5A–N4A
C3A–N4A
C3A–N3A
1.464(2)
1.329(2)
1.383(2)
1.3491(18)
1.390(2)
1.427(2)
1.3209(18)
C3A–N1A
C1A–N1A
N3A–C2A
C2A–N2A
C1A–N2A
C1A–O1A
C2A–O2A
1.321(2)
1.366(2)
1.365(2)
1.376(2)
1.3887(18)
1.237(2)
1.2409(17)
Unit B of 4
C7B–N5B
N5B–C4B
N5B–C6B
C4B–N4B
C5B–N4B
C3B–N4B
C3B–N3B
1.472(2)
1.316(2)
1.3920(19)
1.3444(18)
1.382(2)
C3B–N1B
C1B–N1B
N3B–C2B
C2B–N2B
C1B–N2B
C1B–O1B
C2B–O2B
1.316(2)
1.365(2)
1.369(2)
1.373(2)
1.3852(19)
1.241(2)
1.2454(18)
1.433(2)
1.3200(17)
Table 3
Selected bond angles (°) of the crystallographically independent units, A and B, of the
Zwitterionic salt 4.
Fig. 2. Molecular structure of the triazine-functionalized imidazolium-2-ylidene 4
(unit B) showing 50% probability thermal ellipsoids.
Unit A of 4
C7A–N5A–C4A
C7A–N5A–C6A
C6A–N5A–C4A
N5A–C4A–N4A
C4A–N4A–C3A
N4A–C3A–N3A
N4A–C3A–N1A
C3A–N1A–C1A
124.71(14)
125.49(16)
109.28(14)
107.64(14)
124.07(14)
113.69(14)
114.17(13)
114.74(13)
C3A–N3A–C2A
N3A–C2A–O2A
N1A–C1A–O1A
O1A–C1A–N2A
O2A–C2A–N2A
C2A–N2A–C1A
N2A–C1A–N1A
N2A–C2A–N3A
114.09(14)
121.76(15)
122.86(14)
120.09(17)
120.19(17)
123.76(16)
117.05(15)
118.05(13)
HO---Otrizine (1.998 Å), CHimidazolium---Otriazine (2.569 Å) and
OH---Owater (2.103 Å), forming
a three-dimensional network.
Further, the molecules of 4 possess numerous short contacts
between heteroatoms and the disordered and non-disordered
hydrogen atoms, with bond distances ranging from 2.880 to
3.357 Å. These bond distances are quite normal and are comparable
to the reported values of similar imidazolium salts [25].
Unit B of 4
C7B–N5B–C4B
C7B–N5B–C6B
C6B–N5B–C4B
N5B–C4B–N4B
C4B–N4B–C3B
N4B–C3B–N3B
N4B–C3B–N1B
C3B–N1B–C1B
125.36(14)
125.59(16)
109.02(13)
108.48(14)
123.54(14)
113.56(14)
113.79(13)
114.72(13)
C3B–N3B–C2B
N3B–C2B–O2B
N1B–C1B–O1B
O1B–C1B–N2B
O2B–C2B–N2B
C2B–N2B–C1B
N2B–C1B–N1B
N2B–C2B–N3B
113.32(14)
122.24(16)
123.18(14)
119.73(17)
119.32(17)
123.57(16)
117.07(15)
118.44(13)
3.4. Theoretical studies on complexes 9 and 11 using the DFT method
DFT studies have been performed to understand the structures
of the targeted complexes. The theoretical parameters for com-
plexes 9 and 11 in the gaseous phase, such as the nature of the
molecular orbitals, HOMO and LUMO, and the cis–trans energy
barrier, have been established by this study using the B3LYP
method. Gökçe and co-workers reported the synthesis and crystal
structure of a similar complex, trans-bis[1-benzyl-3-(2,3,4,5,6-
pentafluorobenzyl)benzimidazol-2-ylidene] dibromopalladium(II)
[16]. The palladium(II) centre in this complex is conjugated with
two bromide atoms and two substituted benzimidazole groups.
The benzyl group and the pentafluorobenzyl unit on each benz-
imidazole entity form a trans-conformation. Since the palladium
complexes in this report similarly form complexes with 1,3-disub-
stituted benzimidazole groups and two chlorido ligands, it is
appropriate to build the complexes 9 and 11 accordingly. B3LYP,
as one of the density functional methods, was used to optimize
the starting structures. The common bond distances, angles and
dihedral angles were compared with the crystal structure from
the literature, Table 4.
respectively, which are consistent with earlier reports [24]. In
the extended crystal structure (Fig. 3), the dibasic units of 4
are connected via three types of hydrogen bonds, viz.,
The electronic energies of the optimized complexes in both con-
formations, cis- and trans-, are listed in Table 5. Interestingly, the
energy difference between the cis- and trans-conformations has
shown that the trans-isomer is slightly preferable by 1.2 kcal/mol
for complex 9 and 0.6 kcal/mol for complex 11. It is obvious that
the values of the energy differences between the two conforma-
tions for the two complexes are small, around 1 kcal/mol, and
hence it is difficult to say that the cis-conformers of the two com-
plexes are more stable than the trans-conformers. Experimentally,
the separation difficulty of the two isomers of the complexes might
be due to the low energy difference between them. A similar obser-
vation is reported in case of rhenium(I)–NHC complexes [26].
Fig. 1. Molecular structure of the triazine-functionalized imidazolium-2-ylidene 4
(unit A) showing 50% probability thermal ellipsoids.

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A.W. Salman et al. / Polyhedron 81 (2014) 499–510
Fig. 3. Crystal packing diagram of the triazine-functionalized imidazolium-2-ylidene 4 showing C–Hꢃ ꢃ ꢃO, C–Hꢃ ꢃ ꢃN and O–Hꢃ ꢃ ꢃO hydrogen bonds.
The optimized structures of the studied complexes were used to
HOMO molecular orbital for the two isomers is attributed to the
lone pairs on the chlorido ligands. In the LUMO orbital, in addition
to the localized density from palladium(II) and the two chlorido
ligands, there is some contribution from the carbene carbon atom
of the benzimidazole-2-ylidene groups, as shown in Figs. 5 and 6.
calculate the energies of the molecular orbitals (MOs). In order to
have a general idea about the molecular orbitals, a number of orbi-
tals beside the HOMO and LUMO orbitals are shown in Fig. 4. The
energies of the molecular orbitals and the density distribution may
give some insight into the reactivity and kinetic stability of the iso-
mers. In addition, the energies of the HOMO and LUMO represent
the molecule’s ability to donate or accept an electron, respectively.
Fig. 4 shows the molecular orbitals HOMOꢀ2 to LUMO+2 and the
band gaps for the cis- and trans-conformations of complexes 9
and 11. The band gaps for the two complexes 9 and 11 are very
close, 1.6 and 1.7 eV, respectively.
The molecular density distribution through the molecular
orbitals reflects the contribution of each atom in a molecular orbi-
tal. The localization of the density for the HOMO and LUMO for each
of the two complexes is summarized in Figs. 5 and 6. Interestingly,
the density localization is identical for the corresponding molecular
orbitals for the two complexes. The highest contribution for the
3.5. Catalytic epoxidation studies of complexes 9–11
After obtaining an insight into the coordination behavior of the
carbene ligands in the palladium(II)–NHC complexes 9–11 from
the aforementioned spectral and analytical studies, we undertook
an investigation to access their performances in the catalytic olefin
epoxidation of 1-octene and styrene using aqueous hydrogen per-
oxide as an oxidant in homogeneous medium. The blank experi-
ments without 9–11 show no formation of epoxide or any other
oxidized products with either with 1-octene or with styrene.
Therefore, it is believed that the catalytic performances displayed
are mainly due to the presence of active species of 9–11. The cat-
alytic performances of 9–11 in the epoxidation of 1-octene and sty-
rene are summarized in Tables 6 and 7, respectively. As is evident
from the aforementioned tables, the results of 9–11 towards the
olefins differed significantly due to the different activity of the
Table 4
Bond distances (Å) and angles (°) for the optimized structures of complexes 9 and 11.
Bond distance/angle
9
11
cis-
trans-
cis-
trans-
Table 5
Electronic and relative energies for complexes 9 and 11 according to DFT.
Pd–Cl
2.39
2.395
2.07
1.362
1.463
1.471
88.9
2.387
2.06
1.359
1.464
1.464
89.9
2.392
2.063
1.359
1.464
1.465
89.7
Pd–Ccarbene
C–N (benzimi)
N–CH2 (arom)
N–CH2 (aliph)
Cl–Pd–C
2.066
1.358
1.465
1.468
89.5
Complex
Energy (Hartree)
Relative
Relative
energy (Hartree)
energy (kcal/mol)
cis-9
ꢀ2661.554654
ꢀ2661.556486
ꢀ2818.712916
ꢀ2818.713806
0.00000
0.00183
0.00000
0.00089
–
trans-9
cis-11
trans-11
1.1908
–
0.5785
Pd–C–N
Cl–Pd–C–N
125.2
77.9
124.7
71.1
126.8
74.8
126.3
72.6

A.W. Salman et al. / Polyhedron 81 (2014) 499–510
507
1,2-epoxyoctane selectivity because of its highly active double
bond and the relative stability of the formed epoxide. However,
in the case of epoxidation of an exocyclic olefin, such as styrene,
only 13–19% conversion was observed with 100% selectivity
towards epoxide formation, which was the sole product in this
case. The catalytic reactions in the epoxidation of 1-octene by
9–11 are outlined in the Scheme 4.
911
cis
trans
cis
trans
In order to get more insight on the catalytic behavior of palla-
dium(II) complexes, we have tuned the steric bulk (2-methylpro-
pyl, 2-methylbutyl and n-hexyl) at one N-terminal of the
benzimidazole core, keeping the other (benzyl) constant. Varied
steric bulk around the active species of 9–11 affects the epoxida-
tion activity in the order 9 (2-methylpropyl) > 10 (2-methylbu-
tyl) > 11 (n-hexyl) towards both the olefins. Complex 9 provides
comparatively less steric bulk around the active palladium centers
allowing 1-octene and styrene conversion up to 58% and 19%,
respectively, with a much higher selectivity profile. Conversely,
in the case of complexes 10 and 11 the steric bulk around the
active palladium species is much higher, and as a result the % con-
version of 1-octene and styrene into their corresponding oxidized
products was found to be 46.5%, 42% and 14.5%, 13% respectively,
which is much less compared to complex 9. This phenomenon also
affected the selectivity of the epoxide formation of 1-octene, with
12 and 9.5% as the selectivity values [27]. This may due to the large
hexyl/2-methylbutyl group of 10/11 connected to the benzimid-
azole N-terminal which sterically hinders the olefin approach
towards the active site, while complex 9 with a smaller propyl
group can provide access to the olefin with some ease for the for-
mation of the epoxide. In all the cases of 1-octene epoxidation, the
epoxide was the minor product, whereas, 1,2-octanediol and 2-
octanone were the major constituents. Similar observations were
observed in the literature for numerous heterogeneous catalysts
[28]. The percentage conversion of 1-octene and the formation of
0.16eV
0.16eV
0.17eV
0.17eV
Fig. 4. Energy of the molecular orbitals for HOMOꢀ2 through to LUMO+2 and band
gaps for the cis and trans conformations of complexes 9 and 11.
double bonds. Epoxidation of a linear aliphatic olefin, such as
1-octene, by 9–11 displayed 42–58% conversion and 9.5–17%
Fig. 5. Density distribution for the HOMO and LUMO for complex 9.

508
A.W. Salman et al. / Polyhedron 81 (2014) 499–510
Fig. 6. Density distribution for the HOMO and LUMO for complex 11.
Table 6
Catalytic performance of the palladium complexes 9–11 in 1-octene epoxidation.
Entry
Catalyst
Time (h)
Oxidant
% Conversion
% Selectivity
1,2-Octanediol
2-Octanone
1,2-Epoxyoctane
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
2
3
4
4
4
4
4
4
1
2
3
4
1
2
3
4
1
2
3
4
H2O2
H2O2
H2O2
–
–
–
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
–
–
–
–
–
–
4.0
14.5
32.5
58.0
7.5
10.0
38.5
46.5
6.5
16.5
40.0
42.0
–
–
–
–
–
–
51.0
58.0
60.5
61.0
43.0
44.5
46.5
43.0
63.5
67.0
68.5
69.5
–
–
–
–
–
–
49.0
42.0
36.0
22.0
57.0
55.5
49.0
45.0
36.5
33.0
28.5
21.0
–
–
–
–
–
–
–
9
10
11
9
9
9
tracea
3.5
17.0
–
9
10
10
10
10
11
11
11
11
tracea
4.5
12.0
–
–
3.0
9.5
a
Not determined.
1,2-octanediol and 2-octanone on oxidation using aqueous hydro-
gen peroxide at 70 °C catalyzed by palladium complexes 9–11 is
shown in Fig. 7.
The aforementioned catalytic experiments were also performed
at the lower temperature of 40 °C to study the effect of tempera-
ture on the catalytic efficacies of complexes 9–11, however, no or

A.W. Salman et al. / Polyhedron 81 (2014) 499–510
509
Table 7
Catalytic performance of the palladium complexes 9–11 in styrene epoxidation.
Table 8
Influence of solvent on the palladium complexes 9–11 catalyzed epoxidation
reactions of olefinsb.
Entry
Catalyst
Time (h)
Oxidant
% Conversion
% Epoxide
Entry
Catalyst
Substrate
% Conversion (% of epoxide formation)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
2
3
4
4
4
4
4
4
1
2
3
4
1
2
3
4
1
2
3
4
H2O2
H2O2
H2O2
–
–
–
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
–
–
–
–
–
–
–
3.5
12.5
19.0
–
–
–
–
–
–
–
–
3.5
12.5
19.0
–
CH3CN
CH3COCH3
CH3Cl2
1
2
3
4
5
6
9
9
10
10
11
11
1-octene
styrene
1-octene
styrene
1-octene
styrene
58.0 (17.0)
19
56.5 (17.0)
12.5
53.0 (14.5)
5.5
9
10
11
9
9
9
46.5 (12.0)
14.5
44.0 (11.5)
11.0
40.5 (5.5)
5.0
42.0 (9.5)
13
40.5 (9.0)
11.5
34.0 (3.5)
4.5
9
b
c
% Yields of the products were calculated after 4 h of the reaction.
In the case of styrene epoxidation, epoxystyrene is the sole product.
10
10
10
10
11
11
11
11
–
–
7.5
14.5
–
–
8.5
13.0
7.5
14.5
–
–
8.5
13.0
4. Conclusions
In summary, a series of sterically tuned palladium(II)–NHC com-
plexes, 9–11, were prepared by the technique of transmetallation
from their silver(I) counterparts, 6–8. The latter complexes were
readily prepared by the reaction of silver(I) oxide with benzimidazo-
lium salts, 1–3, under mild conditions. The novel triazine-tethered
(benz)imidazole salts 4 and 5 were prepared by the reaction of cyan-
uric chloride and n-butyl (benz)imidazole under inert conditions.
The solid-state structure of the triazine-tethered Zwitterionic
imidazolium salt 4 was determined by the single crystal X-ray
diffraction method. However, metallation reactions of both the salts
with silver oxide/palladium chloride under different reaction condi-
tions ended with negative results due to the low acidity values of the
azolium-C2 protons. Complexes 9–11 showed good catalytic
activities for the olefin epoxidation reactions under homogeneous
conditions with aqueous hydrogen peroxide as an oxidant. A change
in the solvent affected the product formation, as reflected in the
decreased % conversion of substrates in the order acetonitrile >
acetone > dichloromethane. According to DFT calculations, the
trans-isomers of complexes 9 and 11 are more favorable by 1.2
and 0.6 kcal/mol respectively.
Scheme 4. Catalytic oxidation of 1-octene by the sterically modulated Pd(II)–NHC
complexes 9–11 with hydrogen peroxide.
Acknowledgments
negligible product formations were found. On the other hand, the
catalytic reactions were performed in a variety of solvents to study
the catalytic efficacies of complexes 9–11 in different solvent
media. The trend for the activity remains the same in all the
solvents, whereas the efficiency of 9–11 decreased by 2.5–6%
(1-octene) and 4.5–14.5% (styrene) in the order acetonitrile > ace-
tone > dichloromethane. As is evident from Table 8, the activity
in acetonitrile solvent was the best in all cases for both olefins.
Both the variations in the reaction conditions, temperature and
solvent, followed the general trend that the reaction rate was
enhanced with an increase in the reaction temperature and
polarity of the solvent. Similar results for various catalysts were
found in the literature [29].
N. Abdullah thanks the Malaysian Ministry of Higher Education
for the Fundamental Research Grant Scheme (FP008-2011A). S.
Endud thanks the Universiti Teknologi Malaysia (UTM) for the
Research University grant Q.J130000.2513.05H20 and MOSTI for
the National Nanotechnology Directorate Top-Down Grant
R.J130000.7926.4H007. The authors thank the Ibnu Sina Institute
for Fundamental Science Studies, UTM for research facilities.
Appendix A. Supplementary data
CCDC 901539 contains the supplementary crystallographic data
for Zwitterionictriazinetethered imidazolium salt 4. These data can
Fig. 7. Percentage conversion of 1-octene and the formation of 1,2-octanediol and 2-octanone from epoxidation using aqueous hydrogen peroxide at 70 °C catalyzed by
palladium complexes 9 (A), 10 (B) and 11 (C).

510
A.W. Salman et al. / Polyhedron 81 (2014) 499–510
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retrieving.html, or from the Cambridge Crystallographic Data
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336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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