Synthesis, characterization, density function theory and catalytic performances of palladium(II)-N-heterocyclic carbene complexes derived from benzimidazol-2-ylidenes

Article abstract of DOI:10.1016/j.ica.2015.08.027

1-Benzyl-3-ethylbenzimidazolium iodide (1) and 1-benzyl-3-(2′-nitrilebenzyl)benzimidazolium bromide (2) were prepared by the reaction of 1-benzylbenzimidazole with ethyl iodide or 2-bromomethylbenzonitrile to act as N-heterocyclic carbene (NHC) precursors. Bis-NHC silver(I) complexes having halide (3a and 4a) as well as hexafluorophosphate (3b and 4b) counterions were yielded by the reaction of NHC precursors with silver(I) oxide. Subsequent reactions of the silver(I) halide/hexafluorophosphate complexes with [PdCl₂(CNCH₃)₂] in methanol afforded the NHC palladium(II) complexes (5 and 6) via carbene transfer method. All synthesized compounds were fully characterized by analytical and spectrometric methods. Preliminary catalytic studies evinced that the nitrile-functionalized palladium(II)-NHC complex 6 is highly active in the oxidation of 1-octene as well as styrene in the presence of aqueous hydrogen peroxide as an oxidizing agent. Both the olefins were oxidized to their corresponding oxidized products with 45-52% conversion with moderate selectivity in the presence of NHC palladium complexes, which acted as oxidation catalysts. In order to investigate the suggested structure of the title complexes, density functional theory was used to find the stable structures of the palladium complexes and their isomers. Geometry parameters, molecular orbital energies, electronic energy, band gap, and vibrational frequencies were calculated.

Full text of DOI:10.1016/j.ica.2015.08.027

Accepted Manuscript
Synthesis, characterization, density function theory studies and catalytic per-
formances of palladium(II)-N-heterocyclic carbene complexes derived from
benzimidazol-2-ylidenes
Abbas Washeel Salman, Ghani Ur Rehman, Norbani Abdullah, Srinivasa
Budagumpi, Salasiah Endud, Hassan H. Abdallah
PII:
S0020-1693(15)00433-8
http://dx.doi.org/10.1016/j.ica.2015.08.027
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DOI:
Reference:
Inorganica Chimica Acta
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Please cite this article as: A.W. Salman, G.U. Rehman, N. Abdullah, S. Budagumpi, S. Endud, H.H. Abdallah,
Synthesis, characterization, density function theory studies and catalytic performances of palladium(II)-N-
heterocyclic carbene complexes derived from benzimidazol-2-ylidenes, Inorganica Chimica Acta (2015), doi: http://
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Synthesis, characterization, density function theory studies and catalytic
performances of palladium(II)-N-heterocyclic carbene complexes derived from
benzimidazol-2-ylidenes
Abbas Washeel Salmana,c, Ghani Ur Rehmanb, Norbani Abdullahc*, Srinivasa Budagumpid,
Salasiah Endudb*, Hassan H. Abdallahe
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, 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
Corresponding authors:
Prof. Dr. Salasiah Endud
Professor
Department of Chemistry, Faculty of Science
Universiti Teknologi Malaysia
81310 Skudai, Johor, Malaysia
H/P: +601-9731 1635, Fax: +60-7-5566162
E-mail: salasiah@kimia.fs.utm.my
Dr. Norbani Abdullah
Associate Professor
Department of Chemistry, Faculty of Science
University of Malaya
50603 Kuala Lumpur, Malaysia
H/P: +603-7967 4264
E-mail: norbania@um.edu.my
1

Abstract
1-Benzyl-3-ethylbenzimidazolium
iodide
(1)
and
1-benzyl-3-(2ί-
nitrilebenzyl)benzimidazolium bromide (2) were prepared by the reaction of 1-
benzylbenzimidazole with ethyl iodide or 2-bromomethylbenzonitrile to act as N-heterocyclic
carbene (NHC) precursors. Bis-NHC silver(I) complexes having halide (3a and 4a) as well as
hexafluorophosphate (3b and 4b) counterions were yielded by the reaction of NHC precursors
with silver(I) oxide. Subsequent reactions of the silver(I) halide/hexafluorophosphate complexes
with [PdCl2(CNCH3)2] in methanol afforded the NHC palladium(II) complexes (5 and 6) via
carbene transfer method. All synthesized compounds were fully characterized by analytical and
spectrometric methods. Preliminary catalytic studies evinced that the nitrile-functionalized
palladium(II)-NHC complex 6 is highly active in the oxidation of 1-octene as well as styrene in
the presence of aqueous hydrogen peroxide as an oxidizing agent. Both the olefins were oxidized
to their corresponding oxidized products with 45–52% conversion with moderate selectivity in
the presence of NHC palladium complexes, which acted as oxidation catalysts. In order to
investigate the suggested structure of the title complexes, Density Functional Theory was used to
find the stable structures of the palladium complexes and their isomers. Geometry parameters,
molecular orbital energies, electronic energy, band gap, and vibrational frequencies were
calculated.
Key words: Catalytic oxidation, N-heterocyclic carbene, Silver(I)-NHC complex, Palladium(II)-
NHC complex, DFT study.
2

1. Introduction
Organometallic chemistry of N-heterocyclic carbenes (NHCs) is an emerging field, which from a
general point of view exploits both, the catalytical and biological, potentials of transition as well
as inner-transition metal carbene complexes [1,2]. As an added advantage, NHCs are easier to
functionalize at both the heteroatoms and on azolium ring to tune the associated steric properties.
Interestingly, the NHC ligands react with metal centers through strong ꢀ-donation, and therefore
form more stable metal-carbene bonds. However, a very lesser degree of 4-back-donation from
NHCs is also possible [3]. Benzimidazole cores are among the important building units used in
the NHC ligand design and supramolecular chemistry of carbene complexes [4]. The
benzimidazole-derived NHC complexes are known as robust catalysts in C-C coupling reactions
[5]. These metal complexes were used as precursors for the preparation of other transition metal
NHC complexes via the technique of transmetallation [6]. The benzimidazole and other
functionalized moieties viz., pyridine, pyraole among others, can be integrated as part of many
bidentate donors to afford a class of potential catalysts for numerous catalytic reactions ranging
from various C-C coupling reactions to olefin polymerizations [7].
Palladium(II) complexes with NHC ligands (Chart 1) are widely used in organometallic
chemistry due to their potential catalytic properties and have been successfully applied for olefin
oligo/polymerization and metathesis among other reactions [8,9]. A series of allyl-palladium-
NHC complexes of dibenzyl or diadementyl substituted azolium salts have shown a unique
ability to catalyze selectively the aerobic oxidation of unprotected 1,2- and 1,3-diols, viz., ( )-
1,2-propanediol and 2R,3Rbutanediol and ( )-1,3-butanediol, respectively, and a 1:1 mixture of
meso/rac-2,4-pentanediol to hydroxy ketones [10]. In a similar example, the catalytic
performance of a dibromo palladium complex, which is immobilized on SBA-15 has shown
excellent catalytic activity in the aerobic oxidation of benzyl alcohol [11,12]. Based on seminal
work with [Pd(NHC)2(halide)2] and related functionalized-NHC complexes that show high
selectivity for C-C carbon bond forming and C-H activation reactions, we reasoned that similar
planar, palladium(II) based benzimidazole-derived NHC-complexes could provide a promising
platform for the activation of carbon-oxygen bond formation in the presence of an oxidizing
agent.
<Chart 1 here>
3

On the other hand, oxidation reactions with hydrogen peroxide, one of the ‘greenest’ terminal
oxidants, are of immense significance due to the importance of epoxides in the industrial
manufacture of both bulk and fine chemicals [13,14]. In general, the catalytic reactions that use
hydrogen peroxide in combination with readily available, non-hygroscopic and relatively non-
toxic palladium-derived catalysts are potentially viable for large-scale production in the
industries [15]. On this background, we investigated the synthesis of both, functionalized and
non-functionalized 1,3-disubstituted benzimidazolium salts and their silver(I) and palladium(II)-
NHC complexes. Later complexes were prepared by carbene transfer technique from silver(I) to
palladium(II), and were evaluated for their potential in the epoxidation of 1-octene and styrene in
the presence of 30% aqueous hydrogen peroxide. Theoretical calculations in general and ‘density
functional theory’ in particular was used to predict the best and stable conformation and to shed
light on many theoretical parameters for palladium(II) complexes such as the dipole moment,
electronic energy, cis-trans energy barrier and molecular orbitals. The starting structure was built
according to the crystal structure of the complex trans-bis[1-benzyl-3-(2,3,4,5,6-
pentafluorobenzyl)benzimidazol-2-ylidene]dibromopalladium(II).
2. Experimental
2.1. Reagents and instruments
All the chemicals used for the present investigation were of reagent grade, and the solvents were
dried and distilled before use according to the standard protocols. Benzimidazole, benzyl
bromide, 2-bromomethy benzonitrile, ethyl iodide, potassium hexafluorophosphate, silver(I)
oxide, palladium(II) chloride, 1-octene, styrene and 30% aqueous hydrogen peroxide were
purchased from Sigma–Aldrich and used without further purification. 1-Benzylbenzimidazole
was prepared according to the literature method [16] with slight modifications. 1H and 13C NMR
spectra of all the synthesized compounds were recorded at room temperature using a Bruker 400
MHz spectrometer in d6-DMSO or d3-CD3CN, using TMS as an internal reference. The FTIR
spectra of all the compounds were obtained in potassium bromide disks using a Perkin Elmer
2000 system spectrometer in the range 4000–400 cm-1. The melting points were assessed by
using a Stuart Scientific SMP-1 (UK) instrument. All reported compounds were analyzed for
carbon, hydrogen and nitrogen by the CHN microanalyses using a Perkin Elmer 2400 LS Series
4

CHN/S analyzer. Olefin oxidized products 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-ethylbenzimidazolium iodide (1)
To a solution of 1-benzylbenzimidazole (0.208 g, 1.0 mmol) in acetonitrile (30 mL), an excess of
ethyl iodide (0.21 g, 1.3 mmol) was added while stirring in a 50 mL round bottom flask fitted
with a condenser. The resulting reaction mixture was refluxed for 48 h in an oil bath. The solvent
was removed under reduced pressure by using a rotary evaporator to afford a beige solid, which
o
1
was washed with diethyl ether (3 × 5 mL) and dried. Yield: 83.8 %. M.P.: 125.5 C. H NMR
(400 MHz, d6-DMSO): ꢀ 1.6 (3H, t J = 7.5 Hz, CH3), 4.58 (2H, quartet J = 7.5 Hz, CH2), 5.8
(2H, s, benzylic CH2), 7.30-7.37 (5H, m, benzyl-H), 7.85-7.93 (2H, m, benzimidazole-H), 8.16-
13
8.23 (2H, m, benzimidazole-H) and 10.0 (H, s, benzimidazole-H2a). C{1H} NMR (100 MHz,
d6-DMSO): ꢀ 15.1 (CH3), 51.7 (CH2), 53.2 (benzylic CH2), 125.5 (C2,C6-benzyl), 127.8 (C3,C5-
benzyl), 129.3 (C4-benzyl), 135.9 (C1-benzyl), 118.5 (C4-benzimidazole), 118.8 (C5-
benzimidazole), 126.9 (C6-benzimidazole), 131.3 (C7-benzimidazole), 131.5 (C8,C9-
benzimidazole), and 139.3 (benzimidazolium C2a). FT-IR (KBr disc) cm-1: 2931, 2866 *(C–H),
1626 *(C=N, benzimidazole), 1011 *(C-N, benzimidazole). Anal. Calc. for C16H17N2I: C, 52.8;
H, 4.7; N, 7.7. Found: C, 52.3; H, 5.0; 7.3%.
2.2.2. Synthesis of 1-benzyl-3-(2-benzonitrile)benzimidazolium bromide (2)
This compound was prepared in a manner analogous to that for 1, only with 2-bromomethyl
benzonitrile (0.196 g, 1.0 mmol) instead of ethyl iodide. Preparation, further work up and
purifications were done according to the reported method [17]. Yield: 86.2 %. M.P.: 119-120 oC.
1H NMR (400 MHz, d6-DMSO): ꢀ 5.85 (2H, s, benzylic CH2), 6.1 (2H, s, benzonitrile CH2), 7.3-
7.41 (5H, m, Ar-CHbenzyl), 7.7-7.83 (8H, m, Ar-CHbenzonitrile), 7.91-8.0 Ar-
CHbenzimidazole), and 10.12 (1H, s, benzimidazolium H2´). 13C{1H}NMR (100 MHz, d6-
DMSO): ꢀ 48.2 (CH2), 50.1 (CH2), 111.1 (CꢁN), 112.8 (C2,C6-benzyl), 125.4 (C3,C5-benzyl),
126.4 (C4-benzyl), 135.3 (C1-benzyl), 113.8 (C5-benzonitrile), 127.7 (C4-benzonitrile), 130.4
(C3-benzonitrile), 132.3 (C1-benzonitrile), (C2-benzonitrile), 116.5 (C4-benzimidazole), 126.0
(C5-benzimidazole), 130.4 (C6-benzimidazole), 132.9 (C7-benzimidazole), 136.3 (C8,C9-
5

benzimidazole), 142.9 (benzimidazolium C2ί). FT-IR (KBr disc) cm-1: 2943, ~2860 *(C–H),
2224 *(CꢁN, benzonitrile), 1616 *(C=N, benzimidazole), ~1010 *(C-N, benzimidazole). Anal.
Calc. for C22H18N3Br: C, 65.4; H, 4.5; N, 10.4. Found: C, 65.3; H, 4.3; 10.8%.
2.2.3. Synthesis of bis-(1-benzyl-3-ethylbenzimidazolium)silver(I) iodide/hexafluorophosphate
(3a/3b)
A suspension of silver(I) oxide (0.126 g, 0.055 mmol) and benzimidazolium salt 1 (0.365 g, 1
mmol) in methanol (30 mL) were stirred at room temperature for 30 h in a 50 mL round bottom
flask wrapped with aluminum foil to exclude light. After which the reaction mixture was filtered
through a pad of celite to discard the unreacted silver(I) oxide. Filtrate was evaporated to dryness
under reduced pressure using a rotary evaporator to yield off-white silver(I)-NHC halide
complex 3a as a light-sensitive solid. The complex thus obtained was recrystallized by repeated
o
1
precipitation in methanol and diethyl ether solution. Yield: 67.2 %. M.P.: 202.5 C. ). H NMR
(400 MHz, d6-DMSO): ꢀ 1.45 (6H, t, J = 7.2 Hz, 2×CH3), 4.55 (4H, quartet J = 7.2 Hz, 2×CH2),
5.77 (4H, s, benzylic 2×CH2), 7.26-7.32 (6H, m, 2×benzyl-H), 7.48-7.55 (4H, t, J = 6.0 Hz,
2×benzyl-H), 7.75 (4H, d, J = 7.0 Hz, benzimidazole-H), 7.86 (4H, m, benzimidazole-H).
13C{1H} NMR (100 MHz, d6-DMSO): ꢀ 16.3 (CH3), 51.4 (CH2), 53.8 (benzylic CH2), 119.1
(C2,C6-benzyl), 125.4 (C3,C5-benzyl), 126.9 (C4-benzyl), 135.8 (C1-benzyl), 118.1 (C4-
benzimidazole), 120.2
(C5-benzimidazole), 125.4
(C6-benzimidazole), 132.2
(C7-
benzimidazole), 133.7 (C8,C9-benzimidazole), and 182.2 (C2a benzimidazolium-Ag). FT-IR
(KBr disc) cm-1: ~2935, 2872 *(C–H), ~1620 *(C=N, benzimidazole), 1073 *(C-N,
benzimidazole). Anal. Calc. for C32H32N4IAg: C, 54.3; H, 4.6; N, 7.9. Found: C, 54.8; H, 5.1;
8.1%. Further, the iodide complex 3a was directly converted into its hexafluorophosphate
counterpart by metathesis reaction using KPF6 (0.276 g, 1.5 mmol) in 30 mL methanol. The
resultant solution was stirred for 2 h during which a gray colored precipitate of 3b was formed
and was left to stand overnight. So obtained hexafluorophosphate complex 3b was filtered under
reduced pressure, washed with distilled water (3 x 5 mL) to remove the unreacted KPF6, and air
dried in dark. Spectral data of the complex 3b had no or negligible changes compared to its
iodide counterpart 3a. Microanalysis data of the complex is in well agreement with the proposed
structure.
2.2.4. Synthesis of bis-[1-benzyl-3-(2-benzonitrile)benzimidazolium]silver(I) bromide
6

/hexafluorophosphate (4a/4b)
These complexes were prepared in a manner analogous to that for 3a/3b, only with the
functionalized benzimidazolium salt 2 (0.404 g, 1.0 mmol) instead of non-functionalized salt 1.
Preparation, further work up and purifications of both the complexes were done according to the
o
established procedure [17]. Yield: 55.3 %. M.P.: 221-222 C. Both, 4a and 4b, had almost
1
similar spectral characteristics among which data of the complex 4a is given. H NMR (500
MHz, d6-DMSO): ꢀ 5.7 (2H, s, benzylic CH2), 5.95 (2H, s, benzonitrile CH2), 7.10-7.22 (5H, m,
Ar-CHbenzyl) 7.40-7.52 (4H, m, Ar-CHbenzonitrile), 7.65-7.86 (4H, m, Ar-CHbenzimidazole).
13C{1H} NMR (100 MHz, d6-DMSO): ꢀ 50.3 (CH2-benzyl), 51.9 (CH2-benzonitrile), 110.4
(CꢁN-nitrile), 117.1 (C2,C6-benzyl), 124.4 (C3,C5-benzyl), 127.2 (C4-benzyl), 133.3 (C1-
benzyl), 124.5 (C1-benzonitrile), 128.7 (C3,C4-benzonitrile), 133.7 (C5,C6-benzonitrile), 136.0
(C2-benzonitrile), 128.0 (C4-benzimidazole), 128.9 (C5-benzimidazole), 133.7 (C6-
benzimidazole), 139.3 (C7,C8-benzimidazole), 190.35 (d, J = 72.5 Hz, C2´ benzimidazolium-
Ag). FT-IR (KBr disc) cm-1: ~2940, 2872 *(C–H), 2223 *(CꢁN, benzonitrile), 1601 *(C=N,
benzimidazole), ~1070 *(C-N, benzimidazole). Anal. Calc. for C44H34N6BrAg: C, 63.3; H, 4.1;
N, 10.1. Found: C, 63.3; H, 4.5; 10.5%. Microanalysis data of the complex 4b is in well
agreement with its empirical formula.
2.2.5. Synthesis of bis-(1-benzyl-3-ethylbenzimidazolium)palladium(II) chloride (5)
To a solution of silver(I) bromide complex 3a (0.355 g, 0.05 mmol) in 20 mL dichloromethane
was added PdCl2(CH3CN)2 (0.130 g, 0.05 mmol). The reaction mixture was stirred for 14 h at
room temperature with the exclusion of light. The resulted suspension was filtered through a pad
of celite and the volume of resulting solution was reduced to 4 mL using a rotary evaporator.
Further, diethyl ether was added to the solution to yield the desired palladium(II)-NHC complex
5 as a pale brown powder. So obtained complex 5 was recrystallized by the repeated
precipitation of the complex in methanol using diethyl ether. This complex exhibits two sets of
proton and carbon resonances evidencing the existence of cis- and trans- isomers of complex 5.
Complex 5 is also obtained from silver hexafluorophosphate complex with almost same yield.
o
1
Yield: 48.3 %. M.P.: 263-264 C. H NMR (500 MHz, d3-CD3CN): ꢀ 1.64 (6H, t J = 7.5 Hz,
2×CH3), 1.85 (6H, t J = 7.5 Hz, 2×CH3), 4.85 (4H, quartet J = 7.5 Hz, 2×CH2), 5.03 (4H, quartet
J = 7.5 Hz, 2×CH2), 5.97 (4H, s, benzylic CH2), 6.19 (4H, s, benzylic CH2), 7.1-7.21 (6H, m,
7

benzyl-H), 7.34-7.42 (6H, m, benzyl-H), 7.68-7.75 (4H, m, benzimidazole-H) and 7.87-7.94 (4H,
m, benzimidazole-H). 13C{1H} NMR (125 MHz, d3-CD3CN): ꢀ 16.8, 17.2 (CH3), 50.2, 50.8
(CH2), 52.3, 52.7 (benzylic CH2), 118.6, 119.0, 125.0, 125.4, 126.9, 135.4, 135.8 (Ar-C benzyl),
118.1, 118.3, 120.2, 125.4, 132.2, 132.7, 133.7, (Ar-C benzimidazole), and 178.2 and 178.4
(C2abenzimidazolium-Pd). FT-IR (KBr disc) cm-1: ~2930, ~2870 *(C–H), 1643 *(C=N,
benzimidazole), 1077 *(C-N, benzimidazole). Anal. Calc. for C32H32N4Cl2Pd: C, 59.1; H, 5.0; N,
8.6. Found: C, 59.4; H, 5.1; 8.3%.
2.2.6. Synthesis of bis-[1-benzyl-3-(2-benzonitrile)benzimidazolium]palladium(II) chloride (6)
To a solution of 4a (0.418 g, 0.05 mmol) in 20 mL dichloromethne was added PdCl2(CH3CN)2
(0.130 g, 0.05 mmol). Following the similar procedure to 5, functionalized complex 6 was
isolated as a brown powder. Complex 6 is also obtained from silver hexafluorophosphate
o
1
complex with almost same yield. Yield: 56.3%. M.P.: 261-262 C. H NMR (500 MHz, d3-
CD3CN): ꢀ 6.14 (2H, s, benzylic CH2), 6.26 (2H, s, benzonitrile CH2), 7.1-7.22 (5H, m, Ar-
CHbenzyl), 7.34-7.46 (4H, m, Ar-CHbenzonitrile), 7.7-7.83 (4H, m, Ar-CHbenzimidazole).
13C{1H} NMR (125 MHz, d3-CD3CN): ꢀ 51.5 (CH2-benzyl), 52.3 (CH2-benzonitrile), 112.1
(CꢁN-nitrile), 118.5 (C2,C6-benzyl), 124.5 (C3,C5-benzyl), 128.2 (C4-benzyl), 132.2 (C1-
benzyl), 119.7 (C3-benzonitrile), 123.5 (C4,C5-benzonitrile), 126.7 (C1-benzonitrile), 136.1
(C2-benzonitrile), 127.4 (C4,C7-benzimidazole), 134.1 (C5,C6-benzimidazole), 138.8 (C8,C9-
benzimidazole), 174.2 (C2´ benzimidazolium-Pd). FT-IR (KBr disc) cm-1: 2942, 2864 *(C–H),
2222 *(CꢁN, benzonitrile), 1652 *(C=N, benzimidazole), 1074 *(C-N, benzimidazole). Anal.
Calc. for C44H34N6Cl2Pd: C, 64.1; H, 4.2; N, 10.2. Found: C, 63.9; H, 4.5; 10.4%.
2.3. Catalytic activity
The catalytic epoxidation of alkenes viz., 1-octene and styrene, with 30% aqueous hydrogen
peroxide was executed in a 25 mL round-bottom flask furnished with a condenser. In a
distinctive olefin epoxidation experiment, the flask was loaded with 10 mL of acetone (for 1-
octene) or acetonitirle (for styrene), 1.0 mmol alkene, 1.0 mmol hydrogen peroxide, and 1.0
mmol of palladium complex 5 or 6 as catalyst. The oxidation of olefin was carried out in an oil
bath with continuous stirring at refluxing temperature for 4 h. At appropriate intervals, aliquots
were removed from the reaction mixture and analyzed immediately by GC. Before injecting into
the GC, the aliquots were treated with diethyl ether and centrifuged to confiscate unreacted
8

hydrogen peroxide, water and the palladium complex. Further, the organic part was subsequently
separated and used for GC analysis. Alkene conversion (%) and the reaction products were
quantified using a GC instrument and identified by comparing the retention time of these with
the retention time to those of an authentic standard sample.
2.4. DFT Calculations
The structures of the isomers of the present palladium complexes were built based on the crystal
structure of the complex trans-bis[1-benzyl-3-(2,3,4,5,6-pentafluorobenzyl)benzimidazol-2-
ylidene]dibromopalladium(II)) in which, similarly, the palladium is in square-planar geometry
with a substituted benzimidazole-based NHC ligand field [18]. Gaussian 09 [19] was employed
to execute all the theoretical calculations. 6-31G(d,p) basis set was engaged for the atoms C, H,
N and Cl, whereas Pd was demonstrated with LANL2DZ basis set. The optimization was
achieved without any restriction in symmetry or internal coordinates. Frequencies calculations
have shown no imaginary frequency; and hence the optimized structures of all the isomers are in
their optimum minima. These structures were involved to calculate the full set of the molecular
orbitals, however, it is focused on the orbitals through HOMO-2 to LUMO+2 along with the
band gap energy as this the difference between the HOMO and LUMO set of orbitals, (LUMO-
HOMO).
3. Results and discussion
In the present article new palladium(II)-NHC-complexes of a functionalized and a non-
functionalized benzimidazol-2-lyidene moiety are reported. Both, benzimidazolium salts and
carbene-complexes are air and moisture stable, while silver-complexes were light sensitive. The
use of a nitrile-functionality of the benzimidazolium salt is to know the coordination mode of the
ligand, however, this found away from the metal center. Palladium(II)-NHC complexes were
used as homogeneous catalysts for the olefin epoxidation reactions is in the focus of our work.
CHN elemental analysis data of all the reported compounds is in well agreement with their
suggested structures. So far, only a limited number of palladium(II)-NHC complexes suitable for
olefin oxidation reactions have been studied, nevertheless, nitrile-functionalized complexes
employed in olefin epoxidation reactions is presented for the first time.
9

3.1. Syntheses
N-alkyl- or aryl- or aryl-alkyl substituted benzimidazolium salts are typically prepared by
reacting benzimidazole with alkyl- or aryl- or aryl-alkyl halide in stoichiometric ratio in the
presence of a strong base like sodium hydroxide [16]. Further, N,N’-disubstituted
benzimidazolium salts are prepared by the reaction of one equivalent of the corresponding N-
substituted benzimidazoles with one equivalent of alkyl- or aryl-halide [20]. This method was
successfully utilized to synthesize both, non-functionalized and functionalized, benzimidazolium
compounds as shown in Scheme 1. Refluxing 1-benzylbenzimidazole with ethyl iodide or 2-
bromomethylbenzonitrile in 1,4-dioxane for 2 days afforded the desired 1-benzyl-3-
ethylbenzimidazolium iodide (1) and 1-benzyl-3-(2’-benzonitrile)-benzimidazolium bromide (2),
respectively, as white powders in competitively good yield.
< Scheme 1 here >
< Scheme 2 here >
Further, direct reaction of two equivalents of benzimidazolium salts (1 and 2) with
[PdCl2(CH3CN)2] stirred at different temperatures in different solvents for different periods of
time results in no palladium complex formation. Conversely, the reaction of an imidazolium salt
with palladium acetate could yield palladium NHC complex [21]. Therefore, we followed the
technique of transmetallation to prepare the targeted palladium NHC complexes [22]. Reactions
of salts 1 and 2 with silver(I) oxide in methanol at room temperature for 30 h yielded colorless
silver-NHC halide complexes 3a [Ag(non-functionalized-NHC)2]I and 4a [Ag(nitrile-
functionalized-NHC)2]Br, respectively as shown in Scheme 2. Later, these complexes were
treated with excess potassium hexafluorophosphate in methanol for 4 h at room temperature to
afford silver-NHC hexafluorophosphate complexes 3b [Ag(non-functionalized-NHC)2]PF6 and
4b [Ag(nitrile-functionalized-NHC)2]PF6, in good yields.
The importance of silver-NHC complexes is apparent in their common use as a carbene transfer
agent in the preparation of other transition and inner-transition metal NHC complexes [23,24].
Reaction of 3a and 4a with [PdCl2(CH3CN)2] in methanol at room temperature for 12 h afforded
palladium complexes 5 [Pd(non-functionalized-NHC)2Cl2] and 6 [Pd(nitrile-functionalized-
NHC)2Cl2], respectively (Scheme 3), which is as a result of acetonitrile replacement at
palladium(II) and halide migration to Ag(I). The yields obtained with this carbene transfer
10

method were lower (48.3% for 5, 56.3% for 6), and the same inference is observed in the similar
studies when aforementioned route is used for the preparation of palladium-NHC complexes
[25].
< Scheme 3 here >
3.2. Spectral characterization
The formation of the benzimidazolium salts and their silver and palladium-NHC complexes in
each step can be nicely traced by 1H and 13C NMR, and FTIR spectroscopic techniques followed
by microanalysis data. The successful formation of the benzimidazolium salts 1 and 2 was
corroborated by characteristic downfield signals at ꢀ 10.0 and 10.12 for the benzimidazolium N-
CH-N protons (C2H), respectively. The identity of these NHC precursors 1 and 2 was further
evaluated by characteristic downfield signals of benzimidazolium carbene carbon resonances
(C2) at ꢀ 139.3 and 142.9, respectively. Apart from these characteristic proton and carbon
resonances, spectra of salts also evidenced the presence of benzylic (C12H for 1, C10H and
C17H for 2), aliphatic (C10H, C11H for 1) and other aromatic (C4H-C7H, C14H-C18H of 1 and
C4H-C7H, C12H-C16H, C19H-C22H of 2) proton and carbon resonances which are in well
agreement with the similar reported compounds [26,27].
The reaction of salts 1 and 2 with silver oxide could proceed smoothly in methanol at mild
condition, giving preferably the anti-configured NHC-silver halide complexes 3a and 4a. While
1
further forms complexes 3b and 4b via salt metathesis reactions with KPF6. H-NMR spectrum
of silver complexes in d6-DMSO reveals the complete disappearance of the resonances for the
acidic C2H-benzimidazole protons, evidencing the successful formation of the desired NHC-
silver complexes 3a/b and 4a/b. Further, their 13C NMR spectra also display noteworthy
downfield shifted resonances for the benzimidazolium carbene carbon nuclei to ꢀ 182.2 and
190.35 (C2 for 3a and C2 for 4a), thus pointing to successful NHC-silver complex formation.
Interestingly, the latter resonance is an apparent doublet centered at ꢀ 190.35 with coupling
constant 72.5 Hz. This resonance is presumably due to the coupling between the carbene carbon
of complex 4a and two isotopes of silver, viz., 109Ag and 107Ag nuclei. Almost similar results
were observed in the literature for benzimidazole-derived silver-NHC complexes [28]. Apart
from these major changes, spectra of these complexes also evidenced the presence of resonances
for other aliphatic and aromatic protons and carbon nuclei.
11

Furthermore, the formation of palladium-NHC complexes 5 and 6 was verified using 1H and 13C
NMR spectroscopy, which showed almost similar pattern to that of silver-NHC complexes with
slight variations in the peak positions and their integration values. A bit complicated 1H and 13C
NMR spectrum of complex 5 was mainly attributed to a dynamic process in solution. Both the
spectra evidenced the presence of two sets of resonances for each proton and carbon, which
confirmed the existence of cis- and trans- isomers in solutions. While complex 6 did not show
any isomerism in solution mainly due to more rigid palladium-carbene carbon bond, presumably
that originates from nitrile-functionality. Also, the steric factors originating from the
substitutions perhaps influenced the formation of rigid palladium-carbene carbon bond. Apart
13
from this, appearance of an apparent carbene carbon resonance at ꢀ 178.2 and 174.2 in the C
NMR spectra of palladium complexes 5 and 6, (C2) respectively, indicating the successful
formation of desired complexes [29,30].
Finally, benzimidazolium salts and carbene complexes were also monitored by FTIR spectral
technique over the range 4000–400 cm-1, to ensure their successful formation. Spectra of the
benzimidazolium salts show strong absorption bands at around 1010, 1610, and 2860 and 2930
cm-1 corresponding to the stretching vibrations of benzimidazole C-N, C=N and C-H modules,
respectively. In the case of salt 2, a sharp band with medium intensity was observed at 2222 cm-1
due to the stretching vibrations of nitrile functionality (CꢁN). However, in the complex FTIR
spectra, the band due to benzimidazole ring C-N and C=N vibrations showed a negative shifts
indicating the formation of carbene-complexes. Interestingly, in the case of silver and palladium
complexes of salt 2, nitrile starching vibrations remained unaffected indicating the presence of
aforementioned functionality away from the coordination spear.
3.3. Catalytic epoxidation studies
The activities of the palladium-NHC complexes, 5 and 6, were tested in the oxidation of 1-octene
and styrene. Both, the catalyzed oxidations of 1-octene as well as styrene, were carried out with
aqueous hydrogen peroxide to give mixture of oxidized products in different ratio. Control
experiments revealed that the presence of hydrogen peroxide and palladium complexes 5 and 6
were all essential for the oxidation of aforementioned olefins. Later, the olefin oxidation
character of both the palladium complexes over both the substrates was proven by conducting the
similar experiments with benzimidazolium salts 1 and 2, and their respective silver complexes 3a
12

and 4a. Since no significant catalytic activity was detected by compounds 1-4, hence the
oxidation activities of the two palladium complexes were considered truly by the insertion of
palladium center.
3.3.1. Epoxidation of 1-octene
Taking into account the oxidation of 1-octene, both the palladium complexes were active
(Scheme 4). As evident from the Table 1, the conversion and the selectivity values of both the
complexes were comparable. While the catalytic oxidation of 1-octene by these palladium
complexes did not occur at room temperature even in the presence of excess hydrogen peroxide.
Essentially, since palladium-carbene moieties, in both the cases, are almost similar but they are
having non-functionalized and nitrile-functionalized benzimidazolium cores, which are not
sensitive to the olefin oxidation. Therefore, they show a comparable behavior in terms of activity
and selectivity towards 1-octene. However, the nitrile-functionalized palladium complex 6
showed a higher conversion value (51.2%) with reference to the non-functionalized complex 5
o
(47.3%) at 70 C, which might be originated by the increased steric bulk due to the four benzyl
substitution around the palladium center. Such difference can also be ascribed to the formation of
isomers in solution, which is quite difficult to evaluate. Nevertheless, the activity of these
palladium-carbene complexes found comparable with the heterogeneous catalysts of first row
transition metals reported by us [31] as well as by others [32,33].
< Scheme 4 here >
< Table 1 here >
In terms of selectivity, non-functionalized palladium complex 5 displayed slightly better results
than its functionalized counterpart. Major product in the oxidation of 1-octene by complexes 5
and 6 was the 1,2-octanediol showing ca. 63.2 and 51.8% selectivity, respectively, after 4 h of
reaction. The other main product was the 2-octanone formed with the ca. 34.7 and 45.7%
selectivity along with ca. 2.1 and 2.5% of 1,2-epoxyoctane, respectively. This behavior of
product formation suggests that a similar reactivity takes place at both the palladium centers with
a narrow difference in the acidic character originating from the functionalization. Graphs of
substrate conversion and product formation by complexes 5 and 6 as a function of time are
shown in Figures 1 and 2, respectively. It is found that the yield of 1,2-octanediol increased as
the reaction time is increased in both the cases. It is well known that the formation of 1,2-
13

octanediol from its epoxy counterpart, 1,2-epoxyoctane, is catalyzed by acidic species.
Therefore, it is speculated that, palladium complexes 5 and 6 possess acid sites when activated
with oxidant, showed high yields of 1,2-octanediol. Almost similar catalytic performances are
displayed by palladium-NHC complexes having almost similar ligand field [31a].
< Figure 1 here >
< Figure 2 here >
3.3.2. Epoxidation of styrene
Palladium complexes, 5 and 6, were also tested as catalysts in the epoxidation with styrene with
hydrogen peroxide as terminal oxidant. Both the palladium complexes were active in the
epoxidation of styrene. Both, benzimidazolium salts 1 and 2, and their silver-carbene complexes
3a and 4a, did not induce product formation in the presence and absence of oxidant. Unlike the
formation of mixed products in the case of 1-octene, with styrene complexes 5 and 6 showed
formation of single product, i.e., styrene epoxide. However, the conversion of styrene found
much lesser compared to heterogeneous catalysts [34,35]. Therefore, it is predicted that the
activity associated with these palladium complexes is mainly because of the insertion of
palladium center. While the palladium complexes 5 and 6 did not catalyze styrene epoxidation at
o
room temperature even after for long time, and even at 70 C after 2 h in the presence of excess
hydrogen peroxide. The palladium complexes, 5 and 6, catalyzed the oxidation of styrene in the
o
presence of hydrogen peroxide at 70 C after 3 h with 6.5 and 8%, and after 4 h displayed 7.5
and 9% conversion, respectively. Further, even after prolonged reaction time, complexes did not
show any further increase in the conversion rate even with the varied oxidant to catalyst ratio.
This could be ascribed to the deactivation of active species at higher temperature. In order to
optimize the reaction condition to increase the percentage of substrate conversion, we tried the
catalytic styrene epoxidation reactions with different solvents at different temperatures,
unfortunately all reactions were ended with negative results.
3.4. Theoretical study using DFT method for complexes 5 and 6
A structurally similar palladium(II) complex with two un/substituted benzimidazole, trans-bis[1-
benzyl-3-(2,3,4,5,6-pentafluorobenzyl)benzimidazol-2-ylidene]dibromopalladium(II),
was
reported by Gökçe and coworkers [19]. In this complex, the palladium center was coordinated by
14

two bromido and two benzimidazole-based NHC ligands. The benzyl and the pentafluorobenzyl
substitutions on each benzimidazole core are forming a trans- conformation. Since the present
palladium(II) complexes 5 and 6 possess similar architectures forming a complex with two
substituted benzimidazole-based NHCs and two chlorido ligands, it is appropriate to build the
present palladium complexes 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 of the similar compound [18] from the literature.
< Table 2 here >
As depicted in Table 2, the values of the dipole moment of the complex 5 in the cis- and trans-
conformations are less than 0.5 Debye. The dipole moment of the complex 6 in the cis-
conformation is much higher than its trans- conformer, which shows the difference in the charge
distribution between the two conformations. In contrast, the energy difference or the band gap
between the HOMO and LUMO of the four isomers of both the palladium complexes are very
close with a value around 1.7 eV.
The electronic energies of the optimized complexes in the cis- and trans- conformations with the
cis-trans energy barrier are listed in Table 3. Interestingly, the energy difference between these
two conformations for the complex 5 is less than 0.5 kcal/mol and the trans- isomer is more
stable with that value. The cis- conformation of the complex 6 is more stable than the cis-
conformation with an energy barrier of -11.8 kcal/mol.
< Table 3 here >
The optimized structures of the present palladium complexes were used to calculate the energies
of the molecular orbitals (MOs). In order to have a general idea about the molecular orbitals, a
number of orbitals beside the HOMO and LUMO orbitals are shown in Figure 3. The figure
showing the molecular orbitals through HOMO-2 to LUMO+2 and the band gaps for the cis- and
trans- conformations for the complexes 5 and 6. The band gaps for the two complexes 5 and 6
are close to 1.7 eV.
< Figure 3 here >
The molecular density distribution through the molecular orbital is reflecting the contribution of
each atom in the molecular orbital. The localization of the density for the HOMO and LUMO
15

orbitals for the palladium complexes 5 and 6 are summarized in Figures 4 and 5, respectively.
Interestingly, the density localization is identical for the corresponding molecular orbitals for
these two complexes. The highest contribution for the 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 the palladium and the two chlorido ligands, there is some contribution
from the carbene of the benzimidazole groups, as shown in Figures 4 and 5.
< Figure 4 here >
< Figure 5 here >
4. Conclusions
Two non-symmetrically substituted benzimidazolium salts as NHC precursors can be efficiently
complexed with silver(I) in good yields. Owing to the easier crystallization of the silver-carbene
complexes with bulkier anions like hexaflurophosphate anion, halide complexes were converted
into their hexaflurophosphate counterparts by salt metathesis reaction. Palladium-carbene
complexes of the NHC precursors were prepared by the carbene transfer reaction using silver
complexes. Both, benzimidazolium salts and their carbene-complexes, were evaluated as
catalysts for oxidation of olefins viz., 1-octene and styrene, using hydrogen peroxide as terminal
oxidant. Both the palladium complexes showed moderate substrate conversion values in the
range 47-51% with good selectivity. However, in the case of styrene epoxidation, percentage
conversion of substrate found much lesser compared to heterogonous catalysts. DFT calculations
have shown that the cis and trans energy barrier of the complex 5 is close to 0.5 kcal/mol and the
trans isomer is more stable while the energy barrier for the complex 6 is -11.8 kcal/mol, hence
the cis conformation found more stable.
Acknowledgments
S. Endud thanks Universiti Teknologi Malaysia (UTM) for the Research University grant
Q.J.130000.7113.02H23 and MOSTI for the National Nanotechnology Directorate Top-Down
Grant R.J130000.7926.4H007. Authors thank the Ibnu Sina Institute for Fundamental Science
Studies, UTM for research facilities.
16

Appendix A. Supplementary material
Bar diagrams showing percentage conversion of 1-octene and percentage selectivity as a function
of time are given in the supplementary material.
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19

Chart 1. Catalytically active benzimidazole-derived Pd(II)-NHC complexes.
(Pri)S
Mes
Br
Br
Pd
Br
Br
R
R
N
N
N
N
N
N
Br
N
Pd
N
N
Pd
P R
N
N
N
N
Pd
R
R
Br
Br
Br
R=H,Me,Ph
R=Ph,m-tol,C6H10
R=COOH
O
IV
I
II
III
S(iPr)
Scheme 1. Synthesis of benzimidazol-2-ylidenes.
20

22
18
21
20
23
N
Br
24
2
11
N
N
19
10
17
N
4
4
7
9
8
I
Dioxane
N3
9
5
6
N 3
5
6
Dioxane
Reflux, 48 h
2 I
Br
1
Reflux, 48 h
N
1
N
8
7
10
11
12
18
16
13
17
16
15
14
12
14
1
2
15
13
Scheme 2. Synthesis of bis(carbene)-Ag complexes.
R
R
R
N
N
N
N
N
N
N
Methanol
Methanol
KPF6
Stir, 3-4 h
X
+ Ag2O
Ag
X
Ag
2
Stir, 24-30 h
N
N
N
PF6
R
R
3a: R=CH3, X=I
4a: R=2-CN-Ph, X=Br
1: R=CH3, X=I
2: R=2-CN-Ph, X=Br
3b: R=CH3
4b: R=2-CN-Ph
21

Scheme 3. Synthesis of bis(carbene)-Pd complexes via carbene transfer reaction.
R
R
N
N
N
N
[PdCl2(CH3CN)2]
Ag
X
PdCl2
Methanol
Stir, 10-12 h
2
N
N
R
3: R=CH3, X=I, PF6
4: R=2-CN-Ph, X=Br, PF6
5: R=CH3
6: R=2-CN-Ph
22

Scheme 4. Catalytic oxidation of 1-octene by Pd(II)-NHC complexes 5 and 6 in the presence of
hydrogen peroxide.
OH
HO
1,2-octanediol
+
O
[Cat 5, 6]
H2O2, Me2CO
1-octene
70 oC, 4 h
2-octanone
+
O
1,2-epoxyoctane
Figure 1. Percentage conversion of 1-octene and the formation of 1,2-octanediol and 2-octanone
from epoxidation using aqueous hydrogen peroxide at 70 oC catalyzed palladium complex 5.
23

ꢆꢀ
ꢅꢀ
ꢀ
ꢄꢀ
ꢃꢀ
ꢂꢀ
ꢁꢀ
ꢀ
ꢀ
ꢁ
ꢂ
ꢃ
ꢄ
ꢍ1IVꢉ^._
ꢈ1QCꢉꢊQ`I:ꢋ1QJ
QJ0V`ꢇ1QJ
ꢌHꢋ:JQJVꢉ`Q`I:ꢋ1QJ
Figure 2. Percentage conversion of 1-octene and the formation of 1,2-octanediol and 2-octanone
from epoxidation using aqueous hydrogen peroxide at 70 oC catalyzed palladium complex 6.
24

ꢆꢀ
ꢅꢀ
ꢀ
ꢄꢀ
ꢃꢀ
ꢂꢀ
ꢁꢀ
ꢀ
ꢀ
ꢁ
ꢂ
ꢍ1IVꢉ^._
ꢃ
ꢄ
QJ0V`ꢇ1QJ
ꢈ1QCꢉꢊQ`I:ꢋ1QJ
ꢌHꢋ:JQJVꢉ`Q`I:ꢋ1QJ
25

Figure 3. Energy of the molecular orbitals for HOMO-2 through to LUMO+2 and band gaps for
the cis and trans conformations for the complexes 5 and 6.
Complex 6
cis
Complex 5
trans
cis
0.17 eV
0.16 eV
0.17 eV
0.17 eV
26

Figure 4. Density distribution for the HOMO and LUMO molecular orbitals for the complex 5.
28

Figure 5. Density distribution for the HOMO and LUMO molecular orbitals for the complex 6.
29

Table Captions
Table 1: Catalytic performance of the palladium complexes 5 and 6 in 1-octene oxidation.
Table 2: Bond distances and angles for the optimized complexes 5 and 6 in comparison with the
crystal structure of complex trans-bis[1-benzyl-3-(2,3,4,5,6-pentafluorobenzyl)benzimidazol-2-
ylidene]dibromopalladium(II) [18].
Table 3: Electronic energies and relative energy for the complexes 5 and 6 according to the
density functional theory.
30