Synthesis, characterization and crystal structures of silver(I)- and gold(I)-N-heterocyclic carbene complexes having benzimidazol-2-ylidene ligands

Article abstract of DOI:10.1016/j.jorganchem.2014.01.038

A series of N,N′-disubstituted benzimidazolium salts (1-3) were prepared via the successive N-alkylation method. Cationic, linearly coordinated silver(I)-NHC complexes (4-6) were synthesized in good yield by the reaction of Ag₂O with salts 1-3. The corresponding cationic gold(I)-NHC complexes (7-9) of silver(I)-NHC derivatives were prepared by the technique of transmetallation at mild reaction condition. The new azolium salts and their carbene complexes were characterized by ¹H and ¹³C NMR and FTIR spectroscopy, and elemental analysis. Additionally, benzimidazolium salts 2 and 3, silver(I)-carbene complexes 4a and 6a, and gold(I)-carbene complex 8 were structurally characterized by single crystal X-ray diffraction technique.

Full text of DOI:10.1016/j.jorganchem.2014.01.038

Journal of Organometallic Chemistry 757 (2014) 42e50
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis, characterization and crystal structures of silver(I)e and
gold(I)eN-heterocyclic carbene complexes having benzimidazol-2-
ylidene ligands
,
,
a
Mohammed Z. Ghdhayeb a b, Rosenani A. Haque a , Srinivasa Budagumpi
*
a The School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
b Department of Chemistry, College of Science, University of Kufa, Najaf, Iraq
a r t i c l e i n f o
a b s t r a c t
A series of N,N0-disubstituted benzimidazolium salts (1e3) were prepared via the successive N-alkylation
method. Cationic, linearly coordinated silver(I)eNHC complexes (4e6) were synthesized in good yield by
the reaction of Ag2O with salts 1e3. The corresponding cationic gold(I)eNHC complexes (7e9) of silver(I)
eNHC derivatives were prepared by the technique of transmetallation at mild reaction condition. The
new azolium salts and their carbene complexes were characterized by 1H and 13C NMR and FTIR spec-
troscopy, and elemental analysis. Additionally, benzimidazolium salts 2 and 3, silver(I)ecarbene com-
plexes 4a and 6a, and gold(I)ecarbene complex 8 were structurally characterized by single crystal X-ray
diffraction technique.
Article history:
Received 6 December 2013
Received in revised form
20 January 2014
Accepted 26 January 2014
Keywords:
Ag(I)-complex
Au(I)-complex
N-Heterocyclic carbene
Steric modulation
X-ray diffraction
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
metals to which silver(I)eNHCs are most widely transferred are
gold(I) and palladium(II) [6].
N-Heterocyclic carbenes (NHCs) are interesting class of ligands
in modern organometallic chemistry with diverse ligational be-
haviors. Numerous NHC transition metal complexes are employed
on a huge scale to produce catalytically and biologically relevant
agents, which are demonstrated by several recently published ar-
ticles [1]. Among others, silver(I)eNHC complexes gained special
and considerable attention by the organometallic chemists due to
their versatile applications in catalysis [2] as well as in biology [3].
Wang and Lin were the first to report the use of NHCesilver(I)
complexes to prepare other NHC-transition metal complexes via
carbene transfer method [4]. This transfer route provided accept-
able way of preparing transition metal carbene complexes that are
hard to obtain by free carbene method. Nowadays, the use of NHCe
silver(I) complexes for the preparation of other complexes is
becoming much better than conventional methods especially, for
the preparation of late transition metal carbene complexes [5].
Transmetallation using silver(I)eNHC complexes is advantageous
over free carbene method as this can be performed in air with no
decrease in yield, and affords easy metal complex formation. The
On the other hand, bioorganometallic chemistry of silver(I)e and
gold(I)eNHC complexes is a significant area in both therapeutic and
diagnostic medicine. Development of transition metal carbene
complexes as metal-based drugs is a much focused task, and great
efforts are required to get a compound of interest. In spite of all
limitations and side effects of periodic table metal complexes,
transition metal complexes are still the most widely used chemo-
therapeutic agents and make a large contribution to medicinal
therapeutics. Even though platinum-based complexes had been in
primary focus of research on chemotherapeutic agents [7], the in-
terests in this field have shifted to non-platinum based agents [8] in
order to find different metal complexes with less side effects and
similar cytotoxicity. Thus, a wide variety of metal carbene com-
plexes based on silver, palladium and gold are being intensively
studied as platinum replacements. Among these, silver(I)e and
gold(I)eNHC complexes displayed significant results for anticancer
activity against a number of human derived cancer cell lines [9]. As a
consequence, intense research on the structure activity relationship
of these metal carbene complexes has emerged during the past five
years using sterically and electronically modulated carbene systems
[10]. In this article, we present the synthesis, structure and trans-
metallation studies of biologically relevant benzimidazole-based
salts and their respective silver(I)e and gold(I)eNHC complexes.
* Corresponding author. Tel.: þ60 194118262.
E-mail address: rosenani@usm.my (R.A. Haque).
http://dx.doi.org/10.1016/j.jorganchem.2014.01.038
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
43
Scheme 1. Preparation of carbene ligand precursors 1e3.
2. Results and discussion
salts 1e3 were prepared by the successive N-alkylation method as
shown in Scheme 1. For the symmetrical carbene ligand precursor
1, allylation at both N-terminals of benzimidazole was carried out at
once by refluxing a solution of benzimidazole and allyl bromide
(1:2 ratio) in acetonitrile for 24 h. However, in the case of carbene
ligand precursors 2/3, N-butyl/methyl benzimidazole was treated
with allyl bromide/2-bromomethylbenzonitrile in acetonitrile at
refluxing temperature for 24/20 h, respectively affording the ben-
zimidazolium salts with reasonable yields.
In the corresponding complexes, the silver(I)eNHCs having
bromide counter ion 4e6 were prepared in good yields via the in
situ deportation of benzimidazolium salts by silver(I) oxide. Ben-
zimidazolium salts 1e3 were treated with silver(I) oxide in meth-
anol at mild reaction conditions to afford silver(I)eNHC complexes
4e6 with 78e93% yields (Scheme 2). These reactions were carried
out in equipment wrapped with aluminum foil to avoid the light.
Owing to the fact that, silver(I)eNHC complexes with bulkier
counterions such as, hexafluorophosphate, tetrafluoroborate,
perchlorate, among others are easy to crystallize, complexes 4e6
were subjected to salt metathesis reactions using KPF6 in methanol
to afford silver(I)eNHC hexafluorophosphate complexes 4ae6a in
good yields (Scheme 3). These complexes were found stable to light
for few days.
Considering the practical advantages and wide availability of
simple bis-NHC silver(I)e and gold(I)-complexes of the type
[(NHC)eMe(NHC)]þXꢀ (M ¼ Ag, Au and X ¼ Cl, Br), we were
prompted to study the effect of a steric modulation of the NHCs on
their structure. Among the few spectacular categories of sterically
tuned (benz)imidazole-based NHCesilver(I) and palladium(II)
complexes, we were logically inclined to opt for members of our
previously reported series of similar backbones [11], however,
with different N-substitutions and the metal center. The NHC
precursors and their carbene complexes reported herein are
thermally stable up to their decomposition temperature and are
nonhygroscopic in nature. The complexes are insoluble in water,
but are readily soluble in DMSO, DMF, (m)ethanol and acetonitrile.
Elemental analysis data is in well agreement with the structures of
compounds.
2.1. Syntheses
Successive N-alkyl/arylation of benzimidazole allowed a desir-
able, wide-ranging tunability to access NHC precursors with
various steric and electronic properties. Reported benzimidazolium
Scheme 2. Synthetic pathway to silver(I)eNHC complexes (4e6) from NHC precursors 1e3.

44
M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
Scheme 3. Salt metathesis reactions of silver-carbene complexes 4e6.
Following the transmetallation reactions using silver(I)eNHC
determinations is an indicative of the successful carbene precursor
formation [12].
bromide complexes 4e6, gold(I)eNHC complexes 7e9 were ob-
tained with 77e88% yields after having filtered off the silver(I)
bromide precipitate from a dichloromethane solution as shown in
Scheme 4. The reactions for gold(I)eNHC complexes were con-
ducted at room temperature using gold(I) chloride dimethyl sulfide
as a gold source. This method allows for the preparation of NHCe
Successful formation of silver(I)eNHC complexes 4e6 was
indicated by the complete disappearance of NCHN proton signal
at ca. 9.75 ppm in their 1H NMR spectra, with respect to that of
the carbene precursors. Further, it is also worth notice that the
respective values of the carbon chemical shifts of their NCHN
carbon center are downfield shifted at ca. 190 ppm, nearly 42 ppm
more compared to their corresponding carbene precursors.
These two observations along with the crystal structure de-
terminations collectively evidenced the successful formation of
the desired silver(I)eNHC complexes. Whereas in the case of the
anionic carbene complexes of gold(I) 7e9, carbon chemical shifts
of NCHN carbon center is still more downfield shifted at ca.
191 ppm, which could be due to the more electropositive nature
of the gold(I) center. This observation along with the crystal
structure determination of gold(I) complex collectively confirmed
the formation of target complexes [13]. On the other hand,
aliphatic and aromatic carbon resonances in carbene complexes
were almost similar to that of corresponding carbene precursors
with negligible differences in peak position and intensity. All
carbene complexes and their precursors were stable in solution;
exhibiting good solubility in common deuterated organic
solvents.
gold(I) complexes
7 and 8 having symmetrically and non-
symmetrically substituted benzimidazole-2-ylidenes, respectively.
Whereas, nitrile-functionalized gold(I)eNHC complex 9 was pre-
pared straightforwardly at the same complexation conditions.
Nevertheless, efficient purification could be achieved by the
repeated precipitation of gold(I) complexes in methanol using
diethyl ether.
2.2. NMR spectral studies
All new compounds were fully characterized in solution by NMR
spectroscopic methods and for five of them, namely 2, 3, 4a, 6a and
8, by an X-ray structure analysis. In the 1H NMR spectra of the
carbene precursors 1e3, the NCHN protons resonate at ca.
9.75 ppm, approximately 1.5 ppm more downfield shifted
compared to that of the corresponding acidic C2 proton in the
monosubstituted N-alkyl benzimidazoles. Apart from this, spectra
also evidenced the peaks in the range 1.0e5.5 and 6.9e7.9 ppm,
which are attributed to resonance of aliphatic and aromatic pro-
tons, respectively. The corresponding NCHN carbon signals in the
13C NMR spectra of carbene precursors were also observed at more
downfield positions at ca. 142 ppm compared to those of the more
acidic C2H carbons in the respective monosubstituted N-alkyl
benzimidazoles. This observation along with crystal structure
2.3. FT-IR spectral studies
All compounds were characterized using FT-IR spectral tech-
nique though carbene precursors 1 and 2, and their carbene
complexes do not contain any functional group. However, carbene
precursor 3 and its silver(I)e and gold(I)ecarbene complexes
Scheme 4. Synthetic pathway to gold(I)eNHC complexes (7e9) from silver(I)eNHC complexes 4e6 via transmetallation method.

M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
45
Table 1
Crystal data and structure refinement details for compounds 2, 3, 4a, 6a and 8.
2
3
4a
6a
8
Formula
C14H21N2BrH2O
315.24
Triclinic
P1, (No. 1)
8.5628(19) 9.773(2),
10.250(2)
68.477(4), 84.455(4),
75.437(4)
772.3(3)
C16H14N3Br
328.20
Monoclinic
P21/c, (No. 14)
8.2554(4) 10.5079(6),
17.1114(9)
C26H28AgN4F6P
649.36
Monoclinic
Cc, (No. 9)
10.1366(14), 18.235(3),
15.753(2)
C32H26AgN6F6P
747.43
Triclinic
P-1, (No. 2)
10.1513(3), 12.0531(3),
14.7855(4)
66.742(1), 71.660(2),
72.434(2)
1544.59(8)
2
C28H36AuN4ClO
663.02
Monoclinic
P21/c, (No. 14)
16.8881(4), 14.7745(4),
23.5643(6)
90.00, 94.272(1), 90.00
Formula weight
Crystal system
Space group
ꢀ
a, b, c [A]
a
,
b,
g
[ꢁ]
90.00, 96.392(1), 90.00
90.00, 103.087(2), 90.00
3
ꢀ
V [A ]
1475.13(14)
2
1.478
2836.2(7)
4
1.521
5863.3(3)
4
1.552
Z
2
D (calc) [g/cm3]
1.656
1.607
Mu (MoK
F(000)
a
] [/mm]
2.654
328
2.780
664
0.828
1312
0.774
752
5.137
2720
Crystal size [mm]
Temperature (K)
0.10 ꢂ 0.19 ꢂ 0.34
0.14 ꢂ 0.26 ꢂ 0.44
0.14 ꢂ 0.37 ꢂ 0.49
0.05 ꢂ 0.08 ꢂ 0.38
0.23 ꢂ 0.24 ꢂ 0.74
100
100
100
100
100
ꢀ
Radiation [A]
MoK
a
0.71073
MoKa
a
0.71073
MoK
a
0.71073
MoKa
a
0.71073
MoK a 0.71073
1.2e27.5
46570, 13445
0.042
q
MineMax [ꢁ]
2.14e29.40
11030, 4698
0.042
2.3e30.1
16033, 4316
0.028
2.2e30.1
11808, 7011
0.028
1.9e29.0
32708, 8094
0.057
Tot.; Uniq. Data
Rint
Nref; Npar
R, wR2, S
10685, 303
0.0468, 0.0719, 1.056
4316, 181
0.0221, 0.0581, 1.05
7011, 373
0.0479, 0.1619, 0.98
8094, 417
0.0526, 0.0909, 1.06
13445, 635
0.0472, 0.1474, 1.03
possess a nitrile-functionality, which can take part in the coor-
dination or can make a weak attraction with metal centers. This
possibility can be easily traced by FT-IR spectral technique [14].
The FT-IR stretching vibrational bands for the benzimidazolium
ring in carbene precursors 1e3 were observed at ca. 1550 and
1100 cmꢀ1, whereas aliphatic and aromatic CeH vibrations at ca.
2930 and 2860 cmꢀ1, respectively. The former vibrations shifted
to lower energy region in the corresponding carbene complexes
indicating the formation of the carbene complexes. This shift
is due to the presence of electropositive metal center, which af-
fects the C]N and CeN bond vibrations by dragging electron
density towards itself. The nitrile stretching vibrations in the
carbene precursor 3 was observed at 2224 cmꢀ1. Interestingly, in
the respective carbene complexes this band remains almost un-
altered with peak intensity and position, suggesting its nonin-
volvement in the coordination. This observation is further
supported by the single crystal structure determination of the
silver(I)ecarbene complex 6. Observed stretching vibrational
bands and their assignments are in well agreement with the
previous reports [15].
2.4. Single crystal X-ray diffraction studies
The crystal structures for carbene precursors 2 and 3, silver(I)e
carbene complexes 4a and 6a, and gold(I)ecarbene complex 8 were
obtained, and the crystal data and structure refinement details are
provided in Table 1. Single crystals of these compounds suitable for
X-ray diffraction analysis were obtained either by slow evaporation
of their methanolic solution or by slow diffusion of diethyl ether
into a solution of compound in methanol/acetonitrile at room
temperature. Carbene precursor 2 crystallized in the triclinic space
group P1 having two crystallographically independent structural
units. A perspective view of carbene precursor 2 is shown in Fig. 1.
Pertinent bond distances and angles of 2 are tabulated in Table 2.
An asymmetric unit of 2 consists of two crystallographically inde-
pendent units of benzimidazolium cations, two bromide anions
and two water molecules. While, both benzimidazole rings are
coplanar with slightly different geometric parameters. The internal
ring angle of benzimidazole (N1eC1eN2) at the carbene carbon
center is 109.5(11)ꢁ for N1AeC1AeN2A and 112.0(7)ꢁ for N2Be
C1BeN1B. These bond angles are in well agreement with those
Fig. 1. Molecular structures of two crystallographically units of 2 with displacement ellipsoids drawn at 50% probability.

46
M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
Table 2
Table 3
ꢁ
ꢁ
ꢀ
ꢀ
Selected bond distances (A) and angles ( ) for 2.
Selected bond distances (A) and angles ( ) for 3.
ꢀ
ꢀ
Bond distance (A)
Bond distance (A)
N1AeC8A
N1AeC1A
N2AeC1A
N2AeC12A
N2AeC7A
N1AeC2A
C6AeC7A
Bond angle (ꢁ)
N1AeC8AeC9A
C8AeN1AeC1A
N1AeC1AeN2A
C1AeN2AeC12A
N2AeC12AeC13A
1.473(12)
1.315(16)
1.382(15)
1.425(13)
1.370(10)
1.334(9)
N1BeC8B
N1BeC1B
N2BeC1B
N2BeC12B
N2BeC7B
N1BeC2B
C6BeC7B
1.507(12)
1.313(12)
1.284(12)
1.532(13)
1.376(8)
N2eC16
N2eC7
N2eC6
N1eC7
Bond angle (ꢁ)
C16eN2eC7
C16eN2eC6
N2eC7eN1
C7eN1eC8
C7eN1eC1
1.4576(17)
1.3291(16)
1.3954(17)
1.3365(16)
N1eC1
N1eC8
C10eC15
N3eC15
1.3943(16)
1.4627(16)
1.4427(19)
1.142(2)
1.411(9)
1.3900(7)
125.38(12)
125.99(11)
110.23(12)
125.30(11)
108.38(11)
N1eC8eC9
113.28(10)
118.89(11)
119.73(11)
121.72(12)
178.60(17)
1.3900(11)
C8eC9eC10
C9eC10eC15
C9eC10eC11
N3eC15eC10
114.2(9)
123.3(9)
109.5(11)
126.1(11)
114.3(11)
N1BeC8BeC9B
C8BeN1BeC1B
N1BeC1BeN2B
C1BeN2BeC12B
N2BeC12BeC13B
109.4(8)
129.8(8)
112.0(7)
125.1(8)
108.1(10)
perpendicular to the bc-plane. In the case of complex 4a, one of the
allyl wingtip is disordered over two sets along with fluorides of
hexafluorophosphate unit. However, complex 6a is a well ordered
structure, having almost coplanar benzimidazole units. Both car-
bene ligands adopt a syn-arrangement around the silver(I) center,
which is due to the sterically demanding nature of the benzonitrile
substituents. Similarly, both benzonitrile arms were twisted out of
the CeAgeC module on the side pointing nitrile-functionalities
away from the metal center. This observation is in agreement
with the assignment based on FT-IR spectral analysis. Similar ob-
servations were found in the previously reported nitrile-
functionalized silver(I) carbene and coordination complexes
[17,19,20]. In the extended crystal structure, both complexes
showed similar type of intermolecular hydrogen bonds of CeHeF
observed in the related benzimidazolium salts [16]. In the extended
structure, benzimidazolium cations and bromide anions of 2 are
ꢀ
ꢀ
connected via OHwatereBr (3.286 A), CHbenzimieO (3.315 A) and
ꢀ
CHbenzimieBr (3.131 A) intermolecular hydrogen bonds to form a
three dimensional array.
Carbene precursor 3 is a well ordered bromide salt crystallized
in the monoclinic space group of P21/c comprising one benzimi-
dazolium cation and one bromide anion in an asymmetric unit. A
perspective view of the salt is shown in Fig. 2, and the selected bond
distances and angles are tabulated in Table 3. Planes of benzimid-
azole are benzonitrile rings are almost perpendicular to each other
with a dihedral angle of 113.28(10)ꢁ for N1eC8eC9 module. The
angle at nitrile carbon is found almost linear with the bond angle of
178.60(17) for N3eC15eC10, which is accordance with our previous
reports on nitrile-functionalized azolium salts [17]. Like 2, carbene
precursor 3 shows comparable bond angle (110.23(12)ꢁ for N1e
C7eN2) at carbene carbon center, illustrating the successful salt
formation. In the crystal packing of 3, two types of intermolecular
ꢀ
(2.560e2.665 A) connecting the complex units into three dimen-
sional networks.
Single crystal X-ray diffraction results of gold(I)ecarbene com-
plex 8 revealed that the crystals are comprised of monoclinic unit
cells in the space group P21/c having two crystallographically in-
dependent structural units possessing slightly different geometric
parameters. Molecular structures of complex 8 are depicted in
Figs. 5 and 6. Selected bond distances and angles of complex 8 are
tabulated in Table 6. Gold(I)ecarbene complex 8 comprises two
crystallographically independent gold(I) bisecarbene complex
cations along with two chloride ions and four water molecules in an
asymmetric unit. The central gold(I) ion adopts a distorted linear
ꢀ
ꢀ
hydrogen bonds, CeHeBr (2.868 A) and CeH—Nnitrile (3.189 A) are
observed.
Silver(I)ecarbene complex 4a (Fig. 3) crystallized in the mono-
clinic space group Cc, while complex 6a (Fig. 4) crystallized in the
triclinic space group P1. Pertinent bond distances and angles of
complexes 4a and 6a are tabulated in Tables 4 and 5, respectively.
These complexes possess one bisecarbene complex cation and one
hexafluorophosphate anion in their asymmetric unit. Both com-
plexes show a slightly distorted linear coordination geometry
around the silver(I) center with the bond angle of 172.58(15) (C1e
Ag1eC8) and 176.09(13)ꢁ (C1eAg1eC17) for 4a and 6a, respec-
tively. The internal ring angles at carbene carbon centers are
decreased to 104.9(5)ꢁ (N1eC1eN2), 105.3(5)ꢁ (N3eC8eN4) and
106.0(3)ꢁ (N1eC1eN2), 105.6(3)ꢁ (N4eC17eN5) in both cases
respectively, which is attributed to the formation carbene-silver(I)
bonds [18]. Carbene-silver-carbene module found almost
Fig. 3. Molecular structure of 4a with displacement ellipsoids drawn at 50%
Fig. 2. Molecular structure of 3 with displacement ellipsoids drawn at 50% probability.
probability.

M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
47
Table 5
ꢁ
ꢀ
Selected bond distances (A) and angles ( ) for 6a.
ꢀ
Bond distance (A)
Ag1eC1
Ag1eC17
N1eC1
N2eC1
N4eC17
N5eC17
N1eC8
2.080(3)
2.072(3)
1.350(4)
1.358(4)
1.350(4)
1.364(4)
1.463(4)
N2eC9
N4eC24
N5eC25
N3eC16
N6eC32
P1eF1
1.467(4)
1.463(4)
1.463(4)
1.148(4)
1.144(5)
1.602(2)
1.605(2)
P1eF2
Bond angle (ꢁ)
C1eAg1eC17
Ag1eC1eN1
Ag1eC1eN2
Ag1eC17eN4
Ag1eC17eN5
176.09(13)
126.6(2)
127.2(2)
128.0(2)
126.3(2)
N1eC1eN2
106.0(3)
105.6(3)
114.3(3)
179.0(4)
176.8(4)
N4eC17eN5
N2eC9eC10
C15eC16eN3
C31eC32eN6
arrangement of the carbene ligands. Reported benzimidazolium
salts and their silver(I)e and gold(I)eNHC complexes displayed
similar structure with the biologically relevant complexes showing
potential anticancer activity.
Fig. 4. Molecular structure of 6a with displacement ellipsoids drawn at 50%
probability.
4. Experimental
4.1. Reagents and instruments
coordination sphere with 176.1(3)o for C1AeAu1AeC8A and
178.4(3)ꢁ for C1BeAu1BeC8B. The internal ring angles [106.8(6)ꢁ
for N1AeC1AeN2A, 107.0(6)ꢁ for N3AeC8AeN4A, 105.7(7)ꢁ for
N1BeC1BeN2B and 106.3(6)ꢁ for N3BeC8BeN4B] at carbene car-
bon centers are comparable with that of silver(I)ecarbene com-
plexes and are much lesser than carbene precursors. On the other
hand, gold(I)ecarbene carbon bond lengths were found in the
All chemicals and solvents were obtained from commercial
sources and all reagents and solvents were of analytical grade
and used without further purifications. Benzimidazole, 1-
methylbenzimidazole, ally bromide, 2-bromomethylbenzonitrile,
n-butyl bromide, silver(I) oxide, potassium hexafluorophosphate
and gold(I)chloride(dimethyl sulfide) were purchased from Sigmae
Aldrich. The carbene ligand precursors were prepared according to
an established procedure. Nuclear magnetic resonance spectra
were recorded in d6-DMSO or CDCl3 using a Bruker 500 MHz
Ascend spectrometer at room temperature. The 1H and 13C NMR
peaks were labeled as singlet (s), doublet (d), triplet (t), and
multiplet (m). Chemical shifts were referenced with respect to
solvent signals. Elemental analysis was carried out on a Perkin
Elmer Series II, 2400 microanalyzer. The FT-IR spectra of the
ꢀ
range 2.019(7)e2.026(7) A, which are comparable with that of the
other gold(I)ecarbene complexes [21]. Like silver(I)-carbene com-
plexes 4a and 6a, in the case of gold(I) complex 8 the carbene li-
gands adopt a syn-arrangement around the metal center. In the
crystal packing of complex 8, two types of intermolecular hydrogen
ꢀ
ꢀ
bonds OHwater—Cl (3.253 A) and CHeCl (3.202 A) were observed
along with other short contacts leading to the three dimensional
network.
3. Conclusion
A series of sterically modulated carbene complexes of silver(I)
and gold(I) were prepared and their solid (2, 3, 4a, 6a and 8) as well
as solution state structures were studied. Gold(I)ecarbene com-
plexes were prepared by the technique of transmetallation using
silver(I)ecarbene complexes. Single crystal X-ray diffraction studies
of carbene complexes revealed the presence of distorted linear
coordination geometry around the metal center with syn-
Table 4
ꢁ
ꢀ
Selected bond distances (A) and angles ( ) for 4a.
ꢀ
Bond distance (A)
Ag1eC1
Ag1eC8
N1eC1
N2eC1
N1eC15
N2eC18
N3eC21
2.105(5)
2.056(6)
1.327(7)
1.328(6)
1.349(13)
1.518(8)
1.413(8)
N4eC24
N1eC2
N2eC7
N3eC9
N4eC14
P1eF1
1.485(7)
1.379(5)
1.396(5)
1.333(5)
1.347(5)
1.570(10)
1.613(8)
P1eF2
Bond angle (ꢁ)
C1eAg1eC8
Ag1eC1eN1
Ag1eC1eN2
Ag1eC8eN3
172.58(15)
129.9(4)
124.9(4)
124.9(4)
Ag1eC8eN4
N1eC1eN2
N3eC8eN4
F1eP1eF6
129.8(4)
104.9(5)
105.3(5)
163.9(8)
Fig. 5. Molecular structure of a crystallographically independent unit (A) of 8 with
displacement ellipsoids drawn at 50% probability.

48
M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
removed under a vacuum. The oily residual was then washed
thoroughly with fresh dioxane (2 ꢂ 3 mL) and diethyl ether
(2 ꢂ 3 mL) to yield a viscous brown liquid. Yield: 71%. 1H NMR
(500 MHz, d6-DMSO):
d
5.41 (4H, s, NeCH2), 5.18 (4H, d, J ¼ 13 Hz, ]
CHcise), 5.43 (4H, m, ]CHtranse), 6.12 (2H, m, CH]CH2), 7.71e8.02
(4H, m, benzimi-ArH) and 9.75 (1H, s, benzimi-H20); 13C {1H}NMR
(125 MHz, d6-DMSO):
(CH]CH2), 126.89, 127.90, 129.20, 131.36 (benzimi-ArC) and 140.86
(benzimi-C20). FT-IR (KBr disc) in cmꢀ1: 2881, 2939
(CeH), 1524,
1146 (benzimidazole ring).
d 49.23 (NeCH2), 113.49 (CH2]CHe), 120.70
n
n
4.2.2. Synthesis of 1-allyl-3-butylbenzimidazolium bromide (2)
To a solution of butyl benzimidazole (0.50 g, 1.9 mmol) in
acetonitrile (20 mL), allyl bromide (0.24 g,1.9 mmol) was added and
refluxed for 20 h. The mixture turned brownish like salt 1 after
completion of reaction. Then the solvent was removed under a
vacuum giving the product as brown solid, washed thoroughly with
diethyl ether (2 ꢂ 5 mL) and recrystallized using methanol. Single
crystals suitable for X-ray diffraction analysis were obtained by
slow evaporation of the salt solution in methanol at room tem-
perature. Yield: 86%, M.P.: 161e163 ꢁC. 1H NMR (500 MHz, d6-
DMSO):
d
0.93 (3H, t, J ¼ 7.2 Hz, CH3), 1.35 (6H, m, CH2eCH2eCH3),
1.91 (5H, m, CH2eCH2eCH3), 4.50 (2H, t, J ¼ 7.2 Hz, NeCH2Bu), 5.15
(2H, d, J ¼ 5.8 Hz, NeCH2), 5.39 (2H, d, J ¼ 5.8 Hz, ]CHtranse), 5.43
(2H, s, ]CHcise), 6.24 (2H, m, CH]CH2), 8.00e8.12 (4H, m, ben-
zimi-ArH) and 9.75 (1H, s, benzimi-H20); 13C{1H}NMR (125 MHz,
Fig. 6. Molecular structure of a crystallographically independent unit (B) of 8 with
displacement ellipsoids drawn at 50% probability.
d6-DMSO): d 13.00 (CH3), 19.50 (CH3eCH2eCH2), 30.05 (CH3eCH2e
compounds were recorded in potassium bromide disks using a
Perkin Elmer 2000 system spectrometer in the range 4000 to
400 cmꢀ1. The instruments are available at The School of Chemical
Sciences, Universiti Sains Malaysia (USM). The X-ray diffraction
data were collected using a Bruker SMART APEX2 CCD area-
detector diffractometer. Structures were solved by direct methods
and refined by full-matrix least-squares against F2. Calculations,
structure refinement, molecular graphics and the material for
publication were performed using the SHELXTL and PLATON (Spek,
2009) software packages available at The School of Physics, USM.
CH2), 46.50 (NeCH2Bu), 49.00 (NeCH2allyl), 113.50, 120.50, 126.50,
131.00 (benzimi-ArC) and 142.10 (benzimi-C20). FT-IR (KBr disc) in
cmꢀ1: 2875, 2937
n(CeH), 1522, 1141 n(benzimidazole ring). Anal.
Calcd for C14H19N2Br: C, 56.9; H, 6.5; N, 9.5%. Found: C, 55.5, H, 6.7,
N, 9.0%.
4.2.3. Synthesis of 1-methyl-3-{(20-nitrile)benzyl}benzimidazolium
bromide (3)
This compound was prepared in a manner analogous to that for
2, only with 1-methylbenzimidazole (0.23 g, 1.5 mmol) and 2-
bromomethylbenzonitrile (0.30 g, 1.5 mmol) instead of 1-
butylbenzimidazole and allyl bromide. Single crystals suitable for
X-ray diffraction analysis were obtained by slow diffusion of diethyl
ether into the methanolic solution of salt at room temperature.
4.2. Synthesis of benzimidazolium salts
4.2.1. Synthesis of 1,3-bis-(allyl)benzimidazolium bromide (1)
To a stirred solution of benzimidazole (0.25 g, 2.1 mmol) in
dioxane (10 mL), allyl bromide (0.51 g, 4.2 mmol) was added in one
portion and the mixture was stirred at reflux for one day. A brown
colored oily product was formed, after cooling, the solvent was
Yield: 78.5%, M.P.: 140e141 ꢁC. 1H NMR (500 MHz, d6-DMSO):
d 4.14
(3H, s, NeCH3), 6.05 (2H, s, benzylic-CH2), 7.45, 7.55e7.76 (4H, m,
benzyl Ar-H), 7.89, 7.96, 8.10 (4H, m, benzimi-ArH) and 9.85 (1H, s,
benzimi-H20); 13C {1H} NMR (125 MHz, d6-DMSO):
d 36.10 (NeCH3),
51.00 (benzylic-CH2), 111.10, 112.75, 124.70, 134.81 (benzimi-ArC),
118.63 (N^C), 112.40, 125.10, 130.03 (benzyl-ArC), 141.01 (benzimi-
Table 6
C20). FT-IR (KBr disc) in cmꢀ1: 2935, 2862
n
(CeH), 2224
n(C^N),
ꢁ
ꢀ
Selected bond distances (A) and angles ( ) for 8.
1534, 1118
n(benzimidazole ring) Anal. Calc. for C16H15BrN3: C, 58.4;
ꢀ
Bond distance (A)
Au1AeC1A
Au1AeC8A
N1AeC1A
N2AeC1A
N3AeC8A
N4AeC8A
N1AeC15A
H, 4.6; N, 12.7%. Found: C, 58.1; H, 4.7; N, 12.5%.
2.024(7)
2.026(7)
1.356(9)
1.330(9)
1.332(9)
1.359(10)
1.466(10)
1.468(9)
1.465(10)
1.480(10)
Au1BeC1B
Au1BeC8B
N1BeC1B
N2BeC1B
N3BeC8B
N4BeC8B
N1BeC15B
N2BeC19B
N3BeC22B
N4BeC26B
2.020(8)
2.019(7)
4.3. Synthesis of silver(I)eNHC complexes
1.341(11)
1.349(10)
1.357(10)
1.353(10)
1.485(10)
1.480(11)
1.469(11)
1.469(10)
4.3.1. Synthesis of bis{1,3-bis-(allyl)benzimidazolium}silver(I)
bromide (4)
To a suspension of Ag2O (0.33 g, 1.4 mmol) in methanol (20 mL),
was added 1 (0.20 g, 0.7 mmol)) and stirred at room temperature
for 16 h in the equipment wrapped with aluminum foil to avoid
light. The reaction mixture was filtered through a pad of Celite, and
the resulted colorless filtrate was concentrated to 5 mL under a
vacuum. A white solid was afforded by addition of 75 mL diethyl
ether to the filtrate. So obtained solid was redissolved in acetoni-
trile and 150 mL of diethyl ether was added to reprecipitate the
solid, which was isolated by filtration and dried. Single crystals
suitable for X-ray diffraction analysis were grown by slow diffusion
N2AeC19A
N3AeC22A
N4AeC26A
Bond angle (ꢁ)
C1AeAu1AeC8A
Au1AeC1AeN1A
Au1AeC1AeN2A
Au1AeC8AeN3A
Au1AeC8AeN4A
N1AeC1AeN2A
N3AeC8AeN4A
176.1(3)
127.2(5)
126.1(5)
129.8(6)
123.1(5)
106.8(6)
107.0(6)
C1BeAu1BeC8B
Au1BeC1BeN1B
Au1BeC1BeN2B
Au1BeC8BeN3B
Au1BeC8BeN4B
N1BeC1BeN2B
N3BeC8BeN4B
178.4(3)
127.7(6)
126.6(6)
127.2(6)
122.3(8)
105.7(7)
106.3(6)

M.Z. Ghdhayeb et al. / Journal of Organometallic Chemistry 757 (2014) 42e50
49
of diethyl ether into the methanolic solution of the complex. Yield:
84.3%, M.P.: 144e146 ꢁC. 1H NMR (500 MHz, d6-DMSO):
5.15 (4H,
repeated precipitation in methanol using diethyl ether. Yield: 77.5%,
M.P.: 171 ꢁC. 1H NMR (500 MHz, d6-DMSO):
5.11 (4H, m, NeCH2),
d
d
m, NeCH2), 5.22 (2H, s, 2ꢂ ¼ CHtranse), 5.32 (2H, s, 2ꢂ ¼ CHcise),
5.34 (2H, s, 2ꢂ ¼ CHtranse), 5.47 (2H, s, 2ꢂ ¼ CHcise), 6.28 (2H, m,
6.19 (2H, m, CH]CH2), 7.49, 7.70 (4H, m, benzimi-ArH); 13C{1H}
CH]CH2), 7.86e7.94 (4H, m, benzimi-ArH); 13C{1H}NMR (125 MHz,
NMR (125 MHz, d6-DMSO):
123.90 (CH]CH2), 125.21, 133.08, 133.61 (benzimi-ArC), 189.03
(CcarbeneeAg). FTIR (KBr disc) cmꢀ1: 2868, 2923
(CeH), 1476, 1192
(benzimidazole ring). Anal. Calc. for C26H28N4AgBr: C, 53.3; H, 5.2;
N, 9.6%. Found: C, 53.2; H, 5.4; N, 9.3%.
d
51.00 (NeCH2), 112.07 (CH2]CHe),
d6-DMSO):
123.87,133.11,134.61 (benzimi-ArC),184.33 (CcarbeneeAu). FTIR (KBr
disc) cmꢀ1: 2873, 2936
(CeH), 1467, 1131 (benzimidazole ring).
Anal. Calc. for C26H28N4AuCl: C, 49.6; H, 4.5; N, 8.9%. Found: C, 50.2;
H, 4.9; N, 9.3%.
d 51.71 (NeCH2), 111.60 (CH2]CHe), 121.00 (CH]CH2),
n
n
n
n
4.3.2. Synthesis of bis{1-allyl-3-butylbenzimidazolium}silver(I)
bromide (5)
4.4.2. Synthesis of bis{1-allyl-3-butylbenzimidazolium}gold(I)
chloride (8)
This complex was prepared in a manner analogous to 4, only
with 2 (0.19 g, 0.7 mmol) instead of 1. Yield: 93.1%. M.P.: 230e
This complex was prepared in a manner analogous to 7, only
with 5 (0.35 g, 0.5 mmol) instead of 4. Single crystals of gold(I)
complex 8 suitable for X-ray diffraction studies were grown by the
slow diffusion of ethyl ether into the methanolic solution of the
complex at room temperature. Yield: 88.4%, M.P.: 249e250 ꢁC. 1H
231 ꢁC. 1H NMR (500 MHz, d6-DMSO):
d
1.00 (3H, t, J ¼ 8.1 Hz, CH3),
1.45 (6H, m, CH2eCH2eCH3), 1.97 (5H, m, CH2eCH2eCH3), 4.53 (2H,
t, J ¼ 6.7 Hz, NeCH2Bu), 5.13 (2H, s, NeCH2CH), 5.17 (1H, t,
J ¼ 5.5 Hz, ]CHtranse), 5.33 (1H, t, J ¼ 5.5 Hz, ]CHcise), 6.15 (2H, m,
CH]CH2), 7.45e7.55 (4H, m, benzimi-ArH); 13C{1H}NMR (125 MHz,
NMR (500 MHz, CDCl3):
d
1.00 (3H, t, J ¼ 7.5 Hz, CH3), 1.44 (2H, m,
CH2eCH2eCH3), 2.04 (2H, m, CH2eCH2eCH3), 4.63 (2H, t, J ¼ 7.2 Hz,
NeCH2), 5.31 (2H, m, NeCH2allyl), 5.25 (1H, s, ]CHcise), 5.38 (1H, s,
]CHtranse), 6.16 (1H, m, eCH]CH2) and 7.63 (4H, m, benzimi-ArH),
7.92 (4H, d, J ¼ 7.0 Hz, benzimi-ArH); 13C {1H} NMR (125.0 MHz,
d6-DMSO): d 13.31 (CH3), 20.20 (CH3eCH2eCH2), 33.52 (CH3eCH2e
CH2), 50.08 (NeCH2Bu), 53.21 (NeCH2CH), 113.31, 118.65 (CH]
CH2), 125.61, 127.93, 132.81, 134.20 (benzimiAreC) and 187.41
(CcarbeneeAg). FTIR (KBr disc) cmꢀ1: w2860, 2925
n
(CeH), 1488,
CDCl3):
d 13.30 (CH3), 20.04 (CH3eCH2eCH2), 32.41 (CH3eCH2e
1187
n(benzimidazole ring). Anal. Calc. for C28H36N4AgBr: C, 54.4;
CH2), 49.67 (NeCH2Bu), 51.51 (NeCH2CH), 112.45 (CH]CH2), 119.30
(CH]CH2), 124.72, 131.90, 133.24 (benzimi-ArC) and 190.90 (Ccar-
H, 6.2; N, 9.1%. Found: C, 54.7; H, 6.4; N, 8.8%.
beneeAu). FTIR (KBr disc) cmꢀ1: 2865, 2928
n
(CeH), 1470, 1184
(benzimidazole ring). Anal. Calc. for C28H36N4AuCl: C, 50.7; H, 5.8;
N, 8.5%. Found: C, 51.3; H, 5.9; N, 8.3%.
4.3.3. Synthesis of bis[1-methyl-3-{(20-nitrile)benzyl}
benzimidazolium]silver(I) bromide (6)
n
This complex was prepared in a manner analogous to 4, only
with 3 (0.23 g, 0.7 mmol) instead of 1. Yield: 78.6%, M.P.: 205e
4.4.3. Synthesis of bis[1-methyl-3-{(20-nitrile)benzyl}
benzimidazolium]gold(I) chloride (9)
206 ꢁC. 1H NMR (500 MHz, d6-DMSO):
d 4.05 (3H, s, NeCH3), 5.93
(2H, s, benzylicCH2), 7.20 (2H, d, J ¼ 7.8 Hz, benzimi-ArH), 7.47 (4H,
This complex was prepared in a manner analogous to 7, only
with 6 (0.34 g, 0.5 mmol) instead of 4. Yield: 81.2%, M.P.: 221e
m, benzimi-ArH), 7.57 (2H, d, J ¼ 6.0 Hz, benzyl-ArH), 7.86e7.94
(4H, m, benzyl-ArH); 13C {1H} NMR (125 MHz, d6-DMSO):
d 35.42
222 ꢁC. 1H NMR (500 MHz, CDCl3):
d 3.47 (3H, s, NeCH3), 5.32 (2H, s,
(NeCH3), 50.00 (benzylic-CH2), 117.31 (C^N), 110.42, 124.44, 134.27
(benzimi-ArC), 121.11, 128.10, 129.30, 139.41 (benzyl-ArC), 191.80
benzylic-CH2), 6.94 (2H, d, J ¼ 7.0 Hz, benzimi-ArH), 7.16e7.23 (4H,
m, benzimi-ArH), 7.40 (2H, d, J ¼ 7.2 Hz, benzyl-ArH), 7.64e7.73 (4H,
(CcarbeneeAg). FTIR (KBr disc) cmꢀ1: 2882, 2937
n
(CeH), 2221
(benzimidazole ring). Anal. Calc. for
32H26N6AgBr: C, 56.2; H, 4.1; N, 12.3%. Found: C, 56.4; H, 4.4; N,
11.9%.
m, benzyl-ArH); 13C {1H} NMR (125 MHz, CDCl3):
d 27.52 (NeCH3),
n
(C^N), 1466, 1183 n
42.39 (benzyl-CH2), 110.38 (C^N), 112.31, 125.40, 129.31, 140.00
(benzimi-ArC), 122.04, 128.10, 133.24 (benzyl-ArC), 191.50 (Ccar-
C
beneeAu). FTIR (KBr disc) cmꢀ1: 2874, 2927
n(CeH), 2222
n(C^N),
1476, 1192
n(benzimidazole ring), Anal. Calc. for C32H26N6AuCl: C,
4.3.4. Salt metathesis of silver(I)eNHC bromide complexes (4e6) to
silver(I)eNHC hexafluorophosphate complexes (4ae6a)
52.8; H, 3.7; N, 11.5%. Found: C, 53.0; H, 4.1; N, 11.3%.
A solution of silver(I)eNHC complex 4 (0.30 g, 0.5 mmol) in
methanol (20 mL) was added KPF6 (0.092 g, 0.5 mmol) and stirred
for 3e4 h at room temperature. The solvent was removed under a
vacuum to afford white solid of 4a. This residual solid was thor-
oughly washed with water to remove unreacted KPF6, and was
isolated by filtration and dried. In the same manner, silver bromide
complexes 5 and 6 were converted into their hexafluorophosphate
counterparts 5a and 6a, successfully. Further, NMR and FTIR spec-
tral data of the hexafluorophosphate derivatives are as same as
their bromide derivatives. Elemental analysis results of these
complexes are in well agreement with their structure.
Acknowledgments
R.A.H. thanks Universiti Sains Malaysia (USM) for the Short term
Grant 304/PKIMIA/6311123 and the Research University Grant
1001/PKIMIA/811217. M.Z.G. thanks USM for the RU grant 1001/
PKIMIA/844137 and Kufa University, Najaf, Iraq for a postgraduate
research scholarship. S.B. thanks USM for a postdoctoral research
fellowship.
Appendix A. Supplementary material
CCDC 849996, 849997, 849998, 962984, and 862525 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
4.4. Synthesis of gold(I)eNHC complexes
4.4.1. Synthesis of bis{1,3-bis-(allyl)benzimidazolium}gold(I)
chloride (7)
A mixture of silver(I)ecarbene complex 4 (0.3 g, 0.5 mmol) and
AuCl(SMe2) (0.15 g, 0.5 mmol) in dichloromethane (20 mL) was
stirred at room temperature for 12 h in an equipment wrapped
with aluminum foil to exclude light. It was then filtered through a
pad of Cilite and the solvent was removed under a vacuum to obtain
an off-white solid. So obtained complex was recrystallized by
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