A straightforward synthesis of 4,5-dihalofunctionalized imidazol-2-ylidene/imidazolyl-metal complexes from trihaloimidazolium salts/imidazoles: Structure and catalytic studies

Article abstract of DOI:10.1016/j.jorganchem.2017.09.018

Backbone dihalofunctionalized imidazol-2-ylidene/imidazolyl-metal complexes (5?13) of group–10 metal ions were synthesized through a proficient and single step oxidative addition method using 2,4,5-trihaloimidazolium salts or imidazoles respectively. The donor property of 4,5-diiodofunctionalized imidazol-2-ylidene was described for the first time. The catalytic activity of palladium complexes containing imidazol-2-ylidene (6 and 7)/imidazolyl (10 and 11) groups towards Suzuki-Miyaura coupling reaction of aryl bromides was evaluated and compared with its unsubstituted counterpart. Besides, the investigation of reaction pathway in catalysis was carried out.

Full text of DOI:10.1016/j.jorganchem.2017.09.018

Accepted Manuscript
A straightforward synthesis of 4,5-dihalofunctionalized imidazol-2-ylidene/imidazolyl-
metal complexes from trihaloimidazolium salts/imidazoles: Structure and catalytic
studies
Iruthayaraj Avinash, Vivek Gupta, Vedhagiri Karthik, Ganapathi Anantharaman
PII:
S0022-328X(17)30538-7
10.1016/j.jorganchem.2017.09.018
JOM 20093
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 11 July 2017
Revised Date: 5 September 2017
Accepted Date: 7 September 2017
Please cite this article as: I. Avinash, V. Gupta, V. Karthik, G. Anantharaman, A straightforward
synthesis of 4,5-dihalofunctionalized imidazol-2-ylidene/imidazolyl-metal complexes from
trihaloimidazolium salts/imidazoles: Structure and catalytic studies, Journal of Organometallic Chemistry
(2017), doi: 10.1016/j.jorganchem.2017.09.018.
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ACCEPTED MANUSCRIPT
A Straightforward Synthesis of 4,5-Dihalofunctionalized Imidazol-2-
ylidene/Imidazolyl-Metal
Complexes
from
Trihaloimidazolium
Salts/Imidazoles: Structure and Catalytic Studies
Iruthayaraj Avinash, Vivek Gupta, Vedhagiri Karthik and Ganapathi Anantharaman*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India.
Abstract
Backbone dihalofunctionalized imidazol-2-ylidene/imidazolyl-metal complexes (5–13) of group–10
metals were synthesized through a proficient and single step oxidative addition method using 2,4,5-
trihaloimidazolium salts or imidazoles respectively. The donor property of 4,5-diiodofunctionalized
imidazol-2-ylidene was described for the first time. The catalytic activity of palladium complexes
containing imidazol-2-ylidene (6 and 7)/imidazolyl (10 and 11) groups towards Suzuki-Miyaura coupling
reaction of aryl bromides was evaluated and compared with its unsubstituted counterpart. Besides, the
investigation of reaction pathway in catalysis was carried out.
Keywords: Backbone functionalized NHCs, trihaloimidazoles, oxidative addition, group-10 metals,
imidazolyl-metal complexes, Suzuki Miyaura coupling
1
. INTRODUCTION
Choice of ligands with tunable electronic and steric parameters is one of the crucial requisites for
designing the stable and selective catalysts. N-heterocyclic carbenes (NHCs) offer that advantageous
property in addition to their strong sigma donating ability which enables them to form robust transition
metal complexes and made them to be metamorphosed from a laboratory curiosity to a remarkable and
ubiquitous class of ligands in organometallic chemistry [1]. Functionalization at backbone of the NHC
ring is an effective tool to alter the donor properties which in turn can influence the catalytic activity of
their metal complexes [2-4]. Further, it can provide an additional site for second coordination and scope
for post modification of metal complexes [5]. Recently our group has reported the synthesis of various
backbone heteroatom functionalized imidazolium/imidazolinium salts and their metal complexes along
with the evaluation of their electronic properties and catalytic activity [6].
An interest for functionalization of halogen group at 4,5-position of imidazol-2-ylidene was triggered
Cl
₂
by the first report of an air stable 4,5-dichloro-1,3-dimesitylimidazol-2-ylidene (IMes ) by Arduengo in
1
997 [7]. Subsequently, the influence of chlorine atom at the backbone of imidazol-2-ylidene metal
complexes was manifested in the enhancement of its catalytic activity in numerous organic
transformations [8-11] and also in antimicrobial as well as in cytotoxic studies [12]. Notably, Organ and
1

ACCEPTED MANUSCRIPT
co-workers have introduced the chlorine atoms at the backbone of sterically hindered PEPPSI catalyst
which had a remarkable impact on selectivity and activity in most challenging cross coupling reactions
[
9,10]. Regardless of a large number of reports on 4,5-dichlorofunctionalized NHC–metal complexes, the
study on its bromine counterpart remains less explored [13] and there are no reports in the case of 4,5-
diiodofunctionalized NHC–metal complexes. Recently, Cole’s group has illustrated that the stability of
Br₂
Br₂
4
,5-dibromofunctionalized imidazol-2-ylidenes (IMes and IPr ) and the catalytic activity of their metal
complexes are above par than that of their well-established unsubstituted counterparts (IMes and IPr)
14].
[
Ar
N
Ar
N
Ar
N
X
CX4
MLn
base
H
X
MLn
N
Ar
N
Ar
X
N
Ar
X = Cl, Br
Multistep synthesis
Reactive carbene intermediate
(Previous Work)
X
BF4
X
PPh3 BF4
N
N
N
N
M(PPh3)4
X
M
X
X
X
PPh3
X = Br, I
Single step synthesis
No intermediates
(This Work)
Scheme 1: Synthesis of 4,5-dihalo-functionalized NHC–Metal complexes
The lack of suitable substrates and the handling of free carbenes remain the bottle neck in the
development of these ubiquitous class of 4,5-dihalofunctionalized-imidazol-2-ylidenes and their metal
complexes [14]. Oxidative addition of low valent metal precursors to C2–X bond of imidazolium salt is a
convenient and advantageous route to synthesize metal–NHC complexes as initiated by Stone and co-
workers without involving the reactive free carbenes and was further extended by few other research
groups [15, 16]. But surprisingly, this route has not been used to synthesize backbone heteroatom
functionalized NHC–metal complexes. In addition, mostly the metal complexes containing NHCs with
bulky N-substituents were utilized in catalysis whereas the metal complexes containing NHCs with lesser
bulky N-substituents are scarcely evaluated in catalysis. Recent reports have demonstrated that NHCs
with smaller N-substituents can also be useful precatalysts in cross coupling reactions of aryl halides [17].
Herein, a facile and convenient method to synthesize 4,5-dihalofunctionalized 1,3-dimethylimidazol-2-
ylidene metal complexes of group 10 metal ions using simple 2,4,5-trihaloimidazolium salts is reported
along with the evaluation of the donor property of newly synthesized 4,5-diiodo-1,3-dimethylimidazol-2-
I
₂
ylidene (IMe ). Besides, the synthesis of 4,5-dihalofunctionalized imidazol-2-yl-metal complexes from
corresponding
2,4,5-trihaloimidazoles
is
described. Also,
the
catalytic
activity
of
2

ACCEPTED MANUSCRIPT
imidazolylidene/imidazolyl–palladium complexes in the Suzuki–Miyaura coupling of aryl bromides and
the role of backbone halogen atoms in the catalysis is studied in detail.
2
2
. RESULTS AND DISCUSSION
.1. Synthesis of 4,5-dihalofunctionalized imidazol-2-ylidene-metal complexes (5–9): 2,4,5-Trihalo-1-
methylimidazole (1 and 2) was prepared either from a commercially available 2,4,5-tribromoimidazole or
2
4
,4,5-triiodoimidazole obtained either by direct iodination or as the side product during the synthesis of
,5-diiodoimidazoles [18a]. 2,4,5-Trihalo-1,3-dimethylimidazolium salts [X = Br (3) and X = I (4)] were
synthesized by quaternization of respective 2,4,5-trihalo-1-methylimidazoles (1 and 2) using
trimethyloxonium tetrafluoroborate [18b]. The salts obtained were authenticated through NMR and ESI-
MS. 3 and 4 were treated either with Ni(cod)
2
/PPh
3
or M(PPh
3
)
4
(M = Pd or Pt) to undergo oxidative
Br
I
₂
₂
addition reaction to afford corresponding IMe /IMe –metal complexes (5–9) (Scheme 2).
Scheme 2. Synthesis of backbone halofunctionalized NHC–metal complexes of group 10
Conditions: (a) DCM, rt, overnight; (b) THF, 0 °C, 3h; (c)THF, rt, 8 h; (d) THF, rt, 12 h.
Compared to the complexes (6–9), the corresponding nickel complex (5) was readily obtained in 3 h at
low temperature, however the efforts to prepare iodide analogue of 5 were unsuccessful. The Ni–NHC
and Pd–NHC complexes are soluble in common organic solvents such as dichloromethane and acetone;
but the Pt–NHC complexes were soluble only in highly polar solvents like acetonitrile and
dimethylsulfoxide. The carbene complexes (6–9) were authenticated through spectroscopic, spectrometric
1
and analytical methods. H NMR spectra indicated the formation of the respective complexes by clear
upfield shift of N–CH
3 3 6 5 3
protons and 1:5 ratio of N–CH to P(C H ) peaks. The complexes 5–7 were
13
corroborated by the characteristic carbene carbon peak in C NMR spectra in CD Cl at 163.9, 163.4 and
2
2
1
64.4 ppm respectively. All these values lie in the range that was reported for similar complexes in
literature [19].
3

ACCEPTED MANUSCRIPT
5
6
7
8
Figure 1. ORTEP diagrams showing 50% probability thermal ellipsoids and selected atom labels for
─
complexes 5–8. Hydrogen atoms, counter anions (BF ), and solvent(s) of crystallization have been
4
omitted for clarity.
However, carbene carbon peaks could not be obtained for platinum complexes 8 and 9 even after large
3
1
number of scans. A single peak at P NMR of complexes 5–7 illustrated that the coordinated PPh ligands
3
are arranged in trans fashion to each other. The formation of complexes 6 and 7 was further affirmed by
─
+
their ESI-MS peaks which corresponds to the mass of [M–(BF ) ] ion and their isotopic distribution
4
pattern matches well with the simulated pattern (supporting information). Unlike palladium complexes,
the platinum complexes (8 and 9) are not very stable in their solution medium, the dissociation of
31
1
triphenylphosphine and its subsequent oxidation to phosphine oxide has been observed in their P and H
NMR spectra. However, structure of 8 (Figure 1) and 9 (Figure S1) was confirmed by X-ray diffraction
method and ESI-MS. Despite the presences of three C–X bonds (C2, C4 and C5) in 3 and 4, the oxidative
4

ACCEPTED MANUSCRIPT
addition preferentially occurs at C2 position rather than C4 or C5 position. The electron deficient nature
of C2−X bond which is flanked by two electronegative nitrogen atoms could be one of the reasons for the
observed selectivity.
Table 1. Selected bond lengths (Å) and angles (deg) of 5-8, 10, 11 and 14
Bond
length/Angle
5
6
7
M = Pd
X = I
8
10
M = Pd
X = Br
11
M = Pd
X = I
14
M = Pd
X = Br
M = Ni
X = Br
M = Pd
X = Br
M = Pt
X = Br
N1–C2
N3–C2
1.374(8)
1.351(8)
1.343(6)
1.345(6)
1.342(16)
1.348(17)
1.355(18)
2.016(12)
1.344(11)
1.346(11)
1.313(13)
1.983(8)
1.385(8)
1.321(8)
1.358(10)
2.012(6)
1.362(7)
1.333(7)
1.350(8)
2.002(5)
1.346(14)
1.347(14)
1.336(16)
1.984(12)
C4–C5
1.328(10)
1.850(6)
1.327(8)
M1–C2
1.988(5)
M1–X3
2.3096(10)
2.2328(12)
104.7(5)
2.4648(7)
2.3294(10)
105.9(4)
2.6286(12) 2.4765(10) 2.5084(8)
2.6521(6) 2.4613(13)
M1–P1
2.319(4)
104.8(10)
174.9(4)
2.3193(15) 2.3001(12) 2.3369(10) 2.330(3)
N1–C2–N3
C2–M1–X3
106.1(7)
179.5(2)
110.9(5)
110.8(5)
105.3(10)
176.9(3)
175.65(19)
179.96(14)
174.47(18) 175.16(16)
The structure of complexes 5–9 were unambiguously established by single crystal X-ray diffraction
method. The crystallographic data are given in Table S1 and S2 of supporting information. All the
complexes possess four-coordinate square planar geometry around the central metal atom (Figure 1). The
coordination around the central atom in these complexes reasserts the predicted structural arrangement
through spectroscopic characterization. The nickel complex 5 has Ni1–C2 bond distance of 1.850(6) Å,
Ni1–Br3 bond distance of 2.309(10) Å and N1–C2–N3 bond angle of 104.7(5) which lies closer to the
similar nickel complexes [19a]. The Pd1–C2 bond distance of 6 and 7 was found to be 1.988(5) Å and
2
.016(12) Å respectively which correlates well with the similar type of complexes [19b]. In platinum
complexes 8 (Figure 1) and 9 (Figure S1), the platinum coordination distances and angles are
unexceptional which are closer to the similar compounds. Other selected bond parameters of 5 to 8 are
─
given in Table 1. The metric parameters of 9 were not discussed due to the disordered BF4 anion which
has been treated using SQUEEZE procedure (see section 4.5).
I
Though 4-iodoimidazol-2-ylidene (IMe )–palladium complex was previously reported by our
group [6c], to best of our knowledge, complexes 7 and 9 are the first known examples of 4,5-
I
31
₂
diiodoimidazol-2-ylidene (IMe )–metal complexes. The TEP value of 7 determined from P NMR (in
dichlormethane-d ) chemical shift value through the method described by Albrecht and co-workers based
2
-1
I
₂
on regression analysis was found to be 2053 cm [20]. This value suggests that IMe is a weaker donor
I
than the 1,3-dimethylimidazol-2-ylidene (IMe) and 4-iodoimidazol-2-ylidene (IMe ) (Table 2) [21,6c].
However, TEP value could not be calculated for 6 using the above method due to the variation in the
metal-coordinated halogen atom.
5

ACCEPTED MANUSCRIPT
I
I
₂
Table 2. Comparison of TEP of IMe with IMe and IMe
Complex
ν
CO(avg)
δ
P
TEP
(cm )
Ref.
-
1
(
ppm)
a
Ref. [21]
2
025
−
−
2051
Our
previous
work [6c]
b
18.3
2051
BF
4
I
I
PPh
Pd
N
N
3
I
b
−
17.9
2053
This work
PPh
3
a
-1
TEP =(0.847 x νCO(avg)) + 336 cm
TEP = 2125 – (4.01 x δ P)
b
2
.2. Synthesis of 4,5-dihalofunctionalized imidazol-2-yl–metal complexes (10–13):
Unlike imidazol-2-ylidene, imidazol-2-yl–metal complexes are highly reactive and very rarely isolated
[
22]. Generally, they were prepared by the deprotonation of imidazoles that are bound to the metal
complexes [23]. Later, Hahn has synthesized them using oxidative addition of halogenated N-
methylbenzimidazoles and purine derivatives [24]. In our case, oxidative addition reaction of C2–X of
2
,4,5-trihalo-1-methylimidazoles with M(PPh ) [M = Pd, Pt] in toluene at room temperature leads to 4,5-
3 4
dihalofunctionalized imidazol-2-yl complexes of palladium (10 and 11) and platinum (12 and 13)
Scheme 3). However, the reaction was unsuccessful using nickel(0) precursor.
(
Scheme 3. Synthesis of backbone halo-functionalized imidazol-2-yl–metal complexes
The palladium complexes are highly stable both in solid and solution state and they were crystallized
from dichloromethane to afford yellow colored crystals suitable for X-ray diffraction studies. The
presence of an upfield singlet (N-CH ) and a multiplet [P(C H ) ] in the downfield region in 1:10 ratio in
3
6
5 3
1
31
their respective H NMR spectra authenticates the presence of 10 and 11. A clear singlet in P NMR
spectrum of 10 and 11 at 20.3 ppm and 18.9 ppm respectively suggests the trans arrangement of PPh
3
ligands. On the contrary, the platinum complexes are unstable in solution which decomposed to afford
triphenylphosphine oxide crystals during crystallization. However, the formation of platinum complexes
6

ACCEPTED MANUSCRIPT
1
2 and 13 were confirmed by ESI-MS peaks at 1038.9370 and 1179.8965 respectively which corresponds
+
to [M+H] .
1
0
11
Figure 2. ORTEP diagrams showing 50% probability thermal ellipsoids and selected atom labels for
complexes 10 and 11. Hydrogen atoms and solvent(s) of crystallization have been omitted for clarity.
The molecular structure of 10 and 11 reveals a four-coordinate square planar geometry around
palladium (Figure 2). The bond parameters of 10 and 11 clearly differentiate them from their respective
NHC complexes (Table 1). As expected, Pd–C2 bond distance of 10 [2.012(6) Å] is longer than its
corresponding NHC complex 6 [1.988(5) Å] [25]. In contrast, Pd–C2 bond distance of 11 [2.002(5) Å] is
comparable to its corresponding NHC complex 7 [2.016(12) Å]. C2–N1 and C2–N3 bond distances of 10
[
1.385(8) Å and 1.321(8) Å] and 11 [1.362(7) Å and 1.333(7) Å] are uneven in contrast to almost even
distances of 6 [1.343(6) Å and 1.345(6) Å] and 7 [1.342(16) Å and 1.348(17) Å]. This explains the
2
presence of unsubstituted nitrogen (N3) with a lone pair of electrons in exocyclic sp orbital of nitrogen
°
°
atom in 10 and 11 [25]. Further the N1–C2–N3 bond angle of 10 [110.9(5) ] and 11 [110.8(5) ] are
°
°
substantially greater than their respective NHC–metal complexes 6 [105.9(4) ] and 7 [104.8(10) ].
7

ACCEPTED MANUSCRIPT
Scheme 4. Conversion of backbone functionalized imidazol-2-yl–palladium complexes to their
a
corresponding NHC–Pd complexes. In case of route B, some amount of unreacted starting materials (10
or 11) were present.
The quaternization of imidazol-2-yl–palladium complexes are known to provide NHC–palladium
complexes [24]. The imidazol-2-yl–palladium complexes 10 and 11 were quaternized using
trimethyloxonium tetrafluoroborate in dichloromethane at room temperature to give respective NHC–
palladium complexes 6 and 7 along with some unreacted starting materials (Scheme 4). ESI-MS (positive
1
31
mode) peaks at 1104.8522 and 964.9006 corroborate the formation of 6 and 7. H and P NMR spectrum
of the crude compound indicates the presence of products, starting material and some intractable species.
2
.3. Catalytic activity and effect of halogens at the backbone of imidazol-2-ylidene/imidazol-2-yl–
palladium complexes in Suzuki–Miyaura coupling:
+
3 2
Cationic metal–NHC complexes of [trans-(NHC)MX(PPh ) ] type were known to be an effective
precatalyst for cross-coupling reactions. At first, Welton’s group reported the in situ formation of [trans-
+
(
NHC)PdBr(PPh ) ] as a catalytically active species in Pd–catalyzed Suzuki–Miyaura coupling in ionic
3
2
+
liquid [bmim][BF ] [19a, 26]. Later, a series of [trans-(NHC)MX(PPh ) ] type complexes containing
4
3 2
remote, normal and mesoionic carbenes were synthesized and found to exhibit notable catalytic activity in
various cross-coupling reactions [27]. In the light of aforementioned reports, the cationic Pd–NHC
complexes (6 and 7) and neutral Pd–imidazol-2-yl complexes (10 and 11) along with Pd(PPh ) were
3
4
screened as precatalysts for Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid to
explore their catalytic activity and to investigate the effect of backbone halogen atoms.
The cross coupling of 4-bromoanisole with phenylboronic acid was chosen as representative reaction
for screening the catalysts with various solvents and bases. In most of the reaction condition (during
I
Br
₂
₂
optimization), IMe containing palladium complex 7 exhibited poorer catalytic activity than IMe
containing palladium complex 6 due to the rapid formation of inactive palladium black particles.
However, in the presence of DMF and Cs CO , 7 yielded 81% conversion in catalysis whereas 6
2
3
displayed superior catalytic activity with 94% conversion even at lower catalyst loading (0.1 mol %) at
similar experimental conditions (Table 3, entry 16 and 17). The diiodoimidazol-2-yl–palladium complex
1
1 and dibromoimidazol-2-yl–palladium complex 10 gave 53% and 78% conversion respectively whereas
Pd(PPh ) gave just 69% conversion. The cross coupling of aryl chlorides with phenyl boronic acid was
3
4
also investigated using 6 which gave 70 % product conversion in the case of 4-chloro-nitrobenzene,
however, only 11 % product conversion was observed in the case of 4-chloroanisole (entry 21 and 22).
8

ACCEPTED MANUSCRIPT
a
Table 3. Optimization of reaction conditions
b
Entry
[Pd]Cat.
R
X
Solvent
Base
Cat.
loading
Time
(h)
Conv.(%)
(
mol %)
1
2
3
4
5
6
7
8
9
6
7
6
7
6
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
OCH3 Br
DMF
DMF
K CO3
5
5
5
5
5
5
5
5
5
5
5
5
2
2
2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
24
24
24
24
24
24
24
24
24
24
12
12
12
12
12
12
12
12
12
12
12
12
79
8
73
70
40
5
2
K CO3
2
Dioxane
Dioxane
Toluene
Toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Dioxane
Toluene
DMF
DMF
DMF
DMF
DMF
K
K
2
CO
CO
3
2
3
K CO3
2
7
K CO3
2
10
11
6
7
6
7
6
6
6
K
2
CO
3
56
15
80
40
98
85
95
46
82
94
81
78
53
69
70
11
K CO3
2
K
3
PO
4
1
1
0
1
K PO4
3
Cs CO3
2
1
1
1
1
1
1
1
1
2
2
2
2
3
4
5
6
7
8
9
0
1
2
Cs CO3
2
Cs CO3
2
Cs
Cs
2
2
CO
CO
3
3
6
7
Cs CO3
2
Cs CO3
2
10
11
Pd(PPh
Cs CO3
2
Cs
Cs
2
CO
CO
3
3
)
4
2
3
6
6
NO2
Cl
DMF
DMF
Cs CO3
2
OCH3 Cl
Cs CO3
2
a
Reaction conditions: aryl halide (1 mmol), PhB(OH) (1.2 mmol), catalyst (0.1 mol %), Cs CO (2.0 mmol) and
2
2
3
b
DMF (3 mL). G.C. conversion. % Conversion calculated from aryl halide based on GCMS analysis.
Br
₂
Overall, IMe containing palladium complex 6 stood out to be the best precatalyst for the cross
coupling of aryl bromide among the above investigated complexes. Thus, various aryl bromides (entry 1–
8
in Table 4) were employed in the coupling reactions under the optimized experimental condition using
precatalyst 6. All the coupled products were isolated through column chromatography and the purity was
1
13
confirmed through H and C NMR spectroscopy. The precatalyst 6 manifested remarkable catalytic
activity even with challenging electron rich (entry 3–5) and unactivated aryl bromides (entry 2) along
with electron deficient aryl bromides (entry 1). Moreover, precatalyst 6 effectively catalyzed double
coupling of 1,3-dibromobenzene to give 95% yield of 1,3-diphenylbenzene with just 1% of monocoupled
product. In addition, coupling of heteroatom containing 2-bromothiophene (entry 8) was also successfully
catalyzed by 6.
9

ACCEPTED MANUSCRIPT
Br
₂
Table 4. Catalytic activity of [IMe Pd(PPh
3
)
2
Br] (7) and [IMePd(PPh
3
)
2
Br] (14) in Suzuki–Miyaura
a
Coupling
Cat.
loading
mol %)
Time
Yield
(%)
Entry
1
R–X
Product
Catalyst
b
(
h)
(
8
8
0.1
6
1
98
93
4
4
2
3
0.1
0.1
6
1
95
92
12
6
1
91
87
4
4
4
4
5
12
12
0.1
0.1
6
1
92
85
6
1
93
83
6
7
6
0.1
0.2
6
1
97
91
4
4
12
6
1
95
87
c
c
8
12
0.2
6
1
75
51
4
a
Reaction conditions: aryl halide (1 mmol), PhB(OH) (1.2 mmol), catalyst (0.1 mol %), Cs CO (2.0 mmol) and
2
2
3
b
c
DMF (3 mL). Isolated yield, G.C. conversion.
[
3 2
Trans-(IMe)PdBr(PPh ) ] 14 (IMe = 1,3-dimethylimidazol-2-ylidene IMe) was synthesized to
compare its catalytic activity with that of 6 and to investigate the role of backbone bromo substituents in
the catalysis of coupling reactions. The complexes that are very similar to 14 were reported in the
10

ACCEPTED MANUSCRIPT
literature using deprotonation method at high temperature [19a]. In the present case, oxidative addition of
2
-bromo-1,3-dimethylimidazolium salt to Pd(PPh
of 14 was authenticated by spectroscopic, spectrometric and single crystal X-ray diffraction methods
Figure S2, supporting info). The catalytic activity of 14 was on a par with that of 6 and the isolated
3 4
) in THF is afforded 14 in 90 % yield. The formation
(
yields of the biaryl products were almost comparable in the case of electron deficient substrates and
slightly lower in the case of electron rich substrates and heteroatom containing substrates.
2
.4. Investigation of reaction pathway in catalysis
In order to probe whether the catalysis is occurring through homogeneous or heterogeneous pathway,
(
a) mercury drop experiment, (b) CS test and (c) hot filtration test were conducted [28]. All these tests
2
suggest that the reaction is proceeding through heterogeneous pathway (Figure 3 and Supporting
information Table S3) [29]. In particular, the hot filtration test confirms the separation of active palladium
species/clusters from the reaction medium which is further ascertained by the sudden decline in progress
of the reaction (Figure 3 and Supporting information, entry 10-12 Table S3). The detailed procedures of
these tests are described in section 4.4 (vide infra).
Figure 3. Reaction profile diagram (Time Vs Conversion) of poisoning tests and hot filtration test. The
conversion was monitored by GC-MS analysis based on aryl halides and the data were averaged over
three runs.
Further, the ESI-MS spectra of the reaction mixture indicate mass values corresponding to the
formation of 2,4,5-triphenyl-1,3-dimethylimidazolium salts (in the case of 6 and 7) and 2-phenyl-1,3-
dimethylimidazolium salt (from 14) (Supporting Information). Earlier, it was extensively studied and
observed by Cavell’s group and others that the group 10 metal complexes [(Ar)(NHC)ML ] of NHCs
2
containing less bulky N-substituents can undergo reductive elimination to generate Pd(0) and aryl
substituted imidazolium salts as products [30].
11

ACCEPTED MANUSCRIPT
X
X
PPh3
X
X
PPh3
Pd
N
N
PhB(OH)2
- XB(OH)2
N
N
X
X
N
N
Pd
X
Pd PPh3
PPh
PPh
3
3
PPh
3
trans- (A)
cis- (A)
Pd(0)
+
X
N
X
Reductive
N
X
X
N
N
Elimination
Pd PPh3
X
X
X
N
N
- 2 PPh3
N
PPh
3
N
X
Active Pd(0) Particles
may be stabilized by
imidazolium salts
X
X
N
Scheme 5. Plausible pathway for the formation of Active Pd(0) particles and 2-phenylimidazolium salts
The above results illustrate that the following transformations could have occurred to the precatalyst in
the reaction medium before the Suzuki-Miyaura coupling: (i) Transmetallation reaction occurs between
phenylboronic acid with palladium complex (6 or 7 or 14) to give (aryl)Palladium-NHC complex (A) and
(
ii) (cis-A) undergoes reductive elimination to give 2-phenylimidazolium salts [31] and [Pd(0)] (Scheme
). Recently, Ananikov’s group proposed that the cleavage of M-NHC bond plays a crucial role in the
5
catalytic activity of Heck coupling reactions in generating the active Pd(0) nanoparticles which are
stabilized by imidazolium salts [32]. Based on these information, we infer that in this case, the Pd (0)
species stabilized by imidazolium salts (formed via reductive elimination) act as the active catalyst in
Suzuki-Miyaura coupling (Scheme 5). Moreover, the efficiency of precatalysts 6, 7 and 14 may be
attributed to the stabilization of Pd(0) by various imidazolium salts.
3
. CONCLUSION
In summary, a simple and single step synthesis of backbone dihalofunctionalized imidazol-2-ylidene
metal complexes (5–9) has been achieved through oxidative addition of 2,4,5-trihaloimidazolium
precursors to zero valent group 10 metal precursors. The first example of 4,5-diiodofunctionalized NHC–
metal complexes and its donor property was described. The same method was conveniently utilized for
synthesizing dihalofunctionalized imidazol-2-yl–metal complexes (10 and 11). Complexes 6, 7, and 14
are one of the rare precatalysts containing non-bulky NHC ligand that show excellent activity in Suzuki–
Miyaura coupling of challenging electron rich and heteroatom containing aryl bromides and 6 exhibits
superior activity among them. The heterogeneous catalytic pathway of these precatalysts was
unambiguously established and the active catalyst was found to be Pd(0) species stabilized by
12

ACCEPTED MANUSCRIPT
imidazolium salts. Over all the current synthetic method can be extended towards the preparation of main
group as well as transition metal compounds and their potential application in catalysis. Also, the
backbone halogen atoms of the metal complexes (5–13) can be utilized for introducing different
functional groups or another metal to form bimetallic complexes. These works are currently under
progress in our laboratory.
4
4
. EXPERIMENTAL SECTION
.1 Materials and Methods: All the reactions were carried out under an inert atmosphere of nitrogen gas
using a standard Schlenk line technique. Glove box was used for sample preparation for reactions.
Solvents were dried according to the standard literature procedures and were freshly distilled prior to use.
Bis(1,5-cyclooctadiene)nickel(0), Pt(PPh
3
)
4
,
1-methylimidazole, 2,4,5-tribromoimidazole, and
trimethyloxonium tetrafluoroborate were procured from the commercial sources and used as received.
Starting materials such as Pd(PPh ) [33a], 2,4,5-tribromo-1-methyl-imidazole [18b], 2-bromo-1-
3
4
methylimidazole [33b], 2,4,5-triiodo-1-methyl-imidazole [18a] and 2-bromo-1,3-dimethylimidazolium
tetrafluoroborate [18b] were prepared using the literature procedure. The melting points reported are
1
13
31
uncorrected. H, C, and P NMR spectra were recorded on JEOL (400 or 500 MHz) spectrometers using
any one of the solvents such as CDCl , CD Cl , and DMSO-d . The chemical shifts were referenced with
3
2
2
6
1
13
31
respect to TMS ( H and C NMR) and 85% H PO ( P NMR). The ESI mass spectra were recorded on a
3
4
Waters Q-TOF instrument. IR spectra were recorded in KBr pellets using a Perkin Elmer–Spectrum Two
FTIR spectrophotometer. Elemental analyses were performed with a Perkin Elmer Series–II CHNS/O
analyzer 2400.
4
.2. Synthesis and characterization
Synthesis of 3: 2,4,5-tribromo-1-methyl-imidazole (1) (0.5 g, 1.56 mmol) and trimethyloxonium
tetrafluoroborate (0.25 g, 1.72 mmol) in a Schlenk flask was charged with dichloromethane (30 mL). The
mixture was stirred overnight at room temperature to afford a colorless solid. The volatiles were removed
in vacuum and the resulted solid was washed with diethyl ether (2 X 15 mL) and dried under vacuum to
1
give 3 as colorless powder. Yield: 0.60 g (91%). Mp. 304 °C. H NMR (DMSO-d
6
, 500 MHz,
), 112.0 (4,5-C–Br), 124.9
C2–Br). IR (KBr, cm ): 1637(m), 1554(m), 1504(s), 1445(m), 1396(m), 1329(m), 1295(m), 1056(vs),
δ, ppm):
13
3
3 6 3
.78 (s, 6H, N–CH ). C NMR (DMSO-d , 125 MHz, δ, ppm): 38.1 (N–CH
-1
(
─
+
1
034(vs), 813(w), 641(w), 522(w). ESI MS: m/z 332.8081 [M–(BF ) ] .
4
Synthesis of 4: 2,4,5-Triiodo-1-methyl-imidazole (2) (2.0 g, 4.35 mmol) and trimethyloxonium
tetrafluoroborate (0.707 g, 4.78 mmol) in a Schlenk flask was charged with dichloromethane (50 mL).
13

ACCEPTED MANUSCRIPT
The mixture was stirred overnight at room temperature to afford a colorless solid. The volatiles were
removed in vacuum and the resulted solid was washed with diethyl ether (2 X 20 mL) and dried in
1
vacuum to give 4 as colorless powder. Yield: 2.29 g (93%). Mp.: 285 °C (with decomp.). H NMR
13
(
DMSO-d , 500 MHz,
δ
, ppm): 3.87 (s, 6H, N–CH ). C NMR (DMSO-d , 125 MHz,
δ, ppm): 43.6 (N–
6
3
6
-1
CH ), 95.5 (4,5-C–I), 104.2 (C2–I). IR (KBr, cm ): 1637(m), 1509(m), 1483(s), 1384(m), 1318(m),
3
─
+
1
4
288(m), 1034(vs), 791(m), 628(w), 521(w). ESI MS: m/z 474.7664 [M–(BF ) ] .
Synthesis of 5: The mixture of bis(1,5-cyclooctadiene)nickel(0) (0.05 g, 0.178 mmol) and
triphenylphosphine (0.093 g, 0.356 mmol) in THF (10 mL) was cannulated to a Schlenk flask containing
3
(0.178 mmol, 0.10 g) in THF (5 mL) at 0°C and was stirred at the same temperature for 2.5 h to give
bright yellow precipitate. The resultant precipitate was separated out through filtration and washed with
dry n-pentane (2 x 15 mL) and diethyl ether (1 x 15 mL) and dried in vacuum to give yellow powder of 5.
Crystals of 5 were obtained in dichloromethane /hexane (20:1) mixture at –5 °C. Yield: 0.107 g (60 %).
1
Mp.: 212 °C (with decomp.). H NMR (CD
2
Cl
2
, 500 MHz,
δ, ppm): 3.57 (s, 6H, N–CH
3
), 7.44–7.47 (m,
13
1
2H, Ph–Hmeta), 7.54 (t, 6H, Ph–Hpara), 7.62–7.63 (m, 12H, Ph–Hortho). C NMR (CD Cl , 125 MHz, δ,
2 2
ppm): 37.9 (N–CH ), 99.9, 110.7 (4,5-C–Br), 128.7–129.1 (Ph–C
Ph–Cipso ), 131.8 (Ph–Cpara), 134.0
meta,
3
3
1
−1
(
Ph–Cortho), 163.9 (NCN). P NMR (CD Cl , 202 MHz, δ, ppm): 20.3. IR (KBr, cm ): 3057(w), 2921(w),
2 2
1
1
5
646(w), 1584(w), 1561(m), 1480(m), 1459(m), 1435(s), 1368(w), 1313(w), 1183(w), 1160(w), 1112(s),
089(vs), 1059(vs), 1027(s), 997(s), 819(w), 756(m), 746(m), 707(s), 697(vs), 675(m), 617(w), 524(vs),
+
14(vs), 486(m), 459(w). ESI MS: 254.8974 [C H Br N ] corresponds to the 4,5-dibromo-1,3-dimethyl
5
7
2
2
imidazolium ion fragmented from 5. Due to product decomposition, satisfactory elemental analysis could
not be obtained.
General procedure for the synthesis of complexes 6–9: An equimolar amount of [M(PPh ) ] and 3 or 4
3
4
in a Schlenk flask was charged with THF (20 mL) and was stirred for 8 h ( for 6 and 7) or 12 h (for 8 and
) at room temperature. The resultant yellow precipitate was separated and washed with n-pentane (2 x 15
9
mL) and diethylether (1 x 15 mL) and dried under vacuum to give the product.
Synthesis of 6: [Pd(PPh ) ] (0.206 g, 0.178 mmol) and 3 (0.075 g, 0.178 mmol) were used. The slow
3
4
evaporation of acetone/toluene (4:1) mixture of the title compound afforded colorless crystals. Yield: 0.17
g (91%). Mp.: 275–280 °C (with decomp.). Anal. calcd. for C H BBr F N P Pd: C, 46.83; H, 3.45; N.
41
36
3
4
2 2
1
2
7
.66. Found: C, 46.80; H, 3.56; N, 2.66. H NMR (DMSO-d
6
3
, 500 MHz, δ, ppm): 3.30 (s, 6H, N–CH ),
13
.48–7.54 (m, 12H, Ph–Hmeta), 7.56–7.67 (m, 18H, Ph–Hpara, Ph–Hortho
)
2 2
C NMR (CD Cl , 125 MHz, δ,
ppm): 37.9 (N–CH ), 109.9 (4,5-C–Br), 128.7 (Ph–Cipso), 129.1 (Ph–Cmeta), 132.0 (Ph–Cpara), 133.9 (Ph–
3
31
−1
Cortho), 163.4 (NCN). P NMR (CD Cl , 202 MHz, δ, ppm): 20.6. IR (KBr, cm ): 3057(m), 2945(w),
2
2
14

ACCEPTED MANUSCRIPT
1
818(w), 1645(m), 1585(w), 1561(m), 1464(m), 1435(vs), 1383(m), 1372(m), 1347(m), 1313(m),
1185(w), 1161(w), 1114(s), 1095(vs), 1058(vs), 1027(vs), 997(s), 818(w), 757(s), 745(s), 706.86(s),
─
+
6
96(vs), 668(m), 617(w), 522(vs), 514(vs), 493(m), 459(w). ESI MS: m/z 964.8932 [M–(BF ) ] .
4
3 4
Synthesis of 7: [Pd(PPh ) ] (0.50 g, 0.43 mmol) and 4 (0.23 g, 0 .43 mmol) were used. The slow
evaporation of dichloromethane/toluene (5:1) mixture of the title compound afforded yellow crystals.
Yield: 0.37 g (73%). Mp.: 205 °C (with decomp.). Anal. calcd. for C H BF I N P Pd: C, 41.29; H, 3.04;
41
36
4 3
2 2
1
N, 2.35. Found: C, 41.59; H, 3.01; N, 2.35. H NMR (CD Cl , 500 MHz, δ, ppm): 3.36 (s, 6H, N–CH3),
2
2
1
3
7
.42–7.45 (m, 12H, Ph–Hmeta), 7.51–7.53 (m, 18H, Ph–Hpara, Ph–Hortho,) C NMR (CD
2 2
Cl , 125 MHz, δ,
ppm): 40.8 (N–CH ), 88.7 (4,5-C–I), 129.0 (Ph–Cmeta), 129.8 (Ph–Cipso), 132.0 (Ph–Cpara), 134.1 (Ph–
3
31
−1
Cortho), 164.4 (NCN). P NMR (CD Cl , 202 MHz, δ, ppm): 17.9. IR (KBr, cm ): 3053(w), 2920(w),
2
2
1
1
6
816(w), 1638(w), 1584(w), 1572(w), 1516(m), 1494(w), 1479(s), 1460(m), 1434(vs), 1368(m), 1329(w),
312(m), 1185(w), 1162(w), 1090(vs), 1054(vs), 997(s), 795(w), 759(s), 743(s), 703(vs), 696(vs),
─
+
4
65(m), 617(w), 518(vs), 493(s), 462(w). ESI MS: m/z 1104.8528 [M–(BF ) ] .
Synthesis of 8: [Pt(PPh
3 4
) ] (0.05 g, 0.04 mmol) and 3 (0.017 g, 0.04 mmol) were used. The slow
evaporation of the 8 in acetonitrile/DMSO (10:1) mixture afforded colorless crystals. Yield: 0.033 g (76
─
+
%
). Mp.: 185 °C. ESI MS: m/z 1052.9608 [M–(BF ) ] . Due to the rapid decomposition of the compound
4
in DMSO-d a satisfactory NMR could not be obtained. Due to the hygroscopic nature of 8, elemental
6
analysis could not be carried out.
Synthesis of 9: [Pt(PPh ) ] (0.05 g, 0.04 mmol) and 4 (0.02 g , 0.04 mmol) were used. The slow
3
4
1
evaporation of 9 in DMSO afforded yellow colored crystals. Yield: 0.04 g (79%). Mp.: 170 °C. H NMR
1
3
(
DMSO-d
00 MHz,
Carbon NMR of N–CH is not found which may be due to the merging of peak with the residual DMSO.
6
, 400 MHz,
3 6 5 3 6
δ, ppm): 3.35 (s, 6H, N–CH ), 7.41–7.64 (m, 30H, P(C H ) . C NMR (DMSO-d ,
1
δ
, ppm): 92.0 (4,5-C–I), 129.0–129.5 (Ph–Cmeta, Ph–Cipso), 132.2 (Ph–Cpara), 134.5(Ph–Cortho).
3
─
+
Carbene carbon peak could not be obtained even after 10000 scans. ESI MS: m/z 1193.9130 [M–(BF ) ] .
4
Due to the hygroscopic nature of 9, elemental analysis could not be carried out.
3 4
General procedure for the synthesis of imidazol-2-yl–metal complexes 10–13: [M(PPh ) ] (1.0
equivalent) was added as a solid to a toluene solution (15 mL) of 1 or 2 (1.0 equiv.) in a Schlenk flask and
was stirred overnight at room temperature. The solvent was removed under the vacuum. The resulted
yellow residue was dissolved in DCM (2 mL) and was precipitated by addition of pentane (15 mL),
filtered and dried in vacuum to afford the product as yellow powder.
15

ACCEPTED MANUSCRIPT
3 4
Synthesis of complex 10: [Pd(PPh ) ] (1.0 g, 0.94 mmol) and 1 (0.30 g, 0.94 mmol) were used. Yellow
colored crystals were grown from dichloromethane/toluene (4:1) mixture of 10 by slow evaporation
method. Yield: 0.82 g (92%). Mp.: 260 °C (with decomp.). Anal. calcd. for C H Br N P Pd: C, 50.58;
40
33
3
2 2
1
H, 3.50; N, 2.95. Found: C, 50.61; H, 3.46; N, 2.99. H NMR (CDCl , 500 MHz,
δ, ppm): 2.81 (s, 3H, N–
3
13
CH ), 7.31–7.39 (m, 18H, Ph–H
Ph–Hpara ), 7.60 (d. 12H, Ph–H
)
C NMR (CDCl , 100 MHz, δ,
3
meta,
ortho .
3
3
ppm): 34.8 (N–CH ), 103.0 (C4–I), 118.2 (C5–I), 128.2 (Ph–Cmeta), 130.5 (Ph–Cpara), 132.3 (Ph–Cipso),
3
1
−1
1
2
1
4
3
34.6 (Ph–Cortho), 147.1 (NCN). P NMR (CDCl , 125 MHz, δ, ppm): 20.2. IR (KBr, cm ): 3049(w),
922(m), 2852(w), 1630(w), 1499(w), 1479(m), 1434(s), 1420(m), 1374(m), 1345(w), 1300(w), 1262(w),
198(m), 1158(w), 1095(s), 1027(w), 998(w), 956(w), 803(w), 745(m), 692(vs), 521(vs), 508(s), 496(m),
+
56(w). ESI MS: m/z 950.8801 [M + H] .
Synthesis of complex 11: [Pd(PPh ) ] (1.0 g, 0.87 mmol) and 2 (0.40 g, 0.87 mmol) were used. Yellow
3
4
colored crystals were grown from dichloromethane/toluene (4:1) mixture of 11 by slow evaporation
33 3 2 2
method. Yield: 0.80 g (85%). Mp.: 260 °C. Anal. Calcd. for C40H I N P Pd: C, 44.04; H, 3.05; N, 2.57.
1
Found: C, 44.27; H, 2.99; N, 2.51. H NMR (CD Cl , 500 MHz,
δ
, ppm): 2.88 (s, 3H, N–CH ), 7.33 (m.
2
2
3
13
1
2H, Ph–Hmeta), 7.41 (m. 6H, Ph–Hpara), 7.55 (d. 12H, Ph–Hortho) C NMR (CDCl , 100 MHz,
δ, ppm):
3
3
7.4 (N–CH ), 81.2 (C4–I), 97.4 (C5–I), 127.9 (Ph–Cmeta), 130.5 (Ph–Cpara), 131.3 (Ph–Cipso), 134.6 (Ph–
3
31
−1
C
2 2
ortho), 150.8 (NCN). P NMR (CD Cl , 202 MHz, δ, ppm): 17.1. IR (KBr, cm ): 3053(w), 2921(w),
1
1
1
585(w), 1570(w), 1480(m), 1433(vs), 1417(m), 1365(m), 1354(m), 1339(m), 1176(m), 1093(s),
058(m), 1026(w), 998(m), 934(m), 746(m), 691(vs), 617(w), 520(vs), 510(vs) 459(w). ESI MS: m/z
+
090.8369 [M + H] .
Synthesis of complex 12: [Pt(PPh ) ] (0.05 g, 0.04 mmol) and 1-methyl-2,4,5-tribromoimidazole (0.012
3
4
+
g, 0.04 mmol) were used. Yield: 0.03 g (79%). ESI MS: m/z 1038.9370 [M + H] . Satisfactory NMR
spectrum or could not be obtained for 12 due to its rapid decomposition.
Synthesis of complex 13: [Pt(PPh ) ] (0.05 g, 0.04 mmol) and 1-methyl-2,4,5-triiodoimidazole (0.018 g,
3
4
+
0
.04 mmol) were used. Yield: 0.04 g (82%). ESI MS: m/z 1179.8929 [M + H] . Satisfactory NMR
spectrum could not be obtained for 13 due to its rapid decomposition.
Synthesis of 14: The same procedure as for 6 was followed except for the use of 2-bromo-1,3-
dimethylimidazolium tetrafluoroborate instead of 3. [Pd(PPh ) ] (0.065 g, 0.25 mmol) and 2-bromo-1,3-
3
4
dimethylimidazolium tetrafluoroborate (0.29 g, 0.25 mmol) were used. The slow evaporation of
dichloromethane/toluene mixture (4:1) of the title compound afforded colorless crystals. Yield: 0.20 g
(
4 2 2
90%). Mp.: 150 °C. Anal. Calcd for C41H38BBrF N P Pd: C, 55.09; H, 4.29; N, 3.13. Found: C, 54.99; H,
16

ACCEPTED MANUSCRIPT
1
4
.21; N, 3.21. H NMR (DMSO-d
6
, 400 MHz,
C NMR (DMSO-d
25.6(Ph–Cmeta), 129.5 (Ph–Cipso), 132.1 (Ph–Cpara), 134.3 (Ph–Cortho), 159.0 (NCN). P NMR (DMSO-d ,
δ
, ppm): 3.18 (s, 6H, N–CH
3
), 6.69 (s, 2H, –CH), 7.49–
1
3
7
.54 (m, 30H, P(C
6
H
5
)
3
.
6
, 125 MHz, , ppm): 37.2 (N–CH
δ
3
), 97.4 (4,5-C–I),
31
1
6
−
1
2
02 MHz, δ, ppm): 21.5. IR (KBr, cm ): 3053 (w), 2923(w), 2853(w), 1766(w), 1710(w), 1629(w),
1
586(w), 1571(w), 1480(m), 1434(s), 1396(w), 1312(w), 1284(w), 1227(w), 1186(w), 1163(w), 1095(vs),
1
057(vs), 998(m), 759(m), 743(m), 706(s), 694(s), 673(m), 522(vs), 515(vs), 494(m), 461(w). ESI MS:
─
+
4
m/z 807.0718 [M–(BF ) ] .
4
.3. Procedure for Suzuki–Miyaura coupling: Aryl bromides (1 mmol), phenylboronic acid (1.2
mmol), cesium carbonate (2.0 mmol), catalyst (0.1 mol %), and DMF (3 mL) were charged in a two neck
flask attached with a condenser and the mixture was heated in a oil bath maintained at 80°C for the
specified time as given in the Table 3 and 4. Then the mixture was diluted with 10 mL water and extracted
with 15 mL DCM and filtered over celite. The organic layer was dried over anhydrous magnesium
sulphate, filtered and evaporated to dryness to afford crude product which was purified by silica gel (100–
2
00 mesh) column chromatography. The purity of the products was confirmed by spectroscopic and
spectrometric methods which matched with the reported data.
4
.4. Procedure for poisoning experiments and hot filtration tests
Poisoning tests: The additives, Hg (a small drop)/CS (10 mol%) was added to the reaction mixture
2
containing 4-bromoanisole (1 mmol), phenylboronic acid (1.2 mmol), catalysts (6 or 7 or 14, 0.5 mol%)
and cesium carbonate (2 mmol) at 80 ˚C. Excess of CS was used to prevent the dissociation of CS from
2
2
the metal centre at high temperature. Three different batches of experiments were carried out by varying
the time of addition of the additives (t = 0, 20 min, 40 min). In given time intervals, 0.2 mL of reaction
mixture was injected out, diluted with ethyl acetate and passed through a short column of silica for GC-
MS analysis.
Hot filtration test:
The same cross coupling reaction described above was carried out using three different catalysts (6 or 7 or
1
4) using the same quantity of reactants at 80 ˚C. The hot mixture was immediately filtered at after 20
minutes using frit under inert atmosphere to the Schlenk flask in the pre heated oil bath at 80 ˚C. After
given time intervals, 0.2 mL of reaction mixture was injected out, diluted with ethyl acetate and passed
through a short column of silica for GC-MS analysis.
4
.5. X-ray crystallography: The crystal data were collected on a Bruker SMART APEX CCD
diffractometer. Data were collected using graphite-monochromated MoK radiation ( = 71073 Å) at 100
K. All the structures were solved by direct methods using SHELXTL–97 [34] and refined by full matrix
α
λ
17

ACCEPTED MANUSCRIPT
least-squares on F2. All the hydrogen atoms were included in idealized positions and a riding model was
used. Non-hydrogen atoms were refined with anisotropic displacement parameters. Disordered solvent
─
molecules in 8 and 10 and disordered counteranion BF4 in 9 (supporting information) were treated by
squeeze refinement using PLATON [35]. The CIF and other supplementary information of reported
compounds can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif using CCDC numbers 1561258-1561265.
Supporting information
X-ray crystal structure of 9 and 14, CIF files, crystallographic data, NMR spectra for complexes 5–7, 8-11
and 14, theoretical and experimental isotopic distribution plots of complexes 5–14 and table containing
various tests for investigating reaction pathway.
Acknowledgement
This study was supported by the SERB, Department of Science and Technology (DST), the Government
of India. A.I. (IITK and DST), V. G. (UGC and DST), V.K. (CSIR) thank funding agencies and institute
for their doctoral fellowships.
References
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1. Note: In case of 6 and 7, the backbone halogens consecutively undergo oxidative addition
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A Straightforward Synthesis of 4,5-Dihalofunctionalized Imidazol-2-
ylidene/Imidazolyl-Metal
Complexes
from
Trihaloimidazolium
Salts/Imidazoles: Structure and Catalytic Studies
Iruthayaraj Avinash, Vivek Gupta, Vedhagiri Karthik and Ganapathi Anantharaman*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India.
Highlights
•
A simple and one-step synthesis of backbone dihalofunctionalized NHC-metal complexes of group 10
metals.
•
•
•
The same method is applicable to synthesize dihalofunctionalized imidazolyl-metal complexes.
First example of 4,5-diiodoimidazolylidene-metal complexes.
Suzuki-Miyaura coupling of aryl bromides using Pd complexes and their reaction pathway were
studied.
•
Efficient catalytic activity towards both electron rich and heteroatom containing aryl bromides.