C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes with biphenyl backbone. A comparison of bis(1,2,3-triazol-5-ylidene) and bis(imidazol-2-ylidene) ligands

Article abstract of DOI:10.1016/j.jorganchem.2014.06.023

C₂-Symmetric normal and mesoionic bis-N-heterocyclic carbenes (NHCs) derived from 1,1′-((1,1′-biphenyl)-2,2′- diylbis(methylene))bis(3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium) diiodide (5) and 3,3′-((1,1′-biphenyl)-2,2′-diylbis(methylene))bis(1- phenyl-1H-imidazol-3-ium) dibromide (6) were used as ligands for the synthesis of the corresponding Pd(II) complexes. 2,2′-Disubstituted 1,1′-biphenyl moiety was used as the C₂-symmetric backbone for the synthesis. 2,2′-Bis(bromomethyl)-1,1′-biphenyl was used as a common precursor for the synthesis of both 5 and 6. These salts were treated with Ag₂O for the in-situ generation of the corresponding NHC-Ag(I) complexes which were transmetallated to the corresponding Pd(II) chloro and acetate complexes. The bis(1,2,3-triazol-5-ylidene) derivative gave structurally well-defined mono nuclear chelate complexes that were characterized by spectroscopic and single crystal XRD data. The bis(imidazol-3-ylidene) derivative gave polymeric complex and not the expected mono nuclear chelate complex. These complexes were compared for their reactivity in Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1016/j.jorganchem.2014.06.023

Journal of Organometallic Chemistry 768 (2014) 68e74
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes with
biphenyl backbone. A comparison of bis(1,2,3-triazol-5-ylidene) and
bis(imidazol-2-ylidene) ligands
Sandip Guchhait, Keshab Ghosh, Bemineni Sureshbabu, Venkatachalam Ramkumar,
Sethuraman Sankararaman*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes (NHCs) derived from 1,10-((1,10-
biphenyl)-2,20-diylbis(methylene))bis(3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium) diiodide (5) and 3,30-
((1,10-biphenyl)-2,20-diylbis(methylene))bis(1-phenyl-1H-imidazol-3-ium) dibromide (6) were used as
ligands for the synthesis of the corresponding Pd(II) complexes. 2,20-Disubstituted 1,10-biphenyl moiety
was used as the C2-symmetric backbone for the synthesis. 2,20-Bis(bromomethyl)-1,10-biphenyl was used
as a common precursor for the synthesis of both 5 and 6. These salts were treated with Ag2O for the in-
situ generation of the corresponding NHCeAg(I) complexes which were transmetallated to the corre-
sponding Pd(II) chloro and acetate complexes. The bis(1,2,3-triazol-5-ylidene) derivative gave structur-
ally well-defined mono nuclear chelate complexes that were characterized by spectroscopic and single
crystal XRD data. The bis(imidazol-3-ylidene) derivative gave polymeric complex and not the expected
mono nuclear chelate complex. These complexes were compared for their reactivity in SuzukieMiyaura
coupling reaction.
Article history:
Received 7 April 2014
Received in revised form
20 June 2014
Accepted 21 June 2014
Available online 30 June 2014
Keywords:
N-Heterocyclic carbene
NHCePd complex
C2-Symmetric ligand
Transmetallation
Normal NHC
Mesoionic NHC
© 2014 Elsevier B.V. All rights reserved.
Introduction
several reports have appeared on the use of both 1,10-biphenyl and
1,10-binaphthyl moieties in imidazolylidene based NHC chemistry
Chelating C2-symmetric ligands are very useful for the synthesis
of C2-symmetric metal complexes that find application in metal
catalyzed stereoselective organic synthesis [1,2]. 2,20-Disubstituted
1,10-biphenyl [3,4] and 1,10-binaphthyl [5,6] moieties are commonly
used chiral C2-symmetric backbone structures. In recent times N-
heterocyclic carbene (NHC) ligands have gained prominence over
the conventional phosphane ligands in transition metal chemistry
[7e15]. Their extraordinary stability and superior catalytic activity
of NHCs and their metal complexes have been amply demonstrated
[7,11,16,17]. Among the various NHCs, imidazol-2-ylidenes are the
most widely exploited as organocatalysts and ligands for transition
metal complexes. C2-Symmetric chelating bis(imidazol-2-ylidene)
bearing 1,10-biphenyl and 1,10-binaphthyl moieties are well known
in literature [18e24]. The first chelated chiral bis(imidazol-2-
ylidene) complexes were reported by RajanBabu using 2,20-disub-
stituted-1,10binaphthyl as the chiral backbone [25]. Subsequently
[18e24]. However, the chelating C2-symmetric mesoionic carbenes
(MICs) such as 1,2,3-triazol-5-ylidenes are relatively still scarce
[26,27]. The first example of chelating bis(1,2,3-triazol-5-ylidene)
was reported from this laboratory [28]. In continuation of our in-
vestigations on NHC-metal complexes [28e32], herein we report
the synthesis of 1,10-biphenyl based C2-symmetric bis(1,2,3-triazol-
5-ylidene) (mesoionic NHC) and bis(imidazol-2-ylidene) (normal
NHC) Pd(II) complexes. The structures of bis(1,2,3-triazol-5-
ylidene) Pd(II) complexes have been unambiguously established
by single crystal XRD data. The catalytic activities of the normal and
mesoionic NHC complexes were compared for the SuzukieMiyaura
coupling reaction.
Results and discussion
Synthesis of precursors to NHC ligands
Diphenic acid (1) was converted to 2,20-bis(bromomethyl)-1,10-
biphenyl (2) in three steps according to literature reported methods
[33,34]. Compound 2 was used as a common precursor for the
* Corresponding author. Tel.: þ91 44 22574210; fax: þ91 44 2257 0545.
E-mail address: sanka@iitm.ac.in (S. Sankararaman).
http://dx.doi.org/10.1016/j.jorganchem.2014.06.023
0022-328X/© 2014 Elsevier B.V. All rights reserved.

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74
69
Table 1
synthesis of NHC precursors 5 and 6 (Scheme 1). It was converted to
Crystallographic data of compounds 5, 6, 8 and 9.
the corresponding diazide (3) on treatment with sodium azide.
Diazide 3 on click reaction with phenylacetylene yielded the cor-
responding bis(triazole) 4 in 97% yield. The triazole ring protons
appeared as a singlet at 7.45 ppm and the methylene protons
appeared as an AB quartet centered at 5.23 ppm in the 1H NMR
spectrum of 4. Methylation of 4 was carried out using methyl iodide
in an autoclave to yield the corresponding bis(triazolium) diiodide
5 in 81% yield, as a crystalline solid. The triazolium ring protons
appeared as a singlet at 8.97 ppm in the 1H NMR spectrum of 5. The
methylene protons of 5 appeared as an AB quartet (5.85 and
6.10 ppm) due to their diastereotopic nature in the 1H NMR spec-
trum in 1,1,2,2-tetrachloroethane-d2. When the spectrum was
measured as a function of temperature from 25 ꢀC to 130 ꢀC, the AB
pattern remained unchanged indicating the configurational sta-
bility of the axially chiral biphenyl moiety bearing bulky triazolium
substituents in the 2,20-positions. This suggests that it might be
possible to resolve 5 using a suitable ion exchange method to the
enantiomerically pure form. However, our attempts to resolve 5 as
the corresponding camphor-10-sulfonate salt were unsuccessful.
Compound 2 was treated with 2 equivalents of 1-phenyl-1H-
imidazole to yield the corresponding bis(imidazolium) dibromide 6
as a colorless crystalline solid (Scheme 1). The imidazolium ring
protons appeared as a singlet at 10.53 ppm in the 1H NMR spectrum
of 6. The precursor salts 5 and 6 were thoroughly characterized by
spectroscopic as well as single crystal XRD data (Table 1 and
Supplementary information).
Parameters
Formula
5
6
8
9
C32H30I2N6
C32H34Br2N4O3 C32H28N6Cl2Pd C36H34N6O4Pd
Formula weight 752.42
Radiation (Å) 0.71073
Crystal system Monoclinic
682.45
673.90
0.71073
721.09
0.71073
l
0.71073
Triclinic
P-1
Orthorhombic Monoclinic
Pba2 P2/n
14.2758 (12) 9.5296 (5)
9.5037 (7)
12.1910 (7)
90
Space group
C2/c
a (Å)
b (Å)
c (Å)
aꢀ
bꢀ
Gꢀ
V (Å3)
25.8910(9)
12.6507(4)
18.8966(7)
90
92.0500(10) 94.943(2)
90
6185.4(4)
298(2)
8
11.0768(4)
11.2420(4)
13.6131(5)
95.452(2)
12.1826 (8)
14.1960 (9)
90
97.505 (3)
90
1633.97 (17)
298 (2)
2
12,201
4368/0.0407
0.617
90
90
111.390(2)
1557.87(10)
298(2)
16.54.0 (2)
298 (2)
2
6609
3125/0.0248
0.752
T (K)
Z
2
Reflections/
unique/Rint
m
21,740
18,134
7565/0.0192 5446/0.0299
2.605
2960
1.79 to 28.27 1.964 to 24.996 1.67 to 27.47 1.67 to 29.70
1.015
mmꢁ1
2.640
696
F(000)
range
Goodness-
of- fit on F2
Final R indices R1 ¼ 0.0324 R1 ¼ 0.0501
684
740
q
1.097
1.085
1.012
R1 ¼ 0.0576
R1 ¼ 0.0564
wR2 ¼ 0.0758 wR2 ¼ 0.1314 wR2 ¼ 0.1543 wR2 ¼ 0.1510
R indices
(all data)
R1 ¼ 0.0501 R1 ¼ 0.0705
R1 ¼ 0.0818
R1 ¼ 0.0910
wR2 ¼ 0.0856 wR2 ¼ 0.1403 wR2 ¼ 0.1722 wR2 ¼ 0.1741
disappearance of the triazolium ring proton at 8.97 ppm. The
isotope distribution pattern obtained for the molecular ion in the
ESI-MS agreed with that of the calculated. The Pd(II) complexes
were synthesized by transmetallation reaction [35]. Thus treatment
of the silver NHC complex 7 with PdCl2(CH3CN)2 gave the corre-
sponding trans-Pd(II) dichloro derivative 8 in 96% and with
Pd(OAc)2 gave the corresponding trans-Pd(II) diacetate complex 9
in 90% yield, respectively (Scheme 2). All these complexes were
thoroughly characterized by spectroscopic and single crystal XRD
Synthesis of C2-symmetric bis(triazol-5-ylidene) Pd(II) complexes
Bis(triazolium) salt 5 was treated with silver oxide in CH2Cl2 at
room temperature for 24 h to yield the corresponding silver NHC
complex (7) in 93% yield. Although complex 7 was light sensitive
and decomposed on exposure to room light it could be character-
ized by NMR and HRMS data. 1H NMR spectrum indicated the
Scheme 1. Synthesis of NHC precursor salts 5 and 6 from diphenic acid.

70
S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74
Scheme 2. Synthesis of C2 symmetric chelating bis(triazol-5-ylidene) Ag(I) and Pd(II) complexes.
data. Pd(II) dichloro complex 8 crystallized in orthorhombic system
in Pbc2 space group. Pd center has a distorted square planar trans
geometry (Fig 1). The ClePdeCl bond angle is 173.8(2)ꢀ and Ccar-
beneePdeCcarbene bond angle is 177.3(5)ꢀ. The molecular structure
in the crystal has C2 symmetry, the C2 axis passing through the
center of the biphenyl bond and Pd atom. The CcarbeneePd distance
(2.030(5) Å) is in accordance with the earlier reports of Pd-triazol-
crystallized in monoclinic system in P2/n space group (Fig 2). Pd
center has a distorted square planar trans geometry. The OePdeO
bond angle is 175.5(3)ꢀ and CcarbeneePdeCcarbene bond angle is
176.0(2)ꢀ. Although formation of polymeric metal complexes is a
distinct possibility in all of the above reactions, it did not happen. In
all of the above reactions well defined C2-symmetric mono nuclear
chelate complex alone was formed as evident from the isolated
high yields and 1H NMR spectra of the crude products.
5-ylidene complexes [28e32] Pd(II) diacetate complex
9
Fig. 1. ORTEP diagram of compound 8 with thermal ellipsoid drawn at 30% probability. Selected bond lengths (Å) and bond angles (ꢀ): Pd1eCl1 ¼ 2.341(2), Pd1eC2 ¼ 2.030(5),
Cl1ePdeCl1 ¼ 173.77(2), Cl1ePd1eC2 ¼ 88.61(19), Cl1ePd1eC2(#1) ¼ 91.26(19), C1eC2eN3 ¼ 101.62(5). Hydrogen atoms are omitted for clarity.

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74
71
Fig. 2. ORTEP diagram of compound 9 with thermal ellipsoid drawn at 30% probability. Selected bond lengths (Å) and bond angles (ꢀ): Pd1eO1 ¼ 2.042(2), Pd1eC11 ¼ 2.048(4),
C11ePd1eC11(#1) ¼ 176.00(2), N1eC11eC7 ¼ 102.38(3). Hydrogen atoms are omitted for clarity.
Synthesis of polymeric bis(imidazol-2-ylidene) Pd(II) complexes
of polymeric complexes 10 and 11 instead of monomeric chelate
complex from 6 is surprising considering several mono nuclear
chelate complexes of bis(imidazolylidene) ligands with 1,10-
biphenyl and 1,10-binaphthyl backbones have been reported in the
literature [19]. However no structural characterization by single
crystal XRD has been reported for these complexes.
The complexation behavior of bis(imidazolylidene ligand
derived from bis(imidazolium) dibromide 6 was quite different
from that of bis(triazolylidene) ligand derived from bis(triazolium)
diiodide 5. When reacted with silver oxide 6 gave the corre-
sponding bis(imidazol-2-ylidene) Ag(I) complex as evident from
the disappearance of the imidazolium protons at 10.53 ppm in the
1H NMR spectrum of the reaction mixture. However the resonances
in the 1H NMR spectrum were quite broad and not discernable to
the expected mono nuclear chelate complex. The in-situ generated
Ag(I) complex was treated with PdCl2(CH3CN)2 and Pd(OAc)2 to
yield the corresponding polymeric bis(imidazol-2-ylidene) Pd(II)
dichloride (10) and diacetate (11) complexes, respectively
(Scheme 3). Unlike 8 and 9 these complexes were not soluble in
common organic solvents. 1H NMR spectra of the soluble portion
(in CDCl3) showed broad peaks. Based on these observations we
conclude that unlike the bis(triazolylidene) case which gave
exclusively the chelate complexes 8 and 9 in excellent yields,
polymer formation was observed in case of the bis(imidazolyli-
dene) derivatives. The corresponding chelating mono nuclear
complexes, if formed at all, were only in trace amounts. Formation
Comparison of the catalytic activities for SuzukieMiyaura coupling
of 1,4-dibromobenzene
NHCePd(II) complexes have been shown wide variety of cata-
lytic activities ranging from CeC cross coupling, CeH activation,
oxidation, reduction, hydro-carboxylation, hydroarylation etc.
[7,10,11,29,36e40]. The catalytic activity is very much pronounced
by the nature of the NHC ligands bonded to Pd atom. In comparison
to normal NHCePd complexes the mesoionic NHCePd complexes
showed higher catalytic activities towards cross coupling reactions
[31,41e43]. We examined the catalytic activities of complexes 8 and
10 towards Suzuki-Miyaura coupling of 1,4-dibromobenzene with
phenylboronic acid to yield p-terphenyl (Scheme 4). The dichloro
complexes 8 and 10 were chosen so as to compare their catalytic
efficiencieswith our earlier work in which dichloropalladium
Scheme 3. Synthesis of polymeric bis(imidazol-2-ylidene) Pd(II) complexes.

72
S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74
pump. X-ray crystallographic data were collected on a Bruker-AXS
Kappa CCD-Diffractometer with graphite-monochromator Mo Ka
radiation (
l
¼ 0.71073 Å). The structures were solved by direct
methods (SHELXS-97) and refined by full-matrix least squares
techniques against F2 (SHELXL-97). Hydrogen atoms were inserted
from geometry consideration using the HFIX option of the program.
For thin layer chromatography (TLC) analysis, E-Merck pre-coated
TLC plates (silica gel 60 F254 grade, 0.25 mm) were used.
Scheme 4. Comparative Suzuki-Miyaura coupling using complexes
8 and 10 as
catalysts.
Synthesis and characterization
derivative bearing mesityl wing-tip groups was used [31]. In both
cases p-terphenyl was obtained in excellent yields. With 2% catalyst
loading and under otherwise identical conditions the turnover
number (TON) and turnover frequency (TOF) for catalyst 8 were 88
and 12.5 and for catalyst 10 were 75 and 12.5, respectively. Under
the conditions employed for SuzukieMiyaura coupling practically
no difference in catalytic activity was observed between catalysts 8
and 10 as evident from nearly identical TON and TOF. However, the
TONs of these catalysts were much lower than our earlier report
[31]. These complexes were not effective for the SuzukieMiyaura
coupling of chloroarenes [43].
Preparation of silver oxide (Ag2O) [45]
Silver oxide was prepared by the addition of an aqueous solution
of sodium hydroxide (2 g in 20 mL) to an aqueous solution of silver
nitrate (1 g in 10 mL). Continuous shaking during the addition
ensured completion reaction resulting in a brown precipitate. It
was filtered and washed with 200 mL of water and then with 50 mL
of acetone. Silver oxide (700 mg) thus obtained was dried under
vacuum to yield a dark brown solid.
Synthesis of diazide 3 [46]
2,20-Bis(bromomethyl)-1,10-biphenyl (2) (5 g, 14.1 mmol) was
dissolved in DMF (15 mL) under N2 atmosphere in a two necked
round bottom flask equipped with a condenser. NaN3 (4.78 g,
73.5 mmol) was added and the reaction mixture was heated at
85 ꢀC for 12 h. After completion (TLC), the reaction mixture was
taken in a separating funnel, diluted with water (60 mL) and
extracted with hexane (3 ꢂ 60 mL). The combined organic layer
was dried over Na2SO4 and evaporated under vacuum to give dia-
zide 3 in 92% yield (3.58 g) as a colorless liquid, IR (neat): 2097 cmꢁ1
Conclusions
1,10-Biphenyl based C2-symmetric chelating mono nuclear
bis(1,2,3-triazol-5-ylidene) Pd(II) complexes (8 and 9) bearing
mesoionic NHC ligand were synthesized. The structures of
bis(1,2,3-triazol-5-ylidene) Pd(II) complexes (8 and 9) have been
unambiguously established by single crystal XRD data. In com-
plexes 8 and 9 palladium has distorted square planar geometry
with trans stereochemistry. Bis(imidazol-2-ylidene) Pd(II) com-
plexes (10 and 11) were polymeric in nature as evident from their
poor solubility as well as broad lines in the 1H NMR spectra. In the
variable temperature 1H NMR study of the bis(triazolium) salt 5 in
the temperature range of 25e130 ꢀC the AB quartet signal observed
for the diastereomeric methylene protons remained intact indi-
cating the configurational stability of the biphenyl backbone. This
suggested that it might be possible to resolve 5 using a suitable ion
exchange method. Finally a direct comparison of the catalytic ac-
tivities of complexes 8 and 10 was made in SuzukieMiyaura
coupling of 1,4-dibromobenzene with phenylboronic acid to p-
terphenyl. The catalytic activities of these two complexes were
comparable as indicated by the nearly same values for the TON and
TOF for the above reaction.
(
nazide); 1H NMR (CDCl3, 400 MHz)
d: 4.06 and 4.12 (AB quartet,
JAB ¼ 13.2 Hz, 4H, CH2), 7.21 (d, J ¼ 7.8 Hz, 2H), 7.37e7.47 (m, 6H);
13C NMR (CDCl3, 100 MHz)
133.7, 139.6.
d: 52.6 (CH2), 128.3, 128.5, 129.4, 130.2,
Synthesis of bis(triazole) 4
Diazide 3 (3 g, 11.35 mmol) was treated with phenylacetylene
(2.31 g, 22.7 mmol) in 10 mL degassed DMSO:H2O (9:1, v/v) in a
100 mL round bottom flask. CuI (0.43 g, 2.27 mmol) was added and
the reaction mixture was stirred under N2 atmosphere at room
temperature for 12 h. After completion of the reaction (TLC),100 mL
of water was added to the reaction mixture. The precipitate formed
was filtered through a sintered funnel. The product was washed
with hexane several times and then dissolved in CH2Cl2. It was
dried over Na2SO4 and then solvent was evaporated under vacuum
to obtain pure product in 97% yield (5.16 g) aꢁs1a grey solid. Mp:
Experimental section
154e158 ꢀC; IR(KBr): 1606, 1479, and 1224 cm
;
1H NMR (CDCl3,
Diphenic acid (1) was converted to 2,20-bis(bromomethyl)-1,10-
biphenyl (2) in three steps according to literature reported methods
[33,34] in 79% overall yield. 1-Phenyl-1H-imidazole was prepared
from bromobenzene and imidazole following literature method
[44,19].
500 MHz)
d
: 5.17 and 5.3 (AB quartet, 4H, JAB ¼ 15.5 Hz, CH2),
7.21e7.26 (m, 4H), 7.30 (t, J ¼ 7.5 Hz, 2H), 7.37 (t, J ¼ 7.7 Hz, 4H),
7.42e7.46 (m, 6H), 7.71 (d, J ¼ 7.7 Hz, 4H); 13C NMR (CDCl3,
125 MHz) d: 52.0 (CH2),120.4,125.7,128.3,128.8,128.9,129.0,129.2,
130.1, 130.4, 133.4, 138.8, 147.7; HRMS (ESI): m/z: calcd for C30H25N6
[M þ H]þ 469.2141, found 469.2145.
Instrumentation
Synthesis of bis(triazolium) diiodide 5
Infrared (IR) spectra were recorded on a JASCO 4100 FT-IR
spectrometer. 1H NMR spectra were measured on Bruker AVANCE
spectrometers operating at 400 and 500 MHz for proton and 100
and 125 MHz for carbon-13 nuclei, respectively. Chemical shifts
were reported in ppm from tetramethylsilane as the internal
standard. 13C NMR spectra were recorded with complete proton
decoupling. C-13 chemical shifts were reported in ppm using re-
sidual solvent peaks as internal standard. High-resolution mass
spectra (HRMS) were performed on Micromass ESI Q-TOF micro
mass spectrometer equipped with a Harvard apparatus syringe
Bis(triazole) 4 (2 g, 4.26 mmol) was treated with methyl iodide
(7.26 g, 51.2 mmol) in CH3CN (7 mL) in a teflon coated steel auto-
clave and heated in an oil bath at 85e90 ꢀC for 48 h. The reaction
mixture was evaporated to dryness and the solid product was
washed with hexane and ethyl acetate (5 ꢂ 10 mL) Pure triazolium
salt was obtained as a light brown colour solid in 81% yield (2.6 g).
Mp: 184e188 ꢀC; IR (KBr): 3061, 3031, 1610, 1568, 1491, 1456, and
1432 cmꢁ1. 1H NMR (DMSO-d6, 500 MHz)
d: 4.23 (s, 6H, CH3), 5.65
and 5.76 (AB quartet, 4H, JAB ¼ 15.5 Hz, CH2), 7.22 (d, J ¼ 7 Hz, 2H)
7.51e7.58 (m, 4H), 7.68e7.70 (m, 12H), 8.97 (s, 2H, triazolium H);

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74
73
13C NMR (DMSO-d6,125 MHz)
129.0, 129.1, 129.3, 129.4, 129.5, 130.3, 130.5, 131.5, 138.7, 142.2;
d
: 38.9 (CH3), 54.6 (CH2),122.3,128.3,
Synthesis of polymeric bis(imidazol-2-ylidene) Pd(II) complexes 10
and 11
HRMS (ESI) m/z: (m ¼ 498.2532, z ¼ 2, dicationic salt) calcd for
These complexes were prepared following the same procedure
as described in the synthesis of complexes 8 and 9. Bis(imidazo-
lium) dibromide 6 (300 mg, 0.48 mmol), Ag2O (133 mg, 0.57 mmol)
and Pd(CH3CN)2Cl2 (123 mg, 0.52 mmol) were reacted to obtain
250 mg of yellow solid (10) (80%). Similarly the corresponding ac-
etate complex (11) was prepared using Pd(OAc)2 (123 mg,
0.55 mmol) in 90% yield (300 mg). These complexes were insoluble
in common organic solvents and they were not processed further.
They were used as it is for SuzukieMiyaura coupling reactions.
C
16H15N3 [M þ H]þ 249.1266, found 249.1257.
Synthesis of palladium dichloro complex 8
Bis(triazolium) diiodide 5 (200 g, 0.27 mmol) was treated with
Ag2O (74 mg, 0.32 mmol) in CH2Cl2 and the mixture was stirred for
24 h under N2 atmosphere in the dark. An aliquot of the reaction
mixture was evaporated and 1H NMR and ESI mass spectra were
recorded to check the complete conversion of the iodide salt to the
corresponding Ag complex 6. The absence of the triazolium proton
resonance at 8.97 ppm in the 1H NMR spectrum clearly indicated
the complete conversion of 5 to silver complex 7. 1H (NMR, CDCl3,
Suzuki-Miyaura coupling of 1,4-dibromobenzene and phenylboronic
acid
400 MHz) d: 4.19 (s, 6H, CH3), 5.53 and 5.63 (AB quartet, 4H,
A mixture of 1,2-dibromobenzene (100 mg, 0.42 mmol), phe-
nylboronic acid (124 mg, 1.02 mmol), triphenylphosphine (5 mg,
0.018 mmol), NaOH (68 mg, 1.7 mmol) and Pd-complex 8 (5 mg,
2 mol%) were taken in a 50 mL two necked round-bottom flask.
Dioxane (4 mL) was added to the mixture and heated at 105 ꢀC.
After 6 h the reaction was complete (TLC). Removal of solvent under
reduced pressure followed by aqueous work up gave the crude
product which was purified by crystallization from CH2Cl2/hexane
mixture to yield pure p-terphenyl as a colorless crystalline solid in
88% yield. The reaction was repeated with complex 10 and p-ter-
phenyl was obtained in 75%. The product was confirmed by com-
JAB ¼ 14.4 Hz, CH2), 7.29 (m, 7H), 7.37 (m, 11H); HRMS (ESI) m/z:
calcd for C32H28N6Ag [M]þ 603.1426, found 603.1445. After 24 h
stirring [PdCl2(CH3CN)2] (75.8 mg, 0.29 mmol) was added to the
reaction mixture and stirring was continued for another 24 h. The
reaction mixture was filtered through a sintered crucible and sol-
vent was evaporated under vacuum to give complex 8 as a pale
yellow solid. It was washed with diethyl ether and ethyl acetate
several times to obtained pure product (8) in 96% yield (171 mg).
Mp: 246e250 ꢀC, IR (KBr): 3448, 3065, 1630, 1478, and 1444 cmꢁ1
1H NMR (CDCl3, 400 MHz)
: 4.07 (s, 6H, CH3), 5.55 and 6.55 (AB
;
d
parison with authentic sample. 1H NMR (CDCl3, 500 MHz)
d:
quartet, 4H, JAB ¼ 14.4 Hz, CH2), 7.28e7.45 (m, 14H), 7.94 (d,
J ¼ 7.4 Hz, 4H); 13C NMR (CDCl3, 100 MHz)
d: 37.3 (CH3), 54.6 (CH2),
7.35e7.38 (t, 2H, J ¼ 7.5 Hz), 7.45e7.48 (t, 4H, J ¼ 8 Hz), 7.65e7.66 (d,
4H, J ¼ 7.5 Hz), 7.69 (s, 4H); 13C NMR (CDCl3, 125 MHz)
d
: 127.2,
127.6, 127.7, 128.5, 128.7, 129.0, 129.1, 130.1, 131.3, 134.0, 139.4, 145.1,
160.7 (carbene C); HRMS (ESI): m/z: [M
32H29N6Cl2Pd 673.0866, found 673.0882.
þ
H]þ calcd for
127.5, 127.6, 128.9, 140.2, 140.8.
C
Acknowledgments
Synthesis of palladium diacetate complex 9
Bis(triazolium) diiodide 5 (200 mg, 0.27 mmol) was treated with
Ag2O (74 mg, 0.32 mmol) in dichloromethane and the reaction
mixture was stirred for 24 h under N2 atmosphere in the dark.
Pd(OAc)2 (65 mg, 0.29 mmol) was added in the reaction mixture
and stirring was continued for 24 h. The reaction mixture was
filtered through a sintered funnel and solvent evaporated to obtain
the crude product as a dark brown colour solid. Upon washing the
crude product with ether and ethyl acetate several times pure
product was obtained in 90% yield (172 mg). Mp:198e200 ꢀC, IR
(KBr): 3430, 3055, 2924,1588,1478, and 1440 cmꢁ1. 1H NMR (CDCl3,
We thank CSIR for fellowship (BS), CSIR and DST, New Delhi for
financial support (SS) and the Department of Chemistry, IIT Madras
for infrastructure facilities.
Appendix A. Supplementary data
The following is the supplementary data related to this article:
CCDC numbers 960652, 989618, 960653 and 960654 contain the
supplementary crystallographic data for compounds 5, 6, 8 and 9,
respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
500 MHz)
(AB quartet, 4H, JAB ¼ 14.4 Hz, CH2), 7.32e7.47 (m, 14H), 8.07e8.09
(d, 4H); 13C NMR (CDCl3, 125 MHz)
: 22.6 (OCOCH3), 37.2 (CH3),
d: 1.10 (s, 6H, OCOCH3), 4.50 (s, 6H, CH3), 5.48 and 6.85
d
54.2 (CH2), 127.3, 128.2, 128.4, 128.5, 128.6, 128.9, 130.1, 131.3, 134.8,
139.8, 145.8, 160.0 (carbene C), 176.1 (CO); ESI-MS: m/z 603 (M-
OCOCH3).
Appendix B. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2014.06.023.
Synthesis of bis(imidazolium) dibromide 6
Dibromide 2 (1.4 g, 4.1 mmol) and 1-phenyl-1H-imidazole
(1.31 g,1.2 mL, 9.1 mmol) were dissolved in dioxane (10 mL) and the
mixture was heated under N2 atmosphere at 85 ꢀC for 14 h. The
reaction mixture was cooled to 0 ꢀC to obtain a white solid pre-
cipitate. The solid was collected by filtration and washed with
hexane and ether to obtain pure bis(imidazolium) dibromide 6 in
98% yield (2.5 g). It was further recrystallized from methanol. Mp:
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