Palladium biphenyl N-heterocyclic carbene complexes: Synthesis, structure and their catalytic efficiency in water mediated Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.molcata.2016.03.032

Synthesis of bis-N-heterocyclic carbene ligand with biphenyl backbone [MIBIPhen](Br)₂ (L), [Ag(MIBIPhen)]₂(PF₆)₂ (1) and palladium-NHC complexes, [Pd(MIBIPhen)(Cl)(H₃C-CN)]PF₆ (2), [Pd(MIBIPhen)(H₃C-CN)₂](PF₆)₂ (3) and [Pd(MIBIPhen)Br₂] (4) {MIBIPhen = (3,3′-(biphenyl-2,2′-diylbis(methylene))bis(1-methyl-1H-imidazol-3-ium)bromide)} is reported. All complexes (1-4) are characterized by NMR, ESI-MS, elemental analysis and single crystal XRD. Complex 1 serves as an efficient NHC transfer agent for the preparation of complex 2. Treatment of complex 2 with AgPF₆ drives the exchange of chloride by acetonitrile to result in complex 3. Complex 4 is prepared by in situ deprotonation of MIBIPhen with Pd(OAc)₂. Complex 1 is molecular box type with silver in bent geometry. Complexes 2, 3 & 4 exist as monomers with palladium in distorted-square-planar geometry. All Pd-NHC complexes are explored as catalyst in water mediated Suzuki-Miyaura cross coupling reaction where complex 3 shows efficient catalytic activity.

Full text of DOI:10.1016/j.molcata.2016.03.032

Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium biphenyl N-heterocyclic carbene complexes: Synthesis,
structure and their catalytic efficiency in water mediated
Suzuki–Miyaura cross-coupling reaction
Lavudi Sevaa, Wen-Shu Hwangb, Shahulhameed Sabiaha,∗
a Department of Chemistry, Pondicherry University, Pondicherry 605 014, India
b Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, Republic of China (ROC)
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2016
Received in revised form 13 March 2016
Accepted 22 March 2016
Available online 23 March 2016
Synthesis of bis-N-heterocyclic carbene ligand with biphenyl backbone [MIBIPhen](Br)2 (L),
[Ag(MIBIPhen)]2(PF6)2 (1) and palladium-NHC complexes, [Pd(MIBIPhen)(Cl)(H3C-CN)]PF6 (2),
[Pd(MIBIPhen)(H3C-CN)2](PF6)2 (3) and [Pd(MIBIPhen)Br2] (4) {MIBIPhen = (3,3ꢀ-(biphenyl-2,2ꢀ-
diylbis(methylene))bis(1-methyl-1H-imidazol-3-ium)bromide)} is reported. All complexes (1–4) are
characterized by NMR, ESI-MS, elemental analysis and single crystal XRD. Complex 1 serves as an
efficient NHC transfer agent for the preparation of complex 2. Treatment of complex 2 with AgPF6
drives the exchange of chloride by acetonitrile to result in complex 3. Complex 4 is prepared by in situ
deprotonation of MIBIPhen with Pd(OAc)2. Complex 1 is molecular box type with silver in bent geometry.
Complexes 2, 3 & 4 exist as monomers with palladium in distorted-square-planar geometry. All Pd-NHC
complexes are explored as catalyst in water mediated Suzuki-Miyaura cross coupling reaction where
complex 3 shows efficient catalytic activity.
Keywords:
N-Heterocyclic carbene
Silver-NHC
Palladium-NHC
Biphenyl
Suzuki-Miyaura coupling
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
work on NHC chemistry [10], we tried to explore the organometal-
lic chemistry of silver and palladium complexes with NHC ligand
Over the last few decades, N-heterocyclic carbenes (NHC) are
extensively used as organocatalysts or ancillary ligands in transi-
tion metal catalyzed organic transformations [1]. There are number
of NHC-transition metal complexes comprising mono, bis, and pin-
cer types are known in the literature [2]. These complexes vary in
terms of stability and catalytic activity depending on the sterics
and electronics of the NHC ligands. Considerably, they can be con-
trolled by modification of wingtip substituents and NHC backbones
[1–3]. Having reviewed the NHC ligand design, it is found that biaryl
appended NHC-metal complexes are very much limited in litera-
ture [4,5]. Among the biaryl NHC systems, biphenyl NHC counter
parts are poorly explored in terms of synthesis and structure [6].
Nevertheless, biphenyl ligands are known as prochiral ligands
and chiral auxiliaries in organic transformations [7]. They showed
interesting coordination chemistry, especially with pyridyl donors
[8,9]. Copper complexes with biphenyl ligands are explored in
the hydrolytic cleavage of phosphodiesters [9]. Having inspired
by the interesting crystal motifs of above work and our previous
bearing biphenyl backbone. In this paper, we report the synthesis,
isolation and characterization of biphenyl bis-NHC-Silver and pal-
ladium complexes 1–4 (Schemes 1 and 2). The catalytic efficiency
of Pd-NHC complexes were tried in Suzuki-Miyaura cross-coupling
reaction especially in water.
2. Results and discussion
2.1. Synthesis of ligand and metal complexes
The
bisimidazolium
ligand,
L
(3,3ꢀ-(biphenyl-2,2ꢀ-
diylbis(methylene))bis(1-methyl-1H-imidazol-3-ium) bromide)
was prepared by stirring commercially available 2,2ꢀ-bis(bromo
methyl)-1,1ꢀ-biphenyl and 1-methyl imidazole in acetonitrile
for three hours at room temperature (Scheme 1). This reaction
provides the desired NHC-ligand L in quantitative yield. The
reaction of L with silver oxide (Ag2O) in water, followed by anion
exchange with silver hexa fluorophosphate (AgPF6) yielded the
dinuclear silver bis-carbene complex 1 in 81% yield (Scheme 1).
Complex 1 is stable in air and moisture and it serves as an NHC
transfer agent. The NHC-complex 2 is prepared from silver carbene
complex 1 via transmetalation protocol invented by Wang and
∗
Corresponding author.
E-mail address: sabiahs@gmail.com (S. Sabiah).
http://dx.doi.org/10.1016/j.molcata.2016.03.032
1381-1169/© 2016 Elsevier B.V. All rights reserved.

126
L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
Lin [11]. The reaction of complex 1 with PdCl2(H3C-CN)2 yielded
the mono-nuclear palladium carbene complex 2 in 85% yield. The
dehalogenation of complex 2 by AgPF6 in acetonitrile afforded the
complex 3 in 90% yield. On the other hand, complex 4 is prepared
by in situ deprotonation of bis- imidazolium ligand L with Pd(OAc)2
in acetonitrile (Scheme 2).
Palladium complexes 2–4 are characterized by NMR, ESI-MS and
elemental analysis in addition to single crystal XRD. The 1H NMR
of complex 2 shows two multiplets at ı 5.24–ı 5.61 ppm due to
methylene protons. N-Methyl peaks appeared as two singlets at
ı 3.83 and ı 3.87 ppm and its corresponding carbon appears at ı
163.8 ppm and ı 166.0 ppm in 13C NMR. The ESI-MS of complex
2 displays a base peak at m/z 593 corresponding to mononuclear
palladium bis-carbene monocation [C22H22F6N4PPd]+. The labile
nature of complex 3 prevented us to get clear NMR spectral pattern.
Nevertheless, elemental analysis and single crystal XRD confirmed
the formation of complex 3. The symmetrical dibromo-complex 4
showed two clear doublets at ı 5.08 and ı 5.83 ppm for methylene
protons and a singlet at ı 3.78 ppm for N-methyl protons. The ESI-
MS of complex 4 shows a peak at m/z 609 due to mononuclear
palladium bis-carbene [C22H22Br2N4Pd+H+] species.
2.2. Spectral charaterization
The ligand L was characterized by NMR, ESI-MS (M+ = m/z 527;
[C22H24Br2N4Na]+) and elemental analysis. Its 1H NMR spectrum
shows a singlet at ı 3.81 ppm for two methyl protons and two dou-
blets at ı 5.16 and ı 5.31 ppm for four methylene protons to suggest
that molecule has C2-symmetry. The carbene proton appears at ı
8.91 ppm, while carbene carbon appears at ı 138.3 ppm. The ligand
L easily forms a silver bis-carbene complex 1 which was confirmed
by NMR followed by other techniques. The disappearance of a sin-
glet at ı 8.91 ppm (i.e. NHC-proton) in the 1H NMR of complex 1 is
an evidence for the formation of silver bis-carbene complex. Single-
crystal X-ray diffraction analysis confirmed the solid state structure
as dinuclear bis carbene complex. However, ESI-MS of complex 1
with a base peak at m/z 449 revealed that the complex 1 may exist
as monomeric species [C22H22AgN4]+ in solution.
2.3. X-ray structure analysis
The single crystal X-ray structures of silver and palladium bis-
carbene complexes 1, 2, 3 and 4 are shown in Figs. 1 and 2 and
their crystallographic details are listed in Table 1. Single crys-
tals of complex 1 suitable for diffraction studies were grown in
acetonitrile by slow evaporation method. The solid state struc-
ture represents a dinuclear silver bis-carbene complex 1 with
Scheme 1. Synthesis of complexes 1–3.
Scheme 2. Synthesis of complex 4.

L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
127
Fig. 1. Molecular diagram of complexes 1 and 2 (thermal ellipsoids at 30% level). All hydrogen atoms and anions are omitted for clarity. Selected bond distances (Å) and
bond angles (deg) are as follows. (1): Ag1-C1 = 2.070(7), Ag1-C44 = 2.083(6), Ag2-C22 = 2.085(7), Ag2-C23 = 2.069(7); C1-Ag1-C44 = 174.3(3), C22-Ag2-C23 = 177.9(3). (2):
Pd1-N1 = 2.011(2), Pd1-Cl1 = 2.2671(9), Pd1-C3 = 2.021(2), Pd1-C23 = 2.031(3); C3-Pd1-C23 = 174.92(10), C3-Pd1-Cl1 = 86.92(7), N1-Pd1-Cl1 = 178.39(7).
Fig. 2. Molecular diagram of complexes 3 and 4 (thermal ellipsoids at 30% level). All hydrogen atoms and anions are omitted for clarity. Selected bond distances (Å)
and bond angles (deg) are as follows. (3): Pd1-N1 = 1.977(9), Pd1-N2 = 1.954(9), Pd1-C5 = 2.003(10), Pd1-C26 = 2.010(10); N1-Pd1-N2 = 178.9(4), C5-Pd1-C26 = 179.6(5). (4):
Pd1-Br1 = 2.4414(5), Pd1-C1 = 2.023(4); C1-Pd1-C12 = 176.1(2), C1-Pd1-Br1 = 89.83(11).
molecular box shape (refer Supplementary material). The metal
centre adopts slightly bent geometry with ligation from two car-
benoid carbons. The bond distances from carbenoid carbon to silver
centers are Ag1-C1 = 2.070(7) Å & Ag1-C44 = 2.083(6) Å and Ag2-
C22 = 2.085(7) Å and Ag2-C23 = 2.069(7) Å. It is observed that these
bond distances fall in the range observed for known Ag-bis NHC
carbene complexes [12]. The distortion in geometry is apparent
from the CNHC-Ag-CNHC bond angle of C1-Ag1-C44 = 174.3(3)◦ and
C22-Ag2-C23 = 177.9(3)◦, compared to an ideal angle of 180◦.
The two metal centers are separated by 3.55 Å and is consider-
ably higher than the Van der Waals contact distance of 3.44 Å, which
indicates the nonexistence of silver-silver interaction. Complex 1
shows two different biphenyl twist angles 74.90◦ and 77.86◦ indi-
cating two imidazolium moieties are connected with two different
silver centers.
Complexes 2 and 3 are crystallized from mixture of H3C-
CN/diethyl ether by vapour diffusion method, whereas complex 4
is crystallized from acetonitrile by slow evaporation. All palladium
complexes exist as monomers with Pd in distorted-square-planar
geometry. In complex 2 the ligation is provided by acetonitrile
nitrogen and chloride along with two carbenoid carbons. Apart
from two NHC carbon ligation complex 3 has two acetonitriles and
complex 4 has two bromides, respectively. The carbenoid carbon
to palladium bond distances are [2: Pd1-C3 = 2.021(2) Å, Pd1-
C23 = 2.031(3) Å; 3: Pd1-C5 = 2.003(10) Å, Pd1-C26 = 2.010(10) Å; 4:
Pd1-C1 = 2.023(4) Å] measured from X-ray structures. It is observed
that the complex 3 shows slightly shorter bond distance than com-
plexes 2 and 4. The bond distances fall in the range observed for
the known Pd-bis NHC carbene complexes [13]. The trans-CNHC-Pd-
CNHC bond angles of complexes 2, 3 and 4 are 174.92(10)◦, 179.6(5)◦

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L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
Table 1
Crystal data and structure refinement details for 1–4.
Crystal data
1
2.C4H10
O
3.H3C-CN
4
CCDC No.
742417
C44H44Ag2F12P2
1190.55
orthorhombic
20.4441(12)
13.0820(7)
17.8316(10)
90
757889
901897
C28H31N7PdF12P2
861.94
monoclinic
11.170(3)
12.403(3)
26.296(6)
90
1044238
C22H22Br2N4Pd
608.66
monoclinic
15.9473(11)
9.2174(6)
16.5767(16)
90
chem formula
formula mass/amu
crystal system
a/Å
b/Å
c/Å
C28H35ClN5PdF6PO
744.43
monoclinic
11.7625(9)
17.1288(13)
16.5981(13)
90
␣/deg
␤/deg
90
90
4769.1(5)
293
95.578(2)
90
3328.3(4)
296
100.633(5)
90
3580.5(15)
296
113.949(10)
90
2226.9(3)
298
ꢀ/deg
V/A3
T/K
space group
z
Pca21
4
0.977
P21/n
4
0.749
P21/n
4
0.701
P2/n
4
4.438
ꢁ/mm−1
No. of reflec. collect.
no of indep. reflec.
Rint
final R1(I > 2␴(I))
final wR2(F2)(I > 2␴(I))
final R1 (all data)
final wR (F2)(all data)
goodness of fit on F2
21301
7825
28417
5863
17782
6242
12708
3895
0.0457
0.0492
0.1077
0.0693
0.1186
1.217
0.0256
0.0371
0.1047
0.0488
0.1146
1.047
0.1962
0.0579
0.1081
0.2357
0.1626
0.917
0.0357
0.0330
0.0645
0.0534
0.0708
1.044
reaction. This reaction is one of the most important and broadly
utilized reactions in the construction of biaryls [15]. Although
several other methods (e.g. Kharash, Negishi, Stille, Himaya,
Liebeskind-Srogl and Kumuda coupling) are available, the Suzuki-
Miyaura Reaction (SMR) highlights the mild reaction conditions
and the commercial availability of diverse boronic acids that are
environmentally safer than other organometallic reagents [16].
Interestingly, water mediated SMR reactions are attractive owing
to environmental, economic and safety considerations [17,18].
Hence we turned our interest towards water mediated SMR
reaction. Initially, the coupling reaction was performed between
bromobenzene with 4-chlorophenylboronic acid in different sol-
vents using 0.2 mol% of complex 2 in the presence of potassium
carbonate under reflux condition (Table 2, entries 1–5). Water was
found to be a suitable solvent and it provides the desired product in
90% yield (Table 2, entry 3). Nevertheless, different bases including
sodium carbonate, sodium acetate, potassium hydroxide, triethy-
lamnie, etc. were examined in water (Table 2, entries 6–10) where
K2CO3 was found to be superior over other bases.
Fig. 3. Thermogravimetric Analysis (TGA) of complexes 1, 2, 3 and 4.
Under the same reaction conditions, complex 3 and 4 yielded
the desired coupling product in 95% and 63%, respectively (Table 2,
entries 11 and 12). Optimization of catalyst loading is tabulated
in Supplementary Material and we had proceeded with 0.2 mol%
complex. Comparative results of the catalytic activities of the com-
plexes 2–4 with different aryl boronic acids are shown in Table 3.
It is worthy to note that substitution on the phenylboronic acids
(e.g. electron donating and withdrawing groups) did not affect the
product yield which shows the broadness of the catalytic efficiency
of complex 3 (Table 3, entries 6–10). When we tried the coupling
reaction between simple halobenzene and phenyl boronic acid by
complex 3 (0.2 mol%), bromo and iodobenzene gave 96% biphenyl
where as chlorobenzene afforded only 35% yield.
The order of reactivity of the complexes in C C coupling reaction
is found to be 3 > 2 ꢁ 4. There are various aspects to account for the
difference in catalytic activity of Suzuki Myara cross coupling reac-
tion [14a,19]. The electronic/steric property of NHC, counter anions,
bases, solvents, temperature, catalyst loading, additives, support-
ing/anchoring ligands etc play important roles to bring catalytic
efficiency. In the present study, it is not appropriate to discuss the
electronic influences by NHC since all Pd-NHC carbene (Pd-C) dis-
tances are almost equal [14b]. NHC-steric factors do not contribute
as the wing-tip substituent on nitrogen is only methyl in all Pd-
and 173.0(2)◦ respectively. The biphenyl twist in complexes 2, 3
and 4 are apparent from the dihedral angles of 66.02◦, 64.11◦ and
78.99◦ between the benzene rings.
2.4. Thermal stability by TGA anaysis
To understand the thermal stability of the complexes, thermo-
gravimetric analysis (TGA) was studied (Fig. 3). The decomposition
temperature of complexes 1, 2, 3 and 4 is apparent from 280 ◦C,
285 ◦C, 290 ◦C and 150 ◦C respectively. The complete loss of car-
benes were slow and was observed beyond 400 ◦C. The observed
TGA analysis with decomposition temperature is comparable with
other Pd-NHC complexes reported in literature [14]. Most of the
complexes show decomposition temperature Td = ∼217–310 ◦C.
The present analysis suggests that Pd(II)-NHC complexes are quite
stable with MIBIPhen chelation.
2.5. Catalytic activity towards Suzuki-Miyaura coupling reaction
The catalytic efficiency of synthesized palladium-NHC com-
plexes 2, 3 & 4 were tested in the Suzuki-Miyaura cross-coupling

L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
129
Table 2
Optimization of reaction conditions.
Entry
Solvent
Complex
Bases
Yield (%)b
1
2
3
4
5
6
7
8
DMF
Tolune
H2O
MeCN
THF
H2O
H2O
H2O
H2O
H2O
H2O
H2O
2
2
2
2
2
2
2
2
2
2
3
4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaOAc
NaOH
KOH
82
84
90
57
49
62
41
71
29
33
95
63
9
10
11
12
NEt3
K2CO3
K2CO3
Bold value signifies metal complexes numbers.
aReagents and conditions: arylboronic acid (1.2 equiv.), bromo-benzene (1.0 equiv.), catalyst (0.2 mol%), K2CO3 (6 equiv.).
b
Isolated yield (average of two runs).
Table 3
Suzuki-Miyaura reaction between arylboronic acids and bromobenzene.
Entry
Catalyst
R
Yield (%)b
1
2
3
4
5
2
4-F
4-Cl
H
4-Et
4-OMe
86
90
87
89
87
6
7
8
9
4-F
4-Cl
H
4-Et
4-OMe
94
95
96
97
95
3
4
10
11
12
13
14
15
4-F
4-Cl
H
4-Et
4-OMe
62
63
58
59
56
aReagents and conditions: arylboronic acid (1.2 equiv.), bromo-benzene (1.0 equiv.), catalyst (0.2 mol%), K2CO3 (6 equiv.), reflux.
b
Isolated yield (average of two runs).
NHC complexes. Hence, the difference in catalytic activity may arise
from supporting ligands [19e,f]. In the present study, the activity
increases when the palladium core changes from neutral [LPdBr2]
to [LPd(Cl)(CH3CN)]+ to [LPd(CH3CN)2]2+. We propose that weakly
coordinated CH3CN ligands could enhance the catalytic activity via
easy substrate access at metal centre.
were comparable with literature reports [20] (refer Supplementary
material).
3. Experimental
3.1. Materials and physical measurements
Well-defined NHC-based catalysts for SMR in water were
reported by Tu, Karimi, Wang and Godoy and Peris [16]. More exam-
ples are documented in the recent review of SMR [18b]. The reaction
works well in water may be due to easy replacement of acetonitrile
by water which could enhance the catalytic activity in presence of
K2CO3 [19c] (refer Supplementary material for ESI-MS of complex
3 in acetonitrile + water mixture). When we compared with sim-
ilar biphenyl NHC Pd system (SIBiphen), complex 3 serves as an
efficient catalyst for biaryl construction in SMR reaction [19b]. All
the coupling products were confirmed by NMR/mass and the data
All the reactions were carried out under inert atmosphere. The
solvents and chemicals were purchased from Sigma Aldrich and
used without further purification. NMR spectra were recorded
on a Bruker Avance 400 NMR spectrometer (1H, 400 MHz; 13C,
100 MHz). Mass spectra were measured on water’s Quattro Micro
V 4.1. Elemental analyses were carried out using FLASH 2000
organic elemental analyzer. X-ray crystallography data were
collected on X Calibur Oxford Diffraction Ltd. with Mo K␣ radi-
ation (␭ = 0.71073 Å). Structure solution and refinement were

130
L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
performed with SHELX-97. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were generated according to
stereochemistry and refined using riding model in SHELX-97.
for C24H25ClF6N5PPd (M.Wt 670.33 g/mol): C 43.00, H 3.76, N 10.45;
found: C 43.06, H 3.76, N 10.41.
3.5. Synthesis of complex 3
3.2. Synthesis of ligand L
Complex 2 (0.100 g, 0.15 mmol) was dissolved in acetonitrile
(3 mL) followed by silver hexafluorophosphate (AgPF6) (0.038 g,
0.15 mmol) which induced precipitation. The reaction mixture was
allowed to stir for 30 min at room temperature. The suspension
was filtered by using G3 sintered funnel through celite and the
filtrate was collected in a flask. The solvent was pumped off to
get pale yellow crude which was washed with diethyl ether and
dried under vacuum to get desired pale yellow solid in 90% yield
(0.110 g). MS (ESI) m/z 485 [C22H22N4Pd·2H2O]+; Anal. Calcd for
C26H28F12N6P2Pd (M.Wt 820.90 g/mol): C 38.04, H 3.44, N 10.24;
found: C 38.17, H 3.45, N 10.28.
2,2ꢀ-Bis(bromomethyl)-1,1ꢀ-biphenyl (0.100 g, 0.29 mmol)
was stirred in acetonitrile (5 mL) to which 1-methylimidazole
(0.047 mL, 0.59 mmol) was added through syringe under nitrogen
atmosphere. After stirring three hours at room temperature, the
solvent was pumped off to get crude solid. The crude solid was
washed with DCM (3 × 5 mL) and dried under vacuum to get a white
solid in 97% yield (0.144 g). M.P = 244–245 ◦C; 1H NMR (400 MHz,
DMSO-d6): ı 3.81 (s, 6H, CH3), 5.16 (d, 2H, CH2, J = 15.2 Hz), 5.31
(d, 2H, CH2, J = 15.2 Hz), 7.09 (d, 2H, CHimid, J = 7.6 Hz), 7.36 (d,
2H, CHimid, J = 7.6 Hz), 7.44 (t, J = 7.2 Hz, 2H, CHph), 7.52 (m, 4H,
CHph), 7.71 (s, 2H, CHph), 8.91 (s,2H, C2Himid); 13C NMR (100 MHz,
DMSO-d6): ı 35.9 (s, CH3), 50.4 (s, CH2), 122.5 (s, CHimid), 123.8 (s,
CHimid), 128.8 (s, CHph), 128.9 (s, CHph), 129.1 (s, CHph), 129.8
(s, CHph), 132.2 (s, Cph), 136.8 (s, Cph), 138.3 (s, C2imid). MS (ESI)
m/z 527 [C22H24Br2N4Na]+. Anal. Calcd for C22H24Br2N4 (M.Wt
504.27 g/mol): C 52.40, H 4.80, N 11.11; found: C 52.65, H 4.81, N
11.15.
3.6. Synthesis of complex 4
The ligand L (0.100 g, 0.20 mmol) was dissolved in dry acetoni-
trile (5 mL) to which palladium acetate (0.045 g, 0.20 mmol) was
added as a solution in acetonitrile at room temperature under nitro-
gen atmosphere. The brick red solution was allowed to stir at rt for
1 h and at reflux condition for one day. The reaction mixture was
brought to room temperature, filtered through celite bed and the
filtrate was collected in a 50 mL flask. Evaporation of the solvent fol-
lowed by purification through column chromatography (100% ethyl
3.3. Synthesis of complex 1
Ligand L (0.100 g, 0.20 mmol) was dissolved in distilled water
(5 mL) to which silver(I) oxide (Ag2O) (0.138 g, 0.60 mmol) was
added. It was allowed to stir for two hours at room temperature
under exclusion of light. The suspension was filtered through celite
and the filtrate was collected in a flask to which KPF6 (0.110 g,
0.60 mmol) was added. The formation of white precipitate was
observed which was filtered and dried under vacuum to get com-
plex 1 in 81% yield (0.096 g). 1H NMR (400 MHz, DMSO-d6): ı
3.77 (s, 12H, CH3), 5.10 (d, 4H, CH2, J = 15.6 Hz), 5.33 (d, 4H, CH2,
J = 15.6 Hz), 7.15–7.18 (m, 4H, CHimid), 7.21–7.24 (m, 4H, CHimid),
7.35–7.41 (m, 12H, CHph), 7.82 (s, 4H, CHph); 13C NMR (100 MHz,
DMSO-d6): ı 38.0 (s, CH3), 51.0 (s, CH2), 122.2 (s, CHimid), 124.1 (s,
CHimid), 127.0 (s, CHph), 127.99 (s, CHph), 128.5 (s, CHph), 131.2
(s, CHph), 135.7 (s, Cph), 136.4 (s, Cph), 137.1(s, CNHC). MS (ESI)
m/z 449 [C22H22AgN4]+. Anal. Calcd for C44H44Ag2F12N8P2 (M.Wt
1190.55 g/mol): C 44.38, H 3.73, N 9.41; found: C 44.48, H 3.72, N
9.46.
acetate) yielded the desired yellow solid in 45% yield (0.054 g). 1
NMR (400 MHz, DMSO-d6): ı 3.78 (s, 6H, CH3), 5.08 (d, 2H, CH2,
J = 15.2 Hz), 5.83 (d, 2H, CH2, J = 15.2 Hz), 7.07–7.09 (m, 2H, CHimid
H
,
J = 7.6 Hz), 7.16–7.18 (m, 2H, CHimid, J = 7.6 Hz), 7.23 (m, 2H, CHph),
7.52 (t, 4H, CHph, J = 4.4 Hz), 7.64 (s, 2H, CHph); 13C NMR (100 MHz,
DMSO-d6): ı 36.7 (s, CH3), 50.0 (s, CH2), 121.2 (s, CHimid), 123.2 (s,
CHimid), 126.3 (s, CHph), 127.8 (s, CHph), 128.5 (s, CHph), 130.64 (s,
CHph), 136.75 (s, Cph), 138.5 (s, Cph), 171.3 (s, CNHC); MS (ESI) m/z
609 [C22H22Br2N4Pd+H]+; Anal. Calcd for C22H22Br2N4Pd (M.Wt
608.67 g/mol): C 43.41, H 3.64, N 9.20; found: C 43.46, H 3.97, N
9.18.
3.7. General procedure for Suzuki-Miyaura cross-coupling
reaction
Arylboronic acid (1.2 equiv.), anhydrous potassium carbonate
(6 equiv.) and 0.2 mol% of catalyst were taken into 25 mL round
bottom (RB) flask under N2 atmosphere. Distilled water (5 mL) was
added to the flask followed by bromobenzene (1 equiv.) using a
syringe. The resulted reaction mixture was refluxed under nitrogen
atmosphere for 12 h. The reaction mixture was cooled to room tem-
perature and extracted with ethyl acetate. The organic layer was
separated and evaporated to dryness which was further purified
by column chromatography. The reactions are done twice and the
average of isolated yields is reported. All biphenyl products were
confirmed by 1H and 13C NMR and compared with literature reports
[20].
3.4. Synthesis of complex 2
Complex 1 (0.100 g, 0.08 mmol) was dissolved in acetonitrile
(3 mL) to which [PdCl2(H3C-CN)2] (0.044 g, 0.17 mmol) was added.
The mixture was allowed to stir for one hour at room temperature.
The suspension was filtered using G3 sintered funnel through celite
and the filtrate was evaporated. The resulted solid was washed with
diethylether and dried to obtain a pale yellow solid of complex 2
in 85% yield (0.097 g). 1H NMR (400 MHz, DMSOd6): ı 3.83 (s, 3H,
CH3), 3.87 (s, 3H, CH3), 2.09 (s, 3H, CH3acetonitrile), 5.24–5.33 (m, 2H,
CH2), 5.57–5.61 (m, 2H, CH2), 7.19–7.25 (m, 2H, CHph), 7.31–7.38
(m, 4H, CHimid), 7.44–7.52 (m, 4H, CHph), 7.86 (s, 1H, CHph), 7.93
(s, 1H, CHph); 13C NMR (100 MHz, DMSO-d6): ı 37.17 (s, CH3),
37.36 (s, CH3), 49.38 (s, CH2), 50.29 (s, CH2), 1.17 (s, CH3acetonitrile),
118.12 (s, CNacetonitrile), 122.82 (s, CHimid), 123.41 (s, CHimid), 125.01
(s, CHimid), 125.34 (s, CHimid), 126.81 (s, CHph), 127.59 (s, CHph),
128.19 (s, CHph), 128.51 (s, CHph), 128.82 (s, CHph), 131.06 (s,
CHph), 131.91 (s, CHph), 135.23 (s, CHph), 135.76 (s, CHph), 137.63
(s, Cph), 138.56 (s, Cph), 163.81 (CNHC), 166.00 (CNHC). MS (ESI) m/z
671 [C24H25ClF6N5PPd+H]+ m/z 593 [C22H22F6N4PPd]+; Anal. Calcd
4. Conclusions
In conclusion, we have demonstrated the synthesis of
methylene bridged biphenyl bis-NHC silver(I) and palladium(II)
complexes. All complexes are isolated in good yield and charac-
terized by single crystal XRD, NMR, ESI-MS and elemental analysis
(EA). The Pd(II)-NHC complex 3 is found to be an efficient catalyst
for water mediated Suzuki-Miyaura cross-coupling reaction.

L. Seva et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 125–131
[11] H.M.J. Wang, I.J.B. Lin, Organomtallics 17 (1998) 972–975.
131
Acknowledgements
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We gratefully acknowledge the Department of Science and
Technology, New Delhi for financial support (SERB NO. SR/FT/CS-
102/2010). We thank the Central Instrumentation Facility,
Pondicherry University for NMR spectra and DST-FIST for the
single-crystal X-ray diffraction facility.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.03.
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