Benzimidazol-2-ylidene ligated palladacyclic complexes of N,N-dimethylbenzylamine - Synthesis and application to C-C coupling reactions

Article abstract of DOI:10.1016/j.ica.2016.04.048

Palladacyclic complexes derived from N,N-dimethylbenzylamine (dmba) and the benzimidazol-2-ylidene ligands: 1,3-di(cyclohexyl)benzimidazol-2-ylidene (BzImCy), 1,3-di(tert-butyl)benzimidazol-2-ylidene (BzIm^tBu) and 1,3-di(1-adamantyl)benzimidazol-2-ylidene (BzImAd) were prepared. The yield for (BzImCy)Pd(Cl)dmba (2a) was 72% but yields for the more sterically encumbering analogues: (BzImAd)Pd(Cl)dmba (2b) and (BzIm^tBu)Pd(Cl)dmba (2c) were 34% and 38%, respectively. The complexes were obtained as off-white, moisture- and air-stable crystalline solids and were characterized by ¹H NMR and ¹³C NMR spectroscopy, CHN analysis and IR spectroscopy. Complex 2c was further characterized by X-ray diffraction studies. The catalytic activity of all three complexes was explored for the Suzuki-Miyaura and Heck-Mizoroki coupling reactions of simple aryl bromides.

Full text of DOI:10.1016/j.ica.2016.04.048

Inorganica Chimica Acta 449 (2016) 38–43
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Benzimidazol-2-ylidene ligated palladacyclic complexes
of N,N-dimethylbenzylamine – Synthesis and application
to C–C coupling reactions
Kerry-Ann Green a, Paul T. Maragh a, Kamaluddin Abdur-Rashid b, Alan J. Lough c, Tara P. Dasgupta a,
⇑
a Department of Chemistry, University of the West Indies, Mona, Kingston 7, West Indies, Jamaica
b Kamal Pharmachem Inc, 2395 Speakman Drive, Suite 1001, Mississauga, Ontario L5K 1B3, Canada
c Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladacyclic complexes derived from N,N-dimethylbenzylamine (dmba) and the benzimidazol-2-ylidene
ligands: 1,3-di(cyclohexyl)benzimidazol-2-ylidene (BzImCy), 1,3-di(tert-butyl)benzimidazol-2-ylidene
(BzImtBu) and 1,3-di(1-adamantyl)benzimidazol-2-ylidene (BzImAd) were prepared. The yield for
(BzImCy)Pd(Cl)dmba (2a) was 72% but yields for the more sterically encumbering analogues: (BzImAd)
Pd(Cl)dmba (2b) and (BzImtBu)Pd(Cl)dmba (2c) were 34% and 38%, respectively. The complexes were
obtained as off-white, moisture- and air-stable crystalline solids and were characterized by 1H NMR
and 13C NMR spectroscopy, CHN analysis and IR spectroscopy. Complex 2c was further characterized
by X-ray diffraction studies. The catalytic activity of all three complexes was explored for the
Suzuki–Miyaura and Heck–Mizoroki coupling reactions of simple aryl bromides.
Received 8 March 2016
Received in revised form 22 April 2016
Accepted 25 April 2016
Available online 2 May 2016
Keywords:
Benzimidazol-2-ylidene
N,N-Dimethylbenzylamine
Palladium
Ó 2016 Elsevier B.V. All rights reserved.
Catalysis
Heck–Mizoroki
Suzuki–Miyaura
1. Introduction
[11–15]. Consequently, N,N-dimethylbenzylamine is one such sac-
rificial ligand, which enables the formation of the pre-catalyst, and
The strong
r
-donating properties of N-heterocyclic carbenes
through reductive degradation it can be dislodged in a controlled
manner during the catalyst activation process [8,11]. The chelating
structure of this ligand in palladacycles confers a degree of thermal
stability, minimizing susceptibility to decomposition to metallic
Pd, thereby making them attractive as catalysts for cross-coupling
reactions [16]. Relative to their phosphine counterparts, palladacy-
cles containing NHC ligands are less explored [11]. Despite this,
NHC-ligated palladacycles have successfully catalyzed a variety
of useful reactions such as the Heck–Mizoroki, Suzuki–Miyaura
(NHCs) and the ability to tune their electronic and steric properties
by varying the backbone and N-substituents have made them par-
ticularly useful as ligands, since they form strong metal–carbon
bonds with a variety of metals [1–3]. They are also useful as
organocatalysts [4]. Since the isolation of the first stable free
NHC by Arduengo in 1991 [5], NHCs have emerged as active, robust
and versatile ancillary ligands for several metal mediated homoge-
neous catalytic reactions including Pd-catalyzed C–C coupling
reactions [6–10]. The air-, moisture- and thermally-stable nature
of NHC complexes has led to an increase in their application to
homogeneous catalytic transformations, and arguably rivaling
phosphine ligands as versatile and attractive ligand alternatives
[1,8].
In Pd-mediated cross-coupling reactions, the active form of the
catalyst is believed to be a mono-ligated NHC-Pd(0) species, how-
ever, a variety of other co-ligands such as p-quinone, pyridine (Pyr)
derivatives and chelating ligands such as acetylacetonate (acac) are
necessary for stabilizing the pre-catalyst, and also serve to
facilitate efficient delivery of the metal into the catalytic cycle
coupling,
a-arylation of ketones and dehydrohalogenation reac-
tions [8,10,17].
In this work, we are investigating palladacyclic complexes of
dmba containing benzimidazol-2-ylidene ligands by combining
the strongly donating and sterically demanding properties of the
NHC ligands with the stability of the palladacycle framework.
Complexes containing benzimidazol-2-ylidene ligands such as
(BzIm)Pd(acac)Cl and (BzIm)Pd(Pyr)Cl2 were previously prepared
in our laboratory [18] and our ongoing interest in NHCs and their
palladium(II) complexes prompted us to explore the synthesis
and catalytic behavior of these Pd-dmba-benzimidazol-2-ylidene
complexes. Analogous air- and moisture-stable palladacyclic
complexes, with unsymmetrical benzimidazol-2-ylidenes and
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.ica.2016.04.048
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

K.-A. Green et al. / Inorganica Chimica Acta 449 (2016) 38–43
39
dimethylamino ligands in a mutually trans arrangement have been
reported by Günay et al. [19]. The flexible and robust framework of
palladacycles is believed to be responsible for their high turnovers
in coupling reactions [17]. The choice of N-alkyl substituted NHCs
relative to their aryl counterparts was based on the tendency of
these former systems to have stronger electron donor properties
[20,21].
(s, 6H, N(CH3)2); 2.27 (s, br, 6H, CH (adamantyl)); 1.76 (m, 12H,
CH2 (adamantyl)). 13C NMR (CDCl3) dC 178.4 (Pd-NCN); 151.0,
146.8, 136.1, 134.7, 125.4, 123.1, 121.9, 120.7, 116.2 (Ar-C); 72.6
(CH2 (benzyl)); 62.0 (NC(CH2)3); 51.3 (N(CH3)2); 42.2, 36.2
(CH2 (adamantyl)); 30.6 (CH (adamantyl)).
m
/cmÀ1 3045, 2980,
2905, 2858, 2840, 1578, 1453, 1375, 1332, 1292, 1104, 864, 844,
740, 663, 626. Anal. Calc. for C36H46N3ClPd.CHCl3: C, 56.83; H,
6.06; N, 5.37. Found: C, 56.87; H, 6.05; N, 5.29%.
2. Experimental
2.2.1.3. (BzImtBu)Pd(dmba)Cl (2c). Complex 2c was isolated as an
off-white crystalline solid (yield: 211.5 mg, 38%). 1H NMR (CDCl3)
dH 7.76 (m, 2H, meta-CH (BzImtBu)); 7.15 (m, 2H, ortho-CH (BzImt-
Bu)); 6.94 (d, 1H, J = 7.2 Hz, Ar-CH); 6.85 (t, 1H, J = 7.3 Hz, Ar-CH);
6.59 (t, 1H, J = 7.2 Hz, Ar-CH); 6.02 (d, 1H, J = 7.5 Hz, Ar-CH); 3.85
(s, 2H, CH2 (benzyl)); 2.80 (s, 6H, N(CH3)2); 2.22 (s, br, 18H, C
(CH3)3). 13C NMR (CDCl3) dC 179.2 (Pd-NCN); 150.6, 147.1, 135.8,
135.1, 125.5, 123.3, 122.1, 121.3, 115.4 (Ar-C); 72.4 (CH2 (benzyl));
2.1. General – Materials and instruments
Reactions and manipulations were carried out under an inert,
dry atmosphere of argon using standard air-free techniques with
solvents purified and dried according to standard procedures
[22], and deoxygenated prior to use. Selected reactions were
performed in air and without special anhydrous precautions. The
compounds 1,3-di(1-adamantyl)benzimidazolium chloride, 1,3-di
(cyclohexyl)benzimidazolium chloride and 1,3-di(tert-butyl)benz-
imidazolium chloride, were obtained from Kamal Pharmachem
Inc. and were used as received. Other chemicals were obtained
from Kamal Pharmachem Inc., Sigma–Aldrich and BDH Chemicals
and were used without further purification. 1H and 13C NMR spec-
tra were recorded on a Bruker ACE 200 or 500 MHz Fourier Trans-
form spectrometer and referenced to the residual protons in the
incompletely deuterated solvents. Chemical shifts are reported in
parts per million (d, ppm) relative to tetramethylsilane (TMS). IR
spectra were obtained on Bruker Vector 22 and Tensor 37 spec-
trometers. CHN analyses were performed on a PE 2400 CHNS/O
Analyzer at the UWI, Mona, Jamaica.
60.1 (NC(CH3)3); 51.3 (N(CH3)2); 31.5 (C(CH3)3).
m
/cmÀ1 3055,
3004, 2971, 2912, 2834, 1580, 1477, 1402, 1373, 1332, 1228,
1182, 1128, 1024, 996, 936, 854, 744. Anal. Calc. for C24H34N3ClPd:
C, 56.92; H, 6.77; N, 8.30. Found: C, 57.07; H, 6.59; N, 8.22%. Single
crystals were obtained from a solution of Hexanes:EtOAc:CH2Cl2
(5:5:1) following slow evaporation.
2.2.2. General procedure for the Heck–Mizoroki reaction of aryl
bromides and methyl acrylate
A typical reaction is given below for the Heck–Mizoroki cou-
pling reactions, employing a modified literature protocol [24,25].
(BzIm)Pd(Cl)dmba (4.5 mg, 2 mol%), Cs2CO3 (262 mg,
0.80 mmol) and a stirring bar were placed in a dry RadleyTM tube.
The tube was stoppered and the contents flushed with argon
gas.1 1,4-Dioxane (1 mL) was then added via syringe followed by
methyl acrylate (0.054 mL, 0.60 mmol) and the aryl halide2
(0.05 mL, 0.40 mmol). The resulting mixture was refluxed for 24 h.
After refluxing, the reaction mixture was filtered, and the desired
coupled product purified by column chromatography using hex-
anes:EtOAc (12:1) as eluent.
2.2. Synthesis of complexes
2.2.1. Representative synthesis of NHC-palladacycles
A modified literature procedure [11,23] was employed in the
preparation of the NHC-ligated palladacycles. For a typical reac-
tion, the benzimidazolium chloride (2 equiv.), di-(l-chlorobis(N,
N)-dimethylbenzylamine)dipalladium (1 equiv.), K2CO3 (10 equiv.)
were added to a reaction flask equipped with a stirring bar and
subsequently purged with argon gas. 1,4-Dioxane (3 mL) was then
added and the mixture was refluxed for 24 h. The mixture was then
filtered and the sample purified by flash column chromatography
using hexanes:CH2Cl2:EtOAc (5:5:1) as eluent.
2.2.3. General procedure for the Suzuki–Miyaura reaction of aryl
bromides and phenylboronic acid
A typical reaction is given below for the Suzuki–Miyaura cou-
pling reactions, employing a modified literature protocol [26,27].
(BzIm)Pd(Cl)dmba (4.5 mg, 2 mol%), Cs2CO3 (275 mg,
0.84 mmol), phenylboronic acid (73.6 mg, 0.60 mmol) and a stir-
ring bar were placed in a dry RadleyTM tube. The tube was stoppered
and the contents flushed with argon gas.1 1,4-Dioxane (1 mL) was
then added via syringe followed by the aryl halide2 (0.05 mL,
0.40 mmol). The resulting mixture was heated for 24 h. After heat-
ing, the reaction mixture was filtered, and the desired coupled pro-
duct purified by column chromatography using hexanes:EtOAc
(12:1) as eluent.
2.2.1.1. (BzImCy)Pd(dmba)Cl (2a). Complex 2a was obtained as an
off-white crystalline solid (yield: 287.5 mg, 72%). 1H NMR (CDCl3;
dH (ppm)): 7.63 (m, 2H, meta-CH (BzImCy)); 7.22 (m, 2H, ortho-
CH (BzImCy)); 7.02 (d, 1H, J = 7.1 Hz, Ar-CH); 6.92 (t, 1H,
J = 7.2 Hz, Ar-CH); 6.64 (t, 1H, J = 7.3 Hz, Ar-CH); 6.12 (d, 1H,
J = 7.3 Hz, Ar-CH); 5.68 (m, 2H, NCH); 3.95 (s, 2H, CH2 (benzyl));
2.88 (s, 6H, N(CH3)2); 2.22 (m, 5H, CH2 (cyclohexyl)); 1.89 (m,
9H, CH2 (cyclohexyl)); 1.28 (m, 6H, CH2 (cyclohexyl)); 13C NMR
(CDCl3; dC (ppm)): 184.2 (Pd-NCN); 148.5, 148.0, 136.7, 134.0,
125.1, 123.6, 122.1, 121.9, 112.6 (Ar-C); 72.2 (CH2 (benzyl)); 62.5
(NCH); 50.3 (N(CH3)2); 30.9, 30.7, 26.3, 26.2, 25.5 (CH2 (cyclo-
3. Results and discussion
3.1. Synthesis and characterization of complexes
hexyl)). m
/cmÀ1 3045, 2933, 2856, 1608, 1576, 1475, 1445, 1370,
1352, 1217, 1021, 955, 896, 850, 738. Anal. Calc. for C28H38N3ClPd:
C, 60.02; H, 6.87; N, 7.53. Found: C, 59.34; H, 6.58; N, 7.30%.
The synthesis of the desired (BzIm)Pd(Cl)dmba complexes, was
based on the principle that halide-bridged palladacyclic dimers can
be cleaved by nucleophiles such as NHCs to produce monomeric
Pd-complexes [23,28]. Several NHC-ligated palladacyclic pre-cata-
lysts of N,N-dimethylbenzylamine (dmba) have been prepared
2.2.1.2. (BzImAd)Pd(dmba)Cl (2b). Complex 2b was obtained as an
off-white crystalline solid (yield: 119.2 mg, 34%). 1H NMR (CDCl3)
dH 7.97 (m, 2H, meta-CH (BzImAd)); 7.19 (m, 2H, ortho-CH
(BzImAd)); 6.94 (d, 1H, J = 7.4 Hz, Ar-CH); 6.89 (t, 1H, J = 7.3 Hz,
Ar-CH); 6.63 (m, 1H, Ar-CH); 6.03 (d, 1H, J = 7.4 Hz, Ar-CH);
3.87 (s, 2H, CH2 (benzyl)); 3.15 (m, 12H, CH2 (adamantyl)); 2.86
1
The same procedure was followed for the reactions performed in air, except that
the purging of the reaction tube with argon was omitted.
2
Under inert conditions, for the solid aryl halide, 4-bromoacetophenone (80.0 mg,
0.40 mmol) was weighed and placed in the RadleyTM tube before purging with argon.

40
K.-A. Green et al. / Inorganica Chimica Acta 449 (2016) 38–43
using the principle of the substitution of a bridging chloride of the
dimer [Pd(dmba)( -Cl)]2 by isolated free NHCs [8,11,29]. Conse-
quently, in this study, complexes 2a, 2b and 2c were prepared
via a convenient version of a one-pot synthesis from the dimer
[Pd(dmba)(l-Cl)]2 and the corresponding benzimidazolium salts
for 2a, 2b, 2c, respectively, integrating for two protons distin-
guished the methylene protons of the N,N-dimethylbenzylamine
moiety, compared to dH 3.93 in the parent dimer. The nitrogen-
bound methyl protons occurred as a broad singlet integrating for
6 protons at dH 2.88, 2.86, 2.80 for 2a, 2b, 2c, respectively, relative
l
(ca. 1:2 equiv. respectively) with potassium carbonate as base
(Scheme 1). The complexes were obtained as air- and moisture-
stable solids, in yields of 72%, 34% and 38%, comparable to existing
one-pot synthetic procedures in the current literature [11,19,23].
Purification by column chromatography was necessary before
characterization.
to dH 2.85 for the dimer [Pd(dmba)(l-Cl)]2.
3.3. Crystal structure
In the crystal structure of 2c (Fig. 1), the 1,3-di(tert-butyl)benz-
imidazol-2-ylidene ligand lies trans to the dimethylamino moiety.
Crystal and experimental data are provided in Table 1 and selected
bond-lengths and bond angles are listed in Table 2. The N2–C1–N1
bond angle is 106.85(17)° and is characteristic of singlet benzimi-
dazol-2-ylidenes which usually measures approximately 107° [11].
The C2–C3 (bridge) bond length is 1.398(3) Å. The Pd1–C1 and
Pd1–C10 bond lengths are 1.988(2) Å and 2.000(2) Å, respectively,
and are consistent with the usual range for Pd–C bonds [19,23].
The bond lengths for Pd1–Cl1 and Pd1–N3 are 2.4173(5) Å and
2.1295(17) Å, respectively, and are comparable to the values
observed for similar bonds in the literature [8,19,23]. There is a dis-
torted square planar geometry about the palladium(II) center, a
consequence of the Pd being a part of the five-membered pallada-
cyclic ring. The angles C1–Pd1–Cl1 and C1–Pd1–C10 are 88.53(6)°
and 94.77(8)°, respectively, and the arrangement of the benzimida-
zol-2-ylidene is non-coplanar with respect to the palladacyclic ring
and a more perpendicular relationship exists between the moi-
eties. A search of the literature revealed that complexes 2a, 2b
and 2c have not been previously reported.
The low yields obtained for 2b and 2c can be attributed to the
decomposition of some of the metal precursor to palladium black.
Additionally the greater steric bulk imposed by the adamantyl and
t-butyl N-substituents on the benzimidazole framework, relative
to the cyclohexyl group may also be an influential factor. The steric
repulsion between the bulky backbone (aromatic bridge) and the
sterically demanding N-substituents result in larger values for %
VBur (descriptor characterizing ligand steric demand) [30,31] as
the greater steric demand limits access to, and therefore coordina-
tion/complexation, at the metal center. The greater steric bulk of
the adamantyl and t-butyl substituents relative to the cyclohexyl
moiety has been demonstrated by Scott et al. [32] and Huang
et al. [33] via synthetic, structural and thermochemical studies.
The stereoelectronic properties of NHCs bearing these bulky
N-substituents influence synthetic outcomes, and are directly
related to the carbene lone pair availability and overlap with the
metal orbitals [20,33].
The mechanism of the formation of these NHC-ligated pallada-
cycles is however, still unclear. Kantchev et al. [11] proposed a
mechanism via the formation of a short-lived
p-complex with
3.4. Catalysis
the precursor azolium salt, thereby increasing the acidity of the
proton (im-H2) and facilitating complexation. They discounted
the possibility of an in situ generated NHC intermediate, since
N-alkyl substituted NHCs are known to react sluggishly with the
in situ generated dimer [Pd(dmba)(l-Cl)]2. On the contrary,
Gökçe et al. [23] attributed the formation of some related (NHC)
Pd(dmba)Cl complexes, prepared from the azolium salt and NaH
in the presence of the parent dimer [Pd(dmba)(l-Cl)]2, to the gen-
eration of the free NHC in situ with subsequent cleavage of the
dimer to form the desired complex.
In a preliminary study, the catalytic activity of pre-catalysts
2a, 2b and 2c was assessed for the Heck–Mizoroki and
Suzuki–Miyaura reactions involving simple aryl bromides.
Benzimidazol-2-ylidenes are known to exhibit intermediate
behavior between imidazol-2-ylidenes and imidazolin-2-ylidines
[35]. Results from studies on the electron-donating properties of
various imidazolin-2-ylidenes and imidazol-2-ylidenes suggest
that within a particular class of NHCs, the electronic differences
are relatively small. Experimental data suggest that electron-donor
properties are unlikely to play a significant role in the observed
differences in catalytic activity for imidazol-2-ylidenes versus
imidazolin-2-ylidines [36,37] Consequently, for the benzimidazol-
2-ylidenes explored in this study, the electronic properties of the
ligands are not expected to be significantly different and so any
difference in the catalytic activity is likely attributed to the
difference in steric properties of the benzimidazol-2-ylidenes.
3.2. NMR spectroscopy
Diagnostic resonances in the 1H and 13C NMR spectra confirm
the successful preparation of the NHC-palladacycles of
N,N-dimethylbenzylamine. The carbene carbon of the benzimidazol-
2-ylidene ligands of 2a, 2b and 2c occurred at dC 184.2, 178.4
and 179.2, respectively, such that the observed trend for the car-
bene carbon resonances relative to the benzimidazol-2-ylidenes
is BzImCy > BzImtBu > BzImAd. These carbon resonances are
consistent with assignments found in the literature for similar
complexes [9,23,34]. The methylene carbon of the benzylamine
moiety resonated in the range dC 72.2–72.6. The nitrogen
bound methyl carbons occurred as one peak resonating at dC
50.3, 51.3 and 51.3, for 2a, 2b and 2c, respectively, compared with
two closely spaced peaks for similar carbons in the precursor
3.4.1. Heck–Mizoroki coupling
3.4.1.1. Formation of methyl 4-methoxycinnamate and methyl
4-acetylcinnamate. Table 3 summarizes the results for the Heck–
Mizoroki coupling of 4-methoxybromobenzene and 4-bromoace-
tophenone with methyl acrylate. The reactions performed under
inert atmosphere showed no significant difference in the activities
of the pre-catalysts for the formation of methyl 4-methoxycinna-
mate, and all pre-catalysts 2a, 2b, 2c performed poorly with yields
of only 20%, 18% and 9%, respectively, (entries 1, 3 and 4). Pre-cat-
alyst 2a performed equally poorly in air, for the formation of
methyl 4-methoxycinnamate with a yield of 20% (entry 2). Under
inert conditions, 2b and 2c showed improved performance for
the coupling of the activated aryl halide, 4-bromoacetophenone
with methyl acrylate. The bulkier pre-catalysts were more active
towards the formation of methyl 4-acetylcinnamate; with yields
of 66% and 41% for 2b and 2c respectively, (entries 6 and 8)
whereas a yield of only 11% (entry 5) was obtained for the less
[Pd(dmba)(l-Cl)]2, observed at dC 52.8 and 52.6.
Chemical shifts for the dmba protons of some imidazol-2-ylidene
and imidazolin-2-ylidene analogues of these (BzIm)Pd(Cl)dmba
systems generally occurred further upfield relative to the
parent dimer [8,11,23,34]. This shift to higher fields has been
attributed to the increased
r-donicity of the NHC ligands [23].
However, this general upfield shift was not observed for all the
bulky benzimidazol-2-ylidene ligated systems 2a, 2b, and 2c. In
the 1H NMR spectrum, a singlet signal at dH 3.95, 3.87 and 3.85

K.-A. Green et al. / Inorganica Chimica Acta 449 (2016) 38–43
41
Scheme 1. Synthesis of (BzIm)Pd(dmba)Cl complexes.
Table 1
X-ray experimental data for 2c.
Empirical formula
Formula weight
T (K)
C24H34ClN3Pd
506.39
147(2)
k (Å)
0.71073
Crystal system
Space group
a (Å)
orthorhombic
Pbca
12.5455(4)
b (Å)
15.8780(5)
c (Å)
23.7632(8)
a
(°)
90
b (°)
90
c
(°)
90
4733.6(3)
V (Å3)
Z
8
qcalc (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
1.421
0.912
2096
)
Crystal size (mm)
h (°)
0.43 Â 0.25 Â 0.21
2.24–27.51
Index ranges
À16 6 h 6 16, À16 6 k 6 20,
À30 6 l 6 25
24689
Reflections collected
Independent reflections [Rint
Completeness to h = 27.51°
Absorption correction
Maximum, minimum transmission
Refinement method
]
5430 [0.0321]
99.8%
Semi-empirical from equivalents
0.7456, 0.6823
Full-matrix least-squares on F2
5430/0/270
Fig. 1. An ORTEP representation of chloro[1,3-di(tert-butyl)benzimidazol-2-ylidene]
(N,N-dimethylbenzylamine)palladium(II), 2c. Displacement ellipsoids are repre-
sented at the 30% probability level and hydrogen atoms are omitted for clarity.
Data/restraints/parameters
bulky cyclohexyl derivative. However, pre-catalyst 2b displayed
reduced activity in air, with a moderate 47% yield of methyl
4-acetylcinnamate (entry 7). It is apparent that the greater steric
bulk of 2b and 2c is playing a role in possibly enhancing the
reductive elimination step in the catalytic cycle [20,38] thereby
increasing catalytic efficiency.
Goodness-of-fit (GOF) on F2
1.025
Final R indices [I > 2
r
(I)] R1, wR2
0.0255, 0.0564
0.0431, 0.0645
0.396 and À0.663
R indices (all data) R1, wR2
Largest difference peak and hole
(e ÅÀ3
)
Table 2
3.4.2. Suzuki–Miyaura coupling
Selected bond lengths and bond angles for 2c.
3.4.2.1. Formation of 4-methoxybiphenyl and 4-acetylbiphenyl. Suzuki–
Miyaura coupling reactions generally require milder conditions
relative to the Heck–Mizoroki couplings [39]. Initially, the
reactions were conducted at 50 °C for this study. However, a
higher temperature was required to improve yields for some
pre-catalysts, and so, some reactions were subsequently conducted
at 101 °C, contingent on the performance at lower temperature
(Table 4). For each Suzuki–Miyaura coupling reaction, the yields
of the desired biphenyl derivatives ranged from poor to excellent
for all three pre-catalysts (Table 4).
The bulkier pre-catalysts 2b and 2c performed better than the
less bulky cyclohexyl system, 2a under inert atmosphere for the
formation of 4-methoxybiphenyl at 50 °C, with 2b giving a very
good yield of 83% and 2c a 36% yield (entries 3 and 4). Upon
increasing the temperature to 101 °C, the yield for 2a improved
significantly to 94% (entry 1); but this had little or no effect on
Bond lengths (Å)
Pd(1)–C(1)
Pd(1)–C(10)
Pd(1)–N(3)
Pd(1)–Cl(1)
C(2)–C(3)
1.988(2)
2.000(2)
2.1295(17)
2.4173(5)
1.398(3)
Bond angles (°)
C(1)–Pd(1)–Cl(1)
C(1)–Pd(1)–C(10)
N(2)–C(1)–N(1)
C(1)–Pd(1)–N(3)
88.53(6)
94.77(8)
106.85(17)
176.95(7)
the performance of 2b (85%, entry 3). The increase in yield for 2c
(54%, entry 4) was reasonable, though not remarkably so. At
50 °C, pre-catalysts 2a and 2b outperformed 2c for the formation

42
K.-A. Green et al. / Inorganica Chimica Acta 449 (2016) 38–43
Table 3
under inert conditions. The selected C–C coupling reactions done
in air with pre-catalysts 2a and 2b, without stringent anhydrous
techniques, were effectively mediated and with comparable activ-
ities to those conducted under inert atmosphere, in most instances.
The steric nature of the NHC ligands appears to be a significant fac-
tor, influencing the yields of the complexes as well as their cat-
alytic activities in the coupling reactions.
Evaluation of NHC-Pd complexes for the Heck–Mizoroki coupling of 4-methoxybro-
mobenzene and 4-bromoacetophenone with methyl acrylate.
Br
H2C
NHC-Pd catalyst (2 mol%)
R
O
Cs2CO3, 1,4-dioxane, 101oC, 24 h
+
O
CH3
O
CH3
R
O
R = OCH3, COCH3
Acknowledgements
Entry
Pre-catalyst
R
Yielda (%)
The authors thank the Department of Chemistry at The Univer-
sity of the West Indies, Mona, Jamaica for a research grant and
technical assistance. We acknowledge and thank Kamal Pharma-
chem Inc. for providing some reagents.
1
2
3
4
5
6
7
8
(BzImCy)Pd(dmba)Cl
(BzImCy)Pd(dmba)Cl
(BzImAd)Pd(dmba)Cl
(BzImtBu)Pd(dmba)Cl
(BzImCy)Pd(dmba)Cl
(BzImAd)Pd(dmba)Cl
(BzImAd)Pd(dmba)Cl
(BzImtBu)Pd(dmba)Cl
2a
2a
2b
2c
2a
2b
2b
2c
OCH3
20
20b,*
18
9
11
COCH3
66
47b,*
41
Appendix A. Supplementary material
a
CCDC 1456395 contains the supplementary crystallographic
data for 2c. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/10.
1016/j.ica.2016.04.048.
Yields are based on the aryl-halide.
Reaction performed in air.
Pd black was not observed.
b
*
Table 4
Evaluation of NHC-Pd complexes for the Suzuki–Miyaura coupling of 4-methoxybro-
mobenzene and 4-bromoacetophenone with phenylboronic acid.
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