Bis-diimidazolylidine complexes of nickel: Investigations into nickel catalyzed coupling reactions

Article abstract of DOI:10.1039/c1dt10858a

Air and moisture stable homoleptic bis(diimidazolylidine)nickel(ii) complexes, ([(diNHC)₂Ni]²⁺) 3a,b and their corresponding silver(i) 4a,b and palladium(ii) 5a,b complexes were synthesized and characterized by NMR and single crystal X-ray analysis. The catalytic potential of complex 3a was assessed in Mizoroki-Heck and Suzuki-Miyaura coupling reactions. In the Suzuki-Miyaura coupling reaction, nickel precatalyst 3a was active for the coupling of aryl chlorides as well as aryl fluorides. The analogously synthesized Pd(ii) complexes resulted in formation of (diNHC)PdCl₂ species which were not active for the coupling of aryl fluorides. For the Mizoroki-Heck reaction, it was found that aryl iodides could be activated in the absence of nickel or palladium precatalysts when using Na₂CO₃ or NEt₃ as base while aryl iodides and aryl bromides could be activated in the Suzuki-Miyaura reaction sans precatalyst when K₃PO₄ was used as base. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt10858a

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PAPER
Bis-diimidazolylidine complexes of nickel: Investigations into nickel catalyzed
coupling reactions†
Tressia A. P. Paulose,a Shih-Chang Wu,a Jeremy A. Olson,a Tony Chau,a Nikki Theaker,a Matt Hassler,a
J. Wilson Quailb and Stephen R. Foley*a
Received 7th May 2011, Accepted 5th September 2011
DOI: 10.1039/c1dt10858a
Air and moisture stable homoleptic bis(diimidazolylidine)nickel(II) complexes, ([(diNHC)2Ni]2+) 3a,b
and their corresponding silver(I) 4a,b and palladium(II) 5a,b complexes were synthesized and
characterized by NMR and single crystal X-ray analysis. The catalytic potential of complex 3a was
assessed in Mizoroki–Heck and Suzuki–Miyaura coupling reactions. In the Suzuki–Miyaura coupling
reaction, nickel precatalyst 3a was active for the coupling of aryl chlorides as well as aryl fluorides. The
analogously synthesized Pd(II) complexes resulted in formation of (diNHC)PdCl2 species which were
not active for the coupling of aryl fluorides. For the Mizoroki–Heck reaction, it was found that aryl
iodides could be activated in the absence of nickel or palladium precatalysts when using Na2CO3 or
NEt3 as base while aryl iodides and aryl bromides could be activated in the Suzuki–Miyaura reaction
sans precatalyst when K3PO4 was used as base.
Heck reaction and was found to exhibit high turnover numbers in
the coupling of aryl bromides and showed potential for catalyst
Introduction
recyclability.7 The analogous [(diNHC)2Ni]2+ complexes have, to
date, not been investigated for Heck or Suzuki coupling reactions.
However, a pincer-type [(2,6-(NHC)2-pyridine)NiBr]+ complex
has been reported which demonstrated catalytic activity in Suzuki–
Miyaura cross-coupling reactions.3c These results motivated us to
synthesize cis-chelating bidentate NHCcomplexes ofnickel(II) and
to evaluate their catalytic activity with regards to cross-coupling
reactions.
Herein, we report the synthesis of two [(diNHC)2Ni]2+ com-
plexes, their Ag(I) and Pd(II) analogues, and investigations into
their activity in standard Mizoroki–Heck and Suzuki–Miyaura
coupling reactions. In the course of these investigations, it was
found that select bases could catalyze the coupling reactions of
aryl iodides and aryl bromides in the absence of any Ni(II) or
Pd(II) precatalyst. “Transition-metal free” Suzuki-type coupling
reactions employing Na2CO3 in the presence of Bu4NBr have been
previously described.8 However, the authors later reported that
sub-ppm levels of Pd impurities in commercially available Na2CO3
were likely responsible for catalyzing these reactions.9 These results
highlight the importance of performing control experiments while
investigating the catalytic C–C coupling activity of any potential
nickel precatalyst.
Transition-metal catalyzed carbon–carbon and carbon–hete-
roatom bond forming reactions play a fundamental role in organic
transformations. Over the last four decades a large number of
transition-metal catalysts have been developed for cross-coupling
reactions with palladium-based catalysts emerging as one of the
most efficient and versatile systems.1 One obvious drawback
is the prohibitive cost of palladium which has resulted in the
exploration of less costly transition metals as alternatives to
palladium systems for cross-coupling reactions.2 Nickel provides
an attractive alternative due to its relative low cost and ability
to more readily undergo oxidative addition reactions with C–
Cl and C–F bonds, however relatively little effort has been
dedicated to the development of nickel complexes.1b,3 For example,
a simple SciFinder search for “Heck” and “palladium” yielded
3073 independent references while the corresponding nickel search
resulted in only 87 references.4 One prominent disadvantage to
using nickel is that nickel complexes are known carcinogens.5
Palladium(II) complexes with cis-chelating bidentate N-
heterocyclic carbene (diNHC) ligands are air and moisture stable
compounds that have demonstrated high activity as precatalysts
for cross-coupling reactions.6 A [(diNHC)2Pd]2+ complex was
recently investigated for its catalytic activity in the Mizoroki–
aDepartment of Chemistry, University of Saskatchewan, Canada. E-mail:
stephen.foley@usask.ca
Results and discussion
bSaskatchewan Structural Sciences Centre, 110 Science Place, S7N 5C9,
Saskatoon, Saskatchewan, Canada
Synthesis of diNHC ligands
† Electronic supplementary information (ESI) available. CCDC reference
numbers 697184–697187. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10858a
The diimidazolium salts 1a,b were synthesized in two steps
according to the literature procedure for 1a previously reported
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by Lee et al. (Scheme 1).6b In this procedure, the N-benzyl
imidazoles were first prepared by reacting the substituted benzyl
bromides with imidazole in the presence of 4 equiv of NaH for
12 h. Heating a solution of the resulting N-benzyl imidazole
in neat dibromomethane for 2 days resulted in formation of
the corresponding methylene-bridged diimidazolium salts 1a,b in
high yield. A potentially frustrating problem with diimidazolium
salts and of their resulting diNHC complexes is poor solubility
t
in common organic solvents, thus the Bu groups in 1b were
introduced in the para position of the benzyl moiety to increase
solubility. While diimidazolium salt 1b and the resulting nickel and
palladium complexes remained insoluble in chlorinated solvents,
solubility greatly increased in polar solvents such as DMSO and
1
DMF. The H NMR spectra of 1a,b in DMSO-d6 both exhibit
characteristic resonances at 9.7 for the protons on the C2 (NCHN)
position of the imidazolium rings.
Scheme 1 Synthesis of diimidazolium salts 1a,b.
Deprotonation of 1a,b with 2 equiv of K[N(SiMe3)2] in THF
generated the neutral carbenes 2a,b as evidenced by absence of the
carbenic C2 proton in the H NMR spectra (Scheme 2). The C4
Scheme 2 Synthesis of diNHC complexes of Ag(I), Ni(II), and Pd(II).
1
of the neutral diNHCs 2a,b with one equiv of NiBr2(DME)
afforded the homoleptic [(diNHC)2Ni]2+complexes 3a,b with two
chelating diNHC ligands and two non-coordinating bromide
counterions as the only isolated products (Scheme 2). 3a,b
were the only observed products irrespective of stoichiometry,
thus the yields were optimized when two equiv of ligand were
employed. [(DiNHC)2Ni]2+ complexes 3a,b are air and moisture
stable and can be crystallized by slow diffusion of ether into a
and C5 protons on the imidazolium rings, upon deprotonation,
also shift upfield from the aromatic to the olefinic region of the
spectra consistent with a localized C C bond. The 13C NMR
spectra of 2a,b show a downfield shift of the bridging methylene
carbon and of the methylene carbon of the benzyl groups. As well,
the 13C NMR spectra show the appearance of a peak at 216 and
214 ppm which is characteristic of the carbenic carbons, NCN, for
2a and 2b respectively.
1
concentrated solution of the sample in DMF (Table 1). The H
NMR spectra of 3a,b show non-equivalent NCH2N resonances
for the two methylene bridge protons consistent with retention
of configuration of the boat-shaped six-membered chelate rings
at ambient temperatures. The solid state structures of complexes
3a,b were determined by single-crystal X-ray diffraction and show
a distorted square planar geometry about the metal centers (Fig.
1 and 2). The two C–Ni–C planes defined by the two chelating
diNHC ligands are exactly coplanar with an average ◦Ni–C bond
Synthesis of diNHC complexes of Ag(I), Ni(II) and Pd(II)
Previous attempts to make methylene-bridged (diNHC)NiX2
(X = halogen) complexes from the reaction of Ni(OAc)2 with
diimidazolium salts always yielded a dicationic Ni(II) species
with the general formula [(diNHC)2Ni]2+.10 Formation of the
neutral cis-dihalide complexes was never observed. This is in
sharp contrast to the analogous palladium reaction which readily
yields the (diNHC)PdX2 species as the preferred product.10,11
Successful attempts have been made to synthesize monocationic
[(diNHC)NiCl(PMe3)]Cl complexes12 while an ethylene-bridged
mononuclear dialkylnickel complex, (diNHC)Ni(CH3)2, has also
been synthesized13 Recently however, a series of propyl-bridged
(diNHC)NiX2 complexes were synthesized from the group of
Bouwman et al. These complexes demonstrated good activity in
the Kumada coupling reaction.14
˚
length of 1.90(1) A and a C–Ni–C bite angle of 86.1(1) for 3a and
86.3(1)◦ for 3b for the six-membered chelate rings.
An alternative method for the synthesis of complexes 3a,b
is via a transmetallation route using (diNHC)Ag(I) complexes
(Scheme 2). The advantage of using (diNHC)Ag(I) complexes as
carbene transfer reagents include simple preparation directly from
a diimidazolium salt and ease of handling due to air stability.15 The
new silver complexes 4a,b were synthesized by the reaction of 1a,b
with Ag2O in CH2Cl2 at room temperature based on literature
procedures.15k These silver complexes are sparingly soluble in
highly polar solvents such as DMF and DMSO and display
Rather than use the acetate elimination route, we investigated
if direct reaction of a neutral diNHC ligand with one equiv of a
NiX2 source could yield a cis-dihalide complex. However, reaction
252 | Dalton Trans., 2012, 41, 251–260
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Table 1 Crystal data and refinement parameters for compounds 3a,3b·4DMF, 4a and 5a·0.5DMF
3a
3b·4DMF
4a
5a·0.5DMF
Formula
M
C42H40Br2N8Ni
875.35
C70H100Br2N12NiO4
1392.15
C42H40Ag4Br4N8
1407.94
C45H47Cl4N9OPd2
1084.62
Monoclinic
C2/c
30.461(2)
7.8144(8)
22.228(2)
Crystal system
Triclinic
Triclinic
Triclinic
¯
¯
¯
Space group
P1
P1
P1
˚
a/A
9.8058(3)
11.8395(3)
13.6055(4)
13.8460(3)
80.971(2)
9.8623(4)
˚
b/A
10.6250(4)
11.2120(5)
63.075(2)
67.743(2)
71.563(2)
948.97(7)
1
1.532
173(2)
9.9754(3)
11.6153(4)
92.339(2)
˚
c/A
a (◦)
b (◦)
g (◦)
90
69.9533(2)
73.8540(2)
2007.88(9)
1
1.151
173(2)
96.566(2)
120.533(4)
90
4557.4(7)
4
1.581
173(2)
0.71073
2192
109.941(2)
1063.25(6)
1
2.199
173(2)
0.71073
676
3
˚
V/A
Z
rc/Mg m-3
T/K
˚
radiation, l/A
F(000)
0.71073
446
0.71073
734
q range, deg
reflns collected/unique
Rint
2.18 to 29.64
21018/5312
0.0605
R1 = 0.0423, wR2 = 0.0954
R1 = 0.0674, wR2 = 0.1059
3.12 to 27.53
29325/9199
0.0599
R1 = 0.0495, wR2 = 0.1241
R1 = 0.0711, wR2 = 0.1347
3.01 to 27.52
15555/4868
0.0544
R1 = 0.0666, wR2 = 0.1752
R1 = 0.0834, wR2 = 0.1880
3.06 to 25.02
25932/4021
0.1257
R1 = 0.0464, wR2 = 0.0818
R1 = 0.0787, wR2 = 0.0936
Final R1(I >2sI)
R1 (all data)
Fig. 1 ORTEP plot of 3a at the 50% probability level. Hydrogen
atoms and the two non-coordinating bromide ions have been omitted
◦
˚
for clarity. Selected bond lengths (A) and angles ( ): Ni1–C7 = 1.896(2),
Ni1–C1 = 1.906(2), C7–Ni1–C7i = 180.00(11), C7–Ni1–C1i = 93.90(10),
C7–Ni1–C1 = 86.10(10).
Fig. 2 ORTEP plot of 3b at the 50% probability level. Hydrogen atoms,
remarkable stability towards heat, light, air and moisture. The 1H
NMR spectra of 4a,b show the absence of any signal between 8–
10 ppm indicating the successful deprotonation of the carbenic
protons in 1a,b. The remaining imidazolylidine protons upon
coordination with Ag(I) are shifted upfield while the bridging
methylene protons are broadened relative to their corresponding
signals in 1a,b. The 13C NMR spectra of 4a,b show the appearance
of a singlet at 181 ppm which is characteristic of a carbenic carbon
for 4a,b with no evident coupling between the carbenic carbon and
the silver center. (NHC)Ag(I) complexes containing halide ligands
usually exhibit no coupling between the carbenic carbon and the
silver atom indicating a weak Ag–NHC bond and fast exchange
between the silver centers and the carbene moieties. Therefore,
these complexes are expected to act as good carbene transfer
agents for transmetallation reactions.16 The poor solubility of the
non-coordinating bromide ions and solvent molecules have been omitted
◦
˚
for clarity. Selected bond lengths (A) and angles ( ): Ni1–C10 = 1.894(3),
Ni1–C14 = 1.899(3), C10–Ni1–C10i = 180.00(11), C14–Ni1–C10i
93.75(11), C10–Ni1–C14 = 86.25(11).
=
silver complexes 4a,b was a hurdle in conducting low temperature
NMR experiments to investigate if there was indeed any coupling
observed at lower temperatures. To better understand the coordi-
nation environments in 4a,b the crystal structure of complex 4a
was determined (Fig. 3), which showed that these complexes were
tetranuclear species of the general formula Ag4(m-Br4)(m-diNHC)2
which adopt a tetragonal bipyramidal geometry with Ci symmetry
with four Ag atoms in the equatorial positions and two Br atoms in
the axial positions. The Ag–Ag edges are alternatively bridged by
˚
NHC and Br ligands with weak Ag ◊ ◊ ◊ Ag interactions (2.9–3.1 A).
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Fig. 4 ORTEP plot of 5a·0.5(DMF) at the 50% probability level.
Hydrogen atoms and solvent molecule have been omitted for clarity.
◦
˚
Selected bond lengths (A) and angles ( ): Pd(1)–C(14) = 1.967(5),
Pd(1)–C(10) 1.966(5), Pd(1)–Cl(1) 2.3721(13), Pd(1)–Cl(2)
2.3726(13), C(14)–Pd(1)–C(10) = 84.0(2), Cl(1)–Pd(1)–Cl(2) = 90.25(5).
=
=
=
Fig. 3 ORTEP plot of 4a at the 30% probability level. Hydrogen atoms
◦
˚
have been omitted for clarity. Selected bond lengths (A) and angles ( ):
Table 2 Catalytic activity of complex 3a in the Mizoroki–Heck coupling
reactiona
Ag(1)–C(10) = 2.081(8), Ag(2)–C(14) = 2.152(8), Ag(1)–Br(1) = 2.7237(13),
Ag(1)–Br(2) = 2.8917(12), Ag(1)–Ag(2) = 3.1239(13), Ag(1)–Ag(2)i
2.9433(12), Ag(1)i–Ag(2)–Ag(1) = 81.91(3).
=
This structural motif is unique among structurally characterized
NHC complexes of silver.15a,d,e,j,k,16a,17
The silver complexes 4a,b readily undergo transmetallation
with NiBr2(DME) and PdCl2 to give complexes 3a,b and 5a,b
respectively. While (diNHC)PdCl2 5a is a new complex, the bromo
and iodo analogues have been previously reported.6b,d 5a,b were
synthesized to compare the catalytic activity of the [(diNHC)2Ni]2+
complexes to that of the analogously synthesized palladium
complexes. The solid state structure of complex 5a was determined
by single-crystal X-ray diffraction, which showed the palladium
center in the expected square planar geometry (Fig. 4). 5a co-
crystallized with one half of a molecule of DMF.
Entry
Cmpd 3a (mol %)
Bu4NI
Conversion (%)
1
5
1
5
5
3 eq
3 eq
0
50
0
0
2
3
4*
3 eq
42
a Reaction conditions: aryl halide (0.5 mmol), butyl acrylate (2.5 mmol),
Na2CO3 (1.5 mmol), precatalyst (0.025 mmol), Bu4NI (1.5 mmol), DMF
(3 mL). *Reaction performed under air instead of N2.
Table 3 Mizoroki–Heck reaction of aryl bromides catalyzed by complex
3aa
Catalytic activity of [(diNHC)2Ni]2+ for coupling of aryl bromides
As a first step to evaluate the catalytic activity of [(diNHC)2Ni]Br2
complex 3a, we carried out the Mizoroki–Heck coupling reaction
of bromobenzene with butyl acrylate. The reaction conditions were
optimized using this reaction and it was found that satisfactory
yields were obtained with 5 mol % of 3a in the presence of 3 equiv
of Na2CO3 and 3 equiv of Bu4NI in DMF as solvent. Long reaction
times (2 days) were necessary to maximize the yield. Of the various
bases explored: Na2CO3, NEt3, K2CO3, KOAc, K3PO4, NaOAc,
CsOAc and Cs2CO3, the catalyst was active only in the presence of
Na2CO3 in 50% yield. All other bases did not yield any product.
The catalyst was found to be inactive in the absence of Bu4NI. The
reaction can be carried out aerobically with only a small loss in
activity (Table 2). In all cases, only the E-isomer was observed for
the products of the Mizoroki–Heck coupling reactions.
Entry
R
Conversion (%)
1
2
3
4
H
50
56
44
4
COMe
CHO
MeO
a Reaction conditions: aryl halide (0.5 mmol), butyl acrylate (2.5 mmol),
Na2CO3 (1.5 mmol), precatalyst (0.025 mmol), Bu4NI (1.5 mmol), DMF
(3 mL).
With conditions optimized, we next carried out the nickel-
catalyzed Mizoroki–Heck coupling reaction of a variety of aryl
bromides. In the presence of 3a, activated aryl bromides react
with butyl acrylate to produce the corresponding cinnamates in
reasonable yield (Table 3). However, the catalyst was essentially
inactive towards deactivated aryl bromides and aryl chlorides.
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Table 4 Control experiments for Mizoroki–Heck coupling of aryl halides
Next we investigated if there was any kind of ligand transfer
from precatalyst 3a, to the putative palladium impurities in the
Na2CO3. This was to determine if the catalytic activity was possibly
a result of NHC transfer from the nickel precatalyst to the pal-
ladium impurities. To evaluate this we investigated the Mizoroki–
Heck coupling reaction of bromobenzene with butyl acrylate
in the presence of (diNHC)Ag(I) complex 4a. As mentioned
above, (NHC)Ag(I) complexes have frequently been used as NHC
transfer reagents in organometallic chemistry.15,16 If the catalytic
activity was due to carbene transfer from 3a to the putative
palladium impurities in Na2CO3, then the coupling reaction in the
presence of the corresponding (diNHC)Ag(I) complex 4a should
result in similar yields. However no activity was observed, which
suggested that precatalyst 3a was involved in the activation of aryl
bromides in the Mizoroki–Heck coupling reaction.
The catalytic activity of complex 3a in Suzuki–Miyaura cou-
pling reactions was also investigated. Precatalyst 3a was active
for the coupling of activated aryl bromides, aryl chlorides and
even aryl fluorides with phenylboronic acid in the presence of
K3PO4 as base and dioxane as solvent with moderate to high
yields (Table 5). We also observed a reverse trend in the activities
of activated aryl halides compared to non-activated aryl halides.
The conversions for acetyl aryl halides were lower than their
corresponding aryl halides, for reasons not known to us right now.
The analogously synthesized palladium precatalyst, 5a, was also
investigated in the coupling of aryl fluorides with phenylboronic
acid under conditions otherwise identical for entries 5 and 6 in
Table 5. No activity was observed for C–F activation with 5a
under these conditions.
in the absence of transition metal speciesa
Entry
X
Base
Precat.3a
Bu4NI
Conv. (%)
1
2
3
4
5
6
I
I
Br
I
I
Na2CO3
Na2CO3
Na2CO3
NEt3
NEt3
NEt3
5 mol (%)
0
0
5 mol (%)
0
5 mol (%)
3 eq
0
3 eq
3 eq
0
100
100
0
100
100
0
Br
3 eq
a Reaction conditions: aryl halide (0.5 mmol), butyl acrylate (2.5 mmol),
Na2CO3 (1.5 mmol), precatalyst (0.025 mmol), Bu4NI (1.5 mmol), DMF
(3 mL).
“Transition metal free” coupling reactions: coupling of aryl halides
and the importance of performing control reactions
Quantitative yields were obtained in the Mizoroki–Heck coupling
reaction of iodobenzene with butyl acrylate only when Na2CO3 or
NEt3 were used as bases (Table 4). To investigate if a nickel species
was indeed the active catalyst, several control experiments were
performed with iodobenzene and butyl acrylate. Surprisingly, it
was found that quantitative yields could be obtained with Na2CO3
as well as NEt3 in the absence of nickel species and/or Bu4NI
(Table 4). However, Na2CO3 was not active for Mizoroki–Heck
coupling reactions with aryl bromides. Thus all investigations were
carried out with aryl bromides as aryl iodides can be easily coupled
in the absence of a nickel precatalyst. This finding emphasizes
the importance of performing control reactions when assessing
the catalytic activity of a potential nickel precatalyst. Unfortu-
nately, these tests are often neglected in the literature, especially
given that many of the nickel precatalysts active for Mizoroki–
Heck and Suzuki–Miyaura reactions report the coupling of aryl
iodides.3a,3c–e,18 The commercially obtained Na2CO3 and NEt3 that
were used in our experiments were analyzed for palladium and
nickel content by ICP and the results indicated that Ni and Pd
levels were <5 ppm (the detection limit of the ICP instrument
used). It is likely that sub-ppm levels of Pd is responsible for
the observed coupling reactions in the “transition-metal free”
control reactions as palladium-catalyzed coupling reactions of
aryl iodides and aryl bromides have reported activities greater
than 106.19 While the active species in Na2CO3 and NEt3 remain
unknown, a similar observation of “transition-metal free” Suzuki-
type coupling reactions employing Na2CO3 in the presence of
Bu4NBr has been previously reported.8 However, the authors later
reported that sub-ppm levels of Pd impurities in commercially
available Na2CO3 were likely responsible for catalyzing these
reactions.9
To investigate if a nickel species was the active catalyst in the
Suzuki–Miyaura coupling reactions, several control experiments
were again performed. For the Suzuki–Miyaura reaction, it was
found that aryl iodides and aryl bromides could be activated in the
absence of nickel species when K3PO4 was used as base (Table 6).
No activity for aryl chlorides was observed.
Conclusions
In summary, we have synthesized stable homoleptic [(diN-
HC)2Ni]2+ complexes which are active precatalysts in Mizoroki–
Heck and Suzuki–Miyaura coupling reactions, however long
reaction times and relatively high catalyst loadings are required. By
comparison, state of the art palladium catalyzed Suzuki–Miyaura
reactions require 0.1% catalyst loading for 1 h at ambient tem-
perature for deactivated aryl chlorides.20 In the Suzuki–Miyaura
coupling reaction, nickel precatalyst 3a was active for the coupling
of aryl chlorides as well as aryl fluorides. Analogously synthesized
palladium complexes result in formation of (diNHC)PdCl2 species
which were not active for the coupling of aryl fluorides. More
importantly, these results also emphasize the importance of
performing control reactions when assessing the catalytic activity
of a potential nickel precatalyst as the base employed, or more
likely transition metal impurities in the base, can also act as a
coupling agent in the absence of nickel precatalysts. For example,
the coupling of aryl iodides in the Mizoroki–Heck reaction was
observed using Na2CO3 as base, while the coupling of aryl iodides
and aryl bromides in the Suzuki–Miyaura reaction was possible
employing K3PO4 as base in the absence of any nickel or palladium
precatalyst.
The catalytic activity of the metal precursor for complex
3a, NiBr2(DME), was also evaluated under otherwise identical
conditions to that used for 3a and it was found to be inactive
for the Mizoroki–Heck coupling reaction of bromobenzene with
butyl acrylate.
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Table 5 Suzuki–Miyaura cross-coupling of aryl halides with phenyl-
Table 6 Control experiments for Suzuki–Miyaura coupling of aryl
boronic acida
halides in the absence of transition metal speciesa
Entry
Aryl Halide
Conversion (%)
100
Entry
Aryl Halide
Conversion (%)
87
1
1
2
42
2
49
3
4
37
31
3
4
37
0
5
6
30b
14b
5
0
a Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid
(1.5 mmol), base (2 mmol), precatalyst (5 mol%), solvent (3 mL), 120
◦C, 3 d.
resonances of CDCl3 (1H: d 7.24; 13C: d 77.23), C6D6 (1H: d
7.16; 13C: d 128.4) and DMSO-d6 (1H: d 2.50; 13C: d 39.51).
Coupling constants are given in Hz. Elemental analyses were
performed on a Perkin-Elmer 2400 CHN elemental analyzer. ICP
analyses were done by SRC Analytical, Saskatchewan, Canada.
Dibromomethane and Ag2O were purchased from Alfa-Aesar
Chemical Company and used as received. Imidazole, benzyl
bromide, tert-butylbenzyl bromide, NiBr2.DME, K[N(SiMe3)2],
anhydrous DMF, and NaH were purchased from Sigma–Aldrich
Chemical Company and used as received. PdCl2 was obtained from
PMO Pty Ltd, Australia. N-benzylimidazole and 1,1¢-dibenzyl-
3,3¢-methylenediimidazolium dibromide (1a) was synthesized ac-
cording to literature procedures.6b
7
8
4
3
a Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid
(1.5 mmol), base (2 mmol), precatalyst (5 mol%), solvent (3 ml), 120 ◦C, 3
d. b Determined by 19F NMR based on residual aryl fluoride.
Experimental
General Information
1-(4-tert-butylbenzyl)imidazole
Unless otherwise stated, all reactions were performed under N2
using standard Schlenk techniques or in a N2-filled drybox.
THF and dichloromethane were dried using a MBraun Solvent
Purification System and were stored in a glovebox. C6D6 was
dried over Na/K and was degassed using freeze-pump-thaw
technique and stored in a glovebox. Dioxane was dried over
sodium and distilled prior to use. All temperatures for catalytic
reactions refer to the temperature of pre-equilibrated sand baths.
A Schlenk flask was charged with imidazole (1.85 g, 0.0272
mol) and NaH (2.57 g, 0.107 mol) in THF (50 mL) at ambient
temperature. After 5 min, 4-(tert-butyl)benzyl bromide (5.0 mL,
0.0272 mol) was added. The mixture was stirred for 12 h following
which the solvent was removed under vacuum. Water (20 mL) and
dichloromethane (30 mL) were added to the reaction products
and the organic layer was separated. The organic layer was further
extracted with water (2 ¥ 20 mL) and the solvent was removed
under vacuum to yield the product as a pale yellow solid which
can be used without further purification. (4.93 g, 85%). 1H NMR
(CDCl3): d 7.52 (s, 1H, NCHN), 7.35 (d, J = 8.1, 2H, CHAr), 7.07
1
1H and 13C { H} NMR spectra were recorded on a Bruker 500
MHz Avance spectrometer. Chemical shifts for 1H and 13C NMR
are reported in ppm in reference to the residual 1H and 13 C
256 | Dalton Trans., 2012, 41, 251–260
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(d, J = 8.1, 2H, CHAr), 7.06 (s, 1H, CHimid), 6.89 (s, 1H, CHimid), 5.06
(s, 2H, ArCH2N), 1.29 (s, 9H, CH3). 13C NMR (CDCl3): d 151.59
(CAr(para)), 137.66 (NCHN), 133.44 (CAr(ipso)), 130.00 (CHimid), 127.26
(CHAr), 126.11 (CHAr), 119.50 (CHimid), 50.71 (ArCH2N), 34.82
(C(CH3)3), 31.51 (C(CH3)3). EI-MS (m/z): calcd for C14H18N2
[M]+: 214, found: 214.
6.9, 4H, CHAr), 6.74 (s, 2H, NCH2N), 6.26 (s, 2H, CHimid), 6.03 (s,
2H, CHimid), 5.06 (s, 4H, ArCH2N), 1.16 (s, 18H, CH3). 13C NMR
(C6D6): d 214.40 (Cimid), 150.87 (CAr(para)), 135.97 (CAr(ipso)), 128.69
(CHAr), 126.12 (CHAr), 120.22 (CHimid), 119.50 (CHimid), 65.51
(NCH2N), 55.11 (ArCH2N), 34.80 (C(CH3)3), 31.74 (C(CH3)3).
Bis(1,1¢-methylene-3,3¢-dibenzylimidazole-2,2¢diylidene)nickel(II)
dibromide (3a)
1,1¢-dibenzyl-3,3¢-methylenediimidazolium dibromide (1a)
A Schlenk flask was charged with a mixture of 2a (0.500 g, 1.51
mmol), NiBr2(DME) (0.234 g, 0.757 mmol) and THF (15 mL).
After stirring at ambient temperature for 24 h, a pale yellow
precipitate was obtained. The precipitate was filtered and washed
with THF (3 ¥ 10 mL) and dried under vacuum to yield a
pale yellow powder. Crystals were obtained by slow diffusion of
A Schlenk flask was charged with N-benzylimidazole (1.0 g, 6.32
mmol) and dibromomethane (20 mL) and stirred at 80 ◦C for 2 d.
A white precipitate resulted which was filtered, washed with THF
(3 ¥ 15 mL) and dried under vacuum to yield 1a as a white powder
(1.40 g, 80%). 1H NMR (DMSO-d6): d 9.70 (s, 2H, NCHN), 8.12
(s, 2H, CHimid), 7.93 (s, 2H, CHimid), 7.54–7.37 (m, 10H, CHAr),
6.72 (s, 2H, NCH2N), 5.52 (s, 4H, ArCH2N). 13C NMR (DMSO-
d6): d 137.71 (NCHN), 134.13 (CAr(ipso)), 128.98 (CHAr), 128.88
(CHAr(para)), 128.60 (CHAr), 123.16 (CHimid), 122.54 (CHimid), 58.29
(NCH2N), 52.27 (ArCH2N).
1
ether into a concentrated DMF solution (0.398 mg, 60%). H
NMR (DMSO-d6): d 7.75 (s, 4H, CHimid), 7.39–7.29 (m, 12H,
CHAr), 7.27 (s, 4H, CHimid), 7.13–6.98 (m, 8H, CHAr), 6.53 (d,
J = 13, 2H, NCHaHbN), 6.50 (d, J = 13, 2H, NCHaHbN), 4.85
(d, J = 15, 4H, ArCHaHbN), 4.37 (d, J = 15, 4H, ArCHaHbN).
13C NMR (DMSO-d6): d 171.34 (Cimid), 135.85(CAr(ipso)), 128.88
(CHAr), 128.24 (CHAr(para)), 127.61 (CHAr), 123.50 (CHimid), 122.92
(CHimid), 62.50 (NCH2N), 52.61 (ArCH2N). Anal. Calcd for
C42H40Br2N8Ni: C 57.63; H 4.61; N 12.80. Found: C 57.96; H
4.83; N 12.53. ESI-MS (m/z): calcd for C42H20Br2N8Ni [M-Br]+:
793, found: 793.
1,1¢-di(4-tert-butylbenzyl)-3,3¢-methylenediimidazolium dibromide
(1b)
A Schlenk flask was charged with N-tert-butylbenzylimidazole
(1.0 g, 4.67 mmol) and dibromomethane (20 mL) and stirred at
80 ◦C for 2 d. A white precipitate resulted which was filtered,
washed with THF (3 ¥ 15 mL) and dried under vacuum to yield
1b as a white powder (1.05 g, 75%). 1H NMR (DMSO-d6): d 9.68 (s,
2H, NCHN), 8.11 (s, 2H, CHimid), 7.93 (s, 2H, CHimid), 7.45 (d, J =
8.2, 4H, CHAr), 7.41 (d, J = 8.2, 4H, CHAr), 6.71 (s, 2H, NCH2N),
5.46 (s, 4H, ArCH2N), 1.27 (s, 18H, CH3). 13C NMR (DMSO-
d6): d 151.44 (CAr(para)), 137.60 (NCHN), 131.22 (CAr(ipso)), 128.45
(CHAr), 125.73 (CHAr), 123.14 (CHimid), 122.50 (CHimid), 58.27
(NCH2N), 51.98 (ArCH2N), 34.35 (C(CH3)3), 30.98 (C(CH3)3).
Anal. Calcd for C29H38Br2N4: C, 57.82; H, 6.36; N, 9.30. Found:
C, 57.85; H, 6.21; N, 9.16. ESI-MS (m/z): calcd for C29H38Br2N4
[M-Br]+: 521, found: 521.
Bis(1,1¢-methylene-3,3¢-di-4-(tert-butyl)benzylimidazole-2,2¢-
diylidene)nickel(II) dibromide (3b)
A Schlenk flask was charged with 2b (100 mg, 0.227 mmol),
NiBr2(DME) (35 mg, 0.114 mmol) and THF (10 mL). After
stirring at ambient temperature for 12 h, a pale yellow precipitate
was obtained. The precipitate was filtered and washed with THF
(3 ¥ 10 mL) and dried under vacuum to yield a white powder. Crys-
tals were obtained by slow diffusion of ether into a concentrated
DMF solution (50 mg, 40%). 1H NMR (DMSO-d6): d 7.76 (s, 4H,
CHimid), 7.32 (d, J = 8.1, 8H, CHAr), 7.28 (s, 4H, CHimid), 7.04 (d,
J = 8.1, 8H, CHAr), 6.46 (d, J = 13, 2H, NCHaHbN), 6.01 (d, J =
13, 2H, NCHaHbN), 4.85 (d, J = 15, 4H, ArCHaHbN), 4.37 (d, J =
15, 4H, ArCHaHbN), 1.21 (s, 36H, CH3). 13C NMR (DMSO-d6):
d 171.23 (Cimid), 150.68 (CAr(para)), 133.15 (CAr(ipso)), 127.28 (CHAr),
125.28 (CHAr), 123.35 (CHimid), 123.04 (CHimid), 62.17 (NCH2N),
52.21 (ArCH2N), 33.13 (C(CH3)3), 30.95 (C(CH3)3). Anal. Calcd
for C58H72Br2N8Ni: C 63.33; H 6.60; N 10.19. Found: C 63.67; H
6.78; N 9.94. ESI-MS (m/z): calcd for C58H72Br2N8Ni [M-2Br]2+:
469, found: 469.
1,1¢-dibenzyl-3,3¢-methylenediimidazole-2,2¢-diylidene (2a)
A Schlenk flask was charged with a mixture of 1a (0.1 g, 0.204
mmol), K[N(SiMe3)2] (89.5 mg, 0.449 mmol) and THF (10 mL).
The resulting mixture was stirred at room temperature for 5 min
to give a pale yellow solution which was then filtered. The filtrate
was dried under vacuum to yield a yellow powder of 2a (66.1 mg,
98%). 1H NMR (C6D6): d 7.11–7.00 (m, 10H, CHAr), 6.99 (s, 2H,
NCH2N), 6.20 (s, 2H, CHimid), 6.12 (s, 2H, CHimid), 4.98 (s, 4H,
ArCH2N). 13C NMR (C6D6): d 216.28 (Cimid), 139.16 (CAr(ipso)),
129.08 (CHAr), 128.68 (CHAr), 128.03 (CHAr), 120.28 (CHimid),
119.49 (CHimid), 65.75 (NCH2N), 55.27 (ArCH2N).
(1,1¢-dibenzyl-3,3¢-methylenediimidazole-2,2¢diylidene)tetra-
silver(I) tetrabromide (4a)
A suspension of 1a (0.5 g, 1.02 mmol) and an equivalent amount
of Ag2O (0.236 g, 1.02 mmol) in dichloromethane (15 mL) was
stirred at room temperature for 3 d with exclusion of light under
dinitrogen. The color of the suspension gradually changed from
black to brown. The suspension was then filtered, washed with
dichloromethane (3 ¥ 10 mL) and dried under vacuum to yield 4a
as a light brown powder. Crystals were obtained by slow diffusion
1,1¢-methylene-3,3¢-di-4-(tert-butyl)benzylimidazole-2,2¢-diylidene
(2b)
A Schlenk flask was charged with a mixture of 1b (0.1 g, 0.166
mmol), K[N(SiMe3)2] (0.729 g, 0.365 mmol) and THF (10 mL).
The resulting mixture was stirred at room temperature for 5 min
to give a pale yellow solution which was then filtered. The filtrate
was dried under vacuum to yield a yellow powder of 2b (0.731 g,
98%). 1H NMR (C6D6): 7.19 (d, J = 7.1, 4H, CHAr), 7.09 (d, J =
1
of ether into a concentrated DMF solution (0.646 g, 90%). H
NMR (DMSO-d6): d 7.87 (s, 4H, CHimid), 7.59 (s, 4H, CHimid),
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1
7.24 (m, 12H, CHAr(meta and para)), 7.09 (m, 8H, CHAr(ortho)), 6.70 (br s,
4H, NCH2N), 5.21 (s, 8H, ArCH2N)). 13C NMR (DMSO-d6): d
181.08 (Cimid), 136.58 (CAr(ipso)), 128.68 (CHAr), 128.01 (CHAr(para)),
127.14 (CHAr), 123.38 (CHimid), 122.37 (CHimid), 63.45 (NCH2N),
54.44 (ArCH2N). Anal. Calcd for C42H40Br4N8Ag4: C 35.83; H
2.86; N 7.96. Found: C 36.21; H 2.78; N 8.12.
under vacuum to yield 5a as a white powder (0.143 g, 48%). H
NMR (DMSO-d6): d 7.58 (s, 2H, CHimid), 7.31 (m, 10H, CHAr),
7.27 (s, 2H, CHimid), 6.49–6.25 (2 doublets (overlapping)), 2H,
NCH2N), 6.12 (d, J = 14.6, 2H, ArCHaHbN)), 5.32 (d, J = 14.6,
2H, ArCHaHbN).13C NMR (DMSO-d6): d 162.31 (Cimid), 156.87
(CAr(para)), 138.81 (CAr(ipso)), 128.69 (CHAr), 128.03 (CHAr), 127.97
(CHimid), 121.94 (CHimid), 62.46 (NCH2N), 52.82 (ArCH2N). Anal.
Calcd for C21H20Cl2N4Pd·0.5DMF: C 49.83; H 4.37; N 11.62.
Found: C 49.48; H 4.02; N 11.33. ESI-MS (m/z): calcd for
C21H20Cl2N4Pd [M-Cl]+: 469, found: 469.
Bis(1,1¢-methylene-3,3¢-di-4-(tert-butyl)benzylimidazole-2,2¢-
diylidene)tetrasilver(I) tetrabromide (4b)
A suspension of 2b (0.5 g, 0.830 mmol) and an equivalent amount
of Ag2O (0.192 g, 0.830 mmol) in dichloromethane (15 mL)
was stirred at room temperature for 3 d with exclusion of light
under nitrogen atmosphere. The color of the suspension gradually
changed from black to white. The suspension was then filtered,
washed with dichloromethane (3 ¥ 10 mL) and dried under
(1,1¢-Methylene-3,3¢-di-4-(tert-butyl)benzylimidazole-2,2¢-
diylidene)palladium(II) dichloride (5b)
A suspension of 4b (0.4 g, 0.245 mmol) and two equiv of PdCl2
(87 mg, 0.49 mmol) in DMF (10 mL) was stirred at 90 ◦C for
16 h under nitrogen atmosphere. The color of the suspension
gradually changed from light brown to grey. The solution was then
filtered through a celite column to remove the AgBr precipitate.
The filtrate was concentrated under vacuum. Upon addition of
diethyl ether a white precipitate was obtained which was filtered
and dried under vacuum to yield 5b as a white powder (0.176
g, 58%). 1H NMR (DMSO-d6): d 7.57 (s, 2H, CHimid), 7.38
(d, J = 8.1, 4H, CHAr), 7.31 (d, J = 7.7, 4H, CHAr)), 7.20
(s, 2H, CHimid), 6.33 (2 doublets (overlapping)), 2H, NCH2N),
6.01 (d, J = 14.5, 2H, ArCHaHbN)), 5.32 (d, J = 14.5, 2H,
ArCHaHbN), 1.25 (s, 18H, CH3). 13C NMR (DMSO-d6): d 156.99
(Cimid), 150.46 (CAr(para)), 133.80 (CAr(ipso)), 127.87 (CHAr), 125.44
(CHAr), 122.02 (CHimid), 121.73 (CHimid), 62.44 (NCH2N), 52.76
(ArCH2N), 34.30 (C(CH3)3), 31.08 (C(CH3)3). Anal. Calcd for
C29H36Cl2N4Pd·0.25DMF: C 56.14; H 5.98; N 9.36. Found: C
56.50; H 5.81; N 9.06 ESI-MS (m/z): calcd for C29H36Cl2N4Pd
[M-Cl]+: 581, found: 581.
1
vacuum to yield 3 as a white powder (0.610 g, 90%). H NMR
(DMSO-d6): d 7.86 (s, 4H, CHimid), 7.58 (s, 4H, CHimid), 7.23
(d, J = 8.0, 8H, CHAr), 7.08 (d, J = 7.8, 8H, CHAr), 6.71 (br s,
4H, NCH2N), 5.20 (s, 8H, ArCH2N), 1.15 (s, 36H, C(CH3)3).
13C NMR (DMSO-d6): d 181.20 (Cimid), 150.45 (CAr(para)), 133.15
(CAr(ipso)), 127.28 (CHAr), 125.28 (CHAr), 123.35 (CHimid), 123.04
(CHimid), 63.30 (NCH2N), 54.21 (ArCH2N), 33.13 (C(CH3)3),
30.95 (C(CH3)3). Anal. Calcd for C58H72Br4N8Ag4: C 42.66; H
4.45; N 6.87. Found: C 42.36; H 4.22; N 6.54.
Alternative synthesis of Bis(1,1¢-methylene-3,3¢-dibenzylimidazole-
2,2¢diylidene)nickel(II) dibromide (3a)
A suspension of 4a (0.5 g, 0.710 mmol) and one equiv of
NiBr2(DME) (110 mg, 0.355 mmol) in DMF (15 mL) was stirred
◦
at 90 C for 24 h under dinitrogen. The color of the suspension
gradually changed from light brown to grey. The solution was
then filtered to remove the AgBr precipitate. The filtrate was
concentrated under vacuum. Upon addition of diethyl ether a
pale yellow precipitate was obtained which was filtered and dried
under vacuum to yield 3a as a pale yellow powder (0.140 g, 45%).
General procedure for the Mizoroki–Heck coupling reactions
In a typical experiment, an oven dried 25 mL pressure tube
equipped with a stir bar was charged with 5 mol% of the catalyst
(0.025 mmol), base (Na2CO3, 1.5 mmol), additive (Bu4NI) and
phenylboronic acid (2.5 mmol). Under nitrogen, DMF (5 mL)
and aryl halides (0.5 mmol) were added via syringe. The tube was
sealed with a Teflon cap and placed in pre-heated sand bath at
Alternative synthesis of Bis(1,1¢-methylene-3,3¢-di-4-(tert-butyl)-
benzylimidazole-2,2¢-diylidene) nickel(II) dibromide (3b)
A suspension of 4b (0.5 g, 0.613 mmol) and one equiv of
NiBr2(DME) (94.5 mg, 0.307 mmol) in DMF (10 mL) was stirred
at 90 ◦C for 24 h. The color of the suspension gradually changed
from light brown to grey. The solution was then filtered to remove
the AgBr precipitate. The filtrate was concentrated under vacuum.
Upon addition of diethyl ether a white precipitate was obtained
which was filtered and dried under vacuum to yield 3b as a white
powder (0.118 g, 35%).
◦
150 C. After the specified time the tube was removed from the
sand bath and water (20 mL) added followed by extraction with
ether (3 ¥ 10 mL). The combined organic layers were washed with
saturated aqueous NaCl (10 mL), dried over anhydrous MgSO4,
filtered and the solvent was removed under vacuum. The residue
1
was dissolved in CDCl3 and analyzed by H NMR. Yields were
determined by 1H NMR against the remaining aryl halide.
(1,1¢-Methylene-3,3¢-dibenzylimidazole-2,2¢diylidene)palladium(II)
General procedure for the Suzuki–Miyaura coupling reactions
dichloride (5a)
In a typical run, an oven dried 25 mL pressure tube equipped with
a stir bar was charged with 5 mol % catalyst (0.025 mmol), base
(K3PO4, 1.5 mmol) and phenylboronic acid (2.5 mmol). Under
nitrogen, dioxane (5 mL) and aryl halide (0.5 mmol) were added
via syringe. The tube was sealed with a Teflon cap and placed
in pre-heated sand bath at 120 ◦C. After the specified time the
tube was removed from the sand bath and water (20 mL) added
followed by extraction with dichloromethane (3 ¥ 10 mL). The
A suspension of 4a (0.4 g, 0.568 mmol) and two equiv of PdCl2
(101 mg, 0.568 mmol) in DMF (10 mL) was stirred at 90 ◦C
for 16 h. The color of the suspension gradually changed from
light brown to grey. The solution was then filtered through a
celite column to remove the AgBr precipitate. The filtrate was
concentrated under vacuum. Upon addition of diethyl ether a
white precipitate was obtained which was filtered and dried
258 | Dalton Trans., 2012, 41, 251–260
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combined organic layers were washed with saturated aqueous
NaCl (10 mL), dried over anhydrous MgSO4, filtered and the
internal standard (dodecahydrotriphenylene) was added. Solvent
was removed under vacuum. The residue was dissolved in CDCl3
and analyzed by 1H NMR. Yields were determined by 1H NMR
against dodecahydrotriphenylene as the internal standard.
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X-ray structure determinations
Data was collected at -100 ◦C on a Nonius Kappa CCD diffrac-
tometer, using the COLLECT program.21 Cell refinement and
data reductions used the programs DENZO and SCALEPACK.22
SIR9723]was used to solve the structures and SHELXL9724 was
used to refine the structures. ORTEP-3 for Windows25 was used for
molecular graphics and PLATON26 was used to prepare material
for publication. H atoms were placed in calculated positions with
3 For recent or influential Ni catalyzed coupling reactions, see: (a) P.
L. Chiu, C.-L. Lai, C.-F. Chang, C.-H. Hu and H. M. Lee,
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X. Zhang, W. Chen, S. Fu and D. Wang,, Organometallics, 2007, 26,
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U
iso constrained to be 1.5 times Ueq of the carrier atom for methyl
protons and 1.2 times Ueq of the carrier atom for all other hydrogen
atoms.
For complex 3b·4DMF, there is one A ALERT because of the
Solvent Accessible Void of 291 A3. This was due to a disordered
solvent molecules which could not be modelled. This electron
density was handled by using the SQUEEZE option in PLATON25
There is a SQUEEZE addition at the end of the CIF. The two B
ALERTs are due to the very different types of C atoms in the
structure, ranging from phenyl and other rigid rings to disordered
tert-butyl groups. The H atoms on the C atoms show the same
range of Ueq, because they are determined by the values for the
attached C atoms.
For complex 4a, the Hirshfeld test B ALERTs assume inde-
pendent bonds, not clusters such as Ag4Br4 in this structure. The
high Ueq for Ag1 and Ag2 is possibly due to some disorder which
could not be modelled. The low Ueq for Br2 is the other side of
the possible disorder of the Ag atoms. The nearest atoms to Br2
are Ag1 and Ag2. The low bond precision of C–C bonds is a
consequence of the heavy atom cluster.
4 SciFinder Version 2007 Search performed May 5, 2011.
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CCDC 697184–697187† contain the supplementary crystallo-
graphic data for 3a, 3b·4DMF, 4a and 5a·0.5DMF. The data
can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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Acknowledgements
We gratefully thank the Natural Sciences and Engineering Re-
search Council of Canada (NSERC) for financial support and the
Saskatchewan Structural Sciences Centre (SSSC).
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