Palladium(II) Complexes with Small N-Heterocyclic Carbene Ligands as Highly Active Catalysts for the Suzuki-Miyaura Cross-Coupling Reaction

Article abstract of DOI:10.1002/cctc.201200428

Four new palladium(II) complexes of the type [Pd(NHC)₂X₂] with N-heterocyclic carbene (NHC) ligands of relatively small steric hindrance were prepared and characterized by using spectroscopic and X-ray methods. For [Pd(bmim-y)₂Br₂] (bmim-y=1-butyl-3-methylimidazol-2-ylidene), crystals of both cis and trans isomers were obtained. All the studied complexes demonstrated very high activity in Suzuki-Miyaura cross-coupling in ethylene glycol, which yielded turnover numbers of up to 760000. High activity was also observed if NaBPh₄ was used instead of PhB(OH)₂, and the best results (turnover number=580000) were obtained with [Pd(emim-y)₂Cl₂] (emim-y=1-ethyl-3-methylimidazol-2-ylidene). In the reaction mixture, different forms containing [Pd_x(NHC)_y]⁺ fragments (x=1-4, y=2-5) were identified by using ESI-MS. In the presence of Suzuki-Miyaura reaction substrates, catalytic palladium intermediates with aryl groups-[Pd(NHC)₂Ph]⁺ and [Pd₃(NHC)₄Ph]⁺-were detected. Additional mechanistic investigations, such as TEM observations and mercury poisoning experiments, substantiated the formation of nanoparticles as a catalyst resting state. These heterogeneous particles serve as a reservoir for soluble palladium species-atoms or clusters that function as homogeneous catalysts for the Suzuki-Miyaura reaction.

Full text of DOI:10.1002/cctc.201200428

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DOI: 10.1002/cctc.201200428
Palladium(II) Complexes with Small N-Heterocyclic
Carbene Ligands as Highly Active Catalysts for the Suzuki–
Miyaura Cross-Coupling Reaction
Michał S. Szulmanowicz, Andrzej Gniewek, Wojciech Gil, and Anna M. Trzeciak*[a]
Four new palladium(II) complexes of the type [Pd(NHC)2X2]
with N-heterocyclic carbene (NHC) ligands of relatively small
steric hindrance were prepared and characterized by using
spectroscopic and X-ray methods. For [Pd(bmim-y)2Br2] (bmim-
y=1-butyl-3-methylimidazol-2-ylidene), crystals of both cis and
trans isomers were obtained. All the studied complexes dem-
onstrated very high activity in Suzuki–Miyaura cross-coupling
in ethylene glycol, which yielded turnover numbers of up to
760000. High activity was also observed if NaBPh4 was used in-
stead of PhB(OH)2, and the best results (turnover number=
580000) were obtained with [Pd(emim-y)2Cl2] (emim-y=1-
ethyl-3-methylimidazol-2-ylidene). In the reaction mixture, dif-
ferent forms containing [Pdx(NHC)y]+ fragments (x=1–4, y=2–
5) were identified by using ESI-MS. In the presence of Suzuki–
Miyaura reaction substrates, catalytic palladium intermediates
with aryl groups—[Pd(NHC)2Ph]+ and [Pd3(NHC)4Ph]+—were
detected. Additional mechanistic investigations, such as TEM
observations and mercury poisoning experiments, substantiat-
ed the formation of nanoparticles as a catalyst resting state.
These heterogeneous particles serve as a reservoir for soluble
palladium species—atoms or clusters that function as homoge-
neous catalysts for the Suzuki–Miyaura reaction.
Introduction
High catalytic activity of palladium complexes with N-heterocy-
clic carbene ligands (Pd–NHC) is well documented in the litera-
ture.[1] The wide application of Pd–NHC in the Suzuki–Miyaura
reaction can be explained by the high stability of the PdÀNHC
bond, which prevents the formation of inactive palladium
black. In addition, the strong donating ability of NHC ligands
facilitates the oxidative addition of aryl halide whereas its
steric bulkiness increases the rate of reductive elimination of
the product.[2–4] Consequently, NHC ligands substituted at the
N atoms by groups of big steric hindrance, such as mesityl,
adamantyl, or cyclohexyl, are preferred in the Suzuki–Miyaura
reaction of less reactive chloroarenes.[2–4] In most cases, the cat-
alytic system is composed of a palladium precursor, such as
Pd(OAc)2 or Pd2(dba)3, and free NHC or imidazolium salt
(NHC·HX).[3–5] In such systems, catalytically active species are
formed in situ. There are also examples of the application of
the well-characterized Pd0–NHC or Pd2+–NHC complexes as
catalysts for the Suzuki–Miyaura cross-coupling reaction.[6–8] Ac-
cording to the well-accepted mechanism of the reaction,
a monocarbene Pd0–NHC complex is the key form responsible
for the catalytic activity.[7–10] Nolan proposed that the Pd0–NHC
complex is formed as a result of Pd2+ reduction by an ORÀ
group originating from 2-propanol.[7] Herrmann postulated the
formation of [Pdx(NHC)n] species (x>0.5n) with the Pd0–1,3-bi-
sadamantylimidazolin-2-ylidene complex used as the catalyst
under Suzuki–Miyaura reaction conditions.[11]
Although the Pd–NHC complexes bearing bulky substituents
at the N atoms of the imidazole ring are excellent catalysts for
the Suzuki–Miyaura reaction, much less is known about the
catalytic properties of their smaller analogues. To gain some in-
sight into this field, we decided to prepare Pd–NHC complexes
of relatively small steric hindrance and use them as the cata-
lysts. Our work was aimed not only at the estimation of their
catalytic activity but also at the identification of palladium spe-
cies present in the catalytic system. For phosphane-free palla-
dium catalysts, as those studied here, in principle two mecha-
nisms of the Suzuki–Miyaura reaction can be considered: het-
erogeneous catalysis with catalytically active Pd0 nanoparticles
and homogeneous catalysis involving soluble palladium com-
plexes.[12] It is expected that the strong PdÀNHC bond stabiliz-
es monomolecular palladium species and directs the reaction
mostly to the homogeneous pathway.
To confirm which catalytic mechanism dominates in our
system, the identification of soluble and insoluble palladium
forms is necessary. Thus, the soluble palladium species formed
under reaction conditions were identified by using ESI-MS, and
the formation of Pd0 nanoparticles was followed by using TEM.
Recently, ESI-MS has been used as an effective technique for
the characterization of reaction intermediates,[13] such as the
Suzuki–Miyaura reaction[14]; however, to the best of our knowl-
edge, no mechanistic investigations involving Pd–NHC com-
plexes had been reported.
[a] Dr. M. S. Szulmanowicz, Dr. A. Gniewek, Dr. W. Gil, Prof. Dr. A. M. Trzeciak
Faculty of Chemistry, University of Wrocław
14 F. Joliot-Curie, 50-383 Wrocław (Poland)
Fax: (+48)71-328-2348
E-mail: anna.trzeciak@chem.uni.wroc.pl
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200428.
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Results and Discussion
Structure of [Pd(NHC)2X2] complexes
New palladium complexes of the type [Pd(NHC)2X2] (in which
NHC could be mokt-y=1-methyl-3-octylimidazol-2-ylidene,
miop-y=1-methyl-3-propoxymethylimidazol-2-ylidene,
or
emim-y=1-ethyl-3-methylimidazol-2-ylidene) were obtained in
the reaction of Pd(OAc)2 with NHC·HX in THF. One similar com-
plex, [Pd(bmim-y)2Br2] (bmim-y=1-butyl-3-methylimidazol-2-yl-
idene), had been obtained and characterized earlier by Xiao.[15]
We found that a small modification in the synthetic method
mentioned in the literature, namely, stabilization of the reac-
tion temperature at 708C and washing of the crude product
with water, resulted exclusively in the formation of trans iso-
mers of all the complexes, such as [Pd(bmim-y)2Br2]. The origi-
nal method led to a mixture of trans and cis isomers, and both
these isomers form two rotamers: syn and anti.[15]
Figure 2. The molecular structure and atom numbering scheme of trans-[Pd-
(bmim-y)2Br2]. Displacement ellipsoids are drawn at the 30% probability
level, and H atoms are shown as small spheres of arbitrary radii.
1H NMR analysis of the [Pd(bmim-y)2Br2] complex in the
CDCl3 solution confirmed the presence of trans–syn and trans–
anti rotamers in a 1:1 ratio. However, the same solution con-
tained a mixture of trans and cis isomers in a 1:2.5 ratio after
24 h. The recrystallization of the freshly prepared [Pd(bmim-
y)2Br2] from the CH2Cl2/hexane solution resulted in the forma-
tion of single crystals of trans–syn rotamer. In contrast, very
slow (ꢀ3 weeks) evaporation of the CHCl3 solution produced
cis–anti [Pd(bmim-y)2Br2] crystals. The molecular structures of
cis-[Pd(bmim-y)2Br2] and trans-[Pd(bmim-y)2Br2] are presented
in Figures 1 and 2, respectively. The cis-[Pd(bmim-y)2Br2] com-
pound is one of the few examples of such a structure deter-
mined for palladium complexes with a nonchelating carbene
ligand. The PdC bond length of 1.983(6) ꢁ (Table 1) is very simi-
lar to that in cis-[Pd(mmim-y)2I2] (mmim-y=1,3-dimethylmida-
zol-2-ylidene), 1.990(3), 1.997(3) ꢁ[16] and in cis-[Pd(aaim-y)2I2]
(aaim-y=1,3-diallylimidazol-2-ylidene), 2.004(4), 1.993(4) ꢁ.[17]
The [Pd(emim-y)2Br2] complex in the CDCl3 solution consist-
ed of two trans rotamers, which were converted slowly to the
cis isomers. After 24 h, the trans/cis ratio was equal to 1:0.4.
Single crystals of trans–syn-[Pd(emim-y)2Br2] were obtained
Table 1. Characteristic bond lengths observed in [Pd(NHC)2X2] com-
plexes.
Complex
Bond length [ꢁ]
PdÀC
PdÀX
(X=Br or Cl)
cis-Pd(bmim-y)2Br2
trans-Pd(bmim-y)2Br2
trans-Pd(emim-y)2Br2
trans-Pd(miop-y)2Cl2
1.983(6)
2.021–2.026(4)
2.022(5)
2.4967(8)
2.4306–2.4420(8)
2.4404(9)
2.027–2.036(3)
2.3036–2.3161(9)
through slow evaporation of the CH2Cl2 solution, and the mo-
lecular structure of this complex is shown in Figure 3.
In contrast, [Pd(miop-y)2Cl2] formed only one trans–anti rota-
mer and its isomerization was not observed in the CDCl3 solu-
tion after 24 h. The molecular structure of trans-[Pd(miop-
Figure 1. The molecular structure and atom numbering scheme of cis-[Pd-
(bmim-y)2Br2]. Displacement ellipsoids are drawn at the 30% probability
level, and H atoms are shown as small spheres of arbitrary radii. Unlabeled
atoms are generated through symmetry operation [symmetry code: (i) y, x,
1Àz].
Figure 3. The molecular structure and atom numbering scheme of trans-[Pd-
(emim-y)2Br2]. Displacement ellipsoids are drawn at the 30% probability
level, and H atoms are shown as small spheres of arbitrary radii. Unlabeled
atoms are symmetrically dependent through an inversion center [symmetry
code: (i) 1Àx, 1Ày, 1Àz].
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y)2Cl2] is presented in Figure 4. The PdÀC bond lengths found
for complexes trans-[Pd(bmim-y)2Br2], trans-[Pd(emim-y)2Br2],
and trans-[Pd(miop-y)2Cl2] (Table 1) are observed typically in
most of Pd2+ carbene compounds.[18] The molecular structure
of trans-[Pd(miop-y)2Cl2] consists of three slightly different mol-
ecules, which is characterized by different orientations of the
propoxymethyl chain in the miop-y ligand. This is due to the
presence of numerous CÀH···O and CÀH···Cl hydrogen bonds
(see the Supporting Information). As a consequence, a three-
dimensional network of such interactions forms in the crystal.
Only trans-[Pd(mokt-y)2Cl2] was identified in the CDCl3 solu-
tion, which remained unchanged after 24 h. Thus, the type of
substituents bonded to the N atoms of the NHC ligands affects
the isomerization process in the solution and longer alkyl
groups prevent rotation. Interconversion of other Pd–NHC con-
formers was reported in the literature.[19]
Suzuki–Miyaura reaction
The obtained palladium complexes [Pd(NHC)2X2] were tested
as the catalyst precursors in the Suzuki–Miyaura reaction
(Scheme 1) performed under different conditions; the tempera-
Scheme 1. Suzuki–Miyaura reaction of 2-bromotoluene with phenylboronic
acid.
ture, solvents, and bases were varied. An effect of the solvent
is very similar for all the studied complexes (Figure 5). The best
results were obtained clearly in ethylene glycol, whereas lower
but still attractive conversions (50–60%) were obtained in
methanol and water. Interestingly, 2-propanol and 1,4-dioxane,
which had been very often recommended for the Suzuki–
Miyaura reaction, also with palladium complexes containing
bulky NHC ligands, were not suitable for the studied catalysts.
Different mixtures of water and ethylene glycol can also be
considered as reasonably good reaction media (see the Sup-
porting Information).
Studies of the base effect were performed at 40–1108C. In
the reaction with NaHCO3, a good yield of the product was ob-
tained at 40 and 608C whereas KOH and Cs2CO3 were efficient
at higher temperatures 80–1108C. Importantly, the catalytic re-
sults obtained with [Pd(IMes)2Cl2], a model bulky NHC com-
plex, were similar to those obtained with smaller carbene li-
gands (Table 2). Furthermore, additional studies performed
with use of lower amounts of the catalyst (0.01–0.0001 mol%)
confirmed such observations (Table 3).
If NaBPh4 was used instead of PhB(OH)2 under the same con-
ditions, 2-methyl-biphenyl was obtained with a high yield and
the highest turnover number value was observed for [Pd-
(IMes)2Cl2] (Table 3).
The attempts to use the [Pd(bmim-y)2Br2] complex in the
Suzuki–Miyaura reaction with different aryl halides showed
that it is very active in comparison to bromoarenes; however,
unusually lower yields of the product were obtained if chlor-
oarenes were used as the substrates (Table 4).
Figure 4. The molecular structure and atom numbering scheme of trans-[Pd-
(miop-y)2Cl2]. Displacement ellipsoids are drawn at the 30% probability
level, and H atoms are shown as small spheres of arbitrary radii. Unlabeled
atoms are symmetrically dependent through inversion centers [symmetry
codes: (i) 1Àx, 1Ày, 1Àz; (ii) Àx, Ày, Àz].
Mechanistic studies
The ESI-MS method was selected as a tool to analyze organo-
metallic compounds present in a multicomponent reaction
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the chemical individuals was recognized by compar-
ing the experimental isotopic patterns with those
obtained through computer simulation (see the Sup-
porting Information). All the palladium species typi-
cally contain at least one NHC ligand, which indi-
cates high stability of the PdÀNHC bond in contrast
to triazolylidene palladium complexes, which lose tri-
azolylidene ligands under Suzuki–Miyaura reaction
conditions.[20] In a solvent, only dimeric and trimeric
ions containing NHC ligands were formed. Typically,
the signals of these species were observed after 5–
20 min, and longer heating of the solution resulted
in a decrease in the number of peaks (Figure 6).
Most probably, soluble palladium forms were trans-
formed into insoluble nanoparticles, which precipi-
tated in the solution. The highest stability indicated
the [Pd(mokt-y)2Cl2] solution, the ESI-MS spectrum of
which practically remained the same after 5 min as
well as after 50 min.
Figure 5. Effect of the solvent on the yield of the Suzuki–Miyaura reaction. Reaction con-
ditions: [Pd] 1 mol%, 2-bromotoluene 1 mmol, PhB(OH)2 1.15 mmol, NaHCO3 1.7 mmol,
solvent 10 mL, 1 h, 1108C.
In the reaction of [Pd(NHC)2X2] complexes with
PhB(OH)2 and NaHCO3, new in-
termediates containing an aryl
Table 2. Effect of the base and temperature on the yield of the Suzuki–Miyaura reaction.[a]
group coordinated to palladium
are formed. For example, the
Base
KOH
T
[8C]
Yield [%]
Pd(miop-y)2Cl2
[Pd(NHC)2Ph]+ cation was ob-
Pd(bmim-y)2Br2
Pd(emim-y)2Br2
Pd(mokt-y)2Cl2
Pd(IMes)2Cl2
served for NHC=bmim-y, miop-
y, whereas a trimer of the com-
position [Pd3(mokt-y)4Ph]+ was
observed for NHC=mokt-y. Sim-
ilarly, in the reaction with bro-
motoluene, monomeric cations
bearing aryl ligand—[Pd(mokt-
y)4(C6H4CH3)]+ and [Pd(miop-
y)2(C6H4CH3)]+—were formed.
Under Suzuki–Miyaura reac-
tion conditions, monomeric in-
termediates such as [Pd(mokt-
y)3Ph]+, [Pd(mokt-y)2Ph]+, or
[Pd(emim-y)2Ph]+ dominated in
the ESI-MS spectra, although
mokt-y also formed trimeric spe-
cies containing tolyl ligand
40
60
80
0
1
96
100
0
2
100
100
0
5
100
100
0
0
100
93
0
0
70
84
110
Cs2CO3
40
60
80
22
10
99
1
15
100
99
61
2
100
100
1
6
98
86
1
2
100
90
110
100
NaHCO3
40
60
80
67
95
95
97
43
93
95
96
63
94
93
98
54
96
87
96
62
90
100
100
110
[a] Reaction conditions: [Pd] 0.8 mol%, 2-bromotoluene 1 mmol, PhB(OH)2 1.15 mmol, NaHCO3 1.7 mmol, ethyl-
ene glycol 10 mL, 1 h, 1108C.
mixture. The main advantage of the ESI-MS method is that it
transfers reaction intermediates directly from the solution to
the gas phase in which MS characterization can be per-
formed.[13,14] However, keeping in mind that the formation of
palladium species under the measurement conditions cannot
be ruled out completely, we repeated the ESI-MS experiments
for all the studied complexes in solutions containing only the
solvent and, separately, the substrates of the Suzuki–Miyaura
reaction. The initial results showed that palladium-containing
fragments, recognized through a characteristic isotopic pat-
tern, were observed only in the MS(+) region.
(C6H4CH3) originating from 2-bromotoluene. Thus, in the pres-
ence of the Suzuki–Miyaura reaction substrates, the aggrega-
tion of palladium atoms is prohibited and the clusters that
were formed earlier are transformed into monomolecular spe-
cies participating in the catalytic cycle. The most important in-
termediate identified in these studies was the transmetalation
product [Pd(miop-y)(Ph)(C6H4CH3)]+.
Additional comparative experiments were performed with
the [Pd(mokt-y)2Cl2] complex and 2-propanol as the solvent. In
the reaction with PhB(OH)2 or 2-bromotoluene, the respective
aryl intermediates were found, which confirms again the acti-
vation of the substrates by the Pd–NHC complexes. The pres-
ence of the aryl ligand in the palladium trimer was evident,
which indicated the participation of small palladium clusters in
the catalytic process.
All the identified cationic palladium species formed during
the heating of [Pd(NHC)2X2] complexes at 808C in pure ethyl-
ene glycol and in the presence of 2-bromotoluene and
PhB(OH)2 are summarized in Table 5. The composition of all
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Table 3. Results of the Suzuki–Miyaura reaction with different [Pd-
(NHC)2X2] precursors.[a]
Table 4. Results of the Suzuki–Miyaura reaction with different bromoar-
enes and chloroarenes.[a]
Complex
[Pd]
PhB(OH)2
Yield [%]
NaBPh4
Yield [%] TON
Aryl halide (ArX)
Yield [%]
TON[b]
2-chlorotoluene
4-chlorotouene
4-chloro-2-nitrotoluene
4-bromoanisole
2-bromo-5-nitrotoluene
1-chloro-2-nitrobenzene
2
5
1
100
100
15
Pd(bmim-y)2Br2
0.8
0.01
0.0001
97
96
62
121
9600
620000
96
89
0
120
8900
0
Pd(emim-y)2Br2
Pd(miop-y)2Cl2
Pd(mokt-y)2Cl2
Pd(IMes)2Cl2
0.8
0.01
0.0001
96
65
50
120
6500
500000
85
99
58
106
9900
580000
[a] Reaction conditions: [Pd(bmim-y)2Br2] 1 mol%, ArX 1 mmol, PhB(OH)2
1.15 mmol, NaHCO3 1.7 mmol, ethylene glycol 10 mL, 1 h, 1108C.
0.8
0.01
0.0001
86
68
2
107
6800
20000
80
74
0
100
7400
0
cles were identified, and their size distributions were analyzed
(Table 6). [Pd(emim-y)2Br2], [Pd(miop-y)2Br2], and [Pd(mokt-
y)2Cl2] complexes form rather small particles (4–5 nm), whereas
in the case of [Pd(bmim-y)2Br2] and a bulky carbene complex
[Pd(IMes)2Cl2], slightly larger Pd0 species are observed after
heating in ethylene glycol.
0.8
0.01
0.0001
91
94
76
113
9400
760000
94
91
42
118
9100
420000
0.8
0.01
0.0001
100
82
125
8200
100
100
100
125
10000
1000000
In the second series of TEM experiments, the samples were
taken directly after the Suzuki–Miyaura reaction performed at
808C for 1 h in ethylene glycol. The most important observa-
tion was a remarkable decrease in the average Pd0 particle size
(Table 6) compared to the first series of experiments, in which
86
860000
[a] Reaction conditions: 2-bromotoluene 1 mmol, PhB(OH)2 1.15 mmol or
NaBPh4 0.30 mmol, NaHCO3 1.7 mmol, ethylene glycol 10 mL, 1 h, 1108C;
[b] TON=turnover number.
The ESI-MS measurements in-
dicated the presence of soluble
Table 5. ESI-MS data (m/z) for [Pd(NHC)2X2] complexes obtained under Suzuki–Miyaura reaction conditions at
808C in ethylene glycol.
palladium intermediates con-
taining one to four palladium
atoms, NHC ligands, and aryl
groups originating from the
Suzuki–Miyaura reaction sub-
strates. It should also be pointed
out that only cations of +1
charge were observed. For the
proper interpretation of their
composition, it is necessary to
assume that palladium is pres-
ent in the system as either Pd2+
or Pd0 species. The formation of
Pd0 is accepted commonly in
the mechanism of the Suzuki–
Miyaura reaction as the main
form responsible for the activa-
tion of substrates in an oxidative
addition pathway.
Complex
m/z
Formula[a]
Ethylene
glycol
PhB(OH)2 +
NaHCO3
2-Br-toluene+
NaHCO3
PhB(OH)2 +
2-Br-toluene+
NaHCO3
Pd(bmim-y)2Cl2
459.2
459.2
599.2
459.2
565.0
599.2
Pd(bmim)2Br
Pd2(bmim)2Br
Pd(bmim)3Br
Pd2(bmim)3Br3
Pd2(bmim)4Br3
Pd3(bmim)4Br3
Pd2(bmim)5Br3
599.2
866.9
1005.0
1005.0
1108.9
1141.1
1005.0
1108.9
Pd(emim-y)2Br2
Pd(miop-y)2Cl2
Pd(mokt-y)2Cl2
403.1
403.1
Pd(emim)2Ph
Pd(emim)2Br
Pd2(emim)3Br3
Pd2(emim)4Br3
Pd3(emim)4Br3
405.0
780.0
892.9
996.8
451.1
Pd(miop)2Cl
Pd(miop)2Ph
Pd(miop)2(CH3C6H4)
Pd(miop)2(CH3C6H4)(Ph)
Pd(miop)3Cl
491.2
603.2
505.2
Analysis of catalytic reaction
mixtures by using TEM revealed
further details. Two series of ex-
periments were performed. In
the first series of TEM experi-
ments, the complexes were
heated in ethylene glycol for 1 h
before the observations were
made. Pd0 nanoparticles were
formed as a result of Pd2+ com-
plex reduction. The nanoparti-
583.2
603.2
529.2
529.2
571.3
587.2
723.4
Pd(mokt)2Cl
Pd(mokt)2Ph
571.3
723.4
571.3
587.2
Pd(mokt)2(CH3C6H4)
Pd(mokt)3Cl
Pd2(mokt)3Cl3
Pd3(mokt)4Cl3
Pd3(mokt)3Cl5Ph
Pd3(mokt)4Ph
723.4
901.3
1095.4
1095.4
1153.4
1179.6
[a] In the proposed formulas, imidazolium cations (i.e., mokt) and NHC (i.e., mokt-y) are not distinguished.
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Figure 6. ESI-MS spectra measured after the Suzuki–Miyaura reaction was performed with [Pd(bmim-y)2Br2] at 808C in ethylene glycol for different periods of
time (5–50 min).
Table 6. Size distributions (average diameter and FWHM[a] of Gauss fit) of
nanoparticles obtained by heating the [Pd(NHC)2X2] complexes in ethyl-
ene glycol and after the Suzuki–Miyaura reaction.
Complex
Ethylene glycol
Suzuki–Miyaura reaction
dm
[nm]
6.9
5.7
5.1
FWHM
[nm]
1.6
2.1
1.7
dm
[nm]
4.1
4.1
3.3
FWHM
[nm]
1.1
1.2
1.2
Pd(bmim-y)2Br2
Pd(emim-y)2Br2
Pd(miop-y)2Cl2
Pd(mokt-y)2Cl2
Pd(IMes)2Cl2
3.4
6.7
1.1
1.7
3.3
2.7
1.6
1.1
Figure 7. TEM of a) Pd0 nanoparticles obtained from the [Pd(emim-y)2Br2]
complex in ethylene glycol and b) the same particles after the Suzuki–
Miyaura reaction.
[a] FHWM=full-width at half-maximum.
only the complexes and the solvent were heated. Figure 7 is
representative of all the complexes and shows a micrograph of
nanoparticles obtained from [Pd(emim-y)2Br2] in ethylene
glycol and of the same particles after the Suzuki–Miyaura pro-
cess. The observed change in the diameter of nanoparticles ex-
plains the catalytic activity of the system because the particle
size redistribution requires partial dissolution of particles,
which thus generates dissolved palladium species. Thus, the
homogeneous reaction mechanism is suggested to dominate
in this case.
(IMes)2Cl2], and 100% yield was achieved in approximately
15 min, the same as in the absence of Hg0. Consequently, solu-
ble palladium species with coordinated NHC ligands should be
considered as catalytically active. This is in agreement with the
ESI-MS results, which indicate the presence of such species in
the reaction mixture. In contrast, the formation of Pd0 nanopar-
ticles under Suzuki–Miyaura reaction conditions could be con-
sidered as a catalyst resting state preferred in the absence of
the Suzuki–Miyaura reaction substrates. However, if the sub-
strates are present, Pd0 nanoparticles undergo slow dissolution
and act as a source of soluble palladium complexes.
The homogeneous reaction mechanism was also evidenced
through mercury poisoning experiments.[12a,21] The addition of
Hg0 did not influence the product formation in the Suzuki–
Miyaura reaction catalyzed by [Pd(bmim-y)2Br2] or by [Pd-
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1
[emim]Br (0.19 g, 1 mmol) in THF (15 mL). H NMR (600 MHz, CDCl3,
Conclusions
trans–syn): d=6.81 (s, 2H; NCH), 4.06 (t, 3H; NCH3), 4.52 (m, 2H;
NCH2), 1.61 ppm (m, 3H; CH3); 1H NMR (600 MHz, CDCl3, trans–
anti): d=6.80 (s, 2H; NCH), 4.10 (t, 3H; NCH3), 4.51 (m, 2H; NCH2),
1.60 ppm (m, 3H; CH3); 13C NMR (600 MHz, CDCl3): d=168.9 (N2C=
), 122.1 (s, CH), 120.1 (s, CH), 46.0 (s, NCH2), 16.2 ppm (s, CH3); IR
(KBr): n˜max =3151, 3118, 3098 n(=CÀH); 2931, 2872, 1470, 1339,
1027, 746 n(CÀH); 1263, 1214 n(CÀN); 1635 cmÀ1 n(C=C); yield:
70%; elemental analysis calcd (%) for C12H20Br2N4Pd: C 29.62, H
4.14, N 11.52; found: C 28.90, H 4.14, N 11.52.
Simple palladium complexes of the composition [Pd(NHC)2X2]
(in which NHC=1,3-dialkylimidazol-2-ylidene and alkyl=CH3,
C2H5, C8H17, CH2OCH2CH2CH3) used as the catalyst precursors in
the Suzuki–Miyaura reaction in ethylene glycol demonstrated
excellent catalytic activity at a concentration as low as 1ꢂ
10À4 mol%. The catalytic results are similar to those obtained
with an analogous complex containing bulky NHC ligand [Pd-
(IMes)2Cl2]. During the reaction, Pd2+ was reduced to Pd0; the
MS analysis enabled the identification of polynuclear inter-
mediates containing Pd0 bonded to NHC ligands, which con-
firmed the high stability of the PdÀNHC bond. The palladium
aryl intermediates [Pd(NHC)2Ph]+ and [Pd3(NHC)4Ph]+ were
identified in the presence of PhB(OH)2. At the same time,
larger palladium species were formed—Pd0 nanoparticles.
However, under Suzuki–Miyaura reaction conditions, these
nanoparticles could be dissolved partially, as confirmed by TEM
analysis. Leaching of palladium from these nanoparticles pro-
vides molecular, catalytically active species that are able to cat-
alyze the cross-coupling reaction under relatively mild condi-
tions.
Synthesis of [Pd(miop-y)2Cl2]: The method was the same as for
[Pd(bmim-y)2Br2] but used Pd(OAc)2 (0.05 g, 0.22 mmol) and
[miop]Cl (0.09 g, 0.47 mmol) in THF (25 mL). 1H NMR (600 MHz,
CDCl3): d=4.13 (s, 3H; CH3), 7.03 (s, 1H; CH), 6.86 (s, 1H), 5.90 (d,
2H; CH2), 3.65 (m, 2H; CH2), 1.59 (m, 2H; CH2), 0.89 ppm (t, 3H;
CH3,); 13C NMR (600 MHz, CDCl3): d=169.9 (s, NCN), 37.8 (s, H3CN),
122.9 (s, ÀCH), 119.7 (s, ÀCH), 79.7 (s, OCH2N), 71.0 (s, CH2), 22.7 (s,
CH2), 10.4 ppm (s, CH3À); IR: n˜max =3033–3064 n(=CÀH), 2940, 1472,
1327, 1077, 769 n(CÀH); 1222 n(CÀN); 1639 cmÀ1 n(C=C); yield:
65%; elemental analysis calcd (%) for C16H28Cl2N4O2Pd: C 39.56, H
5.81, N 11.53; found: C 38.90, H 5.45, N 11.20.
Synthesis of [Pd(mokt-y)2Cl2]: The method was the same as for
[Pd(bmim-y)2Br2] but used Pd(OAc)2 (0.12 g, 0.53 mmol) and
[mokt]Cl (0.34 g, 1.7 mmol) in THF (25 mL). 1H NMR (600 MHz,
CDCl3): d=6.74 (d, 2H, CH), 4.45 (m, 2H; CH2), 2.06 (m, 2H; CH2),
4.11 (d, 3H; NCH3), 1.30 (m, 10H; CH2), 0.38 ppm (t, 3H; CH3);
13C NMR (600 MHz, CDCl3): d=169.4 (s, N2C=), 121.7 (s, CH), 120.6
(s, CH), 37.5 (s, CH), 50.7 (s, NCH2), 31.3 (s, CH2), 31.8 (s, CH2), 29.3
(d, CH2ÀCH2), 27.0 (s, CH2), 22.6 (s, CH2), 14.1 ppm (s, CH2); IR:
n˜max =2861–3164 n(=CÀH); 1479, 1387, 1071, 808, 762, 709 n(CÀH);
1249 n(CÀN); 1578 cmÀ1 n(C=C); yield: 80%; elemental analysis
calcd (%) for C24H44Cl2N4Pd: C 50.93, H, 7.84, N 9.90; found: C
50.20, H 7.50, N 9.51.
Experimental Section
[Pd(NHC)2X2] complexes were prepared according to the slightly
modified method described in Reference [15], and [Pd(IMes)2Cl2]
was obtained according to the literature method.[6g] The standard
Schlenk technique was used with N2 as an inert gas.
Synthesis of [Pd(bmim-y)2Br2]: Pd(OAc)2 (0.05 g, 0.22 mmol) was
dissolved in THF (15 mL), and then [bmim]Br (0.106 g, 0.48 mmol)
was added. The resulting mixture was heated to 708C along with
magnetic stirring for 3 h. The color changed from red to yellow,
and the formation of two phases was observed. After cooling
down the mixture to RT, the upper phase containing the product
in THF was separated. From the lower phase, the product was ex-
tracted with three portions of THF (10 mL each). The extracted THF
solutions were combined in a Schlenk tube, and the solvent was
removed under vacuum. The resulting residue was dissolved in
CH2Cl2 and washed with three portions of water (15 mL each) to
remove unreacted [bmim]Br. After evaporation of CH2Cl2, the prod-
Suzuki–Miyaura reaction method
Suzuki–Miyaura reactions were performed in a 50 mL Schlenk tube
in an air atmosphere. The solid substrates base (1.7 mmol) and
phenylboronic acid (1.15 mmol, 0.133 g) or NaBPh4 (0.30 mmol,
0.095 g) were weighed and placed directly in the Schlenk tube.
Next, ethylene glycol (3 mL) and 2-bromotoluene (1 mmol,
0.118 mL) were added. After heating the substrates to the required
temperature (80 or 1008C), the palladium complex (8ꢂ10À3–1ꢂ
10À6 mmol) was added. The Schlenk tube was closed with a rubber
stopper, and the reaction mixture was stirred at 80 or 1108C. After
1 h, the reactor was cooled down and the organic products were
extracted with 3ꢂ3+1 mL of n-hexane (with intensive stirring for
5 min). The extracts (10 mL) were analyzed by using a GC–flame
ionization detector (Hewlett Packard 5890) with dodecane
(0.076 mL) as an internal standard. The products were identified by
using GC–MS (Hewlett Packard 5971A).
1
uct was washed with hexane and finally dried in vacuum. H NMR
(600 MHz, CDCl3, trans–syn): d=6.83 (d, JHH =2.0 Hz, 1H; NCH), 6.81
(d, JHH =2.0 Hz, 1H; NCH), 4.06 (s, 3H; NCH3), 4.43 (t, JHH =7.6 Hz,
2H; NCH2), 2.06 (qi, 2H; CH2), 1.46 (m, JHH =7.6 Hz, 2H; CH2),
1.01 ppm (t, JHH =7.4 Hz, 3H; CH3); 13C NMR (600 MHz, CDCl3,
trans–syn): d=169.3 (N2C), 122.1 (CH), 121.0 (CH), 50.8 (NCH2), 38.0
(NCH3), 32.8 (CH2), 20.0 (CH2), 13.8 ppm (CH3); 1H NMR (600 MHz,
CDCl3, trans–anti): d=6.81 (d, JHH =2.0 Hz, 1H; NCH), 6.83 (d, JHH
=
2.0 Hz, 1H; NCH), 4.09 (s, 3H; NCH3), 4.46 (t, JHH =7.6 Hz, 2H;
NCH2), 2.10 (qi, 2H; CH2), 1.47 (m, JHH =7.5 Hz, 2H; CH2), 1.02 ppm
(t, JH,H =7.4 Hz, 3H; CH3); 13C NMR (600 MHz, CDCl3, trans–anti): d=
169.4 (N2C), 122.1 (CH), 121.0 (CH), 50.9 (NCH2), 38.2 (NCH3), 33.1
(CH2), 20.3 (CH2), 14.0 ppm (CH3); IR (KBr): n˜max =3033–3064 n(=CH);
2940, 1477, 1385, 1241, 1077, 750 n(CH); 1222 n(CN); 1639 cmÀ1
n(C=C); yield: 85%; elemental analysis calcd (%) for C15H26Br2N4Pd:
C 35.41, H 5.20, N 10.32; found: C 35.11, H 4.70, N 9.98.
Mass spectrometry
Spectra were recorded for solutions prepared by heating palladium
precursors (1ꢂ10À2 mmol) in ethylene glycol (5 mL) at 808C for
a given time (5–50 min). Next, the solution (10 mL) was added to
methanol (1.5 mL) and a sample of 250 mL was taken instantane-
ously for the measurement. The palladium concentration was ap-
proximately 2.2 mm.
Synthesis of [Pd(emim-y)2Br2]: The method was the same as for
[Pd(bmim-y)2Br2] but used Pd(OAc)2 (0.11 g, 0.5 mmol) and
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Samples containing the Suzuki–Miyaura reactants were prepared in
the same way by using a palladium precursor (1ꢂ10À2 mmol), 2-
bromotoluene (1 mmol), PhB(OH)2 (1 mmol), and NaHCO3
(1.7 mmol).
Crystallographic data collection and refinement
Single crystals suitable for X-ray measurements were mounted on
glass fibers in silicone grease and cooled to 100 K in a nitrogen
gas stream,[22] and the diffraction data were recorded on a Kuma
KM-4 CCD diffractometer with graphite monochromated MoKa ra-
diation (l=0.71073 ꢁ). The structures were solved subsequently
by using direct methods and developed by using the full-matrix
least-square refinement on F2. Structural solution and refinement
were performed by using the SHELX suite of programs.[23] Analyti-
cal absorption corrections were performed with CrysAlis RED.[24] C,
N, O, Cl, Br, and Pd atoms were refined anisotropically. All H atoms
were positioned geometrically and refined isotropically by using
a riding model with a common fixed isotropic thermal parameter.
The molecular structure plots were prepared by using the ORTEP-3
program.[25] Crystal data and selected details of structure determi-
nation are summarized in Table 7.
Experiments were performed by using a Bruker’s Apex-Qe Ultra 7
Tesla instrument equipped with a dual ESI source. The potential
between the spray needle and the orifice was set to 4.5 kV. In the
MS/MS mode, the quadruple was used to select the precursor ions
fragmented in the hexapole collision cell by applying argon as
a target gas. The product ions were subsequently mass analyzed.
For the CID MS/MS measurements, a voltage of 20 V was applied
on the hexapole collision cell.
Transmission electron microscopy
For the observations of Pd0 nanoparticles formed as a result of re-
duction in the reaction solvent, palladium precursors (1ꢂ
10À2 mmol) were placed in a Schlenk tube and ethylene glycol
(5 mL) was added subsequently. The tube was sealed with
a rubber stopper and heated at 808C for 1 h. Then, the mixture
was cooled down and a single droplet of the solution mixture was
placed on a carbon-coated microscope grid and dried for 40 min.
For observing nanoparticles after the Suzuki–Miyaura reaction,
a droplet of the solution was taken directly from the Schlenk tube
after a 1 h reaction performed at 808C with the same amount of
the catalyst precursor (1ꢂ10À2 mmol).
The supplementary crystallographic data for this article can be ob-
tained from the Cambridge Crystallographic Data Centre (CCDC)
through www.ccdc.cam.ac.uk/data_request/cif. The CCDC reference
codes are as follows: cis-dibromo-bis(1-butyl-3-methylimidazol-2-
ylidene)palladium(II) (CCDC-725475), trans-dibromo-bis(1-butyl-3-
methylimidazol-2-ylidene)palladium(II) (CCDC-725476), trans-di-
chloro-bis(1-propoxymethyl-3-methylimidazol-2-ylidene)palladiu-
m(II) (CCDC-725477), and trans-dibromo-bis(1-ethyl-3-methylimida-
zol-2-ylidene)palladium(II) (CCDC-885699).
TEM measurements were performed with an FEI Tecnai G2 20 X-
TWIN electron microscope operating at 200 kV. The size distribu-
tion of the nanoparticle for each sample was determined by count-
ing the size of approximately 250 palladium nanoparticles from
several TEM images obtained from different places of the TEM
grids. The size distribution plots were fitted by using Gauss curve
approximation.
Acknowledgements
Authors are grateful to Prof. Piotr Stefanowicz and Prof. Zbigniew
Szewczuk for their help with mass spectrometry experiments and
to Mr. Marek Hojniak (Faculty of Chemistry, University of Wro-
cław) for GC-MS analyses.
Table 7. Crystallographic data.
Complex
cis-Pd(bmim-y)2Br2
trans-Pd(bmim-y)2Br2
trans-Pd(emim-y)2Br2
trans-Pd(miop-y)2Cl2
Formula
Mr
Crystal system
Space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
[PdBr2(C8H14N2)2]
542.64
tetragonal
P43212
7.558(2)
7.558(2)
35.236(8)
90
90
90
2012.8(8)
4
1.791
[PdBr2(C8H14N2)2]
542.64
triclinic
[PdBr2(C6H10N2)2]
486.54
monoclinic
P21/n
8.568(3)
8.782(3)
10.930(4)
90
96.72(3)
90
816.8(5)
2
[PdCl2(C8H14N2O)2]
485.72
triclinic
¯
P1
¯
P1
7.895(3)
8.292(3)
15.903(4)
89.15(3)
81.18 (3)
84.44 (3)
1023.9(6)
2
7.293(2)
15.578(4)
19.403(5)
68.83(3)
85.39(3)
83.37(3)
2040.1(10)
4
g [8]
V [ꢁ3]
Z
Dx [gcmÀ3
]
1.760
1.978
1.581
Radiation
MoKa
MoKa
MoKa
MoKa
Reflections
Parameters
Rint
17867
107
0.079
8532
212
0.030
6274
90
0.024
28137
462
0.038
R1 (I>2s)
q [8]
0.044
3.0–27.5
4.90
0.034
3.0–27.5
4.81
0.038
3.0–27.5
6.02
0.030
3.0–27.5
1.19
m [mmÀ1
T [K]
]
100(2)
100(2)
100(2)
100(2)
Color
Shape
light yellow
plate
light yellow
plate
light yellow
plate
light yellow
plate
Size [mm]
0.42ꢂ0.40ꢂ0.12
0.25ꢂ0.20ꢂ0.15
0.24ꢂ0.12ꢂ0.10
0.32ꢂ0.15ꢂ0.09
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Received: June 30, 2012
Published online on October 26, 2012
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