Oligoether substituted bis-NHC palladium and platinum complexes for aqueous Suzuki-Miyaura coupling and hydrosilylation

Article abstract of DOI:10.1016/j.jorganchem.2015.03.027

Abstract The synthesis of four oligo ethylene glycol substituted, bis-NHC palladium(II) and platinum(II) complexes is reported. Two of them were characterized by solid state structures. All complexes show excellent solubility in a variety of organic solvents and water. The palladium complexes were tested in the Suzuki-Miyaura coupling reaction and the platinum complexes in the hydrosilylation reaction of alkynes in aqueous solvent mixtures. All show a high catalytic activity at ppm level catalyst-loading under very mild reaction conditions.

Full text of DOI:10.1016/j.jorganchem.2015.03.027

Journal of Organometallic Chemistry xxx (2015) 1e6
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Oligoether substituted bis-NHC palladium and platinum complexes
for aqueous SuzukieMiyaura coupling and hydrosilylation
Dominik Munz, Christoph Allolio, Dirk Meyer 1, Maik Micksch, Leonard Roessner,
Thomas Strassner*
€
Physikalische Organische Chemie, Technische Universitat Dresden, Bergstrasse 66, 01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of four oligo ethylene glycol substituted, bis-NHC palladium(II) and platinum(II) com-
plexes is reported. Two of them were characterized by solid state structures. All complexes show
excellent solubility in a variety of organic solvents and water. The palladium complexes were tested in
the SuzukieMiyaura coupling reaction and the platinum complexes in the hydrosilylation reaction of
alkynes in aqueous solvent mixtures. All show a high catalytic activity at ppm level catalyst-loading
under very mild reaction conditions.
Received 10 December 2014
Received in revised form
17 March 2015
Accepted 27 March 2015
Available online xxx
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Platinum
NHC
SuzukieMiyaura coupling
Hydrosilylation
Water
ꢀ
ꢀ
Introduction
anions like BF4 or PF6 using the corresponding silver salts [24].
We found that the result was strongly dependent on the coun-
terion. Especially the anion metathesis with AgOAc lead to an
interesting result, since not the desired mononuclear complex, but
instead a trinuclear PdeAgePd carbene acetato complex was iso-
lated [29]. Furthermore, many organometallic reactions like also
the Heck reaction with chelating bis-NHC ligands either produce or
require nucleophilic ions [30]. In order to achieve high solubility
even in the presence of these coordinating counterions, a different
strategy is needed.
PEGylation is a widely used technique in pharmaceutical
chemistry [31,32] and well known for not only improving the solu-
bility, but also changing the physicochemical properties of transi-
tion metal complexes [33e36]. Therefore, we decided to synthesize
oligoether substituted bis-NHC complexes, which we expected to
show a significantly enhanced solubility in both organic solvents
and water.
The SuzukieMiyaura coupling is a very important reaction in
the field of organic synthesis [37,38] and due to safety issues,
environmental aspects, and increasing regulatory pressure, the
development of modern reaction protocols under aqueous condi-
tions has become more and more important in recent years
[39e41]. Therefore, the synthesis and application of water soluble
transition metal NHC complexes is currently of interest [42e46].
N-Heterocyclic carbene (NHC) ligands are ubiquitous in organo-
metallic chemistry [1e3] and have found widespread application in
cross coupling chemistry [4e8]. Chelated palladium and platinum
bis-NHC complexes show exceptional high chemical stability,
favorable electronic and steric properties, and have been shown to
be very efficient catalysts for a variety of organic transformations
[9e11]. They also showed very high catalytic activity in the Heck
reaction [12e14] and the SuzukieMiyaura coupling [15e17]. We
found that bis-NHC platinum complexes are very efficient catalysts
for the hydrosilylation of olefins [18,19], which is of extraordinary
importance for the silicon industry [20e23]. Unfortunately,
chelated palladium- and platinum-bis-NHC catalysts typically
show very limited solubility in solvents like dichloromethane,
tetrahydrofuran or water, which seriously complicates their use in
homogeneous catalysis and also hinders mechanistic organome-
tallic investigations [24e28]. To increase the solubility in organic
solvents, we exchanged the halide anions for non-coordinating
* Corresponding author.
E-mail address: thomas.strassner@chemie.tu-dresden.de (Th. Strassner).
X-ray analysis.
1
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D. Munz et al. / Journal of Organometallic Chemistry xxx (2015) 1e6
Strategies to improve the activity of CeC coupling reactions in an
aqueous environment typically rely either on additives like phase
transfer agents [47,48], solid supported catalysts [48,49], or the
attachment of polar groups [50,51] to the ligand.
Reports on the hydrosilylation of olefins and alkynes in protic
solvents [52e55] or ionic liquids [19,56] and especially under
platinum catalysis [57e59] are, however, extremely rare.
Accordingly, we investigated the oligoether substituted palla-
dium complexes as catalysts in the SuzukieMiyaura coupling and
the corresponding platinum complexes in the hydrosilylation re-
action in aqueous solution.
Results and discussion
Synthesis
Fig. 1. Solid state structure of palladium bis-NHC complex 5. Second molecule of 5 and
hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å) and angles (ꢁ) for molecule A: Pd1AeBr1A 2.4975(7),
Pd1AeBr2A 2.4919(7), Pd1AeC2A 1.984(5), Pd1AeC5A 1.974(5), C2AePd1AeC5A
85.1(2), Br1AePd1AeBr2A 89.85(2). Selected bond lengths (Å) and angles (ꢁ) for
molecule B: Pd1BeBr1B 2.4902(7), Pd1BeBr2B 2.4933(7), Pd1BeC2B 1.983(5),
Pd1BeC5B 1.986(5), C2AePd1BeC5B 83.4(2), Br1BePd1BeBr2B 89.50(2).
The bisimidazolium salts were prepared in tetrahydrofuran (3)
and acetonitrile (4) by alkylation of the corresponding bisimida-
zoles 1 and 2 with 2-[2-(2-methoxyethoxy)ethoxy]ethylbromide in
93% (3) and 68% (4) yield (Scheme 1).
Scheme 1. Synthesis of the bisimidazolium salts.
Subsequently, the platinum and palladium complexes 5e8
could be synthesized from the bisimidazoliums salts with palla-
dium acetate (5, 6) or platinum acetylacetonate (7, 8) in DMSO
(Scheme 2).
Fig. 2. Solid state structure of platinum bis-NHC complex 7. Second molecule of 7 and
hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å) and angles (ꢁ) for molecule A: Pt1AeBr1A 2.4939(7),
Pt1AeBr2A 2.4966(7), Pt1AeC2A 1.970(5), Pt1AeC5A 1.974(5), C2AePt1AeC5A
85.6(2), Br1AePt1AeBr2A 88.63(2). Selected bond lengths (Å) and angles (ꢁ) for
molecule B: Pt1BeBr1B 2.4902(7), Pt1BeBr2B 2.4933(7), Pt1BeC2B 1.983(5),
Pt1BeC5B 1.986(5), C2AePt1BeC5B 83.4(2), Br1BePt1BeBr2B 89.50(2).
1.984(5) Å, 1.974(5) Å))ꢁ; molecule B: 1.983(5) Å, 1.986(5) Å) and the
Ptecarbene (molecule A: 1.970(5) Å, 1.974(5) Å)ꢁ; molecule B:
1.961(5) Å,1.967(5) Å) bond lengths seem to be slightly elongated in
comparison to the complexes with methyl substituents (CePd:
1.983(5) Å, 1.971(5) Å, CePt: 1.963(10) Å, 1.950(10) Å) [60,61].
Scheme 2. Synthesis of the palladium- and platinum-bis-NHC-complexes.
SuzukieMiyaura coupling
All four complexes are well soluble in a large variety of organic
solvents like tetrahydrofuran, dichloromethane, acetone, aceto-
nitrile or ethanol and show moderate solubility in toluene and
water. Single crystals of 5 and 7 were obtained by slow diffusion of
diethyl ether into a concentrated solution of the complexes in
methanol. The solid state structures of both complexes are iso-
structural (Figs. 1 and 2) and both asymmetric units contain two
independent metal complexes.
The palladium complexes 5 and 6 were tested as catalysts in the
Suzuki coupling of phenylboronic acid with 4-bromo toluene,
which is considered a challenging reactant in water due to its
hydrophobicity. We decided to test the catalysts first in a 1:1
mixture of methanol and water with K2CO3 as base, which had been
determined to be more or at least equally effective as Cs2CO3, KOH,
or K3PO4.
In both solid state structures, the oligo ethylene glycol chains are
coiled up and situated below the plane of the bis-NHC moiety. The
CePdeC (molecule A: 85.1(2)ꢁ; molecule B: 83.4(2)ꢁ) and CePteC
(molecule A: 85.6(2)ꢁ; molecule B: 84.4(2)ꢁ) bite angles of the li-
gands are considerably larger than these of the analogous bis-NHC
complexes with methyl substituents (CePdeC: 83.2(2)ꢁ, CePteC:
83.8(4)ꢁ) [60,61]. Likewise, the Pdecarbene (molecule A:
Initially, we investigated the catalyst loading with complex 5
(Table 1) and found for catalyst loadings up to 0.001 mol-% similar
yields of 80% at a reaction temperature of 100 ꢁC (entries 1e4)
demonstrating the high catalytic activity of our system with turn-
over numbers up to 81,000. Subsequently, we investigated the
reaction temperature with a catalyst loading of 0.1 mol-% and ob-
tained for temperatures of 40e100 ꢁC similar yields of around 80%
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D. Munz et al. / Journal of Organometallic Chemistry xxx (2015) 1e6
3
Table 1
yield of 81% for 4-bromo anisole (entry 3) and of 61% for N,N-
dimethyl-4-bromo aniline (entry 4).
SuzukieMiyaura coupling in MeOH:H2O solvent mixture.a
The presence of ortho substituents at the aryl bromide did not
lead to a significant decrease of the yield as 1-bromo-2,4-dimethyl
benzene (entry 5) and 1-bromo-2,4,6-trimethyl benzene (entry 6)
could be converted in comparable yields. When the carboxylate
ester (Table 2, entry 7) was used the corresponding biphenyl
carboxylate ester was obtained in a yield of 72% with additional 8%
of the biphenyl carboxylic acid demonstrating that the hydrolysis of
the carboxylate ester occurred only to a small amount under the
reaction conditions.
We did not observe signs of the formation of palladium black at a
reaction temperature of 60 ꢁC. Consequently the aqueous phase,
which contains the palladium catalyst, could be recycled without a
decrease in catalytic activity (Table 3, entries 1 and 2). Additional
cycles led to reduced yields (entries 3 and 4), which we believe to be
a consequence of the increasing salt load in the aqueous phase. The
high concentration of potassium salt in the aqueous phase led to a
highly viscous system in the case of entry 3 and especially entry 4.
Entry
Catalyst
T [ꢁC]
[mol-%] Cat.
Yield [%]
TON
80
800
8100
81,000
790
800
790
650
2550
280
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
100
100
100
100
80
60
40
RT
40
1
0.1
0.01
0.001
0.1
0.1
0.1
0.1
80
80
81
81
79
80
79
65
51
28
25
12
9
5
5
6
0.05
0.01
0.1
10
11
12
40
40
40
250
120
Pd(OAc)2
0.1
a
Reaction conditions: 0.5 mL H2O, 0.5 mL MeOH, 1 mmol 4-bromo toluene,
1.1 mmol PhB(OH)2, 2 mmol K2CO3, [Pd], 12 h, under argon, GC yield.
(entries 2, 5e7). However, at room temperature the yield decreased
slightly (entry 8). We also reduced the catalyst loading at a tem-
perature of 40 ꢁC but obtained significantly lower yields with
loadings of 0.05 mol-% and 0.01 mol-% (entries 9 and 10). Under the
best reaction conditions (0.1 mol-%, 40 ꢁC) we compared complexes
5, 6 and Pd(OAc)2 (entries 7, 11 and 12). Complex 6 turned out to be
significantly less catalytically active than complex 5 with a yield of
only 25%, which clearly demonstrates that the methylene bridged
ligand in 5 is superior to the phenyl bridged ligand in 6. Pd(OAc)2
gave only small amounts of the cross coupling product, which
emphasizes the increase of activity by the bis-NHC ligand.
Table 3
Recyclability of catalyst.a
Entry
Run
Yield [%]
1
2
3
4
1
2
3
4
81
81
61
54
a
Reaction conditions: 1.5 mL H2O, 1.5 mL MeOH, 1 mmol 4-bromo toluene,
1.1 mmol PhB(OH)2, 0.01 mol-% 5, 2 mmol K2CO3, 12 h, 60 ꢁC, under air, GC yield.
Next, we briefly investigated the catalytic activity without the
addition of methanol to the reaction mixture (Table 4). Although
the yield for catalyst 5 was again considerable higher than for
Pd(OAc)2 and catalyst 6 (entries 1e5) we obtained only moderate
yields of 55%. The addition of the phase transfer catalyst NBu4Br did
not lead to higher yields (entry 7) indicating that the presence of
methanol is very important in order to obtain a highly active cat-
alytic system. Probably methanol increases the solubility of the
organic compounds and increases the reactivity of the boronic acid.
Additionally, we investigated the catalytic activity of 5 with
other aryl bromides (Table 2). We obtained an excellent yield of 99%
for the coupling of the acetophenone derivative (entry 2). Also for
more challenging aryl bromides we obtained good yields, e.g. a
Table 2
Variation of substrate.a
Entry
1
Substrate
Yield [%]
77
Table 4
SuzukieMiyaura coupling in water.a
T [ꢁC]
Yield [%]
2
99
Entry
Catalyst
1
2
3
4
5
6
7
5
6
100
100
100
80
60
40
55
9
3
4
5
81
61
77
Pd(OAc)2
10
30
36
35
30b
5
5
5
5
100
a
Reaction conditions: 1 mL H2O, 1 mmol 4-bromo toluene, 1.1 mmol PhB(OH)2,
0.1 mol-% [Pd], 2 mmol K2CO3, 12 h, GC Yield.
b
Addition of 0.3 mmol NBu4Br.
Hydrosilylation
6
71
The platinum complexes 7 and 8 were tested as catalysts for the
hydrosilylation of phenyl acetylene in a methanol:water ¼ 1:1
mixture (Table 5).
Again, the methylene bridged complex (7) showed higher cata-
lytic activity than the phenyl bridged (8) with yields of up to 83%
(entry 1). The reactions proceeded with an excellent regiose-
lectivity since only b-isomers (A and B) were obtained. However,
we found only a moderate stereoselectivity demonstrated by
trans:cis mixtures of 3:1 to 5.5:1 (entry 1). The lower reaction
7
72b
a
Reaction conditions: 0.5 mL H2O, 0.5 mL MeOH, 1 mmol aryl bromide, 1.1 mmol
PhB(OH)2, 0.01 mol-% 5, 2 mmol K2CO3, 20 h, 60 ꢁC, under air, isolated yield.
b
Methyl (biphenyl carboxylate) was obtained in a yield of 72% next to biphenyl
carboxylic acid in a yield of 8%.
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D. Munz et al. / Journal of Organometallic Chemistry xxx (2015) 1e6
temperature slightly improved the stereoselectivity, but also
decreased the yield.
Conclusion
Four palladium- and platinum complexes with chelating, oligo
ethylene glycol substituted bis-NHC ligands have been prepared.
They show excellent solubility in a variety of organic solvents and
water and could be characterized by their solid state structures. The
palladium complexes show a very high catalytic activity at ppm
catalyst loadings in the SuzukieMiyaura reaction in an aqueous
solvent under very mild reaction conditions with TONs higher than
80,000. The synthesized platinum complexes were tested in the
hydrosilylation reaction in an aqueous solvent mixture as well and
show reasonable catalytic activity.
Table 5
Hydrosilylation in methanol/water solvent mixture.a
Entry
Catalyst
T [ꢁC]
[mol-%] Cat.
Yield [%]
Selectivity A/B/Cb
1
2
3
4
5
7
8
7
7
7
80
80
80
60
40
1
1
0.5
1
1
83
70
68
79
69
4/1/0
3/1/0
3/1/0
4.5/1/0
5.5/1/0
Experimental
General procedure
a
Reaction conditions: 1 mL H2O, 1 mL MeOH, 1 mmol phenyl acetylene, 1.1 mmol
Solvents of at least 99.5% purity were used throughout this
study. All other chemicals were obtained from common suppliers
and used without further purification. The bisimidazoles 1 and 2
and 1-[2-bromoethoxy-(2-methoxyethoxy)ethoxy]ethane were
prepared as reported in the literature [62e64].
triethylsilane, [Pt], 16 h, under air, isolated yields.
b
Determined by 1H NMR.
As other silanes like triethoxysilane or dimethylphenylsilane are
not stable in aqueous reaction mixtures the reaction conditions are
limited to triethylsilane. But we ran the hydrosilylation at 60 ꢁC
with other alkynes to study the selectivities with these alkynes
(Table 6). With 1-octyne (entry 2) an excellent yield of 92% was
obtained. An excellent regioselectivity towards the corresponding
1H and 13C NMR spectra were recorded with a Bruker AC 300
and Bruker DRX 500 P spectrometer. The spectra were referenced
internally to the resonances of the solvent (1H, 13C). Elemental
analyses were performed by the microanalytical laboratory of our
institute using a EuroVektor Euro EA-3000 Elemental Analyzer.
Melting and decomposition points were determined with a Wag-
ner&Munz PolyTherm A melting point apparatus and are uncor-
rected. Positive mode ESI-MS spectra for the synthesized
compounds were recorded on a Bruker Esquire MS with Ion Trap
Detector on samples dissolved in NH4OAc buffered methanol.
b
-isomers was found and the stereoselectivity increased to 9:1.
With internal alkynes the yields decreased significantly, phenyl
propylene (entry 3) led to a yield of 66% and 2-octyne (entry 4) to
38%. For both substrates we obtained an excellent stereoselectivity
as only cis products were found, but only with moderate regiose-
lectivities. Interestingly with phenyl propylene the
obtained as the main product and the -isomer as the minor
product with an ratio of 2:1 indicating that a phenyl ring acti-
vates its -position in an internal alkyne. With 2-octyne there was
no considerable bias for one position and we obtained a 1.2:1
mixture towards the less sterically hindered isomer. Using diphenyl
acetylene (entry 5) only traces of the product were obtained, which
points toward detrimental steric hindrance.
a-isomer was
b
SuzukieMiyaura cross coupling
a:b
a
A 10 mL crimp vial with a magnetic stir bar was charged with
the catalyst, boronic acid (1.1 mmol), aryl bromide (1 mmol), and
powdered base (2 mmol). The vial was capped with a PTFE-faced
butyl rubber septum and evacuated and filled with argon twice.
Distilled water was added through the septum with a syringe and
degased for 2 min. The resulting mixture was stirred at the given
temperature for the given time. For the determination of GCeMS
yields, the reaction mixture was extracted with dichloromethane
(3x 10 mL), dried over Na2SO4, and filtered through a thin pad of
silica gel. Dodecane (170 mg) was added as internal standard.
Table 6
Variation of substrate.a
Entry
1
Alkyne
Products
Hydrosilylation
2
3
A 10 mL screw top vial with a magnetic stir bar was charged
with the catalyst, alkyne (1.0 mmol), triethylsilane (128 mg,
1.1 mmol), and a 1:1 water:methanol mixture (2 mL) and the re-
action mixture was stirred at the indicated temperature. Subse-
quently, the aqueous phase was extracted with diethyl ether (3x
5 mL), dried over MgSO4 and the solvent was removed in vacuo. The
residue was purified by column chromatography on silica gel
(isohexane:ethyl acetate 4:1) and the selectivity was determined by
1H NMR spectroscopy.
4
Synthesis of the bisimidazolium salts
5
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-(1,10-
diimidazolium)}-methylene-dibromide (3)
0.60 g (4.1 mmol) of the methylene bridged bisimidazol 1 was
a
stirred with 2.20
g (9.7 mmol) of 1-[2-bromoethoxy-(2-
Reaction conditions: 1 mL H2O, 1 mL MeOH, 1 mmol alkyne, 1.1 mmol trie-
thylsilane, 1 mol % 7, 60 ꢁC, 16 h, under air, isolated yields.
methoxyethoxy)ethoxy]ethane in 10 mL of THF at 120 ꢁC for
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D. Munz et al. / Journal of Organometallic Chemistry xxx (2015) 1e6
5
72 h in an ACE pressure tube, during which a second phase formed.
Volatiles were removed in vacuo and the residue was dissolved in
acetonitrile. The product was precipitated by addition of diethyl
ether, washed with 2 mL of THF and dried in vacuo to afford the
very hygroscopic, colorless product (2.27 g, 93%). M.p. 103 ꢁC. 1H
with 1 mL of water. The crude product was further purified by
fractionate precipitation: The organic phase was reduced to 4 mL
and 10 mL of diethyl ether were added, upon which a brown pre-
cipitate formed. Solids were rapidly filtered off, and the solvent was
removed in vacuo. Subsequently it was again dissolved in 4 mL of
dichloromethane,10 mL of diethyl ether were added and the yellow
precipitate, which formed, was again filtered off. The solution was
layered with further 30 mL of diethyl ether and stored at 3 ꢁC. After
one week 160 mg (50%) of colorless, hygroscopic crystals had
formed, which were collected and dried in vacuo. M.p. 78 ꢁC. 1H
NMR (300 MHz, DMSO-d6):
d
¼ 9.43 (s, 2H, NCHN), 8.00 (s, 2H,
NCH), 7.86 (s, 2H, NCH), 6.68 (s, 2H, CH2), 4.42 (t, J ¼ 4.7 Hz, 4H,
CH2), 3.79 (t, 4.8 Hz, 4H, CH2), 3.60e3.35 (m, 16H, CH2), 3.24 (s, 6H,
CH3) ppm. 13C NMR NMR (151 MHz, DMSO-d6):
d
¼ 137.8 (CH),
123.6 (CH), 121.8 (CH), 71.2 (CH2), 69.5 (CH2), 69.5 (CH2), 69.4
(CH2), 67.81 (CH2), 58.1 (CH3), 57.8 (CH2), 49.2 (CH2) ppm. Anal.
Calcd. for C21H38Br2N4O6 (602.36): C 41.87, H 6.36, N 9.30% found C
41.86, H 6.59, N 9.41%.
NMR (300 MHz, CDCl3):
d
¼ 7.87 (s, 2H, CH), 7.57 (d, 2H, J ¼ 1.9 Hz,
NCH), 7.15 (d, 2H, J ¼ 1.9 Hz, NCH), 4.93 (m, 2H, CH2), 4.65 (m, 2H,
CH2), 4.07 (m, 2H, CH2), 3.87 (m, 2H, CH2), 3.64 (s, 8H, CH2), 3.62 (m,
4H, CH2), 3.56 (m, 4H, CH2), 3.38 (s, 6H, CH3) ppm. 13C NMR
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-[4,5-dibromo-
(1,10-diimidazolium)}-1,2-phenylene]-dibromide (4)
(75 MHz, CDCl3):
(Cipso), 125.1 (CH), 121.9 (CH), 71.9 (CH2), 70.3 (CH2), 70.3(CH2), 70.1
(CH2), 70.1 (CH2), 59.0 (CH3), 51.6 (CH2) ppm. Anal. Calcd. for
d
¼ 164.0 (NCN), 132.6 (Cipso), 131.3 (CH), 126.3
0.23 g (0.63 mmol) of the dibromo phenylene bridged bisimi-
dazol 2 was stirred with 0.31 g (1.37 mmol) of 1-[2-bromoethoxy-
C
26H36N4Br4O6Pd (926.62): C 33.70, H 3.92, N 6.05% found C 34.02,
(2-methoxyethoxy)ethoxy]ethane in 5 mL of acetonitrile at 120 ꢁ
C
H 3.97, N 6.19%. ESI-MS: m/z ¼ 847.0 [PdL$Br]þ.
for 72 h in an ACE pressure tube. The product was precipitated by
addition of 50 mL of diethyl ether at 3 ꢁC, washed with 5 mL ethyl
acetate and dried in vacuo to afford the very hygroscopic, colorless
product (0.35 g, 68%). M.p. 60 ꢁC. 1H NMR (300 MHz, DMSO-d6):
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-(1,10-
diimidazoline-2,20-diylidene)}-methylene-platinum(II)dibromide(7)
0.29 g (0.75 mmol) of platinum(II) acetylacetonate was stirred
with 0.45 g (0.75 mmol) of the bisimidazolium salt 3 in 10 mL of
DMSO for 3 h at 60 ꢁC, 3 h at 80 ꢁC and 3 h at 110 ꢁC. The solvent was
removed in vacuo at 60 ꢁC, the residue was dissolved in 5 mL of
dichloromethane and washed with 3 mL of water. The crude
product was further purified by fractionate precipitation: Diethyl
ether was added to the organic phase, until a brown precipitate
formed. Solids were rapidly filtered off, and the solution was
layered with further 50 mL of diethyl ether and stored at 3 ꢁC. After
one week 0.34 g (58%) of colorless, hygroscopic crystals had formed,
which were collected and dried in vacuo.
d
¼ 9.54 (s, 2H, NCHN), 8.52 (s, 2H, CH), 7,90 (s, 2H, CH), 7.75 (s, 2H,
CH), 4.43 (t, 4H, J ¼ 4.5 Hz, CH2), 3.81 (t, 4H, J ¼ 4.5 Hz, CH2), 3.57 (m,
4H, CH2), 3.51 (m, 8H, CH2), 3.40 (m, 4H, CH2), 3.20 (s, 6H, CH3) ppm.
13C NMR (75 MHz, DMSO-d6):
d
¼ 138.4 (NCHN), 132.6 (CH), 129.8
(Cipso), 127.2 (Cipso), 123.7 (CH), 122.7 (CH), 71.2 (CH2), 69.6 (CH2),
69.5(CH2), 67.9 (CH2), 67.0 (CH2), 58.1 (CH3), 49.4 (CH2) ppm. Anal.
Calcd. for C26H38N4O6Br4 (822.22): C 37.98, H 4.66, N 6.81% found C
37.54, H 4.40, N 6.68%.
Synthesis of the bis-NHC complexes
M.p. 78 ꢁC. 1H NMR (500 MHz, CDCl3):
NCH), 7.24 (d, J ¼ 1.9 Hz, 2H, NCH), 6.10 (dd, J ¼ 12.9 Hz, J ¼ 105.6 Hz
2H, CH2), 4.75e4.50 (m, 4H, CH2), 4.0e3.7 (m, 4H, CH2), 3.7e3.65
(m, 16H, CH2), 3.46 (s, 6H, CH3) ppm. 13C NMR (126 MHz, CDCl3):
d
¼ 7.31 (d, J ¼ 1.9 Hz, 2H,
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-(1,10-
diimidazoline-2,20-diylidene)}-methylene-palladium(II)dibromide (5)
0.45 g (2.0 mmol) of palladium acetate was stirred with 1.20 g
(2.0 mmol) of the bisimidazolium salt 3 in 15 mL of DMSO for 2 h at
40 ꢁC, 2 h at 60 ꢁC, 2 h 80 ꢁC, 2 h at 100 ꢁC and 2 h at 120 ꢁC. The
solvent was removed in vacuo at 60 ꢁC, the residue was dissolved in
4 mL of dichloromethane and it was washed with 5 mL of water.
The organic layer was dried with Na2SO4, reduced to 3 mL, and the
crude product was precipitated by addition of 40 mL of diethyl
ether at 3 ꢁC. After filtration, the solid was dissolved in 3 mL of
dichloromethane, layered with 40 mL of diethyl ether and stored at
3 ꢁC. After three days 1.34 g (95%) of colorless, hygroscopic crystals
had formed, which were collected and dried in vacuo. M.p. 81 ꢁC. 1H
d
¼ 145.1 (NCN), 122.5 (CH), 119.9 (CH), 71.9 (CH2), 70.8 (CH2), 70.4
(CH3), 70.3 (CH3), 70.1 (CH3), 60.4 (CH2), 59.1 (CH3), 50.6 (CH2) ppm.
ESI-MS: m/z ¼ 715.1 [PtL$Br]þ. Anal. Calcd. for C21H36Br2N4O6Pt
(795.43): C 31.71, H 4.56, N 7.04% found C 31.60, H 4.29, N 7.01%.
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-[4,5-dibromo-
(1,10-diimidazoline-2,20-diylidene)}-1,2-phenylene]platinum(II)
dibromide (8)
0.12 g (0.31 mmol) of platinum(II) acetylacetonate was stirred
with 250 mg (0.30 mmol) of the bisimidazolium salt 4 in 8 mL of
DMSO for 2 h at 40 ꢁC, 2 h at 60 ꢁC, 2 h at 80 ꢁC, 2 h at 80 ꢁC and
3.5 h at 120 ꢁC. The solvent was removed in vacuo at 60 ꢁC, the
residue was dissolved in 5 mL of THF and solids were filtered off.
The THF was removed under reduced pressure and the crude
product was further purified by fractionate precipitation: The
residue was dissolved in 3 mL of dichloromethane and diethyl
ether was added until the precipitation of a brown solid. The so-
lution was filtered off and the solvent was removed in vacuo. The
residue was dissolved in 3 mL of dichloromethane and precipitated
with diethyl ether. 0.09 g (29%) of a light brown solid was collected
and dried in vacuo.
NMR (500 MHz, CDCl3):
d
¼ 7.48 (d, J ¼ 1.9 Hz, 2H, NCH), 7.24 (d,
J ¼ 1.9 Hz, 2H, NCH), 6.39 (dd, J ¼ 13.24 Hz, J ¼ 17.02 Hz, 2H, CH2),
4.7e4.6 (m, 4H, CH2), 4.7e3.7 (m, 4H, CH2), 3.50e3.65 (m,16H, CH2),
3.37 (s, 6H, CH3) ppm. 13C NMR (126 MHz, CDCl3):
d
¼ 159.3 (NCN),
123.0 (CH), 121.2 (CH), 71.9 (CH2), 70.9 (CH2), 70.4 (CH2) 70.3 (CH2),
70.2 (CH2), 64.0 (CH2), 59.1 (CH3), 51.4 (CH2) ppm. Anal Calcd. for
C
21H36Br2N4O6Pd (706.76): C 35.69, H 5.13, N 7.93% found 35.77, H
5.08, N 8.04%. ESI-MS: m/z ¼ 627.1 [PdL$Br]þ.
3,30-Bis{1,10-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-[4,5-dibromo-
(1,10-diimidazoline-2,20-diylidene)}-1,2-phenylene]palladium(II)
dibromide (6)
94 mg (0.35 mmol) of palladium acetate was stirred with
250 mg (0.30 mmol) of the bisimidazolium salt 4 in 8 mL of DMSO
for 12 h at 40 ꢁC and 1 h at 60 ꢁC. The solvent was removed in vacuo
at 60 ꢁC, the residue was dissolved in 5 mL of THF and solids were
filtered off. The THF was removed under reduced pressure, the
residue was dissolved in 10 mL of dichloromethane and washed
M.p. 121 ꢁC. 1H NMR (300 MHz, CDCl3):
d
¼ 7.81 (s, 2H, CH), 7.54
(d, 2H, J ¼ 2.1 Hz, NCH), 7.07 (d, 2H, J ¼ 2.3 Hz, NCH), 4.89 (m, 2H,
CH2), 4.62 (m, 2H, CH2), 4.04 (m, 2H, CH2), 3.86 (m, 2H, CH2),
3.68e3.60 (m, 12H, CH2), 3.59e3.50 (m, 4H, CH2), 3.38 (s, 6H, CH3)
ppm. 13C NMR (75 MHz, CDCl3):
(CH), 126.0 (Cipso), 124.3 (CH), 120.8 (CH), 71.9 (CH2), 70.34 (CH2),
d
¼ 152.1 (NCN), 133.3 (Cipso), 131.0
70.30 (CH2), 70.13 (CH2), 70.10 (CH2), 59.0 (CH3), 50.9 (CH2) ppm.
Please cite this article in press as: D. Munz, et al., Journal of Organometallic Chemistry (2015), http://dx.doi.org/10.1016/
j.jorganchem.2015.03.027

6
D. Munz et al. / Journal of Organometallic Chemistry xxx (2015) 1e6
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Anal. Calcd. for C26H36Br4N4O6Pt (1015.29): C 30.76, H 3.57, N
5.52% found C 30.96, H 3.77, N 5.60%. ESI-MS: m/z ¼ 935.1
[PdL$Br]þ.
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Acknowledgments
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D. Munz thanks the Studienstiftung des Deutschen Volkes and
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Appendix A. Supplementary material
CCDC 1054106 and 1054107 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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Please cite this article in press as: D. Munz, et al., Journal of Organometallic Chemistry (2015), http://dx.doi.org/10.1016/
j.jorganchem.2015.03.027