Copper and palladium complexes with N-heterocyclic carbene ligands functionalised with carboxylate groups

Article abstract of DOI:10.1016/j.jorganchem.2008.07.017

The new imidazolium salts functionalised with the trimethylsilyl ester group 1a-c, were easily obtained by quaternisation of alkyl- or aryl-imidazoles with trimethylsilyl bromoacetate. Salt 1a was isolated and fully characterised. It reacted with mesityl

Full text of DOI:10.1016/j.jorganchem.2008.07.017

Journal of Organometallic Chemistry 693 (2008) 3369–3374
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Copper and palladium complexes with N-heterocyclic carbene ligands
functionalised with carboxylate groups
*
Andreas A. Danopoulos , Pamela Cole, Steven P. Downing, David Pugh
School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2008
Received in revised form 8 July 2008
Accepted 15 July 2008
Available online 22 July 2008
The new imidazolium salts functionalised with the trimethylsilyl ester group 1a–c, were easily obtained
by quaternisation of alkyl- or aryl-imidazoles with trimethylsilyl bromoacetate. Salt 1a was isolated and
fully characterised. It reacted with mesityl copper (Cu5Mes5) under trimethylsilyl abstraction to form the
complex 2. Methanolysis of 1a–c gave good yields of the carboxylic acid functionalised imidazolium salts
3a–c. Deprotonation of the latter in liquid ammonia led to the zwitterionic imidazolium carboxylates 4a–
c. Reaction of 4a with (Cu5Mes5) gave solutions from which the insoluble polymeric 5a crystallised
slowly. Generation of the carboxylate-functionalised NHC in situ followed by reaction with Pd(OOCCH3)2
gave the new complex 6a in which the NHC-carboxylate ligand is chelate bidentate.
Keywords:
NHC
Carboxylate complexes
Palladium complexes
Copper complexes
Organometallic polymer
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Results and discussion
Ester, amide and carboxylate-functionalised N-heterocyclic car-
bene (NHC) ligands are interesting additions to the ever growing
class of mixed donor ligands containing NHC and other ‘classical’
donor groups [1]. The combination of the soft, neutral strongly
2.1. Synthesis of imidazolium salts
The quaternisation of alkyl- or aryl-imidazoles with
BrCH2COOSiMe3 proceeds in good yields in dioxane at 80–90 °C
(see Scheme 1). In one case the imidazolium salt [1a, R = DiPP]
was isolated as moisture sensitive colourless powder and charac-
terised by spectroscopic and analytical methods.
The imidazolium salts 1a–c, were converted directly to the car-
boxylic acid functionalised imidazolium salts 3a–c by methanoly-
sis in CH2Cl2 under mild conditions. The carboxylic acid
derivatives were air stable powders which were characterised
spectroscopically and analytically. The structure of 3a was also
determined crystallographically (see Fig. 1). Metric data for this
compound are listed in the caption and refinement parameters
are given in Table 1.
Preliminary studies show that the imidazolium salts 1a–c are
convenient starting materials for further derivatisation. For exam-
ple reaction with primary alkylamines leads cleanly to imidazoli-
um amides in good yields.
The presence of two acidic sites in 3a–c raised the question of
the optimum conditions for the selective deprotonation at the imi-
dazolium C2 and the carboxylic acid protons. The presence of the
r
-donating NHC and the hard carboxylate, ester or amide donors
may lead to new hemilabile metal complexes, or complexes with
high solubility in polar solvents; formation of polymetallic assem-
blies by virtue of the excellent bridging properties of the carboxyl-
ate group and the stabilisation of higher metal oxidation states are
further attractive features. Even though the electronically related
ester- and carboxylate-functionalised phosphines have already
been studied by various groups [2], amide, [–C(@O)NR2], function-
alised NHC complexes of Pd and Ni have only recently been re-
ported [3]. Methyl imidazolium carboxylic acids and carboxylates
have been studied as ionic liquids [4]. In this paper we report a
mild and general high yielding synthesis of the carboxylic acid/car-
boxylate-functionalised imidazolium salts as precursors to NHC
complexes. We also report the first examples of Pd(II) and Cu(I)
complexes in which the ligand is bidentate NHC-carboxylate
bound and bridging NHC and carboxylate bound giving rise to
polymers. The compounds described in this paper are shown in
Schemes 1–3.
enolisable protons a-to the carboxylic acid may further complicate
selective deprotonation methods. Attempts to deprotonate 3a–c
with one equivalent of KN(SiMe3)2 or LiNPri2 led to intractable
mixtures, possibly due to competing reactions at more than one
sites. Reactions with triethylamine led to the zwitterionic imidazo-
* Corresponding author.
E-mail address: ad1@soton.ac.uk (A.A. Danopoulos).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.017

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A.A. Danopoulos et al. / Journal of Organometallic Chemistry 693 (2008) 3369–3374
O
O
O
(i)
(ii)
(iii)
N
N
Br
Br
N
N
N
N
N
N
OSiMe3
R
R
OH
R
O
R
1a-c
4a-c
3a-c
Scheme 1. The synthesis of trimethylsilylcarboxylate 1a–c, carboxylic acid 3a–c and carboxylate 4a–c functionalised imidazolium salts (a: R = 2,6-di-isopropylphenyl, DiPP
b: R = mesityl, c: R = tert-butyl). Reagents and conditions: (i) BrCH2COOSiMe3, dioxane, 80 °C (ii) methanol, CH2Cl2; (iii) liq. NH3, then CH2Cl2.
DiPP
Br
N
Br
N
O Cu
Cu O
O
O
(i)
N
Cu
O Cu
Br
N
N
N
OSiMe3
DiPP
DiPP
Br
N
Br
1
2
DiPP
(ii)
O
O
O
N
N
DiPP
O
O
N
N
Cu
(iii)
Cu
O
N
O-
3a
DiPP
5a
Scheme 2. The synthesis of the copper complexes 2 and 5a. Reagents and conditions: (i) (CuMes)5 THF, RT; (ii) methanol, CH2Cl2; (iii) (CuMes)5.
by triethylammonium hydrobromide. Clean and selective deproto-
N
N
N
nation of the carboxylic acid was accomplished by suspending the
salts in liquid ammonia. After completion of the reaction and evap-
oration of the ammonia, the resulting colourless solid residue was
extracted with CH2Cl2 giving pure zwitterionic 4a–c after evapora-
tion of the CH2Cl2. In this case the ammonium bromide was com-
pletely insoluble in dichloromethane and can be easily removed by
filtration. The zwitterions were characterised by analytical and
spectroscopic methods. In addition 4a was characterised crystallo-
graphically (Fig. 2). Metric data for this compound are listed in the
caption and refinement parameters are given in Table 1.
DiPP
O
O
(i)
O- (ii)
O
Pd
N
N
O
O
DiPP
DiPP
N
6a
Scheme 3. The synthesis of the palladium complex 6a. Reagents and conditions: (i)
LiNPri2 in THF; (ii) 0.5 equiv. Pd(OOCCH3)2 in THF.
Reaction of the zwitterions 4a–c with one equivalent of base
[KN(SiMe3)2 or LiNPri2] gave intractable mixtures.
2.2. Complexes derived from imidazolium trimethylsilylcarboxylates
The presence of the trimethylsilyl protected carboxylic acid
tethered to imidazolium group seemed attractive for the controlled
synthesis of mononuclear and binuclear NHC complexes. However,
it proved impossible to form selectively complexes in which the
trimethylsilylcarboxylate functionality on the tether remained in-
tact. In all cases desilylation took place even under strictly anhy-
drous conditions.
Reaction of 1a with (CuMes)5 [5] in THF for short reaction times
followed by crystallisation by addition of ether gave moderate
yields of complex 2. Longer reaction times or work up or use of
chlorinated solvents led to insoluble polymers. Complex 2 was
characterised by elemental analysis and crystallography. Spectro-
scopic characterisation was hampered by its low solubility in com-
mon organic solvents. A diagram of the molecule is shown in Fig. 3;
metric data for the complex are listed in the caption and refine-
ment parameters are given in Table 1.
The complex comprises a tetranuclear copper cluster with four
bridging bromides and two bridging acetates; the latter are teth-
ered to non-coordinated imidazolium groups. Therefore, trimethyl-
silyl abstraction has taken place. The four copper atoms lie on the
Fig. 1. ORTEP representation of the structure of 3a showing 50% probability
ellipsoids. H atoms (except imidazolium C-2) are omitted for clarity. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: C(13)–N(1) =
1.339(3); C(13)–N(2) = 1.329(3); C(17)–O(1) = 1.206(3); C(17)–O(2) = 1.320(3);
N(2)–C(13)–N(1) = 108.6(2).
lium carboxylates (see Scheme 1) [4c], however, isolation of ana-
lytically pure samples was hampered by persistent contamination

A.A. Danopoulos et al. / Journal of Organometallic Chemistry 693 (2008) 3369–3374
3371
Table 1
Summary of the crystal data, data collection and refinement for compounds 3a, 4a, 2, 5a and 6a
3a
4a
2
5a
6a
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
17H23BrN2O2
C17H22N2O2
286.37
Monoclinic
P21/c
10.6375(6)
14.4475(11)
11.0317(8)
90
110.835(4)
90
C34H44Br4Cu4N4O4
1146.53
C18H23Cl2CuN2O2
433.82
Monoclinic
P21/c
13.0427(5)
10.2452(4)
14.6127(6)
90
92.874(2)
90
C18H26Cl2N2O2Pd0.50
426.52
Orthorhombic
Pbca
9.7822(3)
18.0772(8)
22.1400(9)
90
90
90
3915.1(3)
8
367.28
Monoclinic
Cc
13.8480(3)
17.2330(6)
7.6383(3)
90
92.497(2)
90
1821.09(10)
4
Triclinic
ꢀ
P1
8.7491(4)
8.9462(3)
14.5575(6)
74.445(2)
82.423(2)
65.198(2)
3978.8(2)
1
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
1584.54(19)
2
1950.17(13)
4
Z
T (K)
120(2)
9312
3866
0.0262
0.0277
0.0622
120(2)
19204
3620
0.1346
0.0702
0.1902
120(2)
20016
4595
0.0587
0.0393
0.0807
120(2)
23051
4462
0.0943
0.0575
0.1706
120(2)
22233
4505
0.0990
0.0661
0.1677
No. data
No. unique
Rint
Final R(|F|) for
Final R(F2)
complex[6a] and [Cu4(l l l
4-I)2( 2-I)2( 2-PP)2], PP = 2-diphenylphos-
phino-phosphinine)[6c] have been structurally characterised.
[Cu4(l
4-Cl)2( 2-Cl)2[ 2-(P5Ph5)]2] [6b] has also recently been iso-
l
l
lated and structurally characterised. Complex 2 is the first example
without phosphorus donor ligands that adopts this geometry.
2.3. NHC complexes of copper(I) and palladium (II)
Alkanolysis of (CuMes)5 by the zwitterion 4a under carefully
controlled conditions (see Section 3) gave yellow solutions, from
which the insoluble polymeric complex 5a was obtained. The char-
acterisation of 5a was carried out by analytical and crystallo-
graphic methods. A diagram of the polymeric repeat unit is
shown in Fig. 4; metric data are listed in the caption of the figure.
Fig. 5 gives a view of the polymeric chain in which the coordination
sphere of the copper atoms can be seen. Refinement parameters
are given in Table 1.
The Cu(I) centres in 5a adopt an almost linear geometry (ca
177.2°); they are coordinated to the C2NHC of one polymer repeat
unit and one oxygen atom of the
repeat unit. In this way a zig-zag polymeric organometallic chain
is obtained. The bond lengths Cu–CNHC (1.859(4) Å) and Cu–Oacetate
(1.864(3) Å) are very similar. Polymeric Cu(I) pyridine functional-
ised NHC complexes have been previously reported [7]. The lower
Fig. 2. ORTEP representation of the structure of 4a showing 50% probability
ellipsoids. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°)
with estimated standard deviations: C(5)–N(1) = 1.329(3); C(5)–N(2) = 1.336(3);
N(1)–C(5)–N(2) = 108.5(2).
j
1-carboxylate of the following
Fig. 3. ORTEP representation of the structure of
2 showing 50% probability
ellipsoids. H atoms (except imidazolium C-2) are omitted for clarity. Selected
bondlengths (Å) and angles (°) with estimated standard deviations: Cu(1)–
Cu(2)#1 = 2.5601(7);
Cu(1)–Cu(2) = 2.8298(7);
Cu(2)–Cu(2)#1 = 2.6425(10);
Br(1)–Cu(1) = 2.2931(7);
Br(1)–Cu(2)#1 = 2.5553(6);
Br(2)–Cu(2) = 2.4607(7);
Cu(1)–O(1) = 1.911(3); Cu(2)–O(2) = 2.022(3); Br(1)–Cu(1) = 2.2931(7); Br(1)–
Cu(2)#1 = 2.5553(6).
same plane in a rhomboidal arrangement bridged unsymmetrically
by
l
4-bromides. The Cu–Cu distances in the Cu4 plane are
Fig. 4. ORTEP representation of the structure of the repeat unit of the polymer 5a
showing 50% probability ellipsoids. H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (°) with estimated standard deviations: C(1)–N(2) =
1.356(5); C(1)–N(1) = 1.364(5); C(1)–Cu(1) = 1.859(4); O(1)–Cu(1) = 1.864(3);
N(2)–C(1)–N(1) = 103.2(3).
2.8298(7) Å (between the acetate bridged Cu atoms) and
2.5601(7) Å (between the Br bridged Cu atoms). Complexes con-
taining analogous Cu(I) halide clusters are very rare. A [Cu4(l4
-
I)2(
l
2-I)2(
l
2-dppm)2],
dppm = bis-diphenylphosphinomethane,

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A.A. Danopoulos et al. / Journal of Organometallic Chemistry 693 (2008) 3369–3374
Palladium carboxylates with monodentate or bidentate NHC li-
gands have been described especially with relevance to C–H activa-
tion [9]. Complex 6a is the first example where the NHC and the
carboxylate functional groups are part of the same chelate provid-
ing additional stabilisation.
3. Conclusions
The Cu(I) and Pd(II) complexes with the new carboxylate-func-
tionalised NHC ligands reveal two different coordination modes of
the ligand: chelating bidentate and bridging two different metal
centres. In the latter case organometallic linear polymeric copper
(I) structures have been observed. The presence of hard (COOÀ)
Fig. 5. View of the structure of the polymer 5a.
and a strong
r-donor (NHC) on the same coordination sphere
may favour the stabilisation of higher oxidation states of various
metals or promote hemilabile behaviour which are areas of future
investigation in our group.
4. Experimental
4.1. General methods
Elemental analyses were carried out by the microanalytical lab-
oratory at London Metropolitan University. All manipulations were
performed under nitrogen in a Braun glove box or using standard
Schlenk techniques, unless stated otherwise. Solvents were dried
using standard methods and distilled under nitrogen prior use.
The light petroleum used throughout had a b.p. 40–60 °C.
The starting materials BrCH2COOSiMe3, But-imidazole, mesityl-
imidazole, 2,6-diisopropylphenyl-imidazole and Cu5Mes5 were
prepared according to literature procedures [10,11,5]. NMR data
were recorded on Bruker AV-300 and DPX-400 spectrometers,
operating at 300 and 400 MHz (1H), respectively. The spectra were
referenced internally using the signal from the residual protio-sol-
vent (1H) or the signals of the solvent (13C).
4.1.1. 3-(2,6-Di-isopropylphenyl)-1-(O-trimethylsilyl-acetato)-
imidazolium bromide (1a)
A solution of O-trimethylsilyl-bromoacetate (2.75 g, 0.013 mol)
and 2,6-di-isopropylphenyl-imidazol (3 g, 0.013 mol) in dioxane
(50 mL) was heated at 80 °C for 8 h. After cooling the reaction mix-
ture to room temperature, the volatiles were removed under re-
duced pressure and the residue was washed with ether and dried
under vacuum to give the product as a colourless moisture sensi-
tive solid. Yield: 3.80 g, 67%. 1H NMR (CDCl3): d 10.35 (s, 1H, C2–
H imidazolium), 7.90 and 7.18 (s, 1H each, imidazolium backbone),
7.48 (t, 1H, DiPP), 7.32 (d, 2H, DiPP), 5.46 (s, 2H, CH2 linker), 2.35
[sept, 2H, CH(CH3)2], 1.12 and 1.23 [d, 6H each, CH(CH3)2], 0.32
[s, 9H, OSi(CH3)3]. 13C{1H} NMR (CDCl3): d 0.30 [OSi(CH3)3], 24.49
and 24.71 [CH(CH3)2], 29.21 [CH(CH3)2], 52.36 (CH2), 124.92,
125.25, 130.78, 132.45, 140.08 and 146.17 (all aromatic), 166.66
(C-2 imidazolium). Calc. for C20H31N2SiO2Br: C, 54.66; H, 7.11; N,
6.37. Found: C, 53.80; H, 6.95; N, 6.22%.
Fig. 6. ORTEP representation of the structure of 6a showing 50% probability
ellipsoids. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°)
with estimated standard deviations: Pd(1)–O(1) = 2.013(3); Pd(1)–C(1) = 2.014(5);
O(1)#1–Pd(1)–C(1)# 1 = 87.44(18); O(1)–Pd(1)–C(1)#1 = 92.56(18); O(1)#1–
Pd(1)–C(1) = 92.56(18).
solubility of the polymers obtained here may be due to longer
chain lengths. Once crystallised, the air and moisture sensitive 5a
could not be redissolved in dichloromethane and decomposed by
attempted dissolution in polar non-protic solvents.
The reaction of Pd(OAc)2 with two equivalents of the NHC li-
gand generated in situ at lower temperatures from 4a and one
equivalent LiNPri2 in THF gave after workup low to moderate yields
of 6a in the form of off-white crystals. The structure of 6a was con-
firmed crystallographically (see Fig. 6).
The complex comprises a square planar Pd centre with two j2
ligands arranged such as the NHC and carboxylate donors are
mutually trans. The Pd–CNHC (2.014(5) Å) and Pd–Oacetate
(2.013(3) Å) bond distances are within the expected range [8].
The six membered chelate rings adopt a boat conformation.
The solid-state structure of 6a is maintained in solutions in non-
polar solvents as suggested by the 1H NMR spectroscopy. The dia-
stereotopic isopropyl methyls of the DiPP substituent of the NHC
functional group appear as two doublets at d 1.05 and 1.27, while
4.1.2. General method for the synthesis of the carboxylic acid
functionalised imidazolium salts 3a–c
Under N2, trimethylsilyl bromoacetate (1 mmol) and dioxane
(ca. 20 mL) were added to a preweighted ampoule. The imidazole
(1 mmol) was then added to the solution, the ampoule was evacu-
ated partially, sealed and heated to 90 °C overnight. On cooling a
white or yellow precipitate emerged. This was dissolved in dichlo-
romethane (100 mL) and methanol (ca. 2 mL) was added to the
solution dropwise with stirring. After the completion of the addi-
tion and stirring for 0.5 h the volatiles were removed under re-
duced pressure leaving a creamy white solid, which was washed
the methine protons as one septet at d 2.32. The protons
a to the
coordinated carboxylates appear as a broad singlet at d 3.27.

A.A. Danopoulos et al. / Journal of Organometallic Chemistry 693 (2008) 3369–3374
3373
with ether and dried under vacuum to afford analytically and spec-
troscopically pure imidazolium bromides.
13C{1H} NMR (chloroform-d): 197.43, 172.97, 157.77, 149.81,
144.86, 140.24, 134.14, 50.25, 20.06, 16.66. IR (Nujol, cmÀ1):
1715 (C@O). MS (ES+) 244.3 (M+H)+.
4.1.3. [3-(2,6-Di-isopropylphenyl)-(1-acetic acid)]-imidazolium
bromide (3a)
4.1.9. [3-(tert-Butyl)-(1-a-acetate)]-imidazolium (4c)
Prepared as above from (2,6-di-isopropylphenyl)-imidazole.
Yield: 0.30 g, ca. 78%. Calc. for C17H23N2O2Br: C, 55.59; H, 7.11;
N, 7.63. Found: C, 55.50; H, 6.04; N, 7.59%. 1H NMR (DMSO-d6): d
9.63 (1H, s, imidazolium), 8.12 and 8.10 (1H each, s, imidazol back-
bone), 7.62 (t, J = 8.1 Hz, 1H, DiPP), 7.44 (d, J = 8.0 Hz, 2H, DiPP),
5.32 (s, 2H, CH2 linker), 2.29 (septet, J = 6.8 Hz, 2H, Pri), 1.14 (dd,
J = 6.9 Hz, 12H, Pri). 13C{1H} NMR (DMSO-d6): 166.92 (COO),
144.11 (aromatic), 138.27 (aromatic), 130.56 (aromatic),
129.53(aromatic), 123.62 (aromatic), 123.46 (aromatic), 49.27
(CH2), 27.06 (Pri), 22.84 (Pri), 22.74 (Pri). IR (Nujol, cmÀ1), 1735
(C@O). MS (ES+) 287.3 (MÀBr).
Yellow oil prepared as above from 3c. Yield 58%.
Calc. for C9H14N2O2: C, 59.32; H, 7.77; N, 15.37. Found: C, 59.43;
H, 7.32; N, 15.31. NMR (DMSO-d6): d 9.19 (1H, s, imidazolium),
7.87 and 7.62 (1H each, t, J = 1.8 Hz, imidazol backbone), 4.42 (s,
2H, CH2 linker), 1.57 (s, 9H, But). 13C{1H} NMR (DMSO-d6):
165.87, 134.59, 130.14, 125.34, 118.70, 52.76, 49.50, 29.09. IR (Nu-
jol, cmÀ1): 1621 (C@O). MS (ES+) 182.3 (M+H)+.
4.1.10. Synthesis of the copper cluster 2
In the glove box 0.30 g (0.71 mmol) of 1a and 0.25 g
(0.30 mmol) of (CuMes)5 were placed in a Schlenk tube and mixed.
To the solid mixture THF (30 mL) was added and stirred at room
temperature for 1 h. After this time the reaction mixture was lay-
ered with ether. Colourless air sensitive crystals appeared after 1
day that were collected and dried. The crystals decomposed in
chlorinared, protic or donor solvents which made the acquisition
of NMR data impossible. Characterisation was carried out analyti-
4.1.4. [3-(Mesityl)-(1-acetic acid)]-imidazolium bromide (3b)
Prepared as above from mesityl-imidazole. Yield: 0.22 g, 65%.
Calc. for C14H17N2O2Br: C, 51.70; H, 5.27; N, 8.61. Found: C,
51.37; H, 5.12; N, 8.07%. 1H NMR (DMSO-d6): d 9.51 (1H, s, imi-
dazolium), 8.07 and 7.94 (1H each, t, J = 2.1 Hz, imidazol back-
bone), 7.14 (2, 2H, mes), 5.29 (s, 2H, CH2 linker), 2.32 (s, 3H, p-
Me), 2.01 (s, 6H, o-Me). 13C{1H} NMR (DMSO-d6): 168.04 (COO),
140.46 (aromatic), 138.74 (aromatic), 134.38 (aromatic), 131.22
(aromatic), 129.24 (aromatic), 124.43 (aromatic), 50.27 (CH2),
20.56 (Me), 16.79 (Me). IR (Nujol, cmÀ1): 1718 (C@O). MS (ES+)
245.3 (MÀBr).
cally and crystallographically. Calc. for
C34H44N4O4Cu4Br4: C,
35.62; H, 3.87; N, 4.89. Found: C, 35.03; H, 3.55; N, 4.41.
4.1.11. Synthesis of the copper polymer 5a
In the glove box 0.30 g (1 mmol) of 4a and 0.19 g (0.20 mmol) of
(CuMes)5 were placed in a Schlenk tube and mixed. THF (50 mL)
was added and the solution became yellow to yellow-green within
1 h. At this stage the THF was removed under reduced pressure and
the solid residue was crystallised by layering dichloromethane
solutions with light petroleum. Longer THF contact times lead to
the precipitation of white powder and should be avoided. Once
the crystalline 5a was obtained it cannot be dissolved in any sol-
vent without decomposition, therefore the acquisition of NMR data
was not possible.
4.1.5. [3-(tert-Butyl)-(1-acetic acid)]-imidazolium bromide (3c)
Prepared as above from tert-butyl-imidazole. Yield: 0.15 g 56%.
Calc. for C14H17N2O2Br: C, 41.08; H, 5.76; N, 10.66. Found: C, 40.85;
H, 5.52; N, 10.10%. NMR (DMSO-d6): d 9.50 (1H, s, imidazolium),
8.08 and 7.83 (1H each, t, J = 1.8 Hz, imidazol backbone), 5.15 (s,
2H, CH2 linker), 1.58 (s, 9H, But). 13C{1H} NMR (DMSO-d6):
167.87 (COO), 135.52 (aromatic), 131.22 (aromatic), 123.92 (aro-
matic), 59.57 (CH2), 49.50 (But) 29.50 (But). IR (Nujol, cmÀ1):
1718 (C@O). MS (ES+) 245.3 (MÀBr).
Characterisation was carried out analytically and crystallo-
graphically. Calc. for C17N2H19O2Cu.CH2Cl2: C, 50.06; H, 4.90; N,
6.49. Found: C, 50.82; H, 5.15; N, 7.01%.
4.1.6. General synthesis of the zwitterions 4a–c
The imidazolium salt (1 mmol) was added to a flask and cooled
to À78 °C. Liquid ammonia (ca. 50 mL) was condensed into the
flask whilst stirring with a stir bar. After 15 min the flask was re-
moved from the cooling bath and the ammonia was allowed to
evaporate. The resulting residue was extracted into dichlorometh-
ane (3 Â 50 mL). The dichloromethane extracts were combined and
the volatiles were removed under vacuum. The solid residue was
dried azeotropically with toluene.
4.1.12. Synthesis of the palladium complex 6a
To a THF suspension of 4a (0.43 g, 1.5 mmol) at À78 °C was
added a pre-cooled solution of LiNPri2 in THF (0.16 g, 1.5 mmol)
followed by a solution of palladium acetate in the same solvent
(0.15 g, 0.67 mmol in 10 mL). The cold reaction mixture was al-
lowed to reach room temperature slowly and stirred overnight.
Evaporation of the volatiles under reduced pressure, extraction of
the solid residue in dichloromethane, filtration and layering with
ether gave off-white air stable crystals. Yield 35%. Calc. for
4.1.7. [3-(2,6-Di-isopropylphenyl)-(1-
a
-acetate)]-imidazolium (4a)
C36H52O4N4Pd: C, 50.67; H, 6.14; N, 6.57. Found: C, 50.02; H,
White solid prepared as above from 3a. Yield: 82%.
5.93; N, 6.22.
Calc. for C17H22N2O2: C, 71.30; H, 7.74; N, 9.78. Found: C, 71.17;
H, 7.32; N, 9.33%. NMR (DMSO-d6): d 9.37 (1H, s, imidazolium),
7.93 (2H each, s, imidazol backbone), 7.61 (t, J = 7.8 Hz, 1H, DiPP),
7.44 (d, J = 7.5 Hz, 2H, DiPP), 4.65 (s, 2H, CH2 linker), 2.50 (septet,
J = 1.8 Hz, 2H, Pri), 1.15 (d, J = 1.9 Hz, 12H, Pri). 13C{1H} NMR (chlo-
1H NMR (dichloromethane-d2): d 8.25 and 8.10 (1H each, s, imi-
dazol backbone), 7.65 (t, 1H, DiPP), 7.45 (d, 2H, DiPP), 3.27 (s, 2H,
CH2 linker), 2.32 (septet, 2H, Pri), 1.27 and 1.05 (dd, 12H, Pri).
4.2. Crystallography
roform-d):
d 238.98, 172.37, 146.01, 139.03, 132.00, 130.73,
124.87, 122.08, 28.84, 24.65, 12.52. IR (Nujol, cmÀ1), 1637 (C@O).
A summary of the crystal data, data collection and refinement
for compounds 3a, 4a, 2, 5a and 6a are given in Table 1.
MS (ES+) 287.3 (M+H)+.
All data sets were collected on a Enraf-Nonius Kappa CCD area
detector diffractometer with an FR591 rotating anode (Mo Ka radi-
ation) and an Oxford Cryosystems low temperature device operat-
4.1.8. [3-(Mesityl)-(1-a-acetate)]-imidazolium (4b)
White solid prepared as above from 3b. Yield: 42%.
Calc. for C14H16N2O2: C, 68.83; H, 6.60; N, 11.47. Found: C,
68.47; H, 6.23; N, 11.01%. 1H NMR (chloroform-d): d 9.59 (1H, s,
imidazolium), 7.15 (2H imidazol backbone), 6.92 (2, 2H, mes),
5.38 (s, 2H, CH2 linker), 2.27 (s, 3H, p-Me), 2.02 (s, 6H, o-Me).
ing in
x scanning mode with w and x scans to fill the Ewald
sphere. The programs used for control and integration were COLLECT
SCALEPACK, and DENZO [12]. The crystals were mounted on a glass fiber
with silicon grease from Fomblin vacuum oil. All solutions and
,

3374
A.A. Danopoulos et al. / Journal of Organometallic Chemistry 693 (2008) 3369–3374
(d) G.J.P. Britovsek, K.J. Cavell, M.J. Green, F. Gerhards, B.W. Skelton, A.H.
White, J. Organomet. Chem. 533 (1997) 201;
(e) H. Werner, J. Bank, B. Windmüller, O. Gevert, W. Wolfsberger, Helv. Chim.
Acta 84 (2001) 3162. and other references cited therein.
[3] (a) C.-Y. Liao, K.-T. Chan, J.-Y. Zeng, C.-H. Hu, C.-Y. Tu, H.M. Lee,
Organometallics 26 (2007) 1692;
refinements were performed using the WINGX package [13] and all
software packages within. All non-hydrogen atoms were refined
using anisotropic thermal parameters, and hydrogens were added
using a riding model.
(b) C.-Y. Liao, K.-T. Chan, Y.-C. Chang, C.-Y. Chen, C.-Y. Tu, C.-H. Hu, H.M. Lee,
Organometallics 26 (2007) 5826.
[4] (a) Z. Fei, W.H. Ang, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Chem. Eur. J. 12
(2006) 4014;
5. Supplementary material
CCDC 689233, 689234, 689235, 689236 and 68923 contain the
supplementary crystallographic data for 3a, 4a, 2, 5a and 6a. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(b) Z. Fei, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Inorg. Chem. 45 (2006)
6331;
(c) Z. Fei, D. Zhao, T.J. Geldbach, R. Scopelliti, P.J. Dyson, Chem. Eur. J. 10 (2004)
4886.
[5] S. Gambarotta, C. Floriani, A. Chiessi-Villa, C. Gaustini, Chem. Commun. (1983)
1156.
[6] (a) A. Camus, G. Nardin, L. Randaccio, Inorg. Chim. Acta 12 (1975) 23;
(b) M. Scherer, D. Stein, F. Breher, J. Geier, H. Schönberg, H. Grützmacher, Z.
Anorg. Allg. Chem. 631 (2005) 2770;
Acknowledgement
We thank Johnson-Matthey Plc for a loan of palladium salts.
(c) N. Mezailles, P. Le Floch, K. Washbisch, L. Ricard, F. Mathey, C.P. Kubiak, J.
Organomet. Chem. 541 (1997) 277.
[7] A.A. Danopoulos, G. Eastham, M.B. Hursthouse, S. Kleinhenz, M.E. Light, A.A.D.
Tulloch, Organometallics 20 (2001) 2027.
References
[8] A.A. Danopoulos, N. Stylianides, D. Pugh, F. Hancock, Organometallics 26
(2007) 5627.
[9] J.C. Slootweg, P. Chen, Organometallics 25 (2006) 5863. and references cited
therein.
[10] G.A. Gornowicz, J.W. Ryan, J. Org. Chem. 31 (1966) 3439.
[11] A.J. Arduengo III, F.P. Gentry Jr., P.K. Taverkere, H.E. Simons, US Patent,
6,177,575, 2001; (b) A.L. Johnson, US Patent, 3,637,731, 1972.
[12] R. Hooft, COLLECT, Nonius BV, 1997–2000; Z. Otwinoski, W. Minor, Methods
Enzymol. 276 (1997), 307.
[1] (a) For reviews covering some heteroatom functionalised NHC complexes see
for example: D. Bourissou, O. Guerret, F.P. Gabbaı, G. Bertrand, Chem. Rev. 100
(2000) 39;
(b) W.A. Herrmann, Angew. Chem., Int. Ed. 41 (2002) 1290;
(c) E. Peris, R.H. Crabtree, Coord. Chem. Rev. 248 (2004) 2239;
(d) D. Pugh, A.A. Danopoulos, Coord. Chem. Rev. 251 (2007) 610;
(e) A.T. Normand, K.J. Cavell, Eur. J. Inorg. Chem. (2008) 2781.
[2] (a) A. Bader, E. Lindner, Coord. Chem. Rev. 108 (1991) 27;
(b) E. Lindner, S. Pautz, M. Haustein, Coord. Chem. Rev. 155 (1996) 145;
(c) P. Braunstein, F. Naud, Angew. Chem., Int. Ed. 40 (2001) 680;
[13] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 83.