Palladium(II)-acetylacetonato complexes with mesoionic carbenes: Synthesis, structures and their application in the Suzuki-Miyaura cross coupling reaction

Article abstract of DOI:10.3390/molecules21111561

A series of novel palladium(II) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(II) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(II) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

Full text of DOI:10.3390/molecules21111561

molecules
Article
Palladium(II)-Acetylacetonato Complexes with
Mesoionic Carbenes: Synthesis, Structures and
Their Application in the Suzuki-Miyaura Cross
Coupling Reaction
Lara Hettmanczyk, Bianca Schmid, Stephan Hohloch and Biprajit Sarkar *
Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36,
Berlin D-14195, Germany; Lara.Hettmanczyk@fu-berlin.de (L.H.); bianca.schmid@chem.tu-berlin.de (B.S.);
Stephan.Hohloch@web.de (S.H.)
* Correspondence: biprajit.sarkar@fu-berlin.de; Tel.: +49-30-838-50919; Fax: +49-30-838-53310
Academic Editors: James Crowley and Derek J. McPhee
Received: 20 September 2016; Accepted: 12 November 2016; Published: 17 November 2016
Abstract: A series of novel palladium(II) acetylacetonato complexes bearing mesoionic carbenes (MICs)
have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I
(MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (
1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-
1,2,3-triazol-5-ylidene ( ), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (
2
3); acac =
acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and
1
palladium(II) acetylacetonate is described herein. All complexes were characterized by H- and
13C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes
were characterized by single crystal X-ray crystallography confirming a square-planar coordination
geometry of the palladium(II) center. A delocalized bonding situation was observed within the
triazolylidene rings as well as for the acac ligand respectively. Complex
2 was found to be an efficient
pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with
phenylboronic acid.
Keywords: mesoionic carbenes; triazolylidenes; palladium; Suzuki-Miyaura cross coupling; click
chemistry; abnormal carbenes
1. Introduction
N-heterocyclic carbenes (NHCs) are a very attractive class of compounds. Because of their
easy accessibility and their high complex stability, these ligands have been used extensively in
organometallic chemistry in the last decades [
catalysis [ ] but also in other fields of chemistry such as material science [
chemistry [10 12] or metallosupramolecular chemistry [13 15]. A special kind of NHCs are mesoionic
carbene (MIC), which have received high popularity in recent years. Apart from imidazol-5-ylidenes,
1,2,3-triazol-5-ylidenes are the most noted MICs found in the literature [16 22]. Their precursors, the
1,2,3-triazoles, are easily accessible via the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC)
reaction [23 27]; subsequent alkylation results in the corresponding 1,2,3-triazolium salts, which can
be converted into mesoionic carbenes [ 22 28]. Metal complexes of 1,2,3-triazol-5-ylidenes are used
as catalysts in a variety of catalytic transformations [16 21]. For example, copper(I) complexes have
been found to be potent catalysts for the “click” reaction [29 34], while cyclization reactions can
be performed using gold(I) 1,2,3-triazol-5-ylidene complexes as catalysts [14 28 35 41]. Iridium(III)
complexes were shown to be efficient catalysts in water oxidation reactions [ 42 45] as well as other
1
–
6]. They are mainly applicable in homogeneous
7,
8
9,
10], bioorganometallic
–
–
–
–
6, ,
–
–
,
, –
9, –
oxidation reactions and transfer hydrogenations [46
–
51], which are also catalyzed by ruthenium(II)
Molecules 2016, 21, 1561; doi:10.3390/molecules21111561
www.mdpi.com/journal/molecules

Molecules 2016, 21, 1561
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and osmium(II) complexes [46
,47
,49
,
50,
52–58]. Furthermore, 1,2,3-triazol-5-ylidenes and their metal
complexes have also been investigated for their photochemical [59–64] and redox properties [60,65].
Especially palladium(II) 1,2,3-triazol-5-ylidene complexes obtained a lot of attention since the
discovery of 1,2,3-triazol-5-ylidene complexes [22]. Various types of palladium(II) complexes show
diversities in the coordination fashion and have different additional ligands (Figure 1) [22,38,65–82].
Almost all palladium(II) complexes were used as catalysts in a variety of cross coupling reactions. Mainly,
they are used as pre-catalysts in reactions like the Suzuki-Miyaura cross coupling reaction [68–75,80–83]
or α-arylation and α-methylation reactions [38,75,80].
R'
N
N
N
R
N
R''
N
X
X
R'
R
X
Pd
X
R''
X
X
N
X
N
R
Pd
R
N
N
N
N
R'
R'
Pd
Pd
R
N
R'
N
X
N
R''
N
R
N
R'
N
N
R''
Sakararaman, 2009 [69]
Fukuzawa, 2010 [68]
Sarkar, 2013 [72]
Albrecht, 2008 [22]
Hong, 2012 [82]
Sankararaman, 2009 [69]
Albrecht, 2011 [66]
Sarkar, 2013 [72]
N
N
N N
R
N
N
N
R
R
R
R'
N
O
O
Pd
Pd
Cl
R''
X
X
N
Pd
I
R'
N
Fukuzawa, 2012 [71]
Mendoza-Espinosa, 2015 [38]
X´
This work
Albrecht, 2012 [70]
Crudden, 2012 [67]
Hong 2012 [82]
Sarkar, 2015 [72]
Figure 1. Overview of selected palladium(II) complexes with 1,2,3-triazol-5-ylidene ligands.
We have been interested in the development of a novel coordination motif of palladium(II)
1,2,3-triazol-5-ylidenes complexes, in which
2-acetylacetonate is coordinated as additional chelating
κ
ligand to the palladium(II) center. For normal N-heterocyclic carbenes (NHC), these kind of
palladium(II) complexes, but with chlorido instead of iodido as ligand, were first reported in 2005 [84].
These NHC complexes showed high catalytic activity in the Buchwald–Hartwig amination and in the
arylation of α-ketones [85,86].
Herein we report on the synthesis of a series of novel palladium(II) acetylacetonato complexes
bearing mesoionic carbenes. The complexes have been characterized by 1H- and 13C-NMR
spectroscopy, mass spectrometry and single crystal X-ray crystallography. Their catalytic activity
in the Suzuki-Miyaura cross coupling reaction are presented as well.
2. Results and Discussion
2.1. Synthesis and Characterization of the Palladium(II) Complexes
The triazolium salts [HL1]I
[
70
,87], [HL2]I
[
68] and [HL3]I
[32] were synthesized according to
procedures reported in the literature. The complexes
1–3
were synthesized in a one-pot reaction
starting from the corresponding triazolium salts via direct metalation by use of [Pd(acac)2] under
inert conditions (Scheme 1). The route used here is similar as for the preparation of related nNHC
complexes [85], but for the triazolylidene complexes the iodides of the triazolium salts were used
because of synthetic ease. One of the acac ligands on [Pd(acac)2] acts as an internal base for the
deprotonation of the triazolium salts. The coordinated iodides on the palladium centers originate
from the corresponding triazolium salts (Scheme 1). The identity and purity of the compounds were

Molecules 2016, 21, 1561
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unambiguously proven by mass spectrometry, 1H- and 13C-NMR spectroscopy. All complexes were
isolated as yellow solids after precipitation or as yellow crystalline solids after recrystallization
from a mixture of dichloromethane and hexane. In contrast to the achieved moderate to good
yields of these MIC complexes, the yields obtained for the nNHC complexes have been nearly
quantitative [85], even under aerobic conditions [86]. The reason for this might be the higher acidity
of the imidazolium-2H [88–90] in comparison with the acidity of the triazolium-5H [19
,91]. While the
solubility of complexes is very high in THF, diethyl ether, toluene or chlorinated solvents, the
1–3
complexes were only moderately to sparingly soluble in hexane or pentane.
R'
N
N
N
N
Pd(acac)2
R'
N
N
O
O
Pd
1,4-Dioxane,
105 °C, 2 days
R
I
R
I
[HL1]I R = Ph, R' = Mes
[HL2]I R = R' = Mes
[HL3]I R = R' = Dipp
1 R = Ph, R' = Mes (60%)
2 R = R' = Mes (33%)
3 R = R' = Dipp (45%)
Ph = Phenyl, Mes = Mesityl, Dipp = 2,6-Diisopropylphenyl
Scheme 1. Synthesis of the palladium(II) MIC complexes.
The first indication for the formation of the palladium(II) MIC complexes in solution was the
disappearance of the triazolium-5H in the 1H-NMR spectra and the appearance of the single proton of
the bound acac ligand at 5.14, 5.12 and 5.13 ppm for
13C-NMR spectroscopy based on the signals of the carbene carbons at 151.1 and 150.2 ppm for
respectively. Unfortunately, the signal corresponding to the carbene carbon for was not resolved in its
1,
2
and
3
respectively. Further proof was given by
2
and
3
1
13C-NMR spectrum. Two signals of the quaternary carbons of the bound acac ligand was observable
for each complex in the range of 183–187 ppm (see Figures S1–S3). For the methyl-group of the acac
ligand two separate signals could be observed in the 1H-NMR as well as in the 13C-NMR spectra.
Additional characterization of the complexes by ESI-MS showed molecular peaks corresponding to
the molecular mass with loss of the iodide (see ESI).
2.2. Structural Characterization
The molecular structures of palladium(II) complexes
1 and 3 in crystal could be confirmed by
single crystal X-ray crystallography (Figure 2). Single crystals were grown by slow diffusion of hexane
onto a concentrated solution in dichloromethane. Crystallographic details for these complexes are
given in Table S1. Important bond length and angles are depicted in Table 1.
(a)
(b)
Figure 2. Perspective view of complexes
1
(
a) and
3
(
b). Ellipsoids are drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity.

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◦
Table 1. Selected bond length (Å) and angles ( ).
Atoms
1
3
C1-C2
C1-N3
C2-N1
N1-N2
N2-N3
Pd1-C1
Pd1-I1
Pd1-O1
Pd1-O2
O1-C23
O2-C21
C23-C22
C21-C22
1.37 (2)
1.36 (1)
1.37 (1)
1.32 (1)
1.32 (1)
1.98 (1)
2.561 (1)
2.027 (7)
2.060 (7)
1.27 (1)
1.27 (1)
1.39 (2)
1.42 (2)
106 (1)
103 (1)
114.9 (8)
112.5 (9)
103.1 (8)
127 (1)
127 (1)
125 (1)
90.4 (3)
89.1 (4)
177.1 (4)
175.3 (2)
88.8 (2)
91.9 (3)
46.7
1.387 (6)
1.376 (6)
1.354 (6)
1.304 (6)
1.351 (6)
1.957 (5)
2.560 (1)
2.054 (4)
2.046 (4)
1.264 (6)
1.281 (7)
1.398 (8)
1.379 (8)
107.5 (4)
103.3 (4)
112.0 (4)
111.9 (4)
105.4 (4)
127.0 (5)
126.8 (5)
127.0 (5)
88.9 (1)
88.7 (2)
178.4 (2)
177.4 (1)
90.5 (1)
91.9 (2)
86.0
C1-C2-N1
C2-C1-N3
C1-N3-N2
C2-N1-N2
N1-N2-N3
C23-C22-C21
O1-C23-C22
O2-C21-C22
C1-Pd1-I1
C1-Pd1-O1
C1-Pd1-O2
I1-Pd1-O1
I1-Pd1-O2
O1-Pd1-O2
trz-Ar (Ctrz) *
trz-Ar (Ntrz) *
trz–acac *
47.5
57.1
71.1
82.7
* Planes measured with the program Diamond 4.0.2; trz = triazolylidene.
Complex
1 crystallized as yellow needles in the orthorhombic space group Pcba, while complex 3
crystallized as yellow plates in the monoclinic space group P21/n (see Table S1). The C–C, C–N
and N–N bond lengths and the corresponding angles within the five-membered triazolylidene
rings indicate a delocalized bonding situation in those rings for both complexes (Table 1). The
aryl-substituents are twisted with respect to the triazolylidene ri◦ng. In complex
1 the dihedral angles
◦
between the aryl-substituents and the triazolylidene ring are 46.7 and 47.5 . The Dipp-substituents in
◦ ◦
complex
3 are almost perpendicular to the triazolyldiene rings with dihedral angles of 86.0 and 71.1 .
The large twists observed in the aforementioned case is likely due to the higher steric demand of the
Dipp substituents (Table 1). Within the acac ligand, the bond length and angles are also indicative of a
delocalized bonding situation.
Both palladium(II) MIC complexes display an almost square-planar coordination geometry, in
which the angles between palladium and the different ligands are in between 88.7(2)◦ and 91.9(3)◦.
The square-planar geometry is typical for palladium(II) centers with a d8 electronic configuration.
The related nNHC complex showed a slightly higher deviation from the perfect square-planar
geometry [84]. In complex
plane, while in complex
1
the triazolylidene ring is twisted about 57.1◦ with respect to the acac-Pd
◦
3
the triazolylidene ring is rotated about 82.7 resulting from the higher steric
demand of the Dipp substituents (Table 1).
The distances between the palladium atoms and the carbene carbon atoms are almost similar
in both complexes with 1.98 (1) Å and 1.957 (5) Å respectively, which is in accordance with other
palladium(II) triazolylidene complexes [65
,75,80,92]. The Pd-C bond length of 1.969 (2) Å for the
related nNHC complex lies in between the Pd-C distances of the MIC complexes reported here [84].

Molecules 2016, 21, 1561
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The Pd-I distances of 2.561 (1) Å and 2.560 (1) Å are a bit lower compared to Pd-I bond distances in
other palladium(II) MIC complexes [65 75 80 92]. While the acac ligand in is unsymmetrically bound
to the Pd center (2.027 (7) Å and 2.060 (7) Å) in the bonding situation was found to be more symmetric
,
,
,
1
3
with Pd-O1 and Pd-O2 being 2.054 (4) Å and 2.046 (4) Å respectively. For the related nNHC complex
the Pd-O bond length are shorter with 2.044 (1) Å and 2.036 (2) Å compared to the MIC complexes [84].
2.3. Catalysis
Palladium(II) NHC complexes in general are known to be effective catalysts in various
transformations and therefore they play a central role in organic chemistry [3,7,93,94]. Different bonds
such as C–X (X = halide) can be activated and new C–C or C–N bonds can be built. Also, many examples
for catalytically active palladium(II) MIC complexes in cross coupling reactions are known in the
literature. The main application was reported in Suzuki–Miyaura coupling reactions [68
–75
,80,81,83],
Mizoroki-Heck reactions [67] or Hiyama coupling reactions [77 78]. Other reactions like
,
α-arylation or
α-methylation [38,75,80] and the Buchwald–Hartwig reaction [79] were presented as well.
To gain an insight into the catalytic activity of our novel palladium(II) MIC complexes we
have chosen the Suzuki-Miyaura cross coupling and the Buchwald-Hartwig amination reactions as
examples and complexes
2 and 3 as pre-catalysts. Both complexes show good to excellent activities in
the Suzuki-Miyaura cross-coupling reaction between aryl bromides and chlorides with phenylboronic
acid (Scheme 2).
Scheme 2. Palladium(II) mesoionic carbene (MIC) complexes
2 and 3 as pre-catalysts in the Suzuki-
Miyaura cross coupling reaction.
For 4-bromobenzaldehyde, the reaction could be performed in the environment-friendly solvent
water at room temperature under air, using potassium carbonate as base. With a catalyst loading of
only 0.5 mol % of
2 a conversion of 82% could be obtained within 24 h. For complex 3, the conversion
was fairly similar at 78%. In the case of 4-chlororbenzaldehyde, which tends to be less easy to activate,
the reaction conditions had to be changed. Therefore, we used 1,4-dioxane as the solvent under an Ar
atmosphere, cesium carbonate as a base and the reaction temperature was increased to 80◦. Applying
these conditions, we were able to achieve a conversion of 78% within 24 h with catalyst loadings of
1 mol % of
2. For
3, the conversion was 56% under the same conditions. Additionally, we investigated
the Buchwald-Hartwig amination reaction with both
2
and as pre-catalysts under microwave
3
irradiation conditions. This was necessary as the Buchwald-Hartwig amination is intrinsically a
more difficult reaction to perform in comparison to the Suzuki-Miyaura cross-coupling reaction.
Unfortunately, with complex 2, no conversion was observed under the used conditions. With complex 3,
however, we were able to observe 30% conversions to product by using 4.4% catalyst loading under
only 15 min of microwave irradiation (Scheme 3). The investigation of these two classes of catalytic
reactions shows that the substituents on the triazolylidene ligands do have a direct or an indirect

Molecules 2016, 21, 1561
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influence on the catalytic activities of the resulting palladium complexes. In the Suzuki-Miyaura
cross-coupling reaction, the activity of the complexes presented here seems to be better than those
of the corresponding PEPPSI complexes containing triazolylidene ligands, in which case a catalyst
loading of 2.5 mol % (in comparison to 0.5 mol %) was necessary with similar substrates [80]. These
results indicate a positive effect of the use of the acac containing complexes for these kinds of catalysis.
Scheme 3. Palladium(II) MIC complex 3 as pre-catalyst in the Buchwald-Hartwig amination.
3. Materials and Methods
3.1. General Procedures, Materials and Instrumentations
The triazolium salts [HL1]I [70,87], [HL2]I [68] and [HL3]I [32] were prepared as described
previously in the literature. Commercially available chemicals were used as purchased, unless
otherwise noted. The solvents used for synthesis and catalysis were dried and distilled under inert
1
gas and degassed by common techniques prior to use, unless otherwise noticed. H- and 13C-NMR
spectra were recorded on a Jeol ECS 400 or Jeol ECZ 400R spectrometer (Joel, Munich, Germany) using
the chemical shift of the solvent as an internal standard. Multiplets are reported as follows: singlet (s),
duplet (d), triplet (t) quartet (q), quintet (quint), and combinations thereof. Mass spectrometry was
performed on an Agilent 6210 ESI-TOF (Agilent, Walbronn, Germany).
3.2. X-ray Crystallography
X-Ray data were collected on a Bruker Smart AXS diffractometer (Bruker, Karlsruhe, Germany).
Data were collected at 140(2) K (see Table S1) using graphite-monochromated Mo K
(λα = 0.71073 Å). The strategy for the data collection was evaluated by using the Smart software
(Bruker, AXS Inc., Madison, WI, USA). The data were collected by the standard ‘ scan techniques’ and
α radiation
ω
were scaled and reduced using Saint+ and SADABS software. The structures were solved by direct
methods using SHELXS-97 or SHELXS_2014/7 and refined by full matrix least-squares, refining on F2.
Non-hydrogen atoms were refined anisotropically [95–101]. If it is noted, bond length and angles were
measured with Diamond Crystal Version 4.0.2 (Crystal Impact GbR, Bonn, Germany) and Molecular
Structure Visualization Version 3.1 (Crystal Impact GbR). CCDC 965893 and 1015507 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk)
3.3. Synthesis
3.3.1. General Procedure for the Preparation of the Palladium(II) 1,2,3-Triazolylidene Complexes
1–3
The corresponding triazolium salt [HL1]I [HL2]I or [HL3]I (0.2 mmol, 1 eq.) and palladium(II)
,
acetylacetonate (60.9 mg, 0.2 mmol, 1 eq.) were dissolved in absolute 1,4-dioxane (3 mL) and stirred for
◦
48 h at 105 C. The reaction mixture was filtered over Celite and volatiles were removed under vacuum.
The residue was re-resolved in dichloromethane (2 mL) and precipitated with diethyl ether (50 mL)
to remove unreacted triazolium salt. The filtrate was reduced and precipitated again with hexane
(50 mL) to remove the unreacted Pd(acac)2, which is soluble in hexane. The yellow precipitate was
collected by filtration and washed with hexane. Recrystallization from a mixture of dichloromethane

Molecules 2016, 21, 1561
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and hexane (1:4) gave the complexes as crystalline yellow solids. Single crystals suitable for X-ray
diffraction analysis were obtained by slow diffusion of hexane onto a concentrated solution of the
complex in dichloromethane.
Palladium(II) complex
1
: The product was obtained as yellow solid in a yield of 60% (73.1 mg, 0.12 mmol).
1H-NMR (401 MHz, CDCl3, 23 ◦C)
δ
= 7.96–7.91 (m, 2H, Ar-H), 7.61–7.50 (m, 3H, Ar-H), 7.04–6.98 (m,
2H, Ar-H), 5.14 (s, 1H, CH of acac), 4.11 (s, 3H, NCH3), 2.37 (s, 3H, CH3), 2.29 (s, 3H, CH3), 2.16 (s, 3H,
CH3), 1.85 (s, 3H, CH3 of acac), 1.76 (s, 3H, CH3 of acac) ppm. 13C-NMR (101 MHz, CDCl3, 24 ◦C)
δ = 186.6, 183.6 (CO, acac), 145.6, 141.4, 140.5, 135.6, 135.3, 130.8, 130.1, 129.0, 127.3 (all Aryl-C), 99.1
(CH, acac), 37.8 (NCH3), 27.6, 26.3, 21.4 (all Alkyl-C) ppm. HRMS (ESI): calcd for [C23H26IN3O2Pd]
[M − I]+ m/z 482.1060, found 482.1065.
Palladium(II) complex
0.066 mmol). H-NMR (401 MHz, CDCl3, 22 C)
2
: The product was obtained as yellow solid in a yield of 33% (43.0 mg,
1
◦
δ = 7.01 (s, 4H, Aryl-H), 5.12 (s, 1H, CH of acac),
3.80 (s, 3H, NCH3), 2.37 (p-d, 6H, CH3), 2.27–2.19 (m, 12H, CH3), 1.81 (s, 3H, CH3 of acac), 1.76 (s, 3H,
CH3 of acac) ppm. 13C-NMR (126 MHz, CDCl3, 21 ◦C)
= 186.4, 183.2 (CO, acac), 151.1 (Carbene-C),
147.8, 144.5, 141.5, 140.5, 140.4, 139.1, 135.6, 135.5, 122.9 (all Aryl-C), 99.0 (CH, acac), 36.5 (NCH3), 29.8,
δ
27.6, 26.2, 21.5, 21.4 (all Alkyl-C) ppm. HRMS (ESI): calcd for [C26H32IN3O2Pd] [M
found 524.1518.
−
I]+ m/z 524.1529,
Palladium(II) complex
3
: The product was obtained as yellow solid in a yield of 45% (66.2 mg, 0.09 mmol).
1H-NMR (400 MHz, CDCl3, 20 ◦C)
δ
= 7.53 (t, J = 7.8 Hz, 2H, Aryl-H), 7.34 (d, J = 7.8 Hz, 4H, Aryl-H),
5.13 (s, 1H, CH of acac), 3.78 (s, 3H, NCH3), 2.99 (hept, J = 6.9 Hz, 2H, CH), 2.92–2.82 (m, 2H, CH), 1.86
(s, 3H, CH3 of acac), 1.81 (s, 3H, CH3 of acac), 1.36–1.32 (m, 12H, CH3), 1.07 (dd, J = 10.8, 6.9 Hz, 12H,
CH3) ppm. 13C-NMR (101 MHz, CDCl3, 22 ◦C)
δ = 186.0, 183.5 (CO, acac), 150.2 (Carbene-C), 146.1,
143.5, 143.3, 136.1, 131.5, 131.3, 124.2, 124.2, 123.4 (all Aryl-C), 98.8 (CH, acac), 37.5 (NCH3), 30.9, 29.1,
27.7, 27.5, 26.0, 25.1, 24.9, 22.9 (all Alkyl-C) ppm. HRMS (ESI): calcd for [C32H44IN3O2Pd] [M − I]+ m/z
608.2468, found 608.2471.
3.3.2. Suzuki-Miyaura Cross Coupling Reactions
The 4-bromobenzaldeyhde (92.5 mg, 0.5 mmol, 1 eq.), phenylboronic acid (73.2 mg, 0.6 mmol,
1.2 eq.) and potassium carbonate (103.9 mg, 0.75 mmol, 1.5 eq.) were mixed in the presence of
water (1.5 mL) under air. Then complexes 2 or 3 (0.5 mol %) was added. The mixture was stirred at
room temperature for 24 h. For the aryl-chloride, 4-chlorobenzaldeyhde (28.1.5 mg, 0.2 mmol, 1 eq.),
phenylboronic acid (20.3 mg, 0.24 mmol, 1.2 eq.) and cesium carbonate (130.3 mg, 0.4 mmol, 2 eq.) were
mixed in presence of absolute 1,4-dioxane (1 mL) under ◦nitrogen atmosphere. After that, complexes
2
or
3 (1 mol %) was added. The mixture was stirred at 80 C for 24 h. The crude reaction mixtures were
poured into dichloromethane respectively and extracted with water five times. Finally, the combined
aqueous layers were extracted one last time with dichloromethane. The combined organic layers were
dried over sodium sulfate, filtered and evaporated to dryness. Afterwards, yields were determined via
1H-NMR spectroscopy with the help of the aldehyde protons.
3.3.3. Buchwald-Hartwig Amination
4-Bromobenzene (34.2 mg, 0.2 mmol, 1 eq.), aniline (21.9
µL, 22.4 mg, 0.24 mmol, 1.2 eq.), NaOtBu
(23.1 mg, 0.24 mmol, 1.2 eq.) and the complexes or (4.4 mol %) were mixed in the presence of
2
3
absolute 1,4-dioxane (2 mL). The reaction mixture was stirred at 100 ◦C for 15 min in the microwave.
The reaction mixture was cooled to room temperature, and the solvent was removed under vacuum.
Afterwards, yields were determined via 1H-NMR spectroscopy with the help of the protons of the
methyl-group of the product with 1,3,5-tribromobenzene as internal standard.

Molecules 2016, 21, 1561
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4. Conclusions
In conclusion, we have presented here the first examples of three palladium(II) MIC complexes
of the type (MIC)Pd(acac)I. Additional to the MIC, the chelating bidentate ligand
2-acetylacetonate
κ
is coordinated to the palladium(II) center. The synthesis and characterization of all complexes by
1
mass spectrometry, H- and 13C-NMR spectroscopy is described herein. The molecular structure
in crystal was verified for two of the three palladium(II) complexes by single crystal X-ray
crystallography. Structural characterization revealed the expected square-planar coordination fashion
of the palladium(II) center. Due to the popular application of palladium(II) MIC complexes as
pre-catalysts in cross coupling reactions, we utilized one of our new complexes as a pre-catalyst
in the Suzuki–Miyaura cross coupling reaction. First attempts showed that these kind of complexes
are excellent pre-catalysts for the cross coupling of phenylboronic acid with an aryl-bromide as well
as an aryl-chloride. The inclusion of the acac ligand on the Pd-MIC unit opens up new possibilities
for the synthesis of novel palladium complexes with mesoionic carbene ligands. Changing the donor
properties of the acac ligand is conceivable, as is its substitution with other chelating anionic ligands.
Such new motifs might open up additional opportunities and perspectives in the chemistry of the
metal complexes of MIC ligands.
Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/21/
11/1561/s1.
Acknowledgments: The Deutsche Forschungsgemeinschaft /DFG) and the Fonds der Chemischen Industrie
(FCI) are kindly acknowledged for the financial support of this work. We thank Samir Patra for preliminary work
on the Buchwald-Hartwig amination reactions.
Author Contributions: L. Hettmanczyk and B. Sarkar designed the project. Experiments and data analysis was
performed by L. Hettmanczyk and B. Schmid. Crystal data were recorded and solved by S. Hohloch. Manuscript
was written by L. Hettmanczyk and B. Sarkar.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds 1–3 are available from the authors.
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