Structure, dynamics and catalytic activity of palladium(II) complexes with imidazole ligands

Article abstract of DOI:10.1016/j.ica.2010.08.037

Several palladium complexes of the type [Pd(im)₂Cl₂], [Pd(im)₃Cl]Cl, and [Pd(im)₄]Cl₂ (im = imidazole 1, 1-methylimidazole 2, 1,2-dimethylimidazole 3, 1-butylimidazole 4, 4a, 1-phenylimidazole 6, 1-phenylimidazoline 7, and 1-methylimidazoline 8) were prepared and structurally characterized. The square planar structure of two new complexes with the composition [Pd(im)₄]Cl₂ (2b, 4b) was confirmed by X-ray analysis. In solution, exchange of imidazole ligands leading to heteroleptic products was evidenced by ESI-MS studies. Two bis-ligated complexes, bearing 1-methylimidazole (2a) and 1-propoxymethylimidazole (5) ligands, were obtained in the reaction of palladium with imidazoles formed by deprotection of one nitrogen atom in the respective imidazolium halides. Catalytic Suzuki-Miyaura reactions were carried out using the obtained palladium complexes in isopropanol-water solution. High yields of the cross-coupling products were obtained at 40 and 60 °C when 2-bromotoluene, 4-bromotoluene, and 4-bromoanizole were used as substrates.

Full text of DOI:10.1016/j.ica.2010.08.037

Inorganica Chimica Acta 363 (2010) 4346–4354
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Structure, dynamics and catalytic activity of palladium(II) complexes
with imidazole ligands
⇑
M.S. Szulmanowicz, W. Zawartka, A. Gniewek, A.M. Trzeciak
University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie Str., 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 26 August 2010
Several palladium complexes of the type [Pd(im)2Cl2], [Pd(im)3Cl]Cl, and [Pd(im)4]Cl2 (im = imidazole 1,
1-methylimidazole 2, 1,2-dimethylimidazole 3, 1-butylimidazole 4, 4a, 1-phenylimidazole 6, 1-phenyl-
imidazoline 7, and 1-methylimidazoline 8) were prepared and structurally characterized. The square pla-
nar structure of two new complexes with the composition [Pd(im)4]Cl2 (2b, 4b) was confirmed by X-ray
analysis. In solution, exchange of imidazole ligands leading to heteroleptic products was evidenced by
ESI-MS studies. Two bis-ligated complexes, bearing 1-methylimidazole (2a) and 1-propoxymethylimi-
dazole (5) ligands, were obtained in the reaction of palladium with imidazoles formed by deprotection
of one nitrogen atom in the respective imidazolium halides. Catalytic Suzuki–Miyaura reactions were
carried out using the obtained palladium complexes in isopropanol–water solution. High yields of the
cross-coupling products were obtained at 40 and 60 °C when 2-bromotoluene, 4-bromotoluene, and
4-bromoanizole were used as substrates.
Dedicated to Achim Muller
Keywords:
Palladium
Imidazole
Imidazoline
ESI-MS studies
Suzuki–Miyaura reaction
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Surprisingly, despite broad application of palladium compounds
with N-heterocyclic carbenes [16–23] as catalysts of C–C bond
The imidazole ring presents a structural fragment that plays an
important role in many biological systems, enzymes, metallo-pro-
teins as well as in natural products and anticancer drugs [1]. It also
serves as a good ligand in various transition metal complexes [2].
Low toxicity, high stability in the presence of moisture and air, rel-
atively strong bonding to metals and the possibility of adjusting
electronic and steric properties by introducing sterically demand-
ing substituents at the N atom are important advantages of these
ligands in catalytic processes in comparison with the less environ-
mentally friendly and relatively easily degradable phosphines
[3–7].
What is a fascinating and unique feature of palladium is its
ability to form complexes of different composition with mono-
dentate imidazole ligands, containing 2, 3 or 4 imidazole ligands
bonded to palladium [3,8–15]. To date, relations between these
species have been reported only scarcely. For example, studies
of Pd(II) complexes with N-allylimidazole ligand have shown that
the [Pd(im)4]Cl2 complex, isolated and stable in solid state,
decomposed in room temperature to the [Pd(im)2Cl2] species
[13].
forming reactions, palladium complexes with N-coordinating
imidazole ligands have to date not been widely explored in this
field [24,25]. Palladium complexes bearing imidazole-based che-
lating ligands have been tested in the Heck reaction [26–28]. Good
yields have been obtained with imidazole-coordinated monoden-
tate Pd–NHC (NHC = N-heterocyclic carbene) complexes [29]. A
recyclable catalytic system for the Heck coupling has been con-
structed using a Pd(II) complex with a bis-imidazole ligand in an
ionic liquid as the reaction medium [30]. Palladium–imidazole
complexes formed in situ from [PdCl2(CH3CN)2] and imidazole li-
gands have effectively catalyzed the Suzuki–Miyaura reaction in
dioxane and in ionic liquids [8,31], and more recently palladium
complexes containing 2-arylbenzimidazole ligands have also been
successfully applied for biphenyl synthesis [32]. The imidazole
precursor [PdL3Cl]Cl (L = 1-methylimidazole) has been tested in
the same reaction in water as a solvent [8]. A palladium complex
with coordinated N-carbamoyl-substituted heterocyclic carbene
and 1-methylimidazole has catalyzed the Sonogashira coupling of
iodo- and bromoaryl derivatives with terminal alkynes [12].
Silica-supported palladium–imidazole complexes gave high cata-
lytic activity in Suzuki–Miyaura and Heck reactions [33].
The potential applicability of palladium complexes containing
imidazole ligands as catalyst of C–C bond forming reactions
prompted us to study their structure and reactivity with a special
emphasis to the lability of these complexes.
⇑
Corresponding author. Tel.: +48 71 3757253; fax: +48 71 3282348.
E-mail address: ania@wchuwr.chem.uni.wroc.pl (A.M. Trzeciak).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.037

M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
4347
2.2.2. Preparation of complexes [Pd(1-butylimidazole)2Cl2], 4a, [Pd(2-
phenylimidazole)2Cl2], 6, [Pd(2-phenylimidazoline)2Cl2], 7, and [Pd(2-
methylimidazoline)2Cl2], 8
2. Experimental
2.1. Methods and materials
CH3CN (5 cm3) was added to PdCl2 (0.18 g, 1 Â 10À3 mol), and
the mixture was heated for 30 min at 70 °C until the formation of
an orange precipitate of [PdCl2(CH3CN)2]. Next, the appropriate
imidazole (2 Â 10À3 mol) was added, and heating continued for
1.5 h. During that time, the product precipitated. After cooling
down to ambient temperature, the product was filtered off, washed
with CH3CN, and dried in vacuo.
C, H, and N analyses were carried out with a Perkin–Elmer 2400
CHN instrument. IR spectra were recorded on a Nicolet FTIR IM-
PACT 400 spectrophotometer, using KBr pellets. FIR spectra were
measured on a IFS 66/S Bruker spectrophotometer, using Nujol
mulls. NMR data (1H, 13C) were recorded on Bruker 500 or
600 MHz spectrometers. Mass spectrometric analyses were per-
formed using an Apex-Qe 7T instrument (Bruker Bremen Germany)
equipped with dual ESI source. Spectra were recorded using water
[Pd(1-butylimidazole)2Cl2], 4a (70% yield)
Anal. Calc. for C14H24N4Cl2Pd: C, 39.50; H, 5.68; N, 13.16; Found:
C, 39.9; H, 6.1; N, 13.3%. 1H NMR (600 MHz, CDCl3): d (ppm): 0.92
(3H, t, JH–H = 7.4 Hz, CH3); 1.31 (2H, sx, JH–H = 7.4 Hz, CH2); 1.71
(2H, CH2, q, JH–H = 7.4 Hz); 3.88 (2H, t, JH–H = 7.4 Hz, NCH2); 6.76
(1H, s, CH); 7.38 (1H, s, CH); 7.97(1H, s, CH).
solutions containing ca. 2.2 lM of palladium. The potential
between the spray needle and the orifice was set to 4.5 kV. The
product ions were subsequently analyzed by the ICR mass
analyzer. GC and GC–MS analyses of organic products were per-
formed using GC–MS instrument HP 5890 II with capillary column
ELITE-5MS and stationary phase 5% diphenylpolysiloxane, 95%
dimethylpolysiloxane.
13C NMR (150 MHz, CDCl3): d (ppm): 13.4 (CH3); 19.6 (CH2);
32.4 (CH2); 48.1 (NCH2); 118.3 (CH); 130.1 (CH); 138.6 (N2C).
[Pd(2-phenylimidazole)2Cl2], 6 [11] (50% yield)
Anal. Calc. for C18H16N4Cl2Pd: C, 46.49; H, 3.46; N, 12.03; Found:
C, 46.5; H, 3.5; N, 12.0%. IR(KBr, cmÀ1) 3210, 3110 (C–H), 2920,
1571 (C@N), 1479, 1117 (C–N), 788, 709 (C–H).
[PdCl2(CH3CN)2] [31] was obtained according to reported
procedure.
Imidazoles were purchased from Aldrich and used without fur-
ther purification.
FIR(nujol, cmÀ1) 326, 355 (Pd–Cl), 280 (Pd–N).
1H NMR (600 MHz, DMSO-d6): d (ppm): 7.38 (2H, s, CH); 7.58
(1H, m, CH); 7.61 (2H, m, CH); 8.71 (2H, d, JH–H = 7.3 Hz, CH);
13.1 (1H, s, NH).
2.2. Synthesis
[Pd(2-phenylimidazoline)2Cl2], 7 [11] (70% yield)
2.2.1. Preparation of complexes [Pd(imidazole)3Cl]Cl 1, [Pd(1-
methylimidazole)3Cl]Cl 2, and [Pd(1-butylimidazole)3Cl]Cl, 4
To a solution of [PdCl2(CH3CN)2] (0.026 g, 0.1 Â 10À3 mol) in
2 cm3 of hot CH3CN, the appropriate imidazole was added
(0.4 Â 10À3 mol). The solution was stirred for 15 min while the
color changed from orange to pale yellow or white. After
cooling down to ambient temperature, a precipitate was formed,
which was filtered off, washed with diethyl ether, and dried in
vacuo.
Anal. Calc. for C18H20N4Cl2Pd: C, 46.03; H, 4.29; N, 11.93; Found:
C, 45.6; H, 4.0; N, 11.7%.
1H NMR (500 MHz, DMSO-d6): d (ppm): 3.51 (2H, t, CH2, JH–
H = 10.3 Hz); 3.87 (2H, t, CH2, JH–H = 10.7 Hz); 7.53 (2H, t, CH, JH–
H = 7.9 Hz); 7.63 (1H, t, CH, JH–H = 7.4 Hz); 7.96 (1H, s, NH); 8.50
(2H, d, CH).
13C NMR (100 MHz, DMSO-d6): d (ppm): 42.6 (NCH2); 55.3
(NCH2); 128.2 (CH); 128.5 (CH); 131.4 (CH); 166.0 (N2C).
[Pd(2-methylimidazoline)2Cl2], 8 (35% yield)
[Pd(imidazole)3Cl]Cl, 1: (70% yield)
Anal. Calc. for C8H16Cl2N4Pd: C, 27.81; H, 4.67; N, 16.21; Found:
C, 27.16; H, 4.37; N, 15.87%.
Anal. Calc. for C9H12N6Cl2Pd: C, 28.33; H, 3.17; N, 22.03; Found:
C, 28.3; H, 3.0; N, 21.8%. IR(KBr, cmÀ1) 3138 (N–H), 2910, 2848, and
1545 (C@N), 1500, 1427, 1341, and 1078 (C–N), 860, 800, 749, 663,
1H NMR (500 MHz, DMSO-d6): d (ppm): 2.24 (3H, s, CH3); 3.33
(2H, t, CH); 3.54 (2H, t, CH); 7.59 (1H, s, NH).
and 605
m(C–H).
13C NMR(100 MHz, DMSO-d6): d (ppm): 15.6 (CH3); 42.82
(CH2); 53.57 (CH2); 166.2 (N2C).
FIR(nujol, cmÀ1) 329 (Pd–Cl), 248, 260 (Pd–N).
1H NMR (600 MHz, D2O): d (ppm): 6.73 (1H, NH); 7.11 (2H, s,
CH); 7.90 (1H, s, N2CH). 13C NMR (150 MHz, D2O): d (ppm):
127.7 (CH); 137.9 (N2C).
2.2.3. Preparation of complexes [Pd(1,2-dimethylimidazole)4]Cl2,
3, [Pd(1-methylimidazole)2Cl2], 2a
[Pd(1-methylimidazole)3Cl]Cl, 2 [8] (70% yield)
Anal. Calc. for C12H18N6Cl2Pd: C, 34.02; H, 4.28; N, 19.84; Found:
C, 34.7; H, 4.8; N, 19.8%. IR(KBr, cmÀ1) 2993, 2927, 1420, 1354, and
1032 (C–N)
C2H5OH (6 ml) and imidazole (2 Â 10À3 mol) were added to
PdCl2 (0.18 g, 1 Â 10À3 mol), and the solution was heated for 3 h
at 70 °C. The product precipitated after cooling down to ambient
temperature was filtered off, washed with C2H5OH, and dried in
vacuo.
FIR(nujol, cmÀ1) 341 (Pd–Cl), 256 (Pd–N). 1H NMR (600 MHz,
D2O): d (ppm): 3.60 (3H, s, NCH3); 6.67 (1H, d, JH–H = 1.6 Hz, CH);
7.00 (1H, d, JH–H = 1.6 Hz, CH); 7.53 (1H, s, N2CH).
13C NMR (150 MHz, D2O): d (ppm): 34.2 (NCH3); 122.3 (CH);
127.9 (CH); 139.0 (N2C).
[Pd(1,2-dimethylimidazole)4]Cl2, 3 (50% yield)
Anal. Calc. for C20H32N8Cl2Pd: C, 42.75; H, 5.74; N, 19.14. Found:
C, 42.7; H, 5.7; N, 19.7%.
[Pd(1-butylimidazole)3Cl]Cl, 4
1H NMR (600 MHz, D2O): d (ppm): 2.23 (3H, s, CH3); 3.44 (3H, s,
NCH3); 6.65 (1H, d, JH–H = 1.6 Hz); 6.88 (1H, d, JH–H = 1.6 Hz).
13C NMR (150 MHz, D2O): d (ppm): 12.0 (CH3); 33.6 (NCH3);
122.5 (CH); 126.1 (CH); 146.7 (N2C).
Anal. Calc. for C21H36N6Cl2Pd: C, 45.87; H, 6.60; N, 15.28. Found:
C, 46.2; H, 7.0; N, 15.4%. IR(KBr, cmÀ1) 3131 (C–H), 2973, 2888, and
1630 (C@C), 1525 (C@N), 1479, 1249, and 1111 (C–N), 847, 769,
and 657 (C–H).
[Pd(1-methylimidazole)2Cl2], 2a (70% yield)
FIR(nujol, cmÀ1) 362 (Pd–Cl), 261 (Pd–N). 1H NMR (600 MHz,
CDCl3): d (ppm): 0.93 (3H, t, CH3, JH–H=7.3 Hz); 1.32 (2H, sx, CH2,
JH–H = 7.5 Hz); 1.73 (2H, qi, CH2); 3.89 (2H, t, NCH2, JH–H = 7.1 Hz);
6.77 (2H, d, NCH); 7.39 (2H, d, NCH); 7.98 (1H, s, N2CH).
Anal. Calc. for C8H12N4Cl2Pd: C, 28.13; H, 3.54; N, 16.40. Found:
C, 28.4; H, 3.2; N, 15.9%.
1H NMR (600 MHz, CDCl3): d (ppm): 3.66 (3H, s, NCH3); 6.74
(1H, d, JHH = 1.7 Hz, CH); 7.38 (1H, d, JHH = 1.7 Hz, CH); 7.95 (1H,
s, N2CH).
13C NMR (150 MHz, CDCl3):
d (ppm): 13.8 (CH3); 20.0
(CH2); 32.9 (CH2); 48.1 (NCH2); 118.3 (CH); 130.1 (CH); 138.6
(N2CH).
13C NMR (150 MHz, D2O): d (ppm): 34.6 (NCH3); 119.6 (CH);
130.4 (CH); 139.4 (N2C).

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2.2.4. Preparation of complex [Pd(1-propoxymethylimidazole)2Cl2], 5
0.49 g (2 Â 10À3 mol) of [dmiop]Cl (dmiop = 1,3-di-propoxyme-
thylimidazolium chloride) was dissolved in THF (15 cm3), and
Pd(OAc)2 (0.22 g, 1 Â 10À3 mol) was added to this solution. The
resulting mixture was heated at 70 °C with magnetic stirring while
the color changed from red to orange. After 3 h, the mixture was
cooled to ambient temperature and condensed in vacuo. The ob-
tained residue was dissolved in CH2Cl2 and washed three times
with water (3 Â 15 cm3) to remove unreacted ionic liquid. The or-
ganic phase was evaporated giving 70% of 5.
tions (performed with CrysAlis Pro [35]) were applied for all of
the compounds. C, N, O, Cl, and Pd atoms were refined anisotropi-
cally. The carbon-bonded H atoms were positioned geometrically
and refined isotropically using a riding model. Crystal data and
structure refinement parameters are summarized in Table 5. The
molecular structure plots (Fig. 1–4) were prepared using the
ORTEP-3 program [36].
2.4. Suzuki–Miyaura reaction procedure
Anal. Calc. for C14H24N4O2Cl2Pd: C, 36.74; H, 5.29, N, 12.24.
Found: C, 36.8; H, 5.2; N, 12.1%.
The S–M reaction was carried out in a 50 cm3 Schlenk tube in an
air atmosphere. The solid substrates: base, KOH (0.117 g,
2.0 Â 10À3 mol), phenylboronic acid (0.133 g, 1.1 Â 10À3 mol) and
the palladium complex (1 Â 10À5 mol) were weighed and placed
in the Schlenk tube. Next, 5 cm3 of the solvent (isopropanol–water,
1:1) and 2-bromotoluene (0.118 cm3, 1 Â 10À3 mol) or 4-bromo-
toluene (0.118 cm3, 1 Â 10À3 mol) or 4-bromoanizole (0.125 cm3,
1 Â 10À3 mol) were added and the Schlenk tube was closed with
a rubber plug, and the reaction mixture was stirred at 40 °C. After
1H NMR (600 MHz, CDCl3): d (ppm): 0.96 (3H, t, JH–H = 7.1 Hz,
CH3); 1.64 (2H, sx, JH–H = 7.1 Hz, CH2); 3.42 (2H, t, JH–H = 7.1 Hz,
OCH2); 5.32 (2H, s, NCH2); 7.02 (1H, s, CH); 7.56 (1H, s, CH); 8.56
(1H, s, N2CH).
13C NMR (150 MHz, CDCl3): d (ppm): 10.3 (CH3); 22.4 (CH2);
71.3 (OCH2); 77.6 (NCH2); 118.0 (CH); 130.8 (CH); 139.1 (N2CH).
2.2.5. Preparation of single crystals of complexes [Pd(1-methyl-
imidazole)2Cl2] 2a, [Pd(1-methylimidazole)4]Cl2 2b, and
[Pd(1-butylimidazole)4]Cl2 4b
C3
i
Cl
Single crystals of 2a were obtained in the following way:
0.0458 g (1.40 Â 10À4 mol) of K2PdCl4 and 0.313 g (1.64 Â
10À3 mol) of 1-methyl-3-propoxymethylimidazolium chloride
were added to a mixture of acetonitrile (1 cm3) and toluene
(0.5 cm3). Next, the reagents were sealed in a Schlenk tube under
a nitrogen atmosphere and placed in a thermostated oil bath
heated to 130 °C. After 1.5 h the mixture was cooled to the ambient
temperature. Twenty-four hour later small needle-shaped crystals
appeared in the bottom (orange) phase of the liquid.
Single crystals of 2b were obtained by slow evaporation of the
CH2Cl2 solution of 2.
N2
C2
N1
C4
O
C1
C5
Pd
C6
Cl
C7
Fig. 2. The molecular structure and atom numbering scheme of trans-dichloro-
bis(1-propoxymethylimidazole-
N3)palladium(II), compound 5. Displacement
j
ellipsoids are drawn at the 30% probability level and H atoms are shown as small
spheres of arbitrary radii. Unlabeled atoms are symmetrically dependent via an
inversion center [symmetry code: (i) 1 À x, 1 À y, 1 À z].
Single crystals of 4b were obtained by slow evaporation of
CH2Cl2 solution of 4.
2.3. Crystal structure determination of 2a, 2b, 5, and 4b
C30
Single crystals of compounds 2a, 5 and 4b suitable for X-ray
measurements were mounted on glass fibers in silicone grease,
cooled to 100 K in a nitrogen gas stream, and the diffraction data
was collected on a Kuma KM-4 CCD diffractometer with graphite
Cl31
Cl32
Cl
monochromated Mo Ka radiation (k = 0.71073 Å). The single crys-
tal of compound 2b was placed inside a glass tube, sealed under
nitrogen atmosphere and the diffraction data was collected at
room temperature. Subsequently the crystal structures were
solved using direct methods and developed by full least-squares
refinement on F2. Structural solution and refinement was carried
out using SHELX set of programs [34]. Analytical absorption correc-
C21
N22
N21
Pd
C24
C22
C23
C13
N11
C11
C3
i
Cl
C2
N1
N2
C4
C12
C1
N12
C14
Pd
Cl
Fig. 3. The molecular structure and atom numbering scheme of tetrakis(1-methyl-
imidazole-
N3)palladium(II) dichloride bis(dichloromethane) solvate, compound
2b. 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 via an inversion center. Disordered methyl groups are represented by
dashed lines.
Fig. 1. The molecular structure and atom numbering scheme of trans-dichloro-
bis(1-methylimidazole-
N3)palladium(II), compound 2a. 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 via an inversion
center [symmetry code: (i) 1 À x, 1 À y, 1 À z].
j
j

M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
4349
1 h, the reactor was cooled down and the organic products were
extracted with 10 cm3 of n-hexane (15 min with stirring) and GC
analyzed with dodecane as a standard (0.076 cm3, 3.3 Â 10À4 mol).
2.5. Preparation of samples for MS studies
A 10-fold molar excess of imidazole was added to 0.001 g of the
palladium complex dissolved in water, and the solution was kept
for 10 min at room temperature. Next, a 10
lected and diluted with 1.5 cm3 of water. Of the resulting solution
(250 l) was used for measurement.
ll sample was col-
Cl
l
For studies of ligand exchange between complexes 5 and 4a, a
sample was prepared in CH3CN. Solutions of both complexes
(0.001 Â 10À3 mol) were prepared separately in CH3CN (1 cm3)
C21
N22
N21
Pd
C24
and next mixed at ambient temperature. Next, a 10 ll sample
was collected and diluted with 1.5 cm3 of CH3CN. Of the resulting
solution (250 l) was used for measurement.
C22
C23
C25A
C26A
C25B
C26B
l
N11
C13
C12
3. Results and discussion
C11
C14
C27A
C27B
3.1. Synthesis and characterization of palladium–imidazole complexes
N12
Two slightly different synthetic approaches, based on
PdCl2(CH3CN)2 or PdCl2 as starting materials, were used to prepare
palladium complexes with imidazole ligands. Complexes 2 [8], 6
[11], and 7 [11] had been obtained before by other procedures.
According to elemental analysis, all reactions led selectively to only
one product, Pd(im)xCl2, with a composition dependent on the
steric hindrance of the imidazole ligand. Thus, the [imidazole]:[Pd]
ratio was 2 for 2-phenylimidazole (6) and for both the imidazoline
complexes (7, 8), 3 for imidazole (1), 1-methylimidazole (2), and
1-butylimidazole (4) (Table 1), and 4 only for the palladium com-
plex with 1,2-dimethylimidazole (3). Application of C2H5OH as
C15
O30
C17
C16
Fig. 4. The molecular structure and atom numbering scheme of tetrakis(1-
buthylimidazole-
N3)palladium(II) dichloride dihydrate, compound 4b. Displace-
ment ellipsoids are drawn at the 30% probability level and H atoms are shown as
small spheres of arbitrary radii. Disordered parts with lower occupancy (20%) are
represented by dashed lines. Unlabeled atoms are symmetrically dependent via an
inversion center.
j
Table 1
Composition and ESI-MS data of palladium–imidazole complexes.
Imidazole
No
Composition according to
elemental analysis
X-ray
ESI-MS (m/z)
[Pd(im)2Cl]+
277
[Pd(im)3Cl]+
345
[Pd(im)2]+
1
[Pd(im)3Cl]Cl
[Pd(met)3Cl]Cl
HN
N
2
2a
2b
305
305
387
429
270
298
[Pd(met)2Cl2]
[Pd(met)4]Cl2
N
N
N
H3C
H3C
3
[Pd(dim)4]Cl2
333
N
CH3
4
4a
4b
[Pd(but)3Cl]Cl
[Pd(but)2Cl2]
389
389
513
513
354
N
N
N
N
[Pd(but)4]Cl2
C4H9
5
[Pd(prop)2Cl2]
[Pd(phen)2Cl2]
[Pd(prop)2Cl2]
421
429
561
574
386
395
CH2OCH2CH2CH3
6
N
N
H
7
8
[Pd(phenim)2Cl2]
[Pd(metim)2Cl2]
433
309
579
393
397
273
N
N
H
HN
N
CH3

4350
M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
the solvent and PdCl2 as the palladium source resulted in the
preparation of the bis-ligated complex 2a, with the composition
[Pd(1-methylimidazole)2Cl2], regardless of the ligand excess used.
Surprisingly, the same synthetic procedure applied for 1,2-dimeth-
ylimidazole led to the formation of the tetra-ligated complex 3,
identical with that obtained from CH3CN solution. Unexpectedly,
recrystallization of the complexes containing three imidazole li-
gands bonded to palladium, namely 2 and 4, produced crystals of
palladium dichlorides containing four imidazole ligands in the first
coordination sphere, 2b and 4b. Consequently, for 2-methylimid-
azole and 2-butylimidazole, it was possible to isolate three differ-
ent products, whereas for 2-phenylimidazole, regardless of the
conditions applied, the same product always formed, containing
two imidazole ligands, as also reported by other authors [11].
Two complexes of the type [Pd(im)2Cl2] were isolated in more
unusual reactions, in which imidazoles originated from imidazoli-
um halides. When K2[PdCl4] was heated for 1.5 h at 130 °C with 1-
methyl-3-propoxymethylimidazolium chloride and the resulting
solution was cooled to ambient temperature, crystals of 2a were
isolated (Scheme 1). The structure of this product was confirmed
by X-ray analysis.
The other unexpected formation of a palladium–imidazole com-
plex consisted in the reaction of Pd(OAc)2 with 1,3-dipropoxyme-
thylimidazolium chloride. This reaction, performed in THF at
70 °C, should produce the palladium–carbene complex
[Pd(NHC)2Cl2] as the main product. However, a bis-imidazole spe-
cies, [Pd(1-propoxymethylimidazole)2Cl2], 5, was formed instead
(Scheme 2).
It is worth noting that in both cases the same propoxymethyl-
ene group from the imidazolium cation was lost, which may sug-
gest that the N–C bond can be split with relative ease in the
presence of palladium. The mechanism of nitrogen deprotection
is not clear; however, one can postulate a nucleophilic attack of
OAcÀ on the CH2 next to the positive charge as a first step of this
transformation.
Very few similar transformations have been reported to date in
the literature. For example, Dupont and co-workers reported the
formation of a [Pd(im)2Cl2] complex as a product of the reaction
of [bmim]2[PdCl4] with [bmim]BF4 in the presence of H2O [37]. A
palladium complex with 1-methylimidazole and N-carbamoyl-
substituted heterocyclic carbene has been obtained in the reaction
of Pd(OAc)2 with carbamoyl imidazolium iodide [12]. It has been
proposed that the elimination of the alkyl group from bis(imidazo-
lium) salts containing a biphenyl backbone facilitates the forma-
tion of imidazole-coordinated dinuclear monodentate Pd–NHC
species [29].
-
Cl
+
N
N
+
Pd(OAc)2
CH2OC3H7
C3H7OCH2
THF
70oC
All of the prepared complexes were characterized by means of
NMR spectra, which confirmed the presence of imidazole ligands
in the coordination sphere of palladium. The presence of a signal
at ca. 140 ppm in 13C NMR, assigned to imino carbon, is consistent
with coordination to palladium via an N atom [15]. It should be
noted that only one set of signals was observed in the 1H and 13C
NMR spectra of the complexes containing three imidazole ligands
occupying non-equivalent positions around palladium. In conse-
quence, practically identical NMR spectra were measured for com-
plexes 4 and 4a.
CH2OC3H7
N
C3H7OCH2
Cl
Pd
Cl
N
N
not observed
N
CH2OC3H7
C3H7OCH2
Table 1 presents all the palladium complexes with imidazole
ligands obtained and characterized in this work.
CH2OC3H7
Cl
Pd
Cl
N
Table 2
N
N
Selected geometric parameters (Å, °) in compound 2a and 5.
N
Compound
2a
2.010 (1)
2.3088 (9)
1.321 (2)
1.346 (2)
5
CH2OC3H7
Pd–N1
Pd–Cl
C1–N1
C1–N2
2.033 (3)
2.316 (1)
1.340 (5)
1.352 (5)
Scheme 1. Formation of complex 5.
Cl–Pd–N1
Cl–Pd–N1i
89.98 (4)
90.02 (4)
89.97 (10)
90.03 (10)
-
Cl
+
N
N
K2[PdCl4]
+
Symmetry code: (i) 1 À x, 1 À y, 1 À z.
CH2OC3H7
H3C
MeCN + toluene
130oC, 1.5 h
Table 3
Selected geometric parameters (Å, °) in compound 2b and 4b.
Compound
2b
1.998 (3)
4b
Pd–N11
2.006 (1)
2.010 (1)
1.324 (2)
1.345 (2)
1.328 (2)
1.343 (2)
93.15 (4)
86.85 (4)
CH3
Cl
Pd–N21
2.008 (3)
1.293 (5)
1.338 (5)
1.304 (5)
1.340 (5)
89.90 (12)
90.10 (12)
N
C11–N11
C11–N12
C21–N21
C21–N22
N11–Pd–N21
N11–Pd–N21i
N
N
Pd
Cl
N
H3C
Symmetry codes: (i) 1 À x, 1 À y, 1 À z.
Scheme 2. Formation of complex 2a.

M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
4351
3.2. Molecular structure of 2a, 2b, 4b, and 5
compound 4b exhibits considerable distortion of the coordination
geometry with the N11–Pd–N21 angle equal to 93.15°. The Pd–Cl
bond lengths (Table 2) fall within a usual range (2.298–2.354 Å) re-
ported for four-coordinate palladium(II) complexes [38]. All the C–
N bond distances in the imidazole rings are intermediate between
the expected single- and double-bond lengths. However, the C1–
N1 bonds in compound 2a (Fig. 1) and 5 (Fig. 2) are slightly shorter
than C1–N2 (Table 2). Similarly, in compound 2b (Fig. 3) and 4b
(Fig. 4), the C11–N11 and C21–N21 bonds are shorter than C11–
N12 and C21–N22 (Table 3), respectively. This suggests that de-
spite delocalization in the heterocyclic ring, the double-bond is
more probably placed between C1–N1 (in 2a, 4) or C11–N11 and
C21–N21 (in 2b, 4b). All other bond distances and angles in the ob-
tained compounds are typical.
The Pd atoms in compounds 2a, 2b, 4b, and 5 are located at
inversion centers. They are four-coordinated in a square geometry.
As a result of presence of the inversion centers the Pd/Cl/N1/Cli/N1i
and Pd/N11/N22/N11i/N21i atoms are strictly planar. The angles
between adjacent ligands in compounds 2a, 2b, and 5 deviate only
slightly from the expected value of 90° (Tables 2 and 3). However,
Table 4
ESI-MS results for reactions of Pd-imidazole complexes 2 and
imidazole ligands and for reactions of complexes 5 with 4a, 4 with 2 and 5 with 3.
5 with excess of
Complex
Imidazole
Palladium species
[Pd(dim)2]
m/z(+) (relative
amount,%)
The obtained crystal of 2a is practically identical to the
previously reported structure of the same compound [14] and it
is isomorphous with trans-dibromo-bis(1-methylimidazole-
2
1,2-dimethylimidazole
(dim)
297 (35)
433 (11)
461 (54)
[Pd(met)4]
[Pd(dim)2(met)2]
[Pd(but)2]
[Pd(but)2Cl]
[Pd(but)3Cl]
[Pd(im)2Cl]
[Pd(met)(im)Cl]
[Pd(met)2Cl]
[Pd(dim)2]
j
N3)palladium(II) [39].
2
2
5
1-butylimidazole (but)
imidazole (im)
354 (50)
389 (9)
513 (41)
277 (44)
291 (43)
307 (13)
297 (61)
333 (26)
342 (9)
429 (3)
389 (20)
407 (32)
513 (48)
370
407
441
531
545
347
430
517
The crystal structure of 4b shows substitutional disorder of the
n-butyl chain attached to N22 atom (Fig. 4). This group is disor-
dered over two positions with the site occupancy factors about
80% and 20%. The methyl H atoms in compound 2b were modeled
as disordered over two equally occupied sets of idealized positions
rotated from each other by 60° (Fig. 3).
1,2-dimethylimidazole
(dim)
[Pd(dim)2Cl]
[Pd(dim)(prop)]
[Pd(dim)3Cl]
3.3. ESI-MS studies of palladium-imidazole complexes
5
1-butylimidazole (but)
–
[Pd(but)2Cl]
[Pd(but)(prop)Cl]
[Pd(but)3Cl]
The ESI-MS method was selected for studies of the palladium-
imidazole complexes in solution. All the complexes under study
presented a similar pattern when dissolved in H2O; namely,
[Pd(im)2Cl]+, [Pd(im)3Cl]+, and [Pd(im)2]+ ions were found. It is
interesting to note that the same MS spectrum pattern was also ob-
tained for complexes 5, 7, and 8, which contained only two imidaz-
ole (or imidazoline) ligands. Clearly, imidazole ligands are easily
exchanged between different palladium ions in solution and such
a process can play in important role in the creation of catalytically
active palladium species. A series of mixed solutions were studied
5 + 4a
[Pd(but)(prop)]
[Pd(but)(prop)Cl]
[Pd(but)(prop)Cl2]
[Pd(but)2(prop)Cl]
[Pd(but)(prop)2Cl]
[Pd(but)(met)Cl]
[Pd(but)(met)2Cl]
[Pd(dim)(prop)Cl]
4 + 2
5 + 3
prop = 1-Propoxymethylimidazole.
met = 1-Methylimidazole.
Table 5
Crystal data and structure refinement parameters for compounds 2a, 5, 2b, 4b.
Compound
2a
5
2b
4b
Formula
Mr
Crystal system
Space group
a (Å)
[PdCl2(C4H6N2)2]
341.52
monoclinic
P21/c
[PdCl2(C7H12N2O)2]
457.68
trigonal
RÀ3
23.735 (8)
23.735 (8)
8.524 (4)
90
90
120
4159 (3)
9
1.645
[Pd(C16H24N8)]Cl2Á2CH2Cl2
675.58
monoclinic
P21/n
[Pd(C28H48N8)]Cl2Á2H2O
710.08
triclinic
ꢀ
P1
5.173 (2)
11.477 (3)
9.893 (3)
90
98.48 (3)
90
580.9 (3)
2
1.952
10.858 (3)
8.628 (3)
15.995 (4)
90
102.80 (3)
90
1461.2 (8)
2
1.535
8.903 (3)
9.012 (3)
11.488 (4)
73.77 (3)
76.66 (3)
89.01 (3)
860.0 (5)
1
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å)3
Z
Dcalc (g cmÀ3)
1.371
0.73
l
(mmÀ1
)
2.03
1.31
1.21
T (K)
100
100
293
100
Crystal size (mm)
Absorption correction
Tminimum/Tmaximum
Measured reflections
Unique reflections
0.20 Â 0.08 Â 0.08
analytical
0.804/0.864
6542
0.35 Â 0.25 Â 0.16
analytical
0.732/0.868
17374
0.28 Â 0.14 Â 0.12
analytical
0.782/0.844
9770
0.18 Â 0.12 Â 0.10
analytical
0.884/0.920
12432
1614
2657
3273
3949
Reflections with I > 2
Rint
r(I)
1514
0.033
2208
0.038
1978
0.064
3780
0.030
h range (°)
3.0–30.0
1614/0/71
1.08
3.0–30.0
2657/0/108
1.11
3.0–27.5
3273/0/153
0.96
3.0–27.5
3949/2/226
1.03
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
R1, wR2 (all data)
0.061
0.166
0.124
0.055
R1, wR2 [I > 2
r
(I)]
0.021
0.61/À0.88
0.056
1.16/À1.41
0.050
0.69/À0.76
0.024
0.44/À0.40
Max/min residual density (e ÅÀ3
)

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M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
to check whether heteroleptic ions containing two different imida-
zoles coordinated to palladium can also be formed.
an equimolar mixture of 5 and 4a, five different mixed species
were identified, containing 1-butylimidazole and 1-propoxyme-
thylimidazole coordinated to palladium (Table 4, Fig. 5). When 4
reacted with 2, two mixed complexes were formed, and only one
was formed when 5 reacted with 3 (Table 4).
When 1,2-dimethylimidazole was added to a solution of 2, the
signal at m/z = 305, assigned to [Pd(1-methylimidazole)2Cl]+,
disappeared entirely, and two new intensive signals appeared at
m/z = 297 and m/z = 461, assigned to [Pd(1,2-dimethylimidaz-
ole)2]+ and [Pd(1,2-dimethylimidazole)2(1-methylimidazole)2]+,
respectively (Table 4). The signal of the palladium ion bearing dif-
ferent imidazole ligands was the most intensive.
When 1-butylimidazole was added to the same complex 2, a
complete substitution of 1-methylimidazole took place, resulting
in the formation of [Pd(1-butylimidazole)2]+ (m/z = 354) and
[Pd(1-butylimidazole)3Cl]+ (m/z = 513) as the main products
together with some amount of [Pd(1-butylimidazole)2Cl]+
(m/z = 389). Imidazole reacted with 2 similarly as 1,2-dimethylim-
idazole, forming ca. 43% of a heteroleptic cation. The exchange of
imidazole ligands was also studied for complex 5, and the results
are summarized in Table 4. In this case, only a partial substitution
of 1-propoxymethylimidazole by 1,2-dimethylimidazole or 1-
butylimidazole was evidenced by analysis of ESI-MS spectra. It
can be concluded that imidazole ligands compete for the place in
coordination sphere of palladium and some preference for the
added free imidazole is observed. Such a process led to the forma-
tion of heteroleptic complexes; however, homoleptic species
dominated.
3.4. Catalytic activity in the Suzuki–Miyaura reaction
The palladium–imidazole complexes were tested as catalysts in
the model Suzuki–Miyaura reaction of selected aryl bromides with
phenylboronic acids (Scheme 3). The first screening of reaction
conditions showed that the reaction of 2-bromotoluene with phen-
ylboronic acid could be performed with high yield already at 40 °C
in 1 h when a mixture of isopropanol and water was used as sol-
vent. Often much harsher conditions have been applied to get rea-
sonable yield of the cross-coupling product when other palladium
catalysts were used [19,40]. Only two complexes, 1 and 6, were not
active under the applied conditions, and 80% of 2-Me biphenyl was
obtained with complex 3. All other complexes afforded a yield of
100%. At 60 °C, complex 6 produced only 10% of 2-Me biphenyl,
which correlated well with its relatively low productivity (61%) ob-
served in S–M coupling at 120 °C over 12 h [11].
High yields of cross-coupling products were also obtained with
most of the palladium complexes when 4-bromotoluene and 4-
bromoanizole were used as substrates (Table 6). In contrast, appli-
cation of 4-vinylphenylboronic acid resulted in lower productivity
of the complexes studied, as shown by the formation of ca. 30% of
the cross-coupling product.
Further studies were performed for solutions containing two
different complexes, with the aim to check whether ligand ex-
change is also possible in such systems. In a solution containing
Intens.
x104
386.1 Pd(prop)2 441.1 Pd(but)(prop)Cl2
2.0
370.1 Pd(but)(prop)
407.1 Pd(but)(prop)Cl
421.1 Pd(prop)2Cl
1.5
561.1 Pd(prop)3Cl
354.1 Pd(but)2
545.2 Pd(but)(prop)2Cl
1.0
248.0
230.0 Pd(buim)
531.2 Pd(but)2(prop)Cl
264.0 Pd(buim)Cl
0.5
314.0
188.0
175.9
0.0
200
250
300
350
400
450
500
550 m/z
Fig. 5. Experimental ESI-MS spectrum obtained for the reaction 5 + 4a in CH3CN (but = 1-butylimidazole, prop = 1-propoxymethylimidazole).
[Pd(im)xCl2]
R,
R,
B(OH)2
+
Br
KOH
R
R
R, = H or vinyl
R = 2-Me, 4-Me, 4-OMe
Scheme 3. Scheme of the Suzuki–Miyaura reaction.

M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
4353
The results presented in Table 6 are in agreement with the ear-
lier observation that imidazoles incorporating an N–H bond do not
catalyze the coupling reaction [8,25]. However, neither of the imi-
dazoline complexes studied, 7 and 8, obeyed this rule, both afford-
ing 100% yield already at 40 °C. The higher activity of the
imidazoline complex 7 compared with 6, which contains imidazole
ligands, had been mentioned before by Hayashi with reference to
Heck and S–M reactions performed under other conditions [11].
Table 6
The yield of cross-coupling products obtained in S-M reaction catalyzed by palladium complexes 1–8.
ArBr
ArB(OH)2
T (°C)
Catalyst
1
2
2a
3
4a
5
6
7
8
2-Br-toluene
2-Br-toluene
2-Br-toluene
4-Br-toluene
4-Br-anisole
PhB(OH)2
PhB(OH)2
4-vinyl-PhB(OH)2
PhB(OH)2
PhB(OH)2
40
60
60
60
60
0
0
0
0
0
100
100
35
100
100
100
100
34
100
100
80
100
25
100
90
100
100
31
100
100
100
100
28
100
100
0
100
100
29
100
100
100
100
29
100
100
10
22
20
10
Reaction conditions: [Pd] 1 Â 10À5 mol, PhB(OH)2 or 4-vinyl-PhB(OH)2 1.1 Â 10À3 mol, ArBr 1.0 Â 10À3 mol, KOH 2.0 Â 10À3 mol, isopropanol–water (1:1) 5 cm3.
O
R
R
R
C
N
N
N
O
N
Pd
O
R
R
N
R
N
CH
X
N
N
N
N
X
N
Pd
X
N
X
Pd
X
CH
HX
R
N
R
N
N
N
R
N
R
O
C
N
N
O
N
N
Pd0
CH
N
Pd
O
R
N
X
R
N
N
R
N
X
N
N
R
CH
HX
R''
R'
R
N
N
Pd0
X
R'
N
N
R
R
N
R
R''
N
N
R
N
N
Pd
N
R
N
R'
N
X
Pd
R'
KOH
R''
B(OH)3
KX
B(OH)2
Scheme 4. Scheme of proposed activation of Pd(II)–imidazole complexes in S–M reaction conditions.

4354
M.S. Szulmanowicz et al. / Inorganica Chimica Acta 363 (2010) 4346–4354
To explain the observed effect of imidazole ligands on the S–M
References
reaction course, a mechanism of Pd–imidazole complex transfor-
mation under catalytic reaction conditions is proposed (Scheme
4). The key step is a reaction of an isopropoxide anion generated
by deprotonation of isopropanol by the base KOH with a Pd(II)–
imidazole complex. Analogously to the mechanism proposed by
Nolan and co-workers [41], b-hydride elimination from coordi-
nated isopropoxide results in the formation of a Pd-hydride spe-
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twice, and finally a Pd(0)–imidazole complex is formed which
can be considered an active species activating aryl halide via the
oxidative addition pathway. Further stages are the same as in the
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formed during reduction of a Pd(II)–imidazole complex by an iso-
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reaction [42]. Alternatively, they can act as an cache of monomo-
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Acknowledgements
Financial support of the Polish Ministry of Science and Higher
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Appendix A. Supplementary material
CCDC 740855–740858 contain the supplementary crystallo-
graphic 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.
Supplementary data associated with this article can be found, in
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