Palladate salts from chiral pyridinium ionic liquids: Synthesis and crystal structures

Article abstract of DOI:10.3390/12081940

This paper reports the preparation, characterization and the crystal X-ray structures of metal-containing ionic liquid complexes based on chiral pyridinium cations and tetrachloropalladate (II) [PdCl₄]^2-.

Full text of DOI:10.3390/12081940

Molecules 2007, 12, 1940-1949
molecules
ISSN 1420-3049
© 2007 by MDPI
www.mdpi.org/molecules
Full Paper
Palladate Salts from Chiral Pyridinium Ionic Liquids: Synthesis
and Crystal Structures
Emmanuel Tourneux 1, Heinz Gornitzka 2, Jean-Daniel Marty 1,* and
Nancy Lauth-de Viguerie 1,*
1
Laboratoire des Interactions Moléculaires, Réactivité Chimique et Photochimique, UMR CNRS
5623, Université Paul Sabatier, 31062 Toulouse Cedex 9, France
2
Laboratoire Hétérochimie Fondamentale et Appliquée, UMR CNRS 5069, Université Paul Sabatier,
31062 Toulouse Cedex 9, France
* Authors to whom correspondence should be addressed; E-mail: marty@chimie.ups-tlse.fr; E-mail:
viguerie@chimie.ups-tlse.fr
Received: 23 May 2007; in revised form: 19 July 2007 / Accepted: 19 July 2007 / Published: 20
August 2007
Abstract: This paper reports the preparation, characterization and the crystal X-ray
structures of metal-containing ionic liquid complexes based on chiral pyridinium cations
and tetrachloropalladate (II) [PdCl4]2-.
Keywords: Chiral Ionic Liquids, Pyridinium, Transition metal complexes, Palladium
Introduction
Ionic Liquids (ILs) are ionic compounds with relatively low melting points. They are characterized
by very low vapor pressures and consequently, are considered as environmentally benign solvents. In
addition ILs possess interesting properties such as tunable polarity, Lewis acidity, coordinating ability,
hydrophilicity/lipophilicity and miscibility with various compounds [1-3]. These properties can be
readily adjusted by a suitable choice of cation/anion. Therefore, the solubilities both of organic
compounds and of metal salts in ILs are important with regard to chemical synthesis and catalytic
processes [4]. The ammonium, phosphonium, pyridinium and imidazolium cations of ILs can
coordinate to MX42- (M=Co,Ni,Cu,Zn,Cd,Pd and X=Cl, Br) complex ions to form ionic liquid crystals

Molecules 2007, 12
1941
(ILCs) [5-18]. These ILCs contain a metal center, and so have been used as catalyst or catalyst
precursors [5]. Moreover, they have a high degree of order, and so they are prime candidates for the
synthesis of inorganic nanostructures [5]. Among them, imidazolium ILCs have been the most widely
used [6-9]. Indeed for catalytical applications, imidazoliums ILs can generate metal-carbene
complexes in situ [10]. Other cores such as pyridinium, ammonium and phosphonium have been
employed to a lesser extent [11]. In the case of transition metal-pyridinium salts (N-alkylpyridinium
tetrahalo-metallates), most of the studies focused on their lyotropic and/or thermotropic liquid crystal
behaviour [12-18]. To our knowledge, the synthesis of chiral pyridinium metal salts has not yet been
described. This paper reports the preparation, characterization and the crystal X-ray structure of
complexes metal-containing ILs based on chiral pyridinium cations and tetrachloropalladate (II)
[PdCl4]2-.
Results and Discussion
The use of Marazano’s route [19], based on the original work of Zincke [20], allowed access to
unique chiral pyridinium salts. Reaction of Zincke’s salt, readily obtained from pyridine and 1-chloro-
2,4-dinitrobenzene, with chiral primary amines give the corresponding enantiopure alkyl pyridinium
chlorides (R)– or (S)–1-phenylethylpyridinium chloride (1R and 1S, Figure 1) [21]. Enantiopure (R)–
and (S)–bis(1-phenylethylpyridinium) tetrachloropalladate complexes (2R and 2S) were prepared in
quantitative yields by reacting palladium chloride with a 2 molar excess of the corresponding
stereoisomer 1R and 1S in acetonitrile during 24 hours under reflux. Procedures used for the synthesis
of 2 were similar to those described in the literature for corresponding imidazolium salts [7,22].
Figure 1. Synthesis of chiral pyridinium ILs and of the corresponding metallates.
NH2
Cl
N
Ph
CH3
NO2
Cl
PdCl2
NO2
N
2
N
N
Cl
PdCl4
CH3
N
Ph
CH3
Ph
Ph
CH3
NO2
(R)-(+) or (S)-(−)
NO2
"Zincke salt"
2R
2S
1R
1S
All the products have been fully characterized by means of C, H, N combustion analysis and IR,
13
1H- and C-NMR spectroscopies (see Experimental section for further details). Crystals suitable for
X-ray crystallography were obtained by recrystallisation of 1R and 2R from acetonitrile solution at
room temperature (respectively colorless and light brown crystals). The crystal structures of 1R and
2R (this one is the first example of metal-containing ILs based on pyridinium chiral salts) are given
respectively in Figures 2 and 3 with 50 % thermal ellipsoids probability. Crystallographic data and
parameters are given in Table 1. Compound 1R is hygroscopic; consequently the asymmetric unit
presents 0.25 molecule of water per molecule of 1R. The water molecules and 1R are involved in a
hydrogen bonding network, as confirmed by FT-IR spectrum that shows a broad band at 3453 cm-1.

Molecules 2007, 12
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Characteristics of these hydrogen bonds were summarized in Table 2. Moreover this asymmetric
unit contains 5 anions and 4 cations due to the presence of anions Cl4 and Cl5 disordered over 2
positions with 50% occupancy factors. In 2R, the metal-containing anion [PdCl4]2-, has a regular
square-planar geometry around the PdII center, the average Cl-Pd-Cl angle being 90.0° (Table 3). The
bond lengths and angles of the pyridinium cation in 2R are similar to those found for 1R. All the
halogen of the anion are part of a three-dimensional hydrogen-bonding network and the shortest H…X
contacts involve only the hydrogen atoms of the pyridine rings. The distances of these H…Cl H-
bonding (Table 4) are in the range of 2.7 to 2.9 Å, and are within the sum of the van der Waals radii of
H and Cl (2.95 Å). This was confirmed by FT-IR spectrum that shows bands between 3100 and 3400
cm-1 characteristic of hydrogen bonding. Moreover, the initial conformation of the asymmetric
pyridinium was not modified during the salt formation as demonstrated by X-ray molecular structure.
Figure 2. Molecular structure and atom labelling scheme of 1R with thermal ellipsoids
drawn at the 50 % probability level (hydrogen atoms are not represented).

Molecules 2007, 12
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Figure 3. Molecular structure and atom labelling scheme of 2R with thermal ellipsoids
drawn at the 50 % probability level (hydrogen atoms are not represented).
The thermal behavior of compounds 1R and 2R was determined by optical microscopy and
differential scanning calorimetry (DSC) and, as expected, is dependent on the nature of the counter
anion. The melting point of (R)–1-phenylethylpyridinium chloride (1R) was found to be equal to
125°C. As described in a previous paper, the substitution of the chloride anion by a bis(trifluoro-
methanesulfonyl)imide anion (CF3SO2)2N- led to a room temperature ionic liquid (RTILs) (Tm
= -30°C) [21]. Here, the substitution of Cl- by [PdCl4]2- (leading to 2R) slightly increased this
temperature to 155 °C, and so ILCs 2R could not be described as RTILs. Moreover, the upper limit of
the liquidus range of these salts which is bounded by the thermal decomposition was also determined:
200°C for compound 1R and 210°C for compounds 2R. Further experiments are in progress to use the
compound 2R (and corresponding gold salts) as a precursor for the formation of nanoparticles [23,24]
surrounded by a chiral shell.
Conclusions
New pyridinium chiral salts: (S) and (R)–1-phenylethylpyridinium chloride and (S) and (R)–bis(1-
phenylethylpyridinium) tetrachloropalladate were synthesized. Crystallographic data of 1R and 2R
were presented and showed that H-bonding interactions play an important role in the formation of both
complexes. The catalytic activities of the palladium complexes and palladium nanoparticles are
currently under investigation.

Molecules 2007, 12
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Acknowledgements
The authors are greatly indebted to C. Routaboul for help in FTIR analysis and C. Claparols for
mass spectrometry analysis.
Experimental.
General.
All chemicals were purchased from Aldrich Fine Chemicals and were used in this study as
1
13
received. Solvents (HPLC grade) were from SDS. H-NMR and C-NMR analysis were conducted
with a Bruker Avance 500 MHz. Chemical shifts were reported as parts per million (ppm). Infrared
measurements were performed on Perkin-Elmer IR FT 1600. Optical rotations measurements were
realized at 20°C on a Perkin Elmer 241 polarimeter (Na lamp, λ = 589 nm) using solutions at 1% in
MeOH. Micro-analysis were performed on a EAH10 apparatus from Carlo Erba and mass
spectrometry measurements on an Applied Biosystem Q TRAP equipped with an electrospray source.
The thermal properties of the samples were determined by polarised-light optical microscopy
(Olympus microscope equipped with a Mettler FP82HT hot stage) and Differential Scanning
Calorimetry (DSC) using a Perkin Elmer PYRIS 1 calorimeter. DSC calibration was done with indium
and lead. DSC measurements were made at heating rates of 10, 20, 30, 40 °C/min and then
extrapolated to 0 °C/min.
Synthesis of (R)– or (S)–1-phenylethylpyridinium chloride (1R and 1S)
To a solution of Zincke salt (15.38 g, 70 mmol) in n-BuOH (200 mL) was added (+)-(R) or (-)-(S)–
1-phenylethylamine (9.38 g, 77 mmol). The mixture was boiled under reflux for 24 h. The solvent was
removed under vacuum. Distilled water was added to the residue. The solid that appeared was filtered
off, and the filtrate was washed three times with dichloromethane. The aqueous solution was
concentrated under reduced pressure. The residue was purified by HPLC (Waters Alliance 2695,
column C18 Delta Park 300Å, 19x300mm, isocratic mode water-acetonitrile 80:20, flow rate
5mL/min.) to afford compounds 1R or 1S as colourless very hydroscopic salts (1R: 92 %; 1S: 93%).
20
20
[α]D (1R–enantiomer) = +44.9 ± 0.5°mL/g/dm, [α]D (1S–enantiomer) = -45.2 ± 0.5°mL/g/dm (1%
1
in methanol). H-NMR (500 MHz, D2O): δ =1.97 (d, J = 7.0 Hz, 3H, CH3), 6.05 (q, J = 7.0 Hz, 1H,
CH), 7.39 (m, 5H, Ph), 7.93 (t, J = 6.8 Hz, 2H, p-py), 8.41 (t, J = 7.0 Hz, 1H, m-py), 8.83 (d, J = 5.5
13
Hz, 2H, o-py) ppm; C-NMR (125 MHz, D2O): δ = 19.7 (CH3), 70.7 (CH), 127.4 (py C3), 128.3 (Ph
C2’), 129.4 (Ph C4’), 129.8 (Ph C3’), 136.7 (Ph C1’), 143.0 (py C2), 145.9 (py C4) ppm. MS (ESI)
positive mode: m/z = 184 [py+]. FT-IR (KBr pellet): 3423, 3054 (νC-H aromatic); 2934 (νC-H aliphatic),
1630, 1478, 1141, 771, 738, 702 and 686 cm-1. Anal. Calcd for C13H14NCl: C, 71.07; H, 6.42; N,
6.37%; Found: C, 71.17; H, 6.52; N, 6.36% (Elemental analysis was obtained from crystals strictly
maintained in dry atmosphere). Melting point: 125°C.

Molecules 2007, 12
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Synthesis of (R)– or (S)–bis(1-phenylethylpyridinium) tetrachloropalladate (2R and 2S)
A mixture of 1-phenylethylpyridinium chloride (500 mg, 2.28 mmol) and palladium chloride (808
mg, 4.56 mmol) in acetonitrile (75 mL) was heated at reflux temperature for 24 h. The formed orange
solution was concentrated to 3 mL under reduced pressure. Addition of toluene (25 mL) affords an
yellow-orange solid that was recovered by filtration, washed with toluene (2 x 10 mL), and dried in
vacuum. Crystals suitable for X-ray diffraction study were obtained by recrystallisation from
1
acetonitrile solution at room temperature (1.2 g, 95% based on Pd). H-NMR (500 MHz, D2O): δ
=2.06 (d, J = 7.0 Hz, 3H, CH3), 6.25 (q, J = 7.0 Hz, 1H, CH), 7.40 (m, 5H, Ph), 7.94 (t, J = 7 Hz, 2H,
p-py), 8.43 (t, J = 8 Hz, 1H, m-py), 8.84 (d, J = 7 Hz, 2H, o-py) ppm; 13C-NMR (125 MHz, D2O): δ =
19.7 (CH3), 70.7 (CH), 127.5 (py C3), 128.3 (Ph C2’), 129.4 (Ph C4’), 129.8 (Ph C3’), 136.7 (Ph C1’),
143.1 (py C2), 145.9 (py C4) ppm. IR (KBr): 3434, 3040 (νC-H aromatic); 2972, 2929, 2870 (νC-H
aliphatic); 1624, 1479, 1451, 1143, 698 cm-1. MS (ESI): Positive mode: 184 [py+]; Negative mode:
581 [Py]2[PdCl4]-. Anal. Calcd for C26H28Cl4N2Pd: C, 50.63; H, 4.58; N, 4.54. Found: C, 50.73; H,
4.53; N, 4.87. Melting point: 155°C.
Crystal data for 1R and 2R
Crystallographic data and details of the structure determination of 1R and 2R are presented in
Table 1. All data for all structures represented in this paper were collected at low temperatures using
an oil-coated shock-cooled crystal on a Bruker-AXS CCD 1000 diffractometer with MoKα radiation
(λ = 0.71073 Å). The structures were solved by direct methods (SHELXS-97, G. M. Sheldrick, Acta
Crystallogr. 1990, A46, 467-473) and all non hydrogen atoms were refined anisotropically using the
least-squares method on F2 (SHELXL-97, Program for Crystal Structure Refinement, G. M. Sheldrick,
University of Göttingen 1997). CCDC 647626 (1R) and CCDC 647627 (2R) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge via
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).
Table 1. Summary of Crystallographic Data and Parameters for [py*][Cl] and for [py*]2[PdCl4].
Identification code
Empirical formula
Formula weight
Temperature
[py*][Cl] (1R)
C13H14.50ClNO0.25
224.21
[py*]2[PdCl4] (2R)
C26H28Cl4N2Pd
616.70
193(2) K
173(2) K
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Tetragonal
Orthorhombic
P212121
P41
Unit cell dimensions
a = 22.369(1) Å
b = 22.369(1) Å
c = 9.570(1) Å
a = 8.059(1) Å
b = 12.211(1) Å
c = 27.872(2) Å
3
3
Volume
4788.9(4) Å
16
2742.7(3) Å
4
Z
3
3
Density (calculated)
1.244 Mg/m
1.494 Mg/m

Molecules 2007, 12
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Table 1. Cont.
-1
-1
Absorption coefficient
F(000)
0.29 mm
1896
1.08 mm
1248
3
3
Crystal size
0.5 x 0.6 x 0.6 mm
5.1 to 23.3°.
-24<=h<=24
-23<=k<=24
-10<=l<=8
0.1 x 0.1 x 0.5 mm
1.5 to 26.4°.
-10<=h<=9
-12<=k<=15
-30<=l<=34
16203
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
Ratio Tmin/Tmax
21441
6428 [R(int) = 0.0184]
(=23.3°) 98.5 %
Semi-empirical
0.694
5600 [R(int) = 0.0225]
(=26.4°) 99.8 %
Semi-empirical
0.776
Refinement method
Full-matrix least-squares Full-matrix least-squares
2
2
on F
on F
Data / restraints / parameters
6428 / 1 / 569
1.18
5600 / 0 / 300
1.05
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R1a = 0.070
wR2b = 0.175
R1 = 0.072
wR2 = 0.176
0.00(10)
R1 = 0.021
wR2 = 0.046
R1 = 0.024
wR2 = 0.047
-0.019(19)
R indices (all data)
Absolute structure parameter
Largest diff. peak and hole
-3
-3
0.76 and -1.24 e.Å
0.30 and -0.23 e.Å
a R1=Σ||Fo| - |Fc||/Σ|Fo|. bwR2=|Σ|Fo|2 - |Fc|2|/Σ|w(Fo)2|1/2 (Fo: observed, Fc: calculated).
w =1/[σ2(Fo)2+(aP)2+bP], where P=(Fo2+2Fc2)/3 and a = 0.0419, b = 18.7495 for 1R and a =
0.0199, b = 0.7641 for 2R.
Table 2. Hydrogen bonds for 1R [Å and °].
D-H...A
d(D-H) d(H...A) d(D...A) <(DHA)
C(2)-H(2B)...Cl(1)
C(12)-H(12)...Cl(1)#1
C(23)-H(23)...Cl(1)#1
C(27)-H(27)...Cl(1)
C(41)-H(41C)...Cl(1)
C(44)-H(44)...Cl(1)#2
C(52)-H(52)...Cl(1)
C(1)-H(1)...Cl(2)
0.98
0.95
0.95
1.00
0.98
0.95
0.95
1.00
0.95
0.98
0.95
0.95
0.98
0.95
0.95
1.00
0.95
0.95
2.80
2.75
2.63
2.49
2.98
2.90
2.51
2.54
2.99
2.96
2.97
2.52
2.75
2.80
2.65
2.80
2.80
2.66
3.687(8)
3.528(7)
3.562(7)
3.478(7)
3.944(9)
3.823(7)
3.419(7)
3.499(6)
3.811(7)
3.909(8)
3.914(7)
3.444(7)
3.665(8)
3.578(7)
3.540(7)
3.710(7)
3.691(7)
3.595(7)
151
139
166
168
167
163
161
161
146
162
173
165
156
139
156
151
157
168
C(13)-H(13)...Cl(2)
C(15)-H(15C)...Cl(2)
C(17)-H(17)...Cl(2)
C(22)-H(22)...Cl(2)
C(28)-H(28B)...Cl(2)#3
C(38)-H(38)...Cl(2)
C(51)-H(51)...Cl(2)
C(14)-H(14)...Cl(3)#4
C(43)-H(43)...Cl(3)#5
C(48)-H(48)...Cl(3)#5

Molecules 2007, 12
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Table 2. Cont.
C(49)-H(49)...Cl(3)
O(1)-H(1O)...Cl(3)#4
O(1)-H(2O)...Cl(3)#6
C(19)-H(19)...Cl(4)#7
C(35)-H(35)...Cl(4)#4
C(36)-H(36)...Cl(4)
C(10)-H(10)...Cl(5)#6
C(45)-H(45)...Cl(5)#8
O(1)-H(2O)...Cl(3)#6
O(1)-H(1O)...Cl(3)#4
C(21)-H(21)...O(1)
0.95
0.97(5)
1.02(5)
0.95
2.74
3.559(8)
3.234(6)
3.226(6)
3.662(7)
3.593(7)
3.766(8)
3.737(8)
3.608(8)
3.226(6)
3.234(6)
3.395(9)
3.506(9)
3.672(9)
3.396(9)
145
157
141
137
122
162
163
136
141
157
164
124
164
139
2.32(6)
2.37(7)
2.90
0.95
2.99
0.95
2.85
0.95
2.82
0.95
2.86
1.02(5)
0.97(5)
0.95
2.37(7)
2.32(6)
2.47
C(25)-H(25)...O(1)#9
C(26)-H(26)...O(1)
0.95
2.88
0.95
2.75
C(40)-H(40)...O(1)#10
1.00
2.57
Symmetry transformations used to generate equivalent atoms:
#1 x,y,z+1 #2 y,-x,z-1/4 #3 x,y,z-1 #4 y,-x+1,z-1/4
#5 -x+1,-y,z-1/2 #6 -y,x,z+1/4 #7 -y+1,x,z+1/4
#8 -x,-y,z-1/2 #9 -x,-y+1,z+1/2 #10 -y+1,x,z-3/4
Table 3. Selected Bond Angles (°) for salt 2R.
2R
Cl(1)-Pd(1)-Cl(2)
Cl(1)-Pd(1)-Cl(3)
90.57(2)
90.39(3)
Cl(2)-Pd(1)-Cl(4)
Cl(3)-Pd(1)-Cl(4)
89.08(2)
90.03(2)
Table 4. Hydrogen bonds for 2R [Å and °].
D-H...A
d(D-H) d(H...A) d(D...A)
<(DHA)
158
C(1)-H(1)...Cl(4)#1
C(3)-H(3)...Cl(2)#2
C(5)-H(5)...Cl(4)#3
C(6)-H(6)...Cl(3)#3
C(7)-H(7)...Cl(2)#1
C(14)-H(14)...Cl(4)#4
C(16)-H(16)...Cl(1)#5
C(16)-H(16)...Cl(3)#5
C(17)-H(17)...Cl(2)
C(18)-H(18)...Cl(1)
C(25)-H(25)...Cl(4)#3
1.00
0.95
0.95
0.95
0.95
1.00
0.95
0.95
0.95
0.95
0.95
2.69
2.55
3.00
2.72
2.70
2.88
2.72
2.73
2.84
2.77
3.00
3.641(2)
3.463(3)
3.614(3)
3.504(3)
3.486(3)
3.744(3)
3.580(2)
3.374(3)
3.559(3)
3.668(3)
3.648(3)
160
123
140
140
144
150
126
133
157
126
Symmetry transformations used to generate equivalent atoms:
#1 x,y+1,z #2 -x+1,y+1/2,-z+1/2 #3 x+1,y+1,z
#4 x+1/2,-y+1/2,-z #5 x+1,y,z

Molecules 2007, 12
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Sample Availability: Samples of compounds 1R, 1S, 2R, 2S are available from authors.
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