Palladate Salts from Chiral Pyridinium Ionic Liquids: Synthesis and Crystal Structures

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-.


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][2][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 MX 4 2-(M=Co,Ni,Cu,Zn,Cd,Pd and X=Cl, Br) complex ions to form ionic liquid crystals (ILCs) [5][6][7][8][9][10][11][12][13][14][15][16][17][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][7][8][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][13][14][15][16][17][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)

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].All the products have been fully characterized by means of C, H, N combustion analysis and IR, 1 H-and 13 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 .
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 [PdCl 4 ] 2-, has a regular square-planar geometry around the Pd II 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 Hbonding (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.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(trifluoromethanesulfonyl)imide anion (CF 3 SO 2 ) 2 N -led to a room temperature ionic liquid (RTILs) (T m = -30°C) [21].Here, the substitution of Cl -by [PdCl 4 ] 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(1phenylethylpyridinium) 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.

General.
All chemicals were purchased from Aldrich Fine Chemicals and were used in this study as received.Solvents (HPLC grade) were from SDS. 1 H-NMR and 13 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)-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 acetonitrile solution at room temperature (1.2 g, 95% based on Pd).

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 F 2 (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).

Figure 1 .
Figure 1.Synthesis of chiral pyridinium ILs and of the corresponding metallates.

Figure 2 .
Figure 2. Molecular structure and atom labelling scheme of 1R with thermal ellipsoids drawn at the 50 % probability level (hydrogen atoms are not represented).

Figure 3 .
Figure 3. Molecular structure and atom labelling scheme of 2R with thermal ellipsoids drawn at the 50 % probability level (hydrogen atoms are not represented).