SYNTHESIS AND CHARACTERISATION OF A NOVEL PHOSPHONATE LIGAND AND ITS PALLADIUM(II) COORDINATION COMPOUND

The novel N-(methylene-2-pyridine)-N,N-bis(diethoxyphosphorylmethyl)amine ligand (L) was obtained through a double Kabachnik-Fields reaction, starting from 2-(aminomethyl)pyridine (2-picolylamine). Based on the L ligand, a new palladium(II) coordination compound [Pd(L)2Cl2] has been synthesized and comprehensively characterized by spectroscopic methods ( 1 H NMR, UV-Vis), ESI mass spectrometry and X-ray crystallography.


Introduction
Phosphonates and phosphonic metal complexes represent an interesting class of coordination compounds which have been described and widely studied for several decades. Their syntheses, structures and potential applications have been reviewed in detail [1][2][3][4].
Phosphonate ligands are considered to be a significant class of molecules which possess a high activity in the medical treatment of bone diseases [5] or contrast agents for magnetic resonance imaging [6]. Moreover the pharmacological use of phosphonates in the prevention of various types of skeletal problems enhances the importance of the requirement of a high oral absorbtion of these compounds. Also, it was determined that the antiresorptive potency of phosphonates can be magnified due to the presence of nitrogen atoms by 10 -10000 times in comparison to non-containing nitrogen phosphonates [7].
From the point of view of coordination chemistry, after the group of carboxylate ligands, the phosphonates are considered to be the biggest class of ligands which can generate a variety of complexes due to their multiple coordination sites. The possible coordination pathway for phosphonate ligands [RPO 3 ] 2is shown elsewhere [2]. The coordination through multiple sites usually contributes to the formation of polymeric coordination compounds or metal-organic frameworks, which are usually insoluble or possess a poor solubility in both organic solvents and aqueous solutions. These complexes may find application in biomedical studies [6,8], and this can be related to the fact that some bisphosphonates may also act as ligands for metals, thus leading them to be employed in the treatment therapy for metal intoxication [9].
It was shown that aminophosphonate ligands easily lead to the formation of metal complexes, which either possess biomedical potential [8], or show thermoluminescent properties [10]. On the other hand, the use of bisphosphonic esters as ligands for metals has been described a lot less [11][12][13][14]. Another interest raised by this kind of ligands is that the presence of bisphosphonate function may influence the solubility of the corresponding complexes in water, therefore allowing the possibility to use such compounds as catalysts in water mediated reactions.
The chemical properties of the 8 th group metal complexes are multilateral, versatile and have a large area of applications. Nevertheless, the chemistry of phosphonate ligands with 8 th group metals is not well studied yet, and there are known only a few examples of palladium based phosphonates.
From another point of view, palladium chemistry is a wide-ranged research area which mediates a big variety of catalytic reactions. Although the design of renewable palladium catalysts for aerobic oxidation is a quite new research area in the field of palladium chemistry. Mekmouche et al. reported a completely new bimolecular system consisting of a redox enzyme laccase and a water soluble palladium complex that is able to perform the reaction of alcohol oxidation into the corresponding aldehyde in an aqueous solution at ambient temperature [15].
In order to synthesize a new potential molecular palladium based catalyst with a high solubility in water,  we  turned  out  towards  the  synthesis  of  a  new  aminobisphosphonate ligand, namely pyridylbisphosphonate, since this ligand may modify the electronic density around the metal and therefore its reactivity, with the aim of preparing the palladium coordination compound for catalytic studies, and/or for biochemical experiments.
In the perspective of developing new catalysts for clean water mediated reactions, herein we report the synthesis of both a novel phosphonate ligand (L) and a new palladium(II) coordination compound [Pd(L) 2 Cl 2 ]. The crystallographic characterization of the [Pd(L) 2 Cl 2 ] complex is also described.

Chemicals
All chemicals were purchased from commercial sources and used as received unless otherwise stated. Basic solvents for synthesis were dried using literature methods. Solvents for spectroscopic investigations were of the highest purity available.

Characterisation
The 1 H, and 31 P NMR spectra were recorded at 25°C on Bruker Avance III nanobay -400 MHz spectrometer. Chemical shifts were reported in parts per million (ppm) referenced to residual solvent peak. The spectra were recorded using CDCl 3 as deuterated solvent.
UV-Vis spectra were recorded on a Varian Cary 50 spectrophotometer from 200-800 nm with samples in 1.0 path length quartz cuvette. Electronic spectra were recorded for solutions of different concentrations in order to be able to calculate the extinction coefficient.
Suitable crystals for single crystal X-ray diffraction study of [Pd(L) 2
We should mention that the reaction was performed in absence of any solvent. This gave the ligand Lan orange oil with a 85% yield after purification, which was fully characterized using the common spectroscopic techniques.
NMR spectra (Figure 2) were performed in CDCl 3 at room temperature. The 31 P NMR (Figure 2, insertion) displays a single peak at 24.4 ppm, whereas the 1 H spectrum is in accordance with the structure. The 1 H NMR spectrum (Figure 2) of L includes eight signals. Four signals in the aromatic region of the spectrum (8.45 -7.14 ppm) are assigned to four protons of the pyridine ring and four signals in the aliphatic region of the spectrum (4.07 -1.25 ppm) are assigned to the protons from CH 2 and CH 3 groups. High resolution mass spectroscopy performed in CH 2 Cl 2 shows a molecular peak at 408. 16 Da, which matches with the required mass.
The UV-Vis spectra for the L ligand, recorded in acetonitrile at different concentrations, are shown in Figure 3. The electronic absorption spectra for the ligand invariably show the presence of a characteristic band in the ultraviolet domain at 260 nm ( Figure 3). The extinction coefficient value is 7700 L mol -1 cm -1 , which corresponds to the energy of 460 kJ/mol.

Synthesis and characterisation of the palladium(II) coordination compound
The palladium(II) complex [Pd(L) 2 Cl 2 ] was easily formed by mixing and stirring a solution of palladium chloride in acetonitrile with a solution of ligand in dichloromethane, at room temperature ( Figure 4). Crystals suitable for single X-ray diffraction analysis were grown from a DMSO solution (after one year).
The NMR spectra for the palladium complex ( Figure 5) are quite similar to those of the free ligand ( Figure 2). 31 P NMR displays a peak at 24.3 ppm. The 1 H NMR spectrum of [Pd(L) 2 Cl 2 ] contains nine signals. Four signals in the aromatic region of the spectrum (9.05 -7.29 ppm) are assigned to four protons of the pyridine ring and five signals in the aliphatic region of the spectrum (5.40 -1.36 ppm): four of which are assigned to the protons from CH 2 and CH 3 groups and one signal to protons from water molecules. In addition, the mass spectrum of this complex leads to the conclusion that a compound with a formula C 32 H 60 O 12 P 4 PdCl 2 is present, corresponding to an exact mass of 994.16 Da.  The electronic absorption spectra for the complex in acetonitrile (Figure 6) display bands in both the ultraviolet region (190 -380 nm) and the visible region of the spectrum (380 -750 nm). The additional band at 400 nm is characteristic for the metalligand charge transfer and is indicating towards the coordination of the ligand to the palladium atom. However the extinction coefficient for this band is rather low: 200 L mol -1 cm -1 and the corresponding energy is 299 kJ/mol.

Structural characterization of palladium(II) coordination compound
The crystallographic structure of the complex [Pd(L) 2 Cl 2 ] could be seen in Figure 7 and a fragment of the crystal packing in the molecule in Figure 8. Compound [Pd(L) 2 Cl 2 ] has a molecular structure in the crystal.
The asymmetric unit consists of a palladium atom with one coordinated L ligand molecule, one coordinated chlorine atom, and one cocrystallized water molecule. The [Pd(L) 2 Cl 2 ] complex crystallizes in the triclinic centrosymmetric space group P-1. The palladium has a square planar coordination sphere provided by two nitrogen and two chlorine atoms in a trans arrangement. The pyridine rings are situated perpendicular to the plane formed by the central metal and its coordination sphere.
The crystallographic data containing the structural parameters and unit cell characteristics are shown in Table 1. CCDC 1518527 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.  The presented data reveals that palladium is coordinated only to the two nitrogen atoms from the two pyridine rings, and is not bound to the phosphonate moiety. This structure is in contrast with the complex described by Jordan et al. [17] in which palladium is bound to the phosphine, and to the oxygen atom of the phosphoryl bond.

Conclusions
In this work, it is described the synthesis of a palladium(II) complex from an aminobisphosphonate ligand incorporating an aminopyridine moiety. This is the first time that such a bisphosphonate is used as a ligand.
The complex was fully characterized by the usual spectroscopic techniques. In the palladium complex, the metal has a square planar coordination sphere and the ligand is bound to the pyridine nitrogen atom only. The use of this catalyst in different organic reactions is under evaluation.