Interaction of 2-aminopyrimidine with dichloro-[1-alkyl-2-(naphthylazo) imidazole]palladium(II) complexes : Kinetic and mechanistic studies

Background The anticancer properties of cisplatin and palladium(II) complexes stem from the ability of the cis-MCl2 fragment to bind to DNA bases. However, cisplatin also interacts with non-cancer cells, mainly through bonding molecules containing -SH groups, resulting in nephrotoxicity. This has aroused interest in the design of palladium(II) complexes of improved activity and lower toxicity. The reaction of DNA bases with palladium(II) complexes with chelating N,N/donors of the cis-MCl2 configuration constitutes a model system that may help explore the mechanism of cisplatin's anticancer activity. Heterocyclic compounds are found widely in nature and are essential to many biochemical processes. Amongst these naturally occurring compounds, the most thoroughly studied is that of pyrimidine. This was one of the factors that encouraged this study into the kinetics and mechanism of the interaction of 2-aminopyrimidine (2-NH2-Pym) with dichloro-{1-alkyl-2-(α-naphthylazo)imidazole}palladium(II) [Pd(α-NaiR)Cl2, 1] and dichloro-{1-alkyl-2-(β-naphthylazo)imidazole}palladium(II) [Pd(β-NaiR)Cl2, 2] complexes where the alkyl R = Me (a), Et (b), or Bz (c). Results 2-NH2-Pym reacts with 1a, 1b, and 1c to yield [{1-alkyl-2-(α-naphthylazo)imidazole}bis(2-aminopyrimidine)]palladium(II) (3a, 3b, 3c) dichloride and with 2a, 2b, and 2c to yield [{1-alkyl-2-(β-naphthylazo)imidazole}bis(2-aminopyrimidine)]palladium(II) (4a, 4b, 4c) dichloride in an acetonitrile (MeCN) medium. The products were characterized using spectroscopic techniques (FT-IR, UV-Vis, NMR). The ligand substitution reactions follow second order kinetics – first order dependence on the concentration of the Pd(II) complex and 2-NH2-Pym. Addition of LiCl to the reaction does not influence its rate. The thermodynamic parameters (standard enthalpy of activation, Δ‡H° and standard entropy of activation, Δ‡S°) were determined from variable temperature kinetic studies. The magnitude of the second order rate constant, k2, at 298 K, was shown to increase thus: b <a <c as well as 1 <2. Conclusion The kinetics of the reaction between Pd(II) complexes (1 and 2) and 2-NH2-Pym were examined spectrophotometrically at 530 nm in MeCN under pseudo-first-order conditions. The reaction rate is largely influenced by the π-acidity of the chelating ligand, with substitution in the naphthyl azoimidazole backbone influencing the rate of the substitution process. The activation parameters, Δ‡H° and Δ‡S°, were determined and support the kinetic rate data.

In order to introduce greater steric crowding around the target metal center, we aim to use different ligands containing the azoimine chelating mode (-N=N-C=N-). This will allow the mechanism of nucleophilic interaction with the metal centre under different local environments to be elucidated. Naphthyl azoimidazoles ((ii) in Figure 1) are chemical analogues to phenyl azoimidazoles ((i) in Figure 1) but with a greater degree of steric crowding and electron donating ability.
Heterocyclic compounds are found widely in nature, being essential to many biochemical processes, with the most thoroughly studied that of pyrimidine. Such ring systems form the building units of many valuable chemotherapeutic agents (Bleomycine), vitamins (Vitamin B 1 ), drugs (hyprotic, antibacterial, antimalarial), nucleic acids (cytosine and uracil). This fact has encouraged us to study the reactions of pyrimidine derivatives with different metal complexes [36,37]. In this study we present the kinetic and mechanistic studies of the reaction of 2-NH 2 -Pym with 1 and 2.

Scheme 1
The reaction kinetics between Pd(N,N / )Cl 2 and 2-NH 2 -Pym were examined spectrophotometrically. The reaction is first order with respect to the Pd(II) complex because k obs -values are almost steady when all variants are constant, other than that of the complex concentration. The k obs -values (Table 1) and the linear plots for the k obs versus initial molar concentration of 2-NH 2 -Pym, [2-NH 2 -Pym] 0 (Figures 2 and 3) indicate the reaction is first order with respect to 2-NH 2 -Pym. The slope of the plot gives the second order rate constant (k 2 ). The small intercept (k 0 ) value indicates the minor existence of a solvent assisted pathway to the overall reaction products, because MeCN is a coordinating solvent.
The reaction rate increases with temperature as expected from the Eyring equation. Activation parameters, standard enthalpy of activation (Δ ‡H°) and standard entropy of  Phenylazoimidazole (i) and Naphthylazoimidazole (ii) Figure 1 Phenylazoimidazole (i) and Naphthylazoimidazole (ii). where k 0 and k 2 are the intercept and the slope of the plot of k obs versus [2-NH 2 -Pym] 0 respectively. The values of k 0 and k 2 are constant when the temperature is constant. The nucleophile is an N donor ligand. It is probable that the basic N coordinates quickly with the positively charged metal centre. The first step of the reaction is the formation of a five-coordinated square pyramidal species (X) from Table 1:  Because the first Pd-Cl bond cleavage is possibly slow, the step is therefore rate determining. This is supported by there being no effect on reaction rate from externally added Clas LiCl. A plausible mechanism, outlined in Scheme 2, initially involves two competing steps : the first, a solvation step where coordinating solvent, MeCN, forms a solvated species Pd(N,N / )Cl 2 (MeCN) (Z); and the second, a nucleophilic attack by 2-NH 2 -Pym (Scheme 2). Though the steric crowding of the α-/β-naphthyl group in N,N / chelating ligand is significant its strong electron withdrawing ability resulting from conjugation, stabilizes the chelated Pd(N,N / ) species, rather than resulting in dechelation.
The kinetic data in Table 1 reveal that the magnitude of k 2 increases thus : b <a <c. This is because of the electron withdrawing tendency of Bz group is greater than that of Me whilst that of the Me group it is slightly greater than for Et.

Conclusion
The kinetics of the interaction between Pd(N,N / )Cl 2 and 2-NH 2 -Pym were examined spectrophotometrically at 530 nm in MeCN under pseudo-first-order reaction conditions. The reactions are first order with respect to the Pd(II) complex and 2-NH 2 -Pym. The rate of reaction is largely influenced by the π-acidity of the chelating ligand; substitution in the naphthyl azoimidazole backbone influences greatly the rate of the substitution. The reaction activation parameters, Δ ‡ H° and Δ ‡ S°, were calculated and correlate well with the kinetic rate data. The products isolated from the reaction between Pd(N,N / )Cl 2 and 2-NH 2 -Pym in MeCN were characterized by spectroscopically, and the composition being confirmed as [Pd(N,N / )(2-NH 2 -Pym) 2 ](ClO 4 ) 2 .  Step 1 Step The decrease in absorbance of the reaction mixture was recorded automatically at ca. 530 nm as a function of time. A ∞ was measured after ~24 h of mixing, when the absorbance became constant. In all experiments, the initial molar concentration of 2-NH 2 -Pym, [2-NH 2 -Pym] 0 was kept at least ten times higher than Pd(II) complex concentration so as to maintain pseudo-first-order kinetic conditions. Pseudo-first-order rate constants, k obs , were obtained from the slopes of the plots of (A t -A ∞ ) versus time ( Figure 8) where A t = absorbance of the reaction mixture at time, t(s) after mixing of 2-NH 2 -Pym solution, and A ∞ = absorbance of same after completion of the reaction.
The resulting mass was then dissolved in methanol, before the addition of an aqueous solution of 1 g of NaClO 4 . The resulting brown precipitate was filtered and washed with water. The dried product was then chromatographed over silica gel, with MeCN eluting an orange band. A yield of 29 mg (65.91%) was obtained.
All other complexes were prepared using the same procedure with yields varying from 58-70%.

Product characterization
To the MeCN solution of the complex, Pd(N,N / )Cl 2 , 2-NH 2 -Pym was added and the orange-colored solution was stirred for about 24 h. Next the solution was filtered and allowed to evaporate to dryness at near to room temperature. The resulting solid was dissolved in methanol, before the addition of aqueous NaClO 4 . The brown precipitate was filtered, washed with water and cold MeCN. The dried product was chromatographed over silica gel with MeCN eluting an orange-red band. Finally the micro-analytical data in addition to U.V.-Vis, IR, NMR (Table 3)  The H NMR spectra of the complexes were recorded in CD 3 CN ( Table 3). The proton-numbering is outlined in Scheme 1. The data reveal that the signals in the complexes are shifted downfield relative to the free ligand val-ues [34] supporting the coordination of ligand to Pd(II). An important feature of the spectra is the general shifting observed for the imidazole protons 4-H and 5-H to lower δ-values relative to naphthyl protons (        Plot of Δ ‡H0 versus Δ ‡S0, i.e., iso-kinetic plot for the reac-tions: Pd(R/aiR)Cl2 + 2-NH2-Pym in MeCN Figure 6 Plot of Δ ‡H0 versus Δ ‡S0, i.e., iso-kinetic plot for the reactions: Pd(R/aiR)Cl2 + 2-NH2-Pym in MeCN. project was based on the ideas of AM and carried out with his guidance and consultation.

Acknowledgements
Financial help from Jadavpur University is gratefully acknowledged. We thank Dr. C. Sinha, Professor of Chemistry, Jadavpur University for his sincere all round help and helpful comments through discussion.