Palladium compounds of xanthine and xanthine derivatives

Abstract

The biological importance of purine bases is well known. The interaction of metal ions with nucleic acids, nucleotides and nucleotides has been an active area of inorganic and structural chemistry during the last few years, and a number of recent reviews exist on the subject [1, 2]. Recently much attention has been paid to palladium-containing complexes, due to their potent anti-tumor activities [3–7]. We report here the synthesis and characterization of Pd(II) complexes with xanthine (XH), theophylline (TH), theobromine (TBH), 3,8-dimethylxanthine (DMH), caffeine (C) and 1,3,8-trimethylxanthine (TMH). Experimental The chemicals theophylline, theobromine, caffeine and PdCl 2 were purchased from Carlo Erba; and were used without further purification. 3,8-dimethylxanthine and 1,3,8-trimethylxanthine were synthesized in our laboratory [8, 9]. The Pd(II) complexes of these ligands were prepared in acid media (HCl 0.25 N), mixing solutions of the ligands and metal salt, PdCl 2 (2:1 mole ratio). Tetrachloro palladates were obtained from solutions with ligand: cation relation equal to 1:1 in acid media (HCl 2.5 N). The precipitates formed in each case were washed with distilled water, ethanol and ether and then air-dried. The IR spectra were run on Beckman 4250. 1H NMR studies were performed in DMSO-d 6 on Hitachi Perkin Elmer R-600 high resolution NMR spectrometer. TMS was used as internal reference. Results and Discussion Table I gives the colour, elemental analysis and the position of the stretching bands PdCl and PdN of the isolated complexes. t001 Colour, Elemental Analysis and ν(PdCl) and (PdN) a (cm −1. Compound Colour C H N Cl Pd ν(PdCl) ν(PdN) [XH 2] 2[PdCl 4]·2H 2O brown 20.32 2.37 18.97 24.05 18.02 310 21.19 2.42 18.74 23.87 17.52 Pd(XH) 2Cl 2·2H 2O yellow 23.27 2.31 21.72 13.77 20.63 350 260 22.90 2.03 21.41 13.85 19.72 [TH 2] 2[PdCl 4] brown 27.52 2.95 18.35 23.26 17.43 330 27.68 2.98 18.53 23.10 16.80 Pd(TH) 2Cl 2 yellow 31.26 2.98 20.84 13.21 19.80 340 250 31.40 2.84 20.88 13.52 19.20 [TBH 2] 2[PdCl 4] brown 27.52 2.95 18.35 23.26 17.43 305 27.39 3.01 18.64 23.35 17.26 Pd(TBH) 2Cl 2 yellow 31.26 2.98 20.84 13.21 19.80 345 255 30.78 3.01 20.69 13.52 19.40 Pd(DMH) 2Cl 2 yellow 31.26 2.98 20.84 13.21 19.80 340 250 30.28 2.89 20.27 12.80 19.80 Pd(C) 2Cl 2 yellow 33.95 3.54 19.81 12.56 18.82 340 260 33.92 3.42 19.60 13.05 18.71 Pt(TMH) 2Cl 2 yellow 31.92 3.99 18.62 11.80 17.70 335 250 32.34 3.96 18.17 11.70 18.20 a Calculated values of elemental analysis in first row. t002 1H NMR Chemical Shifts in ppm. Proton XH Pd(XH) 2Cl 2 TH Pd(TH) 2Cl 2 TBH Pd(TBH) 2Cl 2 C-8-H 7.90 8.30 7.95 8.05 7.97 7.97 C-8-CH 3 Proton DMH Pd(DMH) 2Cl 2 C Pd(C) 2Cl 2 TMH Pd(TMH) 2Cl 2 C-8-H 7.95 7.95 2.40 2.40 C-8-CH 3 2.35 2.70 From magnetic data, (all the compounds are diamagnetic) square planar for PdL 2Cl 2 complexes (where L = XH, TH, TBH, DMH, C and TMH) and tetrachloro palladate anion have been proposed. In PdL 2Cl 2 complexes complexation of the purine base to Pd(II) causes downfield shifts of protons adjacent to the coordination site. Table II shows the proton chemical shifts in parts per million of the ligand and its Pd(II) complexes. The shift to downfields experienced by C-8-H and C-8-CH 3 protons is indicative of complexation at the N 9 position of the ligand (except for the xanthine). The infrared spectra of PdL 2Cl 2 complexes showed single PdCl and PdN stretching bands. This is consistent with a trans-configuration. In the tetrachloro palladate compound only the ν(PdCl) is present.