Thermodynamic and structural characterisation of the complexes formed in the reaction of [Pd(en)(H2O)2]2+ and [Pd(pic)(H2O)2]2+ with N-alkyl nucleobases and N-acetyl amino acids

Abstract

Potentiometric and ¹H NMR studies have been performed on the interaction of [Pd(en)(H₂O)₂]²⁺ and [Pd(pic)(H₂O)₂]²⁺ (pic = 2-picolylamine) with N-alkyl nucleobases and N-acetyl amino acids. The ligands 1-methylthymine, 1-methyluracil and uridine formed only mono and bis(ligand) complexes in the whole pH range (2 to 10) via the co-ordination of N(3) donor atoms of pyrimidine rings. In the case of 1-methylcytosine (MeC), N-acetyl-L-histidine (AcHis) and N-acetylhistamine (AcHm) polynuclear complexes containing a deprotonated exocyclic amino group of MeC or a fully deprotonated imidazole of AcHis or AcHm were also formed. In the [Pd(pic)(H₂O)₂]²⁺–MeC and AcHis/AcHm systems various isomeric species including the cis–trans and linkage isomers were detected by NMR measurements. The thermodynamic equilibrium constants of the complexes of [Pd(pic)(H₂O)₂]²⁺ with nitrogen donors were generally higher than those of [Pd(en)(H₂O)₂]²⁺. The differences in the complex formation reactions of [Pd(en)(H₂O)₂]²⁺ and [Pd(pic)(H₂O)₂]²⁺ were especially high for N-acetyl-L-methionine. The high trans-effect of the thioether donor function of AcMet resulted in the liberation of free ethylenediamine in the [Pd(en)(H₂O)₂]²⁺–AcMet system, while bidentate (S,O) co-ordination of AcMet was proposed for [Pd(pic)(H₂O)₂]²⁺ in acidic solutions. The hydrolytic reactions of the complexes were also followed. The formation of dihydroxo complexes [Pd(en)(OH)₂] and [Pd(pic)(OH)₂] was characteristic in alkaline solutions, while the existence of a dimeric–trimeric and monomeric–dimeric equilibrium was suggested in the neutral pH range for the monohydroxo complexes of [Pd(en)(H₂O)₂]²⁺ and [Pd(pic)(H₂O)₂]²⁺, respectively.