Equilibrium studies of the reactions of palladium(II) bis(imidazolin-2-imine) complexes with biologically relevant nucleophiles. The crystal structures of [(TLtBu)PdCl]ClO4 and [(BLiPr)PdCl2].

The kinetics and the mechanism of the substitution reactions of the complex [(TL(tBu))PdCl](+), where TL(tBu) is 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine, with nucleophiles (guanosine-5'-monophosphate (5'-GMP), l-Methionine (l-Met) and l-Histidine (l-His)) were studied using variable-temperature stopped-flow techniques in aqueous 0.1 M NaClO(4) with 10 mM NaCl at 298 K. The order of reactivity is: l-Met > 5'-GMP > l-His. The formation equilibria of [(BL(iPr))Pd(H(2)O)(2)](2+), where BL(iPr) is 1,2-bis(1,3diisopropyl-4,5-dimethylimidazolin-2-imino)ethane, and [(TL(tBu))Pd(H(2)O)](2+) with some biologically relevant ligands (l-Met, 5'-GMP and l-His) were also studied. The stoichiometry and stability constants of the newly formed complexes are reported, and the concentration distribution of the various complex species has been evaluated as a function of pH. Comparing the values of logβ(1,1,0) for 5'-GMP, l-His and l-Met complexes, the most stable complex is with 5'-GMP followed by l-His and l-Met for both complexes, [(BL(iPr))Pd(H(2)O)(2)](2+) and [(TL(tBu))Pd(H(2)O)](2+). The crystal structures of [(TL(tBu))PdCl]ClO(4) and [(BL(iPr))PdCl(2)] were determined by X-ray diffraction. The coordination geometries around the palladium atoms are distorted square-planar, with the Pd-N1 distance to the central nitrogen atom of the TL(tBu) ligand, 1.944(2) Å, being shorter than those to the other two nitrogen atoms of TL(tBu), viz. 2.034(3) and 2.038(2) Å. The BL(iPr) complex displays similar Pd-N distances of 2.031(2) and 2.047(2) Å.