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]

Abstract

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 NaClO₄ 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(H₂O)₂]²⁺, where BL^iPr is 1,2-bis(1,3diisopropyl-4,5-dimethylimidazolin-2-imino)ethane, and [(TL^tBu)Pd(H₂O)]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(H₂O)₂]²⁺ and [(TL^tBu)Pd(H₂O)]²⁺. The crystal structures of [(TL^tBu)PdCl]ClO₄ and [(BL^iPr)PdCl₂] 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.