The influence of electronic effects on the substitution reactions of N,N′-donor cis-palladium(II) complexes with bidentate (pyridin-2-yl)methyl-aniline ligands

Abstract

The substitution behaviour of bidentate N,N′-donor (pyridin-2-yl)methyl-aniline chelates with different substituents on the mononuclear Pd(II) complexes was investigated. The complexes were synthesized and characterized using 1H NMR, 13C NMR, FTIR, LC–MS, and elemental analysis. The study explored the kinetics and mechanistic behaviour of N,N′-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4). The effect of concentration and temperature on substitution behaviour of coordinated chloride ligands by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M (LiCl) solution of ultra-pure water under pseudo-first order conditions using standard Stopped-Flow and UV–Visible spectrophotometric techniques. The substitution of the chloride ligands from the Pd(II) metal by thiourea nucleophiles was a two-step reaction, where the chloride trans to the pyridine ligand was substituted first due to the strong trans effect on the pyridine ring compared to the amine group. The rate of substitution of the chloride by thiourea nucleophiles increased with the presence of an electron-withdrawing substituent and decreased when an electron-donating group was attached to the para position of the phenyl moiety. The negative activation entropies and positive activation enthalpy for all the substitution reactions support an associative mode of substitution mechanism.