Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.

Mjwara, Pinky Ncomela.

Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.

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Mjwara_Pinky_Ncomela_2023.pdf (3.29 MB)

Date

2023

Authors

Mjwara, Pinky Ncomela.

Abstract

The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored 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-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms 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 solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca.