The role of 8-quinolinyl moieties in tuning the reactivity of palladium(II) complexes: a kinetic and mechanistic study

Abstract

The rate and mechanism of chloride substitution from Pd(II) complexes, chlorobis-(2-pyridylmethyl)aminepalladium(II), 1, chloro-8-[(2-pyridylmethyl)amino]quinolinepalladium(II), 2, chloro-N-(2-pyridinylmethylene)-8-quinolinaminepalladium(II), 3, and chlorobis(8-quinolinyl)aminepalladium(II), 4, are reported. The labile chloride was substituted from the complexes by thiourea nucleophiles viz, thiourea (Tu), N,N′-dimethylthiourea (Dmtu) and N,N,N′,N′-tetramethylthiourea (Tmtu). The reactions were monitored under pseudo-first-order conditions in methanol using stopped-flow spectrophotometry as a function of concentration and temperature. All the reactions obeyed the rate law k_obs = k₂[Nu] following the order 1 > 3 > 2 > 4 with 4 exhibiting the slowest rate of substitution due to the stronger σ-donor effect of 8-quinolyl moiety of the coordinated ligand, which makes the Pd center more electron-rich. This slows the nucleophilic attack by the nucleophiles. The values of the thermodynamic parameters (ΔH^# and ΔS^#) support an associative substitution mechanism. The trends in the DFT calculated data support the experimentally observed order of the reactivity of the complexes.

Keywords:

• π-Conjugation
• electronic effects
• quinoline
• σ-donation

Acknowledgments

We remain grateful to the University of KwaZulu-Natal for financial support and bursary awarded to Daniel Onunga. The authors are equally thankful to C. Grimmer for NMR analysis and C. Janse van Rensburg for elemental and MS analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the University of KwaZulu-Natal, South Africa.