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Containers and Vessels for Supramolecular Catalysis
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
The confinement provided by ligands changes the hydrophobic or hydrophilic microenvironment. It has a large influence on the performance of a catalyst. The labiality associated with a metal—ligand bond and the coordination unsaturation provide access to exchange ligands. The exchanges of ligands in solution contribute to catalytic reactivity. However, it is important that the ligand fields of different ligands decide the electronic configuration of central metal ions and control reactivity. The trans-effect and back-bonding properties of a ligand control the reactivity of a complex by influencing the strength of the metal—ligand bond located at another coordination site within the same metal complex. Hence, the performance of a metal complex in catalytic activity depends on the surrounding environment of the ligand. If a metalcentric reaction is focused, the metal site should have access to a reactant. However, there are exceptions of metal complexes where the reactions may not require direct interaction of substrate with the metal ion. Energy and electron transfer occurring through mediators, space or a part of the ligands perform redox processes. Most importantly, there are ligands that do not directly participate in a catalytic reaction but dictate the course of the redox reaction by changing the rate and selectivity. Those ligands are known as spectator ligands. In the chemistry of mononuclear complexes, spectator ligands contribute significantly to product selectivity.
The role of 8-quinolinyl moieties in tuning the reactivity of palladium(II) complexes: a kinetic and mechanistic study
Published in Journal of Coordination Chemistry, 2019
Daniel O. Onunga, Deogratius Jaganyi, Allen Mambanda
Non-antitumor active monofunctional coordination compounds of chloro and aqua Pd(II) and Pt(II) complexes of the N^N^N tridentate ligands such as 2,2′:6′,2″-terpyridine (terpy), bis(2-pyridylmethyl)amine (bpma), and diethylenetriamine (dien) [8, 20, 23, 30–34] provided useful model substrates for studying the kinetics and mechanisms of ligand exchange reactions of square-planar complexes [30]. For example, it was shown that the reactivity of these complexes could reduce by five orders of magnitude going from terpy spectator ligand to a dien system. This was because of stepwise reduction of π-back bonding by replacing pyridine donor groups by amine ligands hence reducing electrophilicity of the metal center. Moreover, a systematic increase in π-conjugation around the metal center of terpy framework results in either increase or decrease in the rate of substitution depending on the relative position of the π-extension. The acceleration of the reactivity is due to increase in π-back bonding character caused by favorable overlap of the dπ-orbitals of the metal and the ligands’ π*-orbitals [20, 22, 24, 35] while the dampening of reactivity is a result of the net σ-donor effect on the metal center [18, 22, 36].