A kinetic and mechanistic study of cyclic amine functionalized platinum(II) complexes with bio-relevant nitrogen nucleophiles.
Nxumalo, Mthobisi Ian.
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In this study, four square-planar platinum(II) complexes were synthesized namely C1S1, C2S1, C3S1 and C4S1. The novel platinum(II) complex (C4S1) was synthesized using methods detailed in Chapter 4 and characterized using NMR spectroscopy, mass spectroscopy and elemental analysis. The kinetics of the chloride ligand substitution from a series of square-planar platinum(II) complexes named above were investigated using a series of five-membered heterocyclic neutral nitrogen-donor nucleophiles: Imidazole (Im), Pyrazole (Pyz), 1methylImidazole (1-MIm) and L-Histidine (His) in an aqueous solution of constant ionic strength. The kinetics of chloride substitution were investigated as a function nucleophile concentration and temperature under pseudo first-order conditions using UV/Visible spectrophotometry and stopped-flow spectrophotometric techniques. The observed pseudo first-order rate constants obeyed the simple rate law, kobs = k2[Nu]. The observed rate constants along with activation parameters obtained i.e. low activation enthalpies and large, negative activation entropies support an associative mechanism. The data obtained shows that the reactivity of these complexes are influenced by both electronic and steric effects with steric effects being the dominant factor contributing to the overall trend. The kinetic results also show that appended cyclic amine pendant, trans to the leaving chloride group acts as a σ-donor into the terpyridine chelate ligand and is effective only up to the first cyclic amine. The change of the appended cyclic amine to another e.g. from pyrrolidine of C2S1 to piperidine of C1S1 shows that the inductive σdonor ability of the cyclic amine pendant no longer controls the reactivity of the metal centre. The substitution reactivity of the complexes with cyclic amines pendant (C1S1, C2S1 and C3S1) are controlled by the steric influence of the appended cyclic amine which decreases as the cyclic amine pendant gets larger in size, C3S1 < C1S1 < C2S1. This in turn blocks the metal center hence lowering the affinity for the incoming azole nucleophile. The results further support that, the slightest modifications to the terpy moiety leads to changes in the reactivity. The overall trend in the rate of chloride substitution is: C4S1 > C2S1 > C1S1 > C3S1. The observed reactivity trends were supported by density functional theory (DFT) calculation (dipole moment, HOMO-LUMO energy gap, electrophilicity index etc.). In addition, the substitution kinetics was influenced solely by the basicity of the incoming azole nucleophile. The order of reactivity of the nucleophiles with the complexes is in the form, 1-MIm > Im > His > Pyz.