A kinetic and mechanistic study into the substition behaviour of Plantinum (II) Polypyridyl complexes with a series of Azole Ligands.
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Date
2010
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Abstract
A novel platinum(II) complex, [Pt{4-(o-tolyl)-6-(3˝-isoquinoyl)-2,2´-bipyridine}Cl]SbF6 (Pt4) was synthesized and characterised by means of infrared, elemental analysis, mass spectroscopy, and NMR spectroscopy.
Substitution kinetics of chloride ligand from square planar platinum(II) complexes namely; [Pt(2,2´:6´,2˝-terpyridine)Cl]Cl·2H2O (Pt1), [Pt{2-(2´-pyridyl)-1,10-phenanthroline}Cl], (Pt2), [Pt{4´-(o-tolyl)-2,2´:6´,2˝-terpyridine}Cl]CF3SO3 (Pt3) and [Pt{4´-(o-tolyl)-6-(3˝-isoquinoyl)-2,2´-bipyridine}Cl]SbF6 (Pt4) were studied using a series of five-membered heterocyclic neutral nitrogen donor nucleophiles, viz. pyrazole (Pz), triazole (Tz), imidazole (Im), 1-methylimidazole (MIm) and 1,2-dimethylimidazole (DMIm) under pseudo first-order conditions in methanol. The kinetics of the substitution reactions were studied as a function of concentration of the nucleophiles at different temperatures using UV/Visible spectrophotometry and conventional stopped-flow techniques. The observed second-order rate constants, k2 followed a two term rate law kobs = k2[Nucleophile] + k-2 except for 1,2-dimethylimidazole with Pt1, Pt3, Pt4 and Pz and Tz with Pt1. The observed second-order rate constants along with the negative entropies of activation support an associative mode of substitution behaviour.
The results obtained indicate that increasing the π-conjugation in the cis and cis/trans position influences the rate of substitution reactions. Increasing the π-conjugation in the cis position decreases the rate of the substitution reactions by decreasing the π-acceptor property of the terpy moiety. On the other hand, increasing the π-conjugation in the cis/trans position increases the rate of substitution reaction by enhancing the π-acceptor property within the ligand framework. This in turn increases the electronic communication, which consequentially decreases the frontier orbital energy gap between HOMO and LUMO and hence increases the reactivity of the metal centre. Thus the observed trend for the reactivity was Pt2 > Pt1 > Pt3 > Pt4. The observed kinetic trend was further supported by DFT-calculations obtained from the computational analysis carried out for the complexes.
The substitution kinetics was influenced by the basicity of the incoming nucleophiles. Except for the sterically hindered nucleophile, DMIm, the pKa of the entering nucleophiles followed a Linear Free Energy Relationship (LFER) indicating an increase in the substitution reactions with increase in the basicity of the nucleophile. The general trend observed for the reactivity of the nucleophiles is MIm > Im > DMIm > Pz > Tz.
In the reverse reactions the replacement of the azoles by the chloride was controlled by the strength of the Pt—N(azole) bond which is dependent on the strength of the extent of the π-back-bonding between the platinum centre and the azole.
Description
Theses (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
Keywords
Platinum compounds., Theses--Chemistry.