Tuning Reactivity of Platinum(II) Complexes

Chapter 3
The π-Acceptor Effect in the Substitution Reactions of Tridentate N-
Donor Ligand Complexes of Platinum(II): A Detailed Kinetic and
Mechanistic study
3.0 Abstract
The nucleophilic substitution reactions of the complexes of [Pt{4’-(2’’’-CH3-phenyl)-2,2’:
6’,2’’-terpyridine}Cl]CF3SO3,CH3PhPtCl,[Pt{4'-(2’’’-CH3-phenyl)-6-(3’’-isoquinoyl)-2,2’-
bipyridine}Cl]SbF6, CH3PhisoqPtCl,[Pt{2-(2’-pyridyl)-1,10-phenanthroline}Cl]Cl,
pyPhenPtCl, and [Pt(terpy)Cl] + , PtCl with a series of nucleophiles, viz.: thiourea (TU),
N,N-dimethylthiourea (DMTU), N,N,N,N-tetramethylthiourea (TMTU), I ¯, Br ¯, and SCN ¯ in
0.1 M LiCF3SO3 in methanol, have been studied. The order of reactivity of the complexes
under investigation was found to decrease as follows pyPhenPtCl > PtCl > CH3PhPtCl >
CH3PhisoqPtCl. The lability of the chloride of the starting complexes is dependent on
the strength of π-backbonding of the spectator ligands around the platinum centre. This
effect is controlled by how the fused ring system around the terpy moiety is structured.
The experimental data is strongly supported by the DFT-calculations. The dependence of
the second order rate constants on concentration of the nucleophiles and the large and
negative values of the activation entropies (ΔS ≠ ) support an associative mode of
activation for the substitution of the chloride ligand.
3.1 Introduction
Since the discovery of anticancer activity of cis-Pt(NH3)2Cl2 (cisplatin) by Rosenberg et
al., 1 many new Pt(II) complexes have been developed with the aim of obtaining better
antitumor activity, increased solubility and reduced toxicity. 2 This list includes
complexes of Pt(II) with 2,2’:6’,2’’-terpyridine (terpy) or related polypyridine ligands
that have widely recognised spectrochemical interaction with DNA and proteins. 3-5
These Pt(II) terpy complexes have been found to be cytotoxic against Trypanosoma and
1