A trans influence and π-conjugation effects on ligand substitution reactions of Pt(II) complexes with tridentate pendant N/S-donor ligands

Abstract

The rate of displacement of the chloride ligands by three neutral nucleophiles (Nu) of different steric demands, namely thiourea (TU), N,N ' -dimethylthiourea (DMTU) and N,N,N, ' N-tetramethylthiourea (TMTU) in the complexes viz;  [Pt(II)(bis(2-pyridylmethyl)amine)Cl]ClO₄, (Pt1), [Pt(II){N-(2-pyridinylmethyl)-8-quinolinamine}Cl]Cl, (Pt2), [Pt(II)(bis(2-pyridylmethyl)sulfide)Cl]Cl, (Pt3) and [Pt(II){8-((2-pyridylmethyl)thiol)quinoline}Cl]Cl, (Pt4)  was studied under pseudo first-order conditions as a function of concentration and temperature using a stopped-flow technique and UV-Visible spectrophotometry. The observed pseudo first-order rate constants for substitution reactions obeyed the simple rate law. The results have shown that  the chloro ligand in  Pt(N^S^N)  complexes is more labile by two orders of magnitude than  Pt(N^N^N)  complexes due to the high trans labilizing effect brought by the S-donor atom. The quinoline based Pt(II) complexes (Pt2  and  Pt4) have been found to be slow than their pyridine counterparts  Pt1  and  Pt3  due to poor Ï€-acceptor ability of quinoline. Second-order kinetics and large negative activation entropies support an associative mode of activation.

Key words: Substitution, nucleophiles,  pseudo  first-order, associative, entropy