Role of chelate substituents and cis sigma-effect on the rate of ligand substitution at Pt(N-N-N) and Pt(N-N-C) centres

• Published in: Dalton transactions : an international journal of inorganic chemistry no. 2; pp. 299 - 304
• Main Authors: Jaganyi, D, Reddy, D, Gertenbach, J A, Hofmann, A, van Eldik, R
• Format: Journal Article
• Language: English
• Published: England 21.01.2004
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/b311595j

Four complexes of the type [Pt(N-N-X)Cl](X = N or C) were tailor synthesized for mechanistic studies in methanol. The terdentate ligands included terpy, 4'-Ph-terpy, 4'-(2"-CF(3)-Ph)-terpy, and 4'-(2"'-CF(3)-Ph)-6-Ph-2,2'-bipy. The rate of substitution of the chloro ligand by thiourea, N,N'-dimethylthiourea, and N,N,N',N'-tetramethylthiourea was studied as a function of nucleophile concentration, temperature and pressure by using a stopped-flow technique. The observed pseudo-first-order rate constants for the substitution reactions obeyed the simple rate law k(obs) = k(2)[Nu]. Second-order kinetics and negative activation entropies and volumes support an associative substitution mechanism. At 298 K, the values of the second-order rate constant show a slight dependence on the nature of the moiety attached to the terpy ligand. Changing from a nitrogen sigma-donor to a carbon sigma-donor in the cis position, results in a deceleration of the substitution rate. The results suggest that the Pt-C bond in the cis position activates the metal centre in a different way than in the trans position.