Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

Abstract

Abstract The substitution kinetics of the complexes [Pt{4′‐( o ‐CH 3 ‐Ph)‐terpy} Cl]SbF 6 (CH 3 PhPtCl(Sb)), [Pt{4′‐( o ‐CH 3 ‐Ph)‐terpy}Cl]CF 3 SO 3 (CH 3 PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF 6 (PhPtCl), [Pt(terpy)Cl]Cl·2H 2 O (PtCl), [Pt{4′‐( o ‐Cl‐Ph)‐terpy}Cl]SbF 6 (ClPhPtCl), and [Pt{4′‐( o ‐CF 3 ‐Ph)‐terpy}Cl]SbF 6 (CF 3 PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N , N ′‐dimethylthiourea (DMTU), and N , N , N ′, N ′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law k obs = k 2 [Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF 3 PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH 3 PhPtCl. Thiourea was found to be the best nucleophile with N , N , N ′, N ′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008