RESUMEN
The kinetics of chloride substitution from a series of square-planar platinum(II) complexes, viz. [Pt(terpyridine)Cl](+), (Pt1), [Pt{2-(2'-pyridyl)-1,10-phenanthroline}Cl]Cl, (Pt2), [Pt{4'-(2'''-CH3-phenyl)-2,2':6',2''-terpyridine}Cl]CF3SO3 (Pt3) and [Pt{4'-(2'''-CH3-phenyl)-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 using UV/Visible spectrophotometry and conventional stopped-flow techniques. The observed second-order rate constants, k2, followed a two term rate law k(obs) = k2[nucleophile] + k(s) except for DMIm with Pt1, Pt3, Pt4 and Pz, Tz with Pt1. Increasing the π-conjugation in the cis position decreases the rate of chloride substitution by decreasing the π-acceptor property of the terpy moiety. However, increasing the π-conjugation in the cis/trans position increases the rate of substitution by enhancing the π-acceptor property within the ligand framework whereby increasing the reactivity of the metal centre. The observed trend for the reactivity was Pt2 > Pt1 > Pt3 > Pt4. The substitution kinetics was influenced by the basicity of the incoming nucleophiles except for the sterically hindered nucleophile, DMIm. The general trend observed for the reactivity of the nucleophiles is MIm > Im > DMIm > Pz > Tz.
Asunto(s)
Azoles/química , Compuestos de Platino/química , Piridinas/química , Azoles/metabolismo , Cristalografía por Rayos X , Ligandos , Compuestos de Platino/metabolismo , Piridinas/metabolismoRESUMEN
The rate of substitution of the chloride ligand from [Pt(terpy)Cl](+) () (where terpy = 2,2':6',2''-terpyridine) and its corresponding analogue [Pt((t)Bu(3)terpy)Cl](+) () (where (t)Bu(3)terpy = 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) by a series of neutral and anionic nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I(-)) and thiocyanate (SCN(-)), was determined under pseudo first-order conditions as a function of concentration and temperature using standard stopped-flow spectrophotometric techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k(obs) = k(2)[nucleophile]. Second-order kinetics and negative activation entropies support an associative mode of activation. The rate of substitution of chloride is observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by I(-) was observed to be faster than that by SCN(-), in correlation with their polarizability and the softness of the metal centre. A comparison of the second-order rate constants, k(2), at 298 K, obtained for the substitution reactions of and shows that the introduction of strong sigma-donating groups on the periphery of the terpyridyl backbone in results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of theory for the two complexes, and , and a series of similar analogues containing either electron-donating or electron-withdrawing groups in the periphery positions demonstrate that the introduction of electron-donating groups decreases the positive charge on the metal centre and increases energy separation of the frontier molecular orbitals (E(HOMO)-E(LUMO)) of the ground state platinum(ii) complexes leading to a less reactive metal centre whilst the introduction of electron-withdrawing groups has an opposite effect leading to increased reactivity of the metal centre.
RESUMEN
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.