Understanding the role of flexible 4′-functionalized polyethylene glycoxy chains on the behavior of platinum(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine: a kinetic and a mechanistic study

The ligand substitution kinetics of 4 '-functionalized mononuclear Pt(II) (4 '-(ethylene glycoxy)-2,2 ':6 ',2 ''-terpyridine complexes, [Pt(nY-tpy)Cl)Cl] (where Y=ethylene glycoxy, n=number of ethylene, glycoxy units=1, 2, 3, and 4, and tpy=2,2 ':6 ',2 ''-terpyridine), with thiourea, 1,3-dimethyl-2-thiourea, 1,1,3,3-tetramethyl-2-thiourea, and iodide were investigated under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow technique. The observed first-order rate constants followed the simple rate law k(obs)=k(2)[Nu]. The data obtained show that the ethylene glycoxy pendant, trans to the leaving group, acts as a sigma-donor into the terpyridine ligand and is effective only up to n=1, beyond which the substitution reactivity of the complexes are controlled by the steric influence of the appended ethylene glycoxy pendant units, which decreases with increase in the number of ethylene glycoxy units. The activation parameters obtained support an associative mechanism, where bond formation in the transition state is favored. The observed reactivity trends were supported by density functional theory calculations.