Controlling the extent of π-backbonding in platinum(ii) terpyridyl systems: a detailed kinetic, mechanistic and computational approach

Abstract

The rate of substitution of the chloride ligand from [Pt(terpy)Cl]⁺ (Pt1) (where terpy = 2,2′:6′,2″-terpyridine) and its corresponding analogue [Pt(^tBu₃terpy)Cl]⁺ (Pt2) (where ^tBu₃terpy = 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 = k₂[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, k₂, at 298 K, obtained for the substitution reactions of Pt1 and Pt2 shows that the introduction of strong σ-donating groups on the periphery of the terpyridyl backbone in Pt2 results in a corresponding decrease in the reactivity. DFT calculations at the B3LYP/LACVP** level of theory for the two complexes, Pt1 and Pt2, 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.