Solvent effects on ligand exchange between two different pd(II) organometallic complexes

Abstract

We have studied the mechanism of the following transfer of ligands between Pd(II) metal centers of different complex moieties: ▪ at various temperatures and ionic strengths in different solvents. The pseudo-first order rate constants for the disappearance of I fit the complex rate law (2): ▪ indicating a formal mechanism of the type (3): ▪ The pre-equilibrium step (K e is shown by 1H NMR and electronic spectra to be the formation of an associated species, in which a fast η 3 ⇌ η 1 conversion of the allyl of I occurs; this is followed by a slow, rate-determining migration (k 2) of ligands L and Cl −. For this step we propose a trinuclear transition state which requires opening of the five-membered ▪ ring. The k 1 path implies a slow intramolecular rearrangement of I, via either L dissociation of α-diimine chelate ring opening, yielding a reactive intermediate I. The latter reacts rapidly with A to give the final products. An increasing polarity of solvents, on going from 1,2-dichloroethane to acetone, produces a marked increase of both K e and k 2. The pre-equilibrium constant K e is also strongly depressed by increasing ionic strength.