Mechanism of migration of ancillary ligands in binuclear palladium(II) allyl complexes

Abstract

The kinetics and mechanism of the reaction between the binuclear cationic complexes [L₂ClPd{C([graphic omitted]d(η³-CH₂CR⁴CH₂)]ClO₄[1; R¹= R²= C₆H₄OMe-p, R³= H, Me, or Ph, R⁴= H or Me, L = P(C₆H₄X-p)₃, X = Cl, H, or OMe; R¹= C₆H₄OMe-p, R²= R³= R⁴= Me, L = PPh₃ or PMePh₂] and the anionic allyl derivatives [NEt₃(CH₂Ph)][PdCl₂(η³-2-R⁴C₃H₄)], (A), leading to the binuclear zwitterionic compounds [LCl₂Pd{C([graphic omitted](η³-CH₂CR⁴CH₂)], (2), together with the neutral palladium–allyl products [ PdCl(η³-2-R⁴C₃H₄) L], (B), have been studied in 1,2-dichloroethane. This reaction, which involves exchange of the ancillary ligands L and Cl^– between the reactants (1) and (A), proceeds via two parallel steps with rate constants k₁, and k₂. The faster step consists of a rapid pre-equilibrium association (K) to give the intermediate (1 ⋯ A), which then rearranges slowly (k₂)via opening of the five-membered α-di-imino-cycle in a proposed three-metal transition state. The parallel slower step is formulated as a rate-determining (k₁) intramolecular rearrangement of (1) to a labile intermediate (1*), which reacts rapidly with (A) to yield the final products. For this step, the activation process is proposed to involve unassisted α-di-imino ring opening and dissociative exchange of L. The mechanism proposed is based on the activation parameters and on the rate effects exerted by the α-di-imino-substituents R², R³, by the allyl group substituent R⁴, and by the electronic properties of L. In general, the rate constants k₁, k₂ and the pre-equilibrium constant K decrease with increasing electron-releasing abilities of the C- and N-imino-substituents; k₂ is also depressed by increasing steric demands of the R³ and R⁴ groups. An increase in the electron-donor properties of L will also bring about a decrease in the overall reaction rate. The effects are generally related to the ease of opening and rearrangement of the σ,σ-N,N-chelated α-di-imine in (1) together with the lability of the Pd–L bond.