Equilibria and rates of olefin substitution in zerovalent palladium complexes containing a 2-pyridylmethanimine ligand

Abstract

Equilibrium constants K_e have been determined for the olefin susbtitution in palladium(0)α-diimine olefin (ol) complexes [Pd(η²-ol)(L–L)] by ol′ in chloroform [L–L = 2-NC₅H₄CHNC₆H₄OMe-4; ol = dimethyl fumarate (dmf), ol′= acrylonitrile (an), l-chloro-l-cyanoethylene, 1,4-naphthoquinone (nq), tetramethyl ethyleneteracarboxylate (tmetc), fumaronitrile (fn) or maleic anhydride (ma); ol = nq, ol′= tmetc, fn or ma]. The substitution equilibria are rapidly established except when ol = dmf, nq, ol′= tmetc and were studied by UV/VIS and ¹H NMR spectrometry. The K_e values increase with increasing electron affinity of the entering activated olefin. The kinetics of slow approach to equilibrium were monitored by UV/VIS techniques at various temperatures. When ol = dmf, ol′= tmetc, only the second-order rate constant k₂ for the forward step could be measured with sufficient accuracy, whereas for ol = nq, ol′= tmetc the constants for both the forward (k₂) and the reverse (k_–2) steps were determined. Activation parameters indicate an essentially associative mechanism for these paths.