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

Equilibrium constants K-c have been determined for the olefin susbtitution in palladium(0) alpha-diimine olefin (ol) complexes [Pd(eta(2)-ol)(GL)] by ol' in chloroform [L-L = 2-NC5H4CH=NC(6)H(4)OMe-4; ol' = dimethyl fumarate (dmf), ol' = acrylonitrile (an), 1-chloro-1-cyanoethylene, 1,4-naphthoquinone (nq), tetramethyl ethylenetetracarboxylate (tmetc), fumaronitrile (fn) or maleic anhydride (ma); ol = nq, ol' = tmetc, fn or mal. The substitution equilibria are rapidly established except when ol = dmf, nq, ol' = tmetc and were studied by UV/VIS and H-1 NMR spectrometry. The K-c 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 k(2) for the forward step could be measured with sufficient accuracy, whereas for ol = nq, ol' = tmetc the constants for both the forward (k(2)) and the reverse (k(-2)) steps were determined. Activation parameters indicate an essentially associative mechanism for these paths.