Mechanistic Studies of Isomerization Reactions of Tetragonal Planar Tertiary Phosphine Complexes of Palladium(II)

Abstract

The complexes [PdCl2(Me2RP)2] (R = o-tolyl; α-napthyl) may be isolated in both cis and trans forms. In CHCl3 solution cis-trans isomerization processes for these complexes are slow and this is ascribed to steric hindrance by the bulky R groups. The cis-trans isomerization is catalyzed by a variety of added ligands L. These isomerization processes have been extensively studied for the complexes cis-and trans-[PdCl2(Me2o-tolylP)2] in CHCl3 solution. The results of variable temperature n.m.r. and u.v. kinetic studies are fully consistent with a consecutive displacement mechanism with an ionic species of the type [PdCl(Me2o-tolylP)2L]+Cl− as an intermediate. The kinetic characteristics are determined by the nature and relative stability of this ionic intermediate. For L = Me2o-tolylP the rate of isomerization is zero order in complex and second order in added phosphine concentration. (At 38° kcis→trans = 6.2 × 103 l mol−1 s−1, ktrans→cis = 6.3 × 103 l mol−1 s−1) For L = PPh3 the rate of isomerization is considerably slower and first order in both complex and added PPh3 concentrations (kcis→trans6.3 = × 10 l mol−1 s−1, ktrans→cis = 6.7 × 10 l mol−1 s−1). Isomerization is also effected in neat methanol and the rate is first order in complex concentration. N.m.r. studies of mixtures of trans-[PdI2(Me2o-tolylP)2] and cis-[PdCl2(Me2o-tolylP)2] in CHCl3 solution show halogen exchange to occur more rapidly than isomerization. However a cis-diiodo complex is not observed and instead isomerization to the trans-diiodo complex is observed to occur.