Oligomerization and cis-trans isomerization equilibria in dichloropalladium(II) metallacrown ethers and a dichloropalladium(II) complex of 1,12-bis(diphenylphosphino)dodecane

Abstract

Quantitative ³¹P{¹H} NMR spectroscopic studies of monomer–oligomer and cis-trans equilibria in chloroform-d solutions of PdCl₂{Ph₂P(CH₂CH₂O)_nCH₂CH₂PPh₂-P,P′}_n (n = 3, 4, 5) metallacrown ethers and of PdCl₂{Ph₂P(CH₂)₁₂PPh₂-P,P′} are reported. The NMR data for all of the complexes can be adequately modeled using a single cis-trans isomerization equilibrium and two step-polymerization (dimerization, oligomerization) equilibria. As expected, both the dimerization and oligomerization equilibrium constants for the metallacrown ethers increase as n increases. However, the dimerization equilibrium constants of the metallacrown ethers are much smaller than are the oligomerization equilibrium constants. In contrast, the dimerization and oligomerization equilibrium constants for PdCl₂{Ph₂P(CH₂)₁₂PPh₂-P,P′} are nearly identical and are significantly larger than are those of the metallacrown ethers. Kinetic studies of monomer–oligomer and cis-trans equilibria in solutions of cis-PdCl₂{Ph₂P(CH₂CH₂O)₃CH₂CH₂PPh₂-P,P′} indicate that the trans monomer and trans oligomers are formed at approximately the same rates and that the reactions follow reversible first-order kinetics. The very different dimerization constants for the metallacrown ethers and PdCl₂{Ph₂P(CH₂)₁₂PPh₂-P,P′}and the similar rates of formation for the trans monomer and trans oligomers suggest that the isomerization and dimerization reactions have a common rate-determining step that involves cleavage of a palladium–phosphorus bond.