Reversible polymerization and cis-trans isomerization equilibria in [PdCl2{Ph2P(CH2CH2O)4CH2CH2PPh2-P,P′}] metallacrown ethers

The first study of metallacrown ethers with kinetically labile metal centers is reported. The reaction of Ph2P(CH2CH2O)(4)CH2CH2PPh2 with PdCl2 in an acetonitrile-dichloromethane solution yields an equilibrium mixture of cyclic n-mers and monomers with the empirical formula [PdCl2,{Ph2P(CH2CH2O)(4)CH2CH2PPh2-P,P'}]. Both cis and trans coordination geometries are observed for the palladium(II) in these complexes, with the trans being the more abundant. The solution equilibria in both chloroform-d and acetonitrile-d(3) were studied using P-31{H-1}, C-13{H-1}, and H-1 NMR spectroscopy. A model has been developed relating the product distributions at various concentrations, obtained from integration of quantitative P-31{H-1} NMR spectra, to a cis-trans equilibrium constant and equilibrium constants for the dimerization and n-merization reactions of the trans monomer. The concentration and temperature dependence of the equilibrium constants obtained from this model are consistent with the thermodynamics of cis-trans isomerization and reversible step polymerization. One surprising result from this study is that the dimerization equilibrium constant is nearly an order of magnitude smaller than the n-merization equilibrium constant.