Synthesis, characterization, and cis-trans isomerization studies of cis-[PdCl2{Ph2P(CH2CH2O) 3CH2CH2PPh2-P,P′}] and trans-[PtCl2{Ph2P(CH2CH2O) 3CH2CH2PPh2-P,P′}] metallacrown ethers

Article abstract of DOI:10.1002/ejic.200800436

The metallacrown ether cis-[PdCl₂{Ph₂P(CH ₂CH₂O)₃CH₂-CH₂PPh ₂-P,P′}] (cis-1) has been kinetically trapped via precipitation during the reaction of PdCl₂ and Ph₂P(CH ₂CH₂O)₃-CH₂CH₂PPh ₂ in an acetonitrile/THF mixture. The kinetics of the cis-trans isomerization and oligomerization equilibria of cis-1 have been followed using quantitative ³¹P{¹H} NMR spectroscopy. The reaction follows reversible first-order kinetics, and an Eyring analysis was carried out to yield ΔH^?_f (81 ± 2 kJmol ^-1), ΔH^?_r (46 ± 2 kJmol^-1), ΔS^?_f (117 ± 4 Jmol^-1 K^-1), and ΔS^?_r (20 ± 6 Jmol^-1K^-1) for the isomerization reaction. The ΔH^?_f is consistent with a proposed associative transition state involving partial cleavage of a Pd-P bond and partial formation of Pd-O(ether) bond to an adjacent ether oxygen. For comparison, trans-[PtCl₂{Ph₂P(CH₂CH ₂O)₃CH₂-CH₂PPh₂-P, P′}] (trans-2) was isolated from a mixture of cis and trans isomers by column chromatography. An analysis of the isomerization process of trans-2 to cis-2 using quantitative ³¹P{¹H} NMR spectroscopy revealed that no isomerization occurs at 348 K in the absence of an acid catalyst and that the acid-catalyzed trans-cis isomerization is zero order in complex. Crystal structures of cis-1, cis-2, and trans-2 are also presented, and the conformations of the metallacrown ether rings in these structures have been related to the data presented above. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.