Synthetic, structural, and dynamic NMR studies of (bisphosphine)palladium(0) complexes of dibenzylideneacetone

Two complexes of the type (R2PCH2CH2PR2)Pd(dba) have been prepared by the reaction of Pd2(dba)3·CHCl3 with R2PCH2CH2PR2 (R=iPr (1), 74%; Cy (2), 57%; dba=dibenzylideneacetone). X-ray crystallographic studies of 1 and 2 reveal that the coordinated dba ligand adopts an s-trans, s-trans conformation in which the palladium is coordinated to one CC bond in an η2-fashion. Variable temperature, 1H- and 31P{H}-NMR spectroscopy of 1 show two distinct dynamic processes in solution. In the 1H-NMR spectra, a rapid intramolecular exchange of coordinated and uncoordinated CC bonds is observed with the estimated ΔGex‡ being 14 kcal mol−1. In the 31P{H}-NMR spectra, a facile interconversion of the predominant s-trans, s-trans conformer with the minor s-trans, s-cis, and s-cis, s-cis conformers begins to occur at higher temperatures. Molecular mechanics calculations place the relative energies of the three isomers at 0, 0.9, and 4.7 kcal mol−1, respectively. An intramolecular mechanism for double bond exchange is proposed to occur via a symmetric transition state involving the s-cis, s-cis conformer. Strong coordination of dba to palladium in 1 is proposed to account for slow reactions with PhX where the relative rates of oxidative addition were found to increase in the order of X=Cl≪Br