Conformation-sensitive molecular pendulums: variable temperature NMR study of dimeric palladium(I) bisphosphine complexes

Solution and solid state 31P NMR studies were carried out on a series of [Pd2X2(dppm)2] (X = Cl (1a), Br (1b), I (1c)), or [Pd2XY(dppm)2] (X = Cl, View the MathML sourceY=SnCl3- (1d)) complexes and on methyl substituted derivatives such as [Pd2Cl2(dppm)(dppmMe)] (2), syn-[Pd2Cl2(dppmMe)2] (3), and anti-[Pd2Cl2(dppmMe)2] (4) (dppmMe = 1,1-bis(diphenylphosphino)ethane) in order to study and understand the conformational behaviour of the eight-membered Pd2P4C2 rings depending on the substituents and their stereochemistry. These complexes with metal–metal bonds and mutually trans-dppm ligands act as molecular pendulums. On the basis of temperature dependent spectra qualitative correlations have been found between the molecular conformations and the rate of a specific intramolecular motion called “swinging”. While for the extended-boat conformers (2 and 3) this exchange process is of intermediate energy (41–45 kJ mol−1), the barrier is definitely higher (∼54 kJ mol−1) for the extended-chair conformer 4. Changes of symmetry relations are reflected very vividly in the 31P NMR spectra. The observed different chemical shifts, “swinging” rates and activation free energies obtained for the boat and chair conformers are explained by the steric effects and low-temperature conformations of the axial phenyl groups.