η1- and η3-Allylpalladium(II) Complexes Bearing Potentially Tridentate Ligands: Synthesis, Solution Dynamics, and Crystal Structures

The potentially tridentate ligands 2,2′:6′,2"-terpyridine (terpy) and 2,6-bis(diphenylphosphanylmethyl)pyridine (PNP) have been used to prepare novel palladium−allyl complexes of general formula [(terpy)Pd(CH2CHCRR′)](BF4) [R = R′ = H (1); R = H, R′ = Me (2); R = R′ = Me (3)] and [(PNP)Pd(CH2CHCRR′)](BF4) [R = R′ = H (4); R = H, R′ = Me (5); R = R′ = Me (6)], which were characterized by elemental analysis, IR spectroscopy, and 13C, 31P and variable-temperature 1H NMR spectroscopy. The low-temperature 1H NMR spectra show that the configuration of the complexes in solution depends strongly on the nature of the ligand. The terpy complexes 1−3 are η3-allyl species, where terpy is dihapto and one pyridine ring remains uncoordinated, whereas the PNP complexes 4−6 occur as η1-allyl compounds with a trihapto PNP ligand. All complexes are fluxional through η3-η1 exchange processes. Energy barriers of 47.5−48.6 kJ·mol−1 (243−262 K) are reported for the interconversion of terminal allylic protons (complexes 2−4). ΔG‡ is higher (71.1 kJ·mol−1 at 350 K) for the interconversion of the methyl groups in complex 6. A lower-barrier oscillatory mechanism involving terpy (ΔG‡ = 43.9 kJ·mol−1 at 231 K) is also involved in complex 1. The X-ray structures of complexes [(η3-terpy)Pd(η1-C5H9)](BF4) (3a) and [(η3-PNP)Pd(η1-C4H7)](BF4) (5a) are reported; the structure of 3a corresponds to a species that is not observed in solution.