Palladium Complexes of a Novel Family of P,N-Chelates, the 2-(2-Pyridyl)phospholes: Synthesis, Structural Characterization, and Catalytic Activity for Olefin/CO Copolymerization

Abstract

2-(2-Pyridyl)phospholes bearing phenyl or cyclohexyl substituents on the phosphorus atom and 2-pyridyl, phenyl, or 2-thienyl groups on the C 5 atom were prepared according to the Fagan-Nugent route. They reacted with (cod)PdClCH 3 , giving rise to stable complexes 3a-c and 3a'-c' as single diastereoisomers in excellent yields. An X-ray diffraction study performed on complex 3b featuring a 1-phenyl-2-(2-pyridyl)phosphole ligand revealed a cis configuration of the phosphorus atom and the methyl group. The geometry of the coordinated P atom is highly distorted due to the rigidity of the 1,4-P,N chelate backbone. Complexes 3a,a', featuring 2,5-bis(2-pyridyl)phosphole ligands, display fluxional behavior. NMR studies revealed an intramolecular exchange between the pendant and coordinated pyridyl groups, showing that 2-(2-pyridyl)phospholes can behave as hemilabile ligands. Treatment of complexes 3a-c and 3a'-c' with 1 equiv of AgSbF 6 in CH 3 CN afforded the corresponding cationic derivatives 4a-c and 4a'-c' as single diastereoisomers. Complexes possessing no pendant pyridyl moiety are air-stable solids that can be isolated in high yield. They are active for the copolymerization of ethylene and CO in CH 2 Cl 2 , whereas complex 4a, bearing the 2-(2-pyridyl)phosphole ligand 2a, is reduced quantitatively to give the dinuclear [Pd 2 (2a) 2 ] 2 + complex 6. The catalytic activities obtained with complexes 4b,c and 4b',c' are among the highest obtained with P,N chelates. The substitution pattern of the 2-(2-pyridyl)-phosphole ligands has an impact on the productivity of the Pd catalysts. P-cyclohexylphospholes are more efficient ligands than their P-phenyl analogues. Complex 4c, featuring a 2-(2-pyridyl)-5-(2-thienyl)phosphole ligand, catalyzed the copolymerization of CO and norbornene at 85 °C, affording oligomers with a narrow molecular weight distribution. In methanol, the cationic palladium methyl complex 4c afforded the bis-chelate [Pd(2c) 2 ] 2 + complex 5, which has been characterized by an X-ray diffraction study. Although the geometry around the palladium atom is distorted, the metric data of complex 5 are similar to those of 3b.