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

2-(2-Pyridyl)phospholes bearing phenyl or cyclohexyl substituents on the phosphorus atom and 2-pyridyl, phenyl, or 2-thienyl groups on the C5 atom were prepared according to the Fagan−Nugent route. They reacted with (cod)PdClCH3, 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 AgSbF6 in CH3CN 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 CH2Cl2, whereas complex 4a, bearing the 2-(2-pyridyl)phosphole ligand 2a, is reduced quantitatively to give the dinuclear [Pd2(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.磷原子上带有苯基或环己基取代基、且C5位连有2-吡啶基、苯基或2-噻吩基的2-(2-吡啶基)磷杂环戊二烯（2-(2-Pyridyl)phospholes）按照Fagan−Nugent路线合成。该类配体与(cod)PdClCH3反应，以优异收率得到单一非对映异构体形式的稳定配合物3a−c与3a‘−c‘。对带有1-苯基-2-(2-吡啶基)磷杂环戊二烯配体的配合物3b开展的X射线衍射研究表明，其磷原子与甲基呈顺式构型。受1,4-P,N螯合骨架的刚性影响，配位磷原子的几何构型发生显著畸变。带有2,5-双(2-吡啶基)磷杂环戊二烯配体的配合物3a,a‘表现出流变行为。核磁共振研究揭示了悬挂吡啶基与配位吡啶基之间的分子内交换过程，证实2-(2-吡啶基)磷杂环戊二烯可作为半不稳定配体（hemilabile ligands）。以1当量的AgSbF6在CH3CN中处理配合物3a−c与3a‘−c‘，可得到相应的单一非对映异构体形式的阳离子衍生物4a−c与4a‘−c‘。不含悬挂吡啶基的配合物为空气稳定的固体，可高收率分离。该类配合物在CH2Cl2中可催化乙烯与一氧化碳的共聚反应，而带有配体2a的配合物4a会被定量还原，生成双核[Pd2(2a)2]2+配合物6。使用配合物4b、4c与4b‘、4c‘所获得的催化活性，属于P,N螯合配体体系中已报道的较高活性之列。2-(2-吡啶基)磷杂环戊二烯配体的取代模式对钯催化剂的催化产率具有显著影响。P-环己基取代的磷杂环戊二烯配体的催化效率优于其P-苯基取代的同类配体。带有2-(2-吡啶基)-5-(2-噻吩基)磷杂环戊二烯配体的配合物4c可在85℃下催化CO与降冰片烯的共聚反应，得到分子量分布窄的齐聚物。在甲醇溶剂中，阳离子钯甲基配合物4c会生成双螯合型[Pd(2c)2]2+配合物5，该配合物已通过X射线衍射研究完成表征。尽管钯原子周围的几何构型存在畸变，但配合物5的结构度量参数与配合物3b相近。