N−C Cleavage in Pincer PNP Complexes of Palladium

Several new N-methylated diarylamine-based PNP pincer ligands have been prepared.
The synthesis of these ligands is modular and allows incorporation of a variety of substituents
that change the solubility and the stereoelectronic properties of the ligand as well as allow
for the introduction of a sensitive 19F NMR spectroscopic probe. The reactions of PN(Me)P
ligands with PdX2 (X = Cl, OAc) initially proceed with formation of an adduct, (PN(Me)P)PdX2, that may exist in either the neutral or the ionic forms. These adducts are unreactive
in the case of PPh2-bearing ligands, but with the more donating PPri2-bearing ligands, the
adduct evolves into square planar (PNP)PdX with irreversible loss of MeX. Thus, the
feasibility of cleavage of an unstrained N−C bond by PdII is demonstrated. The N−C cleavage
is accelerated by decreasing the solvent polarity. The mechanism may involve either N−C
oxidative addition or a nucleophilic attack (external or internal) of X- on the Me group of
the N-bound PN(Me)P ligand.

已成功制备数种新型N-甲基化二芳基胺基PNP钳形配体。该类配体的合成具有模块化特性，可引入多种取代基，这些取代基既能调控配体的溶解性与立体电子性质，还可引入灵敏的¹⁹F核磁共振光谱探针。
PN(Me)P配体与PdX₂（X=Cl、OAc）的反应初始阶段会生成加合物(PN(Me)P)PdX₂，该加合物可呈中性或离子型两种存在形式。带有PPh₂取代基的配体对应的此类加合物不具备反应活性；而对于给电子能力更强的PPri₂取代配体，该加合物会发生转化，失去MeX并生成平面正方形构型的(PNP)PdX配合物，该过程为不可逆反应。由此证实了Pd(II)可断裂非张力N-C键的可行性。
降低溶剂极性可加速该N-C键断裂过程。该反应的机理可能涉及两种路径：一是N-C键的氧化加成，二是X⁻对N键合PN(Me)P配体甲基位点的亲核进攻（可分为外部亲核进攻与内部亲核进攻两种形式）。