Alkyl(quinolin-8-yl)phosphine Oxides as Hemilabile Preligands for Palladium-Catalyzed Reactions

Preligands of quinolyl-substituted secondary phosphine oxides (SPOs, 2a–d) were prepared and characterized. The unique palladium complex 3, having a distorted-square-pyramidal structure, was obtained from the reaction of 2 equiv of 2c with Pd­(COD)­Cl2 or [Pd­(μ2-Cl)­(η3-allyl)]2. In the crystal structure of 3, an apical chloride ligand and a supramolecular tetradentate ligand composed of a deprotonated 2c′ and a neutral 2c′ were resolved (2c′: PA form of 2c). Intriguingly, the gas-phase optimized geometry of 3 converged to a distorted-square-planar structure, which was predicted by density functional calculations. The solid-state distorted-square-pyramidal structure of 3 can only be explained with the consideration of environmental effects (i.e., the electrostatic interactions between the surrounding molecules). As also evidenced by 31P NMR experiments performed in different deuterated solvents, the crystal structure of 3 is retained in solution. In the crystal structure of 3, a long Pd–Cl bond was analyzed by energy decomposition analysis, showing that the bond is dominated by electrostatic character. Furthermore, application of these SPOs using the Heck reaction shows good reactivity toward common aryl bromides. The hemilabile preligand 2c also tautomerizes to the competent ligand 2c′ for palladium-catalyzed three-component reactions.