Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These “cycloiminophosphanes” possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ¹- and P,N-κ²-tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl₂]₂ demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ¹ and κ² species; treatment with AgOTf gives full conversion to the κ² complex. The potential for catalysis was shown in the Ru^II-catalyzed, solvent-free hydration of benzonitrile and the Ru^II- and Ir^I-catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ²-Rh^III and a P-κ¹-Ru^II complexes.