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‐κ(1)‐ and P,N‐κ(2)‐tungsten(0) carbonyl complexes, by determining the IR frequency of the trans‐CO ligands. Complexes with [RhCp*Cl(2)](2) demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ(1) and κ(2) species; treatment with AgOTf gives full conversion to the κ(2) 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‐κ(2)‐Rh(III) and a P‐κ(1)‐Ru(II) complexes.