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-kappa(1)- and P,N-kappa(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 kappa(1) and kappa(2) species; treatment with AgOTf gives full conversion to the kappa(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-kappa(2)-Rh-III and a P-kappa(1)-Ru-II complexes.

Automated summary

Novel cyclic imine-based ligands with high basicity were synthesized and their donor strength was investigated. These ligands showed potential for catalysis in various reactions, demonstrating flexibility and catalytic activity.