Dynamic Covalent Chemistry Approach toward 18-Membered P4N2 Macrocycles and Their Nickel(II) Complexes.

A dynamic covalent chemistry approach was used for the stereoselective synthesis of 1,10-diaza-3,8,12,17-tetraphosphacyclooctadecanes via condensation reaction of 1,4-bis(organylphosphino)butane, formaldehyde, and primary amines. The obtained 18-membered P4N2 macrocycles were isolated in pure form as meso- (RPSPSPRP) or rac- (RPRPRPRP/SPSPSPSP) isomers. The structural features of the individual stereoisomers were revealed by NMR spectroscopy and X-ray structure analysis. All P4N2 macrocycles form square-planar nickel(II) complexes with the RPSPSPRP isomer only, in which the orientation of the lone pairs of electrons at phosphorus favors this coordination mode, independent of the initial configuration of the ligand, indicating the ability of the 18-membered P4N2 macrocycles to stereoisomerize in the course of the complexation.