Modulating the propeller-like shape of a tripodal C(CH2PPh2)3 fragment by the size of the substituent at the pivotal carbon atom in macrobicyclic tri-λ-phosphazenes

The chiral macrobicyclic tri-λ⁵-phosphazenes formed by tripod-tripod coupling of tris(3-azidobenzyl)amines and 1,1,1-tris[(diphenylphosphino)methyl]methanes present helical topologies as a result of combining two propeller-shaped tripodal fragments with the same sense of twist. The introduction of a series of R_piv substituents of increasing size at the pivotal carbon of the lower tert-butane fragment R_piv-C(CH₂PPh₂)₃ causes a gradual decrease of the helicity in the lower propeller. This phenomenon is revealed in their CDCl₃ solution NMR spectra, and the activation energies for the racemization process of the tri-λ⁵-phosphazenes were calculated by coalescence VT-NMR experiments.