Unprecedented ring-ring interconversion of N,P,C-cage ligands.

The novel N,P,C-cage complexes 5 a-f and 6 a-f have been obtained by the reaction of the P-pentamethylcyclopentadienylphosphinidene complex 2, generated thermally from 2H-azaphosphirene complex 1, with N-methyl-C-arylcarbaldimines 3 a-f. Li/Cl phosphinidenoid complex 8 reacted with 3 a,b to give N,P,C-cage complexes 6 a,b, whereas with 3 c-f, complexes 6 c-f were obtained in negligible amounts only. Both types of ligand N,P,C-cage structures 5 and 6 were found to be in an unprecedented equilibrium, with 5 a,f as the predominant species. Transient electrophilic terminal phosphinidene complexes 10 a-f serve as intermediates in both ligand interconversions (5 a,f↔6 a,f), as evidenced through trapping reactions with phenylacetylene and N-methyl-C-phenylcarbaldimine, thus leading to the novel N,P,C-cage complexes 13 b and 15. DFT calculations predicted a small difference in the relative energies of the two types of N,P,C-cage ligands, and a remarkable stabilisation of the aminophosphinidene complex 10 as the common precursor, thereby providing an insight into this surprising 5-ring-3-ring interconversion. In depth analysis of intermediate 10 revealed the occurrence of both through-bond (conventional inductive/mesomeric effects) and through-space (non-covalent interactions) mechanisms, which amount to 67.8 and 14.4 kcal mol(-1), respectively, and account for the remarkable stabilisation of this intermediate.