Redox-triggered reversible interconversion of a monocyclic and a bicyclic phosphorus heterocycle.

Molecules which change their structures significantly and reversibly upon an oxidation or reduction process have potential as future components of smart materials. A prerequisite for such an application is that the molecules should undergo the redox-coupled transformation within a reasonable electrochemical window and lock into stable redox states. Sodium phosphaethynolate reacts with two equivalents of dicyclohexylcarbodiimide (DCC) to yield an anionic, imino-functionalized 1,3,5-diazaphosphinane 3 a. The oxidation of this anion with elemental iodine causes an intramolecular rearrangement reaction to give a bicyclic 1,3,2-diazaphospholenium cation 6. This umpolung of electronic properties from non-aromatic to highly aromatic is reversible, and the cation 6 is reduced with elemental magnesium to reform the 1,3,5-diazaphosphinanide anion 3 a. Theoretical calculations suggest that phosphinidene species are involved in the rearrangement processes.