Reversible Single Electron Redox Steps Convert Polycycles with a C3P3 Core to a Planar Triphosphinine

Reaction of the imidazolium-stabilized diphosphete-diide IDP with trityl phosphaalkyne affords a mixture which contains the molecules 1 a and 1 b with a central C₃P₃ core, which formally carries a two-fold negative charge. In order to avoid the formation of an antiaromatic 8π electron system within a conjugated dianionic six-membered C₃P₃]²⁻ ring, 1 a adopts a bicyclic 3.1.0] and 1 b a tricyclic 2.2.0.0] structure which are in a dynamic equilibrium. 1 a , b can be reversibly oxidized to a triphosphinine dication 5 ]²⁺ with a central flat aromatic six-membered C₃P₃ ring. This two-electron redox reaction occurs in two single-electron transfer steps via the 7π-radical cation 4 ]⋅⁺, which could also be isolated and fully characterized. The profound reversible structural change observed for the two-electron redox couple 5 ]²⁺/ 1 a , b is in sharp contrast to the C₆H₆/C₆H₆]²⁻ couple, which undergoes only a modest structural deformation.