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

Abstract

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.