Reaction of NHOs with Bisphosphanes - Designing Diradicaloids, Zwitterions and Radicals

The linkage of an imidazole-based N-heterocyclic olefin (NHO), containing a terminal CH₂ donor group, with a phosphorus-centered diradical molecular fragment leads to an open-shell singlet diphospha-indenylide system, a new class of P-heterocycles, which can be interpreted both as a phosphorus-centered diradicaloid and as a zwitterion with a permanent, overall charge separation between the N- and P-heterocyclic ring systems. The rotation of the imidazole ring, which is thermally possible due to a central C-C bond with a weakened π-component, changes both the charge separation and diradical character depending on the dihedral angle, as quantum mechanical calculations indicate. By varying the bulkiness of substituents at the imidazole-based NHO, it was possible to obtain different diphospha-indenylide species with different rotation angles in the solid state and hence varying diradical character. Imidazolium-diphospha-indenylides represent a new class of NHO-based zwitterions with diradical character. Their synthesis, structure, and activation chemistry are described, as well as the quantum mechanical description of the electronic structure in these unusual heterocycles. In addition, along the synthesis route to diphospha-indenylide, we also succeeded in isolating a highly reactive monoradical anion, which was also fully characterized.