Metalation Control of Open-Shell Character in meso-meso Linked Porphyrin meso-Oxy Radical Dimers

Control of open-shell character of meso-meso linked porphyrin meso-oxy radical dimers has been demonstrated by core metalation. Namely, Ni^II -porphyrin dimer 6Ni exhibits a clear ¹ H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H₂ shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature-dependent ¹ H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis-imidazole-coordinated Zn^II -porphyrin dimer 6ZnIm₂ exhibits a perpendicular structure consisting of two planar Zn^II -porphyrins and has been characterized as a distinct open-shell diradical on the basis of its non-observable ¹ H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm₂ is an extremely stable molecule.