Stable Dication Diradicals of Triply‐fused Metallo Chlorin‐porphyrin Heterodimers: Impact of the Bridge on Control of Spin Coupling to Reactivity

Herein we report an unexpected rearrangement, controlled by the nature of bridge, leading to the formation of novel, remarkably stable triply‐fused dinickel(II)/dicopper(II) chlorin‐porphyrin dication diradical heterodimers in excellent yields in which dipyrromethane‐bridge gets completely fused between two porphyrin macrocycles with two new C–C and one C–N bonds. Two macrocycles exhibit extensive π‐conjugation through the bridge which results in an antiferromagnetic coupling between the two π‐cation radicals. In addition, the macrocyclic distortion also favours a rare intramolecular ferromagnetic interaction between the Cu(II) and π‐cation radical spins to form a triplet state. The structural and electronic perturbation in the non‐conjugated dication diradical possibly enables the bridging pyrrolic nitrogen to undergo a nucleophilic attack at the nearby β‐carbon of the porphyrin π‐cation radical with a computed free energy barrier of more than 20 kcal/mol which has been supplied in the form of reflux condition to initiate such rearrangement process. UV‐vis, EPR and ESI‐MS spectroscopies were utilized to monitor the rearrangement process in situ in order to identify key reactive intermediates leading to such an unusual transformation.