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

Abstract

AbstractWe report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin‐porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C−C and one C−N bonds. The 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 CuII and π‐cation radical spins to form a triplet state. The structural and electronic perturbation in the unconjugated 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 >20 kcal mol−1 which was supplied in the form of reflux condition to initiate such a rearrangement process. UV‐vis, EPR and ESI‐MS spectroscopies were used to monitor the rearrangement process in situ in order to identify the key reactive intermediates leading to such an unusual transformation.