Unusual Stabilization of Dication Diradical Intermediate of Dizinc(II) Porphyrin Dimer

Abstract

We have demonstrated here how the nature of a metal ion controls the reactivity of a metalloporphyrin π-cation radical. One-electron oxidations of diethylpyrrole-bridged dicopper(II) and dipalladium(II) porphyrin dimers using iodine as an oxidant result in the formation of strongly interacting cofacial mixed-valent π-cation radical dimers. The mixed-valent cation radical so generated being highly reactive drives a spontaneous and rapid transformation to form an indolizinium-fused chlorin-porphyrin heterodimer. In sharp contrast to this, similar addition of iodine leads to 1e-oxidation of dizinc(II) porphyrin dimer, which is followed by a second oxidation to produce a dication diradical complex. The axial coordination of iodine upon 1e-oxidation of dizinc(II) porphyrin dimer lowers the overall oxidation potential of the system, and thereby, making the second oxidation easily accessible. This has resulted in the stabilization of a dication diradical complex, in which two porphyrin π-cation radicals undergo electronic communication through the bridging pyrrole group. Interestingly, despite being well-separated from each other, the two radical spins undergo strong antiferromagnetic coupling to form a diamagnetic compound. The conjugation also leads to a change in identity of the bridge, which further highlights the critical role played by the bridge in the electronic communication between the two rings. DFT calculations clearly support the experimental observations.