Stepwise Oxidations in a Cofacial Copper(II) Porphyrin Dimer: Through-Space Spin-Coupling and Interplay between Metal and Radical Spins

cis and trans-copper(II) porphyrin dimers have been synthesized, in which two Cu^II porphyrin macrocycles are bridged through a rigid ethene linker for possible through-space and through-bond spin-couplings between the paramagnetic Cu^II centers. It has been found that the two macrocycles come closer after 1 e⁻ oxidation, however, they move far apart upon further 1 e⁻ oxidation leading to transformation of the cis to the trans isomer. Detailed investigations are performed here on the interactions between the two porphyrin macrocycles, between two unpaired spins of closely spaced Cu^II centers, and also between the unpaired spins of metal and porphyrin π–cation radicals. Spectroscopic investigations along with the X-ray structure of the 2 e⁻-oxidized complex displayed strong electronic communications through the bridge between two porphyrin π–cation radicals. The counterion I₉⁻ is stabilized in an unusual trigonal-pyramidal structure in the 2 e⁻-oxidized complex in which the central iodide ion is bound with four iodine (I₂) molecules. Variable-temperature magnetic study revealed strong antiferromagnetic coupling between the two porphyrin π–cation radical spins (J_r–r) in the 2 e⁻-oxidized complex. DFT calculations suggest stabilization of the triplet state, which is also in good agreement with the experiment. Ab initio molecular dynamics allowed the variation of the structural details to be followed upon stepwise oxidations and also the final isomerization process including its associated energy barrier.