Stepwise oxidations of a nickel(ii)–iron(iii) heterobimetallic porphyrin dimer: structure, spectroscopic and theoretical investigation

Abstract

We have reported here the synthesis, structure and spectroscopic properties of the Ni^II–Fe^III heterobimetallic ethene-bridged porphyrin dimer and investigate the effect upon step-wise one- and two-electron oxidations to produce a mixed-valence π–cation radical dimer and a dication diradical complex, respectively. We then investigate their electronic structure and spin coupling model and compare them with the corresponding homobimetallic analog. Detailed UV-vis-NIR, IR, and variable temperature magnetic studies and EPR and NMR spectroscopic investigations along with X-ray structure determination of the 2e-oxidized complex have demonstrated strong electronic communications through the bridge between two porphyrin π–cation radicals. The structure and geometrical parameters revealed the stabilization of the high-spin state of iron and the low-spin state of nickel in the 2e-oxidized complex. The extensive conjugation between the two porphyrin π–cation radicals has also altered the bridge from an ethylene to an exo-methylene moiety. Variable temperature magnetic studies of the 2e-oxidized complex revealed stronger antiferromagnetic coupling between two π–cation radical spins (J_r1–r2) of the Ni^II–Fe^III heterodimer which is in sharp contrast to its diiron(III) analog where the porphyrin π–cation radical undergoes stronger antiferromagnetic coupling predominantly with the corresponding Fe(III) unpaired spin (J_Fe–r). The experimental observations are also strongly supported by DFT calculations.