Electron Spin−Spin Exchange Coupling Mediated by the Porphyrin π System

The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1−3) substituted
with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been
studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin
3 is monoclinic, space group P21/n, with a = 12.239(2) Å, b = 17.819(3) Å, c = 34.445(7) Å, α = 90°, β =
97.466(3)°, γ = 90°, and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent
with |J| ≫ |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as
shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3
exhibits frozen-solution EPR spectra with zero-field splitting and a Δms = 2 transition characteristic of a triplet
state. The intensity of the Δms = 2 transition of 3 was measured as a function of temperature, and the data fit
according to a singlet−triplet model to yield J(3,solution) = −75 cm-1 (H = − 2JŜ1·Ŝ2). Polycrystalline samples
of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were
fit using a modified Bleaney−Bowers equation to give J(3,solid) = −29 cm-1 (H = − 2JŜ1·Ŝ2). The antiferromagnetic
J values are consistent with the π topology of the porphyrin ring.