We report a mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation which is a characteristic of single-ion magnet behaviour. This behaviour originates from the close proximity (≈ 550 cm -1 ) of the intermediate-spin S = 3/2 excited states to the high-spin S = 5/2 ground state. More quantitatively, although the ground state is mostly S = 5/2, a spin-admixture model evidences a sizable contribution (≈ 15%) of S = 3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy ( D = +19.2 cm -1 ). Frequency-domain EPR allowed us to directly access the m S = |±1/2〉 → |±3/2〉 transitions, thus unambiguously measuring the very large zero-field splitting (ZFS) in this 3d 5 system. Other experimental results including magnetization, Mössbauer, and field-domain EPR (HFEPR) studies are consistent with this model, which is also supported by theoretical calculations.
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