Single Ion Magnetic Behaviour in an Iron III Porphyrin Complex A Dichotomy Between High Spin and 5 2 3 2 Spin Admixture

A mononuclear iron III porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single amp; 8208;ion magnet behaviour, is reported. This behaviour originates from the close proximity amp; 8776;550 amp; 8197;cm amp; 8722;1 of the intermediate amp; 8208;spin S 3 2 excited states to the high amp; 8208;spin S 5 2 ground state. More quantitatively, although the ground state is mostly S 5 2, a spin amp; 8208;admixture model evidences a sizable contribution amp; 8776;15 amp; 8201; of S 3 2 to the ground state, which as a consequence experiences large and positive axial anisotropy D 19.2 amp; 8197;cm amp; 8722;1 . Frequency amp; 8208;domain EPR spectroscopy allowed the mS 1 2 amp; 10217; amp; 8594; 3 2 amp; 10217; transitions to be directly accessed, and thus the very large zero amp; 8208;field splitting in this 3d5 system to be unambiguously measured. Other experimental results including magnetisation, Mössbauer, and field amp; 8208;domain EPR studies are consistent with this model, which is also supported by theoretical calculation