Evidence for an unusual thermally induced low-spin (S = 1/2) ⇆ intermediate-spin (S = 3/2) transition in a six-coordinate iron(III) complex: Structure and electronic properties of a (1,2-benzenedithiolato)iron(III) complex containing N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane as Ligand

The reaction of iron(III) chloride with the tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane (L-N₄Me₂) and 1,2-benzenedithiolate yields complex 1, which is, to our knowledge, the first six-coordinate iron(III) complex that is characterized by a thermally induced S = 1/2 ⇆ S = 3/2 spin transition. The spin-crossover phenomenon is demonstrated by structure determinations carried out at two different temperatures and by magnetochemical and Mossbauer experiments. The existence of a thermally accessible S = 3/2 spin state at room temperature is corroborated by ESR and structural evidence. The highly distorted cis octahedral N₄S₂ coordination geometry around the iron ion, which is distinguished by strong equatorial bonds to the thiolate sulfur atoms and the pyridine nitrogen atoms and comparatively weak axial bonds to the amine nitrogen atoms, is thought to be responsible for the occurrence of the rather unusual intermediate-spin state for a six-coordinate iron(III) ion. The rich redox chemistry associated with complex 1 and its inertness towards molecular oxygen are discussed.