EVIDENCE FOR AN UNUSUAL THERMALLY-INDUCED LOW-SPIN (S=1 2)-REVERSIBLE-ARROW-INTERMEDIATE-SPIN (S=3/2) TRANSITION IN A 6-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-benz enedithiolate 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 reversible arrow S=3/2 spin transition. The spin-crossov er phenomenon is demonstrated by structure determinations carried out at two different temperatures and by magnetochemical and Mossbauer exp eriments. The existence of a thermally accessible S=3/2 spin state at room temperature is corroborated by ESR and structural evidence. The h ighly distorted cis octahedral N4S2 coordination geometry around the i ron ion, which is distinguished by strong equatorial bonds to the thio late sulfur atoms and the pyridine nitrogen atoms and comparatively we ak axial bonds to the amine nitrogen atoms, is thought to be responsib le for the occurrence of the rather unusual intermediate-spin state fo r a six-coordinate iron(III) ion. The rich redox chemistry associated with complex 1 and its inertness towards molecular oxygen are discusse d.