Spin‐State Tuning in Iron(II) Triazamacrocyclic Complexes

The structures of [Fe II (tacn) 2 ](OTf) 2 · DMF ( 1· DMF), [Fe II (tacd) 2 ](OTf) 2 ( 2 ), and [Fe II (tacud) 2 ](OTf) 2 ( 3 ) (OTf = triflate, tacn = 1,4,7‐triazacyclononane, tacd = 1,4,7‐triazacyclodecane, tacud = 1,4,8‐triazacycloundecane) are reported. In the solid state, compounds 1 and 2 exhibit temperature‐dependent gradual and reversible one‐step spin crossover between the high‐spin ( S = 2) and low‐spin ( S = 0) states with estimated spin‐conversion equilibria ( T 1/2 ) of 330 and 320 K, respectively. Compound 1 exhibits somewhat reversible magnetic behavior during the heating and cooling cycle, whereas 2 is more reversible. Compound 2 is only the third example of a spin‐crossover compound possessing secondary amine ligands exclusively. Complex 3 is high spin at the temperatures measured. Systematic comparison of the properties of 1 – 3 indicates that the structural–magnetic properties correlate with the ligand‐donor strength. There is also a close correlation with the room‐temperature solution spin state and E 1/2 in acetonitrile for these compounds.