Tuning redox and spin state properties of Fe(II) N-heterocyclic complexes via electronic/steric influence on metal–ligand binding

A series of complexes with the general formula [Fe(L)2]2+, where L represents the tridentating 6-(N-3,5-dimethylpyrazolyl)2,2′-bipyridine (L4); 6-(N-pyrazolyl-1-ylmethyl)-2,2′-bipyridine (L5); and 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,2′-bipyridine (L6), were prepared and characterized. The room temperature solution magnetic susceptibility and redox properties of these compounds were investigated as a function of stepwise variation in the ligand structure. The Fe(III/II) couple was characterized by way of cyclic voltammetry using aprotic solvent conditions (acetonitrile) where each complex was observed to have reversible behavior. NMR methodology was used for measuring the magnetic susceptibilities where both [Fe(L4)2]2+ and Fe(L5)2]2+ exhibited diamagnetic low spin behavior; however, [Fe(L6)2]2+ measured a μeff of 4.1 Bohr-magnetons indicating spin equilibrium predominantly in the high spin state.