Imidazole and Imidazolate Iron Complexes: On the Way for Tuning 3D-Structural Characteristics and Reactivity. Redox Interconversions Controlled by Protonation State

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H3 =
tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H3 = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described:  three
complexes in the L1 series (namely, [Fe(II)(L1H3)]2+ and [Fe(III)(L1H3)]3+ at low pH and [Fe(III)(L1)]0 at high pH)
and three complexes in the L2 series(namely, [Fe(II)(L2H3)]2+ at low pH and [Fe(II)(L2H)]0 and [Fe(III)(L2)]0 at high
pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various
protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by
2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms,
with regard to oxidation and protonation states, were investigated by UV−visible spectroscopy, cyclic voltammetry,
and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in
detail. pKas of the imidazolate/imidazole moieties in MeOH/H2O are reported. Two new species, namely, [Fe(II)(L1)]-
and [Fe(II)(L2)]-, were shown to be obtained in DMSO upon strong base addition and characterized by UV−vis
spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several
protonation states are reported, both in DMSO and in MeOH/H2O. Because of the presence of free imidazole
groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of ΔE =
Edeprot − Eprot ranging from −270 to −320 mV per exchanged proton in DMSO was measured. This study shows
moreover that interconversions (with regard to both redox and protonation states) can be reversed several times.
As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions
with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In
these two series, O2- behaves either as a base or as a reducer and no adducts have been observed.