Biologically Relevant Heterodinuclear Iron–Manganese Complexes

The heterodinuclear complexes [FeIIIMnII(L-Bn)­(μ-OAc)2]­(ClO4)2 (1) and [FeIIMnII(L-Bn)­(μ-OAc)2]­(ClO4) (2) with the unsymmetrical
dinucleating ligand HL-Bn
{[2-bis­[(2-pyridylmethyl)­aminomethyl]]-6-[benzyl-2-(pyridylmethyl)­aminomethyl]-4-methylphenol}
were synthesized and characterized as biologically relevant models
of the new Fe/Mn class of nonheme enzymes. Crystallographic studies
have been completed on compound 1 and reveal an FeIIIMnIIμ-phenoxobis­(μ-carboxylato) core.
A single location of the FeIII ion in 1 and
of the FeII ion in 2 was demonstrated by Mössbauer
and 1H NMR spectroscopies, respectively. An investigation
of the temperature dependence of the magnetic susceptibility of 1 revealed a moderate antiferromagnetic interaction (J = 20 cm–1) between the high-spin FeIII and MnII ions in 1, which was confirmed
by Mössbauer and electron paramagnetic resonance (EPR) studies.
The electrochemical properties of complex 1 are described.
A quasireversible electron transfer at −40 mV versus Ag/AgCl
corresponding to the FeIIIMnII/FeIIMnII couple appears in the cyclic voltammogram. Thorough
investigations of the Mössbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak
antiferromagnetic interaction (J = 5.72 cm–1) within the FeIIMnII pair consistent with
that deduced from magnetic susceptibility measurements (J = 6.8 cm–1). Owing to the similar value of the
FeII zero-field splitting (DFe = 3.55 cm–1), the usual treatment within the strong
exchange limit was precluded and a full analysis of the electronic
structure of the ground state of complex 2 was developed.
This situation is reminiscent of that found in many diiron and iron–manganese
enzyme active sites.