Biologically Relevant Heterodinuclear Iron–Manganese Complexes

The heterodinuclear complexes [Fe<sup>III</sup>Mn<sup>II</sup>(L-Bn)­(μ-OAc)<sub>2</sub>]­(ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>) and [Fe<sup>II</sup>Mn<sup>II</sup>(L-Bn)­(μ-OAc)<sub>2</sub>]­(ClO<sub>4</sub>) (<b>2</b>) 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 <b>1</b> and reveal an Fe<sup>III</sup>Mn<sup>II</sup>μ-phenoxobis­(μ-carboxylato) core. A single location of the Fe<sup>III</sup> ion in <b>1</b> and of the Fe<sup>II</sup> ion in <b>2</b> was demonstrated by Mössbauer and <sup>1</sup>H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of <b>1</b> revealed a moderate antiferromagnetic interaction (<i>J</i> = 20 cm<sup>–1</sup>) between the high-spin Fe<sup>III</sup> and Mn<sup>II</sup> ions in <b>1</b>, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex <b>1</b> are described. A quasireversible electron transfer at −40 mV versus Ag/AgCl corresponding to the Fe<sup>III</sup>Mn<sup>II</sup>/Fe<sup>II</sup>Mn<sup>II</sup> couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex <b>2</b> were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (<i>J</i> = 5.72 cm<sup>–1</sup>) within the Fe<sup>II</sup>Mn<sup>II</sup> pair consistent with that deduced from magnetic susceptibility measurements (<i>J</i> = 6.8 cm<sup>–1</sup>). Owing to the similar value of the Fe<sup>II</sup> zero-field splitting (<i>D</i><sub>Fe</sub> = 3.55 cm<sup>–1</sup>), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex <b>2</b> was developed. This situation is reminiscent of that found in many diiron and iron–manganese enzyme active sites.