Spectroscopic and Electrochemical Characterization of an Aqua Ligand Exchange and Oxidatively Induced Deprotonation in Diiron Complexes

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl2) with Fe(H2O)6(ClO4)2 in the presence of disodium m-phenylenedipropionate (Na2(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe2(L-Bn)(mpdp)(H2O)](ClO4)2 and [Fe2(L-BnCl2)(mpdp)(CH3OH)](ClO4)2, respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P21/n, with a = 13.3095(14) Å, b = 20.1073(19) Å, c = 19.4997(19) Å, α = 90°, β = 94.471(2)°, γ = 90°, V = 5202.6(9) Å3, and Z = 4. The structure of the compound is very similar to that of [Fe2(L-Bn)(mpdp)(H2O)](BPh4)2 determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe2(L-Bn)(mpdp)(H2O)](BPh4)2 reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = −3.2(2) cm-1). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile−water exchange has been investigated by various spectroscopic techniques (UV−visible, NMR, Mössbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mössbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe2(L-Bn)(mpdp)(H2O)]2+ shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mössbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (ΔE Q ≥ 1.0 mm·s-1) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.