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 (Na-2(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe-2(L-Bn)(mpdp)(H2O)](ClO4)(2) and [Fe-2(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 P2(1)/n, with a = 13.3095(14) Angstrom, b = 20.1073(19) Angstrom, c = 19.4997(19) Angstrom, alpha = 90degrees, beta = 94.471(2)degrees, gamma = 90degrees, V = 5202.6(9) Angstrom(3), and Z = 4. The structure of the compound is very similar to that of [Fe-2(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 [Fe-2(LBn)(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, Mossbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mossbauer 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 [Fe-2(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 Mossbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q) greater than or equal to 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.