Structure and Electrochemical Studies of [(trispicMeen)ClFe^IIIOFe^IIICl(trispicMeen)]²⁺. Spectroscopic Characterization of the Mixed-Valence Fe^IIIOFe^II Form. Relevance to the Active Site of Dinuclear Iron−Oxo Proteins

The dinuclear species [(trispicMeen)ClFeIIIOFeIIICl(trispicMeen)]Cl(OH)(H2O)7 (1) (trispicMeen = N,N,N‘-tris(2-pyridylmethyl)-N‘-methylethane-1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a = 33.87(2) Å, b = 17.42(2) Å, c = 23.41(5) Å, β = 132.88(5)°, V = 10 121(25) Å3, and Z = 8. It contains an almost linear unit (Fe−O−Fe angle = 177.4(7)°). The potentially pentadentate ligand is in fact only tetracoordinated with one pyridine not bound to the metal ion. The octahedral coordination of Fe(III) is completed by one chloride ion. The structure of [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl2·CH3COCH3· 2H2O (2) (bispicMeen = N,N‘-bis(2-pyridylmethyl)-N‘-methylethane-1,2-diamine) was also determined. It crystallizes in the monoclinic space group C2/c with a = 11.124(4) Å, b = 22.769(9) Å, c = 15.874(6) Å, β = 97.79(4)°, V = 3984(3) Å3, and Z = 4. The main difference from 1 is that, in 2, the Fe−O−Fe unit is bent with an FeOFe angle = 152.3(3)°. In cyclic voltammetry, 1 exhibits two reduction peaks at −0.230 and −0.960 V/SCE. They correspond respectively to the reduction to the FeIIFeIII and FeIIFeII states. Cyclic voltammetry shows that the mixed-valent form [(trispicMeen)ClFeIIOFeIIICl(trispicMeen)]+ (E° = −0.175 V/SCE) is in equilibrium with another species (E° = +0.065 V/SCE) proposed to be [(trispicMeen)FeIIOFeIIICl(trispicMeen)]2+ in which a chloride ion has been displaced by the originally unbound pyridine. The equilibrium constant was estimated to be 90 M-1, and the rate of the recombination of chloride to the [(trispicMeen)FeIIIOFeIIICl(trispicMeen)]3+ complex was found equal to 3 × 105 M-1 s-1. Controlled potential electrolysis of an acetonitrile solution of 1 allowed the preparation of the mixed-valent Fe(II)−O−Fe(III) form which gives an almost isotropic EPR signal similar to that already observed with oxo-bridged model compounds (Holz, et al. Inorg. Chem. 1993, 32, 5844. Hartman, et al. J. Am. Chem. Soc. 1987, 109, 7387) but different from the rhombic one observed in the mixed valent form of MMO. The mixed-valent forms slowly disproportionate to a mixture of FeIIIFeIII and Fe(II) forms. The mixed-valent forms could be generated by radiolysis at 77 K, and an EPR study of the mixed-valent forms obtained by this procedure demonstrated that these species could not be protonated. Radiolysis of 1 at 77 K afforded the EPR spectrum of [(trispicMeen)ClFeIIOFeIIICl(trispicMeen)]+; upon annealing at 200 K, the solution gave an EPR spectrum very similar to that observed on the electrochemically reduced solution. This is in agreement with the observation of substitution of a chloride ligand. The mixed-valent form was not detected with the analogous complexes of the bispicen family: [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl2·CH3COCH3· 2H2O (2) and [(bispicMe2en)ClFeOFeCl(bispicMe2en)]Cl2 (3). Upon reduction, these complexes quickly (at CV time scale) decompose to mononuclear species.