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

Abstract

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.

Synthesis, Structure and Characterizations in Solid State and Solution of Dinuclear Pentacoordinated FeII and MnII Complexes and of a Linear Tetranuclear FeIII Complex Obtained with the Ligand N,N,N′,N′‐Tetrakis[(6‐methyl‐2‐pyridyl)methyl]propane‐1,3‐diamine

“…[33,34] The other intense band (381 nm, ε ϭ 20000  buted to an oxoǞFe III charge-transfer transition. [35,36] The spectral features reported for µ-oxodiiron() complexes with aminopyridine and chloro ligands, which present an intense band around 320 nm and another at about 380 nm, [37,38] are also consistent with these assignments.…”

Characterization of a Nonheme Mononuclear Peroxoiron(III) Intermediate by UV/Vis and EPR Spectroscopy and Mass Spectrometry

“…The analysis of the set of spectra led to the conclusion that two different species were [18] The decrease in intensity of In this complex, the pentadentate ligand L 5 acts as a tetradentate one as already reported by Nivorozhkin et al [22] proposed recently by Selke and Valentine, [23] by the axial P450, for example, is protonated by a water molecule. [24] [17] The resonance at g ϭ 9.3 is detected only at 4.0 K. The acidity or basicity of the hydroperoxo group must be a 9, 308Ϫ315, the intensity of the spectrum was multiplied by the function of the auxiliary ligand: Negatively charged ligands temperature.…”

Unusual oxidative stability of a multidentate primary amine ligand: facile synthesis of the oxo-bridged diiron(III) complex

“…1. The reduction product was confirmed to be the mixed-valent species by controlled-potential coulometry [17]. The effective magnetic moment at room temperature is l eff ¼ 2:6 lB, and the data of variabletemperature measurements are consistent with strong antiferromagnetic exchange (J ¼ À123ð4Þ cm À1 ) between the two iron centres through the oxo bridge, resulting in a diamagnetic ground state with S T ¼ 0.…”