ELECTRON-TRANSFER IN FE(II)FE(III) MODEL COMPLEXES OF IRON OXO PROTEINS

The nature of valence trapping is studied for a series of six mu-phenoxo-bis(mu-carboxylate)-bridged Fe(II)Fe(III) complexes. The septadentate ligand bimp- binds the two metal ions and provides a phenoxide bridge as well as four imidazole moieties. The X-ray structure of [Fe(II)Fe(III)(bimp)(mu-O2CPh)2](BPh4)2.3/2CH3CN at 295 K is reported. This mixed-valence complex crystallizes in the P1BAR space group with unit cell parameters of a = 15.995 (4) angstrom, b = 23.475 (4) angstrom, c = 11.464 (4) angstrom, alpha = 97.57 (1)-degrees, beta = 101.74 (1)-degrees, gamma = 85.22 (1)-degrees, and Z = 2. A total of 11334 unique data with I > 3-sigma(I) were refined to values of R = 0.045 and R(w) = 0.051. The structure shows distinct octahedral (N3O3) high-spin Fe(III) and Fe(II) ions, with an Fe...Fe distance of 3.440 angstrom. The phenoxide Fe(II)-O-Fe(III) bridging angle is 115.17 (8)-degrees. The BPh4- ions are not symmetrically distributed about the Fe(II)Fe(III) cation; they are closer to the Fe(II) ion than the Fe(III) ion. Sharp H-1 NMR signals spanning a chemical shift range of approximately 350 ppm are seen for CD3CN solutions of the Fe(II)Fe(III) complexes. Relative to the H-1 NMR timescale these mixed-valence complexes are rapidly transferring electrons in solution. This is due to either rapid intramolecular electron transfer or rapid electron transfer between binuclear complexes. A weak intervalence transfer (IT) electronic absorption band is seen at approximately 1300 nm (epsilon = approximately 200 M-1 cm-1) for all six Fe(II)Fe(III) complexes in solution. Two quasireversible one-electron waves corresponding to the Fe(II)Fe(II)/Fe(II)Fe(III) and Fe(II)Fe(III)/Fe(III)Fe(III) couples are seen for each complex by cyclic voltammetry. The two waves observed for the Fe(II)Fe(III)(bimp) complexes occur at potentials 100-300 mV more negative than those reported for the analogous polypyridyl and benzimidazole complexes. Variable-temperature (5-300 K) magnetic susceptibility data are presented for two of the Fe(II)Fe(III) complexes. These data were least-squares fit in a matrix diagonalization approach including the effects of Heisenberg magnetic exchange (H = -2JS1.S2), isotropic Zeeman and axial single-ion zero-field (DS(z2) - 1/3S(S + 1)] for both Fe(II) and Fe(III) interactions. Fitting of the data for one sample of [Fe(II)Fe(III)(bimp)(mu-O2CCH3)2](ClO4)2.2H2O, for example, gave parameters of J = -3.4 cm-1, D(Fe(II)) = 0.63 cm-1, and g(Fe(II)) = 1.86. There is a S = 1/2 ground state; however, the S = 3/2, 5/2, 7/2, and 9/2 excited states are near to the ground state. Fe-57 Mossbauer spectroscopy was used to thoroughly examine the temperature dependence of valence detrapping seen for the Fe(II)Fe(III)(bimp) complexes. It is shown that several of these complexes exhibit valence detrapping (onset of rapid electron transfer relative to the Fe-57 Mossbauer timescale) as the temperature is increased. Furthermore, there is a dependence on which anions and solvate molecules are present. Electron paramagnetic resonance (EPR) signals which are similar to those reported for the Fe(II)Fe(III) forms of iron-oxo proteins such as hemerythrin, ribonucleotide reductase, purple acid phosphatase, and methane monooxygenase are found for liquid-helium-temperature CH3CN solutions of the Fe(II)Fe(III)(bimp) complexes. The EPR signals are sensitive to sample conditions (frozen solution, polycrystals, etc.). The importance of the data observed for the Fe(II)Fe(III)(bimp) complexes in understanding observations on the iron-oxo proteins is discussed.