Dissymmetry effects in .mu.-oxo diiron(III) species: structures and spectroscopic properties of [N5FeOFeX3]+ (X = Cl, Br) and implications for oxo-bridged dinuclear iron proteins

Abstract

We report the first nonbiological unsymmetrical μ-oxo diiron(III) complexes: [(N5)FeOFeX3]X, where N5 is N-(hdyroxyethyl)-N,N’,N’-tris(2-benzimidazolylmethyl)-l,2-diaminoethane and X is C1 (compound I) or Br (compound II). X-ray crystallographic studies reveal structures in which octahedral and tetrahedral iron atoms are bridged by the oxo group with an Fe-0-Fe angle of 149.5 (3)º for I and 150.6 (4)º for 11. A small but significant difference in Fe-0 separations is observed in which the Ion er distances are associated with N5Fe-O [1.784 (4) A in I, 1.797 (9) A in 11] and shorter distances\nwith X,Fe-0 [1.751 (4) 1 in I, 1.726 (9) 8, in 111. Unusually strong antiferromagnetic coupling is observed for I [J = -122 (I) cm-1 and a more typical value for II [J = -106 (1) cm-1]. The nonequivalent iron atoms exhibit distinct Mossbauer spectroscopic parameters for I at 293 K (6 = 0.23, AE, = 1.15 mm/s and 6 = 0.34, AE, = 1.36 mm/s), whereas II at 77 K gives an even better resolved pair of doublets (6 = 0.31, AE, = 1.41 mm/s for FeOBr, and 6 = 0.41, AE, = 1.22 mm/s for NSFeO). The resonance Raman spectrum of I shows a band at 850 cm-l (806 cm-1 for 18O) assigned to the Fe-0-Fe\nasymmetric stretch, vas. Contrary to the behavior of symmetrical dinuclear species, the Y,, mode in I and II has more than 2 times the Raman intensity of the symmetric Fe-0-Fe stretch. A stable one-electron-reduced derivative of I is produced by cyclic voltammetry of the compound in acetonitrile. Many properties of the asymmetric p-oxo diiron(III) complexes are similar to those observed for dinuclear iron proteins (hemerythrin, ribonucleotide reductase, purple acid phosphatase, methane monooxygenase); a comparable inequivalence of the two iron atoms is inferred for several of these proteins.