(MU-OXO)(MU-CARBOXYLATO)DIIRON(III) COMPLEXES WITH DISTINCT IRON SITES - CONSEQUENCES OF THE INEQUIVALENCE AND ITS RELEVANCE TO DINUCLEAR IRON OXO PROTEINS

A new family of (μ-oxo)(μ-carboxylato)diiron(III) complexes has been synthesized with use of the tetradentate tripodal ligand tris(2-pyridylmethyl)amine (TPA) with benzoate, acetate, and diphenyl phosphate as bridging ligands. [Fe2(TPA)2O(OBz)](ClO4)32H2O crystallizes in the monoclinic space group P2fn with a = 11.669 (15) Å, b = 19.568 (20) Å, c = 22.285 (12) Å, and β = 102.24 (8)° the structure was determined at -84 ° from 5445 out of a total of 8854 reflections with R = 0.064 and Rw = 0.080. [Fe2(TPA)2O(OAc)](ClO4)3H2O(CH3COCH3) crystallizes in the monoclinic space group P2ī with a = 20.774 (46) Å, b = 22.312 (5) Å, c = 10.372 (7) Å, and β = 104.14 (12)°. The structure was determined at -88 ° from 6324 out of a total of 9147 reflections with R = 0.078 and Rw = 0.091.[Fe2(TPA)2O(O2P{OPh}2)(ClO4)3(CH3COCH3) crystallizes in the triclinic space group Pī with a = 11.82 (1) Å, b= 13.32 (1) Å, c = 18.03 (1) Å, α = 92.67 (5)Å, β = 94.48 (5)°, and γ = 94.22 (5)°. The structure was determined at -83°C from 6931 out of a total of 9885 reflections with R = 0.049 and Rw = 0.061. These crystal structures establish the presence of a doubly bridged diiron core with distinct iron sites. On one iron, the amine nitrogen of TPA is trans to the oxo bridge, while one of the pyridines is trans to the oxo bridge on the other iron. These complexes exhibit electronic and Mössbauer spectral features and magnetic properties that are very similar to those of (μ-oxo)diiron(III) proteins as well as (μ-oxo)bis(μ-carboxylato)diiron(III) complexes, demonstrating that these properties are not significantly affected by the number of carboxylate bridges and the inequivalence of the iron sites. The iron(III) sites remain distinct in solution as evidenced by 1H NMR and resonance Raman spectroscopies. The inequivalence is manifested in the resonance Raman spectra as an enhancement of the vas(Fe-O-Fe) intensity, which is comparable to those found for (μ-oxo)diiron(III) proteins such as methemerythrin and ribonucleotide reductase. © 1990, American Chemical Society. All rights reserved.