Complexes with Fe^III₂(μ-O)(μ-OH), Fe^III₂(μ-O)₂, and [Fe^III₃(μ₂-O)₃] Cores:  Structures, Spectroscopy, and Core Interconversions

Abstract

We have synthesized the first complexes with bis(μ-oxo)diiron(III) and (μ-oxo)(μ-hydroxo)diiron(III) cores (1 and 2, L = TPA (a), 5-Et3-TPA (b), 6-Me3-TPA (c), 4,6-Me6-TPA (d), BQPA (e), BPEEN (f), and BPMEN (g)) and found them to have novel structural properties. In particular, the presence of two single-atom bridges in these complexes constrains the Fe−Fe distances to 2.7−3.0 Å and the Fe−μ-O−Fe angles to 100° or smaller. The significantly acute Fe−O−Fe angles (e.g., 92.5(2)° for 1c and 100.2(2)° for 2f) enforced by the Fe2O2(H) core endow these complexes with UV−vis, Raman, and magnetic properties quite distinct from those of other (μ-oxo)diiron(III) complexes. Complex 1c exhibits visible absorption bands at 470 (ε = 560 M-1 cm-1) and 760 nm (ε = 80 M-1 cm-1), while complexes 2 show features at ca. 550 (ε ≈ 800 M-1 cm-1) and ca. 800 nm (ε ≈ 70 M-1 cm-1), all of which are red shifted compared to those of other (μ-oxo)diiron(III) complexes. These complexes also exhibit distinct νFe-O-Fe vibrations at ca. 600 and ca. 670 cm-1 assigned to the νsym and the νasym of the Fe−O−Fe units, respectively. The relative intensities of the νsym and νasym bands are affected by the symmetry of the Fe−O−Fe units; an unsymmetric core enhances the intensity of the νasym. Complexes 2 exhibit another band at ca. 500 cm-1, which is assigned to the Fe−(OH)−Fe stretching mode due to its sensitivity to both H218O and 2H2O. Magnetic susceptibility studies reveal J = 54 cm-1 for 1c and ca. 110 cm-1 for 2 (H = JS1·S2), values smaller than those for the antiferromagnetic interactions found in (μ-oxo)diiron(III) complexes. This weakening arises from the longer Fe−μ-O bonds and the smaller Fe−μ-O−Fe angles in the Fe2O2(H) diamond core structure. These spectroscopic signatures can thus serve as useful tools to ascertain the presence of such core structures in metalloenzyme active sites. These two core structures, Fe2(μ-O)2 (1) and Fe2(μ-O)(μ-OH) (2), can also be interconverted by protonation equilibria with pKa's of 16−18 in CH3CN. Furthermore, the Fe2(μ-O)2 core (1) isomerizes to the Fe3(μ2-O)3 core (7), while the Fe2(μ-O)(μ-OH) core (2) exhibits aquation equilibria to the Fe2(μ-O)(μ-H3O2) core (5), except for L = 6-Me3-TPA and 4,6-Me6-TPA. It is clear from these studies that electronic and steric properties of the ligands significantly affect the various equilibria, demonstrating a rich chemistry involving water-derived ligands alone.