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

We have synthesized the first complexes with bis(μ-oxo)diiron(III) and (μ-oxo)(μ-hydroxo)diiron(III) cores (<b>1</b> and <b>2</b>, L = TPA (<b>a</b>), 5-Et₃-TPA (<b>b</b>), 6-Me₃-TPA (<b>c</b>), 4,6-Me₆-TPA (<b>d</b>), BQPA (<b>e</b>), BPEEN (<b>f</b>), and BPMEN (<b>g</b>)) 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 <b>1c</b> and 100.2(2)° for<b> 2f</b>) 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 <b>1c</b> exhibits visible absorption bands at 470 (ε = 560 M^-1 cm^-1) and 760 nm (ε = 80 M^-1 cm^-1), while complexes <b>2</b> 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 ν_Fe-O_-_Fe vibrations at ca. 600 and ca. 670 cm^-1 assigned to the ν_sym and the ν_asym of the Fe−O−Fe units, respectively. The relative intensities of the ν_sym and ν_asym bands are affected by the symmetry of the Fe−O−Fe units; an unsymmetric core enhances the intensity of the ν_asym. Complexes <b>2</b> 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 <i>J</i> = 54 cm^-1 for <b>1c</b> and ca. 110 cm^-1 for <b>2 </b>(<b>H</b> =<i> J</i><b>S</b>₁·<b>S</b>₂), 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 (<b>1</b>) and Fe₂(μ-O)(μ-OH) (<b>2</b>), can also be interconverted by protonation equilibria with p<i>K</i>_a's of 16−18 in CH₃CN. Furthermore, the Fe₂(μ-O)₂ core (<b>1</b>) isomerizes to the Fe₃(μ₂-O)₃ core (<b>7</b>), while the Fe₂(μ-O)(μ-OH) core (<b>2</b>) exhibits aquation equilibria to the Fe₂(μ-O)(μ-H₃O₂) core (<b>5</b>), 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.