Complexes with Fe(III)₂(μ-O)(μ-OH), Fe(III)₂(μ-O)₂, and [Fe(III)₃(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 (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 v(Fe)-O-Fe vibrations at ca. 600 and ca. 670 cm^-1 assigned to the v(sym) and the v(asym) of the Fe-O-Fe units, respectively. The relative intensities of the v(sym) and v(asym) bands are affected by the symmetry of the Fe-O-Fe units; an unsymmetric core enhances the intensity of the v(asym). 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 pK(a)'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.