Complexes with FeIII₂(μ-O)(μ-OH), FeIII₂(μ-O)₂, and [FeIII₃(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-Et 3 -TPA ( b ), 6-Me 3 -TPA ( c ), 4,6-Me 6 -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 Fe 2 O 2 (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 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 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 H 2 18 O and 2 H 2 O. Magnetic susceptibility studies reveal J = 54 cm - 1 for 1c and ca. 110 cm - 1 for 2 ( H = J S 1 · S 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 Fe 2 O 2 (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, Fe 2 (μ-O) 2 ( 1 ) and Fe 2 (μ-O)(μ-OH) ( 2 ), can also be interconverted by protonation equilibria with p K a 's of 16−18 in CH 3 CN. Furthermore, the Fe 2 (μ-O) 2 core ( 1 ) isomerizes to the Fe 3 (μ 2 -O) 3 core ( 7 ), while the Fe 2 (μ-O)(μ-OH) core ( 2 ) exhibits aquation equilibria to the Fe 2 (μ-O)(μ-H 3 O 2 ) core ( 5 ), except for L = 6-Me 3 -TPA and 4,6-Me 6 -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.