Complexes with Fe-2(III)(mu-O)(mu-OH), Fe-2(III)(mu-O)(2), and [Fe-3(III)(mu(2)-O)(3)] cores: Structures, spectroscopy, and core interconversions

We have synthesized the first complexes with bis(mu-oxo)diiron(III) and (mu -oxo)(mu-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 f ound them to have novel structural properties. In particular, the presence of two single-atom bridges in these complexes constrains the Fe-Fe distance s to 2.7 - 3.0 Angstrom and the Fe-mu-O-Fe angles to 100 degrees dr smaller . The significantly acute Fe-O-Fe angles (e.g, 92.5(2)degrees for 1c and 10 0.2(2)degrees for 2f) enforced by the Fe2O2(H) core endow these complexes w ith UV-vis, Raman, and magnetic properties quite distinct from those of oth er (mu-oxo)diiron(III) complexes. Complex Ic exhibits visible absorption ba nds at 470 (epsilon = 560 M-1 cm(-1)) and 760 nm (epsilon approximate to 80 M-1 cm(-1)), while complexes 2 show features at ca. 550 (epsilon approxima te to 800 M-1 cm(-1)) and ca. 800 nm (epsilon approximate to 70 M-1 cm(-1)) , all of which are red shifted compared to those of other (mu-oxo)-diiron(I II) complexes. These complexes also exhibit distinct nu(Fe-O-Fe) vibrations at ca. 600 and ca. 670 cm(-1) assigned to the nu(sym) and the nu(asym) of the Fe-O-Fe units, respectively. The relative intensities of the nu(sym) an d nu(asym) bands are affected by the symmetry of the Fe-O-Fe units; an unsy mmetric core enhances the intensity of the nu(asym). Complexes 2 exhibit an other band at ca. 500 cm(-1), which is assigned to the Fe-(OH)-Fe stretchin g mode due to its sensitivity to both (H2O)-O-18 and (H2O)-H-2. Magnetic su sceptibility studies reveal J = 54 cm(-1) for Ic and ca. 110 cm(-1) for 2 ( H = JS(1).S-2), values smaller than those for the antiferromagnetic interac tions found in (mu-oxo)diiron(III) complexes. This weakening arises from th e longer Fe-mu-O bonds and the smaller Fe-mu-O-Fe angles in the Fe2O2(H) di amond core structure. These spectroscopic signatures can thus serve as usef ul tools to ascertain the presence of such core structures in metalloenzyme active sites. These two core structures, Fe-2(mu-O)(2) (1) and Fe-2(mu-O)( mu-OH) (2), can also be interconverted by protonation equilibria with pK(a) 's of 16-18 in CH3CN. Furthermore, the Fe-2(mu-O)(2) core (1) isomerizes to the Fe-3(mu(2)-O)(3) core (7), while the Fe-2(mu-O)(mu-OH) core (2) exhibi ts aquation equilibria to the Fe-2(mu-O)(mu-H3O2) core (5), except for L = 6-Me3-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 equil ibria, demonstrating a rich chemistry involving water-derived ligands alone .