(TAML)FeIV=O complex in aqueous solution:: Synthesis and spectroscopic and computational characterization

Recently, we reported the characterization of the S = 1/2 complex [Fe-V(O)B*](-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe-IV(O)B*](2-) (2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe-III(H2O)B*](-) (1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe-IV-O-(FeB)-B-IV*](2-) (3), with a pK(a) near 10. In zero field, the Mossbauer spectrum of 2 exhibits a quadrupole doublet with Delta E-Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe-IV state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm(-1) together with the magnetic hyperfine tensor A/g(n)beta(n) = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) angstrom, a distance consistent with a Fe-IV=O bond. DFT calculations for [Fe-IV(O)B*](2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the Fe-57 hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.