(TAML)Fe^IV=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-Fe ^IVB*]^2- (3), with a pK_a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with ΔE_Q = 3.95(3) mm/s and δ = -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_nβ_n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) Å, 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 ⁵⁷Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.