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Article

(TAML)Fe^IVO Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Supporting Info

Arani Chanda^†, Xiaopeng Shan^‡, Mrinmoy Chakrabarti^†, W. Chadwick Ellis^†, Delia L. Popescu^†, Filipe Tiago de Oliveira^†, Dong Wang^‡, Lawrence Que, Jr.*^‡, Terrence J. Collins*^†, Eckard Münck*^†, Emile L. Bominaar*^†

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455

Inorg. Chem., 2008, 47 (9), pp 3669–3678

DOI: 10.1021/ic7022902

Publication Date (Web): April 2, 2008

†

Carnegie Mellon University.

‡

University of Minnesota.

* To whom correspondence should be addressed. E-mail: que@chem.umn.edu (L.Q.), tc1u@andrew.cmu.edu (T.J.C.), emunck@cmu.edu (E.M.), eb7g@andrew.cmu.edu (E.L.B.).

Abstract

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.