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Water-Soluble Iron(IV)-Oxo Complexes Supported by Pentapyridine Ligands: Axial Ligand Effects on Hydrogen Atom and Oxygen Atom Transfer Reactivity

†Departments of Chemistry and ‡Molecular and Cell Biology and the §Howard Hughes Medical Institute,University of California, Berkeley, California 94720, United States
∥Chemical Sciences and ⊥Materials Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
# Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
Inorg. Chem., 2015, 54 (12), pp 5879–5887
DOI: 10.1021/acs.inorgchem.5b00658
Publication Date (Web): June 3, 2015
Copyright © 2015 American Chemical Society
*E-mail: jrlong@berkeley.edu (J.R.L.)., *E-mail: chrischang@berkeley.edu (C.J.C.).

Synopsis


A series of Fe(IV)-oxo complexes supported by pentapyridine ligands with varying electronic densities can be generated photochemically in water. Hydrogen atom transfer and oxygen atom transfer reactivity of these ferryl species toward hydrocarbon substrates correlate with the electron-donating ability of the axial pyridine ligand.

Abstract

Abstract Image
We report the photochemical generation and study of a family of water-soluble iron(IV)-oxo complexes supported by pentapyridine PY5Me2-X ligands (PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine; X = CF3, H, Me, or NMe2), in which the oxidative reactivity of these ferryl species correlates with the electronic properties of the axial pyridine ligand. Synthesis of a systematic series of [FeII(L)(PY5Me2-X)]2+ complexes, where L = CH3CN or H2O, and characterizations by several methods, including X-ray crystallography, cyclic voltammetry, and Mössbauer spectroscopy, show that increasing the electron-donating ability of the axial pyridine ligand tracks with less positive Fe(III)/Fe(II) reduction potentials and quadrupole splitting parameters. The FeII precursors are readily oxidized to their Fe(IV)-oxo counterparts using either chemical outer-sphere oxidants such as CAN (ceric ammonium nitrate) or flash-quench photochemical oxidation with [Ru(bpy)3]2+ as a photosensitizer and K2S2O8 as a quencher. The Fe(IV)-oxo complexes are capable of oxidizing the C–H bonds of alkane (4-ethylbenzenesulfonate) and alcohol (benzyl alcohol) substrates via hydrogen atom transfer (HAT) and an olefin (4-styrenesulfonate) substrate by oxygen atom transfer (OAT). The [FeIV(O)(PY5Me2-X)]2+ derivatives with electron-poor axial ligands show faster rates of HAT and OAT compared to their counterparts supported by electron-rich axial donors, but the magnitudes of these differences are relatively modest.http://pubs.acs.org/doi/pdf/10.1021/acs.inorgchem.5b00658
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