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

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.