Probing the Oxidation Chemistry of Half-Sandwich Iridium Complexes with Oxygen Atom Transfer Reagents

The
new complexes [Ir­(Cp*)­(phpy)­3,5-bis­(trifluoromethyl)­benzonitrile]+ (1-NCAr+) and [Ir­(Cp*)­(phpy)­(styrene)]+ (1-Sty+, Cp* = η5-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC1′-pyridine-κN) were prepared as analogues of reported iridium water oxidation
catalysts, to study their reactions with oxygen atom transfer (OAT)
reagents at low temperatures. In no case was the desired product,
an Ir­(V)­oxo complex, observed by spectroscopy. Instead, ligand oxidation
was implicated. Oxidation of 1-NCAr+ with the OAT reagent dimethyldioxirane (DMDO) yielded dioxygen
when analyzed by GC, but formation of a heterogeneous or paramagnetic
species was simultaneously observed. This amplifies uncertainty over
the actual identity of iridium catalysts in the harsh oxidizing conditions
required for water oxidation. Catalyst stability was then assessed
for a reported styrene epoxidation mediated by [Ir­(Cp*)­(phpy)­(OH2)]+ (1-OH2+). It was found that the OAT reagent iodosobenzene
(PhIO) extensively oxidized the organic ligands of 1-OH2+. Acetic acid was
detected as a decomposition product. In addition, both the molecular
structure and the aqueous electrochemistry of 1-OH2+ are described for the
first time. Oxidative scans revealed rapid decomposition of the complex.
All of the above experiments indicate that degradation of the organic
ligands in catalysts built with the Ir­(Cp*)­(phpy) framework are facile
under oxidizing conditions. In separate experiments designed to promote
ligand substitution, an unexpected silver-bridged, dinuclear Ir­(III)
species with terminal hydrides, [{Ir­(Cp*)­(phpy)­H}2Ag]+ (2), was discovered. The source of Ag+ for complex 2 was identified as AgCl.