Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes

Chemical
and electrochemical oxidation or reduction of our recently
reported Ir­(IV,IV) mono-μ-oxo dimers results in the formation
of fully characterized Ir­(IV,V) and Ir­(III,III) complexes. The Ir­(IV,V)
dimers are unprecedented and exhibit remarkable stability under ambient
conditions. This stability and modest reduction potential of 0.99
V vs NHE is in part attributed to complete charge delocalization across
both Ir centers. Trends in crystallographic bond lengths and angles
shed light on the structural changes accompanying oxidation and reduction.
The similarity of these mono-μ-oxo dimers to our Ir “blue
solution” water-oxidation catalyst gives insight into potential
reactive intermediates of this structurally elusive catalyst. Additionally,
a highly reactive material, proposed to be a Ir­(V,V) μ-oxo species,
is formed on electrochemical oxidation of the Ir­(IV,V) complex in
organic solvents at 1.9 V vs NHE. Spectroelectrochemistry shows reversible
conversion between the Ir­(IV,V) and proposed Ir­(V,V) species without
any degradation, highlighting the exceptional oxidation resistance
of the 2-(2-pyridinyl)-2-propanolate (pyalk) ligand and robustness
of these dimers. The Ir­(III,III), Ir­(IV,IV) and Ir­(IV,V) redox states
have been computationally studied both with DFT and multiconfigurational
calculations. The calculations support the stability of these complexes
and provide further insight into their electronic structures.