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

Abstract

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.

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

“…For instance, quasi‐reversible pre‐catalytic features around 0.9 V vs. NHE often assigned to a molecular Ir III–IV redox couple are more characteristic of amorphous, hydrated iridium‐oxyhydroxide deposites which may form on the surface of the working electrode . The recent electrochemical characterization of well‐defined and stable model complexes showing Ir III–IV redox couples at potentials below 0.7 V vs. NHE and even reversible Ir IV–V transitions around 1.0–1.2 V vs. NHE, call for a revision of the electrochemistry of Cp*Ir based water oxidation precatalysts.…”

Synthesis and Characterization of Iridium(V) Coordination Complexes With an N,O‐Donor Organic Ligand

“…[2] However,t here are few examples of wellcharacterized Ir V coordination complexes.Previous examples include moisture-sensitive homoleptic fluorides, [3] as well as organometallic compounds [4] with potentially non-innocent alkyl or hydride ligands.W erecently reported an Ir IV,V monom-oxo dimer coordinated by the pyalk ligand (pyalk = 2-(2pyridinyl)-isopropanolate) as the first example of an Ir IV,V complex. [5] We attributed the surprising stability of this species to the donor strength [6] and oxidation resistance of the ligand as well as delocalization of the Ir IV,V oxidation state. Nevertheless,t here are no well-characterized examples of non-organometallic purely Ir V complexes with organic ligands.…”

Metalloradicals Supported by a meta‐Carborane Ligand

“…Communications IR. [13,14] Abroad feature at approximately 850 nm is observed (see Figure S25) and it does not change with the polarity of the solvent. This observation is in line with an IVCT-type transition for aC lass III (delocalized) mixed-valence complex.…”