Oxyfunctionalization with Cp*IrIII(NHC)(Me)(Cl) with O2: Identification of a Rare Bimetallic IrIV μ‑Oxo Intermediate

Methanol formation from [Cp*IrIII(NHC)­Me­(CD2Cl2)]+ occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir­(NHC)­Me)2(μ-O)]­[(BArF4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an IrIV–O–IrIV core in which two d5 Ir­(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir–O–Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir–O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium–methyl bond in the [Cp*Ir­(NHC)­Me­(CD2Cl2)]­[BArF4] starting material. Isolation of the diiridium-containing product [(Cp*Ir­(NHC)­Cl)2]­[(BArF4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.