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

Methanol formation from [Cp*Ir-III(NHC)Me(CD2Cl2)] (+)occurs quantitatively at room temperature with air (O-2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)(2)(mu-O)][(BAr4F)(2)], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at similar to 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 Ir-IV-O-Ir-IV core in which two d(5) 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 mu-oxo bridge facilitates communication between the two Ir centers, and sigma/pi mixing yields a nonlinear arrangement of the mu-oxo core (Ir-O-Ir similar to 150 degrees) 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 iridiummethyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr4F] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)(2)][(BAr4F)(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.