title: Oxyfunctionalization with Cp*Ir(III)(NHC)(Me)(Cl) with O₂: identification of a rare bimetallic Ir(IV) μ-oxo intermediate.
abstract: Methanol formation from [Cp*Ir(III)(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)][(BAr(F)4)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 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 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)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)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.
authors.affiliation: ‡Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Lehman
authors.affiliation: †Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, Texas 76203, United States.
authors.forename: Dale R
authors.initials: DR
authors.lastname: Pahls
authors.affiliation: §Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Meredith
authors.affiliation: ‡Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Roger D
authors.initials: RD
authors.lastname: Sommer
authors.affiliation: §Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States.
authors.forename: D Michael
authors.initials: DM
authors.lastname: Heinekey
authors.affiliation: †Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, Texas 76203, United States.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Cundari
authors.affiliation: ‡Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Elon A
authors.initials: EA
authors.lastname: Ison