Oxyfunctionalization with Cp*Ir^III(NHC)(Me)(Cl) with O₂: Identification of a Rare Bimetallic Ir^IV 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.以空气（O₂）为氧化剂、乙醇为质子源，在室温条件下，[Cp*（五甲基环戊二烯基，pentamethylcyclopentadienyl）IrIII(NHC)（氮杂环卡宾，N-Heterocyclic Carbene）­Me­(CD₂Cl₂)]⁺可定量生成甲醇。 本研究分离得到一例罕见的二铱双核金属配合物[(Cp*Ir(NHC)Me)₂(μ-O)][(BArF₄)₂]（记为3），经证实为该反应的中间体。 配合物3的电子吸收光谱在约660 nm处存在一个宽吸收峰，这也是其呈现蓝色的主要原因。 此外，配合物3为反磁性物质，可通过核磁共振（NMR, Nuclear Magnetic Resonance）光谱进行表征。通过X射线单晶衍射表征可知，配合物3具有Ir⁴⁺–O–Ir⁴⁺核结构，两个d⁵构型的Ir(IV)中心由一个氧配体桥联。 密度泛函理论（DFT, Density Functional Theory）与多组态自洽场（MCSCF, Multi-Configuration Self-Consistent Field）计算揭示了配合物3电子结构的多项关键特征：最为显著的是，μ-氧桥可促进两个铱中心之间的电子通讯；σ/π轨道混合使μ-氧桥核呈现非线性排布（Ir–O–Ir键角约为150°），以利于氧原子转移过程。 配合物3的生成源于一个Ir氧/氧自由基中间体，该中间体可通过两种能量相近的竞争成键模型进行描述：两种模型的Ir–O形式键级均为2，但电子结构差异显著。 自由基捕获剂TEMPO（四甲基哌啶氮氧自由基，2,2,6,6-Tetramethylpiperidin-1-oxyl）与1,4-环己二烯并不会抑制配合物3的生成，但TEMPO可阻断由配合物3生成甲醇的过程。 同位素标记实验证实，甲醇产物中的甲基源自起始原料[Cp*Ir(NHC)Me(CD₂Cl₂)][BArF₄]中的铱-甲基键。 反应结束后可高收率分离得到含二铱的产物[(Cp*Ir(NHC)Cl)₂][(BArF₄)₂]（记为4），这表明Cp*配体与NHC配体始终与铱中心结合，在反应条件下未发生明显降解。