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.

Methyl Complexes of the Transition Metals

“…More recently, Ison and Cundari accomplished the insertion of an oxo ligand into an Ir-CH3 bond to form a proposed Ir-OCH3 intermediate (3.3b in Scheme 30) that subsequently released a molecule of methanol, with the overall process following a radical mechanism. 169 Dimethyl ether has also been produced from a Pt(IV)-methyl intermediate in methanolic solutions. 170 A nonradical pathway was suggested for the insertion of an oxo group into an Fe-CH3 bond in the calculated intermediate [Cp*Fe(P(OCH2)3CEt)(O)CH3] derived from the oxidation of [Cp*Fe(P(OCH2)3CEt)2CH3] using O2.…”

Inner‐Sphere Oxygen Activation Promoting Outer‐Sphere Nucleophilic Attack on Olefins

“…It is interesting to note that the chemistry of rhodium(I) and iridium(I) complexes with oxygen is dominated by their high tendency to form peroxide complexes . Notable exceptions are a few rhodium(I) compounds in which oxygen remains coordinated as a simple η 2 ‐ligand, superoxide iridium(III) derivatives, or dinuclear iridium(IV) μ‐oxo complexes . In other instances, oxygenation reactions involving C−O bond formation to 1‐metalla‐2‐oxetanes, highly unstable 1‐metalla‐2,3‐dioxolanes, and alkyl–hydroxy–allyl species have been described, whereas hydroxide or hydroperoxide complexes result from either oxygen insertion into M−H bonds or protonation of peroxide complexes .…”

“…[15] Notablee xceptions are af ew rhodium(I) compounds in whicho xygen remains coordinated as as imple h 2 -ligand, [16] superoxide iridium(III) derivatives, [17] or dinucleari ridium(IV) m-oxo complexes. [18] In other instances, oxygenation reactions [19] involvingC ÀOb ond formation to 1-metalla-2-oxetanes, [20] highly unstable1 -metalla-2,3dioxolanes, [21] and alkyl-hydroxy-allyl species [22] have been described,w hereash ydroxide or hydroperoxide complexes result from either oxygen insertion into MÀHb onds [23] or protonation of peroxide complexes. [24] Therefore, the reactions reported herein, which are promoted by oxygen, have no precedenti n rhodiumand iridium chemistry.…”

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

“…[14,17] DFT computations for 9 confirm that the singlet state is favored by DG = 18.5 kcal mol À1 over the triplet state and the computed structural parameters for S = 0a re in good agreement with the X-ray diffraction results.T he IrÀN(S = 0: 1.887 ; S = 1: 1.992 ) bond length is particularly sensitive to the spin multiplicity.T his observation is in line with the fact that the LUMO (S = 0) represents the highest (p*)MO of the N À Ir À O 3-center-4-electron p interaction (Figure 2). [18,19] MÀHb ond cleavage prior to O 2 splitting results in metal hydroperoxides as the crucial intermediates, [20] which have been isolated occasionally. Synthetic ORR cycle with ab ifunctionalI rdihydride.…”