Iridaoxacyclohexadiene-Bridged Mixed-Valence Iridium Cyclooctadiene Complex: Oxidative Addition and Hydrogen-Transfer to Coordinated Cyclooctadiene

The systematic exploration of the synthesis of heteropentadienyl metal complexes leads us to study the metathesis reaction of [(η⁴-COD)­Ir­(μ₂-Cl)]₂ with lithium 2,4-dimethyl­oxopentadienide, which affords the dinuclear Ir⁰–Ir^II compound (η⁴-COD)­Ir­[η^1:1-μ₂-η⁴-CHC­(Me)­CHC­(Me)­O)]­Ir­(η⁴-COD) (<b>1</b>) with a metal–metal bond. The COD ligands are coordinated η⁴ to each Ir center, whereas the oxopentadienyl ligand is bridging both Ir atoms, allowing the formation of a novel iridapyran complex with Ir^II and bonding η⁴ with Ir⁰. The addition of CO, PMe₃, and PMe₂Ph to the coordinatively unsaturated complex <b>1</b> has led, under mild conditions, to the corresponding dinuclear coordinatively saturated compounds (η⁴-COD)­Ir­[η^1:1-μ₂-η³-CHC­(Me)­CHC­(Me)­O)]­Ir­(η⁴-COD)­(μ₂-CO) (<b>2</b>) and (η⁴-COD)­Ir­[η^1:1-μ₂-η⁴-CHC­(Me)­CHC­(Me)­O)]­Ir­(η⁴-COD)­(PR₃) (R = Me, <b>3</b>; PR₃ = PMe₂Ph, <b>4</b>). Compound <b>3</b> showed a reversible reaction by dissociation of PMe₃, recovering compound <b>1</b>. The reaction of <b>1</b> with H₂ and PMe₃, PMe₂Ph, and P­(<i>n</i>-Bu)₃ allows the isolation of cyclooctenyl derivatives (η⁴-COD)­Ir­[η^1:1-μ₂-η³-CHC­(Me)­CHC­(Me)­O)]­Ir­(μ₂-H)­(η¹:η²-C₈H₁₃)­(PR₃) (R = Me, <b>5</b>; R₃ = Me₂Ph, <b>6</b>; R = <i>n</i>-Bu, <b>7</b>), where the hydrogen promotes the formation of a metal-hydride, as well as hydrogen-transfer to one of the coordinated cyclooctadiene ligands. In the presence of molecular hydrogen, <b>4</b> leads also to the formation of <b>6</b> in better yield. The iridaoxacycle bridging ligand stabilizes these dinuclear iridium complexes, which easily undergo intermolecular insertion into activated C–H bonds. When the more sterically demanding phosphine P­(<i>i</i>-Pr)₃ is added in the presence of H₂, a different reaction takes place, with the displacement of one COD ligand and the formation of (η⁴-COD)­Ir­(μ₂-H)­[η^1:1-μ₂-η³-CHC­(Me)­CHC­(Me)­O)]­Ir­(H)­(P<i>i</i>-Pr₃)₂ (<b>8</b>). The novel complexes <b>1</b>–<b>8</b> have been fully characterized, where <b>1</b> shows dynamic behavior in one of the COD ligands in solution and gives evidence of two different isomers present in the crystalline structure. Molecular structures of <b>1</b>–<b>3</b> and <b>5</b>–<b>8</b> have been determined by single-crystal X-ray diffraction studies