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–IrII compound (η4-COD)­Ir­[η1:1-μ2-η4-CHC­(Me)­CHC­(Me)­O)]­Ir­(η4-COD) (1) 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 IrII and
bonding η4 with Ir0. The addition of CO,
PMe3, and PMe2Ph to the coordinatively unsaturated
complex 1 has led, under mild conditions, to the corresponding
dinuclear coordinatively saturated compounds (η4-COD)­Ir­[η1:1-μ2-η3-CHC­(Me)­CHC­(Me)­O)]­Ir­(η4-COD)­(μ2-CO) (2) and (η4-COD)­Ir­[η1:1-μ2-η4-CHC­(Me)­CHC­(Me)­O)]­Ir­(η4-COD)­(PR3) (R = Me, 3; PR3 = PMe2Ph, 4). Compound 3 showed a reversible
reaction by dissociation of PMe3, recovering compound 1. The reaction of 1 with H2 and PMe3, PMe2Ph, and P­(n-Bu)3 allows the isolation of cyclooctenyl derivatives (η4-COD)­Ir­[η1:1-μ2-η3-CHC­(Me)­CHC­(Me)­O)]­Ir­(μ2-H)­(η1:η2-C8H13)­(PR3) (R = Me, 5; R3 = Me2Ph, 6; R = n-Bu, 7), 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, 4 leads also to the formation
of 6 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-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)­[η1:1-μ2-η3-CHC­(Me)­CHC­(Me)­O)]­Ir­(H)­(Pi-Pr3)2 (8). The novel
complexes 1–8 have been fully characterized,
where 1 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 1–3 and 5–8 have been determined
by single-crystal X-ray diffraction studies.