Unsaturated Iridium(III) Complexes Supported by a Quinolato–Carboxylato ONO Pincer-Type Ligand: Synthesis, Reactivity, and Catalytic C–H Functionalization

The unsaturated σ,π-cyclooctenyl iridium­(III) pincer compound [Ir­(κ³-hqca)­(1-κ-4,5-η-C₈H₁₃)] (<b>1</b>) has been prepared by the reaction of [Ir­(cod)­(CH₃CN)₂]­BF₄ with lithium 8-oxidoquinoline-2-carboxylate (Li₂hqca) and obtained as two isomers derived from the relative disposition of the pincer and the σ,π-cyclooctenyl ligands. Compound <b>1</b> can be prepared as a single isomer by reaction of 8-hydroxyquinoline-2-carboxylic acid (H₂hqca) with [Ir­(μ-OMe)­(cod)]₂. Reaction of [Ir­(μ-OH)­(coe)₂]₂ with H₂hqca gave the square-pyramidal iridum­(III) complex [IrH­(κ³-hqca)­(coe)] (<b>3</b>). This compound exists as dinuclear assemblies [IrH­(κ³-hqca)­(coe)]₂ in noncoordinating solvents and as the corresponding labile mononuclear solvates in more polar solvent solutions. The dimerization of <b>3</b> was established by ¹H-DOSY NMR spectroscopy and an ESI⁺ mass spectrum and supported by DFT calculations. Reaction of <b>3</b> with pyridine gave the adduct [IrH­(κ³-hqca)­(coe)­(py)] (<b>4</b>) and the bis-pyridine complexes [IrH­(κ³-hqca)­(R-py)₂] (R = H (<b>6</b>), 2-Me (<b>7</b>)) by replacement of the coe ligand. Compound <b>4</b> was transformed into the bromo derivative [IrBr­(κ³-hqca)­(coe)­(py)] (<b>5</b>) by reaction with <i>N</i>-bromosuccinimide. Carbonylation of <b>4</b> gave the cyclooctenyl complex [Ir­(κ³-hqca)­(1-κ-C₈H₁₅)­(CO)­(py)] (<b>8</b>), which is stable only under a carbon monoxide atmosphere. The pincer complexes were active in the catalytic borylation of arenes under thermal conditions