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)] (1) 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 1 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)] (3). 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 3 was established
by 1H-DOSY NMR spectroscopy and an ESI+ mass
spectrum and supported by DFT calculations. Reaction of 3 with pyridine gave the adduct [IrH­(κ3-hqca)­(coe)­(py)]
(4) and the bis-pyridine complexes [IrH­(κ3-hqca)­(R-py)2] (R = H (6), 2-Me (7)) by replacement of the coe ligand. Compound 4 was
transformed into the bromo derivative [IrBr­(κ3-hqca)­(coe)­(py)]
(5) by reaction with N-bromosuccinimide.
Carbonylation of 4 gave the cyclooctenyl complex [Ir­(κ3-hqca)­(1-κ-C8H15)­(CO)­(py)] (8), which is stable only under a carbon monoxide atmosphere.
The pincer complexes were active in the catalytic borylation of arenes
under thermal conditions.