ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry

The σ,π-cyclooctenyl iridium­(III) pincer compounds [Ir­(κ³-pydc-X)­(1-κ-4,5-η-C₈H₁₃)] (X = H (1), Cl, Br) have been prepared from [Ir­(μ-OMe)­(cod)]₂ and the corresponding 4-substituted pyridine-2,6-dicarboxylic acids (H₂pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir­(cod)­(CH₃CN)₂]­PF₆. Deuterium labeling studies in combination with theoretical calculations have shown that formation of 1 involves a metal-mediated proton transfer in the reactive intermediate [Ir­(κ²-Hpydc)­(cod)], through the solvent-stabilized hydrido complex [IrH­(κ³-pydc)­(cod)­(CH₃OH)], followed by olefin insertion. The formation of this hydrido intermediate results from solvent-assisted proton transfer through a hydrogen-bonding network, forming an eight-membered metallacycle. In contrast, reaction of [Ir­(μ-OMe)­(cod)]₂ with iminodiacetic acid derivatives, RN­(CH₂COOH)₂, gave the stable iridium­(I) mononuclear [Ir­{κ²-MeN­(CH₂COOH)­(CH₂COO)}­(cod)] (R = Me) complex having a free carboxymethyl group and the tetranuclear complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(coe)₈] has been determined by X-ray analysis. Reaction of 1 with monodentate N- and P-donor ligands gave the compounds [Ir­(κ³-pydc)­(1-κ-4,5-η-C₈H₁₃)­(L)] (L = py, BnNH₂, PPh₃, PMe₃). Reaction of 1 with the short-bite bis­(diphenylphosphino)­methane (dppm) afforded the mononuclear 1-dppm, with an uncoordinated P-donor atom, or the dinuclear complex 1₂-dppm as a function of the molar ratio used. Similarly, the dinuclear complexes 1₂-dppe and 1₂-dppp have been prepared using 1,2-bis­(diphenylphosphino)­ethane (dppe) and 1,3-bis­(diphenylphosphino)­propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-ray structures of 1-py and 1-dppm are reported.