ONO dianionic pincer-type ligand precursors for the synthesis of σ,π-cyclooctenyl iridium(III) complexes: Formation mechanism and coordination chemistry

Article abstract of DOI:10.1021/om400767d

The σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ³-pydc-X)(1-κ-4,5-η-C₈H ₁₃)] (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{κ ⁴-PhN(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN(CH ₂COO)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-C ₈H₁₃)(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 1 ₂-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.