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

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posted on 09.12.2013, 00:00 by Duc Hanh Nguyen, Ingo Greger, Jesús J. Pérez-Torrente, M. Victoria Jiménez, F. Javier Modrego, Fernando J. Lahoz, Luis A. Oro
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 [Ir44-PhN­(CH2COO)2}2(cod)4] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir44-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.