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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Abstract
The
σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ<sup>3</sup>-pydc-X)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)]<sub>2</sub> and the corresponding 4-substituted pyridine-2,6-dicarboxylic
acids (H<sub>2</sub>pydc-X) or, alternatively, from their lithium
salts (X = H) and [Ir(cod)(CH<sub>3</sub>CN)<sub>2</sub>]PF<sub>6</sub>. Deuterium labeling studies in combination with theoretical calculations
have shown that formation of <b>1</b> involves a metal-mediated
proton transfer in the reactive intermediate [Ir(κ<sup>2</sup>-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ<sup>3</sup>-pydc)(cod)(CH<sub>3</sub>OH)], 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)]<sub>2</sub> with iminodiacetic acid derivatives, RN(CH<sub>2</sub>COOH)<sub>2</sub>, gave the stable iridium(I) mononuclear [Ir{κ<sup>2</sup>-MeN(CH<sub>2</sub>COOH)(CH<sub>2</sub>COO)}(cod)] (R = Me) complex
having a free carboxymethyl group and the tetranuclear complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(cod)<sub>4</sub>] (R = Ph) with doubly deprotonated ligands.
The molecular structure of the related cyclooctene complex [Ir<sub>4</sub>{κ<sup>4</sup>-PhN(CH<sub>2</sub>COO)<sub>2</sub>}<sub>2</sub>(coe)<sub>8</sub>] has been determined by X-ray analysis.
Reaction of <b>1</b> with monodentate N- and P-donor ligands
gave the compounds [Ir(κ<sup>3</sup>-pydc)(1-κ-4,5-η-C<sub>8</sub>H<sub>13</sub>)(L)] (L = py, BnNH<sub>2</sub>, PPh<sub>3</sub>, PMe<sub>3</sub>). Reaction of <b>1</b> with the short-bite
bis(diphenylphosphino)methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear
complex <b>1</b><sub><b>2</b></sub><b>-dppm</b> as
a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b><sub><b>2</b></sub><b>-dppe</b> and <b>1</b><sub><b>2</b></sub><b>-dppp</b> 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 <b>1-py</b> and <b>1-dppm</b> are reported
