Ancillary Ligand Effects on C−H Bond Activation Reactions Promoted by β-Diiminate Iridium Complexes

A series of β-diiminate iridium(I) olefin, diolefin, and hydride complexes have been synthesized and evaluated in carbon−hydrogen bond activation reactions. Treatment of [Ir(COE)<sub>2</sub>Cl]<sub>2</sub> (COE = cyclooctene) with the lithio β-diiminate anions [Li(OEt<sub>2</sub>)][BDI] (BDI = ArNC(Me)CH(Me)CNAr; Ar = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>, 2,6-Et<sub>2</sub>C<sub>6</sub>H<sub>3</sub>, 2,6-<sup>i</sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) under an N<sub>2</sub> atmosphere furnished the corresponding iridium(I) cycloctene dinitrogen complexes. Using a similar procedure, the analogous β-diiminate iridium(I) cyclooctadiene compounds have also been prepared and characterized. Addition of the sterically demanding β-diiminate anion (2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)NC(CMe<sub>3</sub>)CH(CMe<sub>3</sub>)CN(2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) to [Ir(COD)Cl]<sub>2</sub> (COD = 1,5-cyclooctadiene) yielded an unusual η<sup>5</sup>-arene complex that is stabilized by a significant contribution from an iminocyclohexadienyl resonance form. The relative electronic influence of each β-diiminate ligand has been evaluated by preparation of the corresponding iridium dicarbonyl complexes and reveals little electronic perturbation among alkyl substituents on the aryl rings. With respect to C−H bond activation, warming the β-diiminate iridium(I) cyclooctene dinitrogen compounds to 50 °C resulted in intramolecular dehydrogenation chemistry, the outcome of which is dependent on the β-diiminate aryl substituents. For the 2,6-dimethyl-substituted complex, transfer dehydrogenation of the cyclooctene ligand is observed, while for the larger diethyl- and diisopropyl-substituted variants, dehydrogenation of the aryl substituents occurs.