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)₂Cl]₂ (COE = cyclooctene) with the lithio β-diiminate anions [Li(OEt₂)][BDI] (BDI = ArNC(Me)CH(Me)CNAr; Ar = 2,6-Me₂C₆H₃, 2,6-Et₂C₆H₃, 2,6-ⁱPr₂C₆H₃) under an N₂ 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-Me₂C₆H₃)NC(CMe₃)CH(CMe₃)CN(2,6-Me₂C₆H₃) to [Ir(COD)Cl]₂ (COD = 1,5-cyclooctadiene) yielded an unusual η⁵-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.