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(OEt ₂)] [BDI] (BDI = ArNC(Me)CH(Me)CNAr; Ar = 2,6-Me ₂C ₆H ₃, 2,6-Et ₂C ₆H ₃, 2,6- ⁱPr ₂C ₆H ₃) under an N ₂ 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 ₂C ₆H ₃)NC(CMe ₃)CH(CMe ₃)CN(2,6-Me ₂C ₆H ₃) to [Ir(COD)Cl] ₂ (COD = 1,5-cyclooctadiene) yielded an unusual η ⁵-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.