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.