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

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posted on 05.12.2005, 00:00 by Wesley H. Bernskoetter, Emil Lobkovsky, Paul J. Chirik
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.