N-Heterocyclic Dicarbene Iridium(III) Pincer Complexes Featuring Mixed NHC/Abnormal NHC Ligands and Their Applications in the Transfer Dehydrogenation of Cyclooctane

The reaction of 1,3-bis(imidazolyl)benzene with excess 1-bromoadamantane at 170 degrees C for 14 h afforded 1,3-bis(1-adamantylimidazolium)benzene dibromide (1), which was characterized by IR, NMR, and X-ray diffraction. Metathetical anion exchange with excess sodium iodide yielded the N-heterocyclic dicarbene precursor 1,3-bis(1-adamantylimidazolium)benzene diiodide, (CHimidCHCHimid)I-2 (2), in high yield. The reaction of [Ir(mu-Cl)(2)(cod)](2) (cod = 1,5-cyclooctadiene) with 2 in the presence of 2.2 equiv of Cs2CO3 in refluxing acetonitrile led to the formation of the unsymmetrical iridium(III) hydride pincer complex [Ir(H)I(CNHCCCaNHC)(NCMe)] (3), which contains a C2-bound NHC ligand and a CS-bound NHC ligand. Recrystallization of 3 from ClCH2CH2Cl/Et2O generated the dihalide Ir(III) pincer complex [IrX2(CNHCCCaNHC)(NCMe)], where X = mixture of I and Cl (4). Its formation probably involves initial formation of an iridium chloro iodo intermediate via solvent-induced substitution of the hydride ligand of 3 by chloride, followed by partial chloride/iodide anion exchanges at both the Ir-Cl and Ir-I sites. Treatment of 4 with excess KI in acetonitrile afforded the diiodide Ir(III) pincer complex [IrI2(CNHCCCaNHC)(NCMe)] (5), which was characterized by IR, NMR, and X-ray diffraction. Dehydroiodination of the monohydride complex 3 with sodium tert-butoxide (NaO-t-Bu) in the presence of hydrogen led to the formation of the new hydrido iridium pincer complex 6, which shows detectable catalytic activity toward the transfer dehydrogenation reaction of cyclooctane.