Transfer hydrogenation reduction of ketones, aldehydes and imines using chelated iridium(III) N-heterocyclic bis-carbene complexes

A series of imidazolium and triazolium chelate ligand precursors are metalated with [Ir(coe)(2)Cl](2) (coe = cyclooctene) or [Ir(cod)Cl](2) (cod = 1,5-cyclooctadiene) to give iridium(III) N-heterocyclic carbene (NHC) chelates [(NHC)(linker)(NHC)IrI2(OAc)]. The triazolium salts metalate more easily and give less electron donating NHCs as shown by IR spectroscopy on analogous iridium(l) carbonyl derivatives. The iridium(III) complexes show activity for the title catalytic reactions in 2-propanol that is very variable, depending on the nature of the linker and terminal N-substituents. Selective reduction of aldehydes over ketones becomes possible with K2CO3, an alternative base promoter to the usual KOH. TOFs of up to 1500/h were achieved. (C) 2004 Elsevier Ltd. All rights reserved.