title: N-Heterocyclic olefins as ancillary ligands in catalysis: a study of their behaviour in transfer hydrogenation reactions.
abstract: The Ir(i) complexes [Ir(cod)(κP,C,P'-NHO(PPh2))]PF6 and [IrCl(cod)(κC-NHO(OMe))] (cod = 1,5-cyclooctadiene, NHO(PPh2) = 1,3-bis(2-(diphenylphosphanyl)ethyl)-2-methyleneimidazoline) and NHO(OMe) = 1,3-bis(2-(methoxyethyl)-2-methyleneimidazoline), both featuring an N-heterocyclic olefin ligand (NHO), have been tested in the transfer hydrogenation reaction; this representing the first example of the use of NHOs as ancillary ligands in catalysis. The pre-catalyst [Ir(cod)(κP,C,P'-NHO(PPh2))]PF6 has shown excellent activities in the transfer hydrogenation of aldehydes, ketones and imines using (i)PrOH as a hydrogen source, while [IrCl(cod)(κC-NHO(OMe))] decomposes throughout the reaction to give low yields of the hydrogenated product. Addition of one or two equivalents of a phosphine ligand to the latter avoids catalyst decomposition and significantly improves the reaction yields. The reaction mechanism has been investigated by means of stoichiometric studies and theoretical calculations. The formation of the active species ([Ir(κP,C,P'-NHO(PPh2))((i)PrO)]) has been proposed to occur via isopropoxide coordination and concomitant COD dissociation. Moreover, throughout the catalytic cycle the NHO moiety behaves as a hemilabile ligand, thus allowing the catalyst to adopt stable square planar geometries in the transition states, which reduces the energetic barrier of the process.
authors.affiliation: Departamento Química Inorgánica - ISQCH, Universidad de Zaragoza - CSIC, 50009 Zaragoza, Spain. miglesia@unizar.es oro@unizar.es.
authors.forename: Amaia
authors.initials: A
authors.lastname: Iturmendi
authors.affiliation: Centre of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia.
authors.forename: Nestor
authors.initials: N
authors.lastname: García
authors.affiliation: Centre of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia.
authors.forename: E A
authors.initials: EA
authors.lastname: Jaseer
authors.affiliation: Departamento Química Física - Instituto de Biocomputación y Física de Sistemas Complejos (BIFI), Universidad de Zaragoza, 50009 Zaragoza, Spain.
authors.forename: Julen
authors.initials: J
authors.lastname: Munárriz
authors.affiliation: Departamento Química Inorgánica - ISQCH, Universidad de Zaragoza - CSIC, 50009 Zaragoza, Spain. miglesia@unizar.es oro@unizar.es.
authors.forename: Pablo J
authors.initials: PJ
authors.lastname: Sanz Miguel
authors.affiliation: Departamento Química Física - Instituto de Biocomputación y Física de Sistemas Complejos (BIFI), Universidad de Zaragoza, 50009 Zaragoza, Spain.
authors.forename: Victor
authors.initials: V
authors.lastname: Polo
authors.affiliation: Departamento Química Inorgánica - ISQCH, Universidad de Zaragoza - CSIC, 50009 Zaragoza, Spain. miglesia@unizar.es oro@unizar.es.
authors.forename: Manuel
authors.initials: M
authors.lastname: Iglesias
authors.affiliation: Departamento Química Inorgánica - ISQCH, Universidad de Zaragoza - CSIC, 50009 Zaragoza, Spain. miglesia@unizar.es oro@unizar.es and Centre of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Oro