Iridium(I) complexes with hemilabile N-heterocyclic carbenes: Efficient and versatile transfer hydrogenation catalysts

16 páginas, 5 figuras, 3 tablas, 3 esquemas.A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]+ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH3)(cod)(MeImR)]+ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h–1. The complexes [Ir(NCCH3)(cod)(MeImR)]+ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.Financial support from Ministerio de Ciencia e Innovación (MICINN/FEDER) is gratefully acknowledged (Projects: CTQ2010-15221, CSD2006-0015, and CSD2009-00050). J.F. T. thanks the Spanish MICINN for a predoctoral fellowship, and S.W. thanks the Eramus program (Univ. RWTH-Aachen, Germany).Peer reviewe

DIGESTIBILIDADE DE INGREDIENTES ALTERNATIVOS PARA TILÁPIA-DO-NILO (Oreochromis niloticus): REVISÃO

A avaliação nutricional dos ingredientes de uma ração destinada à alimentação animal vai desde a escolha dos insumos, sua composição química até sua utilização pelos animais. Para tanto, é necessário saber a digestibilidade dos nutrientes contidos nessa dieta, sendo assim, várias pesquisas vêm sendo desenvolvidas para a avaliação da digestibilidade dos ingredientes usualmente utilizados nas rações para a tilápia-do-nilo, porém, atualmente a utilização de ingredientes alternativos na exploração aqüícola é uma realidade que vêm sendo buscada de forma promissora no Brasil, por conta da grande diversidade e quantidade de eventuais substitutivos a esses ingredientes usualmente utilizados nas rações e da grande disponibilidade desses alimentos

Biomass burning signatures in the atmosphere and snow at Summit, Greenland: An event on 5 August 1994

Two recent reports have suggested that thin layers of ice in Greenland cores with anomalously high concentrations of NH4+ K+ and HCOO− represent deposition from biomass burning plumes advected over Greenland. These interpretations were based primarily on the similarity between the suite of enriched species in the ice and several recent characterizations of biomass burning plumes from various regions around the globe. In August 1994 a biomass burning plume was transported to Summit, Greenland (72°N 38°W) from the Hudson Bay lowlands region of Canada. Gas-phase, aerosol and snow samples impacted by this plume had large enhancements of , and K+. Several other species that have been reported to be enriched in some biomass burning plumes were also enriched in at least one of the three phases (gas, aerosol and snow) at Summit. Comparisons between the plume at Summit and biomass burning plumes sampled in 1990 over the Hudson Bay lowlands suggest that the carboxylic acids may be significantly enhanced by secondary production during the 3–4 days of transport between Canada and Greenland. It also appears that gas to particle conversion during transport may modify the partitioning of the carboxylates, nitrate, and perhaps ammonium and inorganic sulfur between the gas and aerosol phases in the plume. The relative enrichments of these species differ considerably between the atmosphere and snow, but the signal in snow was quite similar to the composition of the anomalous samples previously described in the ice cores

Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]⁺ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH₃)(cod)(MeImR)]⁺ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h^–1. The complexes [Ir(NCCH₃)(cod)(MeImR)]⁺ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(O<i>i</i>Pr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity