- Proton Chemical Shifts and Thermodynamics of the Formation of Hydrogen-Bonded Dimers and Mixed 1:1 Associates in the Ternary System Acetic Acid/Methanol/Tetrahydrofuran-d8
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The chemical shifts of the carboxylic proton of acetic acid and of the hydroxylic proton of methanol dissolved together and separately in tetrahydrofuran-d8 were determined as a function of the temperature and the solute concentrations.The determination of these chemical shifts was possible because in very pure samples the line positions are not affected by proton-exchange reactions.The data can only be explained by the presence of the following association reactions between hydrogen-bonded species which involve the solvent, S: RCOOH...S + RCOOH...S RCOOH...RCOOH...S + S RCOOH...S +ROH...S RCOOH...ROH...S + S ROH...S + RCOOH...S ROH...RCOOH...S + S ROH...S + ROH...S ROH...ROH...S + S The reaction enthalpies are given by +4.2, -1.4, -3.0, and -3.3 kJ mol-1, the reaction entropies at 298 K by +0.3, -3, -9, and -16 J K-1 mol-1.These data are not very far from zero, indicating that the number of hydrogen bonds remains constant during the association.Nevertheless, the extent of association is very low, and the quasi monomers which are hydrogen bonded to the solvent dominate.This arises from the high concentration, cs, of the free solvent.The change of cs at higher solute concentrations has to be taken into account in the treatment of the experimental data.Similarly, hydrogen bonding between the donors and the solvent is the reason that cyclic 1:1 associates and higher associates are not observed and the reason for the slow proton exchange.It is shown that the thermodynamic data of proton donor association in different media obey linear enthalpy-entropy relationships which are typical for the functional proton donor group.
- Gerritzen, Detlef,Limbach, Hans-Heinrich
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- Mono and dimetallic pyrene-imidazolylidene complexes of iridium(III) for the deuteration of organic substrates and the C-C coupling of alcohols
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Three different Ir(iii) complexes with pyrene-containing N-heterocyclic carbenes have been prepared and characterized. Two complexes contain a monodentate pyrene-imidazolylidene ligand, and have the formulae [IrCp?Cl2(pyrene-NHC)] and [IrCp?(CO3)(pyrene-NHC)]. The third complex is a dimetallic complex with a pyrene-di-imidazolylidene bridging ligand, with the formula [{IrCp?(CO3)}2(μ-pyrene-di-NHC)]. The catalytic activity of the three complexes was tested in the H/D exchange of organic substrates, and in the β-alkylation of 1-phenylethanol with primary alcohols. In the deuteration of organic substrates, the carbonate complexes are active even in the absence of additives. The dimetallic complex is the most active one in the catalytic coupling of alcohols, a result that may be interpreted as a consequence of the cooperativity between the two metal centres.
- Iba?ez,Poyatos,Peris
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p. 14154 - 14159
(2016/11/15)
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- Iridium Hydride Complexes with Cyclohexyl-Based Pincer Ligands: Fluxionality and Deuterium Exchange
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Two hydride compounds with aliphatic pincer ligands, (PCyP)IrH2 (PCyP = {cis-1,3-bis[(di-tert-butylphosphino)methyl]cyclohexane}- (1) and (PCyP)IrH4 (2), have been studied, with emphasis on features where such systems differ from arene-based analogues. Both compounds reveal relatively rapid exchange between α-C-H and Ir-H, which can occur via formation of carbene or through demetalation, with nearly equal barriers. This observation is confirmed by deuterium incorporation into the α-C-H position. Complex 1 can reversibly add an N2 molecule, which competes with the α-agostic bond for a coordination site at iridium. The hydrogen binding mode in tetrahydride 2 is discussed on the basis of NMR and IR spectra, as well as DFT calculations. While the interpretation of the data is somewhat ambiguous, the best model seems to be a tetrahydride with minor contribution from a dihydrido-dihydrogen complex. In addition, the catalytic activity of 1 in deuterium exchange using benzene-d6 as a deuterium source is presented.
- Polukeev, Alexey V.,Marcos, Rocío,Ahlquist, M?rten S. G.,Wendt, Ola F.
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p. 2600 - 2608
(2016/09/04)
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- An Air/water-stable tridentate N-heterocyclic carbene-palladium(II) Complex: Catalytic C-H activation of hydrocarbons via hydrogen/deuterium exchange process in deuterium oxide
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While developing novel catalysts for carbon-carbon or carbon-heteroatom coupling (nitrogen, oxygen, or fluorine), we were able to introduce tridentate N-heterocyclic carbene (NHC)-amidate-alkoxide palladium(II) complexes. In aqueous solution, these NHC-Pd(II) complexes showed high ability for C-H activation of various hydrocarbons (cyclohexane, cyclopentane, dimethyl ether, tetrahydrofuran, acetone, and toluene) under mild conditions.
- Lee, Joo Ho,Yoo, Kyung Soo,Park, Chan Pil,Olsen, Janet M.,Sakaguchi, Satoshi,Prakash, G. K. Surya,Mathew, Thomas,Jung, Kyung Woon
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experimental part
p. 563 - 568
(2009/11/30)
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- Iridium-catalyzed H/D exchange into organic compounds in water
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The air-stable complex Cp*(PMe3)IrCl2 efficiently catalyzes the exchange of deuterium from D2O into both activated and unactivated C-H bonds of organic molecules without added acid or stabilizers. Selectivity is observed in many cases, with activation of primary C-H bonds occurring preferentially. A number of new stoichiometric transformations involving the iridiym catalyst precursor are also presented, including an ozidation-decarbonylation reaction with primary alcohols. Copyright
- Klei, Steven R.,Golden, Jeffrey T.,Tilley, T. Don,Bergman, Robert G.
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p. 2092 - 2093
(2007/10/03)
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- First Determination of the Structure of an Ion-Paired Species in Nonpolar Media: 1H, 13C, and 7Li NMR Spectra of Peralkylcyclohexadienyllithium Compounds
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Peralkylcyclohexadienyllithium compounds (2a,b), formed by addition of alkyllithiums to triene 1, are shown to exist as ion-paired aggregates containing conjugated anions bent out of the plane at the saturated ring carbon.Both 13C and 7Li NMR spectra show 2a,b to form two distinctly different ion-paired species - tight peripherally solvated ion-pair aggregates in the presence of mainly tertiary amines and loose separated ion-pair dimers favored with THF, glymes, and HMTP.The latter exhibit two 7Li(+) peaks, one due to Li sandwiched between two anions in a triple ion and the other situated external to it.In the case of 2b with THF both ion-pair species are dimers, and the equlibrium (A(-)Li(+))2*(THF)2 + THF (A2Li)(-) + Li(THF)3(+) is characterized with ΔH = -4 kcal and ΔS = -16 eu.The exchange of ions between ion pairs of 2b with THF is generally slow on the NMR time scale.However, above 0 deg C the lithium exchange rate between the solvated site in the loose ion-pair dimer and the lithium in the tighter ion-pair dimer lies on the NMR time scale.The mean lifetime of lithium in the triple ion is always > 10 s.The slow ion-exchange rates are ascribed to the crowded substitution around 2a,b.
- Fraenkel, Gideon,Hallden-Abberton, Michael P.
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p. 5657 - 5664
(2007/10/02)
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