- Metal-free carbon dioxide reduction and acidic C-H activations using a frustrated Lewis pair
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Activation of CO2 and acidic C-H bonds by the lutidine-tris(pentafluorophenyl)borane [Lut/B(C6F5) 3] frustrated Lewis pair (FLP) are described (lutidine = 2,6-dimethylpyridine). Lut/B(C6F5)
- Tran, Sophia D.,Tronic, Tristan A.,Kaminsky, Werner,Michael Heinekey,Mayer, James M.
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experimental part
p. 126 - 132
(2011/06/24)
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- β-diketiminato scandium chemistry: Attempted deprotonation of cationic amido complexes
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Treatment of the β-diketiminato-supported scandium amido methyl derivatives [ArNC(tBu)CHC(tBu)NAr]Sc(Me)-(NHR) (Ar = 2,6-iPr2-C6H3; R = tBu, 2,6-iPr2-C
- Knight, Lisa K.,Piers, Warren E.,McDonald, Robert
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p. 3289 - 3292
(2008/10/09)
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- Anion exchange in alkyl-zirconocene borate ion pairs. Are solvated alkyl-zirconocene cations relevant intermediates?
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Ion pairs of the type Cpx2ZrMe+···A - containing various ansa-zirconocene methyl cations in contact with Me-B(C6F5)3- or B(C6F5)4- anions have been studied with regard to their anion exchange kinetics by 2D-NMR methods in benzene or toluene solutions. The results - acceleration of anion exchange by added Li+···Me-B(C6F5) 3-, substantial nonproductive exchange between added and Zr-bound Me-B(C6F5)3- anions, an increase of exchange rates at increased zirconocene concentrations, and the exclusively entropic origin of this rate increase - all indicate that anion exchange occurs by way of ion quadruples or higher ionic aggregates, rather than via dissociation to solvent-separated ions. These findings imply that solvent-separated (i.e. anion-free) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing Cpx2ZrMe+···A - ion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of α-olefins.
- Beck,Lieber,Schaper,Geyer,Brintzinger
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p. 1483 - 1489
(2007/10/03)
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- d0 Metal olefin complexes. Synthesis, structures, and dynamic properties of (C5R5)2Zr(OCMe2CH2CH2CH=CH2)+ complexes: Models for the elusive (C5R5)2Zr(R)(Olefin)+ intermediates in metallocene-based olefin polymerization catalysis
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To model the Zr-olefin interaction in the as-yet unobserved (C5R5)2Zr(R)(olefin)+ intermediates in (C5R5)2Zr(R)+-catalyzed olefin polymerization, the coordination of the tethered vinyl group in (C5R5)2Zr(OCMe2(CH2)(n)CH=CH2)+ species has been investigated. The reaction of (C5H5)2Zr(OCMe2CH2CH2CH= CH2)(Me) with B(C6F5)3 or [Ph3C][B(C6F5)4] yields the chelated olefin complex (C5H5)2Zr(OCMe2CH2CH2CH= CH2)+ as the MeB(C6F5)3- (12a) or B(C6F5)4- (12b) salts. In contrast, the reaction of (C5H5)2Zr(OCMe2CH2CH= CH2)(Me) with B(C6F5)3 in CD2Cl2 yields the MeB(C6F5)3- adduct (C5H5)2Zr(+)(OCMe2CH2CH=CH2)(μMe)B(-)(C6F5)3. The reaction of (C5H5)2Zr(OCMe2CH2CH2CH2CH=CH2)(Me) with B(C6F5)3 yields a 1.2/1 mixture (at -90 °C) of the chelated olefin complex (C5H5)2Zr(OCMe2CH2CH2CH2CH=CH2)+ and the MeB(C6F5)3- adduct (C5H5)2Zr(+)(OCMe2CH2CH2CH=CH2)(μ-Me)B(-)(C6F5)3'. The reaction of rac-(EBI)-Zr(OCMe2CH2CH2CH=CH2)(Me) (EBI = ethylene-1,2-bis(1-indenyl)) with B(C6F5)3 or [Ph3C] [B(C6F5)4] yields the chelated olefin complex rac-(EBI)Zr(OCMe2CH2CH2CH=CH2)+ as the MeB(C6F5)3- (20a) or B(C6F5)4-(20b) salts, each as a 1/1 mixture of diastereomers which differ in the relative configuration of the rac-(EBI)-Zr unit and the internal carbon of the coordinated olefin. X-ray diffraction analyses of 12a and the S,S,R/R,R,S isomer of 20a, and NMR data for 12a,b and 20a,b establish that the Zr-olefin bonding in these species is unsymmetrical and consists of a weak Zr-C(term) interaction and minimal Zr-C(int) interaction (12a, Zr-C(term) = 2.68(2), Zr-C(int) = 2.89(2) A; 20a, Zr-C(term) = 2.634(5), Zr-C(int) = 2.819(4) A). X-ray (d(C=C)), IR (v(C=C)), and NMR (1H, 13C) data show that the Zr-olefin interaction does not significantly perturb the structure of the coordinated olefin but does polarize the C=C bond such that positive charge buildup occurs at C(int). Similar unsymmetrical bonding and polarization effects may contribute to the high insertion reactivity of (C5R5)2Zr(R)(α-olefin)+ species. Dynamic NMR studies show that 12a,b and 20a,b undergo olefin face exchange in solution on the NMR time scale. The free energy barrier for face exchange of 20a (ΔG(+)(FE) = 15.4(4) kcal/mol at 43 °C) is significantly greater than that for 12a (ΔG(+)(FE) = 10.7(5) kcal/mol at -55 °C). Possible origins of this difference are discussed. The face exchange of 20a is dissociative, with minimal involvement of anion, solvent, or σ-complex intermediates.
- Carpentier, Jean-Francois,Wu, Zhe,Lee, Chul Woo,Stroemberg, Staffan,Christopher, Joseph N.,Jordan, Richard F.
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p. 7750 - 7767
(2007/10/03)
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