1,721,076 research outputs found
ADC1P
No full textThe Green’s function ADC(3) scheme has been for many years a successful method to predict theoretically
the ionization (and electron affinity) spectrum of molecules. However, a dramatic enhancement of the method’s
power has come only recently, with the development of an approximation method to the one-particle Green’s function
which does not make direct use of the Dyson equation. In the present work, we present an efficient computer
implementation of this novel approach, with first comparative tests demonstrating its enormous computational
advantage over the conventional approach
Linear gold(I) complexes: A systematic NMR/DFT study on the ligand effect on the structure of the ion pair in solution
No full textLigand Effects on Bonding and Ion Pairing in Cationic Gold(I) Catalysts Bearing Unsaturated Hydrocarbons
No full textWe critically review recent experimental and theoretical investigations into some key aspects of the chemistry of gold(I) complexes of the type [L-Au-S]X-+(-) (L = NHC carbenes and phosphanes, S = alkenes and alkynes, and X- = weakly coordinating counterion). These systems are important intermediates formed during gold-catalyzed nucleophilic additions to an unsaturated substrate, and their specific activity is largely governed by two fundamental factors: the nature of the gold-substrate bond and the role of the ion-pair structure in solution. Both are crucially influenced by the nature and properties of the auxiliary ligand L, and on this interplay we focus our discussion. The relative anion-cation orientation, investigated by NOE NMR spectroscopy and DFT calculations, shows that the exact position of the counterion is determined by the natures of the ancillary ligand and substrate: the counterion is located near the substrate in the phosphane complexes, while for the NHC complexes the preferred position of the counterion is near the ligand. This tunable interionic structure opens the way to greater control over the properties and activity of these catalysts. The bond between Au-I and the unsaturated substrate is investigated using an original and powerful theoretical method of analysis. Our approach permits a rigorous definition and assessment of the charge-displacement (CD) components at the heart of the Dewar-Chatt-Duncanson model: substrate-to-metal (sigma donation) and metal-to-substrate (back-donation) and how these change with different ligands. The results consistently reveal that back-donation is a large and crucially important component of the Au-I-substrate bond in all systems: back-donation penetrates the external side of coordinated alkynes, where nucleophile attack is directed, thus partially mitigating the electron depletion caused by sigma donation
Ion pairing in cationic gold catalysts
No full textGold(I) cationic complexes of general formula [LAu+...X-] [L = phosphines or NHCs (N-Heterocyclic Carbenes), X- = weakly coordinating anion] are successfully employed as catalysts in a large variety of organic reactions involving the activation of unsaturated carbon-carbon bonds.[1] A key role in such reactions is played by the counterion [2], which strongly affects activity, regio- and stereo-selectivity. [3]
The linear phosphine gold(I) alkyne [4] complex [(PArF3)Au(2-hexyne)]BF4 (1BF4) [ArF=3,5-bis(trifluoromethyl) phenyl], its analogous [(NHC)Au(2-hexyne)]BF4 (2BF4) [NHC=1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] and the alkenes complexes [5] [(PPh3)Au(4-Me-styrene)]BF4 (3BF4) and [(NHC)Au(4-Me-styrene)]BF4 [4BF4; NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] have been synthesized. Their intramolecular and interionic structures have been investigated by combining 1D and 2D multinuclear NMR spectroscopy and Density Functional Theory calculations.
The relative anion-cation orientation in [LAuS...X] [S= alkenes and alkynes] ,investigated by NMR spectroscopy and DFT, shows that the exact position of the counterion is critically determined by the nature of the ancillary ligand and substrate; this opens the way to a greater control over the properties and activity of these catalysts.
Also, It has been found that unsatured bonds in the phosphine complexes is depleted of electron density [6] to a greater extent than in NHC ones. In the case of alkynes the charge trasferred correlates with the (13C) NMR of the carbons triple bond. 2BF4 is much more “kinetically stable” than 1BF4. All these findings support the view that catalysts with phosphine ancillary ligands are more effective in activating alkyne substrates (higher TOF), while NHC catalysts are more robust (higher TON).
References
[1] Hashmi, A. S. K. Chem. Rev., 107, 3180. (2007)
[2] Macchioni, A. Chem. Rev., 105, 2039. (2005)
[3] Toste, F. D. et al. Science, 317, 496. 2007
[4] Zuccaccia, D. et al Inorg. Chem, , 49, 3080 (2010)
[5] Zuccaccia D. et al. J. Am. Chem. Soc.,131, 3170 (2009)
[6] Belpassi, L. et al. J. Am. Chem. Soc., 130, 1048. (2008
The electronic structure of titanium(IV)chloride and vanadium(IV)chloride by an MS-SCF-Xα method. Assignment of photoelectron and electronic spectra
No full textIon Pairing in Cationic Olefin-Gold(I) Complexes
No full textThe relative anion-cation orientation in [(PPh(3))Au(4-Me-styrene)]BF(4) (1BF(4)) and [(NHC)Au(4-Me-styrene)]BF(4) [2BF(4); NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] has been investigated by combining (19)F, (1)H-HOESY NMR spectroscopy and Density Functional Theory (DFT) calculations incorporating solvent and relativistic effects. It has been found that BF(4)(-) locates on the side of 4-Me-styrene, close to the olefin region that is opposite to the 4-Me-Ph moiety in 1BF(4). In 2BF(4), the counterion approaches the cation from the side of the NHC ligand and is mainly located close to the imidazole ring. In both cases, the counterion resides far away from the gold site, the latter carrying only a small fraction of the positive charge. This indicates that the preferential position of the counterion is tunable through the choice of the ancillary ligand, and this opens the way to greater control over the properties and activity of these catalysts
Synthesis, NMR and theoretical study of Au(I) complexes with Nitrogen Heterocyclic Carbenes (NHC)
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