Anion non coordinant

Résumé

vignette|Structure du [], un anion particulièrement peu coordinant. Un anion non coordinant, ou, de manière plus exacte, faiblement coordinant, est un anion qui interagit peu avec les autres ions et molécules. Se présentant généralement sous la forme de complexes de grande taille, ils sont utiles pour étudier la réactivité des cations électrophiles. On les rencontre généralement sous forme de contre-ions de complexes métalliques cationiques avec une sphère de coordination insaturée. Ce sont des composants essentiels des catalyseurs homogènes de polymérisation des alcènes, ou oléfines, dans lesquels le catalyseur actif est un complexe d'un cation de métal de transition coordonné. On peut les utiliser par exemple comme contre-ions pour les cations à de valence de la forme , où R représente un groupe méthyle ou une chaîne de polyéthylène en croissance. Ce sont également des composants importants de nombreux superacides, qui résultent de la combinaison d'acides de Brønsted et d'acides

Source officielle

https://fr.wikipedia.org/wiki/Anion_non_coordinant

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Cours associés (5)

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Séances de cours associées (5)

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During the last decades, weakly coordinating anions (WCAs) have become a field of great interest both in basic and applied chemistry, as – if compared to "normal" classical anions like Cl- or SO42- – they have a lot of unique and unusual properties, such as high solubility in low dielectric media (like CH2Cl2 or toluene), "pseudo gas phase conditions" in condensed phases and the stabilization of strong electrophilic cations as well as of weakly bound and low charged complexes. Within the present thesis, several contributions to the chemistry of WCAs were made: Based on quantum chemical calculations, the stability of the most widely used WCAs have been investigated, using parameters such as the fluoride ion affinity (FIA), partial charges or frontier orbital energies. One class of the WCAs, the poly- and perfluorinated alkoxyaluminates of the type [Al(ORF)4]- and [(RFO)3Al-F-Al(ORF)3]-, has been used to investigate weakly bound Ag(P4S3) Lewis acid-base adducts and electrophilic cations: When the silver salts of fluorinated alkoxyaluminates are reacted with P4S3, different structure types are found for the adducts, depending on the anion used. The dynamics of these complexes has been explored by different spectroscopic methods (IR, Raman, MAS-NMR, NMR in solution) and X-ray diffraction. The first structurally characterized trihalocarbenium ions CI3+ as salts of different aluminates have been prepared and fully characterized using various spectroscopic techniques, e.g. NMR and vibrational spectroscopy, as well as single-crystal diffraction. The Lewis acidities of the halocarbenium ions have been investigated both on experimental and theoretical grounds and they have been compared to those of the isoelectronic haloboranes. The NO2+ salt of the [Al(OC(CF3)3)4]- anion has been synthesized by a metathesis reaction. It has been reacted with P4 in order to obtain naked phosphorus cations Px+. Another project was the synthesis and characterization of the tetraalkylammonium salts of the [Al(ORF)4]- anions, a promising class of substances for the use in electrochemical applications in non-polar solvents. All subjects treated experimentally within this thesis have been accompanied by quantum chemical calculations.

EPFL2007

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A review on the available candidates in the quest for the least coordinating anion and a summary of new applications, available starting materials, and general strategies to introduce a WCA into a system. Some of the unusual properties of WCA salts such as high soly. in low dielec. media, pseudo gas-phase conditions in condensed phases, and the stabilization of weakly bound and low-charged complexes are rationalized on thermodn. grounds. Limits of the WCAs, i.e., anion coordination and decompn., are shown and a quantum chem. anal. of all types of WCAs is presented which allows the choice of a particular WCA to be based on quant. data from a wide range of different anions. [on SciFinder (R)]

2004

Chargement

Relative stabilities of weakly coordinating anions: A computational study

Ines Raabe

This article describes BP86/SV(P) (DFT) calcns. on a representative set of weakly coordinating anions (WCAs) of type [M(L)n]-, their parent neutral Lewis acids M(L)n-1 and their ate complexes with fluoride, i.e., [FM(L)n]n-1 (M = B, L = F, OTeF5, C6H5, C6F5, C6H3(CF3)2, CF3; M = P, As, Sb, L = F, OTeF5; M = Al, L = OC(CF3)3). Compds. with fluoride bridges, i.e., SbnF5n and [SbnF5n +1]- (n = 2, 3, 4), Al2(L)5F and [(L)3Al-F-Al(L)3]- (L = OC(CF3)3), (F4C6{1,2-B(L)2}2), [F4C6{1,2-B(L)2}2F]-, [F4C6{1,2-B(L)2}2OMe]- (L = C6F5) were also calcd. Based on these BP86/SV(P) and auxiliary MP2/TZVPP, G2, and CBS-Q calcns. the relative stabilities and coordinating abilities of these WCAs were established with regard to the fluoride ion affinities (FIA) of the parent Lewis acids, the ligand affinity (LA) of the WCAs, the decompn. of a given WCA in the presence of a hard (H+, proton decompn. PD) and a soft electrophile (Cu+, copper decompn. CuD), the position of the HOMO, the HOMO-LUMO gap, and population analyses of the anions providing partial charges for all atoms. To obtain data that is more reliable, the assessed quantities were calcd. through isodesmic reactions. If parts of the calcns. could not be done isodesmically, higher levels such as MP2/TZVPP, G2, and CBS-Q were used to obtain reliable values for these reactions. Although the obtained results can not be taken as abs., the relative ordering of the stabilities of all WCAs will undoubtedly be correct, since a single methodol. was chosen for the investigation. To include media effects the decompn. reactions of a subset of 14 WCAs with the SiMe3+ and [Cp2ZrMe]+ ions were also calcd. in PhCl and 1,2-F2C6H4 (COSMO solvation model). We found that in most cases gas-phase calcns. and soln. calcns. give comparable results for the stability of the anion. Applications of the LA and FIA that allow one to decide, on thermodn. grounds, which WCA or Lewis acid is the most suitable for a given problem are sketched. [on SciFinder (R)]

2004

Chargement

Relative stabilities of weakly coordinating anions: A computational study

Ines Raabe

2004

EPFL2007