From weakly coordinating to non-coordinating anions? A simple preparation of the silver salt of the least coordinating anion and its application to determine the ground state structure of the Ag(η2-P4)2+ cation

The unexpected but facile prepn. of the Ag salt of the least coordinating [(RO)3Al-F-Al(OR)3]- anion (R = C(CF3)3) by reaction of Ag[Al(OR)4] with one equiv. of PCl3 is described. The mechanism of the formation of Ag[(RO)3Al-F-Al(OR)3] is explained based on the available exptl. data as well as on quantum chem. calcns. with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)3Al(OC4H8), Cs[(RO)2(Me)AlFAl(Me)(OR)2], Ag(CH2Cl2)3[(RO)3AlFAl(OR)3] and Ag(h2-P4)2[(RO)3AlFAl(OR)3]- are described. From the collected data the [(RO)3AlFAl(OR)3]- anion is the least coordinating anion currently known. With respect to the F- ion affinity of two parent Lewis acids Al(OR)3 of 685 kJ mol-1, the ligand affinity (441 kJ mol-1), the proton and Cu decompn. reactions (-983 and -297 kJ mol-1) as well as HOMO level and HOMO-LUMO gap and in comparison with [Sb4F21]-, [Sb(OTeF5)6]-, [Al(OR)4]- as well as [B(RF)4]- (RF = CF3 or C6F5) the [(RO)3Al-F-Al(OR)3]- anion is among the best weakly coordinating anions (WCAs) according to each value. In contrast to most of the other cited anions, the [(RO)3AlFAl(OR)3] anion is available by a simple prepn. in conventional inorg. labs. The least coordinating character of this anion was employed to clarify the question of the ground state geometry of the Ag(h2-P4)2+ cation (D2h, D2 or D2d). In agreement with computational data and NMR spectra it could be shown that the rotation along the Ag-(P-P-centroid) vector has no barrier and that the structure adopted in the solid state depends on packing effects which lead to an almost D2h sym. Ag(h2-P4)2+ cation (0 to 10.6 Deg torsion) for the more sym. [Al(OR)4]- anion, but to a D2 sym. Ag(h2-P4)2+ cation with a 44 Deg twist angle of the two AgP2 planes for the less sym. [(RO)3AlFAl(OR)3]- anion. This implies that Ag back bonding, suggested by quantum chem. population analyses to be of importance, is only weak. [on SciFinder (R)]

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

Salts of electrophilic cations and Lewis acid-base adducts with weakly coordinating anions

Ines Raabe

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

Structural insights on cholesterol endosynthesis: Binding of squalene and 2,3-oxidosqualene to supernatant protein factor

Michele Cascella, Christos Lamprakis, Maria Josefina Marcaida Lopez

We present the crystal structures of the SEC14-like domain of supernatant protein factor (SPF) in complex with squalene and 2,3-oxidosqualene. The structures were resolved at 1.75 angstrom (complex with squalene) and 1.6 angstrom resolution (complex with 2,3-oxidosqualene), leading in both cases to clear images of the protein/substrate interactions. Ligand binding is facilitated by removal of the Golgi-dynamics (GOLD) C-terminal domain of SPF, which, as shown in previous structures of the apo-protein, blocked the opening of the binding pocket to the exterior. Both substrates bind into a large hydrophobic cavity, typical of such lipid-transporter family. Our structures report no specific recognition mode for the epoxide group. In fact, for both molecules, ligand affinity is dominated by hydrophobic interactions, and independent investigations by computational models or differential scanning micro-calorimetly reveal similar binding affinities for both ligands. Our findings elucidate the molecular bases of the role of SPF in sterol endo-synthesis, supporting the original hypothesis that SPF is a facilitator of substrate flow within the sterol synthetic pathway. Moreover, our results suggest that the GOLD domain acts as a regulator, as its conformational displacement must occur to favor ligand binding and release during the different synthetic steps. (C) 2015 Elsevier Inc. All rights reserved.

Academic Press Inc Elsevier Science2015

Salts of electrophilic cations and Lewis acid-base adducts with weakly coordinating anions

Ines Raabe

EPFL2007

Relative stabilities of weakly coordinating anions: A computational study

Ines Raabe

2004