Relative stabilities of weakly coordinating anions: A computational study

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)]

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

Ines Raabe, Nils Trapp

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)]

2004

Ines Raabe, Nils Trapp

2004