The Facile Preparation of Weakly Coordinating Anions: Structure and Characterisation of Silverpolyfluoroalkoxyaluminates AgAl(ORF)4, Calculation of the Alkoxide Ion Affinity

Abstract

Purified LiAlH4 reacts with fluorinated alcohols HORF to give LiAl(ORF)4 (RF=-CH(CF3)2, 2a; -C(CH3)(CF3)2, 2b; -C(CF3)3, 2c) in 77 to 90% yield. The crude lithium aluminates LiAl(ORF)4 react metathetically with AgF to give the silver aluminates AgAl(ORF)4 (RF=-CH(CF3)2, 3a; -C(CH3)(CF3)2, 3b; -C(CF3)3, 3c) in almost quantitative yield. The solid-state structures of solvated 3a-c showed that the silver cation is only weakly coordinated (CN(Ag)=6-10; CN = coordination number) by the solvent and/or weak cation - anion contacts Ag-X (X=O, F, Cl, C). The strength of the Ag-X contacts of 3a-c was analysed by Brown's bond-valence method and then compared with other silver salts of weakly coordinating anions (WCAs), for example [CB11H6Cl6]- and [M(OTeF5)n]- (M=B, Sb, n=4, 6). Based on this quantitative picture we showed that the Al[OC(CF3)3]4 anion is one of the most weakly coordinating anions known. Moreover, the AgAl(ORF)4 species are certainly the easiest WCAs to access preparatively (20 g in two days), additionally at low cost. The Al-O bond length of Al(ORF)4- is shortest in the sterically congested Al[OC(CF3)3]4- anion-which is stable in H2O and aqueous HNO3 (35 weight%)--and indicates a strong and highly polar Al-O bond that is resistant towards heterolytic alkoxide ion abstraction. This observation was supported by a series of HF-DFT calculations of OR-, Al(OR)3 and Al(OR)4- at the MPW1PW91 and B3LYP levels (R= CH3, CF3, C(CF3)3). The alkoxide ion affinity (AIA) is highest for R=CF3 (AlA=384 +/- 9 kJ x mol(-1)) and R= C(CF3)3 (AIA=390 +/- 3 kJ x mol(-1)), but lowest for R=CH3 (AIA=363 +/- 7 kJ X mol(-1)). The gaseous AL(ORF)4-anions are stable against the action of the strong Lewis acid ALF3(g) by 88.5 +/- 2.5 (RF=CF3) and 63 +/- 12 kJ X mol(-1) (RF=C(CF3)3), while AL(OCH3)4- decomposes with -91 +/- 2 kJ X mol(-1). Therefore the presented fluorinated aluminates AL(ORF)4- appear to be ideal candidates when large and resistant WCAs are needed, for example, in cationic homogenous catalysis, for highly electrophilic cations or for weak cationic Lewis acid/base complexes.

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

“…4 ] -was applied as the simple to use oxidizing agent of choice (eq. 2), which was synthesized according to the procedure established in our group [33].…”

“…To combine a decent stability with sufficient Lewis acidity, we chose the fluorinated alkoxide 1,1,1,3,3,3-hexafluoro-2-propoxy ligand (Ohfip) for the boryl residue. The Ohfip-ligand was proven to provide suitable Lewis acidity to an electron-deficient central atom, e.g., in the related B(Ohfip) 3 , as well as in monomeric Al[OC(CF 3 ) 3 ] 3 or dimeric Al 2 (Ohfip) 6 [30][31][32][33][34]. Carpenter and coworkers suggested that oxidation enhances the Lewis acidity of ferrocenylboranes, so that the B center in certain borylferrocinium compounds abstracts a fluoride from a [BF 4 ] -counteranion [35].…”

“…To avoid this side reaction, we decided to use the silver salt of the perfluorinated alkoxyaluminate anion [Al(OR F ) 4 ] -(R F = C(CF 3 ) 3 ) as an oxidant. The CF bonds of the latter are more resistant against fluoride ion abstraction [33]. Owing to its delocalized negative charge, the [Al(OR F ) 4 ] --anion belongs to the category of weakly coordinating anions (WCAs), which are ideal to investigate the crystal structures of the oxidized compounds due to the minimized interaction between the oppositely charged ions [36].…”

Synthesis and structure of salts of a sterically shielded, lipophilic, C2-symmetric, fluxional aluminate

“…21 and Krossing's "Teflon ball" PFTB. 22 There are only few examples of halogen-free lipophilic anions with low nucleophilicity. One class of these halogen-free anions are carboranates such as the [CB 11 H 12 ] -anion ( Figure 2).…”

Functionalization of P4 in the coordination sphere of coinage metal cations

“…Commercially available IPrMCl (M = Cu, Au; IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene) in combination with Li + [Al(pftb) 4 ] À (pftb = perfluoro-tert-butoxy) 11,12 as chloride scavenger were found to be suitable starting materials allowing the isolation of readily available LA-P 4 adducts. The complexation of P 4 was achieved by dropwise addition of a solution of IPrMCl (1 equiv.…”