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, 2 a; −C(CH3)(CF3)2, 2 b; −C(CF3)3, 2 c) 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, 3 a; −C(CH3)(CF3)2, 3 b; −C(CF3)3, 3 c) in almost quantitative yield. The solid-state structures of solvated 3 a–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 3 a–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 (AIA=384±9 kJ mol−1) and R=C(CF3)3 (AIA=390±3 kJ mol−1), but lowest for R=CH3 (AIA=363±7 kJ 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 mol−1 (RF=C(CF3)3), while Al(OCH3)4− decomposes with −91±2 kJ 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.