The facile preparation of weakly coordinating anions: structure and characterisation of silver polyfluoroalkoxyaluminates AgAl(ORF)4, calculation of the alkoxide ion affinity

Purified LiAlH4 reacts with fluorinated alcs. HORF to give LiAl(ORF)4 (RF = -CH(CF3)2, 2a; -CMe(CF3)2, 2b; -C(CF3)3, 2c) in 77-90% yield. The crude Li aluminates LiAl(ORF)4 react metathetically with AgF to give the silver aluminates AgAl(ORF)4 (RF = -CH(CF3)2, 3a; -CMe(CF3)2, 3b; -C(CF3)3, 3c) in almost quant. yield. The solid-state structures of solvated 3a-c showed that the Ag cation is only weakly coordinated (CN(Ag) = 6-10; CN = coordination no.) 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 analyzed by Brown's bond-valence method and then compared with other Ag salts of weakly coordinating anions (WCAs), for example [CB11H6Cl6]- and [M(OTeF5)n]- (M = B, Sb, n = 4, 6). Based on this quant. picture the Al{OC(CF3)3}4- anion is one of the most weakly coordinating anions known. Also, the AgAl(ORF)4 species are certainly the easiest WCAs to access preparatively (20 g in two days), addnl. 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 aq. HNO3 (35%), and indicates a strong and highly polar Al-O bond that is resistant towards heterolytic alkoxide ion abstraction. This observation was supported by HF-DFT calcns. 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- decomps. 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 homogeneous catalysis, for highly electrophilic cations or for weak cationic Lewis acid/base complexes. [on SciFinder (R)]