The facile preparation of weakly coordinating anions: Structure and characterisation of silverpolyfluoroalkoxyaluminates AgAl(ORF)(4), calculation ofthe alkoxide ion affinity

Purified LiAlH4 reacts with fluorinated alcohols HORF to give LiAl(ORF)(4) (R-F=-CH(CF3)(2), 2a; -C(CH3)(CF3)(2), 2b; -C(CF3)(3), 2c) in 77 to 90% yie ld. The crude lithium aluminates LiAl(ORF)(4) react metathetically with AgF to give the silver aluminates AgAl(ORF)(4) (R-F=-CH(CF3)(2), 3a; -C(CH3)(C F3)(2), 3b; -C(CF3)(3), 3c) in almost quantitative yield. The solid-state s tructures of solvated 3a-c showed that the silver cation is only weakly coo rdinated (CN(Ag) =6-10; CN = coordination number) by the solvent and/or wea k cation-anion contacts Ag-X (X = O, F, Cl, C). The strength of the Ag-X co ntacts of 3a-c was analysed by Brown`s bond-valence method and then compare d 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 quant itative picture we showed that the Al{OC(CF3)(3)}(4) anion is one of the mo st weakly coordinating anions known. Moreover, the AgAl(ORF)(4) species are certainly the easiest WCAs to access preparatively (20 g in two days), add itionally at low cost. The Al-O bond length of Al(ORF)(4) is shortest in th e sterically congested Al{OC(CF3)(3)}(4) anion-which is stable in H2O and a queous HNO3 (35 weight %)-and indicates a strong and highly polar Al-O bond that is resistant towards heterolytic alkoxide ion abstraction. This obser vation was supported by a series of HF-DFT calculations of OR, Al(OR)(3) an d 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 (A TA = 363 +/- 7 kJ mol(-1)). The gaseous Al(ORF)(4) anions are stable agains t the action of the strong Lewis acid AlF3 (g) by 88.5 +/- 2.5 (R-F=CF3) an d 63 +/- 12 kJ mol(-1) (R-F = C(CF3)(3)), while Al(OCH3)(4) decomposes with -91 +/- 2 kJ mol(-1). There fore the presented fluorinated aluminates Al(O RF)(4) appear to be ideal candidates when large and resistant WCAs are need ed, for example, in cationic homogenous catalysis, for highly electrophilic cations or for weak cationic Lewis acid/base complexes.