New reagents to introduce weakly coordinating anions of type Al(ORF)4−: synthesis, structure and characterization of Cs and trityl salts

The facile synthesis of five reagents (Cs+ and Ph3C+ salts) to introduce weakly coordinating anions (WCAs) of type Al(ORF)4− (ORF: OC(H)(CF3)2; OC(CF3)3) or lithium bridged {(RFO)2Al(μ-ORF)2(μ-Li)(μ-ORF)2Al(ORF)2}− (Ph3C+ salt, ORF: OC(H)(CF3)2) is presented. All of the species were characterized spectroscopically, three of the species {Cs[Al(OC(H)(CF3)2)4] (1); Cs[Al(OC(CF3)3)4] (2); [Ph3C]{Li[Al(OC(H)(CF3)2)4]2} (4)} also by a X-ray single crystal structure determination. In contrast to the known Ag+ salts, the solid Cs+ salts 1–2 crystallize unsolvated which makes them suitable starting materials to introduce them as counterions for highly electrophilic and oxidizing cations (i.e. by metathesis with MF6− salts (M: As, Sb); CsMF6 is insoluble in SO2, CH2Cl2, etc.). The anions of 1–2 have thermochemical volumes of 599 Å3 (1) and 758 Å3 (2) with estimated low lattice potential energies of only 379 (1) and 358 (2) kJ/mol (cf. 759 kJ/mol for CsF). The trityl salts [Ph3C][Al(ORF)4] (RF: OC(H)(CF3)2 (3); OC(CF3)3 (5)) and [Ph3C]{Li[Al(OC(H)(CF3)2)4]2 (4)} are extremely robust and sealed NMR samples of 3–5 show no sign of decomposition even after storage at +70°C for 3 months. The basicity of the anions decreases according to Al(OC(H)(CF3)2)4−>Li[Al(OC(H)(CF3)2)4]2−>Al(OC(CF3)3)4−