Novel oxonium compounds of lanthanoids: synthesis of (H3O)3La2F(AsF6)8, and syntheses and crystal structures of (H3O)8La2F(AsF6)13 and (H3O)4La2F(AsF3)2(AsF6)9

Reactions of Ln2O3 with AsF5 in aHF (Ln=La–Lu, aHF=anhydrous hydrogen fluoride) yield solutions of H3O+, solvated Ln3+ and AsF6− ions. Very similar solutions of these ions may be obtained from combination of various other starting materials, e.g. LnF3–H2O–AsF5–aHF, LnFx(AsF6)3−x–H3OAsF6–aHF. From such solutions a new type of oxonium compounds with general formula (H3O)xLnyFz(AsF6)x+3y−z have been isolated at ambient temperature. (H3O)3La2F(AsF6)8 was obtained as a white powder from the solution of La2O3 dissolved in AsF5–aHF after pumping off all volatiles at room temperature. Colourless crystals of (H3O)8La2F(AsF6)13 were prepared by slow static evaporation from a solution obtained after solvolysis of solid (H3O)3La2F(AsF6)8 in aHF at ambient conditions. Colourless crystals of (H3O)4La2F(AsF3)2(AsF6)9 were prepared by the reaction of La2O3 with AsF5 and aHF under solvothermal conditions above the critical temperature of AsF5. Similar reactions of other oxides Ln2O3 (Ln=Nd, Sm, Gd, Dy, Ho, Tm) have also been studied. (H3O)8La2F(AsF6)13 crystallises in the monoclinic space group P21/c (no. 14 ) with a=1079.3(3) pm, b=2859.0(7) pm, c=1068.0(3) pm, β=109.310(8)°, and Z=2. (H3O)4La2F(AsF3)2(AsF6)9 crystallises in the triclinic space group P1̄ (no. 2) with a=1064.8(3) pm, b=1073.3(4) pm, c=1213.9(3) pm, α=75.53(2)°, β=64.79(2)°, γ=70.07(2)°, and Z=1. Both structures are closely related with zig-zag chains of alternating La2F and AsF6 units resembling the main feature. Oxonium ions are arranged among such anionic chains.