Conductometric identification of triple-ion and quadrupole formation by the coordination forces from lithium trifluoroacetate and lithium pentafluoropropionate in protophobic aprotic solvents

Abstract

The molar conductivities (Λ/S cm² mol^–1) of LiCF₃CO₂ and LiC₂F₅CO₂ in acetonitrile, benzonitrile, nitromethane or propylene carbonate have been explained in terms of symmetrical triple-ion formation (2M⁺+ X^–⇄ M₂X⁺, K′_{a, 2} and M⁺+ 2X^–⇄ MX^–₂, K′_{a, 3}; K′_{a, 2}=K′_{a, 3}) and quadrupole formation (M₂X⁺+ X^–⇄ M₂X₂, K′_{a, 4} or M⁺+ MX^–₂⇄ M₂X₂, K′_{a, 5}; K′_{a, 4}=K′_{a, 5}) in addition to the ion pair formation (M⁺+ X^–⇄ MX, K′_{a, 1}) in the concentration range (0.4–6.0)× 10^–3 mol dm^–3. Surprisingly, a great enhancement in quadrupole formation for LiCF₃CO₂ and LiC₂F₅CO₂ was observed in propylene carbonate with the highest relative permittivity (ε_r= 64.4 at 25 °C) of all the solvents. For trifluoroacetate, the limiting molar conductivity (Λ_o= 72.65) given by the Shedlovsky analysis [(0.4–4.0)× 10^–3 mol dm^–3] was much larger than that [Λ_{o, calc}= 26.37] calculated by Kohlrausch's additivity law with strong electrolytes. Lithium pentafluoropropionate gave a similar excess in the Λ_o value. Computer simulations showed an increase in the Shedlovsky Λ_o value with increase in the quadrupole formation constant. At the same time, the apparent association constant (M⁺+ X^–⇄ MX, K_a) calculated by Shedlovsky analysis was 10 times larger than the ion-pair formation constant (K′_{a, 1}) in propylene carbonate (owing to strong quadrupole formation) and was much smaller than the K′_{a, 1} value in the other solvents (mainly owing to strong triple-ion formation). A distinct triple-ion formation from tributylammonium trifluoroacetate or tributylammonium pentafluoropropionate was observed in benzonitrile. Causes of the failure in the Shedlovsky analysis have been discussed from the standpoint of higher-ion aggregates.