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 (εr= 64.4 at 25 °C) of all the solvents. For trifluoroacetate, the limiting molar conductivity (Λo= 72.65) given by the Shedlovsky analysis [(0.4–4.0)× 10–3 mol dm–3] was much larger than that [Λo, calc= 26.37] calculated by Kohlrausch's additivity law with strong electrolytes. Lithium pentafluoropropionate gave a similar excess in the Λo value. Computer simulations showed an increase in the Shedlovsky Λo value with increase in the quadrupole formation constant. At the same time, the apparent association constant (M++ X–⇄ MX, Ka) 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.