Aggregation of Ions in Higher Permittivity Solvents

Abstract

Electrolytic conductances of lithium 4-acyl-5-pyrazolonates (a kind of β-diketonates) and related salts in higher permittivity solvents, acetonitrile (εr = 35.95), acetone (20.7), N,N-dimethylformamide (DMF) (36.71), and propylene carbonate (64.4) were measured at 25 °C. The molar conductivity−concentration (Λ−c1/2) plots of lithium 1-phenyl-3-methyl-4-benzoyl-5-pyrazolonate (LiPMBP) in acetonitrile and acetone display a minimum at 1.51 × 10-3 and 2.0 × 10-3 mol dm-3, respectively, but not in DMF and propylene carbonate. The observed Λ values of LiPMBP in all the solvents are explained by the presence of ion pairs (LiA: A = PMBP) and higher ion aggregates, “symmetrical” triple ions ((Li+)2A-, Li+(A-)2), and quadrupoles (Li2A2). The addition of small amounts of MeOH, H2O, or DMF to acetonitrile caused the cmin value to be higher; with ≥1.0 vol % of these solvents added, the minimum disappeared completely. However, the addition of MeNO2 or PhNO2 caused a more distinct or deeper minimum in the Λ − c1/2 curve. Upon the appearance of the minima, the effects of permittivity and viscosity of the mixed solvents were examined. The conductivity plots of the lithium salts of the 4-trifluoromethoxy derivative (LiPMTFP) and 3-phenyl-4-benzoyl-5-isoxazolonate (LiPBI) did not give minima in the solvents. The observed Λ values could not be fitted without taking into account higher ion aggregations. In DMF, LiPMTFP was much more dissociative. The association constants of the β-diketonates are discussed from the viewpoint of the electron density on the oxygen atoms. Computer calculations indicate that unilateral triple ion formation is impossible for the present system exhibited minima. The formation of bilateral triple ion species (Li+)2A- and Li+(A-)2 (where A- = PMBP- or PBI-) in acetonitrile solution was demonstrated by means of UV−visible and 7Li NMR spectra. The formation of even 5- or 6-coordinated as well as 4-coordinated species was suggested when a large excess of n-Bu4NPBI was present in 0.01 mol dm-3 LiClO4−CD3CN.