Yoshihiko Imai

Triple-ion formation and acid–base strength of ions in protophobic aprotic solvents

The formation of symmetrical triple ions [2M++ X–⇌ M2X+, (K2); M++ 2X–⇌ MX–2, (K3); K2=K3] and the quadrupole [M2X++ X–⇌ M2X2, (K4); M++ MX–2⇌ M2X2, (K5); K4=K5], in addition to ion-pair formation [M++ X–⇌ MX, (K1)] from various uni–univalent salts were examined by means of conductometry in acetonitrile, benzonitrile and propylene carbonate. The salts are made from bases (B) and acids (HX) of varying strength. For diethylcyclohexylammonium chloride [(0.4–6.0)× 10–3 mol dm–3 in benzonitrile], the calculated molar conductivities (Λ/S cm2 mol–1) were fitted to the observed ones within 0.36% of the standard deviation of the relative error, considering the symmetrical formation of the triple ions. In the higher concentration range [(0.4–12.0)× 10–2 mol dm–3] of the salt a minimum was observed in the relation between Λ and C1/2. The observed minimum could be reproduced in terms of the large formation constants of the triple ions and the quadrupole complex by computer simulations. However, a larger formation constant of the quadrupole above the critical value caused the minimum to disappear. Weaker basicities of amines tended to give higher formation constants of the triple ion; however a levelling-off was observed below pKBH+= 18 in acetonitrile. On the other hand, the formation constants for the ion pair and the triple ions from salts with different anion basicities (from perchlorate to 3,5-dinitrobenzoate) were proportional to the acidities of the corresponding acids.

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

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 (Iµr= 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 Kohlrauschs 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.

Polarographic study on the interaction between alkali metal cations and acyclic polyamines in acetonitrile

Abstract Two anodic waves (E 1 2 = −0.50 and −0.60 V vs. Ag/0.1 M AgClO4MeCN) were produced by triethylenetetramine (Trien N4) in acetonitrile containing 0.1 M Et4NClO4 as the supporting electrolyte. The waves were attributed to the following reaction: Hg + Trien ⇆ 1 2 [Hg2 (trien)2]2+ + e− ⇆ [Hg(trien)]2+ + e− Tetraethylenepentamine (Tetren, N5) also gave two anodic waves; however, the second wave (the more positive one) was extremely irreversible. The second wave from diethylenetriamine (Dien, N3) appeared at a much more positive potential. With 0.1 M LiClO4 and NaClO4, the two anodic waves of trien were combined into a single wave (two-electrone process) at −0.40 and −0.49 V, respectively. The complex formation constants between alkali metal cations and trien (1:1) were obtained not only from the positive shift of the anodic wave from trien but also from the positive shift of the cathodic wave of [Hg(trien)]2+ in the prsence of a large exces of Li+ or Na+. The negative shift of the cathodic wave of Na+ in the presence of trien gave the same complex formation constant (log K = 2.7) as those obtained by the above two methods. Dien and even ethylenediamine (En, N2) interacted with Li+ (log K = 5.4 for [Li(en)2]+).

Specific interactions between anions and cations in protophobic aprotic solvents. Triple-ion and higher aggregate formation from lithium and tributylammonium thiocyanates

Triple-ion and quadrupole formation in addition to ion-pair formation from lithium and tributylammonium thiocyanates has been examined by means of conductometry, in several protophobic aprotic solvents: nitrobenzene, benzonitrile, acetonitrile, nitromethane and propylene carbonate. The formation constants of the ion pair (M++ SCN–⇌ MSCN, Ka1; where M+= Li+ or n-Bu3NH+), symmetrical triple ions [2M++ SCN–⇌(M+)2SCN–, Ka2; M++ 2 SCN–⇌ M+(SCN–)2, Ka3; Ka2=Ka3] and the quadrupole [(M+)2SCN–+ SCN–⇌(MSCN)2, Ka4; M++ M+(SCN–)2⇌(MSCN)2, Ka5=(Ka5) were evaluated after correction of the activity coefficients of the ions. Effects of the ratio ΛT/Λ0 on the conductivities were examined, where ΛT and Λ0 are the molar conductivities at infinite dilution of the triple ions and simple ions, respectively. A remarkable enhancement of the triple-ion formation from LiSCN was observed in nitromethane: Ka′1= 5.5 × 104, Ka′2=Ka′3= 8.0 × 107 and ΛT/Λ0= 0.4 (the effects of the ionic atmosphere and viscosity changes are taken into account). At higher salt concentrations, quadrupole formation was observed, which caused the disappearance of the minimum in the Λ–C1/2 curve. In contrast, the degree of triple-ion formation from n-Bu3NHSCN was much smaller than that from LiSCN and quadrupole formation was not observed in all the solvents. The formation of (Li+)2SCN– was suggested by spectrophotometry. Both the donor and acceptor numbers of the solvents were concerned with the formation of higher ion aggregates.

Triple ion formation ability of picrate in protophobic aprotic solvents with very low basicity

Abstract The conditions for the formation of species from monovalent salts in protophobic aprotic solvents have been examined for lithium, triethylammonium and 1,3-diphenylguanidinium picrates (2,4,6-trinitrophenolates) by means of conductometry. The molar conductivities of lithium picrate (Li+ Pic−, (0.4–4.0) × 10−3 mol dm−3) in nitromethane were explained by assuming the formation of “symmetrical” triple ions ((Li+)2Pic− and Li+ (Pic−)2) in addition to ion pairs. However, triethylammonium picrate formed only the ion pair in the same solvent. Symmetrical triple ion formation from lithium picrate was also observed in benzonitrile. The formation of the triple cation (Li+)2 Pic− in benzonitrile was proved by spectrophotometry. Both conductometry and spectrophotometry confirmed that in acetonitrile the lithium salt formed only the ion pair. Distinct triple ion formation was observed for 1,3-diphenylguanidinium picrate ((0.4–6.0) × 10−3mol dm −3) in nitrobenzene, but was not so apparent in benzonitrile. The ability of the picrate ion to form triple ions was found to be as low as that of SCN− or I−.

Polarographic studies on the interaction between macrocyclic compounds and cations or acids in acetonitrile

At the DME, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (Me4[14]aneN4, TMC) in acetonitrile containing 0.1 mol dm−3 Et4NClO4 as the supporting electrolyte gave a single anodic (mercury dissolution) wave at −0.41 V vs. Ag/0.1 mol dm−3 AgClO4+MeCN electrode. The reversible anodic wave is attributed to the reaction: Hg+TMC ⇌; [Hg(tmc)]2+ + 2 e−. The formation of the [Na(tmc)]+ complex has been clarified, and the formation constant of the complex obtained not only by the shift in E12 of the anodic wave from TMC but also by that of the cathodic wave of [Hg(tmc)]2+ (produced by the addition of Hg(ClO4)2 to TMC) in the presence of a large excess of Na+. By the addition of a large excess of benzoic acid (HA) to the TMC solution, the TMC(H+)2(A−(HA)2)2 species was formed. 1,4,8,11-Tetrathiacyclotetradecane ([14]aneS4, TTCT) and [16]aneS4-3,11-diol also gave single anodic waves at more positive potentials to form mercuric complexes. Oxygen-substituted compounds, 18-crown-5 and 12-crown-4, gave incomplete anodic waves.