Triple Ion and Quadrupole Formations from Trialkylammonium Sulfonates and Nitrate in Protophobic Aprotic Solvents with Higher Dielectric Constants

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Abstract
The formation of symmetrical triple ions (2M++X−\ightleftarrowsM2X+, (K2); M++2X−\ightleftarrowsMX2−, (K3); K2=K3) and the quadrupole (M2X++X−\ightleftarrowsM2X2, (K4); MX2−+M+\ightleftarrowsM2X2, (K5); K4=K5), in addition to the ion pair formation (M++X−\ightleftarrowsMX, (K1)) from uni-univalent salts (MX) has been examined in acetonitrile (dielectric constant ε=35.95), benzonitrile (ε=25.2), nitrobenzene (ε=34.82), and nitromethane (ε=36.67) at 25° by means of conductometry. Tho molar conductivities (Λ) of tributylammonium methanesulfonate (n-Bu3NHCH3SO3) of (0.4–6.0) ×10−3M (1 M=1 mol dm−3) in benzonitrile were explained by the formation of triple ions (Ka1=8.4×104, Ka2=Ka3=5.5×105, corrected by the activity coefficients of ions) within ±0.85% relative error, assuming that ΛT=ΛO⁄3 (ΛT: the limiting molar conductivity of the triple ions; ΛO: that of the simiple ions). In nitrobenzene, however, the quadrupole formation must be invoked for the salt in the same concentration range (Ka1=1.38×105, Ka2=Ka3=4.75×107, Ka4=Ka5=8.5×102). The alternative formation constant of the quadrupole (2MX\ightleftarrowsM2X2, (K41)) was evaluated as Ka41=5.0 instead of Ka4. Triple ion formation was observed in acetonitrile and nitromethane, but not in an amphiprotic solvent, ethanol (ε=24.55). The tendency for triple ion formation or higher aggregate formations from trialkylammonium salts was found to decrease as follows: Cl−>CH3SO3−, Br−> TsO−, NO3−>I−, CF3SO3−> (ClO4−). The difference in the aggregate forming ability is discussed from the view point of basicity of the anions, the strength for a proton acceptor in hydrogen bonding, and the localization of electrons in the anion molecule.