Confusing Ions on Purpose: How Many Parent Acid Molecules Can Be Incorporated in a Herbicidal Ionic Liquid?

To expand the application range of herbicidal ionic liquids (HILs), we attempted to increase the amount of the systemic herbicide dicamba in its HILs through the formation of neither purely ionic nor neutral hydrogen-bonded complexes (so-called oligomeric ions) of parent acid and conjugate base. We realized we faced an important question-how far is it possible to increase the amount of active species (thus reducing chemical burden and extra cost) while still retaining the favorable properties of an IL? To answer this question, we prepared and characterized a range of dicamba salts with the cations tetrabutylphosphonium ([P-4444](+)), betainium ([HBet](+)), and carnitinium ([HCar](+)) with varying stoichiometries of excess dicamba acid (HDic). Reactions of [P-4444][OH] or zwitterionic betaine (Bet) or carnitine (Car) with methanolic solutions of HDic in 1:1, 1:2, 1:3, 1:4, and 1:5 stoichiometries led, after solvent removal, to the salts [Cat][Hn-1(Dic)(n)] ([Cat] = [P-4444](+), [HBet](+), [HCar](+); n = 1-4). At 1:1 stoichiometry, the known low melting salts, [P-4444] [Dic], [HBet] [Dic], and [HCar] [Dic], were isolated, of which [P-4444] [Dic] can be classified as an IL. All salts with up to a 3-fold excess HDic, except [P-4444][H(Dic)(2)], were liquid at room temperature when initially prepared, although [HBet] [H(Dic)(2)] crystallized on standing over a period of 8 weeks. Single-crystal X-ray diffraction, H-1 NMR, and Fourier transform infrared spectroscopy revealed that the degree of proton transfer or sharing was dependent on the cation, with phosphonium salts being fully ionic, carnitinium salts being less ionized and retaining some of carnitine's zwitterionic character, and betainium salts sharing protons equally at the 1:1 composition. The results also indicate that the limit of HDic that can be accommodated in these formulations is a 1:4 M ratio since all reactions at 1:5 stoichiometry precipitated HDic upon standing.