Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of “Anti-Electrostatic” Hydrogen Bonding

Abstract

We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH2−py][NTf2]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH2)n–py+)2(NTf2−)2, single-charged (HOOC–(CH2)n–py+)2(NTf2−), and double-charged (HOOC– (CH2)n−py+)2 complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH2)n−py+)2, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol−1, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.

Spectroscopic Evidence for Doubly Hydrogen-Bonded Cationic Dimers in the Solid, Liquid, and Gaseous Phases of Carboxyl-Functionalized Ionic Liquids

“…Density functional theory (DFT) calculations on isolated dicationic dimers, for example, suggest that with spacers of four methylene groups, the repulsive Coulomb forces are almost compensated by attractive hydrogen bonding and dispersion interaction. 19 Consequently, the (c�c) binding motif is likely to be present in the solid and perhaps in the liquid state if counterions are present that can screen the Coulomb repulsion between the cations. For the gas phase, we address the question as to which H-bond arrangements are formed in (2,1) complexes at low temperatures, wherein one counterion is removed and Coulomb repulsion increased.…”

“…This allows us to explore how the attractive (cc) interactions evolve as the ethyl and butyl chains systematically reduce the repulsive Coulomb interactions between the cations (Scheme ). Density functional theory (DFT) calculations on isolated dicationic dimers, for example, suggest that with spacers of four methylene groups, the repulsive Coulomb forces are almost compensated by attractive hydrogen bonding and dispersion interaction . Consequently, the (cc) binding motif is likely to be present in the solid and perhaps in the liquid state if counterions are present that can screen the Coulomb repulsion between the cations.…”