Unusual Cation-Pi Solute Interactions with Deep Eutectic Solvents

Abstract

Deep eutectic solvents (DESs) have emerged as an alternative to both common organic solvents and ionic liquids (ILs). DESs share physicochemical properties with ILs such as low vapor pressure, high thermal stability, high viscosity while offering advantages such as low toxicity, lower cost, and ease of preparation. Moreover, DESs are attractive candidates for electrochemical applications due to their large voltage windows and solubility properties. DESs as a solvent class share a general composition of a hydrogen bond donor (HBD), typically a polyol, amide, or acid, and a hydrogen box acceptor (HBA), usually a quaternary ammonium or phosphonium salt. At a specific molar composition of a HBD and HBD, the DES forms a eutectic mixture resulting in a large melting point depression due to extensive hydrogen bonding between the components.Despite being widely studied, the use and subsequent characterization of DESs as solvents for aromatic phenols and related aromatics has only recently received attention. In this study we have investigated the solubility of a broad class of aromatic solutes as a function of DES composition and solute concentration using 1H, 13C, pulsed field gradient (PFG), and nuclear Overhauser effect (NOE) NMR. The degree to which homogenous mixtures versus heterogeneous and/or multi-phase systems are formed as a function of solute, temperature, and DES composition is examined. Two classes of DESs glyceline (glycerol + choline chloride) and ethaline (ethylene glycol + choline chloride) were investigated.As many as three distinct phases depending on the nature of the aromatic solute are observed by NMR. {1H-1H}-NOESY measurements show strong correlations between the choline chloride ammonium cation and the aryl protons with cation-pi interactions disrupting the hydrogen bonding network of the DES. We will discuss the implications for these systems as green solvents and comment on how clustering of solutes on the nanoscale may find use in catalysis and self-assembly.