Effects of substituent branching and chirality on the physical properties of ionic liquids based on cationic ruthenium sandwich complexes

Abstract

An appropriate understanding of how substituents affect the physical properties of ionic liquids is important for the molecular design of ionic liquids. Toward this end, we investigated how the branching and chirality of substituents affect the physical properties of organometallic ionic liquids. We synthesized a series of ionic liquids bearing a branched or linear alkoxy group with the same number of carbons: [Ru(C5H5)(η(6)-C6H5OR)]X (rac-[1]X: R = -CH(C2H5)(C6H13), [2]X: R = -CH(C4H9)2, [3]X: R = -C9H19), where X = PF6(-), (SO2F)2N(-), and (SO2CF3)2N(-). rac-[1]X are racemic salts. Salts with less symmetrical substituents tend to maintain the liquid state due to suppression of crystallization; crystallization is completely suppressed in most of the rac-[1]X salts and in some of the [2]X salts, whereas not in [3]X salts. The glass-transition temperatures and viscosities of the salts with branched substituents are greater than those with linear substituents. Chiral resolution of rac-[1][PF6] was performed by chiral chromatography. The melting point of rac-[1][PF6] is much lower than that of the enantiopure salt (chiral-[1][PF6]), which we ascribe to the formation of a conglomerate in the solid state. X-ray structure analysis revealed that the solid salts form layered structures.

Structure–Property Relationship for 1-Isopropyl-3-methylimidazolium- and 1-tert-Butyl-3-methylimidazolium-Based Ionic Liquids: Thermal Properties, Densities, Viscosities, and Quantum Chemical Calculations

“…The melting points of several ILs containing branched cations have been reported. Depending on the position of the branching, cation structure, and type of paired anion, T m can either increase or decrease. ,,− For example, 1-(iso-alkyl)-3-methylimidazolium-based ILs tend to show higher T m values than the corresponding linear ILs, while the opposite trend is observed for dicationic ILs . This indicates that branching involves several competing factors that influence T m .…”

“…In addition to the FFV data discussed above, which suggest high viscosity in the branched ILs, there are two competing factors involved in the viscosity of branched ILs, i.e., bulkiness and interaction energy, according to previous works on the viscosity of branched molecules. 13,14,64,73−75,85,92−97 The viscosity increase upon alkyl chain branching has been observed in other ILs 14,64,[73][74][75]92 and some molecular liquids. 85 These behaviors are normally interpreted as being due to the bulkiness of branched chains.…”

Thermal properties, crystal structures, and phase diagrams of ionic plastic crystals and ionic liquids containing a chiral cationic sandwich complex

“…[31][32][33][34] We investigated the effect of chirality on the thermal properties of ILs, using organometallic ILs with chiral long-chain substituents, and revealed that the racemate tends to exhibit low crystallinity and a lower melting point than that of the enantiomer. 35,36 In contrast, the effect of chirality on the thermal properties of IPCs has not been elucidated, though IPCs containing a chiral quaternary ammonium cation have been reported. 37,38 Comparative thermal studies involving racemates have been conducted on neutral chiral plastic crystals such as camphor.…”

Density, Viscosity, Electrical Conductivity, and Surface Tension of Chelate-Based Ionic Liquids [C10mim][M(hfac)3] (M = Co, Ni, Cu) at Different Temperatures

“…Nockemann et al proved that ILs containing the hexafluoroacetylacetonate anion are immiscible with water, and they exhibit strong metal-complexing ability . Mochida et al prepared metallocenium ILs, and the corresponding research was carried out. − They also prepared vapor-responsive ChILs exhibiting reversible changes in color, ChILs with solvatochromic properties, and other metal-containing ILs. , Our group has also been involved in ChIL research. For instance, we developed a class of ChILs which efficiently catalyzed the solvent-free oxidation of cyclohexene by molecular oxygen, which are highly recyclable, and prepared a series of novel alkanolamine-based ILs showing a highly efficient and excellent reversible capture of CO 2 .…”