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.

The physical properties of organometallic ionic liquids containing cationic ruthenium sandwich complexes {[Ru(C5H5)(C6H5R)]X; R = OCH(C4H9)2, OCH(C2H5)(C6H13); X = N(SO2CF3)2, N(SO2F)2} are dependent on the substituents. To obtain detailed information about the motional states and configurations of the constituent ions of these ionic liquids, we performed one-dimensional (1D) heteronuclear Overhauser enhancement spectroscopy (HOESY) NMR experiments and the standard relaxation rate (1/T1 and 1/T2) measurements. We determined the intermolecular cross-relaxation rates between some cation blocks and the anion in each ionic liquid by analyzing the experimental data based on the extended Solomon differential equations. As a result, we were able to estimate several intermolecular distances between the cations and anions in the ionic liquids. This is the first time that NMR has been applied to the determination of the intermolecular cross-relaxation rates and the estimation of intermolecular distances of highly viscous ionic liquids consisting of heteronuclear multiple-spin systems without the support of any X-ray data. The viscosities of these ionic liquids depended largely on the motional correlation times of the cations rather than on the intermolecular distances between the cations and anions.