Average Intermolecular Distances and Dynamics of Ruthenium-Containing Organometallic Ionic Liquids Studied by Nuclear Overhauser Effects between 19F and 1H Nuclei and NMR Relaxation Rate Measurements

Abstract

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.

Organometallic Ionic Liquids Containing Sandwich Complexes: Molecular Design, Physical Properties, and Chemical Reactivities

“…In these ILs, branched substituents increase the viscosity, compared with linear substituents. NMR spectroscopic studies have shown that this tendency correlates with the motion of the alkyl substituents [15d] . Similarly, in the PF 6 and CF 3 BF 3 salts of 5 c , the racemate has a lower melting point than the chiral salt, where these salts formed a conglomerate and racemic compound, respectively [19] .…”