PGSE NMR Diffusion Overhauser Studies on [Ru(Cp*)(η⁶‐arene)][PF₆], Plus a Variety of Transition‐Metal, Inorganic, and Organic Salts: An Overview of Ion Pairing in Dichloromethane

Abstract

PGSE diffusion, 19F, 1H HOESY and 13C NMR studies for a series of [Ru(Cp*)(eta6-arene)][PF6] (1) salts are presented. The solid-state structure of [Ru(Cp*)(eta6-fluorobenzene)][PF6] (1 c) is reported. The extent of the ion pairing and the relative positions of the ions are shown to depend on the arene. For the solvent dichloromethane, new and literature PGSE data for PF6(-) salts of transition-metal, inorganic, and organic salts are compared. Taken together, these new results show that the charge distribution and the ability of the anion to approach the positively charged positions (steric effects due to molecular shape) are the determining factors in deciding the amount of ion pairing. DFT calculations of the charges in four salts of type 1, as well as in a variety of other salts, using a natural population analysis (NPA), support this view. This represents the first attempt, using experimental data, to understand, correlate, and partially explain the various degrees of ion pairing in a widely different collection of salts.

Solvent and temperature effects on the conformational equilibria of a dihydrobenzo[h]quinolinium fluoroborate

“…[5] Diffusion measurements have been employed as a valuable tool in the determination of the extent of association between various ion pairs in different media, providing a broad view of the factors that govern their interactions. [6,7] In line with previous works on the importance of ion-pairing in chemical processes, in the present report we interpret the solventdependent conformational equilibrium of the dihydrobenzoquinolinium cation 1 in terms of the degree of association of the quinolinium system with its tetrafluoroborate counter anion. [1] Complete assignments of their 13 C-and 1 H-NMR spectra in (CD 3 ) 2 CO are given in Table 1.…”

NMR spectroscopy and ion pairing: Measuring and understanding how ions interact

“…Starting with the set of Ru salts, the largest D(cation)/D(anion) ratio in Table 4 The organic salts 19 and 20 are pertinent in that they both show very modest D(cation)/D(anion) ratios. The cyanine salt, 19, has the positive charge distributed over two remote N-C=CH fragments, and this is also supported by the NPA results [18]. This delocalization weakens the ion pairing (the HOESY spectrum [18] shows that the anion sits almost exactly between the two N-atoms such that the two equivalent vinyl protons show the strongest contacts).…”

“…The cyanine salt, 19, has the positive charge distributed over two remote N-C=CH fragments, and this is also supported by the NPA results [18]. This delocalization weakens the ion pairing (the HOESY spectrum [18] shows that the anion sits almost exactly between the two N-atoms such that the two equivalent vinyl protons show the strongest contacts). Once again, there is a specific structure since the single =CH vinyl proton does not show any NOE contacts to the anion.…”

“…To avoid using the r H values from the Stokes-Einstein equation, we have suggested that the ratio D(cation)/D(anion) can be a useful index [18], and this will be shown in many of the tables which appear later (in bold type). In connection with this ratio, we have recently advocated a three-pronged approach in an effort to better appreciate the various subtle factors involved in determining the extent of ion pairing.…”

“…Interestingly, the calculations do not always show a great deal of positive charge on the transition metal and the complexes 11 (Scheme 5) are representative [18]. The scheme shows the measured D values for these Cp* complexes, the solid-state structure for the fluoro-benzene salt, and the calculated charges for the Cp analogs of 11.…”