Ion association effects on the nuclear magnetic resonance parameters of aromatic ions. III. Cyclononatetraenyl anion, cyclooctatetraene dianion, and tropylium cation

Abstract

The effect of counterion, solvent, and temperature on the proton nmr spectrum of the cyclononatetraenyl anion, cyclooctatetraene dianion, and tropylium cation have been investigated. In DME and THF, the lithium and sodium salts of the cyclononatetraenyl anion exist as an equilibrium mixture of contact and solvent-separated ion pairs whereas the potassium, rubidium, and cesium salts exist only as contact ion pairs. Salts of cyclooctatetrene dianion exist as contact ion pairs in all solvents examined. Lithium-7 nmr shifts support this view. The nmr spectrum of several tropylium salts shows the chemical shift of the tropylium cation to be independent of counterion but solvent dependent. These results suggest that the tropylium salts exist in solution as free solvated ions and solvent-separated ion pairs.

Effect of Ion Pairing on Charge Transfer in the Bicyclooctatetraenyl Dianion

“…Ion Pairing in the Ground State . Following Cox et al, we consider a model for 1 2- /2M + that involves at least three different structures: (a) both cations exist in a tight or contact ion triplet with a cyclooctatetraenyl dianion ring, (b) one cation exists in a loose or solvent-separated ion pair (SSIP) while the second is contact ion paired (CIP), and (c) both cations are separated from the dianion ring by a layer of solvent molecules. Cox and co-workers concluded that the Na, K, and Rb salts of the cyclooctatetraene dianion are dicontact ion paired in THF and DME and that a small fraction of SSIPs is formed with the dilithium salt …”

“…Thus, while the lithiums in [dibenzo[ a , e ]COT][Li·TMEDA] 2 , [Li(THF) 2 ][COT·Sm(CH(TMS) 2 ) 2 ], and [Li·DME] 2 [1,4-(TMS) 2 COT] are contact ion paired, those in [Li(THF) 4 ][Ce(COT) 2 ] and [Li(THF) 4 ][Sm(1,4-(TMS) 2 COT) 2 ] are solvent-separated ion paired. The latter are to our knowledge the only known examples of SSIP alkali metal salts of COT in the solid state since [Na(THF) 3 ][Ce(COT) 2 ], [Rb 2 ·diglyme][COT], and seven different potassium salts of COT 2- or 1,3,5,7-tetramethylCOT − all crystallize as CIPs.…”

Effects of ion association upon the solubilities of the cyclooctatetraene dianion

“…From eq 7 it is clear that the addition of alkali metal salts (KI or NaClOs) to solutions of the COT dianion and anion radical in HMPA should result in a decrease in K0bsd-For the over modulated ESR spectra of the anion radical the anion radical concentration is proportional to the amplitude of the ESR line (A), and K0bsd can be expressed as shown in eq 9 where B is simply a proportionality con-Kobsd = (£ )2/( )( 2'), (9) stant.6 Using the dual cavity technique, a simple comparison of A for samples with and without added salt allows us a comparison of K0bad for the samples with various sodium and potassium cation concentrations.…”

Enantiomeric Differences in Ion Association in a Chiral Solvent

“…The resulting deep blue solution shows only one strong 7 Li NMR resonance at −10.4 ppm (relative to lithium iodide in deuterated acetone), Figure . This is even further upfield than the chemical shift observed in THF (−8.55 ppm) or dimethoxyethane (−9.21 ppm) where both cations are bound to the C 8 H 8 dianion as tight or contact ion pairs . The more downfield shift in the case of DME was explained in terms of more molecules of THF solvating the cations being able to disperse the positive charge on the cation better than fewer molecules of DME.…”

“…The planar geometry of C 8 H 8 2- combined with its double negative charge renders it particularly vulnerable to ion association, which has been studied in a variety of ways . Most useful for this study is the fact that the NMR chemical shifts associated with the cyclooctatetraene dianion vary with the degree of ion association 1 …”