Solvent and temperature effects on the conformational equilibria
where the interconversion is rendered more difficult because of a
greater steric hindrance in the free cation.
Conclusions
In conclusion, spectra of the fluoroborate salt 1 were recorded
in CDCl3, C2D2Cl4, CD3CN and CD3COCD3, at VTs. At 300K, as
the solvent polarity was increased, the protons of the ethano
bridge of 1 (a) changed from a singlet in chloroform to an
AAꢁBBꢁ pattern in tetrachloroethylene, acetonitrile and acetone;
(b) split into two methylene signals, with increasing chemical-shift
difference between them. The solvent effects were ultimately
rationalized in terms of a variable degree of ion-pairing in solution.
In CDCl3 a tight ion-pair exists, whereas in CD3CN and CD3COCD3
ion dissociation is nearly complete. In C2D2Cl4 an intermediate
situation prevails. Compound 1 exists as two conformations in
equilibrium. In the tight ion-pair present in chloroform, the N-
phenyl substituent is more perpendicular to the plane of the
pyridinium ring, thus reducing the barrier to interconversion
between the two conformers. As dissociation of the ion-pair
increases in more polar solvents, the dihedral angle between the
pyridinium and the N-phenyl group is reduced, with the effect that
the repulsive steric interaction between the N-phenyl substituent
and the neighboring dihydrobenzo group increases. This has the
effect of increasing the barrier to conformational interconversion
in the free cation.
The effect of temperature on the spectrum of 1 may also
be explained in terms of a conformational interconversion that
is facilitated at higher temperatures, becoming more sluggish
when the temperature is decreased. Thus, the spectrum of 1 in
tetrachloroethylene at 380K resembles the one in chloroform; at
300K, the spectrum of 1 is similar to spectra in more polar media
such as acetone or acetonitrile.
The barrier to conformational interconversion in compound 2 is
higher than in 1, as shown by the fact that the AAꢁBBꢁ pattern of the
ethano protons is not altered by the solvent. This is a consequence
of the greater steric hindrance between the dihydrobenzo groups
and the N-phenyl ring present in compound 2.
Figure 4. Optimized structure, by the AM1 method, of the dihydrobenzo-
quinolinium fluoroborate 1.
These calculations yielded a dihedral angle for the N-phenyl
substitutent with the pyridinium ring of 68◦ in the isolated cation
and of 76◦ in the associated salt. Thus, in a tightly associated
salt, the N-phenyl substituent becomes more perpendicular to the
pyridinium ring than in the free cation, causing a small upfield shift
of the H-1 and H-2 protons as the solvent polarity is decreased
(Table 2). In addition, steric hindrance between the N-phenyl
substituent and the benzo ring is reduced, with the result that the
barrier to interconversion betwee−n the two conformers becomes
smaller. The proximity of the BF4 anion also affects the charge
on the pyridinium ring and on the ethano carbon atoms. As a
result, the charge of the carbon atom attached to H-8, which is
−0.16 in the free cation, changes to −0.14 in the associated salt,
in agreement with the fact that in the associated salt (CDCl3), H-8
is more shielded than in the free cation (CD3COCD3) (Table 2).
Also, in the associated salt, charges on the C-6 and C-5 carbon
atoms were −0.126 and −0.130, respectively. In the free cation,
these charges were −0.126 and −0.137, pointing to a greater
chemical-shift difference between H-6 and H-5 in a more polar
medium, in agreement with the data of Table 2.
The degree of ionic dissociation, as judged by the H-8, H-1 and
H-2 chemical shifts in various solvents, does not correlate entirely
with the solvent dielectric constant, being apparently larger in
acetone (ε ∼ 21) than in acetonitrile (ε ∼ 37). This may point
to a specific effect in the case of the two solvents. Acetone is a
harder, better donor solvent (Kamlet-Taft’s β = 0.4812, Catala´n’s
SB = 0.475[12]) than acetonitrile (β = 0.40,[13] SB = 0.286[12]). This
property might make it a better competing species for displacing
the hard BF4− anion from the ion-pair.
References
[1] A. R. Katritzky, S. S. Thind, J. Chem. Soc., Perkin Trans. 1 1980, 1895.
[2] L. Wittenkeller, W. Lin, C. Diven, A. Ciaccia, F. Wang, D. Mota de
Freitas, Inorg. Chem. 2001, 40, 1654.
[3] C. Ornelas, E. Boisselier, V. Martinez, I. Pianet, J. A. Ruiz, D. Astruc,
Chem. Commun. 2007, 5093.
[4] A. Neudo¨rffer, M. B. Fleury, J. P. Desvergne, M. Largeron, Elec-
trochim. Acta 2006, 52, 715.
[5] M. Higashiyama, K. Inada, A. Ohtori, K. Kakehi, J. Pharm. Biomed.
Anal. 2007, 43, 1335.
[6] P. S. Pregosin, Prog. Nucl. Magn. Reson. Spectrosc. 2006, 49, 261.
[7] A. Moreno, P. S. Pregosin, L. F. Veiros, A. Albinati, S. Rizzato,
Chemistry 2008, 14, 5617.
[8] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
Temperature may also affect the spectra of compound 1 in
a given solvent, as shown in Fig. 3. By raising the temperature,
the conformational interconversion is facilitated, and the ethano
proton signals, just as the H-1 and H-2 multiplets, tend to
merge. Thus, spectra at a higher temperature (375K and
380K) in tetrachloroethylene resemble the spectrum in CDCl3,
where strong ion-pair association facilitates conformational
interconversion. At a lower temperature (300K), the spectrum
in tetrachloroethylene resembles that in acetone or acetonitrile,
J. R. Cheeseman,
V. G. Zakrzewski,
J. A. Montgomery Jr,
R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
C. Gonzalez,
M. Challacombe,
P. M. W. Gill,
B. G. Johnson,
c
Magn. Reson. Chem. 2009, 47, 505–510
Copyright ꢀ 2009 John Wiley & Sons, Ltd.