(i.e., beyond 1 equiv). Moreover, it is noted that the
potassium cation is held tightly in the [9s, K+] complex as
it does not undergo a facile exchange with the uncomplexed
9s on the NMR time scale (at 22 °C). Unfortunately, an
accurate binding constant for the formation of [9s, K+] could
not be determined by the NMR method as it simply showed
complete capture of K+ and suggested that the binding
constant is too large to be measured by NMR spectroscopy.13
-
Single crystals of the [9s, K+] B(C6H5)4 complex were
obtained by a slow evaporation of a CH2Cl2-CH3OH
solution at 22 °C. A molecular structure of the complex with
an additional methanol molecule bound to K+ was established
by X-ray crystallography as shown in Figure 2. The X-ray
Figure 1. Cyclic voltammograms of 2.5 × 10-4 M 9u, 9s, and 10
(as indicated) in 97:3 dichloromethane-acetonitrile at a scan rate
of ν ) 100 mV s-1 (22 °C). In each figure, the corresponding square
wave voltammograms are also shown in red.
Figure 2. ORTEP diagram showing the structure of [9s, K+] -BPh4
with a single CH3OH molecule complexed to K+. The hydrogens
are omitted for clarity.
in 9u (or 9s) are electronically coupled due to their propeller-
shaped (cofacial) arrangement.3,4 As such, the extent of
electronic coupling among the circularly arrayed aryl moi-
eties in 9u (or 9s) can also be gauged by its significantly
lowered oxidation potential, i.e., by 250 mV (or 200 mV)
as compared to the model donor 10 (Eox ) 1.16 V vs
SCE).11,12
Encouraged by the reversibility of the initial electrochemi-
cal oxidations of the symmetrical isomer 9s, we next
examined its binding with the potassium cation as follows.
Thus, an exposure of a solution of 9s in acetone-d6 (0.02
M) to substoichiometric increments of potassium perfluoro-
tetraphenylborate (0.08 M) showed the appearance of a new
set of signals in addition to the initial signals due to the
uncomplexed 9s in the 1H NMR spectra. As shown in Figure
S3 in the Supporting Information, the 1H NMR signals due
to 9s were completely replaced by new signals upon addition
of 1 equiv of K+. It is noteworthy that the 1H NMR spectrum
remained unchanged upon further addition of K+ solution
structure of [9s, K+] shows that a single potassium cation
nestles deep inside the cavity that is comprised of an
(hydrophobic) aromatic bottom (i.e., the central benzene ring)
with a (hydrophilic) polar ethereal fence formed by six
oxygens from the peripheral aryl groups. Such a bipolar
nature of the cavity in 9s allows a tight van-der-Waals fit of
a single K+ cation with a symmetrical η6-coordination of
the K+ to the central benzene ring. The distance between
the mean plane of the central benzene ring and the K+ is
2.808 Å, a distance that is much shorter than the sum of
van-der-Waals/ionic radii of carbon and K+ (i.e., 3.22 Å).
The close K+‚‚‚Ar coordination is a result of the synergy
between the cation-π interaction and the interaction of the
+
potassium cation with all six ethereal oxygens (av dK
∼
···O
2.84 Å) that are prearranged in a manner analogous to
[18]crown-6.
(11) (a) It should be noted that the observed lowering of the first oxidation
potential in 9u/9s does not arise due to the mediation of the central benzene
ring because of the fact that 1,4-bis(2,5-dimethoxytolyl)durene ejects both
of its electrons at a potential of 1.16 V vs SCE. See: Sun, D.; Lindeman,
S. V.; Rathore, R.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 2001, 1585.
Also see: (b) Rathore, R.; Abdelwahed, S. H.; Guezi, I. A. J. Am. Chem.
Soc. 2003, 125, 8712. (c) Rathore, R.; Chebny, V. J.; Kopatz, E. J.; Guezi,
I. A. Angew. Chem., Int. Ed. 2005, 44, 2771.
The intimate electronic interactions of the potassium cation
with the ethereal oxygens from the electronically coupled
pheripheral aryl groups in [9s, K+] provided a unique
opportunity to monitor the binding of K+ using the electro-
chemical method. Thus, Figure 3 shows the square-wave
(12) The nature of electronic coupling, i.e., toroidal delocalization vs
Coulombic repulsion among the circularly arrayed aryl groups in oxidized
9s/9u, is presently under investigation.
(13) Competition experiments indicated that 9s and [18]crown-6 have a
comparable affinity for the potassium cation.
Org. Lett., Vol. 9, No. 7, 2007
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