DOI: 10.1002/chem.200500968
Interplay of p-Sulfonatocalix[4]arene and Crown Ethers En Route to
Molecular Capsules and “Russian Dolls”
Scott J. Dalgarno,
[a]
Julie Fisher,
[b]
and Colin L. Raston*
[c]
Introduction
Calixarenes are a fascinating family of macrocyclic mole-
cules that feature extensively in the field of supramolecular
chemistry.
[1]
This relates to their synthetic availability and
ease of modification, coupled with the presence of cavities/
clefts that are capable of binding a variety of species.
Water soluble calixarenes are readily accessible by func-
tionalising either the upper or lower rim of the polyphenol
macrocycle.
[2]
We, amongst others, have been exploring the
supramolecular chemistry of one such class of calixarene,
the p-sulfonatocalix[n]arenes (in which n = 4,5,6,8). These
molecules display a range of biological activities that include
enzyme inhibition, ion-channel blocking and anti-viral prop-
erties.
[3]
The smallest molecule in this class, p-sulfonatoca-
lix[4]arene (SO
3
[4]), usually crystallises with a truncated
cone conformation, a shape that lends itself to the formation
of self-assembled “up–down” antiparallel bilayer arrays.
[4,5]
These extended solid-state arrays can be built up of super-
molecules based on molecular capsules, “Russian dolls” or
ferris wheels, and can incorporate two- and three-dimension-
al coordination polymers, as well as other structural fea-
tures.
[5]
The series of “Russian doll” structures feature a su-
peranionic capsule (in which two p-sulfonatocalix[4]arenes
shroud a sodium [18]crown-6 complex) that is capable of se-
lectively crystallising polynuclear aquated metal cations
from solution.
[5]
In particular, p-sulfonatocalix[4]arene has been shown to
assemble into spectacular nanospheroids of varied geome-
tries, or nanotubules depending on the guest molecules and
metal ions present in solution.
[6]
Exchanging [18]crown-6 for
pyridine N-oxide in a ternary lanthanide/SO
3
[4]/guest
system results in an icosahedral to cuboctahedral geometric
change in a nanospheroid composed of twelve p-sulfonato-
calix[4]arenes, with a concomitant increase of ~ 30 % in the
internal spheroid volume.
[6]
In addition, the formation of
pores in the outer “shell” of the spheroid associated with
this change allows the tentative analogy to viral mimicry of
the cowpea chloritic mottle virus that has been shown to
Abstract: Diffusion-ordered
1
H NMR
spectroscopy techniques have been
used to determine the binding strength
of p-sulfonatocalix[4]arene (SO
3
[4]) to-
wards a number of charged crown
ether species in aqueous conditions.
For several (doubly) charged (di)aza-
crown ethers, all were bound by SO
3
[4]
either well or very well with binding
constants between 5.1 10
2
–9.9
10
5
m
1
. These results correlate with,
and thus explain the phenomenon of
rapid capture of azacrown ethers in
molecular capsules based on p-sulfona-
tocalix[4]arene and lanthanide metals.
Similarly, the formation of “Russian
doll” superanions in the solution phase
is also elucidated. These superanions
have been shown to selectively crystal-
lise particular polynuclear aquated
metal ions from mixtures in the aque-
ous phase. Neutral [18]crown-6 is not
bound by p-sulfonatocalix[4]arene and
displays a binding constant of 0m
1
.
When sodium [18]crown-6 is examined
in a similar fashion, binding by SO
3
[4]
is observed in solution with K
a
~ 3.1
10
3
m
1
.
Keywords: calixarenes · crown
compounds · host–guest systems ·
NMR spectroscopy · supramolec-
ular chemistry
[a] Dr. S. J. Dalgarno
School of Biomedical and Chemical Sciences
University of Western Australia, Crawley
Perth, WA 6909 (Australia)
Fax : (+ 61)8-9380-1005
[b] Dr. J. Fisher
Department of Chemistry, University of Leeds
Leeds, LS2 9 JT (UK)
Present address: Department of Chemistry
University of Missouri-Columbia, Columbia, MO65211 (USA)
[c] Prof. C. L. Raston
School of Chemistry, University of Leeds
Leeds, LS2 9 JT (UK)
E-mail: clraston@chem.uwa.edu.au
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 2772 – 2777
2772