Highly stable cage-like complexes by self-assembly of tetracationic Zn(II) porphyrinates and tetrasulfonatocalix[4]arenes in polar solvents

Article abstract of DOI:10.1039/b006960o

Tetracationic Zn(II) porphyrinates and tetraanionic calix-[4]arenes can be assembled in polar solvents to obtain cage-like complexes in an entropy driven process. These structures are remarkably stable even in the presence of water or competing salts.

Full text of DOI:10.1039/b006960o

Highly stable cage-like complexes by self-assembly of tetracationic Zn(II
)
porphyrinates and tetrasulfonatocalix[4]arenes in polar solvents†
Roberto Fiammengo, Peter Timmerman,* Feike de Jong and David N. Reinhoudt
Laboratory of Supramolecular Chemistry and Technology, MESA+ Research Institute, P.O. Box 217, 7500 AE
Enschede, The Netherlands. E-mail: d.n.reinhoudt@ct.utwente.nl
Received (in Liverpool, UK) 18th August 2000, Accepted 10th October 2000
First published as an Advance Article on the web 9th November 2000
Tetracationic Zn(II) porphyrinates and tetraanionic calix-
[4]arenes can be assembled in polar solvents to obtain cage-
like complexes in an entropy driven process. These struc-
tures are remarkably stable even in the presence of water or
competing salts.
strongly entropically driven (129 ± 24 J K21 mol21) with a very
small exothermic contribution (28.0 ± 6.7 kJ mol21). There-
fore, we conclude that the release of solvent molecules is the
Metalloporphyrins are well known for their ability to bind a
variety of small molecules1–6 and for their catalytic proper-
ties.7,8 Moreover, they have been intensively studied with
respect to their occurrence in metalloporphyrin-dependent
proteins,9,10 in which they display largely differentiated chem-
ical properties. The chemical behavior of a metalloporphyrin is
strongly affected by the surrounding environment.11 For
example, the peptidic backbone in metalloporphyrin-dependent
proteins is able to fine-tune the chemical properties of the metal
center by means of non-covalent interactions. Most studies with
synthetic porphyrins heavily rely on the covalent modification
of the macrocyclic structure,1,12 while studies on porphyrins
with a non-covalently organized periphery have hardly been
reported so far. In this communication we describe the
formation and characterization of cage-like complexes 1·2,
consisting of cationic porphyrins 1 and anionic calix[4]arenes 2
(see Fig. 1), that are very stable in polar solvents such as MeOH
DMSO and DMPU.‡§
Zinc(II
)
meso-tetrakis(N-alkylpyridinium-3-yl)-porphyrins
1a–c were prepared in three steps starting from pyridine-
3-carboxyaldehyde and pyrrole in an overall yield of 14–18%
following literature procedures.5 Tetrasulfonatocalix[4]arene 2
was obtained in 84% yield by sulfonation of the parent
25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]arene with con-
centrated sulfuric acid using a slightly modified literature
procedure.13
The assembly process was first studied by UV spectroscopy.
Addition of calix[4]arene 2 to a solution of porphyrins 1 in
MeOH (2–3 3 1026 M) causes a bathochromic shift (3–5 nm)
of the porphyrin’s Soret band with a concomitant 20–25%
decrease in e (molar absorptivity) at lmax. This behavior is
characteristic for cationic pyridiniumyl porphyrins that form
ion-pair complexes in water.14 Non-linear least-squares fitting
analysis of the spectral changes using a 1+1 binding model gave
association constants in the order of 107 M21 for the complexes
1·2 (see Table 1). The presence of a well-defined isosbestic
point during the titration strongly supports the proposed
formation of a 1+1 complex. These high values most likely
reflect the perfect complementarity in charge distribution of the
two interacting molecular ions leading to the formation of
strong, cooperative salt bridges. Nevertheless these ion-pair
complexes remain highly dynamic structures as we were unable
to observe any time dependent phenomenon either by NMR or
by UV-vis experiments.
Fig. 1 Molecular structure of components 1, 2, and 1+1 complex 1·2.
Table 1 Association constants for the assembly of cationic zinc porphyrin-
ates 1 with the anionic calix[4]arene 2
Assembly
Solvent
Log K1+1
In order to determine the thermodynamic parameters asso-
ciated with the assembly formation, the stability constant of
complex 1a·2 was determined at different temperatures. Van’t
Hoff analysis of the binding data revealed that the process is
1a·2
1b·2
1c·2
1a·2
1a·2
1a·2
CH3OH
CH3OH
CH3OH
DMSO
DMPU
H2O–DMPU
xwater = 0.26
7.11 ± 0.04
7.04 ± 0.20
7.15 ± 0.09
6.21 ± 0.03
6.16 ± 0.01
6.04 ± 0.02
† Electronic supplementary information (ESI) available: Figs. 3–6. See
http://www.rsc.org/suppdata/cc/b0/b006960o/
DOI: 10.1039/b006960o
Chem. Commun., 2000, 2313–2314
This journal is © The Royal Society of Chemistry 2000
2313

the K1+1 value at 1022 M of Bu4NClO4 clearly showed a smaller
effect in comparison to NaClO4 at the same concentration
(pK1+1 = 5.22 ± 0.08 and 4.56 ± 0.03 for Bu4NClO4 and
NaClO4 respectively). Therefore, it can be concluded that
addition of inert salts, as much as 4500 times the concentration
of the building blocks, only reduces the stability of the complex
by a factor of 10.
Finally, we have found that the influence of the presence of
water on the stability of the assemblies is very small. The log
K1+1 value is 6.04 ± 0.02 in a H2O–DMPU mixture (xwater
=
0.26), which is comparable to the value of 6.16 ± 0.01 obtained
in pure DMPU (see Table 1).
In conclusion we have shown that stable cage-like complexes
are obtained by self-assembly of cationic porphyrins and
anionic calix[4]arenes in polar solvents. These assemblies are
formed as a result of multiple electrostatic interactions between
oppositely charged building blocks and are highly stable in
polar solvents, like MeOH DMSO and DMPU. Furthermore,
they remain stable even in the presence of water and, to a large
extent, upon the addition of electrolytes.
Fig. 2 1H NMR of (A) porphyrin 1a, (B) 1:1 mixture of 1a and 2 at 298 K,
(C) 1:1 mixture of 1a and 2 at 343 K.
These investigations are supported by the Netherlands
Research Council for Chemical Sciences (CW) with financial
aid from the Technology Foundation STW.
main driving force for the formation of the highly stable ion-pair
complexes 1·2 in MeOH.15
Structural investigation of complex 1a·2 by 1H NMR
spectroscopy was performed in CD3CN–D2O (6.5+1.5). At
ambient temperatures the porphyrin proton signals show a
complicated set of signals that indicate the presence of a mixture
of isomeric structures with reduced symmetry. The calix[4]ar-
ene strongly influences the atropoisomerization equilibrium of
the porphyrin,5 without inducing a particular conformational
preference. Coalescence of the various signals occurs at higher
temperatures, whereby the position of the averaged signals is
significantly shifted upfield in comparison to the corresponding
signals in the free porphyrin. The largest shifts are observed for
protons Hb, Hc and Hd (0.3–0.7 ppm) (see Fig. 2), which
suggests that these protons are located in closest proximity to
the calix[4]arene unit. Molecular simulation studies
(CHARMm 24.0) support this view and further show that the
structure in which the alkyl chains are pointing away from the
calix[4]arene minimizes the distance between opposite charges
in the two building blocks.¶
Notes and references
‡ DMPU = 1,3-dimethyl-3,4,5,6-tetrahydropyrimidin-2(1H)-one).
§ Self-assembled molecular capsules formed by means of ionic interactions
have been reported using cyclotriveratrylene building blocks16 or cyclodex-
trin derivatives.17 In the first case the authors did not report the formation
constant for the 1+1 complex and they observed the presence of oligomers
at 7 mM in DMSO-d6. In the paper dealing with cyclodextrins indirect proof
for the existence of the capsules was obtained from the calculation of self-
diffusion coefficients via 1H NMR techniques.
¶ The symmetrical situation as observed from a gas-phase simulation (see
ESI) is likely reflecting the structure of the complex at temperatures higher
than rt.
∑ Coordination of MeOH to zinc(II) porphyrin hosts have been observed in
several cases.18
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The cage-like structure of these ion-pair complexes prompted
us to study their formation in other polar solvents such as
DMSO and DMPU, from which the individual molecules are
more bulky than MeOH. Titration experiments with 1a and 2 in
these solvents also gave well-defined isosbestic spectral
variations. However, no shift in the maximum (DMSO) or a 5
nm blue shift (DMPU) was observed. This indicates a different
solvation shell around the porphyrin upon formation of the
complex in these solvents. Moreover, the K1+1 values are one
order of magnitude smaller than in MeOH (Table 1), which
might indicate that MeOH is probably a very good guest for the
cage-like complex∑ and enhances the stability of the system.
Further experiments on solvent or guest encapsulation in these
cage-like structures are currently ongoing in our laboratories.
Furthermore, we examined the effect of adding salts on the
stability of the ion-pair complexes: association constants were
determined for assembly 1a·2 in DMSO at different concentra-
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shows a bimodal linear relationship with a slope of 1.91 ± 0.11
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Chem. Commun., 2000, 2313–2314