Host-guest driven self-assembly of linear and star supramolecular polymers

Article abstract of DOI:10.1002/anie.200801002

(Figure Presented) Supramolecular plasticity: The remarkable host-guest properties of phosphonate cavitands have been exploited in the self-assembly of supramolecular polymers (see picture) that feature guest-triggered reversibility and template-driven co

Full text of DOI:10.1002/anie.200801002

Communications
DOI: 10.1002/anie.200801002
Supramolecular Polymers
Host–Guest Driven Self-Assembly of Linear and Star
Supramolecular Polymers**
Roger M. Yebeutchou, Francesca Tancini, Nicola Demitri, Silvano Geremia,
Raniero Mendichi, and Enrico Dalcanale*
Angewandte
Chemie
4504
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Chemie
Dynamic materials[1] featuring modular components, nano-
scale dimensions, and controlled responsiveness to external
stimuli are among the most attractive targets in contemporary
materials science. Since supramolecular polymers[2] fulfill
nicely all these conditions, they are in a prominent position in
the list of effective dynamic materials.
While the self-assembly of supramolecular polymers
driven by hydrogen bonding[3] and metal coordination[4] has
undergone explosive development, the host–guest route is by
far less traveled, despite the many different options poten-
tially available. As very high association constants are
required for the self-assembly of truly polymeric materials,
[5] the number of possibilities is greatly reduced. So far, most
of the research has been centered on solvophobic interactions
using cyclodextrins as hosts,[6] and, to a lesser extent, on
pseudorotaxane threading with alkylammonium guests.[7]
More recently calixarenes,[8] cavitands,[9] and donor–acceptor
molecules[10] have been used as monomers, leading to the
formation of oligomeric materials.
In this paper we report a new class of supramolecular
polymers whose self-assembly is driven by the outstanding
complexation properties of tetraphosphonate cavitands
toward methylpyridinium guests. Phosphonate cavitands are
resorcinarene-based molecular receptors presenting one or
more PV moieties as bridging units.[11] Tetraphosphonate
cavitands in their all-inward configuration[12] are able to
complex positively charged species, such as ammonium salts
or inorganic cations with very high association constants
(Kass = 107–109 mꢀ1).[13]
Figure 1. Monomers 1a,b, chain stoppers 2a,b, and model cavitand 3.
In structures 1 and 2 the labels on the pyridinium ring correspond to
1
diagnostic HNMR resonances shown in Figure 6.
=
=
Supporting Information). The P O/P S exchange completely
suppresses methylpyridinium complexation, which is mainly
driven by cation–dipole interactions.
The complexation properties of tetraphosphonate cavi-
tands toward methylpyridinium guests were studied by
isothermal titration calorimetry (ITC) using derivative 3 as
the host. The formation of 2a·3 and 2b·3 dimers represents a
suitable model of a single polymerization step. In both cases
the measured Kass values exceed 107 mꢀ1 in dichloromethane
(Table 1).
Table 1: Thermodynamic parameters deduced from ITC experiments for
the host–guest dimers at 298 K.[a]
Entry
DH [kJmolꢀ1
]
TDS [kJmolꢀ1 DG [kJmolꢀ1 Kass [mꢀ1
] ] ]
The target cavitand monomers 1a,b (Figure 1), which
present four inward-facing phosphonate bridges at the upper
rim and a single methylpyridinium unit at the lower rim, were
synthesized in four steps starting from a monofunctionalized
resorcinarene[14] (see the Supporting Information). The four
methyl groups in the apical positions were introduced to
deepen the cavity and to strengthen CH–p interactions. The
structurally related tetrathiophosphonate analogues 2a,b
were also prepared to be used as chain stoppers. In fact, 2
retains its monomeric form both in solution (Figure 6a) and in
the solid state as shown by the crystal structure of 2a (see the
2b·3[b]
2b·3[c]
2a·3[c]
2b·3[b]
competitive
guest[d]
ꢀ11.9ꢁ0.4
ꢀ30.0ꢁ0.1
ꢀ26.1ꢁ0.2
16.1ꢁ0.5
13.4ꢁ0.5
15.2ꢁ0.9
ꢀ28.0ꢁ0.2 (8.2ꢁ0.7)104
ꢀ43.6ꢁ0.5 (4.0ꢁ0.1)107
ꢀ41.4ꢁ0.8 (1.9ꢁ0.6)107
+
ꢀ99.7ꢁ12.6 ꢀ77.9ꢁ13.6 ꢀ21.8ꢁ0.6 (6.0ꢁ1.4)103
[a] All reported values are the average of three independent measure-
ments. [b] In methanol. [c] In dichloromethane. [d] Competitive guest is
N-butylmethylammonium chloride.
Interestingly, the complexation is driven not only by
enthalpy but also by entropy, indicating that solvation plays a
significant role in the process. In fact, the Kass values are
strongly influenced by the solvent for both guests, increasing
by two orders of magnitude in moving from methanol to
methylene chloride. In contrast the role of the counterion is
[*] Dr. R. M. Yebeutchou, Dipl.-Chem. F. Tancini, Prof. E. Dalcanale
Dipartimento di Chimica Organica ed Industriale
and INSTM UdR Parma, Università di Parma
Viale Usberti 17 A, 43100 Parma (Italy)
Fax: (+39)0521-905472
ꢀ
E-mail: enrico.dalcanale@unipr.it
Homepage: http://www.ssmg.unipr.it
limited, with a small preference for PF6 over iodide.
The crystal structure of the 2b·3 dimer (Figure 2) gives
evidence of the two major interactions responsible for the
complexation: a multiple ion–dipole interaction between the
Dipl.-Chem. N. Demitri, Prof. S. Geremia
Centro di Eccellenza in Biocristallografia
Dipartimento di Scienze Chimiche, Università di Trieste
Viale Giorgeri 1, 34127 Trieste (Italy)
=
inward-facing P O groups and the positively charged methyl-
pyridinium moiety, and directional hydrogen bonds involving
the acidic methyl group with the p-basic cavity (CH3–p
interaction) and the ortho H-pyridinum atoms with two
Dr. R. Mendichi
Istituto per lo Studio delle Macromolecole (CNR)
Via E. Bassini 15, 20133 Milano (Italy)
=
opposite P O groups.
[**] This work was supported by NoE MAGMANet (E.D. and R.M.Y.) and
PRIN 2006034018 (S.G.). We thank Prof. J. de Mendoza and his
team at ICIQ for the use of ITC.
The same interactions responsible for dimer formation are
observed in the homopolymer crystal structure of 1b, where
each linear polymeric chain packs against other four anti-
parallel chains (Figure 3). The charges of the monomers
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 2. Crystal structure of dimer 2b·3.
Figure 4. SLS measurements of Mw versus concentration for the
homopolymer 1a alone (black circles) and along with added chain
stopper 2a (black squares), see the Supporting Information.
bearing the guest functionality are counterbalanced by triflate
anions located in between the lower-rim alkyl chains, close to
the methylpyridinium cations. In the crystal the polymer
assumes a straight and extended (all-trans pyridinium linker)
conformation, by alternately bending the cavity mouth of
each monomer of about ꢁ 458 with respect to the chain-
growth direction.
The weight-average molecular weight Mw of polymer 1a
was experimentally determined at different concentrations in
chloroform by static (elastic) light scattering (SLS, Figure 4)
in batch off-line mode. The Mw of the polymer increases
linearly with the concentration in the range of 1.04–15.0 gLꢀ1
and reaches an upper value of 26300 gmolꢀ1, which corre-
sponds to an average degree of polymerization hNWi of 18
units. Beyond this concentration the SLS signal was saturated
as it is dependent on both concentration and molecular
weight.[5,15] Stepwise addition of stopper 2a led to a pro-
gressive linear decrease of Mw (Figure 4). In contrast, the
presence of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphy-
rintetra(p-toluene sulfonate) (4) as a template led to a
substantial increase in Mw (Table 2). The porphyrin acts as
nucleation unit for the formation of a star-branched polymer
through its four meso pyridinium groups (Figure 5).[16] Por-
phyrin 4 is insoluble in pure chloroform, and it is driven into
solution by complexation with the end cavities of the polymer
chains. The solubility of 4 in solutions of 1a depends on the
concentration of the latter; the maximum 4/1a ratio is 2.5%
molar in a solution containing 1.04 gLꢀ1 1a (Table 2), in
which the concentration of end cavities is higher. In this way
the Mw of the polymer can be dramatically increased without
changing the monomer concentration.
The reversible disassembly of the homopolymer could be
triggered by addition of a competitive guest, N-butylmethy-
lammonium iodide. The Kass of the complex between this
guest and 3 could not be measured by ITC, since its value
exceeds 108 mꢀ1, which is the upper limit of reliable Kass
determination by this method.[17] Therefore the affinity of
this competitive guest for 3 was evaluated by ITC by means of
guest-exchange experiments with complex 2b·3 and N-
butylmethylammonium chloride in methanol (Table 1). The
recorded DG value indicates a large preference for complex-
ation of the
N-butylmethylammonium ion and indicates complete guest
exchange at close to the stoichiometric ratio.
Figure 3. Views of the crystal packing of the 1b homopolymer perpendicular (a) and parallel (b) to the linear polymeric chains. Two
crystallographically independent monomer molecules are present in the cell, each of which gives rise to a similar chain that grows antiparallel
(yellow and light blue chains) (a). Each chain interacts with other four antiparallel chains giving a checkerboard pattern when viewed along the
chain (b).
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Table 2: SLS measurements of the weight-average molecular weight of
the star-branched polymer.
NH and N-CH3 peaks of the ammonium salt undergo large
upfield shifts. These changes prove that the competitive guest
replaced the methylpyridinium moiety inside the cavity,
inducing the depolymerization. Finally, addition of two
equivalents of 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU)
restored the original polymer quantitatively (Figure 6e).
DBU breaks the ammonium–cavitand complex by deproto-
nating the guest. Since DBU is a bulky base, it cannot enter
the cavity; therefore the affinity of its protonated form for 1a
is very low.
Conc. [gLꢀ1
]
Molar ratio 4/1a [%] MW 1a
MW 1a
0
linear polymer[a] star polymer[b]
1.04
2.05
4.10
2.5
2.0
2.0
11850ꢁ150
13490ꢁ230
15940ꢁ90
46210ꢁ580
41600ꢁ180
49700ꢁ180
[a] Mw0 before addition of the template. [b] Mw after addition of template
4.
The same protocol can be followed visually by suspending
the 1a polymer in acetonitrile, where it is insoluble (see vials
in Figure 6). Addition of one equivalent of N-butylmethyl-
ammonium iodide to the suspension was sufficient to obtain a
perfectly homogeneous solution, as expected for disassem-
bled small oligomeric and monomeric species. The solution
became again heterogeneous after addition of one equivalent
of DBU, owing to the presence of the reassembled polymer.
In conclusion, a new family of host–guest supramolecular
polymers has been designed, prepared, and characterized.
The self-association mode leading to the polymer formation
has been clarified both in solution and in the solid state. The
degree of polymerization can be finely tuned by addition of a
chain stopper, in the form of structurally related but complex-
ation-inefficient tetrathiophosphonate cavitand 2. The
remarkable plasticity of the system is proven by the guest-
triggered reversible assembly–disassembly and by the tem-
plate-driven switch from the linear to star-branched polymer.
Figure 5. Template-induced formation of a host–guest star-branched
polymer.
Received: March 1, 2008
Published online: May 5, 2008
1
The polymer disassembly, monitored by H NMR spec-
troscopy in CDCl3,[18] is recorded in Figure 6. Addition of two
equivalents of N-butylmethylammonium iodide to 1a led to
the complete disassembly of the polymer (Figure 6d). The
chemical shifts of pyridine aromatic and N-CH3 protons are
comparable to those of the stopper 2a (Figure 6a), while the
Keywords: host–guest chemistry · phosphonate cavitands ·
self-assembly · supramolecular chemistry ·
supramolecular polymers
.
[1] a) J.-M. Lehn in Supramolecular Science:
Where It Is and Where It Is Going (Eds.: R.
Ungaro, E. Dalcanale), Kluwer Academic
Publishers, Dordrecht, 1999, 287 – 304; b) J.-
M. Lehn, Chem. Eur. J. 1999, 5, 2455 – 2463.
[2] Supramolecular Polymers, 2nd ed. (Ed.: A.
Ciferri), Taylor & Francis, New York, 2005.
[3] a) R. P. Sijbesma, F. H. Beijer, L. Brunsveld,
B. J. B. Folmer, J. H. K. K. Hirschberg,
R. F. M. Lange, J. K. L. Lowe, E. W. Meijer,
Science 1997, 278, 1601 – 1604; b) R. K. Cas-
tellano, D. M. Rudkevich, J. Rebek, Proc. Natl.
Acad. Sci. USA 1997, 94, 7132 – 7137; c) H. Xu,
D. M. Rudkevich, Chem. Eur. J. 2004, 10,
5432 – 5442; d) T. Park, S. C. Zimmerman, J.
Am. Chem. Soc. 2006, 128, 11582 – 11590;
e) E. M. Todd, S. C. Zimmerman, J. Am.
Chem. Soc. 2007, 129, 14534 – 14535.
[4] a) J.-P. Sauvage, Chem. Rev. 1994, 94, 993–
1019; b) I. Manners, Angew. Chem. 1996, 108,
1712 – 1731; Angew. Chem. Int. Ed. Engl. 1996,
35, 1602 – 1621.
[5] L. B. Brunsveld, B. J. B. Folmer, E. W. Meijer,
R. P. Sijbesma, Chem. Rev. 2001, 101, 4071 –
4097.
Figure 6. Section of the 1HNMR spectra (300 MHz, solvent CDCl 3) showing the guest-
triggered assembly–disassembly of the homopolymer 1a (see Figure 1 for assignment of
diagnostic HNMR resonances in 1a/2a): a) 2a, b) homopolymer 1a, c) N-butylmethyl-
ammonium iodide, d) 1a·N-butylmethylammonium iodide complex, e) restored homo-
polymer 1a.
1
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[6] a) M. Miyauchi, Y. Takashima, H. Yamaguchi, A. Harada, J. Am.
[10] G. Fernꢁndez, E. M. Pꢂrez, L. Sꢁnchez, N. Martꢃn, Angew. Chem.
2008, 120, 1110 – 1113; Angew. Chem. Int. Ed. 2008, 47, 1094 –
1097.
[11] R. Pinalli, M. Suman, E. Dalcanale, Eur. J. Org. Chem. 2004,
451 – 462.
[12] P. Delangle, J.-P. Dutasta, Tetrahedron Lett. 1995, 36, 9325 – 9328.
[13] a) P. Delangle, J.-C. Mulatier, B. Tinant, J.-P. Declercq, J.-P.
Dutasta, Eur. J. Org. Chem. 2001, 3695 – 3704; b) J.-P. Dutasta,
Top. Curr. Chem. 2004, 232, 55 – 91.
[14] F. Hauke, A. J. Myles, J. Rebek, Chem. Commun. 2005, 4164 –
4166.
Chem. Soc. 2005, 127, 2984 – 2989; b) H. Takahashi, Y. Taka-
shima, H. Yamaguchi, A. Harada, J. Org. Chem. 2006, 71, 4878 –
4883; c) V. H. Soto Tellini, A. Jover, J. C. Garcia, L. Galatini, F.
Meijide, J. V. Tato, J. Am. Chem. Soc. 2006, 128, 5728 – 5734.
[7] a) N. Yamaguchi, H. W. Gibson, Angew. Chem. 1999, 111, 195 –
199; Angew. Chem. Int. Ed. 1999, 38, 143– 147; b) T. Oku, Y.
Furusho, T. Takata, Angew. Chem. 2004, 116, 984 – 987; Angew.
Chem. Int. Ed. 2004, 43, 966 – 969; c) E. N. Guidry, J. Li, J. F.
Stoddart, R. H. Grubbs, J. Am. Chem. Soc. 2007, 129, 8944 –
8945.
[15] P. J. Wyatt, Anal. Chim. Acta 1993, 272, 1 – 40.
[16] For the crystal structure of a tetrakis(N-methylpyridinium)por-
phyrin linked to four phosphonate cavitands see: R. De Zorzi, B.
Dubessy, J.-C. Mulatier, S. Geremia, L. Randaccio, J.-P. Dutasta,
J. Org. Chem. 2007, 72, 4528 – 4531.
[17] J. L. Sessler, D. E. Gross, W.-S. Cho, V. M. Lynch, F. P.
Schmidtchen, G. W. Bates, M. E. Light, P. A. Gale, J. Am.
Chem. Soc. 2006, 128, 12281 – 12288.
[8] a) T. Haino, Y. Matsumoto, Y. Fukazawa, J. Am. Chem. Soc.
2005, 127, 8936 – 8937; b) S. Ishihara, S. Takeoka, Tetrahedron
Lett. 2006, 47, 181 – 184; c) S. Pappalardo, V. Villari, S. Slovak, Y.
Cohen, G. Gattuso, A. Notti, A. Pappalardo, I. Pisagatti, M. F.
Parisi, Chem. Eur. J. 2007, 13, 8164 – 8173.
[9] a) L. Pirondini, A. G. Stendardo, S. Geremia, M. Campagnolo, P.
Samorꢀ, J. P. Rabe, R. Fokkens, E. Dalcanale, Angew. Chem.
2003, 115, 1422 – 1425; Angew. Chem. Int. Ed. 2003, 42, 1384 –
1386; b) H. Ihm, J.-S. Ahn, M. S. Lah, Y. H. Ko, K. Paek, Org.
Lett. 2004, 6, 3896 – 3896.
[18] The disassembly–reassembly sequence can be traced also by
31P NMR spectroscopy under the same conditions (see Figure S2
in the Supporting Information).
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