Supramolecular polymers based on dative boron-nitrogen bonds

Article abstract of DOI:10.1039/c2cc34231f

Heteroditopic monomers containing an arylboronate ester and a dialkyl-4-aminopyridine group aggregate via dative boron-nitrogen bonds to give main chain supramolecular polymers. The degree of polymerization can be tuned by changing the electronic and steric properties of the boronate ester.

Full text of DOI:10.1039/c2cc34231f

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Supramolecular polymers based on dative boron–nitrogen bondsw
a
a
b
b
b
´
Erin Sheepwash, Nicolas Luisier, Martin R. Krause, Stefanie Noe, Stefan Kubik and
Kay Severin*a
Received 13th June 2012, Accepted 20th June 2012
DOI: 10.1039/c2cc34231f
Heteroditopic monomers containing an arylboronate ester and a
dialkyl-4-aminopyridine group aggregate via dative boron–
nitrogen bonds to give main chain supramolecular polymers.
The degree of polymerization can be tuned by changing the
electronic and steric properties of the boronate ester.
Table 1 Association constants of adducts between dioxaboroles and
pyridyl ligands
Main-chain supramolecular polymers are formed by aggregation of
monomers via directional and reversible non-covalent interactions.1
A limiting factor is the strength of the non-covalent interaction:
binding constants of Ka Z 106 Mꢀ1 between the functional groups
of the monomers are typically required to obtain polymers of
significant molecular weight.1,2 In this regard, dative bonds between
Lewis-acidic arylboronate esters and nitrogen-donor ligands seem
to be less suited as they have been described ‘‘to be of similar
strength to a hydrogen bond’’.3 The quantitative measurements
which have been reported so far provide a slightly more optimistic
picture with binding energies between ꢀ11 and ꢀ25 kJ molꢀ1
Ka [Mꢀ1
]
Ar
R
R0
C6D6
CDCl3
1a
2a
3a
4a
5b
6b
7b
Ph
Ph
2,4,6-F
2,4,6-iPr
Ph
H
Cl
H
H
H
H
Cl
H
H
H
NMe2
NMe2
NMe2
NMe2
5.1(0.5) ꢁ 101
1.1(0.7) ꢁ 104
8.8(6.8) ꢁ 104
4.4(0.5) ꢁ 101
9.1(0.4) ꢁ 104
1.3(0.2) ꢁ 106
4.0(0.1) ꢁ 106
3.6(0.3) ꢁ 101
9.1(2.4) ꢁ 102
9.8(1.1) ꢁ 103
2.2(0.2) ꢁ 100
6.0(0.2) ꢁ 104
6.2(0.3) ꢁ 105
n.d.
2,4,6-F
Ph
a
b
Determined by 1H NMR spectroscopy. Determined by ITC; the
(Ka B 102–104
M
ꢀ1).4,5 Below we report that it is possible
values are averages of three independent measurements.
to increase the strengths of dative B–N bonds substantially
by using complementary electronic effects. This finding was
the basis for the successful synthesis of main-chain supra-
molecular polymers with dative B–N bonds.
and free N-donors was found to be fast on the NMR time scale
and averaged signals were observed. Numerical fitting9 of the
isotherms to a 1 : 1 binding model gave association constants
(Ka), the values of which are summarized in Table 1.
Dative B–N bonds between arylboronate esters and amines
have been studied extensively in the context of boronic acid-
based carbohydrate sensors.3,4,6 Recently, it was shown that
dative B–N interactions between dioxaboroles (condensation
products of boronic acids and catechols) and pyridyl donor-
ligands can be used for the construction of molecularly defined
nanostructures7 and polymeric materials.8 In solution, these
compounds display limited stability because of the weak B–N
bond. In order to gain more insight into the factors which
govern the binding strengths of dioxaboroles and pyridyl
The constants for the binding of pyridine to boronate esters
were found to be in the range of 101–104 Mꢀ1 indicating a
weak to moderate interaction, with electron withdrawing
substituents on the ester increasing the Ka value. A significant
improvement in binding was observed when dimethylamino-
pyridine (DMAP) was employed as the N-donor. The associa-
tion constants were found to be 2 to 3 orders of magnitude
higher than those observed with pyridine (Table 1, entry 1 vs.
entry 5, entry 2 vs. entry 7, and entry 3 vs. entry 6). For the
DMAP adducts of boronate esters containing a halogenated
1
ligands, we decided to perform a quantitative study. H NMR
arylboronic acid or catecholate, binding constants of Ka
=
spectroscopic titration experiments were carried out with different
dioxaboroles and pyridine donors. Benzene-d6 and CDCl3
solutions containing the dioxaborole and a variable amount
of the N-donor ligand were analyzed by 1H NMR spectroscopy
(see ESIw, Fig. S1–S4). In all cases, the exchange between bound
1.3(0.2) ꢁ 106 Mꢀ1 (entry 6) and Ka = 4.0(0.1) ꢁ 106 Mꢀ1
(entry 7) were determined by isothermal titration calorimetry
(ITC). To the best of our knowledge, these are the highest
reported values for a dative B–N bond involving dioxaboroles.
It is also possible to counterbalance electronic with steric
effects. The utilization of a boronate ester with sterically
demanding isopropyl groups resulted in weak binding of
DMAP (entry 4).
a
´
Institut des Sciences et Inge´nierie Chimiques, Ecole Polytechnique
´
Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
E-mail: kay.severin@epfl.ch
´
b Fachbereich Chemie, Technische Universitat Kaiserslautern,
¨
Germany
It is evident from the data summarized above that it is
possible to achieve binding constants of 106 Mꢀ1 by electronic
tuning of the boronate ester and the pyridyl donor ligands.
w Electronic supplementary information (ESI) available: Synthetic
details and analytical data. See DOI: 10.1039/c2cc34231f
c
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Chem. Commun.

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viscosity for monomer 2 compared to 3. The triisopropyl-
substituted 4 gave a comparably shallow curve, with a viscosity
at 100 mM of less than half that of 2.
Assuming that the self-assembly of the monomers occurs
with a Ka value in the range of that measured for the corre-
sponding simple boronate ester–DMAP adducts (Table 1,
entries 5 and 6), the average degree of polymerization can
be calculated.11,12 These calculations yielded values of about
500 (2) and 150 monomeric units (3), which translate into average
molecular weights of 200 000 (2) and 60 000 g molꢀ1 (3).z
The addition of a chain stopper to a supramolecular polymer
should result in a pronounced decrease in solution viscosity,
even at low mole fractions.12 A solution of 2 (75 mM) was
titrated with DMAP. The relative viscosity decreased rapidly to
approximately Zr = 1 at 0.1 equivalents of DMAP (see ESIw,
Fig. S10). The drop in viscosity upon addition of the chain
stopper DMAP indicates that the observed viscosity of solutions
of 2 is not due to unspecific aggregation, but is in fact due to the
formation of boron–nitrogen dative bonds.
Scheme 1 Synthesis of the heteroditopic monomers 2–4 and aggregation
via dative B–N bonds.
To further characterize the polymers, 1H NMR spectra in CDCl3
were taken at varying monomer concentrations. Solutions of 2
showed peak broadening of about 8 Hz when the concentration of
the monomer was increased from 10 to 100 mM (see ESIw,
Fig. S11). The peak broadening is consistent with the formation
of oligomeric and/or polymeric species in solution.13 Furthermore,
diffusion ordered spectroscopy (DOSY) NMR experiments
were carried out for 2 and 3. In both cases, the diffusion
coefficients increased significantly when the monomer concen-
tration was raised from 10 mM to 100 mM (see ESIw,
Fig. S12–S15). This result is in line with the expected concentration
dependence of a supramolecular polymerization process.14
In summary, we have shown that it is possible to increase
the binding strength of pyridyl ligands to boronate esters
substantially by using electronic effects. Importantly, it is
possible to achieve association constants of Z 106 Mꢀ1. This
observation led to the successful development of main-chain
supramolecular polymers based on dative B–N bonds. It is
likely that the B–N binding motif can also be used for
the formation of branched or cross-linked supramolecular
polymers. Furthermore, it should be possible to combine
dative B–N bonds with other supramolecular interactions such
as hydrogen bonds to create complex polymer architectures
via orthogonal self-assembly.15 Studies in this direction are
ongoing in our laboratory.
This finding prompted us to explore the possibility of creating
main-chain supramolecular polymers with dative B–N bonds.
Three monomers were synthesized containing boronate ester
and dialkylaminopyridyl end groups. The preparation of these
monomers was achieved by condensation of the common
precursor 1 with different arylboronic acids (Scheme 1, for
details see ESIw). In organic solvents of low polarity, these
monomers were expected to aggregate via dative B–N bonds.
The strength of the B–N interaction, and thus the degree
of polymerization, should increase in the order 4 o 3 o 2
(4 features sterically demanding isopropyl groups, whereas 2
contains an electron withdrawing 2,4,6-C6H3F3 group).
The aggregation of 2–4 in chloroform was investigated by
concentration dependent viscosity measurements (Fig. 1). The
steep, non-linear increase in the relative viscosities (Zr) with
concentration gives clear evidence for polymer formation
in the case of monomers 2 and 3.10 Further, the observed
trends in viscosities are in line with the association constants
measured for the simple esters (Table 1). A greater degree of
polymerization was observed for the trifluoro-substituted
monomer 2 whose corresponding simple ester in chloroform
provided a Ka value of an order of magnitude higher than that
of the related ester of monomer 3. This difference in Ka values
for B–N bond formation manifests a larger increase in relative
The work was supported by the Swiss National Science
Foundation and by the EPFL. We thank Magdalena Marszalek
for help with viscosity measurements and Dr Pascal Mieville for
help with DOSY measurements.
Notes and references
z It is likely that macrocycles are formed along with polymers. The
former are neglected for the calculations.
1 (a) R. J. Wojtecki, M. A. Meador and S. J. Rowan, Nat. Mater.,
2011, 10, 14–27; (b) S. K. Yang, A. V. Ambade and M. Weck,
Chem. Soc. Rev., 2011, 40, 129–137; (c) J. M. Lehn, Aust. J. Chem.,
2010, 63, 611–623; (d) T. F. A. De Greef, M. M. J. Smulders,
M. Wolffs, A. P. H. J. Schenning, R. P. E. Sijbesma and
E. W. Meijer, Chem. Rev., 2009, 109, 5687–5754; (e) J. D. Fox
and S. J. Rowan, Macromolecules, 2009, 42, 6823–6835;
(f) R. Hoogenboom, D. Fournier and U. S. Schubert,
Fig. 1 Relative viscosities (Zr) of chloroform solutions containing
different amounts of monomer 2 (’), 3 (K), or 4 (m).
c
Chem. Commun.
This journal is The Royal Society of Chemistry 2012

View Online
Chem. Commun., 2008, 155–162; (g) L. Brunsveld, B. J. B. Folmer,
E. W. Meijer and R. P. Sijbesma, Chem. Rev., 2001, 101,
4071–4097.
2 (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 and
E. W. Meijer, Science, 1997, 278, 1601–1604; (b) R. B. Martin,
Chem. Rev., 1996, 96, 3043–3064.
C. Lehen-Ferrenbach and F. Boue, J. Am. Chem. Soc., 2007, 129,
´
15601–15605; (c) H. Hofmeier, R. Hoogenboom, M. E. L. Wouters
and U. S. Schubert, J. Am. Chem. Soc., 2005, 127, 2913–2921;
(d) L. R. Rieth, R. F. Eaton and G. W. Coates, Angew. Chem., Int.
Ed., 2001, 40, 2153–2156; (e) R. F. M. Lang, M. Van and
E. W. Meijer, J. Polym. Sci., Part A: Polym. Chem., 1999, 37,
3657–3670.
3 (a) R. Nishiyabu, Y. Kubo, T. D. James and J. S. Fossey, Chem.
Commun., 2011, 47, 1124–1123; (b) The statement was made with
reference to a computational study: S. Franzen, W. Ni and
B. J. Wang, J. Phys. Chem. B, 2003, 107, 12942–12948.
4 (a) N. Christinat, R. Scopelliti and K. Severin, Chem. Commun.,
2008, 3660–3662; (b) N. Kano, J. Yoshino and T. Kawashima,
Org. Lett., 2005, 7, 3909–3911; (c) L. I. Bosch, T. M. Fyles and
T. D. James, Tetrahedron, 2004, 60, 11175–11190.
5 For a recent theoretical study see: J. D. Larkin, J. S. Fossey,
T. D. James, B. R. Brooks and C. W. Bock, J. Phys. Chem. A,
2010, 114, 12531–12539.
6 (a) B. E. Collins, S. Sorey, A. E. Hargrove, S. H. Shabbir,
V. M. Lynch and E. V. Anslyn, J. Org. Chem., 2009, 74,
4055–4060; (b) L. Zhu, S. H. Shabbir, V. M. Lunch, S. Sorey
and E. V. Anslyn, J. Am. Chem. Soc., 2006, 128, 1222–1232;
(c) W. Ni, G. Kaur, G. Springsteen, B. Wang and S. Franzen,
Bioorg. Chem., 2004, 32, 571–581.
11 (a) P. J. Flory, Principles of Polymer Chemistry, Cornell Univeristy
Press, London, 1953; (b) P. J. Flory, J. Am. Chem. Soc., 1936, 58,
1877–1885.
12 (a) W. Knoben, N. A. M. Besseling and M. A. C. Stuart, J. Chem.
Phys., 2007, 126, 024907; (b) W. Knoben, N. A. M. Besseling and
M. A. C. Stuart, Macromolecules, 2006, 39, 2643–2653; (c) F. Lortie,
S. Boileau, L. Bouteiller, C. Chassenieux and F. Laupretre, Macro-
molecules, 2005, 38, 5283–5287; (d) W. Knoben, N. A. M. Besseling,
L. Bouteiller and M. A. C. Stuart, Phys. Chem. Chem. Phys., 2005,
7, 2390–2398.
13 For selected examples see: (a) Z. Niu, F. Huang and H. W. Gibson,
J. Am. Chem. Soc., 2011, 133, 2836–2839; R. Schmidt, M. Stolte,
M. Grune and F. Wurthner, Macromolecules, 2011, 44, 3766–3776;
¨
¨
(b) R. Dobrawa, M. Lysetska, P. Ballester, M. Grune and
¨
F. Wurthner, Macromolecules, 2005, 38, 1315–1325.
¨
14 (a) N. L. Strutt, H. Zhang, M. A. Giesener, J. Lei and
J. F. Stoddart, Chem. Commun., 2012, 48, 1647–1649;
(b) F. Wang, J. Zhang, X. Ding, S. Dong, M. Liu, B. Zheng,
S. Li, L. Wu, Y. Yu, H. W. Gibson and F. Huang, Angew. Chem.,
Int. Ed., 2010, 49, 1090–1094; (c) T. F. A. de Greef, G. Ercolani,
G. B. W. L. Ligthart, E. W. Meijer and R. P. Sijbesma, J. Am.
Chem. Soc., 2008, 130, 13755.
7 (a) B. Icli, E. Sheepwash, T. Riis-Johannessen, K. Schenk,
Y. Filinchuk, R. Scopelliti and K. Severin, Chem. Sci., 2011, 2,
1719–1721; (b) N. Christinat, R. Scopelliti and K. Severin, Chem.
Commun., 2008, 3660–3662; (c) N. Christinat, R. Scopelliti and
K. Severin, J. Org. Chem., 2007, 72, 2192–2200; (d) N. Christinat,
R. Scopelliti and K. Severin, Chem. Commun., 2004, 1158–1159.
¨
15 For selected examples see: (a) G. Groger, W. Meyer-Zaika,
C. Bottcher, F. Grohn, C. Ruthard and C. Schmuck, J. Am. Chem.
8 (a) J. Cruz-Huerta, D. Salazar-Mendoza, J. Herna
I. F. Hernandez Ahuactzi and H. Hopfl, Chem. Commun., 2012, 48,
´
ndez-Paredes,
¨
Soc., 2011, 133, 8961–8971; (b) F. Grimm, N. Ulm, F. Grohn,
¨
¨
´
¨
4241–4243; (b) E. Sheepwash, V. Krampl, R. Scopelliti, O. Sereda,
A. Neels and K. Severin, Angew. Chem., Int. Ed., 2011, 50,
3034–3037; (c) N. Christinat, E. Croisier, R. Scopelliti,
J. During and A. Hirsch, Chem.–Eur. J., 2011, 17, 9478–9488;
¨
(c) S. K. Yang, A. V. Ambade and M. Weck, J. Am. Chem. Soc.,
2010, 132, 1637–1645; (d) A. V. Ambade, C. Burd, M. N. Higley,
K. P. Nair and M. Weck, Chem.–Eur. J., 2009, 15, 11904–11911;
(e) A. V. Ambade, S. K. Yang and M. Weck, Angew. Chem., Int.
Ed., 2009, 48, 2894–2898; (f) H. Hofmeister, R. Hoogenboom,
M. E. L. Wouters and U. S. Schubert, J. Am. Chem. Soc., 2005,
127, 2913–2921; (g) H. Hofmeister and U. S. Schubert, Chem.
Commun., 2005, 2423–2432.
M. Cascella, U. Rothlisberger and K. Severin, Eur. J. Inorg. Chem.,
¨
2007, 5177–5181.
9 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.
10 (a) G. Groger, W. Meyer-Zaika, C. Bottcher, F. Grohn,
¨
¨
¨
C. Ruthard and C. Schmuck, J. Am. Chem. Soc., 2011, 133,
8961–8971; (b) E. Obert, M. Bellot, L. Bouteiller, F. Andrioletti,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.