Chiral amplification and helical-sense tuning by mono- and divalent metals on dynamic helical polymers

Article abstract of DOI:10.1002/anie.201105769

Ion sensor: A highly dynamic poly(phenylacetylene) bearing α-methoxyphenylacetic acid (MPA) as chiral pendant exhibits selective helix induction and chiral amplification, and gives a material that acts as a sensor for the valence of metal cations. Selecti

Full text of DOI:10.1002/anie.201105769

Communications
DOI: 10.1002/anie.201105769
Helical Structures
Chiral Amplification and Helical-Sense Tuning by Mono- and Divalent
Metals on Dynamic Helical Polymers**
Fꢀlix Freire, Josꢀ Manuel Seco,* Emilio QuiÇoꢁ, and Ricardo Riguera*
In memory of Rafael Suau
The controlled induction of a helix-sense bias in helical
polymers[1] by external stimuli[1–4]—that is, the possibility of
selecting a helix with a preferred handedness—has become a
desirable goal[2,3] owing to the potential applications of these
materials as chiral sensors, molecular devices,[5] chiro-optical
switches, memory elements for information storage, chiral
catalysts, and conductive materials, amongst others.[1–3] Since
the pioneering work by Green et al.[6] on the amplification of
chirality in polyisocyanates, helical polymers with chiral
amplification properties have attracted much attention.[1–3,7]
Herein we report the synthesis and evaluation of a new
and highly dynamic poly(phenylacetylene) (PPA) derivative
that bears chiral pendants. This polymer incorporates the two
aforementioned features (selective helix induction and chiral
amplification) to give a material that acts as a sensor for the
valence of metal cations. In the amplification of chirality
reported herein, the external stimulus—the trigger—is pro-
vided by the selective coordination of the pendants with
mono- or divalent metal cations (achiral agents) in such a way
that the valence of the metal determines the right- or left-
handed helical sense of the polymer and its chiroptical
response.[8]
induced an inversion of the CD spectrum, indicating a
conformational switch from the ap to the sp form. This
inference was corroborated by optical rotation and by 1H and
a-Methoxyphenylacetic acid (MPA), connected to the
À
À
=
phenylacetylene moiety through a (C6H4) NH C( O) amide
bond, was the pendant of choice. This system was selected
because: 1) the 1,2-(amide carbonyl–methoxy) moiety is an
excellent chelating moiety for certain cations (as shown by
CD and NMR spectroscopy studies)[9] and 2) metal chelation
is known to switch the antiperiplanar/synperiplanar (ap/sp)
conformational equilibrium of chiral MPA amides.[9]
In this way, the (R)- and (S)-MPA phenylacetylene
monomers 1 and 2 (Figure 1a) were prepared and we assessed
their capacity to coordinate with mono- and divalent cations
(Li+, Ca2+, Ba2+, Mn2+). The addition of divalent metal cations
[*] Dr. F. Freire, Prof. Dr. J. M. Seco, Prof. Dr. E. QuiÇoꢀ,
Prof. Dr. R. Riguera
Department of Organic Chemistry and Center for Research in
Biological Chemistry and Molecular Materials (CIQUS)
University of Santiago de Compostela
E-15782 Santiago de Compostela (Spain)
E-mail: josemanuel.seco@usc.es
ricardo.riguera@usc.es
Homepage: http://www.usc.es/gi1608/
[**] We thank the Ministerio de Ciencia e Innovaciꢁn [CTQ2008-01110/
BQU, CTQ2009-08632/BQU and Ramꢁn y Cajal contract (F.F.)] and
Xunta de Galicia (PGIDIT09CSA029209PR) for financial support.
Figure 1. a) Structures of 1, 2, poly-1, and poly-2. b) CD spectra of
poly-2 in different solvents (0.1 mgmLÀ1). Poly-1 generates the corre-
sponding mirror-image spectra. c) CD spectra of poly-1 with divalent
metal cations in CHCl3.[14] d) CD spectra of poly-1 with monovalent
metal cations in CHCl3.[14]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105769.
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13C NMR spectroscopy (see full description of the behavior of
the monomers in the Supporting Information).[10]
The addition of monovalent cations to 1 or 2 only led to a
decrease in the intensity of the CD bands but not to an
inversion, thus suggesting that a different type of coordination
was operating. 1H and 13C NMR spectroscopy and optical
rotation data are consistent with this interpretation.
With this information in hand, the two novel PPAs poly-
(R)-MPA (poly-1) and poly-(S)-MPA (poly-2; Figure 1a)
were synthesized.[11] [{Rh(nbd)Cl}2] was employed as a
catalyst (nbd = 2,5-norbornadiene), and the polymeric mate-
rials that were obtained had a high content of the cis isomer,
as was concluded from the chemical shifts of the vinyl protons
(d = 5.7–5.8 ppm) and Raman resonances (1567, 1340,
1003 cmÀ1).[12] The material properties were subsequently
verified.
The helicity of the polymers can be inferred from CD
spectroscopy.[1,12] The small Cotton effects observed at 375 nm
for poly-1 and poly-2 in a series of solvents (Figure 1b)
indicate that, in solution, these polymers have highly dynamic
helices. Only in some solvents a slight predominance of one
sense of helicity was observed and virtually none in CHCl3, as
shown by the absence of a CD effect.
Once evidence for the dynamic behavior had been
obtained, we investigated whether this characteristic could
be modulated by coordination with mono and divalent
cations. Nine perchlorates[13] of divalent metals (M(ClO4)2;
M = Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+ , Ba2+, Hg2+, Pb2+)[14]
were added to solutions of poly-1 and poly-2 in CHCl3. CD,
UV/Vis, and FTIR spectroscopy, and optical rotation studies
indicated the formation of helical polymer–metal complexes
(HPMCs), together with the modulation of the helicity of the
polymer from the initial highly dynamic helix (no CD effect)
to a final helix with a prevalent helical sense (strong positive
Cotton effects at 375 nm after the addition of the salts for
poly-1; negative for poly-2; Figure 1c). Importantly, all the
divalent cations tested induced the same sense of helicity, and
the maximum responses were achieved with just 0.1(M2+)/
1.0(monomer) (equiv/equiv) ratios.
Figure 2. a) AFM image and top and side views of the 3/1 right-
handed helix of poly-1/Ba2+. b) AFM image and top and side views of
the 3/1 left-handed helix of poly-1/Ag+. The values depicted in the top
and side views of the helices were obtained by MMFF94 calculations.
MMFF94[18] molecular mechanics calculations on the
secondary structure of poly-1 (28-mer) showed that a 3/1
right-handed helix resulted when the (R)-MPA pendants
adopted sp conformations. On the other hand, a 3/1 left-
handed helix of similar energy arose when (R)-MPA pendants
adopted ap conformations. In both cases the angles formed by
the oblique stripes and the helical pitches (61.08/62.08 and 3.1/
3.1 nm for sp/ap conformations, respectively) match the
experimental values (Figure 2). These results imply internal
angles close to + 758 and À758 for sp and ap conformations,
respectively, and explain the right- and left-handedness of the
corresponding backbones. Moreover, these angles allow the
formation of hydrogen bonds between the nth and (n+3)th
amide bonds.
On addition of monovalent cations (MClO4; M = Li+,
Na+, Ag+),[15] the polymers adopted the opposite helical sense
in all cases (negative Cotton effects at 375 nm for poly-1;
positive for poly-2) at analogous monomer/metal ratios
(Figure 1d).
A relationship between cation size (ionic radius) and the
observed selectivity bias was not found.
AFM images of poly-1/Ba2+ and poly-1/Ag+ [16] on highly
oriented pyrolytic graphite (HOPG) provided important
insights into the helicity and morphology of the HPMCs
(Figure 2) and are in full agreement with the information
obtained from CD spectroscopy.[17] With Ba2+, the single
chains were packed in a parallel manner, one after the other,
to form a right-handed (clockwise) pendant disposition with
the periodic oblique stripes forming 60.08 angles and a helical
pitch of 3.23 nm (Figure 2a). In the case of Ag+, the chains
were also aligned side-by-side but, in contrast to the previous
case, showed a left-handed (counterclockwise) pendant dis-
position with the periodic oblique stripes forming 60.88 angles
and a helical pitch of 3.21 nm (Figure 2b).
Consequently, the different behavior of the dynamic
polymer in the presence of di- and monovalent cations can
be attributed to the different conformations of the pendants
favored by each type of cation (sp and ap, respectively,
Figure 3).
IR spectra confirmed the different coordination modes of
the cations according to their valence, as evidenced by the
=
shifts of the C O and OMe bands (see Dn values in Table 1).
=
Thus, divalent cations coordinate to both the C O and OMe
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Communications
Figure 3. a) Selective modulation of the helicity induced by control of the conformation of the pendants by complexation with mono- and divalent
cations (poly-1). b) Schematic representation of the amplification phenomenon indicating the ap/sp conformation of the pendants.
Figure 4. Chiral amplification and helix inversion (poly-1) by step-by-step addition: first M+ (Li+), second M2+ (Ni2+) (left) and first M2+ (Ni2+),
second M+ (Li+) (right).
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Angew. Chem. Int. Ed. 2011, 50, 11692 –11696

Table 1: Selected IR spectroscopy data of poly-1 and poly-1/metal cation
(KBr).
Polymer/M
n CO
n OMe
Dn CO
[cmÀ1 [a]
Dn OMe
[cmÀ1 [a]
[cmÀ1
]
[cmÀ1
]
]
]
Poly-1
1682
1650
1672
1669
1665
1658
1638
1099
1099
1100
1100
1063
1074
1074
0
0
0
Poly-1/Ag+
Poly-1/Na+
Poly-1/Li+
Poly-1/Ba2+
Poly-1/Ca2+
Poly-1/Ni2+
32
10
13
17
24
44
À1
À1
36
25
25
[a] Dn=n(poly-1)Àn(poly-1/M).
groups (sp conformation), while monovalent cations only
=
coordinate to the C O unit (ap conformation, Figure 3a).
The low monomer/cation ratios required to achieve a
maximum CD response (less than 0.1 in most cases) imply
that chiral amplification phenomena are operating. The
coordination of one cation to one pendant induces a preferred
conformation (sp or ap depending on the valence), which is
transmitted to the nearby pendant groups by a domino effect,
thus leading to a more stable arrangement of the chain
(Figure 3b). The low energy barrier between the left- and
right-handed helices facilitates the dual behavior of the
polymer, thus allowing its evolution to either one or the other
helical sense according to the valence of the cation.
Additional experiments showed the versatility of the
polymer. For example, once chiral amplification has been
obtained with a cation (i.e. M2+ at a cation/monomer ratio of
0.1), addition of a cation belonging to the other valence type
(M+) at a higher concentration (cation/monomer ratio of 1.0)
reverses the helicity (Figure 4). The opposite addition
sequence (first monovalent, second divalent) works in the
same way and the helicity induced by the cation present in
excess prevails (Figure 4). Addition of mixtures of both types
of cations to the polymer yielded the same final CD spectra as
those obtained by step-by-step additions (see the Supporting
Information).
The full reversibility of the processes was established
using scavenger resins (Figure 5). In these experiments the
CD spectrum of the dynamic polymer was recovered when
the cation was removed after complexation by addition of the
resin (see the Supporting Information). In this way, the
characteristic CD properties of the polymer are recovered (no
memory), and it can be reused for new complexation with
other di- or monovalent cations.
In summary, we present herein a highly dynamic helical
polymer (PPA) with a chiral pendant ((R)- or (S)-MPA),
which shows no CD effect. The discriminating interaction of
the polymer with mono- and divalent metals induces either
left- or right-handed helical senses and opposite chiroptical
responses.[19] This selective modulation (confirmed by AFM)
is triggered by the initial control of the conformation of the
pendants by complexation with the metal cations. This effect
is further transmitted from the pendants to the main polymer
backbone.
Figure 5. Reversible use of poly-2 by means of scavenger resins. For
examples with other metal cations and other addition sequences (i.e.
1st M2+, 2nd resin, 3rd M+ or M2+) see the Supporting Information.
indicating that the chiral polymer amplifies the effect of
metal-ion complexation (chiral amplification). This response
of the polymer to the valence of the ion shows that the
polymer acts as a mono- and divalent metal cation sensor.[20]
Received: June 17, 2011
Published online: October 11, 2011
Keywords: chiral amplification · helical structures ·
.
helix induction · metal cations · sensors
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Communications
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[14] 0.2(M2+ or M1+)/1.0(monomer)(equiv/equiv) ratios taken from
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mechanisms and the helical senses of the copolymers (AFM
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[15] KClO4 and CsClO4 were insoluble under the experimental
conditions.
[16] 0.2(Ba2+)/1.0(monomer) and 0.5(Ag+)/1.0(monomer) (equiv/
equiv) ratios.
[17] To our knowledge, this is the first time that images of both helical
senses are reported for polyphenylacetylenes bearing short
pendants. Usually, long alkyl chains are needed as pendants to
favor the interaction with the HOPG surface. See reference [4a]
and S. Sakurai, K. Okoshi, J. Kumaki, E. Yashima, Angew. Chem.
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[18] Calculations were performed with Spartan 08.
[19] This response is generated from a highly dynamic polymer in
which both helical senses are initially present (no CD signal), so
the phenomenon reported herein cannot be classified as
belonging to the “helix inversion” type because there is not
any “inversion” from a previous helicity.
[20] We recently reported a less dynamic polymer bearing another
kind of pendant with a different connection to the phenyl-
acetylene, where the addition of certain metal cations produced
helix inversion from a previous predominant helical sense, but
neither selectivity associated with the valence of the metals nor
amplification of chirality were observed. Thus, that material did
not behave as valence sensor. See I. Louzao, J. M. Seco, E.
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[10] There has been a recent example of the use of monomers in the
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