+
Na or K . We suggest that the response to Ag seen in gels
+
+
of G1-ene is caused by repulsive electrostatic repulsions
+
between alkene-bound Ag ions, although we cannot rule
out the hypothesis that binding to the alkenes subsequently
+
allows Ag to interact better with the CQO groups within the
gel fibres, hence disrupting them. In summary, this is the first
+
example in which Ag –alkene interactions play a vital role in
mediating a response in soft matter systems, providing funda-
mental insight into the nature of this interaction and acting as
a step on the way to development of heavy-metal-responsive
materials.
We acknowledge EPSRC and University of York for
funding.
13
Fig. 6
C NMR spectra of G1-ene gel in ethyl acetate, and of the gel
Notes and references
6 6
sample after the addition of AgSbPF or LiPF .
1
(a) Molecular Gels: Materials with Self-Assembled Fibrillar Networks,
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+
between the alkene and Ag , with the binding curves being
almost superimposable. Reliable stability constants for amide
(CQO) binding to the metal ion could not be fitted to the data,
due to the ill-defined complex stoichiometry—however, the
binding curves for 1-ene and 1-ane indicated that CQO was
affected by Ag in roughly the same way for both compounds—
2
3
(a) S. Banerjee, R. K. Das and U. Maitra, J. Mater. Chem., 2009,
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Chem. Rev., 2010, 110, 1960–2004.
+
compound 1-ene reached saturation slightly more slowly than
1
+
-ane which would indicate that binding of Ag to the alkene of
1
-ene occurs prior to CQO binding. These data indicate that in
4 (a) H.-J. Kim, J.-H. Lee and M. Lee, Angew. Chem., Int. Ed., 2005,
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4
the solution phase, the binding of the alkene and the amide to
+
+
Soc., 2004, 126, 7009–7014; (c) Y. He, Z. Bian, C. Kang and
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Ag can both occur. For the binding of 1-ene to Ag ions, we
therefore propose that (i) the alkene acts as the primary binding
site (ca. 1:1 binding) as in Octene and, (ii) an ill-defined number
+
of CQO groups can then bind weakly to the alkene-bound Ag .
In the gel-phase, it is clear from the different responses of G1-ene
and G1-ane, that the primary interaction between the alkene
+
group and Ag must lie at the heart of the sensory response.
To finally test the mechanism of cation-induced gel break-
5
(a) D. Braga, S. d’Agostino, E. D’Amen and F. Grepioni, Chem.
Commun., 2011, 47, 5154–5156; (b) J. Dash, A. J. Patil, R. N. Das,
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1
3
down, we used C NMR to monitor the samples. In the native
gel of G1-ene in ethyl acetate, the resonances associated with
the gelator are broadened as the gelator is immobilised within
(
2
c) M. O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter,
011, 7, 2412–2418; (d) P. Casuso, P. Carrasco, I. Loinaz,
6 6
the gel fibres. When either AgSbF or LiPF was added to the
H. J. Grande and I. Odriozola, Org. Biomol. Chem., 2010, 8,
5455–5458; (e) K. Chen, L. Tang, Y. Xia and Y. Wang, Langmuir,
2008, 24, 13838–13841.
1
3
gel, however, the sample showed C NMR gelator peaks as
the gelator became mobile (Fig. 6). Importantly, the resulting
chemical shifts closely resemble the chemical shifts of 1-ene
6
(a) S. V. Brignell and D. K. Smith, New J. Chem., 2007, 31,
1
5
243–1249; (b) G. M. Dykes and D. K. Smith, Tetrahedron, 2003,
9, 3999–4009; (c) K. Murata, M. Aoki, T. Nishi, A. Ikeda and
+
+
when bound to either Ag or Li —with the distinctive upfield
13
shifts of the alkene C resonances associated with the formation
+
of Ag –alkene interactions being clearly observed. This proves
S. Shinkai, J. Chem. Soc., Chem. Commun., 1991, 1715–1719.
W. Deng and D. H. Thompson, Soft Matter, 2010, 6, 1884–1887.
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7
8
the cations complex to G1-ene in the proposed manner, which
leads to the gel–sol conversion.
836–847; (b) N. J. Barnett, L. V. Slipchenko and M. S. Gordon,
J. Phys. Chem.A, 2009, 113, 7474–7481.
+
In conclusion, this study has shown that the Ag –alkene
9 J. Burgess and P. J. Steel, Coord. Chem. Rev., 2011, 255, 2094–2103.
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1
1
interaction is essential for the response of G1-ene to AgSbF6.
This is demonstrated by G1-ane, which is unable to respond to
+
+
Ag but still able to respond to Li . There is no response of
any of these gels to larger, less charge dense Group 1 metal
12 (a) J. R. Moffat, I. A Coates, F. J. Leng and D. K. Smith,
Langmuir, 2009, 25, 8786–8793; (b) J. R. Moffat and
D. K. Smith, Chem. Commun., 2009, 316–318.
+
+
ions such as Na and K , and we propose that the selectivity
amongst Group 1 metals is mediated by the higher charge
1
3 D. Stacchiola, G. Wu, M. Kaltchev and W. T. Tysoe, Surf. Sci.,
001, 486, 9–23.
14 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.
2
+
density of Li , meaning it is a stronger binder to CQO than
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2767–2769 2769