10.1002/chem.201700030
Chemistry - A European Journal
FULL PAPER
observed over iodide. When analogous titrations with 3.XB4 and
azide, acetate, nitrate, perrhenate or sulphate were performed in
a solution of 2% D2O in [D6]acetone, no evidence of binding was
observed (see Supporting Information Fig. S3-8), suggesting
that the interlocked binding cavity surrounded by four XB donor
groups is complementary for spherical halides, in particular
bromide, over other possible anion geometries. Repeating the
halide binding studies in the more competitive aqueous solvent
mixture of 5 % D2O in [D6]acetone shows that neutral rotaxane
3.XB4 displays a distinct Hofmeister bias binding trend with
iodide selectivity (Table 2), underscoring the importance of anion
hydration in the binding process.
opens up new possibilities for the design and preparation of
elaborate XB interlocked receptors capable of strong anion
binding for various analytical and nanotechnological applications.
Acknowledgements
J.Y.C.L acknowledges the Agency for Science, Technology and
Research (A*STAR), Singapore, for a postgraduate scholarship. T.B.
thanks the Development and Promotion of Science and Technology
Talents (DPST) Project, Thailand and Wadham College, Oxford, for
financial funding.
Commenting further on the importance of hydration in
determining the anion affinities and selectivities by these neutral
XB rotaxanes, introducing just 2 % water by volume in acetone
led to a significant reduction in anion binding strength of 1.XB2 in
particular for the more hydrated anions chloride and sulfate.
Clearly, these results show that the stabilities of the XB
rotaxane-anion complexes are highly dependent on solvent
polarity, which contrasts the observations by Hunter and co-
workers regarding the solvent-independence of XB interaction
strengths.[56] In their study, XB association constants between
neutral molecular iodine and tetramethylthiourea decreased only
by a factor of three-fold when the solvent was changed from n-
octane to methanol.[57] While the different XB-donor properties of
molecular iodine and iodotriazoles may play a part in accounting
for the observed contrasting solvent dependency, the
augmented sensitivity of XB-anion binding affinity to solvent
polarity highlights the much higher solvation energies of anions
as compared with neutral species. This is especially the case
with protic solvents, where anions are significantly more
sensitive to changes in the solvent’s hydrogen bond-donor ability.
Notably, Table 2 shows the stabilities of neutral XB rotaxane-
anion complexes with ‘soft’ and poorly-hydrated anions such as
iodide are relatively less sensitive to variations in solvent polarity,
behaving more similarly to the neutral XB donor-acceptor
associated species in Hunter’s study.
Keywords: anion binding • halogen bonding • rotaxanes •
supramolecular chemistry • neutral
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