8
,18b
assembly 1•CB[7] (conformation II) and the transition
ill-defined coordination shell of 4 to 11 ligands;
intermediate rates for the remaining alkali metals and
(2)
crown
state resembling and preceding intermediate 1•CB[7]
.
2þ
7
Mg , which are prone to hexacoordination; and (3) fast
rates with Ca , Sr , and Ba , which often interact with
In the latter case, CB[7] would provide the
metallic cation with up to three coordination sites in addi-
tion to the benzo-15-crown-5 stopper and, hence, further
stabilize the key transition state of the dethreading process.
Finally, despite a doubly positive charge and a radius
similar to that of Ca , Cd has only a minor enhancing
effect on dethreading rates (6.3-fold compared to a cation-
Interactions between cations and assembly 1•CB[7] were
too weak to be measured by NMR spectroscopy or
isothermal titration calorimetry; moreover, the high con-
centrations of salts (up to 1.0 M) used in titrations trig-
gered dethreading and prevented us from determining any
relevant binding affinities. For lack of a quantitative
assessment, we will neglect cation interactions with assem-
bly 1•CB[7] and assume that dethreading rate enhance-
2þ
2þ
2þ
7,18c
eight ligands.
2þ
2þ
ments are mostly due to the stabilization of the transition
crown
2þ
state resembling intermediate 1•CB[7]
that in the presence of large cations (Rb , Cs , NH ),
. We propose
þ
free environment, 74 times weaker than Ca ). Contrary
2þ
þ
þ
to alkali and alkali-earth metals, Cd is a soft Lewis acid;
it is thus expected to interact weakly with the hard oxygen
4
1
6
which are perched above the crown ether ring, optimal
CB[7] cooperativity in the stabilization of the 15-crown-5/
metal adduct takes place before the key transition state of
the dethreading process (i.e., when CB[7] is closer to the
1
9
ligands of the benzo-15-crown-5 and CB[7] units.
Furthermore, at a concentration of 0.10 M, cadmium is
present as a mixture of Cd , CdCl , CdCl , and possibly
2
20
CdCl ; the low concentration of Cd , and possible
2þ
þ
ꢀ
2þ
1
of Na and K , CB[7] cooperativity is optimal at the
,4-butane diammonium central station); in the presence
þ
3
þ
repulsion between the carbonyl portal of CB[7] and the
þ
þ
þ
transition state, and with small cations such as D , Li ,
chloride ligand of crown ether-bound CdCl , may also
contribute to the slow dethreading process.
1
6
which nest inside the crown ether unit, optimal, yet
weak CB[7] stabilization occurs after the transition state.
These considerations are consistent with the observed bell-
shaped trend between dethreading rates and the ionic radii
of alkali metals (see Figure 2b). This trend is also reminis-
cent of a similar sequence observed with binding affinities
In conclusion, we found that metallic and organic
cations can be applied as “supramolecular lubricants” to
facilitate the translation of macrocycles along organic
axles, as long as the cations stabilize the highest energy
transition state of the dethreading process better than the
stable interlocked assemblies. Key factors include the
radius, valence, coordination number, and hardness of
the cations, as well as the exact location of the transition
state along the dethreading pathway. To the best of our
knowledge, this study constitutes the first case of a me-
chanical motion between two interlocked structures facili-
tated on-demand by a third molecular partner.
1
7
of alkali metals toward hemispherand 4 that resembles
crown
intermediate 1•CB[7]
like unit and the overlooking p-methyl anisole in host 4
(see Figure 2; the crown ether-
mimic the benzo-15-crown-5 stopper and one glycoluril
crown
section of CB[7] in intermediate 1•CB[7]
, respectively).
In fact, a logarithmic plot of dethreading rate constants
from [2]pseudorotaxane 1•CB[7] vs the binding affinities
of macrocycle 4 toward ammonium and alkali cations
(
in chloroform-d saturated with D O) shows a significant
2
Acknowledgment. This work was supported by the
Department of Chemistry and Biochemistry, the College
of Arts and Sciences, the Vice President for Research at
Ohio University, and in part by an allocation of computing
time from the Ohio Supercomputer Center in Columbus.
level of linearity (see Figure 2c). A major structural
difference between both systems is of course the movability
of CB[7] along its axle to better adapt to the cations,
compared to the steadier p-methyl anisole unit in host 4.
As shown in Figure 2b, dethreading is faster in the
presence of alkali-earth cations compared to alkali metals;
this effect is likely due to (1) stronger Coulombic inter-
actions with both crown ether and CB[7] units and (2) a
higher entropy gain associated with the desolvation of the
Supporting Information Available. Preparation and
characterization of axles 1 and 3, and their CB[7]-inter-
locked assemblies; kinetic and computational proce-
dures; coordinates of assemblies shown in Figure 2.
This material is available free of charge via the Internet
at http://pubs.acs.org.
7
cations upon binding.
Coordination of the metallic cations may also affect the
dethreading rate of [2]pseudorotaxane 1•CB[7]: a horizon-
tal view of Figure 2b shows (1) poor rate enhancements
(
18) (a) Varma, S.; Rempe, S. B. Biophys. Chem. 2006, 124, 192–199.
þ
18a
(b) Ohtaki, H.; Radnai, T. Chem. Rev. 1993, 93, 1157–1204. (c) Owen,
J. D.; Wingfield, J. N. J. Chem. Soc., Chem. Commun. 1976, 318–319.
(19) Lehn, J. M.; Montavon, F. Helv. Chim. Acta 1978, 61, 67–82.
with Li , whose coordination number is often four,
þ
and with NH4 , which is commonly surrounded by an
(
20) Bazarkina, E. F.; Pokrovski, G. S.; Zotov, A. V.; Hazemann,
J. L. Chem. Geol. 2010, 276, 1–17.
(
16) Moore, S. S.; Tarnowski, T. L.; Newcomb, M.; Cram, D. J.
J. Am. Chem. Soc. 1977, 99, 6398–6405.
17) Lein, G. M.; Cram, D. J. J. Am. Chem. Soc. 1985, 107, 448–455.
(
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 18, 2012
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