Formation of a hetero[3]rotaxane by a dynamic component-swapping strategy

Article abstract of DOI:10.1039/c4cc03612c

Acid-catalysed scrambling of the mechanically interlocked components between two different homo[3]rotaxanes, constituted of dumbbells containing two secondary dialkylammonium ion recognition sites encircled by two [24]crown-8 rings, each containing a couple of imine bonds, affords a statistical mixture of a hetero[3]rotaxane along with the two homo[3]rotaxanes, indicating that neither selectivity nor cooperativity is operating during the assembly process. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03612c

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Formation of a hetero[3]rotaxane by a dynamic
component-swapping strategy†
Cite this: Chem. Commun., 2014,
50, 9665
Eleanor A. Wilson, Nicolaas A. Vermeulen, Paul R. McGonigal,
Received 13th May 2014,
Accepted 28th June 2014
Alyssa-Jennifer Avestro, Amy A. Sarjeant, Charlotte L. Stern and J. Fraser Stoddart*
DOI: 10.1039/c4cc03612c
www.rsc.org/chemcomm
Acid-catalysed scrambling of the mechanically interlocked compo- cooperativity being observed9 in [n]rotaxanes where n = 4 and 5
nents between two different homo[3]rotaxanes, constituted of dumb- and most likely in the higher homologues where n = 8, 12,
bells containing two secondary dialkylammonium ion recognition sites 16 and 20. The ordered cofacial arrangement of the rings along
encircled by two [24]crown-8 rings, each containing a couple of imine the dumbbells as a result of cumulative p–p stacking interac-
bonds, affords a statistical mixture of a hetero[3]rotaxane along with tions11 has, not only allowed us to observe emergent rigid-rod
the two homo[3]rotaxanes, indicating that neither selectivity nor properties, but has also presented us with the opportunity to
cooperativity is operating during the assembly process.
investigate p-orbital communication along the length of appro-
priately designed oligorotaxanes.12 Here, we describe (i) the
If mechanically interlocked molecules,1 such as rotaxanes, are template-directed synthesis4 (Scheme 1) and characterisation,
going to find applications outside the sanctuary of the research both (ii) in solution and (iii) in the solid state, of two homo[3]-
laboratory, then their preparation has to begin with inexpensive rotaxanes—one carrying aromatic donors (D) and the other
starting materials and their production has to be highly efficient. acceptors (A) on their two ring components—which were then
Dynamic covalent chemistry2 (DCC) provides the platform from employed successfully (iv) in an acid-catalysed, thermodynami-
which it is possible to start meeting these two criteria. One3 of the cally controlled scrambling of the D and A rings to afford a
most efficient ways of synthesising a [2]rotaxane is to template4 hetero[3]rotaxane in a statistical mixture with the two parent
the clipping5 of a [24]crown-8 ring, formed from the reversible
condensation of 2,6-pyridine dicarboxaldehyde and tetraethylene
glycol bis(2-aminophenyl)ether, around a secondary dialkyl-
+
ammonium ion (–NH2 –) positioned in the middle of a pre-
formed dumbbell. The reaction, which is all but complete
(quantitative) in acetonitrile at room temperature inside five
minutes, has been extended to the template-directed synthesis4
of multiply interlocked rotaxanes,6 as well as to the use of other
diformyl derivatives.5 Recently, we have reported7 the dynamic
assembly of two series of oligorotaxanes in which repeating
+
–NH2 – cationic centres are separated along the rod sections
of dumbbells by either (i) paraxylylene8,9 (–CH2C6H4CH2–) or
(ii) tris-methylene10 (–CH2CH2CH2–) spacers. In the case of the
latter, the spacing of approximately 3.5 Å between the recognition
+
sites within the (–CH2NH2 CH2CH2–) repeating unit allowing
for the incorporation of p–p stacking interactions between
the aromatic residues on contiguous rings, leading to positive
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
IL 60208-3113, USA. E-mail: stoddart@northwestern.edu; Fax: +1 847 491 1009;
Scheme 1 (a) Template-directed synthesis of Ar[3]R2+ by the direct
Tel: +1 847 491 3793
mixing of 2D2+ with ArDA (purple) and BA. (b) Proposed assembly of Ar–
† Electronic supplementary information (ESI) available. CCDC 999555 and
Ar0[3]R2+ by the direct mixing of 2D2+ with ArDA (purple) and Ar0DA
999556. For ESI and crystallographic data in CIF or other electronic format see
(green), and BA.
DOI: 10.1039/c4cc03612c
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homo[3]rotaxanes, which (v) offers advantages over simply mixing
(Scheme 1b) stoichiometric amounts of the acyclic rotaxane
precursors with a view to forming a hetero[3]rotaxane directly.
The 4-methoxyphenyl- (MDA) and 3,5-difluorophenyl- (FDA)
substituted pyridine dialdehydes were chosen (ESI†) as precur-
sors for the D and A rings, respectively. The phenyl-substituted
pyridine dialdehyde (HDA) was also available (ESI†) for com-
parison studies. The three homo[3]rotaxanes H[3]R2+, M[3]R2+
and F[3]R2+ were prepared (Scheme 1a) using standard procedures,
starting from HDA, MDA, and FDA (all 2 equiv.), respectively, the
dumbbell 2D2+ (1 equiv.) and bis(2-aminophenyl)tetraethylene
glycol (BA, 2 equiv.) in CD3CN (8.0 mM) at room temperature.
The crude 1H NMR spectra of the reaction mixtures, recorded
after 5 min, revealed near-quantitative conversion of the starting
materials to the respective homo[3]rotaxanes, H[3]R2+, M[3]R2+ and
F[3]R2+, as evidenced by (i) the complete consumption of BA and
the disappearance of the aldehyde resonance at ca. 10.2 ppm
+
and (ii) the shift of the broad signal for the NH2 protons from
d 7.0 to 9.5 ppm, indicating their encirclement by the crown ether
rings. Single crystals of M[3]Rꢀ2PF6 and F[3]Rꢀ2PF6, suitable for
X-ray analysis, were grown by diffusing iPr2O into MeCN solutions.
The solid-state structures13 (Fig. 1) of both homo[3]rotaxanes show
Fig. 2 Partial 1H NMR spectra (500 MHz, CD3CN, 298 K) of (a) 2D2+ (b) BA
(c) MDA and (d) FDA. (e) Products of the direct mixing of 2 equiv. of BA with
1 equiv. each of 2D2+, MDA and FDA in CD3CN at 298 K at 3.0 mM.
control over the component stoichiometry, (ii) minimize the
concentration of free aldehyde and amine intermediates at any
giving time, and (iii) avoid the irreversible loss of viable compo-
nents for hetero[3]rotaxane formation.
Mixing M[3]Rꢀ2PF6 and F[3]Rꢀ2PF6 in a 1 : 1 molar ratio in
CD3CN (3.0 mM) at room temperature14 in the presence of
HPF6 (5 mol% per imine bond) facilitates slow imine exchange
over 48 h and leads (Scheme 2) to the formation of the
hetero[3]rotaxane M–F[3]Rꢀ2PF6. This strategy resulted in
exchange to form M–F[3]R2+ in equilibrium with M[3]R2+ and
F[3]R2+, as indicated by 1H NMR spectroscopy (Fig. 3) and mass
Fig. 1 Tubular representations of the solid-state structures of (a) M[3]R2+
and (b) F[3]R2+ showing the centroid-to-centroid distances of 4.6 and
3.8 Å, respectively, between the pyridyl units.
aromatic p–p stacking interactions between the two rings with spectrometry where three peaks at m/z 821.4757, 827.4466, and
average centroid-to-plane distances between the two pyridine units 824.4614, corresponding to M[3]R2+, F[3]R2+ and M–F[3]R2+,
of 3.4 and 3.5 Å, respectively, for M[3]R2+ and F[3]R2+. The 1H NMR respectively, were identified in the high resolution electrospray
spectroscopic data (ESI†) reveals that an attempt to prepare the ionisation (HR-ESI) mass spectrum. See Fig. S30 in the ESI.†
hetero[3]rotaxane M–F[3]R2+ in CD3CN (8.0 mM) from 2D2+ Similar results were obtained on mixing H[3]Rꢀ2PF6 and M[3]Rꢀ
(1 equiv.), BA (2 equiv.), MDA (1 equiv.) and FDA (1 equiv.) resulted
in the formation of a complex mixture of products. Product
selectivity improved when the reaction was repeated at lower
concentration (3.0 mM), as observed by 1H NMR spectroscopy
(Fig 2), as well as in the presence (Fig. S21, ESI†) of a catalytic
amount of HPF6. It became increasingly difficult, however, to
control the molar ratios of the starting materials at this lower
concentration. More importantly, byproducts were observed, both
in solution and in the form of precipitates—presumably kinetically
trapped oligomers—resulting in a mass loss of ca. 34%. This
situation encouraged us to focus on an alternative preparation of
the hetero[3]rotaxane by subjecting an equimolar mixture of the
two homo[3]rotaxanes to dynamic exchange, exploiting all the
attributes of DCC. By adopting this strategy, we impose (i) precise
Scheme 2 Dynamic component-swapping strategy. The acid-catalysed
equilibration of the two homo[3]rotaxanes M[3]R2+ and F[3]R2+ gives rise
to hetero[3]rotaxane M–F[3]R2+
.
9666 | Chem. Commun., 2014, 50, 9665--9668
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2PF6 to give H–M[3]Rꢀ2PF6, and H[3]Rꢀ2PF6 and F[3]Rꢀ2PF6 to
give H–F[3]Rꢀ2PF6. See Fig. S31 and S32 in the ESI.† The
1H NMR spectrum (Fig. 3b) of the equilibrated mixture
obtained by this approach revealed two sets of signals asso-
ciated with M[3]R2+ and F[3]R2+, in addition to the emergence
of an additional set of resonances arising from M–F[3]R2+. In
particular, in the region of chemical shift from 7.5 to 8.0 ppm,
four equal intensity signals were observed at 7.70, 7.75, 7.76
and 7.81 ppm for the pyridyl protons present in M[3]R2+,
M–F[3]R2+ (middle two resonances) and F[3]R2+, respectively.
The two peaks at 7.75 and 7.76 ppm, which occur approxi-
mately half way between those of M[3]R2+ and F[3]R2+, can be
assigned to M–F[3]R2+. Moreover, the fact that the ratio of
M[3]R2+, M–F[3]R2+ and F[3]R2+ is 1 : 2 : 1, based on the integra-
tion of these pyridyl proton resonances, is consistent with
statistical scrambling of the rings in a dynamic exchange
process. In order to verify the constitution of the hetero[3]-
rotaxane, all the imine bonds were reduced,3 locking the rings
into place around the dumbbells and allowing the products
to be isolated. Addition of a methanolic solution of NaBH4 to
Fig. 4 Partial 1H NMR spectra (500 MHz, CD3CN, 298 K) of (a) reduced
M[3]R2+, (b) reduced M–F[3]R2+ (notable components of M and F rings are
highlighted in purple and green, respectively), and (c) reduced F[3]R2+
.
the crude rotaxane mixture in CH2Cl2 at room temperature, of this control experiment, it follows that the D and A rings in
followed by purification by reverse-phase HPLC, resulted M–F[3]R2+ are not experiencing any stabilising aromatic p–p
(Fig. 4) in the isolation of the reduced products.
stacking interactions, i.e., the rotaxane-to-rotaxane transforma-
In order to establish the statistical generality of the dynamic tion is devoid of cooperativity and selectivity in both a kinetic
component-swapping strategy, we treated an equimolar mixture and thermodynamic sense. It remains to be established if self-
(3.0 mM) of H[3]Rꢀ2PF6 and its d5-phenyl analogue D[3]Rꢀ2PF6 sorting, driven by favourable D–A interactions,11 occurs as the
with a catalytic amount of HPF6 (5 mol% per imine bond), number of recognition sites and rings increases in extended
only to discover that HR-ESI mass spectrometric analysis of the hetero[n]rotaxanes. Such an outcome would be reminiscent9 of
reaction mixture reveals a 1 : 2 : 1 ratio of peaks at m/z 791.9658, the emergence15 of p-mediated positive cooperativity16 in the
794.4820 and 796.9972 for H[3]R2+, H–D[3]R2+ and D[3]R2+, analogous homo[n]rotaxanes.
respectively. See Fig. S33 in the ESI.† In light of the outcome
We have demonstrated a modular synthetic strategy for the
formation of mixed ring hetero[3]rotaxanes in a rotaxane-to-
rotaxane transformation. The acid-catalysed dynamic component-
swapping of two homo[3]rotaxanes, which was exemplified by
the formation of four different hetero[3]rotaxanes, could become
a versatile technique for the production of hetero[n]rotaxanes17
containing ordered, mixed ring components. Hetero[3]rotaxanes
assembled in this manner may also be looked upon as a new
emerging family of molecular torsional balances18 for investigating
p–p stacking interactions between donating and accepting
aromatic rings.
This research is part (Project 34-949) of the Joint Center of
Excellence in Integrated Nano-Systems (JCIN) at King Abdul-Aziz
City for Science and Technology (KACST) and Northwestern
University (NU). The authors would like to thank both KACST
and NU for their continued support of this research. P. R. M.
acknowledges support from the National Science Foundation
(NSF) (CHE-138107). A.-J. A. thanks the NSF for a Graduate
Research Fellowship (GRF) under Grant No. DGE-0824162.
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9668 | Chem. Commun., 2014, 50, 9665--9668
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