Rapid thermally assisted donor-acceptor catenation

Article abstract of DOI:10.1039/c2cc34427k

Charged donor-acceptor [2]catenanes containing cyclobis(paraquat-p- phenylene) as the ring component can be synthesised in yields of up to 88% in under one hour by heating two precursors in the presence of macrocyclic polyether templates in N,N-dimethylfo

Full text of DOI:10.1039/c2cc34427k

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Rapid thermally assisted donor–acceptor catenationw
Albert C. Fahrenbach,a Karel J. Hartlieb,a Chi-Hau Sue,a Carson J. Bruns,a Gokhan Barin,a
Subhadeep Basu,a Mark A. Olson,b Youssry Y. Botros,acd Abdulaziz Bagabas,ad
Nezar H. Khdaryad and J. Fraser Stoddart*a
Received 20th June 2012, Accepted 17th July 2012
DOI: 10.1039/c2cc34427k
Charged donor–acceptor [2]catenanes containing cyclobis(paraquat-
p-phenylene) as the ring component can be synthesised in yields of up
to 88% in under one hour by heating two precursors in the presence of
macrocyclic polyether templates in N,N-dimethylformamide at 80 8C.
Table 1 Thermally assisted synthesis of the [2]catenanes 5 & 6a–dꢀ4PF6
Catenanes,1 topologically nontrivial molecules containing two or
more mechanically interlocked rings, have been the focus of
attention now for half a century because of their aesthetic value,2
synthetic challenge, and more recently, potential application as
artificial molecular switches3 in molecular electronic devices,4 metal-
organic frameworks5 (MOFs), and polymers.6 Aside from the
statistical7 synthetic methods, which were employed initially, the
syntheses of catenanes have, for the most part, involved the delicate
interplay of various noncovalent bonding interactions involved
in template-directed protocols,8 relying on metal9 and anion10
templation, hydrogen-bonding,1c,11 donor–acceptor,1b,3a,8e,12
radical–radical13 and hydrophobic interactions.14
Yield (%) R = H/N3
1a/5a
88/–
In our own laboratories, charged donor–acceptor [2]catenanes
containing the tetracationic cyclophane, cyclobis(paraquat-p-
phenylene)15 (CBPQT4+), mechanically interlocked with a wide
range of macrocyclic polyethers have been produced employing a
template-directed strategy, often with the aid of ultra-high
pressures (12–15 kbars), using a clipping mechanism between
two precursors that involves the formation of the p-electron
deficient CBPQT4+ ring around p-electron rich donors – for
example, hydroquinone16 (HQ), 1,5-dioxynaphthalene17 (DNP),
and tetrathiafulvalene18 (TTF) units present in the macrocyclic
polyethers. The times taken to produce these mechanically
interlocked molecules (MIMs), however, are in the range of
days, and the yields obtained19 often leave much to be desired.
1b/5b & 6b
63/65
1c/5c
84/–
1d/5d & 6d
34/34
A one-step, heat-assisted catenation is outlined in Table 1. In one
specific instance, macrocyclic polyether 1a20 (63.6 mg, 0.1 mmol)
was mixed with a five-fold excess of 1,10-[1,4-phenylenebis-
(methylene)]bis(4,40-bipyridinium) bis(hexafluorophosphate)15a
2ꢀ2PF6 (354 mg, 0.5 mmol) and 1,4-bis(bromomethyl)benzene 3
(132 mg, 0.5 mmol) in DMF (5 mL) and heated with stirring at
80 1C for 1 h. Analytical high performance liquid chromato-
graphy (HPLC) on the reaction mixture reveals (Fig. 1) almost
complete conversion of 1a to the corresponding [2]catenane
5a4+. The solution was filtered to remove insoluble highly-charged
polymeric bipyridinium (BIPY2+) by-products and injected into
preparative HPLC for purification. The pure fractions collected
were subjected to a counterion exchange step to afford the
degenerate [2]catenane as its hexafluorophosphate salt 5aꢀ
4PF6 (152.5 mg, 0.09 mmol) in 88% isolated yield, with respect
to the macrocyclic polyether. The same protocol was applied
(see Fig. S7–S9, ESIw) to the macrocyclic polyethers 1b, 1c and
1d: the yields are reported in Table 1. Compounds 5a–d4+ were
a Department of Chemistry and Department of Materials Science and
Engineering, Northwestern University, 2145 Sheridan Road,
Evanston, IL 60208-3113, USA. E-mail: stoddart@northwestern.edu
b Department of Physical & Environmental Sciences, Texas A&M
University-Corpus Christi, 6300 Ocean Drive, Corpus Christi,
Texas 78412-5774, USA
c Intel Labs, Building RNB-6-61, 2200 Mission College Blvd,
Santa Clara, CA 95054-1549, USA
d King Abdulaziz City for Science and Technology (KACST),
P.O. Box 6086, Riyadh, 11442, Kingdom of Saudi Arabia
w Electronic supplementary information (ESI) available: 1H and 13C
NMR spectroscopy, mass spectrometry, analytical HPLC and X-ray
crystallography. CCDC 887694 and 887695. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
c2cc34427k
c
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reaction mechanism, which leads to the making of the mechanical
bond. The mechanism of formation (Scheme 1) of the [2]catenanes
involves the host–guest inclusion complexes [iC1a–d]3+
between the macrocyclic polyethers and the proposed half-closed
intermediate i3+. After formation of the acyclic oligomer i3+
,
inclusion of its BIPY2+ unit inside the cavity of the electron-rich
macrocyclic polyethers proceeds spontaneously. The host–guest
complex is stabilised by the [pꢀ ꢀ ꢀp] donor–acceptor and
[C–Hꢀ ꢀ ꢀO] interactions arising from the BIPY2+ unit of i3+
sandwiched between the two donor units in the macrocycle,
and, likely to some extent, by side-on interactions with the
1-methylene-4,40-bipyridin-1-ium unit. The donor–acceptor
interactions that transpire in these inclusion complexes lower
the activation energy of the final SN2 substitution leading to
catenation, to such an extent that the rate at which this
reaction proceeds is fast enough to compete successfully with
the formation of higher order acyclic oligomeric by-products.
This template effect was described in quantitative detail by
Ercolani21 who also observed that the rate of reaction is much
faster in the case of catenanes compared with pseudorotaxanes.
Fig. 1 Analytical HPLC traces for (a) the pure macrocyclic polyether
1a, (b) the pure dication 22+, (c) the pure 1,4-bis(bromomethyl)benzene
3 and (d) the crude reaction mixture after 1 h of heating in DMF at
80 1C. The peak intensities correspond to the optical absorbance
recoded at 300 nm. The asterisk indicates the presence of acyclic
oligomeric side products.
1
fully characterised (see Fig. S1–S4, ESIw) by H NMR spectro-
scopy and mass spectrometry, as well as by analytical HPLC. All
the spectroscopic and spectrometric data matches that reported
previously. Similar yields were observed when this methodology
was applied with 2-azido-1,4-bis(bromomethyl)benzene (4)
starting material.
Fig. 2 The X-ray crystal structures of (a) 6b4+ and (b) 6d4+. The
azido group does not interrupt the [C–Hꢀ ꢀ ꢀO] and [pꢀ ꢀ ꢀp] interactions
and explains why these catenanes are obtained in yields comparable to
The X-ray crystal structures of the azide-functionalised
[2]catenanes 6b4+ and 6d4+, which are shown in Fig. 2, reveal
that the azide group does not interrupt the noncovalent
bonding interactions – namely, [pꢀ ꢀ ꢀp] stacking and [C–Hꢀ ꢀ ꢀO]
interactions – typically observed1b,3b,17 in these types of donor–
acceptor [2]catenanes. This observation lends support to the
hypothesis that the presence of the azide does not disrupt
significantly the noncovalent bonding interactions which stabilise
the transition state geometry during the cyclisation step and
provides an explanation for why yields comparable to the
unfunctionalised catenanes can be obtained. For a detailed
discussion of the crystal structures, see the Fig. S13 (ESIw).
The fact that catenation can occur even in the face of elevated
temperatures can be understood from a qualitative evaluation
of the template effect and its ability to stabilise significantly the
transition state energy of the second SN2 substitution in the
those of their unfunctionalised counterparts 5b4+ and 5d4+
.
The co-polymerisation of the two acyclic starting materials,
namely 22+ and 3 or 4, still leads to the formation of poly- and
oligomeric by-products, the most soluble of which are observed
(Fig. 1) in the analytical HPLC traces, the reason why 5 equiv.
of these starting materials is necessary. We hypothesise that
the lower yields obtained for the catenanes incorporating
the macrocyclic polyether 1d, which incorporate two HQ
units within its constitution can be explained by the fact that
the HQ units do not stabilise the transition state geometry as
effectively as do the TTF and DNP units.
When considering the intermediate in the general case of
rotaxanes, namely a donor–acceptor heterodimer,21 the stability
of these intermolecular noncovalent bonding interactions is
Scheme 1 Proposed mechanism for the thermally assisted catenation. A pre-equilibrium exists between 1a–d and the intermediate i3+ forming a
pseudorotaxane template, before an SN2 reaction with the benzyl bromide and the pyridyl lone pair becomes accelerated on heating.
c
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much less when compared with its corresponding host–guest
complex, and so, the transition state energies for the final SN2
substitutions leading to rotaxanes are significantly less stabilised.
As a control experiment, we employed the same reaction conditions
in the presence of a polyether dumbbell containing a DNP unit,
and we were unable to observe (see Fig. S12, ESIw) the formation
of any of the [2]rotaxane. In this case, the formation of acyclic
oligomers clearly outcompetes the formation of the targeted
mechanically interlocked rotaxane, and so the reaction has to be
carried out at lower temperatures for longer periods of time in
order for it to proceed efficiently.
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,
which represent, respectively, the degenerate, switchable, and
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We thank Dr Turki S. Al-Saud and Dr Mohamed B.
Alfageeh at the King Abdullah City for Science and Technology
(KACST) in Saudi Arabia for their joint collaborative efforts.
A.C.F. and C.J.B. acknowledge support from an NSF Graduate
Research Fellowship. G.B. thanks the International Center
for Diffraction Data for the award of a 2012 Ludo Frevel
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Chem. Commun.