Crown-ether- and porphyrin-attached gel-phase resins in thermodynamically controlled rotaxane assembly

Article abstract of DOI:10.1039/b816115a

Results of gel-phase proton high resolution magic angle spinning (HR MAS) NMR spectroscopy are described for a systematic study of the reversible, thermodynamically-controlled assembly of surface-attached neutral rotaxanes and pseudorotaxanes based on a t

Full text of DOI:10.1039/b816115a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Crown-ether- and porphyrin-attached gel-phase resins in thermodynamically
controlled rotaxane assembly†
Kathleen M. Mullen,a Ken D. Johnstone,a Dilip Nath,a Nick Bampos,b Jeremy K. M. Sandersb and
Maxwell J. Gunter*a
Received 15th September 2008, Accepted 23rd October 2008
First published as an Advance Article on the web 25th November 2008
DOI: 10.1039/b816115a
Results of gel-phase proton high resolution magic angle spinning (HR MAS) NMR spectroscopy are
described for a systematic study of the reversible, thermodynamically-controlled assembly of
surface-attached neutral rotaxanes and pseudorotaxanes based on a three-component system
consisting of a naphthodiimide thread unit, a naphthalene crown shuttle, and metalloporphyrin
stoppers. Further to the previous systems based on an immobilised thread unit, we report here on the
alternative systems where firstly a crown shuttle unit, and secondly metalloporphyrin stopper units, are
attached to polystyrene beads, with the other two rotaxane components supplied in the surrounding
solution phases in each case. Variations in concentration, temperature, and the effects of the addition of
alkali metal salts are investigated. Within some limitations imposed by the technique itself, these results
confirm that for each of the single entities attached in turn to polystyrene beads, rotaxane formation
parallels that observed in the analogous solution-phase systems.
a dinaphtho crown ether DN38C10 1 to act as the shuttle, and a
Introduction
ruthenium carbonyl porphyrin 2a as the stoppering unit (Series I
In the evolution of nanotechnology towards the construction of
in Fig. 1). HR MAS spectroscopy allowed a real-time monitoring
working devices and machines1 through a supramolecular chemi-
of the equilibrating system, with both solid-attached and solution
cal approach,2 there is an increasing need for a full understanding
phase components clearly observable in the well-resolved proton
and knowledge of the structure and function of solid-attached
spectra. Perturbations to the system such as varying concentration,
supramolecular systems.3 This is especially critical for dynamic
temperature change, and addition of alkali metal salts could be
self-assembled systems where a direct correspondence between
monitored, and it was demonstrated that there is a close parallel
the solution-phase and solid-state behaviour is assumed, in the
between these tethered systems and their all-solution analogues.
absence of definitive structural characterisation methods.
We now extend this study of the three-component rotaxane
Our approach to addressing this deficiency has been to use gel-
systems to the alternative combinations, where each of the crown
phase high resolution magic angle spinning 1H NMR (HR MAS)
ether shuttle (Series II in Fig. 1), and then the porphyrin stopper
spectroscopy to study the equilibrium-controlled construction
(Series III), are immobilised on the beads, and the other two
of neutral rotaxanes tethered to gel-phase polystyrene resin
components in each case are supplied in the surrounding solution
supports, and to monitor their self-assembly from solution-phase
phases.
components.4–7 In this way, our aim is to establish a congruency
that can allow a direct comparison when the same components
are used in thermodynamically-controlled assembly processes at
Results and discussion
other surfaces and interfaces where more direct characterisation
Crown-tethered systems
methods are not available.
In our previous studies,6,7
a naphthodiimide-containing
In this case, it was expected that thermodynamic assembly of a
tethered rotaxane of the type indicated in Series II (Fig. 1) would
be achieved by exposing the crown-loaded beads to a solution of
the bis-pyridine terminated naphthalene diimide thread 3, with
subsequent addition of ruthenium(II) carbonyl 2a or rhodium(III)
iodide 2b porphyrin (Fig. 1 and Scheme 1). The resulting solid-
tethered rotaxane differs from the previously reported thread-
tethered rotaxane in the fact that the beads do not act as one
stoppering group for the rotaxane as it does in the Series I; in
Series II the beads act only to tether the crown, and stoppering
is effected by two ruthenium(II) carbonyl or rhodium(III) iodide
porphyrins (Fig. 1 and Scheme 1).
thread unit of a potential rotaxane was covalently attached to
a gel-phase polystyrene resin bead, which acts as a stopper
at one end; the other end is terminated in a pyridine group,
a potential ligand for a metal ion. For the thermodynamic
rotaxane assembly, these thread-tethered beads were swollen in
a solvent containing the other components of the rotaxane, viz.
aDepartment of Chemistry, School of Science and Technology, the University
of New England, Armidale, NSW 2351, Australia. E-mail: mgunter@une.
edu.au; Fax: +612 6773 3268; Tel: +612 6773 2767
bUniversity Chemical Laboratory, University of Cambridge, Lensfield Rd,
Cambridge, UK CB2 1EW. E-mail: nb10013@cam.ac.uk, jkms@cam.ac.uk;
Fax: +44 1223 336017; Tel: +44 1223 336411
The carboxylic acid crown ether 4 provided a suitable func-
tionality for bead attachment. Although the methyl ester 5 has
been shown to bind naphthodiimides more weakly than the
† Electronic supplementary information (ESI) available: Spectra of 6 and
HR MAS spectrum of 10 and 3 (1 page). See DOI: 10.1039/b816115a
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Fig. 1 Possible rotaxane structural types resulting from the three-component system consisting of naphthodiimide thread, crown ether shuttle, and
metalloporphyrin stopper units, with each one of the three components in turn tethered to a polymer resin, and with the other two components in the
surrounding solution phase.
dinaphtho crown 1,8,9 it has been successfully incorporated into
naphthodiimide-based [2]catenanes in reasonable yields,8 despite
the electron-withdrawing ester functionality which inherently
weakens the p-donor capacity of the crown macrocycle. It was
also shown to form interlocked supramolecules in solution
when attached covalently to diimide and porphyrin moieties.9
Thus, attachment of 4 to ArgoGel beads via a general ester
forming reaction with N-hydroxybenzotriazole (HOBT), 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide (EDC) and triethy-
lamine in chloroform successfully produced the crown-tethered
derivative 6.
The HR MAS proton spectrum of 6 swollen in CDCl3 (see ESI,
Fig. S1†) showed aromatic peaks at d 7.83 (Hi), d 7.27 (Hj), d
7.12(Hq), d 6.75 (Hk) and d 6.42 (Hr) indicating the successful
formation of 6 with a reasonable loading on the beads. However,
addition of the thread component 3 in solution did not shift the
crown aromatic peaks upfield as expected for a diimide-threaded
crown. Addition of the ruthenium carbonyl stopper 2a yielded
similar results, showing unshifted uncomplexed crown aromatic
peaks without their diimide-threaded counterparts (although the
coordinated pyridine resonances were shifted well upfield as
expected). This result suggests the monofunctional crown does
not bind significantly strongly with the naphthalene diimide and
is consistent with the low binding in the solution-phase rotaxane
analogue containing the methyl ester crown 5, naphthalene diimide
thread 3 and ruthenium carbonyl porphyrin stoppers 2a.9 Indeed,
it was apparent that an alternative mono-functional crown that
binds more strongly at room temperature with the naphthalene
diimide thread would be required to observe the solid-bound
crown rotaxane formation using HR MAS NMR.
as it has been shown that diimide binding is comparable for
this alternative linkage isomer in unsubstituted dinaphthocrown
ethers.10
The synthesis of 7 was achieved by the reaction of 1,5-bis-(2-(2-
(2-(2-p-toluenesulfonylethoxy)ethoxy)ethoxy)ethoxy)-naphtha-
lene with methyl 3,7-dihydroxynaphthalene-2-carboxylate in the
presence of potassium carbonate and caesium carbonate in
refluxing acetone. The caesium carbonate templates the crown for-
mation; indeed, in its absence the yield of 7 decreased dramatically
(from 41% to less than 5%).
Nevertheless, this crown was also shown to bind only weakly to
various naphthalenediimide thread units, including 3, both in the
presence and absence of porphyrin stopper units 2a. A weak broad
charge transfer band was observed (~460 nm) in a mixture of 7 and
3, and a binding constant of 13.0 3.0 M-1 was calculated from an
NMR titration experiment. On the other hand, it has been shown
that the binding of both naphthodiimides and pyromellitimides
to 1,5-DN38C10 1 and its 3,7-linkage isomer is remarkably
enhanced by the presence of Li+ or Na+ ions in solution, and
the solid state and solution structures of a series of pseudoro-
taxanes involving these components have been reported in some
detail.10,11
Thus the addition of lithium iodide to the mixture of mono-
functional crown 7, naphthalene diimide thread 3 and ruthenium
porphyrin stoppers 2a similarly provided a high yield of the fully
assembled rotaxane 8a in solution. The resulting NMR spectra
showed significant upfield shifts for both the aromatic crown from
7.71 (Hp), 7.63 (Hi), 7.55 (Hv), 7.51 (Hm), 7.22 (Hn), 7.09 (Hw), 6.96
(Hj), 6.84 (Hl,o), 6.57 (Hx) and 6.42 (Hk) ppm for 7 in 2% CD3OD,
98% CDCl3 with a LiI excess to 6.84 (Hi), 6.67 (Hv), 6.52 (Hj,m),
6.33 (Hw,n ), 6.12 (Hk) and d 5.98 (Hx) ppm for the mixture of 7,
3, 2a and LiI in the same solvent mixture. Likewise the diimide
proton signal shifted from d 8.7 in 3 (typical of an unbound
naphthalene diimide) to d 8.34 (Ha) in the rotaxane mixture,
indicative of an extremely strong and almost quantitative room
temperature threading interaction. The sharp nature of the peaks
indicate that the crown–diimide interaction is in slow-exchange on
Thus an alternative dinaphtho-crown 7 was synthesised. Al-
though the p-electron donicity of this crown is still compro-
mised by the electron-withdrawing ester group, the effect is
counterbalanced by the replacement of the phenyl group of
5 with a naphtho group. The slightly larger binding cavity
resulting from the alternative 3,7-linkage isomer 7 compared to
the more usual 1,5-pattern of 1 was not considered problematic,
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Scheme 1
the NMR time-scale, confirming that the ruthenium porphyrins
successfully stopper the system, as we have observed previously in
related systems.5,6,9
The ArgoGel-OH bead-tethered analogue 10 of this dinaphtho-
38-crown-10 was synthesised by first hydrolysing the methyl ester
7, by refluxing in a mixture of 2 M sodium hydroxide and glycerol.
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Fig. 2 HR MAS NMR spectra of 10 (top) and a mixture of 10 and 3 (bottom). Colours and labelling follow those described in Scheme 1.
The resulting carboxylic acid functionalised crown 9 was tethered
to the ArgoGel-OH beads using the general ester forming reaction
with HOBT, EDC and triethylamine in chloroform.
The crown peaks were again shifted only slightly upfield by
between 0.01 and 0.07 ppm (8.00, 7.68, 7.33, 7.26, 7.09, 7.02, 6.91,
6.45 ppm).
HR MAS NMR analysis of the resulting beads 10 showed a
good loading of the crown with expected crown peaks at 8.08, 7.72,
7.33, 7.29, 7.10, 7.03, 6.94 and 6.46 ppm in 2%MeOD/CDCl3. In
this case the addition of a solution of the bis-pyridyl-substituted
diimide thread 3 resulted in small upfield shifts in the crown proton
positions to 8.01, 7.65, 7.32, 7.25, 7.07, 7.01, 6.88 and 6.43 ppm
(Fig. 2).
Addition of LiI also resulted in only minimal shifts in the NMR
spectrum, although some changes in peak intensity are evident
(see Experimental section). Nevertheless it is clear that the extent
of the LiI effect is far less than that observed in solution.
Porphyrin-tethered systems
The third possible variation on the singly-tethered, double
solution-phase rotaxanes of the Series III illustrated in Fig. 1
required the attachment of the porphyrin stopper unit to the
gel-phase beads, with the crown and thread components in the
surrounding swelling solvent. In this case, however, since the
porphyrin acts as a stopper at each end of a di-functional
thread unit such as 3, there are several possible outcomes for the
assembled rotaxanes (these are discussed in detail in the ESI†).
For the Type I structure, shown in Fig. 4 which corresponds to
the Series III structure cartooned in Fig. 1, the bis-pyridyl diimide
thread 3 in the solution phase coordinates to two adjacent tethered
porphyrin molecules on a single bead, and the crown completes
the rotaxane formation in a fully reversible thermodynamically
controlled assembly. It should be noted that this structure may
in fact be formally termed a catenane or pseudo-catenane, as
the connection between the thread and the tethered porphyrins
creates a pseudo-macrocycle. However, a variety of alternative
structures are also possible (Types II, III, IV, V and VI, see
ESI†).
The addition of LiI resulted in further small upfield shifts in the
crown and diimide peaks. The diimide proton peak shifted from
8.71 to 8.69 ppm indicating a slight increase in its binding to the
crown; however, such a minimal shift can be accounted for simply
as a result of the excess thread 3 added to the solution, rather than
enhanced binding caused by the Li+ ions. In any event, it is clear
that the extent of these shifts is less than that seen in analogous
solution studies.
An increased concentration of diimide thread 3 and LiI resulted
in a further upfield shift and broadening of the crown resonance,
as expected for a system in fast exchange. At lower temperatures
(263 K) the resonances associated with the tethered crown shifted
further upfield again, with the most substantial shifts being those
for proton k, moving from 6.30 to 6.15, proton l from 6.90 to
6.74 and proton o from 7.08 to 6.92 ppm (see ESI, Fig. S2†).
This clearly indicates that pseudorotaxane formation is more
favoured at lower temperatures, a result consistent with solutions
studies and analogous bead experiments on the tethered diimide
thread.
Differentiating between these species using HR MAS NMR
is not trivial (see ESI†), and to circumvent these difficulties, the
mono-stoppered mono-pyridine diimide thread 117 (Scheme 2)
was used as the thread component. This can result in only one
type of rotaxane, allowing a clearer understanding of the factors
influencing the assembly of rotaxanes using porphyrin tethered
beads.
The tethered porphyrin components that were used were the
free base, zinc, rhuthenium(II) carbonyl, and rhodium(III) iodide
derivatives of the meso-tritolyl porphyrin attached to the beads
though a meta-connected long chain sebacoyl diester, 12a–d.
Stoppering of this crown-tethered pseudorotaxane was achieved
with the addition to beads 10 of a mixture containing diimide
thread 3 with two equivalents of either ruthenium carbonyl
porphyrin 2a or rhodium iodide porphyrin 2b. The addition of
either of these mixtures resulted in typically large upfield shifts of
the coordinated terminal pyridine proton signals (for the mixture
containing 2b, these appear at 6.66, 5.31, 2.16 and 1.74 ppm).
Only one set of peaks for the crown and diimide protons could be
seen in the 1H HR MAS spectrum indicating that, as in solution,
this system is still in fast exchange, due to the inherently weak
interaction between the diimide and crown (Fig. 3).
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Fig. 3 HR MAS NMR spectra obtained with mixtures of tethered crown 10, porphyrin 2b and diimide thread 3 (top), and the same mixture plus LiI
(bottom). All spectra recorded in 2%MeOD/CDCl3. Small residual unlabelled peaks are due to uncoordinated pyridine resonances due to a less than
stoichiometric amount of added porphyrin stopper.
Fig. 4 A possible rotaxane structural type resulting from metalloporphyrin-tethered beads, with thread 3 and shuttle 1 units in the surrounding solution
phase. Other possible structures are illustrated in the ESI†.
The free base tethered beads 13a were produced by an EDC
condensation between mono-sebacoyloxy tetratolyl porphyrin 12a
and ArgoGel polymer beads. The HR MAS spectrum of 13a was
of good quality, showing good signal-to-noise ratio. Shifts for the
porphyrin protons were as expected with the b and b¢ protons
appearing at 8.82 ppm and the other porphyrin protons at 8.04
(a, a¢) and 7.51 ppm (b, b¢). From COSY spectra, protons (c–f)
could be easily identified as having chemical shifts of 8.03, 7.92,
7.70, 7.51 ppm (Fig. 5).
Zinc was inserted directly into the free-base tethered beads,
resulting in the disappearance, in the resultant beads 13b, of
the free base NH proton peak at -2.84 ppm that was present
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Scheme 2 Assembly of rhodium porphyrin tethered rotaxane 14, showing the non-systematic aromatic labelling used for the NMR descriptions.
Fig. 5 Partial HR MAS NMR spectra of porphyrin tethered beads 13a–d. Peaks marked s and x are due to solvent, and residual bead peaks incompletely
filtered by the CPMG sequence (see Experimental).
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in the HR MAS spectrum of 13a. However, despite having
the identical porphyrin loading as the free base beads 13a, the
signal-to-noise ratio in the HR MAS NMR spectrum is reduced.
This indicates that metallation of the porphyrin impacts on the
quality of the NMR spectrum, and that spectroscopic quality of
metalloporphyrin beads is not necessarily indicative of loading.
complication due to carboxylic acid coordination to the metal ion.
Although the beads themselves are highly coloured in both cases,
indicative of a good loading, the ruthenium derivative showed
poorer quality HR MAS spectra than those of the rhodium or
free base, and it is assumed that the low signal to noise ratio is
again an inherent function of the ruthenium porphyrin, rather
than due to poor loading (see Experimental Section).
Thus, because of the inherent ligand lability of the zinc
derivatives, and the poor spectroscopic quality of the ruthenium
derivative, the rhodium derivative 13d was used for the self-
assembly study. In its HR MAS NMR spectrum, the b,b¢
porphyrin protons appeared in the typical position of 8.83 ppm.
The porphyrin protons a and a¢ had split into two broad doublets at
8.06 and 7.97 ppm, due the facial differentiation of the porphyrin
imposed by the iodide ligand. The porphyrin protons b and b¢, and
the aromatic protons (c–f) showed characteristic chemical shifts
of 7.49 and 8.06, 7.97, 7.69 respectively. Residual bound pyridine
(from the synthetic procedure) was also apparent in the NMR
spectrum at 6.05, 5.12 and 1.99 ppm.‡
A combined equimolar solution of diimide thread 11 and
crown ether 1 added in large excess to the beads resulted in
displacement of the coordinated pyridine, and formation of the
rotaxane (Scheme 2), as evident by the appearance of bound
pyridine resonances of the mono-stoppered thread unit 11 at 6.65,
5.23, 2.16 and 1.74 ppm (Fig. 6). Counterpart unbound pyridine
peaks were also evident at 9.19, 8.71, 8.24, 7.31 ppm due to the
excess diimide thread added.§
The assembled rotaxane 14 showed slow exchange on the NMR
chemical shift timescale as evident by the presence of both bound
(8.19 ppm, ain) and unbound (8.63 ppm, aout) diimide proton
peaks as well as bound (6.02, 6.57 and 6.76 ppm) and unbound
(6.45, 7.14, and 7.71 ppm) crown peaks (k,j,i) (Fig. 6). This
‡ The coordinated pyridine was not easily removed, even by extensive
washing of the beads with dilute HCl. Washing with neat trifluoroacetic
acid (TFA) successfully removed the pyridine, but it also caused some
decomposition of the porphyrin, either by partial demetallation of the
porphyrin or ligand exchange of the iodide anion for trifluoroacetate.
Similar effects have been seen in solution studies with associated rhodium
iodide porphyrins when using TFA in large excesses.
§ Application of the 2K CPMG pulse sequences during the spectroscopic
acquisition significantly decreases the intensity of these tethered pyridine
peaks, an effect seen previously for the diimide tethered beads, and
discussed in the Experimental Section.
Since insertion of ruthenium(II) and rhodium(III) into por-
phyrins is generally accompanied by the production of black
insoluble by-products which would be difficult to separate from the
polymer beads, the metallo-derivatives 13c and 13d were obtained
by direct attachment of the pre-formed metallated porphyrins 12c
and 12d respectively, using identical concentration conditions as
for the free base derivatives, but in the presence of 1 equivalent
of pyridine per mole of Ru or Rh porphyrin to obviate any
Fig. 6 Partial HR MAS NMR spectrum of rhodium porphyrin beads 13d with an addition of an excess equimolar mixture of diimide 11 and crown 1.
Colours and numbers refer to Scheme 2. Bound crown, diimide and pyridine peaks associated with rotaxane 13 are labelled jin, ain, etc, while unbound
are iout, aout etc. Unlabelled peaks are due to the aromatic stopper groups.
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parallels the solution-phase behaviour of these self-assembled
thermodynamically controlled systems where addition of rhodium
or ruthenium porphyrin to an equimolar mixture of crown and
diimide in solution results in a change from fast to slow exchange
on the NMR chemical shift timescale and gives a similar pattern
in the NMR spectrum to that seen here.
evaporated. The resulting residue was taken up in chloroform and
water. The organic layer was separated, dried (MgSO4), filtered
and the solvent removed. The residue was subjected to column
chromatography (SiO2: 30 : 69 : 1 CHCl3 : Et2O : MeOH) to yield
7 (579 mg, 41%); m/z (EI-MS) [M]+ 694.2974 C38H46O12 (calc.
694.2989); 1H NMR (300 MHz, CDCl3) d 8.08 (1H, s, Ar-H), 7.77
(2H, dd, Ar-H), 7.35 (1H, d, Ar-H), 7.18 (2H, dd, Ar-H), 7.08
(1H, dd, Ar-H), 6.97 (2H, m, Ar-H), 6.52 (2H, dd, Ar-H), 4.17
(2H, m, OCH2), 4.04 (6H, m, OCH2), 3.93 (8H, m, OCH2), 3.93
(3H, s, CH3), 3.75 (16H, m, OCH2).
Limitations of the technique
Although this HR MAS technique produces reliable and com-
prehensive information about the dynamics of these equilibrating
systems, it has some limitations. An impediment to the general ap-
plicability of the technique is the lack of precise quantitation, both
in sample preparation, and in spectroscopic integration. There
are practical difficulties in determining the bead loading, and the
precise quantity of beads contained in each particular sample after
it is loaded into the rotor. Although the concentrations of each
of the solution-phase components can be easily measured, their
relationship to the accessible loaded sites on the beads is not easily
established. This is especially exacerbated when additional solid
salts such as LiI or NaI are added in excess to the solution phase.
These issues are discussed in the Experimental Section for some
specific cases and more generally.
1-(2,6)-(3-Carboxynaphthalena)-15(1,5)-naphthalena-2,5,8,11,
14,16,19,22,25,28-decaoxacyclo-octacosaphane 9. The ester 7
(570 mg) was dissolved in a mixture of 2 M NaOH (20 mL)
◦
and glycerol (20 mL) and heated to 120 C and stirred for 24 h.
The mixture was cooled to room temperature, neutralised with
HCl (5 M aq, 15 mL) and extracted with DCM (3 ¥ 50 mL).
The organic extracts were combined, washed with brine (50 mL),
dried over MgSO4 and evaporated to give a yellow oil. This was
recrystallised from hexane/DCM to give 9 as an off-white solid
◦
(477 mg, 85%), mp 133–135 C; m/z (EI-MS) [M]+ 680.2830
1
C37H44O12 (calc. 680.2833); H NMR (300 MHz, CDCl3) d 8.37
(1H, s, Ar-H), 7.74 (1H, dd, Ar-H), 7.67 (1H, dd, Ar-H), 7.40 (1H,
d, Ar-H), 7.22 (1H, dd, Ar-H), 7.16 (2H, dd, Ar-H), 6.95 (1H, m,
Ar-H), 6.74 (1H, m, Ar-H), 6.57 (1H, dd, Ar-H), 6.50 (1H, dd,
Ar-H), 4.07 (6H, m, OCH2), 3.93 (10H, m, OCH2), 3.75 (16H, m,
OCH2); 13C NMR (75 MHz, CDCl3) d 165.8, 156.2, 154.2, 152.0,
133.9, 131.7, 129.2, 127.8, 126.5, 125.0, 124.9, 122.5, 118.7, 114.5,
114.4, 108.6, 107.8, 105.3, 71.3, 71.1, 70.9, 70.8, 70.7, 70.5, 69.8,
69.7, 69.5, 69.1, 68.9, 67.8, 67.7.
Experimental
General bead attachment procedure
ArgoGel-OH or TentaGel-OH beads (50 mg) and monocarboxylic
acid component(s) (300 mmol) were stirred at room temperature
under nitrogen with CHCl3 (5 mL). Triethylamine (62 mL,
450 mmol) was added to the mixture via a syringe, followed by
HOBT (81 mg, 600 mmol) and EDC (115 mg, 600 mmol). The
mixture was heated to 50 ◦C and stirred for 7 days under N2. The
beads were filtered and washed successively with CHCl3 (5 mL),
acetone (5 mL), water (5 mL), 2 M HCl(aq) (5 mL), sat. sodium
bicarbonate (5 mL), water (5 mL) acetone (5 mL), petroleum spirit
(5 mL) and CHCl3 (5 mL). The beads were then dried under high
vacuum.
ArgoGelTM-OH bound 1-(2,6)-(3-carboxynaphthalena)-15-(1,5)-
naphthalena-2,5,8,11,14,16,19,22,25,28-decaoxacyclo-octacosaph-
ane 10. The general bead attachment procedure was followed
reacting the crown carboxylic acid 7 (204 mg, 300 mmol) with
ArgoGel-OH beads (100 mg). The resulting component-attached
beads 10 were coloured off-white. HR MAS 1H NMR (400 MHz,
CDCl3) d 8.06 (1H, s, Ar-H), 7.71 (2H, dd, Ar-H), 7.28 (1H, d, Ar-
H), 7.12 (2H, dd, Ar-H), 7.04 (1H, dd, Ar-H), 6.92 (2H, m, Ar-H),
6.42 (2H, dd, Ar-H), 4.44 (2H, m, OCH2), 4.15 (2H, m, OCH2),
3.42–3.99 (28H, m, OCH2). Identical results were obtained using
TentaGel-OH beads in place of ArgoGel-OH.
ArgoGelTM-OH bound 1-(1,5)-naphthalena-15-(1,3)-(5-carboxy-
benzena)-2,5,8,11,14,16,19,22,25,28-decaoxacyclooctacosaphane
(6). The general bead attachment procedure was followed re-
acting 1-(1,5)-naphthalena-15-(1,3)-(5-carboxybenzena)-2,5,8,11,
14,16,19,22,25,28-decaoxacyclo-octacosaphane 49 (189 mg,
300 mmol) with ArgoGel-OH beads (20 mg). The resulting
5-[3-(9-Carboxynonionyloxy)phenyl]10,15,20-tris-(p-tolyl)por-
phyrin 12a. Sebacic acid (752 mg, 3.72 mmol), HOBT (200 mg,
1.5 mmol), EDC (285 mg, 1.5 mmol) and NEt3 (150 mg, 1.5 mmol)
were dissolved in a dry 1 : 1 chloroform/THF mix (50 mL).
5-(m-Hydroxyphenyl)10,15,20-tris-(p-tolyl)porphyrin9 (500 mg,
743 mmol) in chloroform (100 mL) was added dropwise over
45 min. The solution was stirred at room temperature under
nitrogen for 2 days. The solvent was evaporated and the residue
taken up in 2% HCl/CHCl3. The organic layer was separated
and washed with sodium bicarbonate solution (sat, aq) followed
by water, then dried with Na2SO4 and evaporated. The resulting
residue was subjected to column chromatography (SiO2: DCM to
2% MeOH/DCM) to yield 12a as a purple crystalline solid which
was recrystallised from methanol (545 mg, 85.6%), mp 248–250 ◦C;
m/z (ES-MS) [M + H]+ 857.4035 C57H53N4O4 (calc. 857.4066);
1H NMR (300 MHz, CDCl3) d 8.90 (8H, d, b-H), 8.11 (6H, d,
Ar-H), 8.11 (1H, dd, Ar-H), 7.97 (1H, s, Ar-H), 7.75 (1H, dd,
1
component-attached beads 6 were coloured white. HR MAS H
NMR (400 MHz, CDCl3) d 7.82 (2H, dd, Ar-H), 7.28 (2H, dd, Ar-
H), 7.12 (2H, d, Ar-H), 6.75 (2H, dd, Ar-H), 6.42 (1H, t, Ar-H),
4.70 (2H, m, OCH2), 4.42 (2H, m, OCH2), 4.23 (4H, m, OCH2),
3.96 (4H, m, OCH2), 3.45–3.85 (20 H, m, OCH2).
1-(2,6)-(3-Carbomethoxynaphthalena)-15-(1,5)-naphthalena-2,
5,8,11,14,16,19,22,25,28-decaoxacyclo-octacosaphane 7. 1,5-
Bis-(2-(2-(2-(2-p-toluenesulfonylethoxy)ethoxy)ethoxy)ethoxy)-
naphthalene (1.62 g, 1.97 mmol) and methyl-3,7-dihydroxy-2-
naphthoate (0.43 g, 1.97 mmol) in dry acetone (50 mL) was added
dropwise over 3 h to a refluxing suspension of K2CO3 (2.0 g,
14.4 mmol) and Cs2CO3 (0.64 g, 1.97 mmol) in acetone (150 mL).
The reaction was stirred at reflux under nitrogen for 3 days. The
mixture was cooled to room temperature, filtered and the solvent
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Ar-H), 7.56 (6H, d, Ar-H), 7.56 (1H, dd, Ar-H), 2.67 (2H, t,
CH2), 2.28 (2H, t, CH2), 1.80 (2H, q, CH2), 1.60 (2H, q, CH2),
1.45 (4H, m, CH2), 1.29 (4H, m, CH2), -2.74 (2H, broad-s, NH);
13C NMR (75 MHz, CDCl3) d 172.4, 149.3, 143.6, 139.2, 137.4,
134.5, 132.1, 131.0, 127.9, 127.4, 120.9, 120.3, 34.5, 33.4, 29.0,
28.9, 24.9, 24.6, 21.5; UV (lnm (e M-1 cm-1), CH2Cl2) 419 (4.13 ¥
105), 516 (1.76 ¥ 104), 551 (9.59 ¥ 103), 590 (6.19 ¥ 103), 646
(6.25 ¥ 103).
ArgoGelTM-OH tethered free base 5-(3-(9-carboxy-nonionyloxy)-
phenyl)-10,15,20-tris-(p-tolyl)porphyrin 13a. The general bead
attachment procedure was followed using free base 5-(3-(9-
carboxy-nonionyloxy)-phenyl)-10,15,20-tris-(p-tolyl)porphyrin
(260 mg, 304 mmol) and ArgoGel-OH beads (40 mg). The resulting
purple coloured beads were then dried under vacuum. HR MAS
1H NMR (400 MHz, CDCl3) d 8.82 (8H, dd, b-H), 8.04 (7H,
d, Ar-H), 7.92 (1H, s, Ar-H), 7.70 (1H, s, Ar-H), 7.51 (7H, s,
Ar-H), 2.81 (2H, m, CH2), 2.63 (2H, m, CH2), 2.19 (4H, m,
CH2), 1.71 (2H, m, CH2), 1.54 (2H, m, CH2), 1.24 (8H, m,
CH2).
beads were filtered and washed successively with CHCl3 (5.0 mL),
acetone (5.0 mL), 2 M HCl (5.0 mL), water (5.0 mL), sodium
bicarbonate (sat, aq, 5.0 mL), water (5.0 mL), acetone (5.0 mL),
petroleum spirit (5.0 mL) and CHCl3 (5.0 mL). The resulting red
coloured beads were then dried under vacuum. HR MAS 1H NMR
(400 MHz, CDCl3) d 8.65 (8H, d, b-H), 8.1 (1H, s, Ar-H), 8.0 (6H,
d, Ar-H), 7.7 (1H, dd, Ar-H), 7.5 (1H, dd, Ar-H), 7.42 (6H, d, Ar-
H), 7.42 (1H, dd, Ar-H), 2.25 (4H, m, CH2), 1.54 (4H, m, CH2),
1.23 (8H, m, CH2).
5-(3-(9-Carboxy-nonionyloxy)-phenyl)-10,15,20-tris-(p-tolyl)-
porphyrinatorhodium(III) iodide 12d. Rhodium(III) iodide was
inserted into 5-(3-(9-carboxy-nonionyloxy)-phenyl)-10,15,20-tris-
(p-tolyl)porphyrin 12a by a variation on published procedures.13
A mixture of free base porphyrin 12a (320 mg, 0.35 mmol),
anhydrous sodium acetate (284 mg, 3.8 mmol) and [Rh(CO)2Cl]2
(180 mg, 0.45 mmol) was evacuated under high vacuum and then
purged with N2. Freshly distilled, dry CHCl3 (100 mL) was then
cannulated in and the mixture was stirred under N2 for 4 hours.
Solid I2 (270mg, 1.1 mmol) was then added and the mixture
was stirred overnight. The insoluble material was removed by
filtration and the remaining solution was washed with sat. KI
(100 mL) and H2O (3 ¥ 25 mL). The organic layer was dried
over Na2SO4 and the solvent removed in vacuo.then subjected to
column chromatography (SiO2: DCM to 2% MeOH/DCM) and
final crystallisation from CH2Cl2/MeOH to afford 12d as a deep
ArgoGelTM-OH tethered zinc 5-(3-(9-carboxy-nonionyloxy)-
phenyl)10,15,20-tris-(p-tolyl)porphyrin 13b. Zinc was inserted
into the free base porphyrin beads 13a by gently stirring the
beads in a solution of DCM followed by addition of a solution of
methanol saturated with zinc acetate. The mixture was stirred at
room temperature overnight before filtering the beads. The beads
were then washed with water (3 ¥ 10 mL) to remove any excess
zinc acetate, acetone (5 mL) and CHCl3 (5 mL). The resulting
purple coloured beads were then dried under vacuum. HR MAS
1H NMR (400 MHz, CDCl3) d 8.87 (8H, d, b-H), 8.03 (7H, d, Ar-
H), 7.90 (1H, dd, Ar-H), 7.50 (7H, d, Ar-H), 2.83 (2H, m, CH2),
2.65 (2H, m, CH2), 2.18 (4H, m, CH2), 1.70 (2H, m, CH2), 1.55
(2H, m, CH2), 1.23 (8H, m, CH2).
◦
red solid (282 mg, 78%) mp 163–165 C; m/z (ESI-MS) [M]+
957.2851 C57H50N4O496Rh (calc. 957.2887);1H NMR (300 MHz,
CDCl3) d 8.89 (8H, s, b, b¢), 8.06–8.10 (7H, m, a, a¢, j), 7.91 (1H, s,
i), 7.74 (1H, br t, k), 7.58 (6H, d, J 9, b, b¢), 7.46 (1H, br d, l),
2.56 (2H, m, CH2), 1.96 (2H, m, CH2), 1.76 (2H, m, CH2), 1.50
(2H, m, CH2), 1.38 (2H, m, CH2), 1.29 (2H, m, CH2), 1.11 (2H,
m, CH2), 0.98 (2H, m, CH2), 0.86 (2H, m, CH2), -0.76 (1H, br s,
COOH); 13C NMR (75 MHz, CDCl3) d 178.8, 178.4, 172.6, 143.7,
143.4, 143.2, 139.0, 137.4, 134.5, 133.7, 132.9, 132.6, 132.3, 131.2,
128.0, 122.6, 121.0, 120.8, 53.5, 46.9, 34.4, 33.5, 33.1, 28.8, 24.9,
24.1, 22.1; UV (lnm (e M-1cm-1), CH2Cl2) 421 (1.71 ¥ 105), 531
(2.11 ¥ 104), 568 (7.76 ¥ 103).
Ruthenium carbonyl 5-(3-(9-carboxynonionyloxy)phenyl)-10,
15,20-tris-(p-tolyl)porphyrin 12c. Ruthenium carbonyl was in-
serted into 5-[3-(9-carboxynonionyloxy)phenyl]10,15,20-tris-[p-
tolyl] porphyrin (500 mg) using a general procedure,12 then
subjected to column chromatography (SiO2: DCM to 2%
MeOH/DCM) to afford 553 mg (96%) of the deep red solid
◦
product, mp >320 C; m/z (ES-MS) [M + Na]+ 1007.274
ArgoGelTM-OH tethered 5-(3-(9-carboxy-nonionyloxy)-phenyl)-
10,15,20-tris-(p-tolyl)porphyrinatorhodium(III) iodide 13d. Argo-
Gel-OH beads (50 mg), rhodium iodide 5-(3-(9-carboxynonio-
nyloxy)-phenyl)10,15,20-tris-(p-tolyl)porphyrin 12d (283 mg,
260 mmol) and pyridine (60 mg, 810 mmol) were stirred at room
temperature under nitrogen with CHCl3 (5 mL). Triethylamine
(61 mL, 456 mmol) was added to the mixture via a syringe, followed
by HOBT (78 mg, 644 mmol) and EDC (111 mg, 644 mmol). The
mixture was heated to 50 ◦C and stirred for 7 days. The beads
were filtered and washed successively with CHCl3 (5 mL), acetone
(5 mL), 2 M HCl (5 mL), water (5 mL), sodium bicarbonate
(sat aq, 5 mL), water (5 mL), acetone (5 mL), pet. spirits (5 mL)
and CHCl3 (5 mL). The resulting red coloured beads were then
dried under water pump vacuum. HR MAS 1H NMR (400 MHz,
CDCl3) d 8.83 (8H, d, b-H), 8.06 (5H, d, Ar-H), 7.97 (4H, dd,
Ar-H), 7.69 (1H, dd, Ar-H), 7.49 (6H, d, Ar-H), 2.29 (4H, m, CH2),
1.56 (4H, m, CH2), 1.23 (8H, m, CH2). This compound was also
attached to TentaGel resin beads giving a similar HR MAS NMR
spectrum.
C58H50N4O5Ru1Na1 (calc. 1007.272); 1H NMR (300 MHz, CDCl3)
d 8.64–8.74 (8H, broad-m, b-H), 7.92–8.09 (6H, broad-m, Ar-H),
7.92–8.09 (1H, broad-s, Ar-H), 7.92–8.09 (1H, broad-dd, Ar-H),
7.66 (1H, broad-dd, Ar-H), 7.42–7.55 (6H, broad-m, Ar-H), 7.55
(1H, broad-dd, Ar-H), 1.3–3.0 (16H, broad-m, CH2); 13C NMR
(75 MHz, CDCl3) d 144.3, 144.0, 143.8, 139.7, 137.0, 136.8, 134.2,
133.9, 132.0, 131.7, 131.4, 127.3, 127.0, 121.4, 34.4, 29.7, 29.3,
29.0, 24.9, 21.5; UV (lnm (e M-1 cm-1), CH2Cl2) 412 (1.70 ¥ 105),
532 (1.50 ¥ 104), 565 (4.05 ¥ 103).
ArgoGelTM-OH tethered ruthenium carbonyl 5-(3-(9-carboxy-
nonionyloxy)phenyl)-10,15,20-tris-(p-tolyl)porphyrin 13c. Argo-
Gel-OH beads (60 mg), ruthenium carbonyl 5-(3-(9-carboxyno-
nionyloxy)phenyl)-10,15,20-tris-(p-tolyl)porphyrin 12c (88.6 mg,
90 mmol) and pyridine (21 mg, 270 mmol) were stirred at room
temperature under nitrogen with CHCl3 (2.0 mL). Triethylamine
(21 mL, 152 mmol) was added to the mixture via a syringe, followed
by HOBT (27 mg, 200 mmol) and EDC (38 mg, 200 mmol).
The mixture was heated to 50 ◦C and stirred for 6 days. The
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Gel-phase HR MAS measurements
which itself may be caused by intra-bead substituent aggregation
(metallo-porphyrins including zinc porphyrins are known to
aggregate in solution). This may introduce a paradoxical situation,
in that a higher loading could lead to a decrease in the signal to
noise ratio through more aggregation.
General experimental protocols for, and limitations of, the HR
MAS measurements are as reported previously.7 More specific
details for sample preparations and spectroscopic interpretations
are given here.
For the samples relating to Fig. 2 and 3, a solution of diimide
thread 3 in 2%MeOD/CDCl3 and excess solid LiI was sonicated
for several minutes to solubilise the lithium iodide and then this
solution was added to the crown tethered beads 10 and the HR
MAS NMR spectrum recorded.
Conclusions
These studies have now confirmed the viability of using gel-
phase proton HR MAS spectroscopic techniques for a systematic
study of the reversible, thermodynamically-controlled assembly of
surface-attached neutral rotaxanes. Thus, for the three-component
system consisting of a naphthodiimide thread unit, a naphthalene
crown shuttle, and metalloporphyrin stoppers, we have demon-
strated that it is feasible to immobilise each of the single entities
in turn by attachment to polystyrene beads, and that rotaxane
formation is analogous to that observed in entirely solution-phase
systems.
For the crown-tethered systems, with the diimide thread unit
and the porphyrin stoppers in the surrounding solution phase,
it was found that a dinaphtho-crown unit was necessary to
ensure sufficiently strong intra-annular binding of the diimide
thread unit; replacement of one of the naphtho-groups by a
phenyl did not result in adequate proportions of the assembled
pseudorotaxanes in the equilibrating system. Even so, for the
dinaphtho-crown derivative, the equilibrium could be shifted in
favour of the pseudorotaxanes by increasing concentration of
the thread unit, by lower temperatures, and by the addition of
lithium iodide, despite some limitations of the technique which
does not allow accurate quantitation. The observed behaviour
in stoppering of the pseudorotaxane with either ruthenium or
rhodium porphyrin units was easily interpreted and paralleled the
all-solution system.
For the alternative system, with the metalloporphyrin stop-
per units immobilised by covalent attachment to the beads,
spectroscopic quality was influenced by the metal ion, and signal-
to-noise ratios did not necessarily reflect bead loading. This was
a result of the CPMG signal-filtering acquisition sequence, and
was found to be limiting in the case of zinc and ruthenium(II) car-
bonyl derivatives of the attached porphyrin, but was satisfactory
for the freebase and rhodium(III) iodide analogues. Additional
complications arise from the fact that for a thread unit with ligand
moieties at both ends, several structural variations are possible,
and these are not easily distinguishable on the basis of chemical
shifts. Accordingly, a thread unit with one end stoppered with an
innocent bulky group, and the other with a pyridine ligand, was
used in the first instance to allow a clearer understanding of the
dynamics of the equilibrating system. The tethered rotaxane that
was formed from the bead-attached rhodium iodide porphyrin,
with the mono-stoppered thread and dinaphtho-38-crown-10 in
the surrounding solution phase was easily identified by HR MAS,
and its spectroscopic characteristics were shown to be analogous
to those observed for the solution-phase. These results can now
be correlated directly to more complicated systems containing bis-
pyridine terminated thread units such as 3.
For the HR MAS spectra shown in Fig. 3, it can be seen that
the resonances for the diimide and terminal pyridine protons
have low intensity despite excess thread having been added and
indeed spectra run on this sample without employing a CPMG
pulse sequence showed much higher proportions of the diimide
3. We have previously shown that the application of the CPMG
sequence, while effectively filtering out the broad proton peaks of
the ArgoGel beads, can in some cases also filter out “non-bead”
resonances.7 This is also a difficulty in attempting to distinguish
between the possible structural Types I to VI in the spectra of
the porphyrin-tethered systems described in the ESI.† Due to
these relaxation effects, and their implication in the NMR pulse
sequences used in this method, relative signal integrations are
not only an unreliable quantitative measure even within a single
tethered molecular species, but they cannot be used to quantify
differently structured components.
After the addition of the LiI, the increase in intensity of the
diimide and terminal pyridine proton peaks of thread 3 in Fig. 3
is obvious. This is however not a result of additional thread as not
only was the same concentration of thread added, but also if an
increased amount of thread had been added it would result in a
downfield shift in the NMR spectrum rather than the observed
slight upfield shift observed. Furthermore, small upfield shifts
in the crown resonances are evident due to a slight increase in
the binding of the diimide to the crown. Of particular interest is
the splitting of the crown resonance k into two peaks, which is
indicative of a loss of symmetry of the naphthalene moiety as a
result of the crown–diimide interaction (an effect which has been
seen in analogous solution studies).
A possible difference between the solution-phase and solid-
tethered behaviour on the addition of LiI could be due to
insufficient LiI being solubilised in solutions made up in the
absence of crown, as it is known that the presence of crown
facilitates LiI solubilisation. One attempt to overcome this was
to add solid LiI to the NMR rotor containing crown beads in
the expectation that the presence of crown would enhance the
availability of the LiI to the system. HR MAS NMR analysis
showed that, whilst slightly further upfield shifts in both the
crown and diimide resonances were observable, indicating some
enhancement of the binding by the addition of the LiI (presumably
due to more being solubilised by the crown), nevertheless its effect
remained less than that observed in solution. It is unlikely that
the full effect of the addition of LiI can be achieved in these
types of HR MAS NMR experiments due to the limitations of the
experimental procedure.
Together with the results from our previous study on the
thread-attached analogous systems, we have now established that
the equilibrium-controlled assembly of each of thread-, crown-
and porphyrin-tethered rotaxanes proceeds analogously to the
The variation in spectroscopic quality and signal-to-noise ratios
for the various HR MAS spectra of the attached porphyrin
species 13a–d shown in Fig. 5 may be due to restricted flexibility,
302 | Org. Biomol. Chem., 2009, 7, 293–303
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solution-phase systems, and a direct comparison is justified,
albeit with some modified behaviour in certain cases with added
alkali metal salts. This now adds confidence to assuming the
structural integrity of solid-bound structures assembled under
thermodynamically controlled conditions, and the outcomes can
be translated to more complex systems and to other solid supports
and interfaces. This leaves the way open to further tethered systems
where two or three of the individual components are immobilised,
which is the subject of on-going current research.
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Acknowledgements
This work was funded by a grant from the Australian Research
Council.
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