Restricting shuttling in bis(imidazolium)…pillar[5]arene rotaxanes using metal coordination

Article abstract of DOI:10.1039/c8dt04096f

Metal coordination to a series of bis (imidazolium)…pillar[5]arene [2]rotaxanes through the formation of metal-carbene bonds facilitates a new strategy to restrict the shuttling motion in [2]rotaxanes. Whereas the pillar[5]arene macrocycle rapidly shuttle

Full text of DOI:10.1039/c8dt04096f

Volume 48 Number 1 7 January 2019 Pages 1–348
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An international journal of inorganic chemistry
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ISSN 1477-9226
PAPER
Neil R. Champness et al.
Restricting shuttling in bis(imidazolium)…pillar[5]arene
rotaxanes using metal coordination

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PAPER
Restricting shuttling in bis(imidazolium)…pillar[5]
arene rotaxanes using metal coordination†
Cite this: Dalton Trans., 2019, 48, 58
Philipp Langer, Lixu Yang,
Neil R. Champness
Constance R. Pfeiffer,
William Lewis
and
*
Metal coordination to a series of bis (imidazolium)…pillar[5]arene [2]rotaxanes through the formation of
metal–carbene bonds facilitates a new strategy to restrict the shuttling motion in [2]rotaxanes. Whereas
the pillar[5]arene macrocycle rapidly shuttles along the full length of the bis (imidazolium) rod for the
parent [2]rotaxane, Ag(I) coordination to the imidazolium groups through the formation of N-heterocyclic
carbenes leads to restricted motion, effectively confining the shuttling motion of the [2]rotaxane. The
Ag(I) coordinated [2]rotaxanes can be reacted further, either removing the Ag–carbene species to recreate
the parent [2]rotaxane, or reaction with more bulky Pd(II) species to further restrict the shuttling motion
through steric inhibition.
Received 12th October 2018,
Accepted 31st October 2018
DOI: 10.1039/c8dt04096f
rsc.li/dalton
understanding and controlling this shuttling process is one of
the most important themes of the field.
Introduction
The importance of molecular machines has been well estab-
Herein we report a series of [2]rotaxanes composed of a
lished. Over the past two decades the synthesis of mechani- decamethylpillar[5]arene (heretofore simply termed pillar[5]
cally-interlocked molecules (MIMs) has exceeded mere curio- arene) and bis-imidazolium rods of varying lengths prepared
sity and has become a field of increasing significance in both in high yields. Pillar[n]arenes are a relatively new class of
understanding and mimicking key biological processes. macrocycles17 which are highly symmetrical, rigid, tubular
Catenanes and rotaxanes are at the forefront of the field1 with structures. Pillar[5]arenes have received growing interest
applications in molecular switches,2 molecular sensing,3 drug and have been employed in a variety of applications.17–22
delivery4 and molecular electronic devices.5 With molecular Pillar[n]arenes, n = 5–10, are known, however pillar[5]arene17
machines and switches, a conformational or configurational has gained increased popularity over the others, in part, due to
change occurs as a consequence of applying an external stimu- its facile synthesis and wide availability. Pillar[5]arenes
lus. Reported stimuli include light,6 electrochemistry,7 pH,8 possess electron rich cavities which give rise to selective host–
heat,9 solvent polarity,10 cation/anion binding,11 allosteric guest chemistry with both positively and neutrally charged
effects12 and reversible covalent bond formation.13 The coordi- electron accepting molecules18 such as pyridinium and viologen
nation of metals has a rich history in catenane and rotaxane derivatives23 as well as imidazolium,24 bis(imidazolium)
synthesis due to their role in early template-based synthetic salts,25 n-alkanes26 and bis(dicyanobutane).27 The tubular
routes. Additionally, macrocycles have been shown to shuttle structure of pillar[n]arenes include two rims which can be
from one ligand/binding site to another along a rotaxane rod selectively28 or entirely29 functionalised in order to tune the
upon metal exchange,14 redox processes15 or protonation.16 properties or build supramolecular materials. The rich host–
The stimuli can affect either the dumbbell rod or the macro- guest chemistry of pillar[n]arenes has been utilised in the syn-
cycle that it threads. An important feature of such systems is thesis of mechanically interlocked molecules30–32 and have
the shuttling of the macrocyclic component on the rod and been applied as molecular shuttles33 and molecular springs.34
Metals have been used previously in conjunction with pillar[n]
arene pseudorotaxanes,35 but examples are scarce.
In this study we demonstrate that bis (imidazolium)…pillar
School of Chemistry, University of Nottingham, University Park, Nottingham,
[5]arene [2]rotaxanes are proligands for N-heterocyclic car-
benes (NHCs) and effectively coordinate Ag(I) and Pd(II) cations
at either end of the rod. The introduction of metal complexes to
the rod restricts shuttling motion of the pillar[5]arene along the
full length of the [2]rotaxane, providing a simple method of
influencing shuttling motion in this class of MIMs.
NG7 2RD, UK. E-mail: Neil.Champness@nottingham.ac.uk
†Electronic supplementary information (ESI) available: Synthetic methods,
including additional NMR studies and comparison of shielding effects in all [2]
rotaxane systems. CCDC 1553404 (1a), 1553405 (3b), 1553406 (1b), 1553407 (1c),
1553408 (2a-Br), 1553411 (2a-Cl), 1553412 (2b), 1553413 (2c), 1553414 (3c) and
1860505 (3a). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8dt04096f
58 | Dalton Trans., 2019, 48, 58–64
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A series of [2]-rotaxanes 1a–c were synthesised employing the imidazolium groups at either end of the rod. Neither imid-
rods of varying lengths (Scheme 1). Building blocks bis-imid- azolium is preferred over the other leaving the pillar[5]arene
azole rods, pillar[5]arene, and 2-(iodomethyl)-1,3,5-trimethyl- to shuttle along the length of the rod. Although the alkane
benzene, were each synthesised using one-step procedures (see chain rod protons Hh–k experience a significant shielding
ESI† for Experimental details). 2-(Iodomethyl)-1,3,5-trimethyl- effect, shown by the upfield shift in their NMR peaks, the
benzene decomposes in light, therefore the subsequent rotax- extent of shielding is not as high as expected if the pillar[5]
ane syntheses were performed whilst shielding the reaction arene was positioned solely in the centre of the rod (Fig. 2).
from light. [2]Rotaxanes 1a–c were synthesised by dissolving The lack of peak splitting and desymmetrisation suggests that
the constituent building blocks in the minimum amount of the pillar[5]arene oscillates along the rod faster than the time-
chloroform required to dissolve all reagents at −15 °C. scale of the NMR experiment. The only exception to this is the
Purification of products was achieved employing flash chrom- signal observed for Hd which is observed as an AB pattern. The
atography with yields as high as 84%. Characterisation was protons of the methylene group that links the imidazolium to
1
1
completed using H, 13C and H–1H COSY NMR spectroscopy the stopper group, Hd/H′d, are diastereotopic and non-equi-
and high-resolution mass spectrometry (HRMS) (see ESI†). In valent due to the chirality of the pillar[5]arene. The chirality
each case single crystals were obtained by layering hexane onto transfer observed indicates that the pillar[5]arene is positioned
a chloroform solution of the [2]rotaxane, 1a–c, and their struc- in close proximity to the methylene group despite relatively
tures confirmed by single crystal X-ray diffraction studies (see small upfield shift. Shuttling in rotaxanes has been extensively
ESI† for full details). Despite the possibility of dethreading by studied37–43 including symmetric [2]rotaxanes with two recog-
dynamic nucleophilic substitution in the presence of an iodo nition sites separated by alkyl linkers.37 Although confor-
counter anion, rotaxanes 1a–c were bench stable.36
mational flexibility of alkyl linkers can lead to disruption of
It is important to note that the rotaxanes 1a–c behave differ- shuttling, shorter linkers, such as those described in this study
ently in the solid-state than in solution. The single crystal data are not anticipated to inhibit fast shuttling processes.37,38 Thus,
for 1a–c shows the pillar[5]arene positioned in the centre of the observation of interactions between the pillar[5]arene and
the rod (see ESI†) and positioned over the linking alkyl chain. the entirety of the rod is consistent with fast shuttling rates.
The energetics of crystal structures are dominated by inter- Indeed, the shuttling behaviour was found to be unaffected by
molecular interactions and efficient packing of molecules, temperature within the temperature range studied. VT 1H NMR
commonly termed packing effects. Although crystal structures measurements over a range of temperatures, 293 K–193 K,
reveal the connectivity of a molecule they do not always exhibited no variation in chemical shifts and only a small
provide a valuable indication of the molecule’s solution-based broadening of individual peaks (see ESI† for spectra).
preferred conformational arrangement. Thus, the crystal struc-
In order to provide a consistent method of comparison we
1
tures of the rotaxanes discussed in this study do not necess- calculated a relative shift for each proton environment [rod H
arily provide a good indication of the solution-phase position- chemical shift (ppm) – rotaxane 1H chemical shift (ppm)],
ing of the pillar[5]arene with respect to the bis-imidazolium corresponding to a shielding factor, and then plotted these for
rods.
each environment along the rod (Fig. 2 and ESI†). The plots
Indeed, in solution shuttling of the pillar[5]arene with contain a mirror plane, reflecting the symmetrical nature of
respect to the rod is both anticipated and observed by 1H NMR the rotaxanes studied herein. We suggest that a greater shield-
spectroscopy (Fig. 1). Although the whole rod is shielded to ing factor indicates a greater interaction of the pillar[5]arene,
some degree, the pillar[5]arene resides preferentially closer to and hence residency over, the given proton environment of the
Scheme 1 Metal coordination to the imidazolium salts of rotaxanes 1a–c as N-heterocyclic carbenes leads to restriction of the pillar[5]arene shut-
tling. Ag(I) coordination can be readily achieved at room temperature and is reversible following addition of acid. Pd(II) coordination (3b, 3c) is readily
achieved from the corresponding Ag(I) complex and in the case of 3a only a mono-Pd(II) complex is observed due to steric restriction.
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Fig. 1 1H NMR spectra (400 MHz, CDCl3, 298 K) schematic showing the NMR shifts of 1c (second row) due to the confinement of the pillar[5]arene
by the coordination of Ag(I) 2c (third row) or Pd(II) 3c (bottom row). Bis-imidazolium rod 4c (top row) is included for comparison.
Fig. 2 Calculated shielding effects [rod 1H chemical shift (ppm) – rotaxane 1H chemical shift (ppm)] for [2]rotaxanes 1a–c indicating the favoured
residency of the pillar[5]arene on the rod.
rod. To measure the relative shifts it was required to synthesise enough to maximise binding interactions with the pillar[5]
model compounds for each species without the pillar[5]arene arene without requiring co-conformational motions.
component. Thus, we prepared the analogous bis-imidazolium
rods, 4a–c (see ESI†), and these were used to calculate the rela- dent as demonstrated by H NMR spectra recorded for 1c (see
tive shifts. The pillar[5]arene clearly has a greater shielding ESI†). H NMR spectra in CDCl3 indicate greater shielding of
The shielding effects of the pillar[5]arene are solvent depen-
1
1
effect on each end of the rod, rather than the centre of the the rod termini, as discussed above, with the centre of the
alkyl linker, in 1b and 1c. In contrast, the butyl linker in 1a is alkyl chain less affected. Spectra recorded in (CD3)2CO exhibit
sufficiently short such that the imidazolium groups are close similar behaviour to those in CDCl3 but in (CD3)2SO larger
60 | Dalton Trans., 2019, 48, 58–64
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shielding effects are observed for the central alkyl chain. This associated with the alkyl chain. We assessed the shielding
suggests that the cationic imidazolium groups are more effects in 2a–c, using the approach discussed above. This
exposed in the polar (CD3)2SO solvent and is consistent with required the synthesis of Ag(I) complexes of the bis-NHC rods,
previous observations in related systems.33 We also investi- 5a–c, which were readily prepared from 4a–c using an analo-
gated the influence of the counter anion by exchanging iodide gous reaction to those used for the synthesis of 2a–c.
for hexafluorophosphate (see ESI†). In contrast to the effects Calculation of the shielding effects in 2a–c clearly indicate that
observed for solvent polarity anion exchange made negligible the greatest shift is observed for the protons associated with
changes to the shielding effects.
the alkyl chain linking the metal-binding NHCs. Negligible
It was anticipated that metal coordination to the imidazo- shifts are observed for the proton environments associated
lium rings would prevent the free shuttling of the pillar[5] with NHCs, bearing in mind the Ag(I) coordination to this
arene along the entirety of the rod and instead confine it to a group. Additionally the protons of the methylene group
central position. Thus, a series of metal-coordinated [2]rotax- linking the imidazolium to the stopper group, Hd, no longer
anes were prepared coordinated to the NHC of the imidazo- exhibit an AB pattern, showing only a sharp singlet, indicating
lium salts. The coordination of Ag(I) to imidazolium salts, as that the pillar[5]arene is no longer in close proximity to these
NHCs, is well established.44 Such Ag–NHC coordination occurs protons and is therefore unable to transfer chirality, as
at room temperature in dichloromethane and does not require observed for 1a–c. It is clear from these results that the pillar
additional reagents (i.e. additional bases). Thus, two equiva- [5]arene is no longer able to shuttle along the full length of the
lents of imidazolium salt were reacted at room temperature rod in 2a–c and is therefore confined to the central alkyl chain
with one equivalent of Ag2O in the presence of KBr to give component of the rotaxane. However, adding a small amount
Ag(I) coordinated [2]rotaxanes 2a–c in high yield (see ESI†). of a proton source, such as trifluoroacetic acid (TFA), the silver
Halide scrambling is possible in these systems so an excess of coordination can be reversed (see ESI†) releasing the pillar[5]
KBr was added to ensure the formation of bromide complexes. arene wheel to shuttle freely along the full length of the
[2]Rotaxanes 2a–c crystallised readily from vapour diffusion of [2]-rotaxane.
hexane into a solution of the complex in chloroform. Single
Ag(I) NHCs are known to be reactive and are thus useful
crystal X-ray diffraction (Fig. 3 and ESI†) along with and 1H, starting materials for further functionalisation.45 Thus, we
13C and 1H–1H COSY NMR confirmed the structure of the were able to convert 2a–c to related Pd(II) complexes 3a–c at
target compounds. To determine the extent of shuttling in room temperature by reaction with Pd(CH3CN)2Cl2, NaI and
1
2a–c, in comparison to the nonmetallated [2]rotaxanes 1a–c, H anhydrous pyridine. The resulting complexes contain Pd(II)
NMR spectroscopy was employed to track the position of the bound to the NHC, from the [2]rotaxane, a pyridine ligand,
pillar[5]arene with respect to the rod. As anticipated the spec- and two iodides. Single crystals were obtained by vapour
trum shows significant shifts in 1H NMR signals (see Fig. 1 for diffusion (hexane/chloroform solution) and the structures of
representative example of 2c) and noticeably so in the signals 3a–c were confirmed by single crystal X-ray diffraction (Fig. 3
and ESI†). Whereas the [2]rotaxanes with longer rods, 3b and
3c, each NHC underwent transmetalation, coordinating Pd(II)
complexes, for 3a only one of the NHC groups transmetalated
with palladium whilst the other was protonated. It is also
noticeable that in this instance although we observe Pd-pyri-
dine coordination during the reaction we were only able to
isolate a zwitterionic complex in which the Pd(II) centre is co-
ordinated by the NHC and three halides, two iodides and a
chloride. Mono-metalation is a result of the shorter linker
between the two imidazolium groups which does not allow
two sterically bulky Pd(II) complexes and the pillar[5]arene on
the same rod. Indeed the presence of the pillar[5]arene can be
considered to be controlling the reactivity of the [2]rotaxane
and is therefore acting as a protecting group for one of the
imidazolium moieties. It is also possible to prepare 3b and 3c
directly from 1b and 1c, respectively, through heating solutions
of the parent [2]rotaxanes to reflux in pyridine with
Pd(CH3CN)2Cl2, NaI and K2CO3. However this led to noticeably
lower overall yields of the Pd(II) complexes. In the case of the
analogous reaction of 1a the steric bulk and the high reaction
temperature led to dethreading of the pillar[5]arene generating
a dimetalated Pd(II)- bis (NHC) species, 6a.
Fig. 3 Views of the crystal structures of (a), (b) 2c; (c), (d) 3c; (e), (f) 3a.
(a), (c) and (e) ball-and-stick; (b), (d), (f) space-filled representations.
The shielding effects in 3a–c were evaluated by comparison
to a series of Pd(II) complexes of bis-NHC rods, 6a–c, which
Blue = imidazolium/NHC rod, red = pillar[5]arene wheel, and green =
mestiyl stopper group.
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Fig. 4 Calculated shielding effects [rod 1H chemical shift (ppm) – rotaxane 1H chemical shift (ppm)] for [2]rotaxanes 3a–c indicating the favoured
residency of the pillar[5]arene on the rod.
could be prepared from reaction of 5a–c using analogous con- [2]rotaxane. Our simple strategy represents an approach to
ditions to the reactions used for 3a–c (see ESI†). As with the influencing and restricting shuttling in these new and poten-
corresponding Ag(I) complexes, 2b/2c, the resulting calcu- tially diverse class of rotaxanes. Transmetalation of Ag–NHC
lations confirm a significant upfield shift for the 1H shifts complexes has been shown to provide a versatile pathway for
associated with the alkyl chain in 3b/3c (Fig. 4). Due the larger the coordination of a variety of metals, including Au,46 Cu,47
coordination sphere of the Pd(II) complex the shifts are more Fe47 and Pd,48 indicating a wide potential variety of target
pronounced in 3b/3c than 2b/2c indicating more restricted metal-coordinated rotaxane species allowing a mechanism for
movement of the pillar[5]arene along the [2]rotaxane rod. As fine tuning shuttling in these systems.
with 2a–c, the signal for Hd is observed as a singlet due to the
absence of interaction with the pillar[5]arene. 3a behaves quite
differently to the other [2]rotaxane complexes due to the asym-
metric substitution of the rod, with only one NHC group co-
Conflicts of interest
ordinated to Pd(II), the other reverting to an imidazolium There are no conflicts to declare.
group. Calculations of the shielding effects clearly indicates
this desymmetrisation with the pillar[5]arene shifting away
from the Pd(II)–NHC complex and deshielding the opposing
end of the alkyl chain and the non-coordinated imidazolium
Acknowledgements
group (Fig. 4). As anticipated, bearing in mind the discussion We thank the Engineering and Physical Sciences Research
above, only one of the methylene proton environments demon- Council (EP/K01773X/1), LY, and the Leverhulme Trust
strates chirality transfer from the pillar[5]arene. Chirality trans- (RPG-2014-317), CRP, for financial support. NRC gratefully
fer is also observed for the protons of the alkyl chain (Hh–Hk), acknowledges receipt of a Royal Society Wolfson Merit Award.
in contrast to the symmetrical [2]rotaxanes in this study.
In summary, a high yielding and versatile strategy for the
synthesis of rotaxanes based on pillar[5]arene and imidazo-
lium salts has been developed and used to synthesise [2]-rotax-
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