Nitrone [2]rotaxanes: Simultaneous chemical protection and electrochemical activation of a functional group

Article abstract of DOI:10.1021/ja1034683

Abstract: We report on the use of the hydrogen-bond-accepting properties of neutral nitrone moieties to prepare benzylic amide macrocycle-containing [2]rotaxanes in yields as high as 70%. X-ray crystallography showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and the two nitrone groups of the thread. Dynamic ¹H NMR studies of the rates of macrocycle pirouetting in nonpolar solutions indicated that the amide-nitrone hydrogen bonds are particularly strong (1.3 and 0.2 kcal mol ^-1 stronger than similar amide-ester and amide-amide interactions, respectively). In addition to polarizing the N-O bond through hydrogen bonding, the rotaxane structure affects the chemistry of the nitrone groups in two significant ways: first, the intercomponent hydrogen bonding activates the nitrone groups to electrochemical reduction, a one-electron-reduction of the rotaxane being stabilized by a remarkable 400 mV (8.1 kcal mol ^-1) with respect to the same process in the thread; second, however, encapsulation protects the same functional groups from chemical reduction with an external reagent (and slows electron transfer to and from the electroactive groups in cyclic voltammetry experiments). Mechanical interlocking with a hydrogen-bonding molecular sheath thus provides a route to an encapsulated polarized functional group and radical anions of significant kinetic and thermodynamic stability.

Full text of DOI:10.1021/ja1034683

Published on Web 06/16/2010
Nitrone [2]Rotaxanes: Simultaneous Chemical Protection and
Electrochemical Activation of a Functional Group
Daniel M. D’Souza,† David A. Leigh,*,† Lo¨ıc Mottier,‡ Kathleen M. Mullen,†
Francesco Paolucci,‡ Simon J. Teat,§ and Songwei Zhang†
School of Chemistry, UniVersity of Edinburgh, The King’s Buildings, West Mains Road,
Edinburgh EH9 3JJ, United Kingdom, Dipartimento di Chimica G. Ciamician, UniVersita` degli
Studi di Bologna, Via Selmi 2, 40126 Bologna, Italy, and CCLRC Daresbury Laboratory,
Warrington, United Kingdom
Received April 23, 2010; E-mail: David.Leigh@ed.ac.uk
Abstract: We report on the use of the hydrogen-bond-accepting properties of neutral nitrone moieties to
prepare benzylic amide macrocycle-containing [2]rotaxanes in yields as high as 70%. X-ray crystallography
showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the
macrocycle and the two nitrone groups of the thread. Dynamic 1H NMR studies of the rates of macrocycle
pirouetting in nonpolar solutions indicated that the amide-nitrone hydrogen bonds are particularly strong
(∼1.3 and ∼0.2 kcal mol-1 stronger than similar amide-ester and amide-amide interactions, respectively).
In addition to polarizing the N-O bond through hydrogen bonding, the rotaxane structure affects the
chemistry of the nitrone groups in two significant ways: first, the intercomponent hydrogen bonding activates
the nitrone groups to electrochemical reduction, a one-electron-reduction of the rotaxane being stabilized
by a remarkable 400 mV (8.1 kcal mol-1) with respect to the same process in the thread; second, however,
encapsulation protects the same functional groups from chemical reduction with an external reagent (and
slows electron transfer to and from the electroactive groups in cyclic voltammetry experiments). Mechanical
interlocking with a hydrogen-bonding molecular sheath thus provides a route to an encapsulated polarized
functional group and radical anions of significant kinetic and thermodynamic stability.
and components in multifunctional materials.8 However, with
Introduction
the notable exception of the R-nitronyl nitroxide organic magnet
systems,9 the use of nitrones and related synthons in molecular-
level assembly processes remains largely unexplored, certainly
in comparison with the use of ethers, amides, imines, ureas,
and guanidinium units.10 This is somewhat surprising given their
desirable intrinsic properties: nitrones possess one of the largest
dipole moments known for any functional-group type (3.37-3.47
D11), making them potentially useful for NLO applications and
control of molecular orientation; they offer a simple route to
stable radicals via one-electron electrochemical reduction12 or
conjugate addition to CdN followed by oxidation;13 and the
N+-O- motif is a powerful hydrogen-bond acceptor.14
The rich and diverse covalent chemistry of nitrones1 [gener-
ally depicted as CdN+(R)-O-, although the C--N+(R)dO
canonical form makes a significant contribution to their proper-
ties] has led to their being exploited in many different ways:
they serve as potent oxidants in many chemical reactions,2 are
readily reduced to amines and hydroxylamines,3 and act as
starting materials for various heterocyclic skeletons via cy-
cloaddition reactions.4 They have also found utility as diagnostic
spin traps,5 potential antioxidant therapeutics,6 precursors to
radical initiators for living and other controlled polymerizations,7
† University of Edinburgh.
‡ Universita` degli Studi di Bologna.
§ CCLRC Daresbury Laboratory.
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A R T I C L E S
D’Souza et al.
Hydrogen bonding has previously been used15-24 to assemble
benzylic amide macrocycles around various amide,16,17 ester,17,18
squaraine,19 phenolate,20 urea,21 pyridone,22 and ion-pair23
templates to generate rotaxanes and catenanes.25 Tuning of
structural rigidity and preorganization has been reported to effect
yields as high as 97% for [2]rotaxanes incorporating amide
threads.17 Although threading protocols using preformed mac-
rocycles have been successfully used in some cases,21,22,24 the
poor solubility of most benzylic amide macrocycles in the
nonpolar solvents needed to promote intercomponent hydrogen
bonding has meant that templated assembly of building blocks
about the thread to form the macrocycle is most often used to
construct such rotaxanes.15 These five-component “clipping”
Scheme 1. Hydrogen-Bonding Modes of Dipeptide and Bisnitrone
Templates for Rotaxane Synthesis
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Murphy, A.; Smart, J. P.; Slawin, A. M. Z. Angew. Chem., Int. Ed.
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S. J.; Wong, J. K. Y. J. Am. Chem. Soc. 2001, 123, 5983–5989.
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reactions (Scheme 1) produce interlocked architectures because
multipoint hydrogen bonding between the open-chain precursor
1 (which in the absence of a suitable template preferentially
adopts a linear syn-anti conformation) and the thread 2
promotes a conformational change that brings the reactive end
groups close together, leading to rapid cyclization of 1 about
the axle.16a,b,i,17 The key factors determining the efficiency of
rotaxane formation in such reactions are the following:
(i) the spatial arrangement of hydrogen-bonding sites on the
thread (ideally chosen so a low-energy conformation of the
macrocycle precursor 1 can bind in a multidentate manner to
the thread, as shown in I in Scheme 1);16a,b
(ii) the rigidity of the template unit (as few as possible internal
degrees of freedom of the thread or intramolecular hydrogen
bonds should be lost upon complexation with 1 to form I).17
(iii) the efficacy of the hydrogen-bonding motifs in the thread
(e.g., amides are much more effective than esters).17
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2003, 5, 1907–1910. (b) Linnartz, P.; Bitter, S.; Schalley, C. A. Eur.
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Pe´rez, E. M.; Wong, J. K. Y.; Saiz, C. G.; Slawin, A. M. Z.;
Carmichael, A. J.; Haddleton, D. M.; Brouwer, A. M.; Buma, W. J.;
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Zhu, D. Org. Lett. 2005, 7, 4835–4838. (f) Marlin, D. S.; Gonza´lez
Cabrera, D.; Leigh, D. A.; Slawin, A. M. Z. Angew. Chem., Int. Ed.
2006, 45, 77–83. (g) Marlin, D. S.; Gonza´lez Cabrera, D.; Leigh, D. A.;
Slawin, A. M. Z. Angew. Chem., Int. Ed. 2006, 45, 1385–1390. (h)
Chatterjee, M. N.; Kay, E. R.; Leigh, D. A. J. Am. Chem. Soc. 2006,
128, 4058–4073. (i) Zhou, W.; Chen, D.; Li, J.; Xu, J.; Lv, J.; Liu,
H.; Li, Y. Org. Lett. 2007, 9, 3929–3932. (j) Leigh, D. A.; Thomson,
A. R. Org. Lett. 2006, 8, 5377–5379. (k) Gonza´lez Cabrera, D.;
Koivisto, B. D.; Leigh, D. A. Chem. Commun. 2007, 4218–4220. (l)
Gassensmith, J. J.; Barr, L.; Baumes, J. M.; Paek, A.; Nguyen, A.;
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Goldup, S. M.; Leigh, D. A.; Slawin, A. M. Z. J. Am. Chem. Soc.
2008, 130, 1836–1838.
The majority of neutral hydrogen-bond-accepting groups
employed in such threads to date have been amides, squaraine
(25) For examples of bisanilide macrocycle-based catenanes and rotaxanes,
see: (a) Hunter, C. A. J. Am. Chem. Soc. 1992, 114, 5303–5311. (b)
Vo¨gtle, F.; Meier, S.; Hoss, R. Angew. Chem., Int. Ed. Engl. 1992,
31, 1619–1622. (c) Ja¨ger, R.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl.
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C. A.; Mayers, P. C. Tetrahedron 1999, 55, 5265–5293, and references
therein. (e) Blight, B. A.; Van Noortwyk, K. A.; Wisner, J. A.;
Jennings, M. C. Angew. Chem., Int. Ed. 2005, 44, 1499–1504. (f)
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Nitrone [2]Rotaxanes
A R T I C L E S
Scheme 2. Synthesis of Bisnitrone [2]Rotaxanes 4-6
Figure 1. X-ray crystal structure of nitrone [2]rotaxane 4. Selected bond
lengths (Å): N39-O39 ) N42-O42, 1.30. Distance between hydrogen-
bond-acceptor groups (Å): O39-O42, 4.87. Intramolecular hydrogen-bond
lengths (Å): O42-HN20, 2.92; O39-HN2, 2.92. Intramolecular hydrogen-
bond angles (deg): N2-H-O39 ) N20-H-O42, 173.3.
units (which have significant oxocarbon anion character), and
esters.16-19 However, the hydrogen-bond basicity of these
carbonyl-based functionalities can be exceeded by other groups
with significant ionic or mesomeric character, such as S+-O-,
P+-O-, and N+-O-.26 Accordingly, we decided to determine
the effectiveness of employing nitrones as a hydrogen-bonding
template for rotaxane formation and in turn investigate the
effects of the mechanically interlocked architecture on the
chemistry of nitrones.
Results and Discussion
Synthesis. The bisnitrone thread 3, which features the two
oxygen atoms at a separation and relative orientation similar to
those of the amide carbonyls in a dipeptide, was prepared in
three steps from diphenylacetaldehyde (see the Supporting
Information). Solutions containing 6.0 molar equiv of isoph-
thaloyl dichloride and 6.6 molar equiv of p-xylylenediamine
were slowly added to 3 in a stirred anhydrous solution of 5:1
CHCl3/CH3CN (Scheme 2). After the addition was complete,
no unconsumed thread 3 could be detected in the reaction
mixture by thin-layer chromatography. Filtration and purification
by flash chromatography on silica gel (CHCl3/MeOH as eluent)
yielded the bisnitrone [2]rotaxane 4 in 70% yield.16e Under
analogous conditions, replacing isophthaloyl dichloride with 2,6-
or 3,5-pyridinedioyl dichloride afforded the corresponding
[2]rotaxanes 5 and 6 in 21 and 40% yield, respectively.
X-ray Crystallography. The solid-state structures of the three
bisnitrone rotaxanes were determined by X-ray crystallography
(Figures 1-3). Suitable single crystals were obtained in each
case by slow diffusion of water vapor into solutions of the
rotaxanes in N,N-dimethylformamide (DMF). Despite the close
similarities in the structural formulas of the rotaxanes and the
similar conditions of crystal growth, significant differences in
the solid-state structures are apparent. The hydrogen-bonding
motifs and relative positions of the components in the solid-
Figure 2. X-ray crystal structure of endopyridyl macrocycle nitrone
[2]rotaxane 5. Selected bond lengths (Å): N39-O39 ) N42-O42, 1.35.
Hydrogen-bond lengths (Å): O42-HN2 ) O42-HN11 ) O39-HN29 )
O39-HN20,2.11;N5-HN2)N23-HN20,2.39;N5-HN11)N23-HN29,
2.33. Hydrogen-bond angles (deg): O42-H-N2 ) O39-H-N20, 149.1;
O42-H-N11 ) O39-H-N29, 156.9, N5-H-N2 ) N23-H-N20, 101.4,
N5-H-N11 ) N23-H-N29, 100.5.
state structures of rotaxanes 5 (Figure 2) and 6 (Figure 3) are
essentially as predicted by design, with two sets of bifurcated
hydrogen bonds between the 1,3-diamide groups of the mac-
rocycle and the nitrone oxygen atoms. The isophthalamide
macrocycle-based rotaxane 4 (Figure 1) adopts a different solid-
state structure than the other two rotaxanes, maximizing
intermolecular amide-amide hydrogen bonds at the expense
of the bifurcated intramolecular amide-nitrone hydrogen bonds.
This is presumably a result of the interplay between a number
of factors: the intermolecular amide-amide hydrogen bonds
formed in 4 are all strong (short, linear, and to regions of high
electron density, i.e., amide carbonyl lone pairs); each amide
hydrogen-bond donor to a nitrone group also acts as a hydrogen-
bond acceptor, with the polarization caused by each interaction
making the other stronger;27 and the structural changes that the
hydrogen-bonding motif brings about may increase van der
(26) (a) Taft, R. W.; Bordwell, F. G. Acc. Chem. Res. 1988, 21, 463–469.
(b) Abraham, M. H. Chem. Soc. ReV. 1993, 22, 73–83. (c) Abraham,
M. H.; Platts, J. A. J. Org. Chem. 2001, 66, 3484–3491. (d) Hunter,
C. A. Angew. Chem., Int. Ed. 2004, 43, 5310–5324.
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protons (see Figure 4c) also confirm the chair conformation of
the macrocycle in solution and indicate that HE2 is the equatorial
site.
Spin polarization transfer by selective inversion recovery
(SPT-SIR)29 measurements on the resolved HE resonances for
the isophthalamide rotaxane 4 gave a value of 12.2 ( 0.1 kcal
mol-1 for the energy barrier for macrocycle pirouetting at 271
K in C2D2Cl4. Fumaric thread bisamide and bisester [2]rotaxanes
having the same macrocycle and near-identical spacing and
orientation of the hydrogen-bond-accepting groups have energy
barriers for pirouetting of 11.4 ( 0.1 and 7.2 ( 0.4 kcal mol-1
,
respectively, in chlorinated solvents at 298 K.17 If it is assumed
that the structures and mechanism of ring rotation in solution
are similar for all of these rotaxanes, the differences in the
energy barriers can largely be attributed to the difference in
strength between the four intercomponent hydrogen bonds
present in the three systems: an amide-nitrone
[-CONH· · ·-O-N+(R)dC-] hydrogen bond is ∼0.2 kcal
mol-1 stronger than the corresponding amide-amide (-CONH· · ·
OdCNH-) hydrogen bond and ∼1.3 kcal mol-1 stronger than
the analogous amide-ester (-CONH· · ·OdCO-) interaction.
Figure 3. X-ray crystal structure of exopyridyl macrocycle nitrone
[2]rotaxane 6. Selected bond lengths (Å): N39-O39 ) N42-O42, 1.30.
Hydrogen-bond lengths (Å): O39-HN2 ) O42-HN20, 2.31; O39-HN11
) O42-HN29, 2.03. Hydrogen-bond angles (deg): O39-H-N2 )
O42-H-N20 127.4, O39-H-N11 ) O42-H-N29, 114.2.
Chemical and Electrochemical Effects of Encapsulation. After
the structures of the nitrone rotaxanes in both solution and the
solid state had been established, investigations to probe the redox
chemistry of the nitrone functional group within the rotaxane
structure were undertaken. The threaded architecture proved to
have significant influences on the chemical and electrochemical
reduction of the nitrone functional group in contrasting ways.
The nitrone groups of thread 3 are quantitatively reduced to
give bishydroxylamine 7 in 30 min by sodium borohydride in
aqueous isopropanol under reflux (Scheme 1). However, no
reaction was observed in the case of rotaxane 4 after refluxing
under identical conditions for >12 h (Scheme 3). The macrocycle
clearly provides a highly effective mechanical barrier to the
reagent, and the intercomponent hydrogen bonding may also
play an important role both by holding the macrocycle in
position and by preventing boron coordination to the nitrone
oxygen atoms. Although encapsulation within rotaxane archi-
tectures has previously been shown to stabilize reactive regions
of a rotaxane axle,30 most notably for chromophores30c or
enzyme-degradable peptide sequences,30e this is one of the most
dramatic examples of the effectiveness of a molecular sheath
in protecting a functional group from a small chemical reagent.
The electrochemical behavior of the bisnitrone thread and
rotaxane also differ significantly from one another. Figure 5
shows the cyclic voltammetry curves for 1.0 mM solutions of
thread 3 and rotaxane 4 in DMF obtained at 298 K using a
scan rate of 500 mV s-1. In both cases, a reversible one-electron
reduction peak corresponding to reduction of the nitrone groups
was observed. The fact that only one electron is consumed in
the reduction is indicative of effective conjugation between the
two nitrone groups that leads to extensive delocalization of the
Waals or π-π stacking interactions in the crystal packing
(intercomponent stacking interactions between the NdC bonds
of the thread and the xylylene rings of the macrocycle are
apparent in all three rotaxane X-ray structures but are consider-
ably distorted in 4).
Double bifurcated hydrogen bonding to the nitrone groups
in the rotaxanes might be expected to increase the contribution
of the CdN+(R)-O- canonical form relative to that of the
C--N+(R)dO form in comparison with simple nitrones. This
is indeed what is seen in the crystal structures of 5 and 6, as
manifested in the lengthening of the N-O bonds (in the case
of 5, to the longest reported to date for a nitrone system) to
1.30 Å in 4 and 6 and 1.35 Å in 5 [cf. 1.28 Å28 in
4-ClC6H4CHdN+(Me)-O-] and shortening of the CdN bonds
to 1.31 Å in 4, 1.26 Å in 5, and 1.28 Å in 6 [cf. 1.31 Å28 in
4-ClC6H4CHdN+(Me)-O-]. The significant increase in the
polarization of the N-O bond by double hydrogen bonding
could prove useful for the design of high-dipole-moment systems
(e.g., with nonsymmetrical hydrogen-bond-acceptor rotaxanes).
Dynamic 1H NMR Spectroscopy. In nonpolar solvents, unlike
the situation in the solid state, the isolated rotaxanes can only
adopt intramolecular hydrogen-bonding patterns to lower their
energy, so the most stable coconformations in each case involve
two sets of bifurcated hydrogen bonds similar to those in the
solid-state structures of 5 and 6. In C2D2Cl4 at room temperature,
the 1H NMR spectra of all three bisnitrone rotaxanes are
relatively simple. The isophthalamide (in 4) and exopyridyl (in
6) rotaxane macrocycles spin rapidly on the NMR time scale,
but rotation of the endopyridyl macrocycle in rotaxane 5 is slow
under the same conditions. The resolution of the HE protons
into two noninterconverting magnetically distinct environments
can arise only if pirouetting of the macrocycle is slow on the
NMR time scale (a 180° rotation of the macrocycle accompanied
by a chair-chair flip maps the HE1 protons onto the HE2 protons;
Figure 4). The different amide couplings to the HE1 and HE2
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Nitrone [2]Rotaxanes
A R T I C L E S
Figure 4. 1H NMR spectra (400 MHz, C2D2Cl4, 295 K) of (a) bisnitrone thread 3, (b) isophthalamide [2]rotaxane 4, and (c) endopyridyl rotaxane 5. The
assignments correspond to the lettering shown in Scheme 2.
Scheme 3. Chemical Reduction of Nitrone Thread 3 and Nitrone
[2]Rotaxane 4
Figure 5. Cyclic voltammetry curves of 1.0 M 4 (red curve) and 3 (black
curve) in 0.05 M tetraethylammonium tetrafluoroborate DMF solution at
298 K using a scan rate of 500 mV s-1with a platinum ultramicroelectrode
(125 µm) as the working electrode.
electrochemically reversible in thread 3, in the nitrone rotaxane
negative charge over both nitrone groups, in line with the
4 the reduction is quasi-reversible (see the Supporting Informa-
tion). Simulations of the CV curves for the thread and rotaxane
reported voltammetric behavior of other bisnitrone derivatives
in aprotic media.12 Significantly, the reduction was anodically
shifted by 400 mV for the rotaxane relative to the thread (E1/2
) -1.70 V for 3 and -1.30 V for 4). This effect can be
(33) For some recent examples of the electrochemistry of rotaxanes, see:
(a) Altieri, A.; Gatti, F. G.; Kay, E. R.; Leigh, D. A.; Martel, D.;
attributed directly to the stabilizing effect of hydrogen bonding
on the nitrone group.31 Since the injected electron is expected
to increase the negative charge on the nitrone oxygen atoms,
hydrogen bonding is stronger in the reduced state than for the
neutral nitrone, with the stabilization being ∼8 kcal mol-1 on
the basis of the 400 mV anodic shift.31,32
The kinetics of electron transfer are also affected by
encapsulation within the rotaxane architecture.33 The CV
measurements showed that while the nitrone reduction is
Paolucci, F.; Slawin, A. M. Z.; Wong, J. K. Y. J. Am. Chem. Soc.
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B. A.; Xu, K.; Shik Shin, Y.; Tseng, H.-R.; Stoddart, J. F.; Heath,
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A R T I C L E S
D’Souza et al.
based on the Butler-Volmer rate law provided the following
investigation. The hydrogen-bond-directed synthesis of nitrone
rotaxanes is simple and, in the case of the isophthaloyl dichloride
building block, high-yielding. The methodology provides an
unusual functional addition to the diversity of templates available
for assembling amide-macrocycle rotaxanes and could poten-
tially lead to new generations of electrochemically driven
molecular machines and superstable radicals.
values for the heterogeneous electron-transfer standard (i.e., at
E ) E°) rate constant: k° ) 0.01 cm s-1 for the rotaxane and
et
1 cm s-1 for the thread. Moreover, an R value of 0.3 was
obtained for the rotaxane, while for the thread R ) 0.5, as
expected for a reversible process. Rotaxane formation is
therefore responsible for lowering the standard rate constant by
2 orders of magnitude. The macrocycle probably acts as an
insulating sheath through which electron transfer to the redox
center must take place by tunneling. In analogy to heterogeneous
electron transfer to redox centers occurring through adsorbed
blocking monolayers on electrodes34 or to proteins,35 the
standard rate constant of the redox process should decrease in
4 relative to 3 by exp(-ꢀd), where ꢀ is a parameter defined by
the medium and d is the thickness of the insulating barrier.35a
Experimental Section
General Method for the Preparation of Bisnitrone
[2]Rotaxanes. The bisnitrone thread 3 and triethylamine were
dissolved in anhydrous 5:1 CHCl3/CH3CN and stirred vigorously
while solutions of the amine in anhydrous CHCl3 and the acid
chloride in anhydrous CHCl3 were added over 3 h using motor-
driven syringe pumps. The reaction mixture was filtered and the
solvent removed under reduced pressure. The crude material was
then subjected to column chromatography (silica gel, CHCl3/MeOH
as eluent) to give the pure [2]rotaxanes 4, 5, and 6 in 70, 21, and
40% yield, respectively. Further details are given in the Supporting
Information.
Conclusions
The powerful hydrogen-bond-accepting properties of nitrones
have been utilized in the synthesis of a series of [2]rotaxanes.
1
Dynamic H NMR experiments show that the amide-nitrone
Acknowledgment. This work was supported by the ERC Ad-
vanced Grant WalkingMols and the EPSRC. We thank the Deutsche
Akademia der Naturforscher Leopoldina (BMBF LPD 9901/8-166)
and Peter und Traudl Engelhorn-Stiftung for a postdoctoral fellowship
to D.M.D. D.A.L. is an EPSRC Senior Research Fellow and holds a
Royal Society-Wolfson Research Merit Award. F.P. acknowledges
financial support from MIUR (PRIN 2008N7CYL5 and 20085M27SS)
and the University of Bologna.
hydrogen bonds present in these systems are significantly
stronger than analogous amide-ester and even amide-amide
hydrogen-bonding interactions. The rotaxane architecture sig-
nificantly alters the reduction and oxidation properties of the
nitrone functional group, simultaneously shielding it from
chemical reduction by external agents while activating it toward
electrochemical reduction. Other properties of these rotaxanes,
including the variation in the Kerr effect under an alternating
electric field, which has been linked to the rate of pirouetting
of the macrocycle about the thread,16e are currently under
Supporting Information Available: Experimental procedures
and spectroscopic data for nitrone [2]rotaxanes 4, 5, and 6; full
crystallographic data (CIF); and details of the electrochemical
experiments. This material is available free of charge via the
Internet at http://pubs.acs.org.
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97, 7075–7080.
(35) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265–
322. (b) McLendon, G. Acc. Chem. Res. 1988, 21, 160–167.
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