Fast pirouetting motion in a pyridine bisamine-containing copper-complexed rotaxane

Article abstract of DOI:10.1002/chem.201304434

The present work reports the introduction of pyridine bisamine terdentate ligands in the structure of a pirouetting copper rotaxane. Rotaxane 2[PF ₆] constitutes the first example of the incorporation of imine-based dynamic covalent chemistry in the synthesis of switchable copper-complexed interlocked systems. In this rotaxane, the substitution of the classical terpyridine terdentate unit by a pyridine bisamine moiety has led to a significant stabilization of the pentacoordinated site. That fact has been evidenced by EPR spectroscopy and cyclic voltammetry. Regarding the tetracoordinated site, the congestion around the coordination sphere has been reduced to accelerate the typically slow reorganization of the Cu^II. Ethynyl-3,8-substitution on the axis phenanthroline along with the 2,9-diphenyl-1,10-phenanthroline (dpp) present in the macrocycle afforded a very stable coordination environment for Cu^I, which is at the same time labile upon oxidation. In summary, the incorporation of a pyridine bisamine unit as a terdentate ligand and the optimization of the bidentate ligand of the axle not only has led to a simplification of the synthetic procedures, but it has also given rise to a bistable systems with an enhanced energetic separation between states and an acceleration of the reorganization processes. Thus far, rotaxane 2[PF₆] presents the fastest switching cycle reported to date in copper-interlocked dynamic systems.

Full text of DOI:10.1002/chem.201304434

DOI: 10.1002/chem.201304434
Full Paper
&
Rotaxanes
Fast Pirouetting Motion in a Pyridine Bisamine-Containing
Copper-Complexed Rotaxane
Eugenio Coronado,[a] Pablo GaviÇa,*[b] Julia Ponce,[a] and Sergio Tatay*[a]
Abstract: The present work reports the introduction of pyri-
dine bisamine terdentate ligands in the structure of a pir-
ouetting copper rotaxane. Rotaxane 2[PF6] constitutes the
first example of the incorporation of imine-based dynamic
covalent chemistry in the synthesis of switchable copper-
complexed interlocked systems. In this rotaxane, the substi-
tution of the classical terpyridine terdentate unit by a pyri-
dine bisamine moiety has led to a significant stabilization of
the pentacoordinated site. That fact has been evidenced by
EPR spectroscopy and cyclic voltammetry. Regarding the tet-
racoordinated site, the congestion around the coordination
sphere has been reduced to accelerate the typically slow re-
organization of the CuII. Ethynyl-3,8-substitution on the axis
phenanthroline along with the 2,9-diphenyl-1,10-phenan-
throline (dpp) present in the macrocycle afforded a very
stable coordination environment for CuI, which is at the
same time labile upon oxidation. In summary, the incorpora-
tion of a pyridine bisamine unit as a terdentate ligand and
the optimization of the bidentate ligand of the axle not only
has led to a simplification of the synthetic procedures, but it
has also given rise to a bistable systems with an enhanced
energetic separation between states and an acceleration of
the reorganization processes. Thus far, rotaxane 2[PF6] pres-
ents the fastest switching cycle reported to date in copper-
interlocked dynamic systems.
Introduction
two stable states involves a change in the relative position of
the interlocked components through translational or rotational
motions that are electrochemically-triggered. Once the applied
electrochemical impulse is removed, the system does not
return spontaneously to the initial state, presenting thus a hys-
teretic behavior that can be exploited for the development of
single-molecule memory units.[8] The bistable behavior of
copper-based redox-active interlocked systems relies on the
different coordination requirements for CuI and CuII ions. On
passing from CuI to CuII, the system undergoes an intramolecu-
lar rearrangement from tetracoordinated to pentacoordinated
configurations.[9,10] These interconversions can be induced by
the application of an electrochemical potential and are charac-
terized to be near quantitative. However, when compared with
the purely organic systems, their major drawback is their long
response times.[11–13] A great synthetic effort has been devoted
to overcome this handicap.
There is an increasing interest within materials science in the
use of molecular structures for information processing.[1–3] Any
chemical system that can exist in at least two states with dis-
tinguishable spectral, electrochemical, or magnetic properties
can be considered a molecular switch, as long as the transition
between the different states could be controlled by an external
stimulus and does not occur spontaneously.[4] In addition, for
practical reasons electronic devices should operate at low re-
sponse times and in a controlled and reproducible manner.
Thus, the application of molecular systems as functional logic
gates requires the switching processes to be externally activat-
ed, quantitative, reversible, and fast.
Bistable rotaxanes and catenanes constitute a family of inter-
locked systems that in solution present a switching behavior,
and early attempts of constructing electronic devices with
these molecules have been already performed.[5–7] In redox-
active catenanes and rotaxanes, the switching between the
The earliest examples of bistable copper-based interlocked
systems were based on bidentate 2,9-disubstituted phenan-
throline and tridentate 5,5’’-disubstituted-2,2’:6,2’’-terpyridine
units.[14,15] These systems presented a marked redox bistability,
which could be switched under electrochemical control. How-
ever, the reorganization processes were found to be slow on
the cyclic voltammetry timescale. In particular, the limiting
step in the switching cycle consisted in the slow reorganization
of the CuII ion from the tetra to the pentacoordinated site. It
was found that this process could be accelerated by the use of
bidentate coordinating units, such as 3,8-disubstituted-2,2’-bi-
pyridines[11] and 8,8’-diphenyl-3,3’-biisoquinolines,[12,16] which
facilitate the approximation of nucleophiles to the metal
[a] Prof. E. Coronado, J. Ponce, Dr. S. Tatay
Instituto de Ciencia Molecular (ICMol)
Universidad de Valencia
C/Catedrꢀtico Josꢁ Beltrꢀn 2, 46980 Paterna (Spain)
E-mail: Sergio.tatay@uv.es
[b] Prof. P. GaviÇa
Centro de Reconocimiento Molecular y Desarrollo Tecnologico (IDM)
Universidad Politecnica de Valencia, Universidad de Valencia
C/Dr. Moliner 50, 46100 Burjassot (Spain)
E-mail: pablo.gavina@uv.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304434.
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center. However, this increment
in the reorganization rate con-
stants was always accompanied
by a lowering of the oxidation
potential. Regarding the triden-
tate coordination chelate, the
5,5’’-substituted
terpyridine
moiety has been maintained in-
variably throughout the years.
Although the preparative meth-
ods of substituted terpyridine li-
gands have been greatly im-
proved, especially by the intro-
duction of cross-coupling reac-
tions, in general, they still in-
volve the use of time-consuming
multi-step synthetic routes and
highly toxic organostannane re-
agents.[17] This report aims to
ease some of these aspects by
the use of non-hindering 3,8-bi-
Figure 1. Chemical structure of rotaxanes 1[PF6] and 2[PF6].
s(phenylethynyl)phenanthroline
and pyridine bisamine chelating
units.
Pyridine bisimine chelators have been proposed as a feasible
alternative to tridentate terpyridine ligands.[18,19] These ligands
can be easily formed by one-pot condensation of commercially
available 2,6-pyridinedicarbaldehyde with primary amines[20]
and form very stable ML2 octahedral complexes with a great
number of soft metal ions such as MnII, FeII, CoII, NiII, CuII, ZnII,
CdII, and HgII.[21] The reduction of the imine bonds to the corre-
sponding amines, transforms this chemically labile chelate into
the non-exchanging pyridine bisamine form.[22] In general,
imine nitrogen atoms lead to larger metal binding constants
than their saturated analogues due to the ability of unsaturat-
ed nitrogen to act not only as a s-donor but also as a p-ac-
ceptor.[23] That fact has been evidenced by the evaluation of
a tridentate pyridine bisamine unit and a 2,9-diphenylphenan-
throline bidentate moiety. To better evaluate the dynamic
properties of 2+ we have also synthesized rotaxane 1+, in
which the macrocycle (M1) contains only a phenanthroline-
based bidentate chelate as shown in Figure 1.
Herein we describe the synthesis of rotaxanes 1[PF6] and
2[PF6] and the characterization of their reduced and oxidized
forms by using UV/Vis, IR, and EPR spectrometry. Moreover, the
electrochemically controlled pirouetting motion of the bistable
copper-complexed rotaxane 2+ has been studied by cyclic vol-
tammetry.
the stability constants of a series of divalent metal complexes Results and Discussion
in which the pyridine units have been sequentially introduced
in linear polyamine ligands.[24] On the other hand, the substitu-
Chemical synthesis
tion of imine by amine nitrogen atoms increases the flexibility
of the chelate. In the particular case of CuII, this is particularly
important because it enables the adoption of less-strained
square-pyramidal geometries in pentacoordinated spheres.
Thereby, pyridine bisamines provide a larger stabilization of
the cupric state than terpyridines, pyridine bisimines, or fully
saturated triamine ligands.[25,26]
We decided to synthesize rotaxanes 1[PF6] and 2[PF6] by the
threading, followed by stoppering approach.[27] The three mac-
rocycles used in this work are presented in Figure 2. Macrocy-
cle M1 (for rotaxane 1) was synthesized as described else-
where.[28] The synthesis of heteroditopic ring M2 was carried
out in four steps, as shown in Figure 2, starting from 2,9-bis(p-
hydroxyphenyl)-1,10-phenanthroline (3), which was prepared
following previously described procedures.[29] In a first step,
a substitution reaction of 3 with 4-(Boc-amino)butyl bromide
(Boc=di-tert-butyl dicarbonate) under basic conditions afford-
ed 4 in reasonable yield (42%). Subsequent deprotection of
the Boc-protected amines with trifluoroacetic acid in dichloro-
methane afforded diamine 5 in 85% yield. Next, the ring clo-
sure of 5 was promoted by its double condensation with 2,6-
pyridinedicarbaldehyde. This reaction was carried out under
high dilution conditions to avoid oligomers as byproducts. Fi-
nally, the imine bonds were reduced in situ with NaBH4 to
Such considerations have motivated the design of pirouet-
ting rotaxane 2+, represented in Figure 1. Pirouetting rotax-
anes compose a class of switchable interlocked molecules in
which the intramolecular rearrangement involves the rotational
motion of a macrocyclic ring around the axis of the molecule.
In the linear component we have introduced the rigid 3,8-bis-
(substituted)phenanthroline unit to reduce the steric hindrance
around the copper center. This unit has been symmetrically
bonded to the stoppers by a phenylethylene spacer. As a mac-
rocycle, we have designed a bis-chelating ring (M2) containing
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catalyzed azide–alkyne cycload-
dition (CuAAC) between diacety-
lenic axle precursor 11 and
azide-functionalized stopper 14.
These reactions take place under
very mild basic conditions,[31]
which make this chemistry very
suitable for the stoppering of
CuI-threaded species.[32–37] The
synthesis of 11 and 14 are sum-
marized in Figure 3. Rigid linear
phenanthroline derivative 10
was obtained in 50% yield from
the reaction of 3,8-dibromo-1,10-
phenanthroline[38] (8) and ((4-
ethynylphenyl)ethynyl)trimethyl-
silane[39] (9) under Sonogashira
cross-coupling
conditions.[40]
Cleavage of the terminal trime-
thylsilyl groups under basic con-
ditions quantitatively yielded di-
acetylene 11. Azide-terminated
stopper 14 was synthesized in
two steps from 2-(4-(tris(p-tert-
butylphenyl)methyl)phenoxy)e-
thanol (12).[15] Thus, alcohol 12
was mesylated and then directly
treated with NaN3 in DMF to
obtain the azide-functionalized
stopper 14 in good yield
(74%).[41]
Figure 2. Coordinating macrocycles used in this study: M1, M2, and M3 (top). Synthesis of macrocycles M2
(middle) and M3 (bottom): a) 4-(Boc-amino)butyl bromide (2 equiv), Na2CO3, DMF, 608C (42%); b) TFA, CH2Cl2
(85%); c) 2,6-pyridinedicarbaldehyde, MeOH/CH2Cl2; d) NaBH4 (92%); e) 6-Aminohexanol (2 equiv), MeOH;
f) NaBH4; g) Di-tert-butyl dicarbonate, triethylamine, heated at reflux (97%); h) Isophthaloyl chloride, CH2Cl2, tri-
ethylamine (68%); i) Trifluoroacetic acid, CH2Cl2 (98%).
obtain the final non-labile pyridine bisamine macrocycle M2 in
high yield.[22]
Rotaxane 1+ was prepared as described in Figure 4. In a first
step, rod-like fragment 11 was threaded into macrocycle M1 in
the presence of CuI to yield the copper-complexed pseudoro-
taxane intermediate. Next, [Cu(M1)11]+ was treated with two
equivalents of the azide-terminated stopper 14 in the presence
of a CuI catalyst under copper-catalyzed azide-alkyne cycload-
dition (CuAAC) conditions. Previous work performed on the
preparation of metal-interlocked species by CuAAC has report-
For comparative reasons, a third macrocycle, named M3,
containing just one coordinating pyridine bisamine tridentate
site was also prepared. This macrocycle was prepared in three
steps starting from 2,6-pyridine carboxaldehyde in a good
overall yield of 65% (Figure 2). In the first step, compound 6
was prepared in one-pot by using a double imine condensa-
tion between 2,6-pyridine car-
boxaldehyde and 6-aminohexa-
nol, followed by reduction with
NaBH4 and subsequent protec-
tion of the amino group with di-
tert-butyl dicarbonate. Next, con-
densation of diol 6 with iso-
phthaloyl chloride under high di-
lution conditions promoted the
ring closure through a diester
formation to yield 7. Finally, the
Boc-amino protecting groups in
7 were cleaved with trifluoroace-
tic acid under standard condi-
tions[30] to give the pyridine bisa-
mine-containing macrocycle M3.
For the capping reaction, we
Figure 3. a) [Pd(PPh3)2Cl2], CuI, THF, iPr2NH (50%); b) K2CO3, MeOH (91%); c) MsCl, TEA, CH2Cl2 (93%); d) NaN3,
decided to perform a copper- DMF (74%).
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synthesized stoppered axle 16 in
70% yield by treating fragment
11 with two equivalents of
azide-terminated stopper 14
under CuAAC conditions. The in-
tertwined copper-complex pre-
cursor [Cu(5)(16)]+ was obtained
upon mixing molecular axle 16,
diamine precursor 5, and CuI in
a 1:1:1 ratio. In this complex,
copper coordination holds 16
and 5 together in the suitable
orientation for clipping. In situ
addition of 2,6-pyridine bisalde-
hyde promoted the condensa-
tion of the two pendant primary
amines around the axis, afford-
ing the pyridine bisimine triden-
tate moiety. Finally, in situ reduc-
tion to the corresponding pyri-
dine bisamine with NaBH4, fol-
lowed by water quenching and
Figure 4. Rotaxane 1+ prepared by the threading followed by a capping approach: a) [Cu(Me6tren)][Br], Na2CO3,
sodium ascorbate, CH3CN, CH2Cl2; b) KCN (aq); c) [Cu(CH3CN)4][PF6], CH3CN/CH2Cl2 (97%).
ed an increase in the yield of the stoppering reaction by the
use of a coordinatively saturated CuI catalyst.[42–44] That reason
motivated the use of [Cu(Me6tren)][Br], which has been de-
scribed as an efficient catalyst in
column chromatography afforded rotaxane 2+ in 22% yield.
Macrocycle M3 was treated with 3,8-bis((trimethylsilyl)ethyn-
yl)-1,10-phenanthroline (TMSPhen)[47] and Cu(OAc)2 to prepare
CuAAC,[45] and whose synthesis
is readily accessible from com-
mercial
ligand
tris(2-
(dimethylamino)ethyl)amine
(Me6tren) and CuBr. Once the
capping process was complete,
the resulting mixture was treat-
ed with aqueous KCN to remove
the copper salts and then puri-
fied by silica column chromatog-
raphy affording demetallated ro-
taxane 15 in 34% yield. Finally,
compound 1+ was obtained by
the re-metallation of 15 with
one equivalent of [Cu(CH3CN)4]-
[PF6]
under
an
argon
atmosphere.
Unfortunately, when the syn-
thesis of 2+ was attempted by
the threading of 11 into M2 fol-
lowed by stoppering under the
same conditions used for 1+, the
desired final product could not
be obtained. As an alternative,
and taking advantage of the dy-
namic nature of the reversible
imine covalent bond, we decid-
ed to use a clipping methodolo-
gy[46] for the preparation of 2+.
The clipping procedure used is
Figure 5. Rotaxane 2+ prepared in one pot by the clipping approach: a) 5, [Cu(CH3CN)4][PF6], CH3CN, CH2Cl2;
illustrated in Figure 5. First, we b) 2,6-pyridinedicarbaldehyde, MeOH; c) NaBH4 (22%).
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the pentacoordinated [Cu(M3)(TMSPhen)]2+ (17) complex.
Complex 17 was used as a reference in the characterization of
the oxidized rotaxane 22+(see later).
served and further confirmed by using comparative UV/Vis, IR,
and EPR studies, as shown below.
The structures of rotaxanes 1[PF6] and 2[PF6] were confirmed
1
by using mass spectrometry and 1D and 2D H NMR spectros-
UV/Vis characterization
1
copy (COSY, NOESY). Comparative H NMR spectra of both ro-
taxanes and axle 16 are shown in Figure 6. The appearance of
the aromatic signals below d=6.0 ppm corresponding to the
ring phenoxy moieties (Figure 6, Mm protons) confirmed un-
ambiguously the coordination of copper to the ring and the
axis.[15] Furthermore, the down- and upfield shifts experienced
by protons P2 and P5, respectively, clearly reveals the partici-
pation of the axis phenanthroline in the complex. NOE experi-
ments (see the Supporting Information) were carried out to
assign e, f, g, h, and i protons. Two new aromatic signals corre-
sponding to the pyridine chelate protons, Py3 and Py4, ap-
Figure 7a shows the UV/Vis spectra of macrocycles M1 and
M2, axis 16, and rotaxanes 1[PF6] and 2[PF6]. The intense ab-
sorption bands (e>50000 Mꢀ1cmꢀ1) below 400 nm are associ-
ated with p–p* ligand-centered (LC) transitions.[48]
Upon coordination, the LC band experienced a redshift of
approximately 25 nm. Moreover, several new bands responsi-
ble for the characteristic red color of tetrahedral [Cu(phen)2]+
complexes appeared in the visible region.[49] These broader
and less-intense bands (e<2000mꢀ1 cmꢀ1) are assigned to
metal-to-ligand charge-transfer (MLCT) transitions. Complexes
of the [Cu(phen)2]+ family typically present at least three
bands in the visible region: band I (above 500 nm), band II
(maximum around 430–480 nm, the most prominent), and
band III (390–420 nm, often hidden by the onset of band II),
whose intensities are related to the symmetry of the com-
plex.[49] As can be seen in Figure 7, in 1+ and 2+ the II and III
bands are masked by the tail of the strongly absorbing LC
bands. However, band I is still visible above 500 nm. In addi-
tion, pronounced low-energy shoulders extending down to
650 nm are characteristic of copper complexes with 2,9-aryl
substituted phenanthrolines.[49] Spectral data are summarized
in Table 1.
1
peared in the H NMR spectra of 2[PF6]. These signals present-
ed chemical shifts comparable to those of bare M2 molecule,
confirming the lack of participation of the pyridine diamine
moiety in the coordination to the copper ion.
Chemical oxidation and spectroscopic characterization
To elucidate the coordination environment of the CuII center in
the oxidized form of rotaxanes 1+ and 2+, compounds 12+
and 22+ were prepared by the treatment of dichloromethane
solutions of both rotaxanes with NO[BF4] and further precipita-
tion with KPF6. The identity of the target threaded-CuII species
was evidenced by HRMS(ES) analysis of dichloromethane solu-
tions containing 1[PF6]2 or 2[PF6]2, in which peaks correspond-
ing to 12+ (m/z: 1081.10) and 22+ (m/z: 1125.79) could be ob-
Chemical oxidation of 1+ and 2+ with NOBF4 proceeded by
with no appreciable change in the UV region, but with an in-
stantaneous color change from red to pale yellow associated
with the disappearance of the MLCT bands characteristic of CuI
complexes and the appearance
of a very weak band centered
around 670 nm and approxi-
mately
e=70mꢀ1 cmꢀ1 (Fig-
ure 7b). This band is similar to
that reported for tetracoordinat-
ed [Cu(dpp)2]2+ (dpp=2,10-di-
phenylphenanthroline)[14] or pen-
tacoordinated
(L=macrocyclic triethylenamine
derivate, lmax =670 nm, e=
[Cu(phen)(L)]2+
40mꢀ1 cmꢀ1)[50] and [Cu(phen)-
(terpy)]2+ (lmax =645 nm, e=
81mꢀ1 cmꢀ1).[51] Hence, although
the UV/Vis data supported the
existence of coordinated CuII
metal ions, given the minimal
expected differences in the UV/
Vis spectrum of 12+ and 22+ spe-
cies, we were unable to discrimi-
nate if CuII was tetra- or penta-
coordinated in 22+. Owing to
that, we decided to characterize
the rotaxane oxidized forms by
using EPR and IR spectroscopy.
Figure 6. 1H NMR spectra in CD2Cl2 of 16, 1[PF6], and 2[PF6]. Dashed lines denote the most representative proton
displacements.
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Figure 8. X-band EPR spectra of 12+ and 22+ recorded at 4 K in a) frozen
acetonitrile solution and in b) powder samples. Microwave frequency was
9.4723 GHz.
Table 2. Values of g and A of rotaxanes 12+ and 22+. [Cu(dpp)2][PF6]2
jj
and [Cu(dpp)(terpy)][PF6]2 parameters have been added for compari-
son.[52]
g
g
A [G]
jj
?
jj
1[PF6]2
2[PF6]2
[Cu(dpp)2][PF6]2
[Cu(dpp)(terpy)][PF6]2
2.065
2.063
2.073
2.045
2.269
2.225
2.276
2.233
150
172
Figure 7. a) Absorption UV/Visible spectra of macrocycles M1 and M2, mo-
lecular axis 16 and rotaxanes 1[PF6] and 2[PF6]. Data recorded in dichlorome-
thane (c=1ꢁ10ꢀ5 m); b) Absorption spectra of rotaxanes 1[PF6] and 2[PF6]
and their oxidized forms. Data recorded in dichloromethane (c=1ꢁ10ꢀ3 m)
(oxidant: NOBF4).
166
stronger (or the axial weaker).[53] This trend matches with the
Table 1. Absorption spectroscopic data in dichloromethane for macrocy-
cles M1 and M2, axle 16 and CuI complexes 1[PF6] and 2[PF6].
additional coordination of one imine nitrogen in 22+ with re-
spect to 32+. Moreover, the values of A and g obtained for
lmax [nm]
(e [mꢀ1 cmꢀ1])
jj
12+ perfectly match these reported for tetracoordinated
[Cu(dpp)2]2+ [52] whereas 22+ parameters better fit those report-
,
M1
324 (17400)
ed for other pentacoordinated CuII complexes in square-pyra-
midal coordination geometries.[50,53,54] In addition, when com-
pared to other CuII terpyridine-containing complexes,[51] our
M2
16
1[PF6]
2[PF6]
324 (14300)
363 (68800)
370 (58900), 505 (1200)
355 (38100), 605 (530)
higher g value supports the idea that the pyridine bisamine
jj
chelate leads to final less-strained square-pyramidal geometries
than in the terpy. Hence, EPR spectra fully confirmed that
upon oxidation M2 pirouettes around the central axis and
copper changes his coordination geometry from tetrahedral to
square pyramidal.
EPR studies
Any possible change in the metal coordination geometry leads
to a modification of the d-orbital splitting pattern that would
be reflected in the electron paramagnetic resonance (EPR)
spectra. Figure 8 shows the powder and frozen acetonitrile so-
lution EPR spectra of 12+ and 22+. Extracted g and parallel hy-
IR Spectroscopy
perfine coupling constants (A ) from solid samples of both ro-
jj
taxanes are compiled in Table 2. Analogous values were ob-
tained from frozen solution samples.
Near-IR spectra of rotaxanes 1[PF6] and 2[PF6] showed charac-
teristic absorption bands associated with aromatic heterocyclic
compounds. The oxidized forms of both rotaxanes presented
a very similar IR spectrum with the exception of the 1800–
1600 cmꢀ1 region, which is expanded in Figure 9, in which
a new intense band at 1720 cmꢀ1 appeared in 2[PF6]2. Such
a band was also found in the IR spectra of the reference CuII
complex 17 and has been previously observed in other CuII
pyridine bisamine complexes.[55,56] This observation further sup-
ports the idea of the generation of a new pentacoordinated
For both complexes we obtained the characteristic pattern
of axially elongated mononuclear CuII complexes, with g >g
jj
?
>2.04 and a well-resolved hyperfine coupling of the parallel
component with the 3/2 nuclear spin of the copper ion.[51,53]
However, on passing from 12+ to 22+ a decrease of g and an
jj
increase of A were observed. As predicted by ligand field
jj
theory and confirmed experimentally,[52] this is the expected
evolution of g and A as the equatorial ligand field becomes
jj
jj
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than in tetracoordinate CuI complexes with two 2,9-diphenyl-
phenanthrolines.[56] This responds to the reduced steric hin-
drance of the ethynyl groups, which facilitates the rearrange-
ment into the more flattened geometry adopted by CuII-
bis(phenanthroline) complexes.[60] Nevertheless, the redox po-
tential of 18 is still well above that of [Cu(phen)2]+ (0.19 V vs.
SCE). This fact has been attributed to the electron-donating
character of trimethylsilylacethylene groups.[61] Regarding the
pentacoordinate environment presented by complexes 17 and
II, the high negative potential observed in 17 (ꢀ0.52 V vs. SCE)
is indicative of the great stabilization of the cupric state pro-
vided by the pyridine bisamine chelate. Such a redox potential
is even lower than that reported for the hexacoordinate di-
valent [Cu(terpy)2]2+ complex I (ꢀ0.41 V vs. SCE).[56] This fact
has been ascribed to the greater basicity of amines compared
with pyridines and to the enhanced flexibility of the chelate
that minimizes the strain in the preferred square-pyramidal ge-
ometry.[62] Reported electrochemical data of comparable bis-
amine coordination spheres are in accordance with the mea-
sured redox potential.[25–26]
Figure 9. IR spectra between 1800 and 1525 cmꢀ1 of macrocycle M2, refer-
ence compound 17 and of rotaxanes 1+ and 2+ and their oxidized forms.
environment involving the new pyridine bisamine chelate
upon oxidation of rotaxane 2+.
Dichloromethane solutions of rotaxane 1+ presented a rever-
sible peak at 0.64 V versus SCE associated with the copper(II/I)
redox couple in a tetrahedral environment. The proximity of
this value to the one reported in CH2Cl2/CH3CN mixtures for
III[57] is noteworthy. This fact reveals that, due to the ability of
ethynyl substituents to stabilize cooper in low oxidation states,
the mixed 2,9-dpp/3,8-diethynyl-substituted bis(phenanthro-
line) environment, although it is less congested, is equally
Electrochemical studies
The electrochemical behavior of rotaxanes 1+ and 2+ was
studied by using cyclic voltammetry. As it has been shown, the
existence of two different coordination sites in M2 leads to the
electrochemically induced movement of the ring around the
axis in 2+, which is not possible in the case of 1+. Since tetra-
and pentacoordination geome-
tries afford well-separated po-
tential values for the CuI/CuII
couple, the position of the ring
can be monitored by using
cyclic voltammetry.[57] It has
been observed that subtle struc-
tural factors, like the electron-
donating character of the li-
gands, the geometry, or the
steric congestion around the
metal, can have a very signifi-
cant influence on the general
behavior of copper(II/I)-based
dynamic systems.[12,58] To better
evaluate these factors we mea-
sured the redox potentials of
complexes [Cu(M3)(TMSPhen)]-
[PF6]2 (17) and [Cu(TMSPhen)2]-
[PF6] (18) (TMSPhen=3,8-bis((tri-
methylsilyl)ethynyl)-1,10-phenan-
throline), whose chemical struc-
ture and redox potentials are
shown in Figure 10. Synthetic
data for 17 and 18 is included in
Figure 10. Copper(I/II) redox potentials of complexes 17 and 18, and 2[PF6]. Potentials of reference complexes I–
III in acetonitrile solution were extracted from the literature and are added for comparison.[56] Redox potentials
were measured in tetrabutylammonium hexafluorophosphate (TBAPF6) 0.1m dichloromethane solution. A silver
wire was used as pseudo-reference electrode. [Cu(dmp)2] (dmp=dimethylphenanthroline) was used as reference
the Supporting Information.
We can see from Figure 10
that the stabilization of the cu-
prous state is less accused in 18 (0.64 V vs. SCE in dichloromethane solution).[59]
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stable to oxidation than [Cu(dpp)2] environments. By contrast,
as shown in Figure 11, in the case of rotaxane 2+ the peak at
0.60 V associated to the oxidation of CuI in the tetrahedral en-
vironment becomes irreversible and a new peak at ꢀ0.56 V is
now apparent. Compared to the data obtained for reference
compound 17, the cathodic wave at ꢀ0.56 V was assigned to
the reduction of the CuII ion in the pentacoordinate environ-
ment. Upon scan reversal, no anodic wave was observed at
this potential and the anodic peak at 0.64 V was recovered
without any appreciable loss of intensity, thus confirming the
reversible rearrangement of the CuI ion into its original tetra-
coordinate site.
scribed an anodic shift with increasing the scan rate, from
which a value for the rate constant of the rearrangement of
tetrahedrally coordinated CuII to a pentacoordinated environ-
ment of k=6.2ꢁ102 sꢀ1 could be estimated (see the Support-
ing Information). Thus, the half-life (t1/2 =(ln2)/k) of the unsta-
ble tetrahedral CuII species is 1.1 ms. Assuming that this re-
arrangement constitutes the limiting step, these data prove
that rotaxane 2+ operates at the millisecond timescale even in
a non-coordinating solvent such as dichloromethane. The ki-
netic rates for the pirouetting motion are higher than those of
the fastest pirouetting rotaxanes reported to date.[11]
Upon repeated cycling in the ꢀ1.0/+1.2 V potential window
we did not observe any change in the shape of the voltammo-
grams, confirming the robustness of the gliding process. The
cyclic voltammograms recorded between 50 and 1000 mVsꢀ1
(the maximum scan rate accessible in our setup) did not show
the appearance of reversible waves in either oxidation or re-
duction processes. The evolution of the oxidation wave poten-
tial with the scan rate in the +0.2/+1.0 V range is illustrated in
Figure 12. As predicted by Nicholson and Shain,[63] the peak de-
Conclusion
In this report we have described the synthesis and dynamic
properties of fast-moving pirouetting copper rotaxane 2[PF6],
based on the use of a 3,8-bis(phenylethynyl)phenanthroline
axle and a macrocyclic component with two different recogni-
tion sites, a phenanthroline moiety and a pyridine bisamine
chelate. Compared with the use of classical terpyridine terden-
tate ligands in copper-based switchable systems, the introduc-
tion of the pyridine bisamine chelate offers not only a greater
stabilization of the cupric state derived from its higher flexibili-
ty, but also an easier synthesis. Moreover, the dynamic nature
of the imine bond enabled the self-assembly of bistable rotax-
ane 2+ by a clipping methodology. Regarding the tetrahedral
coordination site of CuI, the use of a 3,8-ethynyl-phenanthro-
line moiety has afforded a less congested environment around
the copper ion without lessening its redox stability. Thereby,
the thermodynamic stability of the reduced state is maintained
whereas that of the oxidized state is increased, affording a bi-
stable switchable rotaxane in which the electronic states are
separated by more than 1 V. Moreover, the estimated rates for
the electrochemically triggered switching processes take place
in the millisecond timescale. Thus, the introduction of 3,8-eth-
ynyl-phenanthroline and pyridine bisamine ligands as new
building blocks have afforded a synthetically accessible bista-
ble copper rotaxane with the highest switching rate constants
reported to date.
Figure 11. Cyclic voltammogram of rotaxane 2[PF6] (c=1ꢁ10ꢀ3 m) in TBAPF6
0.1m dichloromethane buffer solution. Scan rate: 0.1 Vsꢀ1
.
Experimental Section
General procedures
All chemicals used were purchased from commercial sources and
used without further purification, unless specially mentioned. An-
hydrous dichloromethane, acetonitrile, and tetrahydrofuran sol-
vents were freshly distilled under argon over the appropriate
drying agent (calcium chloride, calcium hydroxide, and sodium, re-
spectively). Column chromatography was carried out on silica gel
(60 ꢂ, 230–400 mesh).
1H NMR spectra were acquired on a Bruker AVANCE DRX 300 spec-
trometer. The spectra were referred to residual proton-solvent ref-
erences. Electrospray (ES) mass spectra were obtained with
a Waters Micromass ZQ spectrometer in the positive ion mode.
The extraction cone voltage was set to 10 V to avoid fragmenta-
tions. High-resolution ES mass spectra were recorded on a MICRO-
MASS QUATTRO TM (Waters) spectrometer in the positive ion
Figure 12. Cyclic voltammograms in TBAPF6 0.1m dichloromethane buffer
solution of rotaxane 2[PF6] at different scan rates (c), and rotaxane 1[PF6]
at 0.1 Vsꢀ1 (a) (c=1ꢁ10ꢀ3 m).
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mode. High-resolution (MALDI-TOF/TOF) mass spectra were record-
ed in a 5800 MALDI TOF/TOF (ABSciex) in positive reflector mode.
All photochemical measurements were carried out in dichlorome-
thane. Absorption spectra were recorded on a Shimadzu UV-
2501PC spectrophotometer using 1 cm path length cuvettes. First
derivative EPR spectra were recorded at 4 K in a Bruker ELEXYS
E580 equipped with continuous-flow cryostats for liquid helium.
EPR samples were measured as powder or solutions of approxi-
mate 1ꢁ10ꢀ3 m in dry acetonitrile. IR spectra were recorded in the
4000–400 cmꢀ1 frequency range on a Nicolet 5700 FTIR spectro-
photometer as pressed KBr pellets.
Diphenol phenanthroline 3,[64] macrocycle M1,[28] 3,8-dibromo-1,10-
phenanthroline 8[38] and 2-(4-(tris(p-tert-butylphenyl)methyl)phe-
noxy)ethanol 12,[15] were prepared according to described proce-
dures. The synthesis of 3,8-bis((trimethylsilyl)ethynyl)-1,10-phenan-
throline (TMSPhen) and complexes 17 and 18, is included in the
Supporting Information.
time, NaBH4 (0.045 g, 1.2 mmol) was added in small portions and
the yellowish solution was stirred overnight. Solvents were re-
moved under reduced pressure affording a yellow oil that was re-
dissolved in dichloromethane and washed with 0.5m NH4Cl until
the pH of the discarded aqueous phase was 7. The organic layer
was dried over Na2SO4 and concentrated under reduced pressure
1
to yield macrocycle M2 as an off-white solid (0.23 g, 92%). H NMR
(300 MHz, CDCl3): d=8.41 (d, J=8.9 Hz, 4H), 8.25 (d, J=8.4 Hz,
2H), 8.06 (d, J=8.4 Hz, 2H), 7.74 (s, 2H), 7.65 (t, J=7.6 Hz, 1H),
7.24 (d, J=7.6 Hz, 2H), 7.10 (d, J=8.9 Hz, 4H), 4.17 (t, J=6.7 Hz,
4H), 3.96 (s, 4H), 2.81 (t, J=7.5 Hz, 4H), 1.98–1.86 (m, 4H), 1.84–
1.70 ppm (m, 4H); IR: (KBr): n˜ =3403 (br), 3185 (br), 2932 (m), 2864
(m), 1601 (s), 1572 (s), 1487 (s), 1400 (s), 1240 (s), 1174 (s), 828 cmꢀ1
(m); MS (ES): m/z (%): 610.48(100) [M+H]+
Boc-protected pyridine-2,6-diylbis(methylene)bis(6-hydroxyhex-
yl) (6): 2,6-Pyridine carboxaldehyde (0.34 g, 2.50 mmol) was added
to a solution of 6-aminohexanol in anhydrous methanol (50 mL).
After stirring for 1 h at room temperature, NaBH4 (0.26 g,
7.00 mmol) was added and the reaction mixture was stirred over-
night. Triethylamine (TEA) (5 mL) and di-tert-butyl dicarbonate
(2.9 mL) were added to the mixture and then it was heated at
reflux during 4 h. Solvents were removed under reduced pressure
and the resulting white semisolid was dissolved in dichlorome-
thane (40 mL) and washed four times with distilled water. The or-
ganic layer was dried over Na2SO4 and concentrated under reduced
pressure to afford the targeted product as colorless oil (1.30 g,
Synthesis
N-Boc-protected diamine 4: A solution of 4-(Boc-amino)butyl bro-
mide (2.00 g, 9 mmol) in dry DMF (10 mL), was added dropwise to
a flask containing 3 (1.11 g, 3 mmol) and K2CO3 (3.37 g, 24.4 mmol)
in dry DMF (20 mL). The solution was stirred at 608C for 5 days.
The solvent was removed under reduced pressure and the result-
ing solid was dissolved in dichloromethane (40 mL). The organic
phase was washed twice with aqueous NH4Cl 0.5m and then with
distilled water. Then it was dried over Na2SO4 and concentrated
under reduced pressure. The resulting yellow oil was purified by
silica column chromatography (CH2Cl2/MeOH) to afford 0.32 g
(15%) of the targeted di-substituted product 4, 0.49 g (30%) of the
mono-substituted product and 0.37 g (21%) of the starting materi-
al. The starting material and the mono-substituted product were
made react again with 4-(Boc-amino)butyl bromide under the
1
97%). H NMR (300 MHz, CDCl3): d=7.61 (t, J=7.8 Hz, 1H), 7.07 (m,
2H), 4.49 (m, 4H), 3.61 (t, J=6.3 Hz, 4H), 3.26 (d, J=28.2 Hz, 4H),
1.65–1.40 (m, 12H), 1.38 ppm (s, 18H).
Boc-protected macrocycle 7: A solution of isophthaloyl chloride
(0.51 g, 2.50 mmol) in distilled dichloromethane (100 mL) was
added dropwise over a stirred solution of 6 (1.30 g, 2.50 mmol)
and TEA (1.6 mL, 11.25 mmol) in distilled dichloromethane
(250 mL). The reaction mixture was stirred under argon at room
temperature for three days. The solvent was partially evaporated
and the remaining solution was washed four times with water and
dried over Na2SO4. Evaporation of the solvent led to a colorless oil
that was purified by column chromatography (silica gel, CH2Cl2/
MeOH) to yield the pure product as a white solid (1.14 g, 68%).
1H NMR (300 MHz, CDCl3): d=8.59 (t, J=1.6 Hz, 1H), 8.25 (dd, J=
7.8, 1.6 Hz, 2H), 7.66–7.51 (m, 2H), 7.22–7.03 (m, 2H), 4.49 (m, 4H),
4.35 (t, J=6.0 Hz, 4H), 3.95 (s, 4H), 3.26 (m, 4H), 1.88–1.68 (m, 4H),
1.50–1.46 (m, 8H), 1.39 ppm (s, 18H); MS (ES): m/z (%): 668.46
[M+H]+; 690.52 [M+Na]+.
1
same conditions to afford 4 in global 42% yield. H NMR (300 MHz,
CDCl3): d=8.41 (d, J=8.8 Hz, 4H), 8.24 (d, J=8.5 Hz, 2H), 8.06 (d,
J=8.5 Hz, 2H), 7.72 (s, 2H), 7.09 (d, J=8.8 Hz), 4.67 (s, 2H), 4.09 (t,
J=6.2 Hz, 4H), 3.23 (dd, J=12.9, 6.5 Hz, 4H), 1.87 (m, 4H), 1.71 (dt,
J=13.1, 6.4 Hz, 4H), 1.46 ppm (s, 18H). MS (ES): m/z (%): 707.25
(100) [M+H]+. Mono-substituted derivative: 1H NMR (300 MHz,
CDCl3): d=8.44 (d, J=8.8 Hz, 2H), 8.41 (d, J=8.8 Hz, 2H), 8.26 (d,
J=8.5 Hz, 2H), 8.09 (d, J=8.5 Hz, 2H), 7.12 (d, J=8.8 Hz, 2H), 7.09
(d, J=8.8 Hz, 2H), 4.10 (t, J=6.2 Hz, 2H), 3.15 (dd, J=13.3, 6.6 Hz,
2H), 1.87 (m, 2H), 1.71 (m, 2H), 1.46 ppm (s, 9H). MS (ES): m/z (%):
536.33 (100) [M+H]+.
Macrocycle M3: Boc-protected macrocycle 7 (0.18 g, 0.28 mmol)
was dissolved in a mixture of dichloromethane and trifluoroacetic
acid (9:1, 10 mL) and stirred at room temperature for 24 h. The sol-
vent was then removed under reduced pressure affording an oil
that was mixed with 1m aqueous NaOH and extracted with di-
chloromethane. The organic phase was washed with water, dried
over Na2SO4, and concentrated under reduced pressure to afford
macrocycle M3 as a white solid (0.13 g, 98%). 1H NMR (300 MHz,
CDCl3): d=8.59 (t, J=1.7 Hz, 1H), 8.25 (dd, J=7.8, 1.7 Hz, 2H),
7.54 (dd, J=11.8, 4.3 Hz, 1H), 7.26 (s, 1H), 7.06 (d, J=7.6 Hz, 2H),
4.35 (t, J=6.3 Hz, 4H), 3.88 (s, 4H), 2.67 (t, J=6.7 Hz, 4H), 1.79 (dd,
J=8.4, 6.3 Hz, 4H), 1.69–1.36 ppm (m, 13H); MS (ES): m/z (%):
468.57 [M+H]+.
Phenanthroline diamine 5: Boc-amino protected compound 4
(0.91 g, 1.29 mmol) was dissolved in a mixture of dichloromethane
and trifluoroacetic acid (9:1, 30 mL) and stirred at room tempera-
ture for 24 h. The solvent was removed under reduced pressure af-
fording a yellow oil that was then mixed with aqueous NaOH 1m
and extracted with dichloromethane. The organic phase was
washed with water, dried over Na2SO4 and concentrated under re-
1
duced pressure to yield 5 as a yellow solid (0.53 g, 85%). H NMR
(300 MHz, CDCl3): d=8.48 (d, J=8.9 Hz, 4H), 8.26 (d, J=8.5 Hz,
2H), 8.08 (d, J=8.5 Hz, 2H), 7.74 (s, 2H), 7.10 (d, J=8.9 Hz, 4H),
4.11 (t, J=6.4 Hz, 4H), 2.81 (t, J=7.1 Hz, 4H), 1.98–1.81 (m, 4H),
1.67 ppm (dt, J=10.3, 7.1 Hz, 4H); MS (ES): m/z (%): 507.65 (25)
[M+H]+; 254.37 (100) [M+H]2+
.
((4-Ethynylphenyl)ethynyl)trimethylsilane (9): A solution of 1,4-di-
ethynylbenzene (2.00 g, 16 mmol) in anhydrous THF (200 mL) was
placed in a two-neck round-bottom flask under argon, and cooled
to ꢀ788C. n-BuLi was then added dropwise (10 mL, 1.6m in
hexane, 16 mmol). After stirring for 1 h at ꢀ788C, chlorotrimethyl–
silane (3 mL, 24 mmol) was added dropwise and the solution was
Macrocycle M2: Diamine 5 (0.20 g, 0.40 mmol) was placed in
a round-bottom flask under argon and dissolved in a mixture of di-
chloromethane (40 mL) and anhydrous methanol (20 mL). Then,
2,6-pyridinedicarbaldehyde (0.054 g, 0.4 mmol) was added and the
mixture was stirred at room temperature for 30 min. After that
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then allowed to warm to room temperature overnight. The reac-
tion was quenched with water and THF was eliminated under re-
duced pressure. Diethyl ether (70 mL) was added, and the organic
phase was washed with water and dried over Na2SO4. Evaporation
of the solvent led to a yellow solid composed of 68% of the de-
sired compound 9, 12% of the diprotected derivative and 20% of
starting material. The excess of starting material was evaporated
under vacuum and the resulting mixture of mono and di-protected
compounds was used in the following stage without further purifi-
cation. 1H NMR (CDCl3, 300 MHz): d=7.41 (s, 4H), 3.15 (s, 1H),
0.25 ppm (s, 9H). 1,4-bis((trimethylsilyl)ethynyl)benzene: 1H NMR
(CDCl3, 300 MHz): d=7.38 (s, 4H), 0.24 ppm (s, 18H).
methane (4 mL). After 30 min of stirring under argon, a degassed
solution of phenanthroline diacetylene 11 (0.02 g, 0.05 mmol) in di-
chloromethane (20 mL) was added through a cannula. The mixture
immediately turned dark brown and was left to react for another
30 min at room temperature. Then the solvents were removed
under reduced pressure. Azide stopper 14 (0.07 g, 0.13 mmol),
Na2CO3 (0.003 g, 0.025 mmol), and sodium ascorbate (0.02 g,
0.10 mmol) were added to the remaining solid and the mixture
was dissolved in degassed dichloromethane (4 mL). In a separate
flask, CuBr (3.7 mg, 0.025 mmol) and Me6tren (6.7 mL, 0.025 mmol)
in degassed acetonitrile (2 mL) were heated for 30 min at 608C
and the resulting green solution was added through a cannula to
the reaction mixture. After 7 days of stirring under argon at room
temperature, the crude was treated with aqueous KCN (20 mL,
120 mg, 0.46 mmol) and dichloromethane (10 mL). After 4 h of vig-
orous stirring, the two phases were separated and the organic
layer was washed with water, dried over Na2SO4, and concentrated
under reduced pressure. The crude product was purified by silica
column chromatography using CH2Cl2/MeOH as eluent to yield de-
metallated rotaxane 15 as a yellow solid (0.354 g, 34%). 1H NMR
(300 MHz, CDCl3): d=9.35 (s, 1H), 9.25 (s, 1H), 9.04 (s, 1H), 8.67 (s,
1H), 8.31 (s, 1H), 8.26 (d, J=8.3 Hz, 2H), 8.02 (s, 1H), 7.91–7.76 (m,
11H), 7.73–7.43 (m, 10H), 7.21 (t, J=6.9 Hz, 14H), 7.15–6.90 (m,
20H), 6.72 (d, J=8.7 Hz, 9H), 4.81 (s, 2H), 4.38 (s, 5H), 4.22 (s, 3H),
4.11 (s, 5H), 3.76 (s, 9H), 3.56 (s, 5H), 1.29 ppm (s, 27H); MS (ES):
m/z (%): 2099.02(100) [M+H+].
3,8-Bis((4-((trimethylsilyl)ethynyl)phenyl)ethynyl)-1,10-phenan-
throline (10): The previous mixture of mono and disylilated deriva-
tives (1.62 g, ca. 6.80 mmol of 9), 3,8-dibromo-1,10-phenanthroline
(8) (0.92 g, 2.72 mmol), [CuI] (0.05 g, 0.27 mmol), [PdCl2(PPh3)2]
(0.11 g, 0.16 mmol), (iPr)2EtN (8 mL), and anhydrous THF (20 mL)
were heated under argon at 608C for four days. The resulting mix-
ture was filtered and the solid washed with dichloromethane
(40 mL). The combined organic solutions were washed with water
(4ꢁ15 mL) and dried over Na2SO4. The solution was filtered and
the solvent was evaporated. The resulting solid was chromato-
graphed over silica gel using CH2Cl2/MeOH as eluent to yield 10 as
1
a yellow solid (0.77 g, 50%). H NMR (300 MHz, CDCl3): d=9.26 (d,
J=2.1 Hz, 2H), 8.36 (d, J=2.1 Hz, 2H), 7.55 and 7.49 (AB system,
8H), 0.26 ppm (s, 18H); HRMS (ES): m/z (%): 573.2168 (100)
[M+H]+.
Rotaxane 1[PF6]: Under an argon atmosphere, an acetonitrile solu-
tion (1 mL) of [Cu(CH3CN)4][PF6] (3.3 mg, 0.01 mmol) was added
through a cannula over a solution of demetallated rotaxane 15
(0.02 g, 0.01 mmol) in distilled dichloromethane (2 mL). The result-
ing brown solution was stirred for 1 h and the solvents were
evaporated under reduced pressure affording the metallated rotax-
3,8-Bis((4-ethynylphenyl)ethynyl)-1,10-phenanthroline (11):
A
suspension of the bis-silylated phenanthroline 10 (0.77 g,
1.35 mmol) and K2CO3 (0.19 g, 1.35 mmol) in methanol (30 mL) was
stirred at room temperature for 1 h. Evaporation of the solvent
yielded a solid which was suspended in water, filtered and washed
with plenty of water affording the pure product as an off-white
solid (0.50 g, 91%). 1H NMR (300 MHz, CDCl3): d=9.30 (d, J=
2.1 Hz, 2H), 8.42 (d, J=2.1 Hz, 2H), 7.84 (s, 2H), 7.60 and 7.55 (AB
system, 8H), 3.23 ppm (s, 2H).
1
ane 1[PF6] as a red/brown solid (2.2 mg, 97%). H NMR (300 MHz,
CD2Cl2): d=8.89 (d, J=1.7 Hz, 2H), 8.66 (d, J=8.3 Hz, 2H), 8.39 (d,
J=1.7 Hz, 2H), 8.26 (s, 2H), 8.16 (s, 2H), 8.10 (d, J=8.3 Hz, 2H),
8.08 (s, 2H), 7.89 (d, J=8.3 Hz, 4H), 7.64 (d, J=8.2 Hz, 4H), 7.25 (d,
J=8.5 Hz, 12H), 7.20–7.05 (m, 20H), 6.79 (d, J=8.9 Hz, 4H), 5.83 (d,
J=8.6 Hz, 4H), 4.81 (t, J=4.6 Hz, 4H), 4.39 (t, J=4.6 Hz, 4H), 3.93
(d, J=4.9 Hz, 4H), 3.82 (d, J=4.9 Hz, 4H), 3.49 (t, J=4.8 Hz, 4H),
3.10 (t, J=4.8 Hz, 4H), 1.28 ppm (s, 54H); IR: (KBr): n˜ =3440 (br),
3143 (br), 2958 (m), 2866 (m), 2207 (w), 1718 (w), 1603 (m), 1505
(m), 1400 (m), 1246 (s), 842 (s), 557 cmꢀ1 (m); MS (ES): m/z (%):
2161.93(100) [M+].
Mesylate-derivative stopper 13: TEA (13 mL) and methanesulfonyl
chloride (0.62 mL, 8.0 mmol) were added dropwise over a stirred
solution of 12 (1.13 g, 2.00 mmol) in dry dichloromethane (20 mL)
at 08C. The mixture was stirred for 2 h at 08C, and then it was al-
lowed to reach room temperature overnight. The resulting organic
solution was washed with water, dried over Na2SO4, and concen-
trated under reduced pressure to obtain the pure product as
1
a white solid (1.19 g, 93%). H NMR (300 MHz, CDCl3): d=7.23 (d,
Oxidized rotaxane 1[PF6]2: A freshly prepared acetonitrile solution
of NOBF4 (0.5 mL, 0.01m) was added over a solution of 1[PF6]
(8.8 mg, 0.005 mmol) in dichloromethane (3 mL). After 10 min of
stirring, the color of the solution completely changed from red/
brown to green/yellow. Once the solvents were evaporated, the
solid was re-dissolved in acetonitrile (3 mL) and precipitated with
a 0.1m aqueous solution of K[PF6] (8 mL,). The resulting precipitate
was filtered and washed with water affording the pure product as
a yellow solid (1.2 mg, 98%). IR: (KBr): n˜ =3445 (br), 3201 (br), 2960
(m), 2209 (w), 1635 (m), 1505 (w), 1400 (m), 1247 (m), 843 (m),
558 cmꢀ1 (m); HRMS (ES): m/z (%): 1081.10 (10) [M2+].
J=8.5 Hz, 6H), 7.11 (d, J=8.9 Hz, 2H), 7.07 (d, J=8.5 Hz, 6H), 6.76
(d, J=8.9 Hz, 2H), 4.59–4.53 (m, 2H), 4.25–4.19 (m, 2H), 3.08 (s,
3H), 1.30 ppm (s, 27H).
Azide-derivative stopper 14: A mixture of 13 (0.40 g, 0.62 mmol)
and NaN3 (0.20 g, 3.10 mmol) was dissolved in DMF (10 mL) and
stirred at 508C for 24. After evaporation of the solvent, the result-
ing solid was dissolved in dichloromethane and the organic layer
was washed with water, dried over anhydrous Na2SO4 and concen-
trated under reduced pressure affording a white solid that was pu-
rified by column chromatography (silica gel, Hexane/CH2Cl2) to
1
yield the pure product 14 as a white solid (0.27 g, 74%). H NMR
Axle 16: Phenanthroline diacetylene 11 (0.04 g, 0.1 mmol), azide-
functionalized stopper 14 (0.14 g, 0.25 mmol), and hydrazine
monohydrate (1.2 mL, 0.025 mmol) were dissolved in dichlorome-
thane (10 mL). In a separate flask, [CuBr] (7.0 mg, 0.05 mmol) and
Me6tren (13 mL, 0.05 mmol) were dissolved in degassed acetonitrile
(4 mL) and stirred for 30 min. The greenish catalyst mixture was
added through a cannula to the reaction mixture and stirred under
(300 MHz, CDCl3): d=7.23 (d, J=8.7 Hz, 6H), 7.09 (m, 8H), 6.78 (d,
J=9.0 Hz, 2H), 4.13 (t, J=6.0 Hz, 2H), 3.58 (t, J=6.0 Hz, 2H),
1.30 ppm (s, 27H); HRMS (ES): m/z (%): calcd for 591.4051(100)
[M+NH4]+; elemental analysis calcd (%) for C39H47N3O: C 81.63; H
8.26; N 7.32; found: C 80.22; H 8.04; N 6.93.
Demetallated rotaxane 15: A degassed solution of [Cu(CH3CN)4]-
[PF6] (0.02 g, 0.05 mmol) in dry CH3CN (4 mL) was added to a de-
gassed solution of macrocycle M1 (0.03 g, 0.05 mmol) in dichloro-
argon at room temperature for 5 days. Aqueous KCN (6 mgmLꢀ1
5 mL) and dichloromethane (10 mL) were added to the crude in
,
Chem. Eur. J. 2014, 20, 6939 – 6950
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6948
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
order to eliminate the copper. After 4 h of stirring, the two phases
were separated and the organic phase was washed with water,
dried over Na2SO4 and concentrated under reduced pressure. The
crude product was purified by silica column chromatography using
CH2Cl2/MeOH as eluent to afford 14 as a yellow solid (0.11 g, 70%).
1H NMR (300 MHz, CDCl3): d=9.30 (d, J=2.0 Hz, 2H), 8.40 (d, J=
2.0 Hz, 2H), 8.04 (s, 2H), 7.89 (d, J=8.4 Hz, 4H), 7.82 (s, 2H), 7.69
(d, J=8.4 Hz, 4H), 7.23 (d, J=8.7 Hz, 12H), 7.11 (d, J=9.0 Hz, 4H),
7.06 (d, J=8.7 Hz, 12H), 6.77 (d, J=9.0 Hz, 4H), 4.83 (t, J=4.6 Hz,
4H), 4.40 (t, J=4.6 Hz, 4H), 1.29 ppm (s, 54H); MS (MALDI TOF):
m/z (%): 1576.98 (100) [M+].
versidad de Valencia), and especially Dr. Isabel Solana, for their
collaboration.
Keywords: amines
· copper · cyclic voltammetry · EPR
spectroscopy · rotaxanes
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The present work has been funded by the EU (Project ELFOS
and ERC Advanced Grant SPINMOL), the Spanish MINECO
(grants MAT2011-22785, MAT2007-61584, and the CONSOLIDER
project on Molecular Nanoscience), and the Generalidad Va-
lenciana (Prometeo and ISIC Programmes of excellence). J.P
and S.T thank the Spanish Ministerio de Educaciꢃn, Cultura y
Deporte for their predoctoral fellowship and JdlC contract. S.T
also thanks the EU for the FP7-PEOPLE-2012-CIG-321739 grant.
Prof. Antonio Domenech (Universidad de Valencia) is kindly ac-
knowledged for fruitful discussions. We also thank SCSIE (Uni-
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Received: November 12, 2013
Published online on April 23, 2014
Chem. Eur. J. 2014, 20, 6939 – 6950
www.chemeurj.org
6950
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim