Rotationally Active Ligands: Dialing-Up the Co-conformations of a [2]Rotaxane for Metal Ion Binding

Article abstract of DOI:10.1002/anie.201607281

A novel [2]rotaxane was constructed that has a bidentate N,N′-chelate as part of a rigid, H-shaped axle and a 24-membered crown ether macrocycle containing six ether O-atoms and an olefinic group as the wheel. This unique topology produces a ligand with t

Full text of DOI:10.1002/anie.201607281

Angewandte
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International Edition: DOI: 10.1002/anie.201607281
German Edition: DOI: 10.1002/ange.201607281
Supramolecular Chemistry
Rotationally Active Ligands: Dialing-Up the Co-conformations of
a [2]Rotaxane for Metal Ion Binding
Giorgio Baggi and Stephen J. Loeb*
Abstract: A novel [2]rotaxane was constructed that has
a bidentate N,N’-chelate as part of a rigid, H-shaped axle and
a 24-membered crown ether macrocycle containing six ether O-
atoms and an olefinic group as the wheel. This unique topology
produces a ligand with the ability to dial-up different donor sets
for complexation to metal ions by simply rotating the wheel
about the axle. The solution and solid-state structures of the
+
+
free ligand and complexes with Li and Cu show how the
ligand adopts different rotational co-conformations for each.
+
+
The Li ion uses the N,N’-chelate and O-donors while the Cu
center is coordinated to both O-donors and the olefinic group.
This concept of rotationally active ligands should be possible
with a wide variety of donor sets and could find broad
application in areas of coordination chemistry, such as catalysis
and metal sequestration.
O
ne of the pioneering methods for the preparation of
mechanically interlocked and topologically non-trivial mole-
cules, such as catenanes, rotaxanes, and molecular knots,
[
1]
involved the use of transition metal ion templates. Most
notably the utilization by Sauvage of tetrahedral Cu com-
I
plexes with orthogonally positioned 1,10-phenanthroline
[
2]
ligands. Although these types of templates have been
subsequently exploited to make, for example, higher-order
[
3]
knots, redox and light switchable mechanically interlocked
[4]
molecules (MIMs), there has been very little research into
exploiting the interlocked topology of MIMS for ligand
design in applications such as selective metal ion sequestra-
Figure 1. a) An axle with a chelating site, b) struts as stoppers, c) a
macrocycle with donor atoms, d) a rotationally active ligand (three
arbitrary color sets are shown for illustrative purposes), e) three co-
conformations for coordination to a metal ion via rotation of the
macrocycle wheel. The long arrow represents direct coordination while
shorter arrows represent weaker second-sphere interactions.
[5]
[6]
tion or catalysis. In particular, we were intrigued by the
possibility of organizing disparate donor sets into the three-
dimensional cavity created between a linear axle and a macro-
cyclic wheel of a [2]rotaxane. The interlocked nature of the
system provides the possibility of rotating the macrocyclic
wheel relative to the axle and preferentially dialing-up donor
sets utilizing different co-conformations of the MIM. This
concept of a rotationally active ligand is outlined in Figure 1.
We chose an initial design based on our recent success in
preparing rigid [2]rotaxane linkers for incorporation into
membered crown ethers to create effective [2]rotaxane
[
8]
molecular shuttles. By removing the spacer group used to
separate the recognition sites, we can generate an axle in
which two benzimidazole groups are directly bonded to each
other resulting in a single recognition site that is a bidentate,
N,N’-chelate. Scheme 1 outlines how this new chelating axle
was prepared and a macrocyclic crown ether wheel clipped
around it employing Grubbsꢀ ring-closing metathesis (RCM)
[7]
metal–organic frameworks (MOFs). Terphenylene struts,
such as those shown in Figure 1, have been successfully
combined with benzimidazolium recognition sites and 24-
[
9]
to yield a [2]rotaxane ligand (L). Herein, we demonstrate
that this type of ligand can coordinate to different metals by
utilizing the axle chelate and rotating the macrocycle to access
the most suitable donor set. Importantly, given the variety of
known macrocyclic ligands and the excellent templating
ability of this orthogonal approach, this concept of rotation-
ally active ligands might be useful for an assortment of
chemical applications.
[
*] Dr. G. Baggi, Prof. Dr. S. J. Loeb
Department of Chemistry and Biochemistry, University of Windsor
Windsor, Ontario, N9B 3P4 (Canada)
E-mail: loeb@uwindsor.ca
Homepage: http://www.uwindsor.ca/loeb
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201607281.
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then applied to a larger scale, separation by column chroma-
I
tography on Cu -adsorbed silica (see the Supporting Infor-
I
I
mation). Although the adsorption of Cu and Ag salts on
silica, zeolites, resins, and other solid supports has been
exploited for the industrial separation of olefin/paraffin
[
11]
mixtures,
and AgNO -impregnated silica is a powerful
3
tool for the separation of mixtures of unsaturated com-
[
12]
pounds, there are no reports describing the preparation and
I
use of Cu -adsorbed silica for the separation of complex
mixtures of olefins by column chromatography. The novelty
of this procedure resides in the ease of preparation of the
stationary phase and in the high separation efficiency of
[
13]
structurally similar isomers.
cis
trans
Once separated, L and L
H and C NMR spectroscopy and ESI mass spectrometry.
were fully characterized by
1
13
Unambiguous identification was made by single-crystal X-ray
diffraction. Figure 2 compares the X-ray structures of isomers
cis
trans
Scheme 1. Preparation of a mixture of [2]rotaxane ligands L and L
via Grubbs’ RCM.
cis
trans
The isolated mixture of L and L
was initially screened
for binding with alkali metal cations using UV/Vis and NMR
titrations. Of the alkali metals, only the smallest was bound
significantly in the predictably tight pocket between axle and
cis
trans
Figure 2. Single-crystal X-ray structures of: left, L and right L
.
O red, N blue, C black; gold bonds=axle, silver bonds=wheel. Only
H-atoms involved in H-bonding or part of the olefinic moiety of the
+
À1
+
wheel, Li (r= 0.90 ꢁ, logK = 5.95m ), Na (r= 1.16 ꢁ,
[
14]
À1
+
À1
macrocycle are shown for clarity.
logK = 2.63m ), K (r= 1.52 ꢁ, logK < 0.1m ); larger ions
resulted in only small changes in chemical shift and broad
resonances that were interpreted as weak external interac-
tions with crown ether oxygen atoms only. Given this size
selectivity, a similar sized transition metal cation was
cis
trans
L
and L . Both axles are centrosymmetric with the two
benzimidazole NH groups involved in H-bonding to the first
and fifth ether O-atoms of the 24-membered macrocycle
+
À1
cis
screened; Cu (r= 0.91 ꢁ, logK = 3.29m ) also appeared to
linkage (L NH···O; 2.93 ꢁ (1608) and NH···O; 3.00 ꢁ
cis
trans
trans
be suitable for coordination to the L /L
A surprising observation was made from the H NMR
spectrum of the L /L
excess of Cu ions. It was apparent that the olefinic double
bond of only one of the isomers was participating in
coordination to the metal center, while the other isomer
ligand mixture.
(1608); L
NH···O; 2.93 ꢁ (1608) and NH···O; 2.98 ꢁ (1578).
Figure 3 shows a comparison of the H NMR spectra of
after addition of an excess of Cu (BF salt). It
forms a stable complex, [Cu-
(L )] , while L becomes protonated due to small amounts
of water, which in combination with Lewis acidic Cu ions and
a basic bis-benzimidazole rotaxane, promotes proton transfer.
The H NMR spectrum of [Cu(L )] shows that the olefinic
1
1
cis
trans
cis
trans
I
À
mixture upon the addition of an
L
and L
4
I
trans
was determined that L
trans
+
cis
I
I
was becoming protonated. Cu olefin complexes are well-
[
10]
1
trans
+
known, but this significant difference in behavior of two
closely related species was puzzling. It was also reasoned that
this difference in coordination chemistry might be used to
separate the two isomers. To this end, preparative TLC plates
trans
H-atoms of L
are shifted upfield (Dd = À0.93 ppm),
indicating a significant interaction of the double bond with
I
the Cu center.
(
see the Supporting Information) were pre-treated with
At this juncture, it was of interest to determine exactly
I
trans
a solution of Cu ions in CH CN and the mixture eluted
with a 9:1 (v:v) CHCl :CH CN solution. One isomer
appeared to have no interaction with the Cu ions and
eluted close to the solvent front while the other isomer eluted
more slowly (R values: L 0.78, L
how L
binds to the most strongly interacting metal cations,
3
+
+ trans + +
trans
Li and Cu . Firstly, both [Li(L )] and [Cu(L )] were
3
3
I
1
1
fully characterized in CDCl solution by H- H 2D ROESY
3
1
1
and H- H 2D COSY NMR experiments. Figure 4 shows the
relative positions of the axle and wheel components as
cis
trans
0.45). This method was
f
2
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trans
+
Figure 5. Single-crystal X-ray structure of [Li(L )] : left, N O tetrahe-
2
2
dral geometry and right, N O trigonal bipyramidal geometry. O red,
2
3
N blue, C black, Li green; gold bonds=axle, silver bonds=wheel. Only
H-atoms involved in H-bonding or the olefin are shown and anions are
[14]
omitted for clarity.
1
Figure 3. H NMR spectra (500 MHz, 298 K, 1:1 CD CN:CDCl ) of:
3
3
cis
cis
a) L ; b) L after the addition of 30 equiv of [Cu(CH CN) ][BF ]; c) [H-
3
4
4
cis +
trans
trans
L ] ; d) L ; e) L
after the addition of 30 equiv of [Cu(CH CN) ]-
3 4
[
BF ]. Only imidazole (NH), olefinic (i) and aromatic (c) protons are
4
labeled; for full assignments, see the Supporting Information.
trans
+
Figure 6. Single-crystal X-ray structure of [Cu(L )] O red, N blue,
C black, Cu teal; gold bonds=axle, silver bonds=wheel. Only H-
atoms involved in H-bonding or the olefin are shown and anions are
[14]
omitted for clarity.
bonded to both the bidentate chelate of the axle and several
donor atoms from the wheel. However, the ligand co-
+
+
conformations used to bind Cu and Li are quite different.
trans
+
The [Li(L )] cation displays two distinct co-conforma-
trans
Figure 4. The different co-conformations of the [2]rotaxane ligand L
tions within the single lattice. In one of the co-conformations,
+
in solution as determined by 2D NMR experiments in CDCl for the
the Li cation adopts a distorted tetrahedral geometry
3
trans
trans
+
trans
+
species: left, neutral L ; middle, [Li(L )] ; right, [Cu(L )] (see
chelating to two O-donor atoms of the macrocycle (LiÀO
the Supporting Information for details).
1
.97(2) and 1.92(2) ꢁ) and the two N-atoms from the axle
chelate (LiÀN 2.03(2) and 2.08(2) ꢁ). In the other co-
+
determined by the ROE cross-peak analysis of the 2D NMR
conformation, the Li center is in distorted trigonal bipyr-
trans
experiments for the free ligand L
and the two complexes
amidal environment coordinating to three O-donor atoms of
the macrocycle (LiÀO 2.27(2), 2.07(2) and 1.99(2) ꢁ) and the
two N-atoms of the axle (LiÀN 2.08(2) and 2.09(2) ꢁ).
trans
+
trans
+
[
Li(L )] and [Cu(L )] (see the Supporting Informa-
tion).
Secondly, single-crystal X-ray structures were determined
trans
+
+
In stark contrast, for [Cu(L )] the Cu cation adopts
a distorted tetrahedral geometry chelating to the axle (CuÀN
trans
trans
for both [Li(L )][BF ] and [Cu(L )][OTf]. Figures 5 and 6
4
trans
+
trans
+
show the structures of the [Li(L )] and [Cu(L )]
2.013(3) and 2.391(3) ꢁ) but utilizing the olefin moiety of the
cations, respectively. The encapsulated metal ions are
macrocycle (CuÀC 2.079(4) and 2.050(4) ꢁ), as was originally
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inferred by their downfield shift in the H NMR spectrum,
[
1] a) J.-P. Sauvage, Acc. Chem. Res. 1998, 31, 611 – 619; b) T. J.
Hubin, D. H. Busch, Coord. Chem. Rev. 2000, 200 – 202, 5 – 52;
c) J.-P. Collin, C. Dietrich-Buchecker, P. GaviÇa, M. C. Jimenez-
Molero, J.-P. Sauvage, Acc. Chem. Res. 2001, 34, 477 – 487; d) C.
Dietrich-Buchecker, M. C. Jimenez-Molero, V. Sartor, J.-P.
Sauvage, Pure Appl. Chem. 2003, 75, 1383 – 1393; e) F. Aricꢂ,
J. D. Badjic, S. J. Cantrill, A. H. Flood, K. C.-F. Leung, Y. Liu,
J. F. Stoddart, Top. Curr. Chem. 2005, 249, 203 – 259; f) J. F.
Stoddart, Chem. Soc. Rev. 2009, 38, 1521 – 1529; g) J. D. Crowley,
S. M. Goldup, A.-L. Lee, D. A. Leigh, R. T. McBurney, Chem.
Soc. Rev. 2009, 38, 1530 – 1541; h) J. E. Beves, B. A. Blight, C. J.
Campbell, D. A. Leigh, R. T. McBurney, Angew. Chem. Int. Ed.
along with a single O-atom (CuÀO 2.068(3) ꢁ).
Thus, solution and solid state results demonstrate how the
interlocked structure of a [2]rotaxane can be utilized to create
rotationally active ligands with orthogonal donor sets at-
tached to highly dynamic but independent components. The
trans
two major degrees of freedom for L
are the facile rotation
about the single bond between the two benzimidazole groups
and the large amplitude rotation of the 24-membered macro-
cycle about the rigid H-shaped axle. In the free ligand
(
Figure 7a), two NH···O hydrogen bonds orient the macro-
2011, 50, 9260 – 9327; Angew. Chem. 2011, 123, 9428 – 9499;
i) S. F. M. van Dongen, S. Cantekin, J. A. A. W. Elemans, A. E.
Rowan, R. J. M. Nolte, Chem. Soc. Rev. 2014, 43, 99 – 122; j) G.
Gil-Ramꢃrez, D. A. Leigh, A. J. Stephens, Angew. Chem. Int. Ed.
2015, 54, 6110 – 6150; Angew. Chem. 2015, 127, 6208 – 6249.
[
2] a) C. O. Dietrich-Buchecker, J.-P. Sauvage, J.-P. Kintzinger,
Tetrahedron Lett. 1983, 24, 5095 – 5098; b) C. O. Dietrich-
Buchecker, J.-P. Sauvage, J.-M. Kern, J. Am. Chem. Soc. 1984,
106, 3043 – 3045; c) M. Cesario, C. O. Dietrich-Buchecker, J.
Guilhem, C. Pascard, J.-P. Sauvage, J. Chem. Soc. Chem.
Commun. 1985, 244 – 247; d) A.-M. Albrecht-Gary, Z. Saad,
C. O. Dietrich-Buchecker, J.-P. Sauvage, J. Am. Chem. Soc. 1985,
107, 3205 – 3209; e) A.-M. Albrecht-Gary, C. Dietrich-
Buchecker, Z. Saad, J.-P. Sauvage, J. Weiss, J. Chem. Soc.
Chem. Commun. 1986, 1325 – 1327; f) C. O. Dietrich-Buchecker,
A. Khemiss, J.-P. Sauvage, J. Chem. Soc. Chem. Commun. 1986,
1376 – 1378; g) C. O. Dietrich-Buchecker, J.-P. Sauvage, Tetrahe-
dron 1990, 46, 503 – 512; h) B. Mohr, J.-P. Sauvage, R. H. Grubbs,
M. Weck, Angew. Chem. Int. Ed. Engl. 1997, 36, 1308 – 1310;
Angew. Chem. 1997, 109, 1365 – 1367; i) P. GaviÇa, J.-P. Sauvage,
Tetrahedron Lett. 1997, 38, 3521 – 3524; j) M. Weck, B. Mohr, J.-
P. Sauvage, R. H. Grubbs, J. Org. Chem. 1999, 64, 5463 – 5471;
k) P. Mobian, J.-M. Kern, J.-P. Sauvage, J. Am. Chem. Soc. 2003,
Figure 7. The rotational co-conformations “dialed-up” for the [2]rotax-
ane ligand: a) neutral ligand L , b) and c) Li complexes, d) Cu
complex.
trans
+
+
cyclic wheel to optimize these non-covalent interactions. In
the complexes (Figure 7b–d), the coordination requirements
of the metal ion influences the choice of donors from the
macrocycle and residual hydrogen bonds provide comple-
mentary second-sphere interactions.
125, 2016 – 2017.
[
3] a) C. O. Dietrich-Buchecker, J.-P. Sauvage, Angew. Chem. Int.
Ed. Engl. 1989, 28, 189 – 192; Angew. Chem. 1989, 101, 192 – 194;
b) C. O. Dietrich-Buchecker, J. Guilhem, C. Pascard, J.-P.
Sauvage, Angew. Chem. Int. Ed. Engl. 1990, 29, 1154 – 1156;
Angew. Chem. 1990, 102, 1202 – 1204; c) C. O. Dietrich-
Buchecker, J.-F. Nierengarten, J.-P. Sauvage, N. Armaroli, V.
Balzani, L. De Cola, J. Am. Chem. Soc. 1993, 115, 11237 – 11244;
d) C. O. Dietrich-Buchecker, G. Rapenne, J.-P. Sauvage, J.
Chem. Soc. Chem. Commun. 1997, 2053 – 2054; e) P. E. Barran,
H. L. Cole, S. M. Goldup, D. A. Leigh, P. R. McGonigal, M. D.
Symes, J. Wu, M. Zengerle, Angew. Chem. Int. Ed. 2011, 50,
This unique combination of a mechanically interlocked
structure and flexibility imparts a single ligand with the ability
to dial-up the appropriate donor atoms required to form
stable complexes of metals cations with very different
+
+
electronic requirements, in this case Li and Cu . A variety
of conceptually similar, rotationally active ligands should be
possible and may find application in catalysis and metal
sequestration.
12280 – 12284; Angew. Chem. 2011, 123, 12488 – 12492; f) J.-F.
Ayme, J. E. Beves, D. A. Leigh, R. T. McBurney, K. Rissanen, D.
Schultz, Nat. Chem. 2012, 4, 15 – 20; g) J.-F. Ayme, J. E. Beves,
C. J. Campbella, D. A. Leigh, Chem. Soc. Rev. 2013, 42, 1700 –
1
712; h) D. A. Leigh, R. G. Pritchard, A. J. Stephens, Nat. Chem.
Acknowledgements
2014, 6, 978 – 982.
[
4] a) A. Livoreil, C. O. Dietrich-Buchecker, J.-P. Sauvage, J. Am.
Chem. Soc. 1994, 116, 9399 – 9400; b) N. Armaroli, V. Balzani, J.-
P. Collin, P. GaviÇa, J.-P. Sauvage, B. Ventura, J. Am. Chem. Soc.
1999, 121, 4397 – 4408; c) M.-J. Blanco, M. C. Jimꢄnez, J.-C.
Chambron, V. Heitz, M. Linke, J.-P. Sauvage, Chem. Soc. Rev.
S.J.L. thanks the Natural Sciences and Engineering Research
Council of Canada for financial support of this research
through their Discovery, Accelerator Supplement and
Canada Research Chair programs.
1999, 28, 293 – 305; d) M. C. Jimꢄnez, C. O. Dietrich-Buchecker,
J.-P. Sauvage, Angew. Chem. Int. Ed. 2000, 39, 3284 – 3287;
Angew. Chem. 2000, 112, 3422 – 3425; e) I. Poleschak, J.-M.
Kern, J.-P. Sauvage, Chem. Commun. 2004, 474 – 476; f) P.
Mobian, J.-M. Kern, J.-P. Sauvage, Angew. Chem. Int. Ed. 2004,
Keywords: co-conformation · coordination chemistry ·
mechanically interlocked molecules · rotaxanes
43, 2392 – 2395; Angew. Chem. 2004, 116, 2446 – 2449; g) S.
Bonnet, J.-P. Collin, M. Koizumi, P. Mobian, J.-P. Sauvage, Adv.
Mater. 2006, 18, 1239 – 1250.
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
[
5] a) M. J. Langton, P. D. Beer, Acc. Chem. Res. 2014, 47, 1935 –
949; b) K. Hiratani, M. Kaneyama, Y. Nagawa, E. Koyama, M.
[9] a) A. F. M. Kilbinger, S. J. Cantrill, A. W. Waltman, M. W. Day,
1
R. H. Grubbs, Angew. Chem. Int. Ed. 2003, 42, 3281 – 3285;
Angew. Chem. 2003, 115, 3403 – 3407; b) V. N. Vukotic, K. J.
Harris, K. Zhu, R. W. Schurko, S. J. Loeb, Nat. Chem. 2012, 4,
456 – 460.
Kanesato, J. Am. Chem. Soc. 2004, 126, 13568 – 13569; c) Y.
Nagawa, J. Suga, K. Hiratani, E. Koyama, M. Kanesato, Chem.
Commun. 2005, 749 – 751; d) Y. Nakatani, Y. Furusho, E.
Yashima, Angew. Chem. Int. Ed. 2010, 49, 5463 – 5467; Angew.
Chem. 2010, 122, 5595 – 5599; e) S.-Y. Hsueh, C.-C. Lai, S.-H.
Chiu, Chem. Eur. J. 2010, 16, 2997 – 3000; f) A. V. Leontiev, C. J.
Serpell, N. G. White, P. D. Beer, Chem. Sci. 2011, 2, 922 – 927;
g) N.-C. Chen, P.-Y. Huang, C.-C. Lai, Y.-H. Liu, Y. Wang, S.-M.
Peng, S.-H. Chiu, Chem. Commun. 2007, 4122 – 4124; h) N.-C.
Chen, C.-C. Lai, Y.-H. Liu, S.-M. Peng, S.-H. Chiu, Chem. Eur. J.
[10] a) M. Berthelot, Ann. Chim. Phys. 1901, 23, 32 – 39; b) W.
Manchot, W. Brandt, Justus Liebigs Ann. Chem. 1909, 370, 286 –
296; c) H. Tropsch, W. J. Mattox, J. Am. Chem. Soc. 1935, 57,
1102 – 1103; d) E. R. Gilliland, H. L. Bliss, C. E. Kip, J. Am.
Chem. Soc. 1941, 63, 2088 – 2090; e) M. J. S. Dewer, Bull. Soc.
Chim. Fr. 1951, 18, C79; f) M. A. Bennett, Chem. Rev. 1962, 62,
611 – 652; g) R. G. Salomon, J. K. Kochi, J. Am. Chem. Soc. 1973,
95, 1889 – 1897; h) J. S. Thompson, R. L. Harlow, J. F. Whitney, J.
Am. Chem. Soc. 1983, 105, 3522 – 3527; i) M. Munakata, S.
Kitagawa, S. Kosome, A. Asahara, Inorg. Chem. 1986, 25, 2622 –
2627; j) X.-S. Wang, H. Zhao, Y.-H. Li, R.-G. Xiong, X.-Z. You,
Top. Catal. 2005, 35, 43 – 61.
[11] a) D. J. Safarik, R. B. Eldridge, Ind. Eng. Chem. Res. 1998, 37,
2571 – 2581; b) A. Takahashi, R. T. Yang, Langmuir 2001, 17,
8405 – 8413; c) E. I. Basaldella, J. C. Tara, G. Aguilar Armenta,
M. E. PatiÇo-Iglesias, E. Rodrꢃguez Castellon, J. Sol-Gel Sci.
Technol. 2006, 37, 141 – 146.
[12] a) J. M. Cubero, H. K. Mangold, Microchem. J. 1965, 9, 227 –
236; b) B. M. Lawrence, J. Chromatogr. A 1968, 38, 535 – 537;
c) T.-S. Li, J.-T. Li, H.-Z. Li, J. Chromatogr. A 1995, 715, 372 –
375; d) C. M. Williams, L. N. Mander, Tetrahedron 2001, 57,
425 – 447.
[13] Along with cis and trans isomers of the double bond, there are
possible cisoid and transoid conformations of the N,N’-imidazole
chelate. The latter are easily interconverted by rotation about
the single bond between the imidazole groups. Only the cisoid,
N,N’-imidazole chelate is used to bind to a metal cation.
[14] CCDC 1496411– 1496414 contain the supplementary crystallo-
graphic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
2008, 14, 2904 – 2908.
[
6] a) E. A. Neal, S. M. Goldup, Chem. Commun. 2014, 50, 5128 –
5142; b) D. A. Leigh, V. Marcos, M. R. Wilson, ACS Catal. 2014,
4, 4490 – 4497; c) Y. Tachibana, N. Kihara, T. Takata, J. Am.
Chem. Soc. 2004, 126, 3438 – 3439; d) Y. Suzaki, K. Shimada, E.
Chihara, T. Saito, Y. Tsuchido, K. Osakada, Org. Lett. 2011, 13,
3774 – 3777; e) C. B. Caputo, K. Zhu, V. N. Vukotic, S. J. Loeb,
D. W. Stephan, Angew. Chem. Int. Ed. 2013, 52, 960 – 963;
Angew. Chem. 2013, 125, 994 – 997; f) B. Lewandowski, G.
De Bo, J. W. Ward, M. Papmeyer, S. Kuschel, M. J. Aldegunde,
P. M. E. Gramlich, D. Heckmann, S. M. Goldup, D. M. D’Souza,
A. E. Fernandes, D. A. Leigh, Science 2013, 339, 189 – 193; g) M.
Galli, J. E. M. Lewis, S. M. Goldup, Angew. Chem. Int. Ed. 2015,
54, 13545 – 13549; Angew. Chem. 2015, 127, 13749 – 13753.
[
7] a) K. Zhu, V. N. Vukotic, C. A. OꢀKeefe, R. W. Schurko, S. J.
Loeb, J. Am. Chem. Soc. 2014, 136, 7403 – 7409; b) K. Zhu, V. N.
Vukotic, C. A. OꢀKeefe, R. W. Schurko, S. J. Loeb, Nat. Chem.
2
015, 7, 514 – 519; c) V. N. Vukotic, C. A. OꢀKeefe, K. Zhu, K. J.
Harris, C. To, R. W. Schurko, S. J. Loeb, J. Am. Chem. Soc. 2015,
37, 9643 – 9651; d) N. Farahani, K. Zhu, C. A. OꢀKeefe, R. W.
1
Schurko, S. J. Loeb, ChemPlusChem 2016, 81, 836 – 841.
[
8] a) K. Zhu, V. N. Vukotic, S. J. Loeb, Angew. Chem. Int. Ed. 2012,
5
1, 2168 – 2172; Angew. Chem. 2012, 124, 2210 – 2214; b) N.
Noujeim, K. Zhu, V. N. Vukotic, S. J. Loeb, Org. Lett. 2012, 14,
484 – 2487; c) K. Zhu, V. N. Vukotic, N. Noujeim, S. J. Loeb,
2
Chem. Sci. 2012, 3, 3265 – 3271; d) N. Farahani, K. Zhu, S. J.
Loeb, ChemPhysChem 2016, 17, 1875 – 1880.
Received: July 27, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Supramolecular Chemistry
G. Baggi, S. J. Loeb*
A rigid, H-shaped [2]rotaxane ligand with
different sets of donor atoms on the axle
and wheel components was prepared.
The dynamics of the rotaxane and the
orthogonal arrangement of the donors
allows access to different co-conforma-
tions by simple rotation of the wheel
about the axle. Three different rotational
isomers were observed and structurally
characterized for the neutral ligand and
&&&& — &&&&
Rotationally Active Ligands: Dialing-Up
the Co-conformations of a [2]Rotaxane for
Metal Ion Binding
+
+
coordination complexes with Li and Cu
ions.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!