Chiral borromeates

Article abstract of DOI:10.1002/anie.200600817

The fellowship of the rings: Circular dichroism measurements reveal that chirality is transferred from 12 stereogenic centers to 6 octahedral zinc(II) centers (see picture) in two enantiomeric pairs of chiral Borromeates, one of which has been found to be

Full text of DOI:10.1002/anie.200600817

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Dynamic Chemistry
centers (i.e., (R,R) or (S,S)) and either 2,6-diformylpyridine
(DFP) or its 4-chloro derivative Cl-DFP[12] to give the (R)12
and (S)12 enantiomers of the Borromean link compounds
BR·12TFA and BRCl6·12TFA (TFA = trifluoroacetate); that
is, (R)12-BR·12TFA, (S)12-BR·12TFA, (R)12-BRCl6·12TFA,
and (S)12-BRCl6·12TFA. It transpires that, as a consequence
of introducing four stereogenic centers into each of three
identical rings in these chiral Borromeates[13] such that they
pair up in locations close to the six ions, these metal ions are
surrounded by chiral coordination spheres as revealed by one
X-ray crystal structure and several circular dichroism (CD)
spectra.
DOI: 10.1002/anie.200600817
Chiral Borromeates**
Cari D. Pentecost, Andrea J. Peters, Kelly S. Chichak,
Gareth W. V. Cave, Stuart J. Cantrill, and
J. Fraser Stoddart*
The quest for topologically interesting molecules has led to
the template-directed synthesis[1] of a wide range of molecular
catenanes and knots.[2] Such molecules with interlocked and
intertwined structures have nonplanar molecular graphs and
can exhibit[3] topological chirality.[4] Although the classic
example is the trefoil knot,[2,4] catenanes—characterized in
knot theory by the Hopf link—can also display topological
chirality, provided that both rings exhibit structural direction-
ality.[5] By contrast, the Borromean rings (BRs)—characterized
in knot theory by the Brunnian link—remain amphicheiral[6]
even after orientation. Thus, the only way by which we can
render BRs chiral is to introduce chirality in the form of
elements that are planar, axial, or point (stereogenic) in nature.
Although the incorporation of stereogenic centers into
mechanically interlocked molecules can be inconsequential,
especially if the stereogenic centers are far removed from the
rest of the functional groups in the molecule, the effects of
such centers can often be amplified, particularly when they
are able to influence nearby molecular structure, sometimes
even inducing chirality.[7] Relatively simple transition-metal
complexes containing 2,2’-bipyridyl ligands with appended
stereogenic centers have been termed chirality generators (or
CHIRAGENs)[8] because the absolute configuration of these
centers can determine the absolute configuration induced at
the metal coordination site. This kind of chirality transfer has
been identified in tight-fitting, mechanically interlocked
molecules such as the catenate investigated recently by
Sauvage and co-workers[9] that contains an axially chiral
binaphthyl unit. Herein, we report the preparation of two
enantiomeric pairs of Borromean linked compounds[10,11]
using ZnII ions to template their formation from diaminobi-
pyridine (DAB) ligands with one of two like stereogenic
The (R,R)- and (S,S)-DAB-Hn·nTFA precursors were
each synthesized (see Supporting Information) in five steps
from the (R) and (S) enantiomers, respectively, of p-hydroxy-
phenylglycine methyl ester hydrochloride. In four separate
reactions, the chiral diaminobipyridine ligands, as their TFAÀ
salts, were treated at 708C for 24 h in iPrOH with equimolar
amounts (0.10 mm) of either DFP or Cl-DFP, in the presence
of Zn(OAc)2 as the template to afford stereospecifically (R)12-
BR·12TFA and (S)12-BR·12TFA, and (R)12-BRCl6·12TFA
and (S)12-BRCl6·12TFA (Scheme 1). High-resolution electro-
spray ionization mass spectrometry (HR-ESI-MS) revealed
peaks at m/z 1585.3302 and 1160.2263, and 1584.2427 and
1160.1778, respectively, for the triply and quadruply charged
molecular ions from (R)12-BR·12TFA and (S)12-BR·12TFA:
the analogous m/z values for (R)12-BRCl6·12TFA and (S)12-
BRCl6·12TFA are 1653.5458 and 1211.9171, and 1652.5632
and 1212.4001, respectively. Although the 1H NMR spectra of
all these compounds recorded in CD3OD at 600 MHz were
characterized by broad resonances, a singlet for the 12 imine
protons was discernible at d = 9.0 ppm in all four spectra and
peaks for most[14] of the other heterotopic protons in the chiral
BR·12TFA and BRCl6·12TFA compounds could be identi-
fied and assigned (Figure 1).
Single colorless crystals[15] of (R)12-BRCl6·12TFA, suitable
for X-ray crystallography,[16] were obtained by vapor diffusion
of Et2O into a methanolic solution of the compound.[17] The
solid-state structure of the original, achiral Borromean
rings[10a] indicated that all three macrocycles adopt chairlike
conformations, such that they confer S6 symmetry on the
molecule. In the chiral Borromean rings, we expected the
C3 axis, which is collinear with the S6 axis in the achiral
progenitor, to be present in the solid-state structure. How-
ever, although the same chairlike conformations can also be
observed (Figure 2a) in the orthogonal arrangement of the
three macrocycles in (R)12-BRCl612+, the C3 axis is not
evident: the symmetry is reduced to C1 on account of one of
the three macrocycles adopting a flipped chairlike conforma-
tion (Figure 2b). On inspection of the three chairlike
conformations in BRCl612+, the CH2OH groups are oriented
in pairs with the pseudo-axial and pseudo-equatorial arrange-
ments alternating between neighboring 608 segments
(Figure 3). Moreover, beyond the molecule, the packing
diagrams (Figure 4) show that these CH2OH groups result
in the formation of two very different hydrophilic solvent
channels that run all the way through the lattice such that the
channels are lined with only pseudo-axial CH2OH groups or
only pseudo-equatorial CH2OH groups.
[*] C. D. Pentecost, Dr. A. J. Peters, Dr. K. S. Chichak, Dr. S. J. Cantrill,
Prof. J. F. Stoddart
The California NanoSystems Institute, and
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)
Fax: (+1)310-206-1843
E-mail: stoddart@chem.ucla.edu
Dr. G. W. V. Cave
School of Biomedical and Natural Sciences
Nottingham Trent University
Nottingham, NG118NS (UK)
[**] The research at UCLA was supported by the National Science
Foundation (NSF).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 1. Schematic representation of (R)12-BR·12TFA and (S)12-BR·12TFA, in which only the blue ring depicts all of the atoms and bonds
present in the macrocycles that make up the Borromeate ring structure. By using either the (R,R)- or the (S,S)-DAB-Hn·nTFA ligand, enantiomeric
BRs withtwelve stereogenic centers are formed.
The CD spectra[18] (Figure 5a) of the Boc-protected
enantiomeric ligand precursors, (R,R)-DAB and (S,S)-DAB,
constitute plots that are mirror images of each other, and both
display optical activity at l < 255 nm. Although not as simple
to interpret, the enantiomeric BRs, (R)12-BRCl6·12TFA and
(S)12-BRCl6·12TFA, also provide plots (Figure 5b) that are
mirror images of each other. The trace from (R)12-
BRCl6·12TFA shows strong Cotton effects at lmin = 224 nm
À27.6mÀ1 cmÀ1) corresponding to (R)12-BRCl6·12TFA and
(S)12-BRCl6·12TFA, respectively. Equally interesting are the
strong Cotton effects observed at l = 310 nm, with De =
À38.8mÀ1 cmÀ1
for
(R)12-BRCl6·12TFA
and
De =
42.6mÀ1 cmÀ1 for (S)12-BRCl6·12TFA. This absorption range
is especially characteristic of chirality associated with the
metal centers, a situation which suggests that chirality has
been induced by the stereogenic centers to the zinc coordi-
nation sphere.[19] Furthermore, the coordination spheres
around the metals are themselves enantiomers of each
other, as reflected by their opposing Cotton effects in the
CD spectra of the enantiomeric BRs.
(De = À29.75mÀ1 cmÀ1)
and
lmax = 239 nm
(De =
49.0mÀ1 cmÀ1), most likely arising from the stereogenic
centers appended to the bipyridyl ligand, considering that
this higher-energy region is within the same range as the
absorptions observed for the free ligand in Figure 5a. Like-
wise, the trace for (S)12-BRCl6·12TFA shows intense Cotton
effects in that region: lmax = 220 nm (De = 32.9mÀ1 cmÀ1) and
lmin = 237 nm (De = À49.0mÀ1 cmÀ1). At lower energy (l =
272 nm), where the free ligand is optically inactive, a pair of
antipodal CD absorptions are evident (De = 41.17,
To eliminate the possibility that the size and shape of the
Borromeate framework transmits chiral information of any
form, the CD spectrum of the achiral progenitor was
measured. It showed no CD response at all. As an additional
verification that chirality is being induced at its metal centers,
the (R)12-BRCl6·12TFA enantiomer was demetallated by
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reducing the 12 imine bonds. Applica-
tion of a recently established proce-
dure[13] for demetallation and reduc-
tion gives a mixture of single rings,
linear fragments, and intact BRs—in
this case, (R)12-BRCl6H24—but without
the metals; that is, the Borromeands.
The CD spectrum (Figure 6) of the
crude demetallated chiral BRs exhibits
much weaker absorptions (at the same
concentrations of 8.0 mm) in the
absence of the metal. In fact, only
two main absorptions are apparent at
lmin = 248 nm
and at
(De = À8.09mÀ1 cmÀ1)
lmax = 277 nm
(De =
6.61mÀ1 cmÀ1): the first bands are
within the optical activity range of
the free ligand. Furthermore, the CD
trace of demetallated (R)12-BRCl6H24
is conspicuously lacking any absorp-
tion at 310 nm, further suggesting that
chirality was induced at the metal
center. These results prompted us to
examine the octahedral disposition of
the ligands surrounding the ZnII ions
from the X-ray crystallographic data
(Figure 7). From this distorted octahe-
dron, it is apparent that the chiral
ligands wrap around the metal in a
helical, and thus chiral, fashion.[20] For
comparison, we re-examined the X-ray
crystal data for the original Borro-
meate and confirmed that each octa-
hedron around the zinc in the achiral
BR is an ideal octahedron with no
distortion. Hence, we conclude that
the presence of the nearby stereogenic
centers disturbs the neighboring coor-
dination sphere to produce chirality at
metal centers in the optically active
Borromeates.
Figure 1. Partial 1H NMR spectra (600 MHz) recorded in CD3OD of a) the freshly deprotected
(R,R)-DAB-Hn·nTFA ligand, b) Cl-DFP, and c) the (R)12-BRCl6·12TFA isomer. If the structure of the
chiral Borromeate has C1 symmetry in solution, then all the protons in the molecule are
heterotopic. The observed line widths in the spectrum are consistent with this structural
assignment.
Figure 2. X-ray crystal structure of (R)12-BRCl6·12TFA viewed a) from an orthogonal perspective
and b) down the core, revealing the low symmetry (C1) resulting from one “flipped” chairlike
macrocycle.
In summary, two pairs of enantiomerically related Borro-
meates were synthesized stereospecifically from enantiomeric
phenylglycine derivatives. All four optically active com-
pounds have three macrocycles each containing four stereo-
genic centers. The one example whose crystal structure has
been determined adopts an asymmetric conformation. All
four compounds exhibit intense CD spectra in solution.
Furthermore, the ZnII octahedra, found in the X-ray crystal
structure of one of the compounds, deviate substantially from
ideal geometry, strongly suggesting that chirality is being
transferred rather efficiently from the 12 stereogenic centers
to the six ZnII centers. Removal of the ZnII ions and reduction
of the imine bonds present in the macrocyles affords a
Borromeand with a significantly different CD spectrum. In
the fullness of time, the chiral Borromeates could find
applications[21] in chemical biology and materials science; or
they could remain what they are presently—that is, stereo-
chemical curiosities.
Figure 3. View of the X-ray crystal structure of (R)12-BRCl6·12TFA down
the c axis withpendant alternating axial (ax) and equatorial (eq)
CH2OH groups.
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Figure 6. CD spectra of (R)12-BRCl6·12TFA (blue) and the crude
reduced chiral Borromeand (brown). Both were obtained at a concen-
tration of 8.0 mm in MeOH (228C).
Figure 4. Packing diagram of (R)12-BRCl6·12TFA viewed down the
c axis revealing alternating hydrophilic solvent channels. Depicted in
the wire view are the equatorial (eq) and axial (ax) solvent channels.
Hydrogen atoms, TFA counterions, and solvent molecules have been
omitted for clarity.
Figure 7. Distorted octahedron around the ZnII atom, drawn from the
X-ray crystal structure data (left) and its mirror image. The imine
nitrogen atoms in the coordination sphere are not equivalent—one is
flanked witha nearby pseudo-equatorial CH 2OH (Ceq) and the other is
situated nearest a stereogenic center witha pseudo-axial CH 2OH (Cax),
thereby eliminating any planes of symmetry from the octahedron.
Experimental Section
(R)12-BRCl6·12TFA: The Boc protecting groups in (R,R)-6 were
removed by the addition of neat CF3CO2H (0.5 mL) to (R,R)-6
(155 mg, 0.235 mmol) with stirring at room temperature for 15 min.
The reaction mixture was concentrated to dryness, leaving a light pink
oil. The excess of CF3CO2H was removed by two repeated addition
and removal cycles of MeOH (5 mL) by rotary evaporation under
reduced pressure. Any residual CF3CO2H was removed under high
vacuum to leave (R,R)-DAB-Hn·nTFA as a pink sticky solid that is
moisture-sensitive and was therefore carried onto the next step
without delay. Zn(OAc)2 (43 mg, 0.235 mmol) was added to a stirred
solution (10 mL) containing (R,R)-DAB-Hn·nTFA (0.235 mmol) and
4-chloro-2,6-diformylpyridine (40 mg, 0.235 mmol) in iPrOH, and the
reaction mixture was heated at 658C for 16 h to produce a pale yellow
solution along with a white precipitate. The mixture was allowed to
cool down to room temperature and filtered, and the precipitate was
washed with iPrOH (3 ” 5 mL) and Et2O (3 ” 5 mL). This procedure
afforded (R)12-BRCl6·12TFA as a white powder (117 mg). Crystals
suitable for X-ray crystallography were grown by vapor diffusion of
Figure 5. Circular dichroism spectra of a) the chiral ligand precursors,
Boc-protected (R,R)-DAB (blue) and (S,S)-DAB (red), bothrecorded at
a concentration of 0.02 mm in MeOH (228C), b) the enantiomeric
chiral BRs (R)12-BRCl6·12TFA (blue) and (S)12-BRCl6·12TFA (red). The
CD spectrum of the achiral (unmodified) BRs was taken as a control
(green). All three samples were measured at a concentration of 8.0 mm
in MeOH (228C). Boc=tert-butyloxycarbonyl.
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Et2O into a solution of (R)12-BRCl6·12TFA in MeOH. Yield: 83%;
[a]2D2 = À6.19 (c = 0.2, MeOH); 1H NMR (600 MHz, CD3OD): d =
4.01 (s, 24H), 6.54 (br, 12H), 6.74 (m, 48H), 8.02 (s, 12H), 8.49 (s,
12H); 9.00 ppm (s, 12H); resonances for the hydroxy protons were
not observed as a result of exchange with the deuterium in the
solvent. A resonance for the CH at the stereogenic center was not
observed because it was obscured by the H2O resonance; HR-ESI-
MS: m/z calcd for (C66H52Cl2N10O8)3Zn6(CF3CO2)12: 5300.8528; found
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(%): 1653.5458 (90) [MÀ3TFA]3+, 1211.9171 (100) [MÀ4TFA]4+
,
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947.1281 (24) [MÀ5TFA]5+, 769.9425 (6) [MÀ6TFA]6+
.
(R)12-BRCl6H24: The ZnII-containing BR complex (R)12-
BRCl6·12TFA (50 mg, 0.01 mmol) was dissolved in anhydrous
EtOH under an Ar atmosphere at 228C. NaBH4 (15 mg, 0.4 mmol)
was added in one portion. The reaction mixture became cloudy
immediately. It was stirred at 228C for 5 days. The reaction was then
quenched by the addition of H2O (5 mL), and the mixture was treated
with an excess of ethylenediamine tetraacetic acid (EDTA; 175 mg)
and heated under reflux for 30 min. Thereafter, the reaction mixture
was allowed to cool down to room temperature and the solvents were
removed under reduced pressure. The crude product was suspended
in H2O (15 mL) and filtered. The white filter cake was washed with
H2O (10 ” 5 mL) to remove excess of salts and then finally with Et2O
(3 ” 5 mL). This procedure afforded 26 mg of the crude product as a
white solid containing both the reduced (R)12-BRCl6H24 and the free
macrocycle in
a 1.0:1.8 molar ratio, as determined from the
1
[6] Defined as “topologically achiral in 3-space”, amphicheiral is a
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Guthrie Tait, one of the pioneers of knot theory. With its original
spelling intact, amphicheiral is still widely used in current
mathematical literature on topology. See Ref. [4c].
integration of the resonances in the H NMR spectrum. Therefore,
this observation suggests that approximately 53% of the (R)12-
BRCl6·12TFA followed a pathway in which all 12 imine bonds were
reduced and the three rings remained interlocked to produce (R)12-
BRCl6H24. The remaining 47% followed a pathway in which at least
one of the rings was cleaved during the borohydride reduction to
produce the two reduced macrocycles and one linear fragment. (R)12-
BRCl6H24 and the component macrocyle mixture exhibited low
solubilities in most solvents and therefore had to be used as the crude
mixture. [a]2D2 = À5.1 (c = 0.1, MeOH); selected 1H NMR data
(600 MHz, CD3SOCD3): d = 3.59 (s, 24H), 3.70 (s, 24H), 4.24 (dd,
J = 4.5, 1.3 Hz, 12H), 4.10 (bs, 12H), 6.38 (m, 12H), 6.70 (d, J =
9.0 Hz, 24H), 7.01 (d, J = 9.0 Hz, 24H), 7.28 (s, 12H), 7.46 (d, J =
2.4 Hz, 12H), 7.89 ppm (d, J = 6 Hz, 12H) (see Supporting Informa-
tion for full assignments). MALDI-MS: m/z calcd for (C66H60N10O8)3:
1256.4; found: 1255.3 (100%) [3M+H+Na]+.
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Keywords: chirality · Schiff bases · self-assembly ·
supramolecular chemistry · template synthesis
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the metal ions, the term “Borromeand” is suggested for the
resulting metal-free compound.
[14] No resonance was observed for the methine protons attached to
the stereogenic centers because it was obscured by a water
resonance. It was also not possible to identify one of the two
vicinal protons located on the bipyridyl ligands.
[15] Crystal
(CF3CO2)6·10.5CH3OH·1.5H2O, trigonal, a = b = 31.293(4), c =
24.348(5) Š, V= 20.649(6) Š3, space group R3, Z = 3, 1calcd
1.362 gcmÀ3
l(MoKa) = 0.71073 Š, F(000) = 8636, T=
data
for
[(C66H52Cl2N10O8)3(ZnO2CCF3)6]-
=
,
120(2) K, 9818 unique reflections (2qmax = 54.98), of which 4944
were observed [Io > 2s(I)]. Absolute structure parameter=
0.0121(15). Final R factors: R1 = 0.1661, wR2 = 0.3865 for 611
parameters.
[16] Single crystals of (R)12-BRCl6·12TFA, suitable for X-ray crys-
tallography, were obtained by vapor diffusion of Et2O into a
methanolic solution of the compound. A single colorless crystal
(0.32 ” 0.24 ” 0.185 mm3) was attached with oil to a thin glass
fiber. Suitable crystals were extremely difficult to mount.
Solvent loss was immediately evident as soon as the crystal
was removed from the mother liquor. Data were collected on a
Bruker-Nonius FR591 rotating anode diffractometer with MoKa
radiation using the phi and omega scan modes (150-second
exposure per frame). Data were corrected for absorption using
the SADABS program, and structure solution and refinement
were performed using the SHELX-97 software package. All
non-hydrogen atoms, except those in the solvent molecules and
counterions, were refined anisotropically while the hydrogen
atoms were included at geometrically calculated positions and
allowed to ride on their parent atoms. CCDC 299519 contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif.
[17] The enantiomeric pairs of BRs, (R)12-BRCl6·12TFA and (S)12-
BRCl6·12TFA, were prepared in the form of the chlorinated
derivatives in order to aid the growth of single crystals suitable
for X-ray crystallography. Analysis of the X-ray data supported
this notion (see Supporting Information). The molecular param-
eters (distances, angles, etc.) are not reported here, however they
are not all that dissimilar from those found in the original
Borromean rings (see Ref. [10a]). The X-ray crystal structure
will be discussed in more detail in a full paper.
[18] CD Spectra were recorded on a JASCO J715 spectropolarimeter
using a Xe lamp equipped with a PS-150 J power supply.
[19] a) H.-R. Mꢀrner, H. Stoeckli-Evans, A. von Zelewsky, Inorg.
Chem. 1996, 35, 3931 – 3935; b) H.-R. Mꢀrner, A. von Zelewsky,
G. Hopfgartner, Inorg. Chim. Acta 1998, 271, 36 – 39; c) O.
Mamula, F. J. Monlien, A. Porquet, G. Hopfgartner, A. E.
Merbach, A. von Zelewsky, Chem. Eur. J. 2001, 7, 533 – 539;
d) K. Campbell, C. A. Johnson II, R. McDonald, M. J. Ferguson,
M. M. Haley, R. R. Tykwinski, Angew. Chem. 2004, 116, 6093 –
6097; Angew. Chem. Int. Ed. 2004, 43, 5967 – 5971.
[20] a) C. J. Hawkins, Absolute Configuration of Metal Complexes,
Wiley-VCH, New York, 1971; b) A. von Zelewsky, Stereochem-
istry of Coordination Compounds, Wiley-VCH, Chichester,
1996.
[21] The fact that (R)12-BRCl6·12TFA is asymmetric in the solid
state, and probably also in solution, raises an important issue—
namely that these kinds of BRs can exist in more than one
diastereoisomeric conformation. As such, we have a necessary
yet insufficient property to be able to construct molecular
switches.
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Angew. Chem. Int. Ed. 2006, 45, 4099 –4104