Controlling the transmission of stereochemical information through space in terphenyl-edged Fe4L6 cages

Article abstract of DOI:10.1021/ja205254s

A series of terphenyl-edged Fe₄L₆ cages were synthesized from substituted 4,4″-diamino-p-terphenyls, 2-formylpyridine, and iron(II). For the parent diaminoterphenyl, all three possible diastereomers, with T, S₄, and C

Full text of DOI:10.1021/ja205254s

ARTICLE
pubs.acs.org/JACS
Controlling the Transmission of Stereochemical Information through
Space in Terphenyl-Edged Fe4L6 Cages
Wenjing Meng,† Jack K. Clegg,† John D. Thoburn,†,‡ and Jonathan R. Nitschke*,†
†Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
‡Department of Chemistry, Randolph-Macon College, Ashland, Virginia 23005, United States
S Supporting Information
b
ABSTRACT: A series of terphenyl-edged Fe4L6 cages were
synthesized from substituted 4,400-diamino-p-terphenyls, 2-for-
mylpyridine, and iron(II). For the parent diaminoterphenyl, all
three possible diastereomers, with T, S4, and C3 point symme-
tries, were formed in nearly equal amounts, as determined by 1H
13
and C NMR. When 2,200-dimethylterphenylenediamine was
used, the T-symmetry diastereomer was observed to predominate. The use of 20,30,50,60-tetramethylterphenylenediamine generated
predominantly the S4 cage diastereomer, whereas 20,50-dimethylterphenylenediamine produced the C3-symmetric cage to a greater
degree than the other two diastereomers. The factors contributing to the transfer of chiral information between metal vertices were
analyzed, and the general principles underlying the delicately balanced thermodynamics were determined.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
Self-assembled polyhedral metalꢀorganic cages1ꢀ8 have received
recent attention because of their ability to alter the chemical
reactivity of other species in solution.9ꢀ11 Recent examples include
the use of cages as catalysts12ꢀ15 or photoreactors,16,17 the stabiliza-
tion of reactive species,18ꢀ21 and the generation of unusual reaction
products.21ꢀ24 The search for new functions has driven the explora-
tion of new cage geometries, including boxes, prisms, cubes,25ꢀ29
and large spherical structures reminiscent of virus capsids.30
The importance of being able to manipulate the stereochem-
istry of chiral cages is highlighted by recent applications of cages
in stereoselective transformations23 and chiral recognition,22
which both involve tetrametallic cages with approximately tetra-
hedral symmetry. The great majority of M4L6 cages reported so
far31ꢀ36 exhibit simple chirality in solution, forming homochiral
species with all four metal ions at each vertex within the cage
possessing the same Λ or Δ chirality, giving the molecule T point
symmetry.32,35,37ꢀ42 Here we present an example of a large
tetrahedral cage that exists in solution as a mixture of diastere-
omers, wherein the chirality of each iron(II) vertex influences
only weakly the stereochemistry of its distant neighbors.43ꢀ45
This stereochemical coupling may be enhanced, however,
through the simple expedient of grafting methyl groups onto
the terphenyl groups that form the cage’s edges. Methylation
thus gave rise to chiral communication46ꢀ48 between vertices
over a distance of ∼1.7 nm such that variations in the place-
ment of the methyl groups allowed the system’s delicately
balanced thermodynamics to be perturbed to allow for the
selection of either the homochiral T (ΔΔΔΔ/ΛΛΛΛ), the
heterochiral C3 (ΔΔΔΛ/ΛΛΛΔ), or the achiral S4 (ΔΔΛΛ)
cage diastereomer (Figure 1) as the self-assembly reaction’s
major product.
The reaction between p-terphenylenediamine A, 2-formylpyr-
idine, and iron(II) tetrafluoroborate in acetonitrile (Scheme 1)
produced cage 1 as the only product observed by one- and two-
dimensional NMR spectroscopy. Both the 1H (Figure S1,
Supporting Information) and 13C (Figure S2) NMR spectra
were complicated; NMR signals were observed as clusters of
peaks, suggesting that 1 existed as a mixture of diastereomers in
solution, in contrast to other cages prepared via subcomponent
self-assembly.27,34,35
8+
Evidence for the tetrahedral structure of 1 included Fe4L6
and Fe4L6(BF4)7+ peaks in the FT-ICR (Fourier transform ion
cyclotron resonance) mass spectrum, whose isotopic distribu-
tion corresponded well to calculated values (Figure S6), as well
as the DOSY (diffusion-ordered spectroscopy)49 spectrum of 1,
which showed that all peaks between 5.0 and 9.5 ppm had the
same diffusion coefficient (Figure S7). The presence of an
4+
alternative structure, such as an Fe2L3 triple helicate,50ꢀ55
would lead to an increase in the intensity of every second peak
8+
in the ESI-MS signal for Fe4L6 and the presence of DOSY
peaks having different diffusion coefficients, neither of which
was observed.
In order to quantify the amount of each diastereomer present
in solution, the 1H NMR peak clusters were deconvoluted using
WINNMR56 software (Figure 2a). Each diastereomer can give
rise to different numbers of peaks in NMR spectra according to
its symmetry: one set of ligand peaks for T, four sets for C3, and
three sets for S4. Eight peaks are thus expected in total for each
proton. When fewer than eight peaks were observed, we assumed
Received: June 7, 2011
Published: July 26, 2011
r
2011 American Chemical Society
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Figure 1. Schematic representations of the three possible diastereomers of M4L6 cages. Purple and green metal centers are of opposing chirality. Blue
lines represent anti-linkages between homochiral metal centers, while orange lines represent syn-linkages between heterochiral metal centers.
Scheme 1. Synthesis of Tetrahedral Cage 1 (Major T
Diastereomer Shown)
1
peak overlap. For example, the four possible H resonances of
1-C3 were generally observed as three peaks in a 2:1:1 integrated
ratio (see Supporting Information).
The three isomers of 1 were found to have nearly equal
concentrations in solution, with the calculated percentages of
33%, 33%, and 34% for 1-T, 1-C3, and 1-S4, respectively. This
conclusion was further supported by similar analysis of the 13C
NMR spectrum of 1, wherein C10 and C8 (Scheme 1) were
observed to give rise to all eight expected signals (Figure 2b).
Analogous carbon atoms across the three diastereomers were
observed to have similar T1 values, allowing us to interpret the
deconvolution of the 13C peak clusters to obtain a diastereo-
meric distribution of 32%, 33%, and 35% for 1-T, 1-C3, and 1-S4,
in good quantitative agreement with the 1H results. The
CAChe57 MM2-optimized structures of the three diastereo-
mers of 1 were calculated to be nearly isoenergetic. The
deviation of diastereomer populations from a statistical dis-
tribution (consisting of 12.5% T, 50% C3, and 37.5% S4)
therefore reflects the presence of slight stereochemical coupling
between metal centers.
Figure 2. Deconvolution of peak clusters for (a) phenylene H8
(1H NMR spectrum) and (b) phenylene C8 (13C NMR spectrum) in
1. The blue line is the measured spectrum. The orange, pink, and green
lines are calculated Lorentzian peaks. The black line is the difference
between the observed spectrum and the sum of all the Lorentzian peaks.
A good fit should show only baseline noise.
three cages 2ꢀ433ꢀ35,59 that have bis(iminopyridine) ligands
of the same type as 1 (Figure 3).
We attribute the homochirality of cages 2ꢀ4 to stereochem-
ical coupling between the metal centers mediated by the arylene
rings that compose the cages’ edges. Each of 2ꢀ433ꢀ35,59 is built
with a shorter biaryl bridge in place of 1’s 4,400-diaminoterphenyl;
each of these ligands adopts an anticlinal conformation. In 2 the
phenylene rings are observed to gear together next to the FeII
centers with dihedral angles between terminal phenylene rings
and the NꢀFeꢀN chelate planes of 65 ( 7°.19,35,60 In the crystal
In contrast to 1, the great majority of tetrahedral M4L6 structures
reported so far are homochiral, exhibiting strong stereochem-
ical coupling between chiral metal centers,39,43,58 including the
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Figure 3. Previously reported homochiral bis(iminopyridine)-based FeII4L4 cages 2,35 3,34 and 4.33,59
structures of 3 and 4, the pyridylꢀpyridyl dihedral angles are
about 57° (3) and 52° (4). The adoption of a T-symmetric
structure allows these ligands to avoid either the transannular
eclipsing of hydrogen atoms in 3 and 4 or steric and Coulombic
interactions between sulfonates in 2, necessitated by the coplanar
arrangements in the meso arrangements of the ligands in the S4
and C3 diastereomers.
asymmetric unit are enantiomers and interact via edge-to-face
πꢀπ interactions. The methyl groups are located along the faces
of the cage cavity. No anions, solvent, or significant electron
density was located within the cage.
In the crystal structure of 5 (Figure 4), the three phenylene
rings around each iron center are observed to twist out of
planarity and conjugation with the imine groups to which they
are bonded in order to avoid steric clash with each other and to
In order to test the hypothesis that stereochemical coupling46ꢀ48
could be modulated in order to control the influence of the
chirality of one FeII vertex upon its neighbors, methyl groups
were introduced onto the terphenyl ligand backbone at various
positions. The methyl group was chosen because its steric bulk
has been demonstrated to constrain the conformations of
biphenyl systems61 and because methylated terphenyls are
synthetically accessible. Three new C2-symmetric methylated
p-diaminoterphenyls were designed and synthesized (Scheme 2;
synthetic details are given in the Supporting Information). The
corresponding tetrahedral cages were prepared following the
general synthetic method shown in Scheme 1 and characterized
by NMR, ESI-MS, and elemental analysis.
benefit from weak CH π stabilizing interactions between
3 3 3
neighboring phenylenes. This twisting also allows the methyl
groups to be directed toward the nearest-neighbor phenylene
ring, in order to benefit from weak internal solvation and van der
Waals stabilization.
When crystalline 5 was redissolved in CD3CN, only one set of
1H NMR signals was observed (Figure 5b), consistent with a
stereochemically pure compound, 5-T. The equilibrium mixture
of diastereomers was regenerated after 5 days in solution at room
temperature, resulting in a 1H NMR spectrum indistinguishable
from that of the freshly prepared cage.
Employing C (Scheme 2) in the subcomponent self-assembly
reaction described for 5 yielded 6 (Scheme 4). In the 1H NMR
spectrum (Figure 6a) of 6, a set of three peaks with equal relative
integrated intensity is observed as the major signal for many
protons, consistent with the presence of 6-S4 as the most
abundant species in solution. NMR spectra revealed no detect-
able amounts of 6-T; the relative abundances of 6-S4 and 6-C3
were thus measured to be 88% and 12% at room temperature.
In contrast to 1 and 5 the peaks corresponding to both the
terminal phenyl rings (Figure 6, δ 5.5ꢀ7 ppm) and the methyls
were significantly broadened, suggesting that the terphenylene
unit of the ligand as a whole rotates slowly on the NMR
time scale.
The use of B led to the preparation of cage 5 as the uniquely
observed product of the self-assembly reaction shown in
Scheme 3. Both the 1H (Figure 5a) and 13C (Figure S9) NMR
spectra of cage 5 appeared simpler than those of cage 1. One
major signal was observed for each ligand hydrogen or carbon
atom, although in the 1H spectrum each major peak was flanked
by smaller peaks. Deconvolution analysis again suggested that
three diastereomers, 5-T, 5-C3, and 5-S4 were present. The ratio
of isomers present, however, was different from that observed for
1, with the proportion of the T isomer present doubling such that
the 5-T/5-C3/5-S4 ratio was observed to be 67%:23%:10%.
Vapor diffusion of diisopropyl ether into a nitromethane
solution of cage 5 allowed the isolation of cube-shaped dark
purple crystals suitable for X-ray analysis. Compound 5 crystal-
lized in the monoclinic space group P21/n with two cage
molecules in the asymmetric unit. Each cage molecule is homo-
chiral with either ΛΛΛΛ or ΔΔΔΔ configurations at the metal
centers, and each has idealized T symmetry. The ligands bridging
the metal centers adopt a helical twist with the helicity reflecting
the handedness of the metal centers they bridge; Λ centers are
linked by M-helices and Δ by P. Each ligand has an idealized anti-
configuration. With ∼17 Å between adjacent metal centers, this
species is among the largest M4L6 tetrahedra yet to be character-
ized by X-ray crystallography.38,41,67ꢀ70 The two molecules in the
Fine crystals of 6 (not suitable for X-ray crystallography) were
obtained via liquidꢀliquid diffusion of diethyl ether into an
acetonitrile solution. The recorded 1H NMR spectrum of freshly
dissolved crystals showed the S4 isomer only (Figure 6b).
Monitoring the decrease of 6-S4 in solution by 1H NMR
spectroscopy over 12 h at 298 K revealed a gradual return to
the equilibrium population of the S4 and C3 isomers (Figure 6c)
with a rate constant for conversion of 6-S4 to 6-C3 of (7.1 ( 0.7)
ꢁ 10ꢀ6
s
ꢀ1, corresponding to an activation barrier, ΔGq, of
102 kJ molꢀ1 at 298 K.
This behavior for cage 6 is similar to what was observed for cage 5:
one isomer crystallized preferentially, with possible elimination
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Scheme 2. Synthetic Routes to the New Methylated Diami-
noterphenyls B, C, and Da
Scheme 3. Synthesis of FeII4L6 Cage 5 (Major T Diastereo-
mer Shown)
a Conditions: (a) 2 equiv of Ag2CO3, 0.05 equiv of Pd(PPh3)4, THF,
DeanꢀStark trap, reflux in the dark, 1 day;62 (b) H2, 1 atm, 0.1 equiv of
10% Pd/C, EtOAc, room temp, 1.5 days;63 (c) 0.02 equiv of I2, 2.5 equiv
of Br2, DCM, reflux, 1.5 h;64 (d) 2.5 equiv of 4-nitrobenzene boronic
acid, 0.1 equiv of Pd(PPh3)4, 5 equiv of Cs2CO3, 1,4-dioxane/H2O 5:1,
reflux, 12 h;65 (e) 50 equiv of N2H4 H2O, 0.1 equiv of 10% Pd/C,
3
EtOH, reflux, 12 h.66
Figure 4. Schematic representation of the crystal structure of the
ΔΔΔΔ enantiomer of 5-T. For clarity, anions and solvent of crystal-
lization are removed and only methyl hydrogen atoms are shown.
of the less-symmetric isomers from the dynamic library71,72 of
diastereomers in solution during crystallization, as we have
observed in the case of a chiral dicopper helicate.73
Cage 7 was synthesized from diamine D (Scheme 5) under
conditions analogous to those used in the preparation of 1, 5, and
6. The 1H NMR spectrum of 7 (Figure 7) revealed a mixture of
three diastereomers, with the ratio 7-T/7-C3/7-S4 measured to
be 21.4%:43.9%:34.7%, indicating a preference for the C3
diastereomer over T compared to nonmethylated 1.
Factors Governing Stereocontrol. Figure 8 summarizes the
differences in diastereoselectivity observed between the four
terphenyl-edged Fe4L6 1cages reported in this study, as measured
at equilibrium, when H NMR analysis revealed no further
changes in diastereomeric ratios (in all cases within 24 h at
298 K). Whereas nonmethylated 1 gave nearly equal proportions
of the three diastereomers, methylation on the terminal pheny-
lene rings (5) favored the T-diastereomer, tetramethylation on
the central ring (6) favored S4, and dimethylation on the central
phenylene (7) favored C3.
The thermodynamics of these cages are more finely balanced
than what has been observed previously in the cases of homo-
chiral cages 2ꢀ4 (Figure 3). The diastereoselectivities observed
in the present system reflect energetic differences of 1ꢀ10 kJ
molꢀ1 between diastereomers, similar to what might be observed
between diastereomeric transition states leading to the two
product enantiomers of a diastereoselective reaction.43 Under-
standing the subtle factors that underlie the transfer of stereo-
chemical information within cages of this type may help to shed
light upon the mechanism of stereoselective transformations,74ꢀ77
as well as more fundamental questions of chiral information
transfer.46,78ꢀ81
As shown in Figure 1, each ligand within an M4L6 cage bridges
two metal centers either of the same chirality in anti fashion or of
opposite chiralities in syn fashion. These two ligand con-
formations are shown in Figure 9a and Figure 9b. Factors that favor
a syn ligand conformation also favor the S4 and C3 diastereomers,
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Figure 6. Comparison of 1H NMR spectra of cage 6 (only the aromatic
region is shown, all recorded at 298 K): (a) 6 formed as shown in
Scheme 3; (b) crystals of 6 5 min after dissolution; (c) 12 h after
dissolution.
Figure 5. Comparison of 1H NMR spectra of cage 5 in CD3CN
(all recorded at 298 K): (a) 5 formed as shown in Scheme 2; (b) crystals
of 5 10 min after dissolution; (c) 5 days after dissolution.
Scheme 5. Synthesis of Tetrahedral Cage 7 (Major C3 Dia-
stereomer Shown)a
Scheme 4. Synthesis of Tetrahedral Cage 6 (Major S4 Dia-
stereomer Shown)a
a The green FeII center is of opposite handedness to those colored
purple.
a Green FeII centers are of opposite handedness to those colored purple.
configuration between two metal ions of opposite handedness.
Although this configuration does not preclude the central pheny-
lene from lying at an optimal 55° angle to both of the terminal
phenyls, the required cis orientation of the methyl groups appears
to be very slightly disfavored with respect to the trans methyl
orientation of the anti ligands of 5-T.83 In order to adopt such a
trans orientation, the methyl groups would need to break van der
Waals contact with neighboring phenyl rings. This frustrated state
thus appears to raise slightly the energy of the syn ligand
conformation, and thus the 5-C3 or 5-S4 diastereomers, with
respect to the anti ligand conformation and the 5-T diastereomer.
In the parent (tetramethyl)terphenylene of diamine C, the
steric bulk of the four methyl groups situated on its central
phenylene ring obliges the two terminal phenylene rings to lie
orthogonal to the central ring, and thus coplanar with each other,
in the lowest-energy conformation.84 In such a coplanar arrange-
ment, tight gearing between the terminal phenylene groups
precludes them both from adopting an optimal 74° dihedral
angle to two imines linked to FeII centers of the same handedness
(as in Figure 9a). In the meso configuration imposed by a syn
linkage (Figure 9b), however, any dihedral angle may be adopted
between the terminal phenyl groups and the imines linked to FeII
which contain four and three syn-bridging ligands respectively,
whereas factors that favor an anti ligand conformation favor the T
diastereomer, which contains only anti ligands. The stereochem-
ical coupling phenomena that favor either the syn or anti
configuration within the terphenylenediamine subcomponents
AꢀD are discussed below.
In the crystal structure of 5 the dihedral angles between the
NꢀFeꢀN chelate planes and terminal phenyl rings average
74 ( 7° (Figure 9a); the average of all 84 analogous dihedrals
for structures containing similar fac-coordinated iron(II) tris-
(2-pyridyl-phenylimine) motifs reported in the 2011 Cambridge
Structural Database82 is 69 ( 9°, suggesting that these dihedrals
are not strained in 5. Each central phenylene in 5 thus lies at a
60 ( 9° dihedral angle to each of its neighbors, close to the
optimal value of 55° for the parent dimethylterphenyl.83 All
dihedral angles in the anti ligands of the T diastereomer of 5 thus
lie close to optimal values, with each ligand taking on a chiral
twist between metal ions.
The syn ligand orientation required to generate S4 and C3 cage
diastereomers (Figure 9b), in contrast, approximates a meso
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Figure 7. 1H NMR spectrum for cage 7 (recorded at 313 K after the
system was observed to have reached equilibrium; only the aromatic
region is shown) with an expansion of the imine region.
Figure 9. Two linking modes: (a) an anti ligand connecting two FeII
centers with the same stereochemistry in 5; (b) a syn ligand connecting
two FeII centers with opposite stereochemistry in 6, showing the
pseudo-mirror plane.
Figure 8. Equilibrium distribution of diastereomers for cages 1, 5, 6,
and 7 at 298 K.
centers of opposite handedness; these dihedrals are the isoener-
getic mirror images of each other. The syn arrangement and thus
the S4 and to a lesser degree the C3 diastereomers are therefore
favored for any ligand whose terminal phenyl groups are coplanar
in the lowest-energy conformation, as with 6.
The parent diaminoterphenyl D, whose central phenylene
group bears two methyl groups, also shows coplanarity between
its terminal phenyl groups in the lowest-energy conformation.85
The central ringis not obligedtolie orthogonal tothe twoterminal
rings, however, allowing the ligands of 7 readily to adopt either a
syn or anti configuration. As a result, the observed population of
isomers in solution approaches the statistical ratio, as with cage 1,
with a slightly lower predominance of the T diastereomer and an
increase of the C3-symmetry diastereomer in the case of 7, possibly
as the result of a slight favoring of the syn configuration.
In summary, the use of diamine ligands that preferentially
adopt an anti arrangement, as in cage 5, should lead to the
preferential formation of homochiral T-symmetric cages, which
contain all anti ligands. The use of ligands that preferentially
adopt a syn arrangement, such as the meso-favoring ligands of
cage 6, should lead to the preferential formation of S4-symmetric
cages, which contain mostly ligands in this orientation. The
lowest-symmetry C3 diastereomer is favored on statistical
grounds and will prevail at the expense of the T diastereomer if
a slight syn bias is applied, as in the case of cage 7.
Variable Temperature Behavior. The equilibrated states of
cages 1, 5, 6, and 7 in acetonitrile at 10 different temperatures
ranging from 298 to 343 K were studied. Equilibration was
monitored by 1H NMR spectroscopy; each system was noted
to have reached equilibrium when its NMR spectrum no
longer changed. In contrast with other M4L6 tetrahedra
reported in the literature,41,43 no coalescence was observed
for any of these cages up to 343 K, suggesting that the
activation barrier to racemization of the FeII stereocenters
Figure 10. Temperature dependence of diastereomer distribution for
cage 5.
was greater than 85 kJ molꢀ1, consistent with the barrier to
diastereomer interconversion of 102 kJ molꢀ1 for 6 at 298 K,
as noted above. The mechanism of racemization of Fe-
(diimine)3 centers has been reported to involve a combination
of predominantly intramolecular (R^ay-Dutt twist or Bailar
twist) and minor intermolecular (dissociative) rearrangements.
The interconversion barriers observed in the present systems are
consistent with these prior reports, suggesting a predominantly
intramolecular isomerization process.43,86ꢀ89 The molar percen-
tages of each diastereomer for each cage at equilibrium were
calculated and compared at different temperatures. As the tem-
perature was increased, the population of 5-T decreased gradually
as the populations of both 5-C3 and 5-S4 were observed
to increase (Figure 10); in contrast, no significant changes in
diastereomer populations were observed for cages 1, 6, and 7.
Van’t Hoff analysis for the equilibrium 5-T a 5-C3 (presented in
the Supporting Information) revealed enthalpic and entropic
contributions of ΔH = 19 kJ molꢀ1 and ΔS = 54 J Kꢀ1 molꢀ1
,
respectively, whereas those for 1, 6, and 7 are minimal (ΔH
<7 kJ molꢀ1, ΔS <18 J Kꢀ1 molꢀ1).
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The temperature dependence of the diastereomeric intercon-
version of 5 is a consequence of the greater entropy change
associated with this process. Each of 5’s methyl groups may indepen-
dently be oriented either inward, as all are observed to point in
the crystal structure (Figure 4), or outward, following a 180°
rotation of the imineꢀphenylene dihedral. An outward orienta-
tion would disrupt the weak van der Waals interaction between a
methyl group and the phenyl group of its neighboring ligand,
leading to a disruption of the stereochemical coupling effects to
which we attribute the stereoinduction observed in 5. More
fundamentally, we would expect that for 5 and indeed for any
system in which the T diastereomer is favored at lower tempera-
tures, the amount of the most ordered and thus most entropically
disfavored T should decrease in favor of the more entropically
favored S4 and C3 diastereomers as the temperature increases.
with ω and ψ scans to approximately 46° 2θ at 120(2) K.98 Data
integration and reduction were undertaken with S HKL Denzo and
Scalepack.98,99 Subsequent computations were carried out using the
WinGX-32 graphical user interface.100 Structures were solved by direct
methods using SIR97.101 Multiscan empirical absorption corrections
were applied to the data set using the program SADABS.102 Data were
refined and extended with SHELXL-97.103 Hydrogen atoms were
refined using a riding model with the hydrogens on the methyl groups
modeled as disordered over two positions. The crystals diffracted poorly
and rapidly suffered solvent loss. Despite rapid handling times and a low
temperature collection, the quality of data was less than ideal. Because of
the low quality of the data, numerous rigid body restraints were required
in the phenyl and pyridyl rings and only the iron atoms were refined
anisotropically. Nevertheless, the quality of the data is more than
sufficient for establishing the connectivity of the structure. The anions
and nitromethane solvent of crystallization within the lattice were
significantly disordered, and they could not be successfully modeled.
Therefore, the SQUEEZE function of PLATON104 was employed
to remove the contribution of the electron density associated with
these anions and solvent from the model, which resulted in far more
satisfactory residuals.
’ CONCLUSIONS
Our recently developed tetrahedral-cage-forming procedure35
thus proved extendable to the preparation of larger structures.
These structures formed cleanly but exhibited complex stereo-
isomery, controllable through the introduction of methyl groups
onto the linear diamine subcomponent. This work thus builds
upon, and contributes to, fundamental studies of the transfer of
chiral information between subunits of larger systems,78,79,81,90,91
which has been shown to underlie such complex phenomena as
creating twisted ribbons from self-assembling dendrons,92 fixing
the stereochemistry of a photocycloaddition reaction product
within a subtly chiral host,23 determining the direction of travel
within a mechanically interlocked molecular machine,93 and
converting light energy into macroscopic rotational motion.94
The ability to manipulate diastereomer distribution offered by
this study provides an additional means to exert control over the
form of a host’s inner phase.95 As enantiomerically defined cages
have provided useful chiral spaces for enantioselective catalysis and
the differential molecular recognition of enantiomers,22,23,39,40,67,96
so may diastereomerically defined cages permit the discrimina-
tion and transformation of diastereomeric guests.
Crystallographic data for 5 are as follows: formula C212H216B8F32Fe4-
N44O40, M = 4938.16, monoclinic, space group P21/n (No. 14),
a = 32.3941(19) Å, b = 47.504(3) Å, c = 36.205(2) Å, β = 104.098(2)°,
V= 54036(6) Å3,Dc =1.214gcmꢀ3,Z= 8, crystal size 0.41 mm ꢁ 0.36 mm ꢁ
0.16 mm, color purple, habit block, temperature 120(2) K, λ(Mo KR) =
0.71073 Å, μ(Mo KR) = 0.302 mmꢀ1, T(SADABS)min,max of 0.4904 and
0.7452, 2θmax = 31.72, hkl range from ꢀ24 to 24, from ꢀ36 to 36, and
from ꢀ27 to 26, N = 123856, Nind = 24261 (Rmerge = 0.1020), Nobs
=
13 465 (9I > 2σ(I)), Nvar = 1066, residuals R1(F) = 0.1354, wR2(F2) =
0.3726, GoF(all) = 1.176, DFmin,max of ꢀ0.345 and 0.398 e Åꢀ3
.
Cage 1. To a Teflon-stoppered NMR tube was added 4,400-diamino-
p-terphenyl (2.60 mg, 10 μmol, 6 equiv), 2-pyridinecarboxaldehyde (1.9
μL, 2.14 mg, 20 μmol, 12 equiv), iron(II) tetrafluoroborate hexahydrate
(2.25 mg, 6.7 μmol, 4 equiv), and deuterated acetonitrile (0.5 mL). The
solution was degassed by three evacuation/N2 fill cycles. The tube was
kept at 50 °C overnight. 1H NMR (CD3CN, 400 MHz, for atom
assignments see Scheme 1): 8.97ꢀ8.82 (12H, imine H6), 8.58ꢀ8.51
(12H, py-H4), 8.35ꢀ8.44 (12H, py-H3), 7.95ꢀ7.83 (24H, Ph-H12),
7.83ꢀ7.69 (12H, py-H2), 7.69ꢀ7.49 (24H, Ph-H9), 7.49ꢀ7.36 (12H,
py-H1), 5.66ꢀ5.41 (24H, Ph-H8). 13C {1H} NMR (125 MHz,
CD3CN): 175.87ꢀ175.19 (12C, C6), 159.31ꢀ159.03 (12C, C5),
157.07ꢀ156.52 (12C, C1), 150.81ꢀ150.48 (12C, C7), 141.22ꢀ141.03
(12C, C11), 140.76ꢀ140.60 (12C, C3), 139.81ꢀ139.31 (12C, C10),
132.28ꢀ131.96 (12C, C4), 130.95ꢀ130.68 (12C, C2), 128.72ꢀ128.30
(36C, C12 and C9), 123.25ꢀ122.67 (24C, C8). FT-ICR MS: [Fe4L6]8+
356.86, [Fe4L6(BF4)]7+ 420.12. Found: C, 59.21; H, 4.02; N, 9.38%. Calcd
for C180H132B8F32Fe4N24 6H2O CH3CN: C, 59.11; H, 4.01; N, 9.47%.
’ EXPERIMENTAL SECTION
General. Chromatographic separations were performed on silica gel
60 (particle size 0.040ꢀ0.063 mm) purchased from Breckland Scientific
Supplies. TLC was performed on silica gel 60 F254 purchased from Merck;
visualization was under ultraviolet light (ν = 254 nm). 1H NMR spectra
1
13
were recorded at 400 MHz. 13C NMR, 1Hꢀ H COSY, 1Hꢀ C HMQC
(heteronuclear multiple quantum coherence), DEPT-135 (distortionless
enhancement by polarization transfer at pulse angle θ = 135°), and
DOSY (diffusion-ordered spectroscopy) spectra were recorded on a 500
MHz spectrometer. NMR spectra were referenced to the residual 1H or
13C NMR signal of the solvent.97 Electrospray ionization mass spectra
were obtained on a Micromass Quattro LC instrument, infused from a
Harvard syringe pump at a rate of 10 μL per minute. Mass spectra
provided by the EPSRC National Mass Spectrometry Service Centre at
Swansea, U.K., were acquired on a Thermofisher LTQ Orbitrap XL.
Satisfactory elemental analyses required the inclusion of water mol-
ecules; this water was observed in 1H NMR spectra of crystallized
products. Possible sources of water include the imine condensation
reaction and the solvents employed. The syntheses of terphenylenedia-
mines BꢀD are reported in the Supporting Information.
3
3
Cage 5. To a Teflon capped NMR tube was added diamine B (11.4
mg, 39.5 μmol, 6 equiv), 2-pyridinecarboxaldehyde (7.56 μL, 8.47 mg,
79.1 μmol, 12 equiv), iron(II) tetrafluoroborate hexahydrate (8.89 mg,
26.4 μmol, 4 equiv), and deuterated acetonitrile (0.5 mL). The solution
was degassed by three evacuation/N2 fill cycles. The tube was kept at
50 °C overnight. The product was purified by recrystallization: diethyl
ether was diffused into an acetonitrile solution of the cage. The desired
product methyl cage 5 was isolated by filtration as purple solid (11 mg,
1
45%). H NMR (CD3CN, 500 MHz): 8.85 (12H, s, imine H), 8.54
(12H, d, J 7.5, py-H), 8.39 (12H, t, J 7.5, py-H), 7.75 (12H, t, J 6.25, py-
H), 7.38 (36H, py-H and Ph-H), 7.11 (12H, d, J 8.0, Ph-H), 4.9ꢀ5.6
(24H, br, Ph-H), 5.06 (12H, s, Ph-H), 1.90 (36H, s, CH3). 13C {1H}
NMR (125 MHz, CD3CN): 175.3, 159.3, 156.7, 150.2, 140.6, 140.2,
137.9, 131.7, 131.2, 130.5, 129.8, 123.7, 119.6, 20.57. ESI-MS: [Fe4L6]8+
377.74, [Fe4L6(BF4)]7+ 444.23. Found: C, 58.14; H, 4.54; N, 8.29%.
Crystallography. Data for 5 were collected on a Bruker-Nonius
APEX2-X8-FR591 diffractometer employing confocal mirror mono-
chromated Mo KR radiation generated from a rotating anode (0.710 73 Å)
Calcd for C192H156B8F32Fe4N24 14H2O: C, 58.09; H, 4.67; N, 8.47%.
3
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dx.doi.org/10.1021/ja205254s |J. Am. Chem. Soc. 2011, 133, 13652–13660

Journal of the American Chemical Society
ARTICLE
Cage 6. To a Teflon capped NMR tube was added diamine C
(8 mg, 25.3 μmol, 6 equiv), 2-pyridinecarboxaldehyde (4.8 μL, 5.4 mg,
50.6 μmol, 12 equiv), iron(II) trifluoromethanesulfonate (6.0 mg, 16.8 μmol,
4 equiv), and deuterated acetonitrile (0.5 mL). The solution was
degassed by three evacuation/N2 fill cycles. The tube was kept at
50 °C overnight. The product was purified by recrystallization: diethyl
ether was diffused into an acetonitrile solution of the cage. The desired
product methyl cage 6 was isolated by filtration as purple solid (12 mg,
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1
65%). H NMR (CD3CN, 400 MHz): 8.90ꢀ9.16 (12H, imine H),
8.52ꢀ8.65 (12H, py-H), 8.37ꢀ8.43 (12H, py-H), 7.73ꢀ7.82 (12H, py-
H), 7.31ꢀ7.52 (12H, py-H), 6.80ꢀ7.00 (24H, Ph-H), 5.60ꢀ6.00 (24H,
Ph-H), 1.70ꢀ2.20 (36H, CH3). 13C {1H} NMR (125 MHz, CD3CN):
175.7ꢀ176.2, 159.2ꢀ159.3, 156.4ꢀ156.9, 150.0ꢀ150.7, 143.4ꢀ144.1,
140.5ꢀ141.1, 130.7ꢀ133.0, 120.7ꢀ123.3, 19.0 (12C, CH3). ESI-MS:
[Fe4L6]8+ 398.82, [Fe4L6(CF3SO3)]7+ 477.12, [Fe4L6(CF3SO3)2]6+
581.42. Found: C, 56.33; H, 4.10; N, 7.31%. Calcd for C212H180F24Fe4-
2, 51–56.
N24O24S8 8H2O: C, 56.24; H, 4.36; N, 7.42%.
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3
Cage 7. To a Teflon capped NMR tube was added diamine D
(6.4 mg, 22.2 μmol, 6 equiv), 2-pyridinecarboxaldehyde (4.2 μL, 4.7 mg,
44.4 μmol, 12 equiv), iron(II) trifluoromethanesulfonate (5.3 mg, 14.8 μmol,
4 equiv), and deuterated acetonitrile (0.5 mL). The solution was
degassed by three evacuation/N2 fill cycles. The tube was kept at
50 °C overnight. The product was purified by recrystallization: diethyl
ether was diffused into an acetonitrile solution of the cage. The desired
product methyl cage 7 was isolated by filtration as purple solid (8 mg,
1
51%). H NMR (CD3CN, 500 MHz): 8.87ꢀ9.00 (12H, imine H),
8.52ꢀ8.61 (12H, py-H), 8.37ꢀ8.41 (12H, py-H), 7.74ꢀ7.78 (12H, py-
H), 7.33ꢀ7.55 (12H, py-H), 7.08ꢀ7.25 (36H, Ph-H), 5.47ꢀ5.60 (24H,
Ph-H), 2.17ꢀ2.28 (36H, CH3). 13C {1H} NMR (125 MHz, CD3CN):
176.0ꢀ175.4, 159. 4ꢀ159.0, 157.0ꢀ156.5, 150.4ꢀ150.0, 142.8ꢀ142.6,
141.0ꢀ140.4, 134.0ꢀ133.5, 132.7ꢀ131.9, 131.2ꢀ130.6, 122.5ꢀ121.9,
20.2ꢀ19.8. ESI-MS: [Fe4L6]8+ 377.75, [Fe4L6(CF3SO3)]7+ 453.07,
[Fe4L6(CF3SO3)2]6+ 553.39. Found: C, 54.20; H, 3.77; N, 7.23%. Calcd
for C200H156F24Fe4N24O24S8 12H2O: C, 54.21; H, 4.09; N, 7.59%.
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3
(22) Fiedler, D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. J.
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’ ASSOCIATED CONTENT
(23) Nishioka, Y.; Yamaguchi, T.; Kawano, M.; Fujita, M. J. Am.
Chem. Soc. 2008, 130, 8160–8161.
(24) Ziegler, M.; Davis, A. V.; Johnson, D. W.; Raymond, K. N.
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S
Supporting Information. Synthetic procedures, NMR
b
spectra and deconvolution analyses, Van’t Hoff analyses, kinetics
data and analysis, MS data, crystal structure data for 5. This
material is available free of charge via the Internet at http://pubs.
acs.org.
(26) Bell, Z. R.; Harding, L. P.; Ward, M. D. Chem. Commun.
2003, 2432–2433.
’ AUTHOR INFORMATION
(27) Meng, W.; Breiner, B.; Rissanen, K.; Thoburn, J. D.; Clegg, J. K.;
Nitschke, J. R. Angew. Chem., Int. Ed. 2011, 50, 3479–3483.
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Commun. 1998, 1681–1682.
Corresponding Author
jrn34@cam.ac.uk
(29) Stephenson, A.; Ward, M. D. Dalton Trans. 2011, 40, 7824–
7826.
’ ACKNOWLEDGMENT
(30) Sun, Q.-F.; Iwasa, J.; Ogawa, D.; Ishido, Y.; Sato, S.; Ozeki, T.;
Sei, Y.; Yamaguchi, K.; Fujita, M. Science 2010, 328, 1144–1147.
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(34) Hristova, Y. R.; Smulders, M. M. J.; Clegg, J. K.; Breiner, B.;
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This work was supported by the Walters-Kundert Charitable
Trust, the Marie Curie Incoming International Fellowship
Scheme of the 7th EU Framework Program (J.K.C.), and the
UK Engineering and Physical Sciences Research Council
(EPSRC). We thank the EPSRC National Mass Spectrometry
Service at Swansea, U.K., for FT-ICR MS experiments and the
National Crystallography Service at Southampton for collecting
X-ray data.
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