Angewandte
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unambiguously shows a single band for a single species with
a hydrodynamic radius of 1.3 nm (Figures S7d and S11d).
Slow diffusion of Et2O into a solution of 1 in CH3CN
results in the cage crystallizing in the centrosymmetric
monoclinic space group P21/c as a pair of DDDD/LLLL
cages in the asymmetric unit. The single-crystal X-ray analysis
confirms the expected T-symmetric configuration of the cage,
with all the metal centers having the same D or L stereo-
chemistry (Figure 2a). The X-ray structure deviates slightly
[Fe4L4OTf]7+ ion also clearly shows that the cage breaks
before the triflate ion is released, thus unambiguously
confirming the endo nature of the triflate anion (Figure 3).
Variable-temperature 19F NMR measurements suggest that
the OTfÀ ion is in fast exchange at room temperature, as
indicated by a single signal for the anion. At 253 K, the OTfÀ
signal splits into two, thus indicating encapsulated and free
OTfÀ ions (Figure S13).
Figure 2. The crystal structure of 1-ACN [only one cage, LLLL-1, is
shown in (a) and (b)] and the molecular model of [(OTfÀ + 2ACN)ꢀ1]
(c, SPARTAN,[14] MM level).
Figure 3. The ESI-QTOF CID spectrum for the isolated [Fe4L4(OTf)]7+
from the perfect T symmetry, with the Fe···Fe distances
varying between 14.75 and 15.49 (average: 15 ). The
ion (marked with a star) and its fragmentation pathway.
3
very electron-deficient cavity has a volume of 330
(calculated by PLATON).[13]
Despite the poor diffraction quality of the crystals, the X-
ray structure unambiguously reveals one OTfÀ ion (Fig-
ure 2a,b) entrapped inside each cage. The overall shape and
size of the cage determined from the X-ray structure and from
molecular modeling studies (Figures S16 and S17) are very
similar. As only one triflate anion is found to reside inside the
cage, molecular modeling studies of the cage with guest
molecules were performed. In the X-ray structure, the triflate
anion resides in the cavity close to the corner of the cage, with
some unresolvable electron density elsewhere in the cavity,
while the molecular modeling study suggests that a triflate
anion alone would occupy the center of the cavity (Fig-
ure S17). The packing coefficient Pc for only one triflate anion
(VOTf = 86 3) is 0.26, which is much lower than the optimal
Pc value of 0.55.[15] The inclusion of two triflate ions would
result in a much more reasonable Pc value (0.52), but the
charge repulsion would prevent the simultaneous encapsula-
tion of two triflate anions in the cavity. The corner location
and the impossibility of encapsulating two anions suggest the
inclusion of an auxiliary guest(s). This proposal is supported
by the DOSY NMR spectrum, where a signal, possibly
originating from entrapped solvent molecules, has the same
diffusion coefficient as the cage (Figures S7d and S11d).
Molecular modeling studies of a triflate anion and two
acetonitrile molecules inside the cavity results in a very
similar location of the triflate anion: close to the corner of the
cage (Figure 2c).
Interestingly, when changing the crystallization conditions
to a slow diffusion of iPr2O into a CH3OH solution,
unexpected spontaneous resolution of the homochiral cages
occurs. Under these conditions, the cage crystallizes in the
cubic chiral space group I23 and Z = 2 (1/12 part of the cage in
the asymmetric unit). The cage has perfect T symmetry, with
Fe···Fe distances of 15.09 . Despite the poor crystal quality
(the two triflate anions and possible solvent molecules could
not be located, see the Supporting Information), the cage is
unambiguously the DDDD enantiomer of 1 (Figure S18).[16]
The monoclinic crystal lattice holds a pair of DDDD and
LLLL enantiomers (Figure S16), which interact through
nesting of the [Fe(bipy)3]2+ moieties of the DDDD cage in the
open side of the LLLL cage. These DDDD/LLLL nesting
pairs are then connected to the adjacent row of enantiomeric
pairs through packing of the [Fe(bipy)3]2+ moieties of the
LLLL cages (Figure S19a,b). The third dimension is com-
pleted through a weak sideways oriented interaction of the
[Fe(bipy)3]2+ moiety with the F3Ph moiety of the adjacent
cage (Figure S19c).
The packing in the chiral cubic I23 crystal lattice is
completely different, as the lattice contains only DDDD cages,
with no nesting between the cages. The four [Fe(bipy)3]2+
moieties of the cage cap the F3Ph moieties of the four
adjacent cages perpendicularly (Figure S20a). Correspond-
ingly, the four F3Ph moieties of the cage are capped by the
[Fe(bipy)3]2+ moieties of four adjacent cages (Figure S20b).
These eight tetrahedrally oriented (for one cage) “head-to-
wall” interactions result in a chiral MOF-like structure.
Our study introduces a new family of stable electron-
deficient tris(bipyridyl) ligands which can form robust M4L4
cages with highly electron-deficient cavities. Cage 1 crystal-
Encapsulation of one triflate anion inside the cage is
supported by the ESIMS spectrum of 1 (Figure S15), in which
the ion containing a single triflate anion, [Fe4L4OTf]7+, is
observed as the most abundant signal. The gas-phase colli-
sion-induced dissociation (CID) experiment on the
Angew. Chem. Int. Ed. 2015, 54, 14890 –14893
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