Tetrahedral metal-organic cages with cube-like cavities for selective encapsulation of fullerene guests and their spin-crossover properties

Article abstract of DOI:10.1039/C8CC06652C

Tetrahedral FeII4L₆ coordination cages containing organic linkers with rigid π-electron aromatic rings and flexible alkyl units were constructed. Interestingly, these tetrahedral cages have rare cube-like cavities for selective encapsulation of fullerene guests. In addition, the solid state spin-crossover properties were influenced by guest binding of fullerene C₆₀.

Full text of DOI:10.1039/C8CC06652C

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Tetrahedral metal–organic cages with cube-like
cavities for selective encapsulation of fullerene
guests and their spin-crossover properties†
Cite this: Chem. Commun., 2018,
54, 12646
Received 15th August 2018,
Accepted 12th October 2018
b
b
a
Wang-Kang Han,a Hai-Xia Zhang,a Yong Wang,
Wei Liu,
Xiaodong Yan,
ac
Tao Lia and Zhi-Guo Gu
*
DOI: 10.1039/c8cc06652c
rsc.li/chemcomm
Tetrahedral FeI4IL6 coordination cages containing organic linkers considered: (i) the cavity shape and size of the cages should
with rigid p-electron aromatic rings and flexible alkyl units were be suitable for capturing target fullerene guests; (ii) the inner
constructed. Interestingly, these tetrahedral cages have rare cube-like voids should be surrounded by p-electron-rich ligands, which
cavities for selective encapsulation of fullerene guests. In addition, the could enhance the affinity towards spherical fullerene guests
solid state spin-crossover properties were influenced by guest binding through aromatic stacking interactions; (iii) the window of the
of fullerene C60
.
cage should be flexible enough to allow guests to enter and exit
the host for isolating guests from the environment. In order to
construct the target metal–organic cages, the ligands should
combine with rigid p-electron groups and flexible linkers
(Scheme 1). With this in mind, we incorporated O–benzene or
O–naphthalene moieties and flexible alkyl units into the
ligands. The O–benzene or O–naphthalene moieties could
provide favorable binding sites for the fullerene guest through
aromatic stacking and donor–accepter interactions, while the
alkyl chains on both sides of the aromatic panels were expected
to enhance the flexibility of the ligands.
Employing these strategies, rigid-flexible di(imidazole aldehyde)
components, (R)-1-(naphthalen-2-yl)ethanamine and iron(II) ions,
were chosen as the building blocks for the construction of metal–
organic cages through a multicomponent self-assembly process
(Fig. 1). Herein, we described the design, syntheses, crystal structures
and host–guest behaviors of flexible FeI4IL6 (L = 1,2-di((imidazol-2-
ylmethylene)-(R)-1-phenylethanamine)ethane derivatives) tetrahedral
metal–organic cages. The coordination cages presented rare
Considerable attention has been paid to the construction of
container molecules in recent years for their wide ranging
applications in separation,1 recognition,2 catalysis,3 gas storage,4
stabilization of reactive species5 and so on.6 Metal–organic cages
(or coordination cages) with porous surfaces and abundant
recognition sites in their central cavities can typically bind guest
molecules through specific host–guest interactions and molecular
recognition.7,8 The design and modulation of the size and shape
of the cage cavity to selectively accommodate guest molecules,
fullerenes for example, is still an enormous challenge due to their
similar physicochemical properties. To date, research on metal–
organic cages for the separation and purification of fullerene guests
has been mainly focused on those cages with rigid ligands.9,10
However, the composition and structure of rigid ligands are quite
limited; moreover, the rigid coordination cages always lack flexibility
and selectivity for encapsulation of different fullerene guests. There-
fore, the design of metal–organic cages with enough flexibility to
accommodate fullerene guests is important.
To develop metal–organic cages capable of selective encap-
sulation of fullerene guests, three main factors need to be
a The Key Laboratory of Synthetic and Biological Colloids, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
P. R. China. E-mail: zhiguogu@jiangnan.edu.cn
b College of Chemistry, Chemical Engineering and Material Science, Soochow
University, Su Zhou 215123, P. R. China
c International Joint Research Center for Photoresponsive Molecules and Materials,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
general characterizations, additional plot and discussion. CCDC 1847958 and
1847959. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8cc06652c
Scheme 1 Schematic representation of the building blocks used for the
design of metal–organic cages with selective encapsulation of fullerene
guests.
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Fig. 1 Design and synthesis of tetrahedral metal–organic cages 1 and 2
with cube-like cavities through a multicomponent self-assembly approach.
cube-like cavities, and displayed selective encapsulation of
fullerene guests. In addition, the solid state spin-crossover
behaviors and the influence of the fullerene guest on the
magnetic properties of the two types of FeI4IL6 coordination
cages were also investigated.
The self-assembly reactions of the di(imidazole aldehyde)
components (6 equiv.), iron(II) trifluoromethanesulfonate (Fe(OTf)2,
4 equiv.) and (R)-1-(naphthalen-2-yl)ethanamine (12 equiv.), in
acetonitrile solution resulted in the formation of tetrahedral Fe4II
L6 cages 1 and 2 (see the ESI†). The microstructures of cages 1 and
2 were characterized by FT-IR, UV-vis, NMR spectroscopy, mass
Fig. 2 Two views of the cationic parts of the X-ray crystal structures:
(a) and (b) for cage 1; (c) and (d) for cage 2. All H atoms, counter anions,
solvent molecules and disorder have been omitted for clarity (C, grey;
N, blue; O, red; Fe, purple).
spectroscopy, and single crystal X-ray diffraction analysis. FT-IR The average center-to-center distance of the p–p stacking was
spectra of cages 1 and 2 showed strong absorption in the region 3.88 Å for cage 1 and 3.72 Å for cage 2.
around 1573–1601 cmÀ1, which was typical for stretching of
Interestingly, unlike the reported edge-capped M4L6 coordi-
imidazole-imine (CQN) groups. The peaks at about 1258 cmÀ1 nation cages with rigid linkers and tetrahedral cavities, cages 1
and 636 cmÀ1 revealed the existence of OTfÀ anions in the metal– and 2 possessed rare cube-like cavities due to the flexible alkyl
organic cage complexes (Fig. S9, ESI†). High-resolution mass chains such that the linkers could be rotated and distorted. As a
spectra (HRMS) showed ion peaks at m/z = 2541.18, 1644.47 and result, six O–benzene (or O–naphthalene) moieties are situated
1196.12 (Fig. S22–S25, ESI†), corresponding to [1(OTf)6]2+, at each face, and four iron(II) ions occupied half of the vertices
[1(OTf)5]3+ and [1(OTf)4]4+, respectively, while the ion peaks of the artificial cube. For cage 1, four O–benzene moieties are
observed at m/z = 2691.24, 1744.50 and 1271.14 (Fig. S26–S29, ESI†) parallel to the cube face in a ‘‘face to face’’ state, while another
correspond very well to [2(OTf)6]2+, [2(OTf)5]3+ and [2(OTf)4]4+, two O–benzene moieties are vertical to the cube face in
respectively. The observed ion peaks agreed well with the simulated an ‘‘edge to edge’’ state (Fig. S32, ESI†). For cage 2, all the
isotopic patterns, further verifying the formation of a Fe4L6 O–naphthalene moieties are vertical to the artificial cube face
stoichiometry of cages 1 and 2.
in an almost ‘‘edge to edge’’ state (Fig. S33, ESI†). The resulting
Single crystal X-ray diffraction analysis at 173 K confirmed cube-like cavities in cages 1 and 2 surrounded with O–benzene
the edge-capped capsule structures of the [Fe4L6]8+ for cages 1 or O–naphthalene moieties could provide favorable binding
and 2. As shown in Fig. 2, each iron(II) centre coordinated with sites for aromatic guest molecules through p–p stacking
six nitrogen atoms from three imidazole-imine Schiff-base type interactions and donor–acceptor interactions. In addition, the
ligands, forming a distorted octahedral FeN6 coordination windows of cages 1 and 2 were opened with large pores to allow
geometry, and the four Fe(II) metal nodes were bridged by six the guests to enter and exit (Fig. 2). The cavity volumes were
ligand linkers, forming a tetrahedral cage with approximate T calculated to be 608 and 563 Å3 for cages 1 and 2, respectively.
point symmetry. The metal centres occupied the vertices and The large cube-like void volume with p-electron density
the linkers situated at the edges of the tetrahedron. All metal revealed by single crystal structural analysis stimulated us to
centres displayed facial coordination and share the same L explore the capability of cages 1 and 2 to act as receptors for
handedness. The average Fe–N bond length of cage 1 was fullerene guests. NMR spectroscopy and high-resolution mass
1.96 Å, and the average Fe–N bond length of cage 2 was slightly spectrometry characterizations were thus carried out to inves-
longer at 1.98 Å. These values were consistent with the typical tigate their encapsulation of fullerene guests.
low-spin iron(II) centres at 173 K.11 The metal–metal spacing was
Upon addition of C60 guests to cage 1 or cage 2 in CD3CN,
in the range of 11.588–12.846 Å for cage 1, and 12.052–12.351 Å for the signals on the 1H-NMR spectra displayed a significant
cage 2. In each [Fe4L6]8+ cation, twelve intramolecular face-to-face chemical shift, especially the aromatic C–H signals between
p–p stacking interactions existed between each parallel naphthalene 5.2 ppm and 8.5 ppm (Fig. 3). After equilibrating the mixture of
ring and imidazole ring of the adjacent ligands, further cage 1 and fullerene C60 at 50 1C for 24 h, a new set of 1H-NMR
stabilizing the supramolecular structure (Fig. S30–S31, ESI†). single peaks was observed at different chemical shifts while the
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Nevertheless, no C70 encapsulation in the cage 1 or 2 was
detected, as indicated by the 1H NMR spectra of the hosts
which appeared nearly at the same chemical shifts as in the
absence of the C70 guest (Fig. S42 and S43, ESI†).
Furthermore, the formation of 1 : 1 host–guest complexes,
[C60C1] and [C60C2], was also supported by the HRMS analysis.
The former exhibited intense signal peaks which corresponded
to three different species: [C60C1(OTf)5]3+, [C60C1(OTf)4]4+, and
[C60C1(OTf)3]5+ at M/Z = 1884.47, 1376.13, and 1071.10 (Fig. 4a),
respectively. The latter, [C60C2], gave ion peaks at M/Z = 1984.83,
1451.40, and 1131.12 corresponding to the [C60C2(OTf)5]3+,
[C60C2(OTf)4]4+, and [C60C2(OTf)3]5+ charge states, respectively
(Fig. 4b). Besides, the isotopic distributions of the experimental
results were also well consistent with the simulated isotopic
patterns (Fig. S44–S51, ESI†).
We inferred from the significant differences of the guest
binding results, that the smaller C60 proved to be a better guest
than C70, could be rationalized by comparing the size and
shape match between the host cavities and the fullerene guests.
The van der Waals volume of C60 is 345 Å3, taking up 56.74% of
the cavity of cage 1, while the van der Waals volume of C70 is
390 Å3, occupying 64.14% of the cavity of cage 1.14 On the other
hand, the volumes of C60 and C70 are about 61.28% and 67.27%
of the cavity of cage 2, respectively. According to Julius Rebek’s
55% rule, which is based on the volume ratio of the guest and
the host,15 it is thus obvious that the cavities of cages 1 and 2
are good match for C60, and that both cages 1 and 2 are not
suitable for C70. For cages 1 and 2, the C60 guest was closer to
the optimum occupation in favor of forming host–guest com-
plexes, while the C70 guest was too large to situate in the host
cavities. Therefore, cages 1 and 2 were expected to show good
Fig. 3 Partial 1H-NMR spectra (400 Hz, CD3CN, 298 K) of (a) a mixture of
cage 1 and C60 (about 5 equiv.) and (b) a mixture of cage 2 and C60 (about
5 equiv.), allowed to equilibrate at 50 1C for 0 h, 24 h and one week.
peaks corresponding to the empty host had partially disap- performance for selective encapsulation of C60 and C70 guests.
peared, which could be attributed to the formation of [C60C1] Magnetic interaction and guest dependent spin-crossover
host–guest complexes.12 Further equilibrating the mixture for (SCO) properties in multinuclear SCO compounds have aroused
one week, nearly all the peaks corresponding to the empty cage particular interest. And magnetic susceptibilities for cages 1
disappeared, indicating that most of the cages were converted and 2 were determined. Both cages 1 and 2 exhibited typical
to the host–guest complexes (Fig. 3a). Similar trends in the gradual and incomplete SCO behaviors. As shown in Fig. 5, the
1H-NMR spectra were also observed after equilibrating the wMT values of cages 1 and 2 remain almost constant (about
mixture of cage 2 and C60, as evidenced by the disappearance 1.64 cm3 K molÀ1 for cage 1 and 1.55 cm3 K molÀ1 for cage 2)
of the peaks corresponding to the free host and concurrent below 200 K, suggesting that most of the Fe(II) centers were in
appearance of a new set of peaks corresponding to the host– the low-spin state at low temperatures. Upon further heating,
guest complex [C60C2]. However, even after the mixture of cage the wMT value gradually increased and reached its maximum
2 and C60 was allowed to equilibrate at 50 1C for one week,
the 1H-NMR signals from the empty cage 2 and host–guest
complexes [C60C2] still coexisted (Fig. 3b). This observation
indicated that cage 1 had a better preference to C60 than cage 2.
We proposed that C60 was bound more strongly within cage 1
than in cage 2, possibly due to a better match in terms of size
and shape and thus the stronger p–p interactions. In addition,
new NMR signals and chemical shifts were also detected in the
13C-NMR spectra after equilibrating the mixture of cage hosts
and C60 guests (Fig. S34, S35, S38 and S39, ESI†). It was worth
noting that the 13C-NMR spectra showed an intense peak at
142 ppm from C60 despite the negligible solubility of C60 in
CD3CN, which further provided evidence for the encapsulation
Fig. 4 High-resolution mass spectra of (a) [C60C1] and (b) [C60C2]
showing the +3, +4 and +5 ion peaks.
of C60 by the host cages 1 and 2 (Fig. S34 and S38, ESI†).9b,13
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Project for Jiangsu Scientific and Technological Innovation Team,
and the MOE & SAFEA for the 111 Project (B13025).
Conflicts of interest
There are no conflicts to declare.
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This work was supported by the National Natural Science
Foundation of China (21771089), the Fundamental Research Funds
for the Central Universities (JUSRP51725B and JUSRP51513), the
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