Size-selective encapsulation of C60 and C60-derivatives within an adaptable naphthalene-based tetragonal prismatic supramolecular nanocapsule

Article abstract of DOI:10.1039/c8cc07886f

A novel naphthalene-based 5·(BArF)₈ capsule allows for the size-selective inclusion of C₆₀ from fullerene mixtures. Its size selectivity towards C₆₀ has been rationalized by its dynamic adaptability in solution that has been investigated by molecular dynamics. Additionally, 5·(BArF)₈ encapsulates C₆₀-derivatives such as C₆₀-PCBM and N-methylpyrrolidine-C₆₀. The latter can be separated from C₆₀ since 5·(BArF)₈ displays distinct affinity for them.

Full text of DOI:10.1039/c8cc07886f

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Size-selective encapsulation of C60 and
C60-derivatives within an adaptable
naphthalene-based tetragonal prismatic
supramolecular nanocapsule†
Cite this:DOI: 10.1039/c8cc07886f
Received 3rd October 2018,
Accepted 12th December 2018
b
Cristina Garcıa-Simon, ‡*a Alba Monferrer,‡a Marc Garcia-Borras,
´
´
`
DOI: 10.1039/c8cc07886f
c
cd
a
Inhar Imaz,
Daniel Maspoch,
Miquel Costas *a and Xavi Ribas
*
rsc.li/chemcomm
A novel naphthalene-based 5ꢀ(BArF)8 capsule allows for the size- HPLC separation. C60-derivatives such as [6,6]-phenyl C61 butyric
selective inclusion of C60 from fullerene mixtures. Its size selectivity acid methyl ester (PCBM-C60) and fulleropyrrolidines present
towards C60 has been rationalized by its dynamic adaptability promising applications in materials science, but they need to be
in solution that has been investigated by molecular dynamics. available in a pure form to display full efficiency.9,10 To date,
Additionally, 5ꢀ(BArF)8 encapsulates C60-derivatives such as C60-PCBM numerous examples of 3D molecular receptors based on metal–
and N-methylpyrrolidine-C60. The latter can be separated from C60 ligand coordination bonds and capable of hosting fullerenes
since 5ꢀ(BArF)8 displays distinct affinity for them.
have been reported.11–14 However, the separation of a single
fullerene by size-confinement effects is not straightforward since
The metal–ligand coordination approach has led to the prepara- the molecular receptors display high affinity for different-sized
tion of sophisticated discrete three-dimensional (3D) structures, fullerenes simultaneously.15 In most reported examples a
which are of great interest due to their intrinsic complexity, higher affinity towards Cn (n Z70) over C60 is observed, due
which conveys multifunctionality, and potential applications in to the more significant solvophobic effects and the more
fields such as molecular catalysis, sensing and purification.1–3 extended p-surface of larger fullerenes that enhances capsule–
Indeed, the possibility of tuning the dimensions of the confined fullerene interaction.16–20 The reverse trend is observed in very
space by modifying the 3D molecular structure is an attractive limited examples.21
feature to gain control over the guest affinity, and allows for the
Our group previously reported a coordination nanocapsule
preparation of extended libraries of coordination capsules.4,5 (4ꢀ(BArF)8) capable of encapsulating different-sized fullerenes
22
On the other hand, the purification of fullerenes is remarkable from C60 to C84
.
This lack of size selectivity was attributed to
due to their applications in materials science and medicine.6,7 its pronounced breathing ability and large cavity size. In light of
Even though fullerene extracts are easily available, the selective the importance of C60 and its derivatives, herein we report a
purification of a specific fullerene cage from a mixture of Cn nanocapsule (5ꢀ(BArF)8), which bears a smaller cavity, obtained
(n 4 70) cages is still a challenging task basically accomplished by tuning the length of its spacers. The newly designed nano-
by means of high performance liquid chromatography (HPLC) capsule with naphthalene spacers is capable of exclusively
techniques, which are generally tedious, time-consuming and encapsulating a smaller C60 fullerene and its corresponding
energy consuming.8 Another challenging issue is the purifica- mono-adduct derivatives in a selective manner.
tion of fullerene derivatives, which is also mainly restricted to
New metallo-capsule 5ꢀ(X)8 (X = CF3SO3, BArF) was synthe-
sized by coordination driven self-assembly of the carboxylate
units from ZnII-porphyrin (2) and PdII-based macrocyclic
synthons (Pd-1c). 5ꢀ(X)8 differs from our previously reported
capsules 3ꢀ(X0)8 (X0 = ClO4, CF3SO3) and 4ꢀ(X)8 in the nature of
the hexaaza macrocyclic ligands used to synthesize ligands 1 and
1b and therefore the corresponding PdII-clip building blocks
(Scheme 1).22,23 Nanocapsule 3ꢀ(X0)8 contained a phenyl ring in its
macrocyclic ligands (1), and the crystallographic distance between
its porphyrins was 7.5 Å. On the other hand, 4ꢀ(X)8 included
a biphenyl moiety in 1b, rendering a Porph-Znꢀ ꢀ ꢀPorph-Zn
distance of 14.1 Å measured from the X-ray structure (Fig. 2).
While the former capsule 3 was only capable of encapsulating
a
´
`
Institut de Quımica Computacional i Catalisi (IQCC) and Departament de
´
Quımica, Universitat de Girona, Campus Montilivi, Girona, E-17003, Catalonia,
Spain. E-mail: cristina.simon@udg.edu, miquel.costas@udg.edu,
xavi.ribas@udg.edu
b Department of Chemistry and Biochemistry, University of California, Los Angeles,
CA90095, USA
c
`
`
Institut Catala de Nanociencia i Nanotecnologia, ICN2, Campus UAB,
08193 Bellaterra, Catalonia, Spain
d
´
ICREA, Pg. Lluıs Companys 23, 08010 Barcelona, Catalonia, Spain
† Electronic supplementary information (ESI) available. CCDC 1561152 and
1561153. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8cc07886f
‡ These authors contributed equally to this work.
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Fig. 1 HRMS of 5ꢀ(BArF)8 and its host–guest adducts. HRMS in CH3CN of
(a) 5ꢀ(BArF)8, (b) C60C5ꢀ(BArF)8, (c) [(mono)-N-pyrrolidine-C60]C5ꢀ(BArF)8
and (d) [PCBM-C60]C5ꢀ(BArF)8.
Scheme 1 Schematic representation of the library of coordination nano-
capsules 3–5. (a) Spacers used to obtain (b) the tetragonal-prismatic
supramolecular capsules 3ꢀ(X0)8 (X0 = ClO4, CF3SO3), 4ꢀ(X)8 and 5ꢀ(X)8
(X = CF3SO3, BArF) (BArF: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate).
tetragonal prismatic geometry bearing D4 symmetry. The
crystallographic Porph-Znꢀ ꢀ ꢀPorph-Zn distance in 5ꢀ(CF3SO3)8
is 8.2 Å. As expected, this distance lies between those observed
in capsules 3ꢀ(X0)8 and 4ꢀ(X)8 (see Scheme 1 and Fig. 2a). In light
of the size contraction with respect to 4ꢀ(X)8, it was predicted
flat anionic p-guests, capsule 4 could host fullerene molecules up that 5ꢀ(X)8 will offer a higher affinity to smaller fullerenes.
to C84 with very high association constants (up to Ka 4 108 Mꢁ1).
Once 5ꢀ(BArF)8 was fully characterized, its ability to host
For the new nanocapsule 5ꢀ(X)8, which contains 2,6-disubstituted fullerenes was tested starting with the encapsulation of C60.
naphthalene units in the macrocyclic ligand (1c) (Fig. S1–S27, Here, the fast formation of 1 : 1 host : guest adducts was observed
ESI†), an intermediate capsule-size was envisioned (Scheme 1).
after mixing a 1 : 1 molar solution of 5ꢀ(BArF)8 in CH3CN and C60
The formation of nanocapsule 5ꢀ(CF3SO3)8 was first con- in toluene. HRMS of the 1 : 1 C60 : 5ꢀ(BArF)8 mixture showed
firmed by high-resolution mass spectrometry (HRMS) (Fig. S28, peaks corresponding exclusively to C60C5ꢀ(BArF)8 (Fig. 1b and
ESI†). Then, CF3SO3ꢁ anions were exchanged with BArF anions Fig. S39, ESI†). Data obtained from the UV-Vis titration of C60
to increase its solubility. 5ꢀ(BArF)8 was obtained in good yield, and 5ꢀ(BArF)8 fitted to a 1 : 1 binding model with an association
effectively showing an enhanced solubility in different organic constant of 1.29 (ꢃ0.42) ꢂ 105 Mꢁ1 (all UV-vis titrations were
solvents, including CH3CN and dichloromethane (DCM). HRMS performed in a toluene/acetonitrile solvent mixture (9/1) and
of 5ꢀ(BArF)8 showed ions corresponding to the cages with a the host concentration was kept constant, Fig. S40, ESI†).24
consecutive loss of counteranions (Fig. 1a and Fig. S29, ESI†), The formation of the host : guest adduct was also evidenced
demonstrating its integrity in solution. Full NMR characteriza- in the 1H-NMR spectrum, which showed several signals shifted,
tion was also performed for 5ꢀ(BArF)8 (Fig. S30–S36, ESI†). especially those corresponding to the aromatic protons of
Remarkably, the DOSY NMR experiment (CH3CN, 298 K) afforded the phenyl rings of the porphyrin pointing into the cavity
a diffusion coefficient of D = 3.1 ꢂ 10ꢁ10 m2
s
ꢁ1, indicating (Fig. S41, ESI†).
Afterwards, the encapsulation of C70 was also attempted and
that the dimensions of 58+ correspond to a hydrodynamic
radius of 18.5 Å in solution (Fig. S37 and S38, ESI†), in line monitored by HRMS. Remarkably, when a solution of C70 in
with the value estimated from crystallographic data (see below toluene was mixed with a solution of 5ꢀ(BArF)8 in CH3CN in a
and Fig. S38b, ESI†).
2 : 1 molar ratio (298 K, 48 h), peaks corresponding to the
Crystallographic data were obtained from 5ꢀ(CF3SO3)8 crystals
C
70C5ꢀ(BArF)8 adduct as well as peaks belonging to the remain-
at the XALOC beamline of the ALBA Synchrotron (Fig. 2a ing empty capsule were observed (3 : 2 ratio, respectively, see
and ESI† for Single-Crystal X-Ray Diffraction – SCXRD details). Fig. S42, ESI†). These results indicated a notable lower affinity
Nanocapsule 58+ consists of two parallel tetracarboxylated of the capsule towards C70 compared to C60. UV-Vis titration
ZnII-porphyrins linked by four macrocyclic dinuclear PdII com- with C70 was in line with HRMS, since negligible changes in the
plexes. The four-carboxylate residues of each porphyrin are Soret band were detected (Fig. S43, ESI†).
linked by means of Z1-O monodentate coordination to one PdII
The encapsulation of C60 might seem surprising based solely
center (Fig. S38, ESI†). As in 38+ and 48+,19,20 58+ presents a on the short Porph-Znꢀ ꢀ ꢀPorph-Zn distance measured in the
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Fig. 2 Crystal structures of the library of capsules 38+–58+ and illustrations of the MD simulation of capsule 5. (a) Lateral views of the crystal structures of
3ꢀ(ClO4)8, 4ꢀ(BArF)8 and 5ꢀ(CF3SO3)8 (counterions and hydrogen atoms are omitted for clarity). (b) Znꢀ ꢀ ꢀZn distance measured along the 500 ns of
Molecular Dynamics (MD) trajectory for 5ꢀ(Cl)8 (purple), C60C5ꢀ(Cl)8 (green) and C70C5ꢀ(Cl)8 (orange) in explicit CH3CN solvent. For more clarity, the
10-step averaged value for each point is represented by the purple line in the plot. (c) Representative snapshots taken along the MD trajectory to illustrate
the conformational flexibility of 58+. Znꢀ ꢀ ꢀZn distances are given in Å, and hydrogen atoms are omitted for clarity.
crystal structure. This distance, 8.2 Å, is much smaller than the
Taking advantage of the higher size-selectivity for C60,
van der Waals diameter of C60 (10.1 Å), thus indicating that nanocapsule 5ꢀ(BArF)8 was used to selectively separate C60 from
5ꢀ(BArF)8 must undergo important structural changes in solution fullerene extracts (extract composition: 70% C60, 28% C70, 2%
to be able to accommodate C60 (Fig. 2a). The latter was explored higher fullerenes). To this end, a solution of 5ꢀ(BArF)8 in
through molecular dynamics (MD) simulations using CH3CN as CH3CN was mixed with a solution of the fullerene extract in
explicit solvent and Clꢁ as counterions (see the ESI† for details). toluene, in a 1 : 2 molar ratio (capsule : fullerenes). Delightfully,
MD simulations revealed that the Porph-Znꢀ ꢀ ꢀPorph-Zn distance HRMS experiments showed that C60 was exclusively encapsu-
in empty 5ꢀ(Cl)8 swings from 10.5 to 12.5 Å in solution (Fig. 2b lated from all the fullerene mixtures (Fig. S46, ESI†). No peaks
and c and Video VS1 in the ESI†). On the other hand, MD corresponding to adducts with C70 or with other higher full-
simulations on C60C5ꢀ(Cl)8 and C70C5ꢀ(Cl)8 host–guest complexes erenes present in the soot (i.e. C72, C76 or C84) were observed,
showed a Porph-Znꢀ ꢀ ꢀPorph-Zn distance of 12.3–13 Å for the even using extended reaction times. The same experiment was
C60 system and 12.6–13.3 Å for the C70 system (Fig. S44, ESI†). repeated using solid 5ꢀ(CF3SO3)8, in which C60 fullerene was also
Porph-ZnꢀꢀꢀPorph-Zn distances explored during the MD trajectory solely encapsulated (capsule : fullerene-extract in a 1 : 8 molar
in C70C5ꢀ(Cl)8 are larger than the Porph-ZnꢀꢀꢀPorph-Zn distances ratio, see Fig. S47, ESI†). In contrast, when a larger 4ꢀ(BArF)8
measured in empty 5ꢀ(Cl)8 during MD, while C60C5ꢀ(Cl)8 dis- capsule is added, in either the solid or liquid form, to a fullerene
tances are in the breathing range (Fig. 2b). Indeed, when com- extract solution, both C60 and C70 and other higher fullerenes are
paring these Porph-Znꢀ ꢀ ꢀPorph-Zn distances to those observed encapsulated, displaying no size exclusion.22
in the previously reported crystal structures of C60C4ꢀ(BArF)8
Finally, the ability of 5ꢀ(BArF)8 to encapsulate C60 derivatives
and C70C4ꢀ(BArF)8, 13.1 Å and 13.7 Å for C60 and C70, was also explored. Host–guest studies were carried out with
respectively,22 the idea that C70 fullerene imposes a more severe PCBM-C60 and N-methylpyrrolidine-C60 (the latter was prepared
distortion on the nanocapsule 5 than C60 is reinforced. This as previously reported).25 When a solution of the corresponding
large distortion of the nanocapsule 5 required to encapsulate derivative in toluene was mixed with a solution of the capsule
C70 is in agreement with the hampered encapsulation of C70 in acetonitrile in a 1 : 1 molar ratio, a clean formation of the
within 5ꢀ(BArF)8 and the higher selectivity towards C60.
host–guest complexes was observed by HRMS after 5 min
Encapsulation experiments with C60 were also performed stirring (Fig. 1c and d and Fig. S48, 49, ESI†). UV-Vis titration
using 5ꢀ(CF3SO3)8 in the solid state. Interestingly, C60 was experiments were performed in order to estimate the asso-
trapped after adding the solid capsule to a solution of the ciation constant between the 5ꢀ(BArF)8 receptor and the C60
-
fullerene in toluene (1 : 8 capsule : fullerene molar ratio, a derivatives. Interestingly, the UV-Vis experiment following the
higher excess of fullerene was applied in the solid : liquid formation of the [N-methylpyrrolidine-C60]C5ꢀ(BArF)8 complex
experiments to facilitate the less favoured encapsulation) (Fig. S45, gave an association constant of 6.60 (ꢃ0.32) ꢂ 104 Mꢁ1 (2-fold
ESI†). The full formation of the C60C5ꢀ(CF3SO3)8 adduct was smaller than the one obtained for C60, Fig. S50, ESI†) and the
observed after 1 h stirring. Note here that the liquid/liquid [PCBM-C60]C5ꢀ(BArF)8 complex gave an association constant of
complexation occurs more rapidly (o5 min, 1 : 1 capsule : fullerene 1.27 (ꢃ2.04) ꢂ 105 Mꢁ1 (Fig. S51, ESI†), which is very similar to
ratio) than the solid/liquid one. The slower rate of the solid/ the one obtained for C60. Encapsulation of both functionalized
liquid encapsulation is most likely due to the higher rigidity fullerenes was also performed using the 5ꢀ(CF3SO3)8 capsule in
of the capsule in the solid state, which restricts its dynamic the solid state. Both species showed complete encapsulation
adaptability needed to accommodate C60, as revealed by MD after 1 h stirring a capsule suspension in a fullerene-derivative
simulations.
toluene solution (Fig. S52 and S53, ESI†); however, encapsulation
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This work was supported by MINECO-Spain (CTQ2016-77989-P
to X. R., CTQ2015-70795-P to M. C., MAT2015-65354-C2-1-R to
D. M. and I. I., 2017 SGR 264 to X. R., and 2014SGR80 to D. M.), the
`
CERCA Programme/Generalitat de Catalunya, ICREA-Academia
´
(X. R. and M. C.), the Ramon Areces Foundation (M. G.-B.) and
the Severo Ochoa Center of Excellence Program (ICN2, Grant
SEV-2017-0706).
Conflicts of interest
There are no conflicts to declare.
Fig. 3 HRMS spectra of the competition experiments of (a) C60 vs. PCBM-C60
and (b) C60 vs. N-methylpyrrolidine-C60
.
Notes and references
of PCBM-C60 was faster as was evidenced by the HRMS spectrum
recorded after 30 min reaction time compared to the one
obtained for N-methylpyrrolidine-C60 (Fig. S52 and S53, ESI†).
The latter observation is in agreement with the higher associa-
tion constant obtained for PCBM-C60.
A competition experiment between C60 fullerene and the
PCBM-C60 derivative further supports the similar values
obtained for their association constants. When 1 equivalent of
the 5ꢀ(BArF)8 capsule was mixed with 2.5 eq. of C60 and 2.5 eq. of
PCBM-C60, in toluene/acetonitrile (4/1) for 2.5 h, very similar
intensity peaks on the HRMS were observed in line with the
similar Ka values (see Fig. 3a and Fig. S54, ESI†). Interestingly,
when an analogous experiment was performed using N-methyl-
pyrrolidine-C60 instead of PCBM-C60, selective encapsulation of
´
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