Key words supramolecular chemistry - host–guest complexes - fullerenes
Introduction
Fullerene C60 is a spherical molecular allotrope of carbon with plenty of applications.[1 ] For instance, the carbonaceous ball is a widely used electron-accepting material
in photovoltaics.[2 ] Tuning of the molecular orbital energies of C60 (and its derivatives) is a critical factor to tune the efficiency of exciton and
electron transfer processes in materials for molecular electronics and photovoltaics.[3 ] Chemical modification of C60 can be employed to tune both its electronic structure as well as its solubility and
mode of embedding into composite materials, thus, many reactions to chemically modify
C60 have been developed.[4 ],[5 ] For instance, PC61 BM (4 ) is one of the most utilized C60 derivatives as an electron-accepting material.[6 ] While covalent modification is a straightforward way to tune the electronic properties
of C60 , chemical reactions that produce stereochemically defined products are often difficult
to control due to a comparable reactivity of carbon positions on pure C60 and its derivatives, causing the formation of multi-adduct isomers.[6 ],[7 ] To achieve regio-controlled modification, various strategies have been developed
such as tether-directed syntheses and supramolecular masking methods.[8 ]–[16 ] Furthermore, examples of C60 derivative encapsulation inside coordination cages have been reported.[17 ]–[19 ] Among them, especially the coordination cages reported by Ribas and Yoshizawa were
examined for their propensity to encapsulate and release C60 derivatives.[17 ],[19 ]
Tailored purification methods for C60 derivatives are still scarce, and thus, there is demand for further strategies to
be explored. Recently, our group has reported a new family of coordination cages based
on organic ligands having a curved π-surface.[20 ]–[23 ] These self-assembled hosts provide a cavity of suitable size and shape to strongly
bind C60 /C70 via convex–concave π-interactions. Tight encapsulation of fullerenes inside these
cages allows for a multitude of applications. For example, the generation and long-term
stabilization of the C60
•– radical anion by nano-confinement inside triptycene-based cage Pd2
1
4 in organic solvents has been demonstrated.[23 ]
For some reported coordination cages composed of ligands with curved π-surfaces, binding
of carbon-rich guests has been demonstrated in the past[24 ]; however, the encapsulation capability of Pd2
1
4 has only been investigated for C60 so far. We envisaged that this coordination cage should be able to accommodate not
only pristine C60 , but also other carbon-rich guest compounds including C60 derivatives. Stimulated by the idea of widening the scope of guest uptake, the encapsulation
capability of Pd2
1
4 has been further investigated in the herein-described study ([Figure 1 ]). In the course of this study, Pd2
1
4 was found to be capable of encapsulating two molecules of corannulene ([Figure 1a ]). Furthermore, Pd2
1
4 displayed high to quantitative affinity towards a variety of C60 derivatives such as PC61 BM ([Figure 1b ]). Stimulated by the fact that Pd2
1
4 can encapsulate PC61 BM but not PC62 BM, which is a bis-adduct analogue of PC61 BM, we explored a facile method to purify PC61 BM by selective uptake and extraction from the cavity. We herein report that addition
of CS2 is able to liberate encapsulated PC61 BM from the cage in a recycling, yet non-disruptive, manner.
Figure 1 Encapsulation of (a) two molecules of corannulene and (b) a C60 derivative inside Pd2
1
4 .
Results and Discussion
The triptycene-based Pd2 L4 coordination cage was prepared following our previous work.[23 ] We started investigating the guest scope of Pd2
1
4 with a selection of rather small neutral polyaromatic hydrocarbons (for details,
see Figures S24 and S25). Among these, only corannulene, representing a substructure
of C60 , was found to be encapsulated within the cavity ([Figure 2a ]). In detail, an excess amount of powdered corannulene was added into an acetonitrile
solution of Pd2
1
4 and heated at 70 °C for 24 h.[25 ] In the 1 H NMR spectrum, a new set of peaks which could be assigned to (Cor)2 @Pd2
1
4 was observed besides parental Pd2
1
4 , which means that encapsulation of corannulene occurs pairwise in a cooperative fashion
with exchange kinetics slower than the NMR time scale ([Figure 2b ]). Two molecules of corannulene were found to be incorporated inside Pd2
1
4 according to the 1 H NMR signal integration ratio between host and guest signals and the results of an
NOESY experiment (Figures S28 and S30). Noteworthy, the signals assigned to the Pd2
1
4 host not containing the corannulene pair showed slightly different chemical shifts
as compared to the cage sample in the absence of corannulene. We assume that this
is caused by loose encapsulation of a single corannulene in fast exchange for this
fraction of species in the equilibrium.
Figure 2 (a) Encapsulation of corannulenes inside Pd2
1
4 . An optimized geometry is shown for (Cor)2 @Pd2
1
4 . (b) 1 H NMR spectra (CD3 CN, 0.7 mM, 500 MHz, 298 K) of Pd2
1
4 (top) and Pd2
1
4 with excess amount of corannulene (bottom). (c) 1 H NMR spectra (CD3 CN, 500 MHz) of (Cor)2 @Pd2
1
4 at 303 K (top) and at 263 K (bottom).
The protons of the encapsulated corannulene guests display an upfield shift by 2.42 ppm
compared to the free corannulene existing in the solution, comparable to what was
observed with other hosts.[24 ],[26 ] In addition, the Ha signal of the pyridine donors, pointing inward the cavity, also undergoes an upfield
shift by 3.12 ppm, probably due to direct interactions between corannulene and these
hydrogen substituents, further supporting the encapsulation of corannulene within
the cavity.[23 ] Furthermore, diffusion-ordered spectroscopy (DOSY) analysis revealed that the encapsulated
corannulenes show a smaller diffusion coefficient compared to free corannulene in
the same acetonitrile solution (Figure S31). Further, encapsulation was found to be
strongly temperature-dependent. Upon cooling, the ratio of (Cor)2 @Pd2
1
4 increased from 39% (303 K) up to 77% (253 K, both at 0.70 mM cage concentration and
excess of powdered corannulene). Intriguingly, during the VT-1 H NMR experiment, a host–guest complex of Pd2
1
4 and single corannulene, namely Cor@Pd2
1
4 , was not observed as a distinguishable species ([Figure 2c ]). To elucidate the dynamic behavior of guest exchange, a 1 H NMR titration experiment was performed (Figure S37). Aliquots of an acetonitrile
solution of corannulene were titrated into an acetonitrile solution of Pd2
1
4 . As a result, peaks assigned to the host–guest complex (Cor)2 @Pd2
1
4 appeared over the addition of 7 equiv of corannulene, alongside with all remaining
peaks of Pd2
1
4 showing slight shifts (Δδ
max = − 0.02 ppm), probably indicating a fast equilibrium of the empty host with a labile
mono-guest adduct.
To gain a further insight into this process, density functional theory (DFT) calculations
at the M06-2X/Lanl2dz level of theory were performed. As a result, encapsulation of
two corannulene molecules was found to lead to a more than two times higher gain in
stabilization energy than encapsulation of only a single corannulene inside the host
(Table S2). In the calculated geometry, convex–concave interactions between the encapsulated
corannulenes and the ligands are clearly visible.
Next, we investigated the encapsulation of various C60 derivatives inside Pd2
1
4 . Therefore, C60 derivatives were dispersed in an acetonitrile solution of Pd2
1
4 at 70 °C for 48 h, after which the residual powdered C60 derivative remains were removed. Compounds 2 –4 , comprising different C60 mono-adducts, were bound in 87 – 100% yield, determined by 1 H NMR analyses measured at 298 K ([Figure 3a, b ]).[27 ] In the 1 H NMR spectra of the resulting solutions, a new set of signals was found besides empty
Pd2
1
4 . As shown in [Figure 3a ], the cage should be desymmetrized due to the encapsulation of these C60 derivatives, containing a rather bulky substituent. Indeed, in the 1 H NMR spectra of 2 –4 @Pd2
1
4 , two sets and four sets of signals were observed for the pyridine and triptycene-backbone
protons, respectively, which indicates the encapsulation of the C60 derivatives with slow exchange dynamics (see [Figure 3c ] and Figures S2, S8 and S16 for complete NMR assignments). In addition, DOSY 1 H NMR shows that all of the newly appearing signals belong to a single species, having
a similar hydrodynamic radius to C60 @Pd2
1
4 (Figures S5, S13, and S21).[23 ] The formation of the host–guest complexes was further confirmed by ESI-MS measurements
([Figure 3d ]). The encapsulation yield of 4 was 87% under the chosen conditions, while quantitative encapsulation of 2 was achieved. In addition, the small apertures found in the densely packed, modelled
geometry of 4 @Pd2
1
4 suggested that encapsulation of bulkier derivatives such as PC62 BM, which can be found as side-products in the course of the synthesis of 4 , should not be possible ([Figure 4a, b ]).[6 ] To test this hypothesis, an excess amount of PC62 BM, available as a mixture of regio-isomers, was dispersed in an acetonitrile solution
of Pd2
1
4 for 24 h at 70 °C. In the resulting 1 H NMR spectrum, only one set of signals for empty Pd2
1
4 was observed (Figure S45). Hence, for steric reasons the bis-adduct does not seem
to be able to bind. This result implies that Pd2
1
4 is able to recognize C60 mono-adducts over corresponding bis-adducts. In fact, when the same equimolar amount
of 4 and PC62 BM were dispersed in an acetonitrile solution of Pd2
1
4 (with minute amounts of CS2 as an additive to accelerate guest uptake in the heterogeneous system) at room temperature
for 24 h, 4 @Pd2
1
4 was obtained as a major species (66% encapsulation yield), but again no host–guest
complex with PC62 BM was formed. As can be seen in the molecular model of 4 @Pd2
1
4 calculated by DFT, the four ligands are forced close together to leave an enough
space for accommodating the single appendix of 4 ([Figure 4a, b ]). We assume that this structural detail then precludes encapsulation of bulkier
PC62 BM. Often, encapsulation of lipophilic guest molecules such as fullerenes within a
coordination cage dissolved in a very polar solvent is governed by solvophobic interactions,
as these guests prefer a rather non-polar cavity environment.[28 ] Therefore, addition of a better solvent for C60 to the host–guest complex solution was envisaged to shift the equilibrium between
confined guest and free guest towards releasing of the guest molecule into the solvent.
Figure 3 (a) Encapsulation of 2 –4 in Pd2
1
4 . (b) Chemical structures of 2 –4 with encapsulation ratio indicated below each structure. (c) 1 H NMR spectra (CD3 CN, 500 MHz, 298 K) of 2 @Pd2
1
4 (top), 3 @Pd2
1
4 (middle) and 4 @Pd2
1
4 (bottom). (d) Excerpts of ESI-MS spectra (positive mode) of 2 @Pd2
1
4 (left), 3 @Pd2
1
4 (middle) and 4 @Pd2
1
4 (right) with a calculated isotopic pattern for each species.
Figure 4 Optimized geometry of 4 @Pd2
1
4 obtained by gas-phase DFT calculations at the B3LYP/Lanl2dz level of theory for Pd
atoms and 6 – 31 G(d,p) for all other atoms; (a) front and (b) top views.
Based on this assumption, we tested a variety of solvents which are commonly utilized
to solubilize C60 . Indeed, addition of CS2 was found to liberate encapsulated guest 4 . Once 33 vol% of CS2 was added to an acetonitrile solution of 4 @Pd2
1
4 , the mixture was shaken and was left to stand for a few minutes. After this period
of resting time, two layers were obtained, where the upper layer is a transparent
acetonitrile solution of the empty coordination cage and the bottom layer is a reddish
CS2 solution containing the liberated compound 4 ([Figure 5 ]). The purity of the extracted guest molecule was confirmed by 1 H NMR measurement (Figure S44). Note that this method is non-disruptive with respect
to the host system, as can be seen in the 1 H NMR spectrum of intact Pd2
1
4 recovered from the upper layer (Figure S43). Finally, we challenged the repetitive
encapsulation and release of 4 over 4 cycles (Table S3). After extracting 4 from 4 @Pd2
1
4 , the mixture was cooled to −78 °C (to conveniently freeze the acetonitrile) and the
CS2 layer was removed by decanting. The recovered acetonitrile solution containing Pd2
1
4 was further utilized for the next extraction experiment. Although a decline of the
encapsulation yield was observed over repetitive cycles, Pd2
1
4 was proven to accommodate and liberate 4 in a recycling yet non-disruptive manner ([Figure 5 ]). We presume that the observed decrease of the encapsulation yield can be attributed
to losing some host by a slight miscibility of CS2 in the acetonitrile solution.
Figure 5 Recycling encapsulation and release of 4 using Pd2
1
4 ; 4 @Pd2
1
4 was obtained in 80.5%, 80.3%, 62.9%, and 52.2% yields after the 1st to 4th cycle,
respectively. The yields were determined by 1 H NMR.
Conclusions
We have investigated the encapsulation capability of coordination cage Pd2
1
4 towards corannulene and several C60 -derivatives. Owing to the curved π-surface of the triptycene backbone of 1 , Pd2
1
4 can encapsulate such non-planar aromatic compounds in high to quantitative yields.
Pd2
1
4 binds two molecules of corannulene in solution. Furthermore, Pd2
1
4 incarcerates C60 derivatives 2 –4 , all mono-adducts of C60 , in a way that the guestsʼ substituents point outside the cavity through the space
between the ligands, leading to a breaking of the fourfold symmetry of the cage. Encapsulation
and liberation of 4 utilizing Pd2
1
4 were demonstrated in a recycling manner. The recycling process can be accomplished
in a layer-to-layer fashion, using two different solvents. In addition, Pd2
1
4 does not encapsulate bulkier bis-adducts of fullerene derivatives, which should make
Pd2
1
4 a candidate for potent and sustainable fullerene derivative purification systems.
Funding Information
This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research
Foundation), under Germanyʼs Excellence Strategy EXC 2033 – Project No. 390 677 874
– RESOLV and GRK2376 “Confinement-controlled Chemistry” – Project No. 331 085 229.
