Synthesis of M12L8 poly-[n]-catenane using non-conventional crystallization method: Instant synthesis of TPB and ZnI2 using aromatic solvents.
Using the extremely fast crystallization method, the instant synthesis26,27 at room temperature conditions in the preparation of mechanically interlocked large M12L8 cages is uncommon. The instant synthesis method allows the selective synthesis using kinetic control, with which it is possible to obtain the poly-[n]-catenane of interlocked M12L8 nanocages as homogeneous phase (i.e., selectively). Although, we have demonstrated the instant synthesis using TPB and ZnCl2 or ZnBr2, using ZnI2 has not been reported. Changing one of the building blocks in the self-assembling components (i.e., metal ions and organic ligands) can have a drastic effect in the final products. For instance, it can lead to different and unexpected structures, particularly in self-assembling systems where the building blocks are more than twenty components, as it is in the poly-[n]-catenanes self-assembled of M12L8 icosahedral nanocages including solvent.
Addition of a methanolic solution of ZnI2 into the vigorous stirring light-yellow solution of TPB in nitrobenzene instantaneously formed a suspension which was left to stir for 5 mins (Figure 1a). After filtration, the solid was very difficult to dry using pump filtration and the sample was dried flowing dry N2 for 30 minutes (Figure S5). Powder XRD analysis revealed that the new product is amorphous (hereafter named a1) with a very large bump covering the 10-30 range in 2θ / ° Bragg diffraction angles and with absence of sharp Bragg reflections (Figure 1b). The diffuse scattering shows that there is short-range ordering that is different from the starting crystalline materials (Figure 1). This differs from what was observed in the chloride and bromide version of poly-[n]-catenanes, which upon instant synthesis using the same solvents, yield always crystalline phases. Notably, the structure of the amorphous phase is not known but it can be inferred via an amorphous-to-crystalline phase transition if a1 uptakes aromatic guest molecules producing the M12L8 poly-[n]-catenane periodic structure.
Therefore, a1 was immersed in a toluene/methanol (4 ml : 1 ml) solution in a round bottom flask and stirred for 12 h (overnight experiment). Then, the suspension was filtered and the solid analysed by powder XRD. The diffractogram showed that the amorphous phase a1 transformed to a crystalline material as sharp Bragg reflections are observed (Figure 1c). The experimental powder XRD pattern corresponds to that of a poly-[n]-catenane. This indicates that the product obtained upon the instant synthesis is the amorphous poly-[n]-catenane of interlocked M12L8 cages. It is important to recall that a1 does not dissolve in toluene/methanol followed by a recrystallization yielding the polycatenane but traps the solvent in the M12L8 cages rearranging in an ordered manner.
Using TG analysis, we quantified the included guest molecules which amount to 1.3 guest molecules in the asymmetric unit (considering only toluene) (13.8 % weight loss) and 7.8 guests per M12L8 nanocage (Figure 1d). 1H NMR of the sample analysed by TGA did not show any presence of methanol, thus corroborating that the weight loss corresponds to toluene (Figure S6). The TG is also informing about the stability of the structure and from 250 to 400 °C there is no weight loss indicating that the M12L8 poly-[n]-catenane is fully evacuated and after ca. 400 °C the material decomposes. Therefore, using nitrobenzene as templating solvent does not lead to a crystalline poly-[n]-catenane nor to another crystalline phase (vide ante) using the instant synthesis method as observed by the repetition of three experiments. This behavior is different to that observed using ZnCl2 and ZnBr2 that under the same experimental conditions formed the crystalline M12L8 poly-[n]-catenane.
The instant synthesis has been also carried out using chlorobenzene and toluene as templating solvent following the same protocol as in the nitrobenzene synthesis (see ESI). The obtained product is in both cases is crystalline, but the quantity of product is small (low yields). Interestingly, the sample obtained with chlorobenzene remained crystalline after one month of being synthesized (Figure S9), with 19 % of weight loss of guest molecules included as shown by TG (Figure S10). However, the sample obtained using toluene as a template, after one month was amorphous and included very few guests (6 % of weight loss with a not well-defined weight loss release step from TG (Figure S8)). Clearly, these two examples of TPB-ZnI2 poly-[n]-catenanes with different stabilities depending on the solvent.
Single crystal X-ray structure at room temperature of M12L8 poly-[n]-catenane using TPB and ZnI2 using nitrobenzene.
The proof that the poly-[n]-catenane can be formed using TPB and ZnI2 using slow crystallization experiments has been carried out preparing single crystals by self-assembling TPB and ZnI2. Pristine single crystals of TPB-ZnI2 poly-[n]-catenane were obtained by layering diffusion of Zn(II) to a solution of TPB in nitrobenzene. In a crystallization tube, as a bottom layer, TPB was dissolved in a nitrobenzene/MeOH mixture, then as a middle layer, methanol was added until a clear biphasic solution was formed (Figure S4). As the top layer a methanolic solution of ZnI2 was added dropwise in such a way that no precipitate was observed during the layering. The crystallization tube was left standing and after 5 days large crystals attached in the middle layer of the tube were obtained. A single crystal of good quality was selected and mounted for SC-XRD analysis. Importantly, the crystals covered in the mineral oil used to mount in the loop did not crack even after being in contact to the atmosphere for more than one hour. Thus, we attempted the structure solution at room temperature.
The single crystal X-ray structure (300 K) was solved in the trigonal space group R-3 with the following unit cell parameters: a = 38.6805(7) Å; b = 38.6805(7) Å; c = 16.0202(3) Å; α = β = 90° and γ = 120°; V = 20758 Å3. Crystallographic analysis reveal the formula [ZnI2)12(TPB)8]·6(C6H5NO2) (1)25 with one ZnI2, and one TPB ligand and one third of a second TPB ligand in the asymmetric unit (Figure 1a). The structure of 1 corresponds to a poly-[n]-catenane with interlocked M12L8 icosahedral cages. The large cages include one nitrobenzene in the asymmetric unit that can be resolved by X-ray crystallography (Figure 2a). Reports of guest inclusion in small voids are widely reported due to efficient host-guest interactions, but the binding and precise three-dimensional (3D) resolution of guests in large cavities is very challenging due to the lack of good host-guest and/or guest-guest non-covalent interactions.28 The simulated powder XRD pattern match well with the reported M12L8 poly-[n]-catenane structures, indicating that the sample is isostructural (Figure S1).
Regarding the M12L8 cage-framework, it is formed of twelve Zn(II) metal centers at the vertices of the icosahedron with a tetrahedral geometry with three Zn-N bonds (2.046 Å, 2.058 Å, and 2.052 Å) and four Zn-I bonds ranging from (2.523 to 2.563 Å). The icosahedron is defined as “opened icosahedrons”20 due to the large windows formed by the absence of 12 of the 20 TPB ligands forming the faces (triangles) of the icosahedron. This large windows allows a good interpenetration of M12L8 cages and thus the mechanical bond formation, resulting in 1D chains of interlocked icosahedral cages. The 1D chains extend along the crystallographic c-axis and pack in the crystalline state in such a way that the interactions among the rods are through weak C-H···I-Zn electrostatic interactions.
The mechanical bond among the cages takes place because the efficient π-stacking among the benzenic core of the TPB ligand (d = 3.868 Å) of adjacent cages. The good benzene-benzene interactions among TPB ligands is one of the key factors contributing to the formation under kinetic control18 of the poly-[n]-catenanes of interlocked M12L8 nanocages through the mechanical bond. This aspect is favourable from the enthalpic point of view. It is known that switching from benzene to triazine core in the exotridentate ligand (i.e., 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT)), the outcome of the self-assembly is a completely different structure that does not form the M12L8 interlocked cages but a 3D coordination polymer with doubly interpenetrated (10,3-b) networks.29
The packing of the 1D chains of interlocked M12L8 cages does not lead a porous structure but an array of isolated cages with large internal volumes of ca. 2600 Å3 (Figure 3). The volume occupied by the solvent molecules corresponds to 36 % of the total cell volume (7553 Å3), being similar to the other reported isostructural structures.18,20,21 Interestingly, the inter-chain interactions for the zinc iodide catenane has an important role in the structure stabilization. In fact, the terminal ligand I in the Zn metal interacts with the aromatic rings of TPB via C-H···I interactions with 3.169 Å and 3.145 Å distances from adjacent chains.
Even though the crystal structure has been solved at room temperature, the M12L8 cages can be resolved with reasonable small atomic thermal parameters (see ESI). One of the four pyridine rings in the asymmetric unit is disordered over two positions which is important to explain the guest exchange reactions despite not having continuous channels (Figure 3).20 The pyridine ring motion along with the dynamic behaviour of the 1D chains of interlocked M12L8 cages, has been proposed to facilitate the guest exchange and the dynamic behaviour of the M12L8 poly-[n]-catenane systems.
The nitrobenzene guest interacts with the TPB host framework via aromatic-aromatic interactions. The host-guest distance among the centroids of the ring in the nitrobenzene and the benzene ring of TPB ligand is 4.135 Å (Figure 4a). The nitrobenzene is oriented with the part of the guest molecule that is more electropositive towards the more electronegative part of the coordinated TPB ligand according to the maps of electrostatic potential (MEPs) for nitrobenzene guest and TPB host as shown in Figure 2.21
There are additional electrostatic interactions involving the nitrobenzene guests which are among the aromatic C-H and one oxygen atom from the -NO2 group. The C-H···O distance is 2.556 Å which from an enthalpic point of view are contributing to the stabilization of the M12L8 cage (Figure 4b). The oxygen that is engaged in the electrostatic interaction is the shorter in the -NO2 group, indicating its involvement in the stabilization of the guests within the M12L8 nanocage.
Screening of single crystals self-assembling TPB and ZnI2 in various aromatic solvents.
In our interest to gauge more structural information in the self-assembly of TPB and ZnI2 and to compare the results from instant synthesis, several slow crystallization experiments were carried out with TPB and ZnI2 with various aromatic solvents acting as a templating agents (Figure S4). The solvents screened were: 1,2-dichlorobenzene, toluene and chlorobenzene. Interestingly, in all the crystallization tubes the M12L8 poly-[n]-catenanes are formed (as indicated by the unit cells obtained which were all trigonal with lattice parameters like those of 1), but in all cases different crystal structures were identified as evidenced by the different unit cells.
Slow crystallization by layering diffusion method using chlorobenzene as templating solvent, gave after 5 days, a mixture of two macroscopically different crystals: i) the M12L8 poly-[n]-catenane which was confirmed by the unit cell parameters as block-like crystals, and ii) a new colourless crystalline phase (2) easily distinguishable due to their thin-plate habit. Crystals of 2 (covered with mineral oil) are stable at room temperature for several hours, therefore, due to their thermal stability, the SC-XRD was recorded at room temperature (300 K). The thin plates (Figure 5a) have different lattice parameters and space group symmetry to those of 1: a = 24.1055(4) Å, b = 14.4843(2) Å, c = 18.2201(2) Å, β = 100.8630(10), V = 6247.58 Å3, crystallizing in the monoclinic system in the C2/c space group (Table S2).
In the asymmetric unit there is one TPB ligand and one and a half ZnI2, as one of the Zn atoms sits at a special position on a 2-fold axis. There is also one chlorobenzene guest molecule (Figure 5b). The chemical formula, according to the SC-XRD data, is [(TPB)1(ZnI2)1.5]n·(C6H5Cl) (2). The minimum metal-ligand circuit is formed by four TPB ligands and four Zn(II) metal centers expanding along the c-axis, which leads to infinite 1D channels including chlorobenzene. The smallest TPB-ZnI2 circuits form pockets of rectangular shape with windows of ca. 10 Å × 15 Å dimensions (Figure 5c).
The minimum circuits expand through the other two Zn-N coordination bonds to give a tubular structure expanding along the c-axis (Figure 6a and 6b). The tubes are large enough to form 1D channels of dimensions ca. 16 Å x 6 Å. Adjacent 1D chains expand along the a- and b-axis via aromatic (pyridine) C-H···I interactions (d = 3.045 Å). The included guest does not form a particular interaction with the host structure but guest-guest interactions through π-π electrostatic contacts with distances among the centroids of each chlorobenzene of 3.743 Å (Figure 6c and 6d). The close guest-guest interactions are contributing to the stabilization of the whole structure. Therefore, 2 can be regarded as a coordination polymer with a 1D channel structure with voids space (void volumes are calculated using a spherical probe of 1.2 Å diameter)30 corresponds to the 30 % of the total unit cell volume (Figure S3).
Importantly, if instead of chlorotoluene, toluene is used as aromatic templating solvent, also a mixture of crystals is obtained where the M12L8 poly-[n]-catenane (large and stable blocks) co-exist with a coordination polymer (thin plates) with unit cell parameters similar to the those of the crystal including chlorobenzene: a = 29.721(4) Å, b = 13.3437(15) Å, c = 17.2043(15) Å, β = 99.804(17), V = 6723 Å3. Details of this structure along with other coordination polymers will be reported elsewhere.
Room temperature kinetic control in the synthesis of M12L8 poly-[n]-catenane using TPB and ZnI2 using nitrobenzene.
Our attempts to obtain the coordination polymer 2 or any of the other coordination polymers observed, using instant synthesis were unsuccessful. The M12L8 poly-[n]-catenane as crystalline or amorphous phases were always obtained instead of coordination polymers. This clearly demonstrates that the instant synthesis crystallization is an effective method to prepare the 1D M12L8 poly-[n]-catenane in a selective manner as it does not allow the error-checking process.31 The error-checking mechanism is possible due to the labile nature of the coordination bond. During the slow crystallization (stratification method), the formation of alternative structures such as the above-mentioned coordination polymers or other potential structures can take place because the coordination bond can be broken and re-formed until the most thermodynamically stable structure is self-assembled (i.e., error checking). It is important to see that the crystal structures 1 and 2 are completely different because the self-assembling process follows different reaction coordinate (Figure 7). Kinetic products tend to form structures that have large voids32 as in 1, while thermodynamic products form denser structures with smaller channels/pores such as in 2. This also can be seen by the different densities 1.690 g/cm3 and 1.904 g/cm3 in 1 and 2 respectively and the difference in void space (36 % vs. 30 %). Another important aspect is that kinetic products tend to be dynamic and hence can perform guest exchange/inclusion reactions, but also can undergo further transformations towards more stable structures upon external stimuli (i.e., upon heating) following crystal-to-amorphous-to-crystal transformations.33 Thus, the crystallization process has a direct effect on the final structure formed and is therefore very important to gain control. The ability to direct at will the product formed in a chemical reaction has a crucial role both in the chemical synthesis and material sciences.
Solid-state synthesis (neat grinding) of the amorphous M12L8 poly-[n]-catenane using TPB and ZnI2.
The solid-state synthesis of poly-[n]-catenanes is not common and even less in systems like the M12L8 interlocked cages. While it has been demonstrated that it is possible to obtain the TPB- ZnBr2 poly-[n]-catenane as an amorphous material, both the TPB-ZnI2 and TPB-ZnCl2 structures have not been yet reported in the solid-state following a solvent-free approach. Here we report, for the first time, the synthesis of the TPB-ZnI2 poly-[n]-catenane by means of neat grinding.
TPB and ZnI2 were mixed in a 1:1.5 molar ratio (TPB 30 mg : ZnI2 47 mg) and grinded using a mortar and pestle for 15 minutes. During the grinding process the reagents and products mixture were at all the time solid. The product a2 (74 mg) was washed with a mixture of methanol (4 ml) and chloroform (4 ml) and left to equilibrate for 1 day. The weight after the washing process was 60 mg.
The yellowish solid a2 (Figure S11) was analysed by powder XRD which showed two broad bumps denoting that there is no long-range order but only short-range ordering as in a1 (Figure S12). To corroborate that the M12L8 poly-[n]-catenane was formed, the amorphous phase a2 was immersed in a mixture MeOH/toluene and stirred for 4 hours. Then the solid was filtered and immediately checked again by powder XRD. The similarity among the experimental powder XRD of the new crystalline phase (Figure S13a) and the simulated from single crystal XRD of 1, clearly indicates that the M12L8 poly-[n]-catenane is obtained (Figure S13b). The same synthetic approach has been done for the solid-state synthesis of the TPB-ZnCl2 amorphous M12L8 poly-[n]-catenane (see ESI).
Inclusion of xylene isomers by amorphous (a2) TPB-ZnI2 M12L8 poly-[n]-catenane.
Because toluene can be included in the M12L8 cages of the amorphous phase, our interest geared towards the inclusion of toluene derivatives such as the three isomers of xylenes by the uptake of a2 using heterogeneous solid-liquid reactions. The inclusion and separation of xylenes is an interesting topic in material sciences as from industrial point of view it is relevant. Some reports have been focusing on the use of MOFs and discrete complexes (0D) for such separation.34,35,36 To the best of our knowledge the inclusion of xylenes in poly-[n]-catenanes, and in particular in M12L8 poly-[n]-catenanes, has not been yet reported. Thus, an amorphous polycatenane synthesized by neat grinding was used to adsorb o-xylene, m-xylene and p-xylene molecules using heterogeneous solid-liquid reactions at room temperature.
In a typical experiment 30 mg of a2 were immersed in 4 ml of o-xylene and 1 ml of methanol. The suspension was stirred overnight, filtered, and analysed by powder XRD analysis. As observed in the diffractograms, a2 transformed from an amorphous to a crystalline phase whose diffractogram corresponds to that of the M12L8 poly-[n]-catenanes (Figure 8a). The same behavior is observed when m-xylene and p-xylene are used instead of o-xylene in the presence of the amorphous M12L8 poly-[n]-catenane (Figure 8b and 8c). The amorphous-to-crystalline transformation is a valid tool to differentiate if guest inclusion takes place (Figure S17). Thus, the a2 phase can uptake the three isomers, para-, ortho- and meta- xylenes that are included in the large M12L8 nanocages.
The instant synthesis was also attempted using o-xylene, m-xylene and p-xylene. However, because of the poor solubility of TPB in xylenes, chloroform and methanol had to be used to obtain an homogenous solution of TPB for the instant synthesis. The instant synthesis produced low quantity of crystalline materials (≈ 10 mg). Thus, we retain that the use of xylenes as templating solvents in the instant synthesis is not ideal. The inclusion of xylenes in the M12L8 nanocages is much more efficient using the amorphous phases a1 or a2 and from an industrial point of view, the amorphous poly-[n]-catenane obtained in the solid-state is preferable.
Solid-State QM Density Functional Theory (DFT) calculations of interaction energies among M12L8 nanocages, lattice energies, host-guest energies, and guest-guest energies.
The availability of the room temperature structure of 1 including nitrobenzene, allows DFT calculations to be carried out giving insights about structural stability and intrinsic local dynamic behaviour of the TPB-ZnI2 M12L8 poly-[n]-catenane system. Therefore, DFT calculations specific for solid crystalline states have been carried out considering the energy interactions among M12L8 cages, among the 1D chains, and host-guest energy interactions. DFT calculations have been carried out PBE/DNP level, (where PBE is the functional of Perdew, Burke and Ernzerhof, DNP states for a standard numerical basis set inserted into Dmol3 package, roughly comparable to the 6-31G** gaussian set).37 The strategy adopted here showed to be good in several recent studies of crystalline systems such as molecules, polymers, and hybrid metal-organic materials.38,39,40,41,42,43,44 Explicit van der Waals contribution, according to the approach proposed by Grimme was determined.45,46
It is important to define the model system to be adopted in DFT calculations. First, the “interaction energies” (E) among interlocked and non-interlocked M12L8 nanocages which can give us a view on the stabilities of the 1D ribbons and of individual (i.e., isolated) M12L8 cages are considered. The second aspect is the “lattice energy” (E*) which considers the energy of one single M12L8 cage or a 1D chain of interlocked cages immersed within the crystalline lattice (1). The third point considers the interaction energy among the M12L8 host and the guest and guest-guest interactions, referred hereafter as Ehost-guest, and Eguest-guest, respectively.
Interaction Energies calculation (E). E can be analysed considering the interaction energies of two close interacting dimers: two interlocked M12L8 cages or two first neighbour cages that are not mechanically linked, but that are both stable. The interaction energy into 1 considering the interlocked M12L8 cages is ca. 89 kcal/mol that is almost 178 kcal/mol per 2 cages and this interaction appears to be strongly affected by the aromatic ring interactions (d = 3.868 Å) from the central TPB ring. The interaction energies among the non-interlocked M12L8 cages which interact via van der Waals interactions correspond to 47 kcal/mol, or 23.5 kcal/mol per cage.
Lattice Energies calculation (E*). The average energy required to extract a single M12L8 cage immersed into the crystalline structure is very high: about 505 kcal/mol. In a similar way, if we consider the polycatenated chain along the c-axis (i.e., mechanically interlocked) and calculate the energy required to remove a single infinite chain of M12L8 interlocked nanocages from the crystalline structure, the calculated E* is 398 kcal/mol, indicating that the model of an infinite 1D chain “immersed” in the structure is a good model explaining the high stability of the poly-[n]-catenane architecture.
Because it has been seen that nitrobenzene has an important role templating the poly-[n]-catenane, we carried out the same sort of calculations but including the six crystallographic nitrobenzene guest molecules in the M12L8 cages. In this way it is possible to compare the previously calculated energies against the empty model not including guest molecules. The interaction of the M12L8 cages with the solvent molecules, the Ehost-guest, is also important in this case. The Ehost-guest is about 46 kcal/mol for each solvent molecule, which is comparable to the energies observed among the non-interlocked cages interacting via electrostatic and van der Waals interactions (i.e., the interactions among neighbouring 1D chains). The Eguest-guest interactions (Figure 4b) are, as expected, much lower, 4.3 kcal/mol but not negligible. Thus, the Ehost-guest and Eguest-guest interaction energies of aromatic guests are very important for the stabilization of the whole poly-[n]-catenane structure. While the lattice energy (E*) values for a single cage and a single chain are higher, and the interaction energies (E) for interlocked cages is also higher (Table 1), the interaction energy of non-interlocked cages shows a lower interaction energy, from 47 kcal/mol without guest to 27 kcal/mol including guests.
Interaction energies among 1D chains of interlocked M12L8 nanocages and the role of the included guest nitrobenzene. Among catenanes formed of molecular rings, 1D catenanes are very interesting because of the dynamic behavior due to their high conformational degrees of freedom through rotations, translations and rocking motions of the molecular rings.47 However, when the chains are formed of interlocked MOCs like the M12L8 large nanocages, the translation, rotation and rocking motions are somehow limited by the free window space left after the mechanical bond formation. In the M12L8 poly-[n]-catenanes the free window space is small and therefore the 1D rods are quite stable. Thermodynamically, the interlocking process is favoured from the enthalpic but not form the entropic point of view. We note that the 1D chains of interlaced M12L8 nanocages are not completely rigid as it has been observed that guest molecules can be exchanged,20 and stability studies demonstrated that included guest molecules come out from the cages under vacuum conditions.12
From the DFT calculations, it has been seen that almost all the interactions increase in the presence of guest molecules. This confirms that when the solvent is included in the M12L8 cages the structure gains in stability within the 1D chains. In particular, the energy interactions increase in single cages, the interlocked cages, and the chain of interlocked cages gaining stability, but the interactions among non-interlocked M12L8 cages, that is, among neighbouring chains, become weaker. In our opinion the enhancement in structural stability brought by the 6 included and ordered guest interactions per M12L8 cage, influences the electrostatic interactions (i.e., lowering) among chains for the structure 1 as observed by DFT. This has a direct effect into the guest exchange/inclusion properties and the dynamic behavior of the 1D chains of interlocked cages which might help to explain the guest inclusion/exchange observed in this systems formed of large, interlocked cages.
It has been reported that the relative displacement among 1D chains of interlocked M12L8 cages considering two main directions, one perpendicular to the chain propagation (c-axis) and the second parallel to the chain extension (i.e., sliding chains) have a low energy cost. While 1Å displacement in the orthogonal to the 1D rod direction has an energy penalty of 10 kcal/mol, in the sliding direction, the energy penalty is 0.2 kcal/mol per 2 Å translation. This calculations were carried out considering a TPB-ZnBr2 model system of two M12L8 cages belonging to different chains (i.e., non-interpenetrated).24 The DFT results reported herein go in that direction as the van der Waals interactions among neighbouring chains are much weaker when the solvent is included in the DFT calculations.
Table 1. Computed DFT energy interactions in 1 with (TPB-ZnI2) and without (TPB-ZnI2(NB)) included nitrobenzene solvent.
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TPB-ZnI2
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TPB-ZnI2(NB)
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Interaction energies (E)
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Interlocked: 89 kcal/mol
Non-Interlocked: 47 kcal/mol
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Interlocked: 111 kcal/mol
Non-Interlocked: 27 kcal/mol
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Lattice energy (E*)
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Single cage: 505 kcal/mol
Chain: 398 kcal/mol
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Single cage: 556 kcal/mol
Chain: 417 kcal/mol
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Host-Guest Energies Ehost-guest
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TPB-NB: n.a.
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TPB-guest: 46 kcal/mol
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