As already described in Chapter 4.6, short interactions between the coordination cage and the encapsulated guest were observed in the X-ray structure, with intermolecular distances between 2.0 and 3.0 Å. These close contacts in the supramolecular host-guest complex were visualized via a Hirshfeld surface analysis with the software Crystal Explorer.[134] The Hirshfeld surfaces were mapped with a Van-der-Waals-radius-normalized distance dnorm around the guest molecules for host-guest complexes [G1@Pd2L24] and [G5@Pd2L24], respectively (see Figure 4.15a and b).
Close contacts, contributing to the stabilization of the host-guest complex are highlighted in red.
Major contacts between the sulfonate oxygen atoms and the inward pointing pyridine hydrogen atoms were found. Formation of the metal containing coordination complex leads to the polarization of the pyridine protons, which can serve as hydrogen bond donors (which is also expressed by the changes in their 1H NMR chemical shifts upon guest binding, see Figure 4.5). Furthermore, close contacts between the guests’ hydrogen substituents or π-faces and the surrounding adamantyl hydrogens were observed. This is no surprise, since close contacts could be detected via 1
H-1H NOESY measurements and X-ray structure analysis. The Hirshfeld surface analysis supports the experimental results for this dense packing inside the sterically congested cage. Further ele-ment-filtered surface pictures and fingerprint plots are given in the experimental section to further visualize the range of non-covalent interactions at the interface of the guest and the surrounding host.
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Figure 4.15 Visualization of non-covalent host-guest interaction. Hirshfeld dnorm surfaces for the guest molecules in (a) [G1@Pd2L24] and (b) [G5@Pd2L24] (plotted with isovalues from −0.4 (red: short contact) to 1.4 (blue: long contact); red dotted lines and arrows indicate some close non-hydrogen bond interactions that are shorter than 3.0 Å). Below: Depiction of dispersion interaction densities (DIDs) between the guests and the surrounding adamantyl groups in (c) [G4@Pd2L24] and (d) [G5@Pd2L24] (red: high DID; blue: low DID). Reprinted with permission from reference [107]. Copyright © 2016 Royal Society of Chemistry.
Furthermore, Prof. Dr. R. Mata and coworkers carried out electronic structure calculations. Aim of these calculations was to obtain a deeper insight into the binding of the guest molecules and the role of dispersion forces in the encapsulation process. The parent cage containing only BF4− coun-ter anions as well as two host-guest systems were considered, namely [G4@Pd2L24] and [G5@Pd2L24]. Initially, the influence of dispersion interaction on the stability of the coordination cage was computed. Calculations at the SCS-LMP2/aug’-cc-pVTZ level of theory[95–97,135,136] gave a dis-persion contribution of 14.3 kJ·mol−1 for the [Pd2L24] system. Furthermore, the dispersion contribu-tion in the [G4@Pd2L24] host-guest complex was calculated to be 9.3 kJ·mol−1. Consequently, the contribution in the host-guest complex [G4@Pd2L24] is 5 kJ·mol−1 smaller than in the [Pd2L24] cage.
Due to the guest-induced expansion of the cage, the four ligands are pushed further apart in the host-guest complex and the dispersion contribution declines. Since this effect is rather small and it
will counterbalance the steric repulsion of the adamantyl group, it will not be a major factor in the cage formation itself.
Furthermore, the influence of dispersion forces on the guest binding was studied. For the host-guest systems [G4@Pd2L24] and [G5@Pd2L24] the overall interaction energies between the ada-mantyl residues and the guest were calculated to be 52.8 kJ·mol−1 and 67.2 kJ·mol−1 (cation-anion interactions were not consider in this calculation). The dispersion energy contributions obtained are 86.3 kJ·mol−1 and 64.0 kJ·mol−1 for [G4@Pd2L24] and [G5@Pd2L24], respectively. These correspond to a strong contribution in binding, even exceeding the total interaction in the case of [G4@Pd2L24].
In the G5 incorporated host-guest system [G5@Pd2L24], the adamantyl groups are pushed stronger outside and so the relative weight of the van-der Waals forces is reduced. Therefore, it is expected, that the main contributions for the adamantyls’ interaction with the guests are Pauli repulsion and dispersion, the two of opposite sign. The conducted calculations demonstrate that dispersion forces can easily add up to large values in such a supramolecular assembly.
For a better visualization of this effect, a plot of the Dispersion Interaction Densities (DIDs), calcu-lated between the guest and the four adamantyl units, was prepared (see Figure 4.15c/d and sec-tion 7.5.7.3 for a definisec-tion of the Dispersion Interacsec-tion Density plots). The DID images show the interaction between the π-systems of the guest molecules and the surface of the pocket. This is in accordance with the Hirshfeld analysis (see page 62). In particular, some hot spots are identifiable where the adamantyl groups are in close contact with the encapsulated guests.
4.8 Conclusion
This Chapter described the synthesis of the bis-monodentate pyridyl ligand L2, which possesses a bulky adamantyl group protruding sideways from its concave face. Addition of square planar metal cations (Pd2+, Pt2+) leads to the clean formation of the discrete coordination cages [Pd2L24] or [Pt2L24]. The adamantyl residues occupy the four entries of the cage and part of the internal cavity.
In the free ligand fast flipping dynamics of the adamantyl substituent was observed. Surprisingly, the flipping mechanism occurs in the supramolecular coordination cage as well, but at a much lower velocity. The latter process did not require a ligand detachment and a sequential flipping mecha-nism of all adamantyl groups was postulated. The coordination cage was found to encapsulate different (bis)anionic guest molecules G1-G9 inside the center of the supramolecular structure.
Guest uptake further decreased the rate constant of the flipping dynamics in the host-guest com-plexes. Single crystal X-ray structure analyses of the free cage and three host-guest complexes showed that the size of the encapsulated guest influences the overall structure of the supramolec-ular assembly. The bigger the guest, the further the adamantyl groups are pushed aside and thereby they reduce the Pd-Pd distance (compression of the cage). Furthermore non-covalent con-tacts between the guest molecules and the host were investigated and substantial contribution of attractive dispersion interactions conveyed by the adamantly groups was identified.
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5 I NFLUENCE OF L IGAND L ENGTH ON
C AGE F ORMATION
5.1 Introduction
The outcome of a self-assembly process between organic ligands and metal cations depends on several factors. Most important influence factors are the type of ligand (e.g. geometry, size, flexi-bility, coordination site) and metal cations. In addition, choice of solvents, kind and concentration of counter anions and temperature influence the topology of the self-assembled coordination struc-ture (see Chapter 1.3). Thus, small variations can make a significant impact on the resulting supra-molecular structure.
In the previously described example, an interpenetrated coordination cage is formed as the ther-modynamic product after mixing eight banana-shaped ligands L1 with four square planar palla-dium(II) cations. The coordination cage is activated by halide anions to encapsulate neutral guest molecules inside its central pocket. Ligand L1 contains an acridone backbone, which is connected via an ethynyl bridge to the coordinating pyridine residues. In the following Chapter, the influence of the size of the organic ligand the cage formation will be studied. A series of new ligands contain-ing different bridgcontain-ing groups (e.g. aryl) will be synthesized and the self-assembly towards supra-molecular cage structures investigated. Furthermore, the resulting structures will be tested for their ability to form host-guest complexes with anionic and neutral guests.