Suzanne M. Jansze

The design of structurally defined heteroleptic coordination cages is a challenging task, and only few examples are known to date. Here we describe a selection approach that allowed the identification of a novel hexanuclear Pd cage containing two types of dipyridyl ligands. A virtual combinatorial library of [Pd_nL_2n](BF₄)_2n complexes was prepared by mixing six different dipyridyl ligands with substoichiometric amounts of [Pd(CH₃CN)₄](BF₄)₂. Analysis of the equilibrated reaction mixture revealed the preferential formation of a heteroleptic [Pd₆L₆L′₆](BF₄)₁₂ assembly. The complex was prepared on a preparative scale by a targeted synthesis, and its structure was elucidated by single-crystal X-ray diffraction. It features an unprecedented trigonal-antiprismatic cage structure with two triangular Pd₃L₃ macrocycles bridged by six L′ ligands. A related but significantly larger [Pd₆L₆L′₆](BF₄)₁₂ cage was obtained by using metalloligands instead of organic dipyridyl ligands.

Nanometer-sized polycarboxylate ligands are interesting building blocks for metallasupramolecular chemistry, but access to these compounds is often limited by complicated synthetic pathways. Here, we describe a simple two-step protocol, which allows preparing linear and bent dicarboxylate ligands with lengths of up to 3 nm from commercially available compounds. The ligands are prepared by iron-templated polycondensation reactions involving arylboronic acids and nioxime. The final products contain two iron clathrochelate complexes and two terminal carboxyphenylene groups. To demonstrate that the new ligands are suitable for the construction of more complex molecular nanostructures, we have prepared a Cu-based metal–organic polyhedron, which represents the largest M₄L₄ cage described so far.

The stability of five different [Pd n(N-donor) m]2 n+ assemblies was examined by performing disassembly experiments with pyridine and with trifluoroacetic acid. Pyridine-induced disassembly was found to be most pronounced for Pd complexes containing N-donor ligands of low basicity. At the same time, these assemblies displayed high acid resistance. The contrasting stability in the presence of acid or pyridine can be used for the pH-controlled switching between different metallosupramolecular structures.

The term “clathrochelate” describes a complex in which a coordinatively saturated metal ion is surrounded by a macropolycyclic ligand. First examples of clathrochelate complexes were reported 50 years ago. Meanwhile, the synthesis and reactivity of clathrochelates have been investigated in detail, and numerous applications have been explored. In this Account, we summarize work on the utilization of transition metal clathrochelates as metalloligands in supramolecular chemistry and materials science, with special focus on results from our group. First, we discuss the chemistry of boron-capped clathrochelates. These complexes are facile to synthesize by metal-templated condensation reactions. The synthesis is modular, and it is straightforward to implement structural variations. Importantly, it is possible to attach functional groups such as amines, pyridines, or carboxylic acids to the ligand periphery. Other noteworthy features of boron-capped clathrochelates are high thermodynamic and kinetic stability, tunable redox potential, and good solubility. Next, we show that clathrochelate-based metalloligands can be used to build molecularly defined metal–ligand assemblies of nanoscale dimensions. Different molecular architectures are described, including coordination cages with unusual gyrobifastigium or square orthobicupola-like structures. Metalloligands containing multiple clathrochelate complexes are particularly well suited to build large metal–ligand assemblies (>3 nm) with minimal synthetic efforts. Boron-capped clathrochelates have also been investigated in the context of materials chemistry. Linear or cross-linked clathrochelate polymers were found to display permanent porosity. Furthermore, such polymers were used to prepare conducting films on electrodes. Clathrochelate metalloligands are well suited to prepare metal–organic frameworks (MOFs). The high stability of clathrochelates ensures compatibility with harsh reaction conditions, and it mitigates potential problems such as exchange reactions. Boron-capped clathrochelates can be decorated with functional groups in lateral and apical position, and it is possible to use these complexes as multiconnected nodes in polymeric structures. Overall, we hope to convey the utility of clathrochelate complexes in supramolecular chemistry and materials science. The work published thus far gives a first glimpse of the potential of these compounds, but there are other directions, which are waiting to be explored. For example, it will be interesting to study the properties of nanostructures based on chiral clathrochelate complexes. Furthermore, the redox and magnetic properties of clathrochelates may give rise to novel functional materials. Given that clathrochelates are straightforward to prepare, we hope that others will join the efforts to explore the supramolecular and materials chemistry of these interesting molecular building blocks.

Using easy-to-access metalloligands, we were able to prepare Pd ₆ L ₈ -type coordination cages of unprecedented size.

The addition of a metastable-state photoacid to solutions containing metal–ligand assemblies renders the systems light responsive. Upon irradiation, proton transfer from the photoacid to the ligand is observed, resulting in disassembly of the metallasupramolecular structure. In the dark, the process is fully reversed. Light-induced switching was demonstrated for six different metal–ligand assemblies containing Pd^II, Pt^II or Ru^II complexes and bridging polypyridyl ligands. The methodology allows liberating guest molecules with light.

The synthesis and characterisation of a range of non-symmetric liquid crystal dimers designed to exhibit the twist-bend nematic phase is reported. Beginning with 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl) hexane, each nitrile group is exchanged systematically for a methoxy group. The behaviour of these dimers is interpreted in terms of their bent shape being the predominant driving force for the formation of the twist-bend nematic phase, and the small differences between the twist-bend nematic–nematic transition temperatures are attributed to the differences between the interaction strength parameters of the mesogenic units. The 4-alkyloxyphenyl 4-[6-(4′-cyanobiphenyl-4-yl)hexyloxy]benzoates with ethyl, butyl, hexyl and octyl chains show the twist-bend nematic phase, whereas the corresponding 4-alkyloxyphenyl 4-[5-(4′-cyanobiphenyl-4-yloxy)pentyloxy]benzoates do not. This difference in behaviour is attributed to the more bent structure of the former. Increasing the terminal chain length initially decreases the twist-bend nematic–nematic transition temperature and this suggests that the chain disrupts the interactions between the mesogenic units. Subsequent increases in chain length have a smaller effect suggesting that the chain can be accommodated within an intercalated arrangement. The transitional behaviour of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-butyloxybiphenyl-4′-yl) hexane is compared to that of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-((S)-2-methyl)butyloxybiphenyl-4′-yl) hexane, and it is shown that chain branching strongly destabilises the twist-bend nematic phase. This is attributed to difficulties associated with packing the dimers. Graphical Abstract

The utilization of easily accessible metalloligands allows the construction of Pd^II₂L₄-type coordination cages of unprecedented size.

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe^II₈L₁₂ cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe^II₄L₆ cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd^II-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd^II₄L₈ assembly, whereas an octahedral Pd^II₆L₁₂ cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd^II cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd^II₄L₈ and Pd^II₆L₁₂ cages, whereas the other did not.

The phase behaviour of equimolar mixtures of 6-(4ʹ-cyanobiphenyl-4-yl)hexyloxybenzoic acid (CB6OBA) with either 1-(4-butylazobenzene-4ʹ-oxy)-5-(4-oxypyridine)pentane (BuABO5OPyr) or 1-(4-butylazobenzene-4ʹ-oxy)-6-(4-oxypyridine)hexane (BuABO6OPyr) is reported. CB6OBA shows a monotropic twist-bend nematic phase and an enantiotropic nematic phase whereas the two pyridyl-based components do not exhibit liquid crystallinity. Both equimolar mixtures (CB6OBA/BuABOnOPyr) show enantiotropic nematic phases. The nematic-isotropic transition temperature and associated entropy change are higher for the CB6OBA/BuABO6OPyr mixture than for the CB6OBA/BuABO5OPyr mixture. This may be accounted for in terms of the average shapes of the hydrogen-bonded 1:1 complexes formed between the two differing components in the mixtures. However, Fourier transform infrared spectroscopy reveals that this complex is not formed quantitatively, but instead a complex mixture exists over the complete temperature range studied, involving the 1:1 complex, both cyclic and open acid dimers, free acid and hence, free BuABOnOPyr molecules.