Capsules and Star‐Burst Polyhedra: An [Ag₂L₂] Capsule and a Tetrahedral [Ag₄L₄] Metallosupramolecular Prism with Cyclotriveratrylene‐Type Ligands

Abstract

Cyclotriveratrylene (CTV) is a macrocyclic host molecule that strongly favors a crown conformation, which gives it a bowl shape with a shallow molecular cavity.[1] Indeed, despite CTV being known for over 40 years, the alternative saddle conformation has only recently been isolated by the rapid quenching of a hot solution or melt, [2] or within a novel CTVbased cryptophane.[3] The bowl-shaped CTV is a cyclic trimer which has a distinctly spiked, pyramidal aspect. We wish to exploit this characteristic to create self-assembled capsules and polyhedra through the formation of metallosupramolecular assemblies. These would likewise have a spiked aspect that allows for significantly more internal space than could be afforded from a similar flat or flexible ligand. A range of spectacular polyhedral structures have been reported from the self-assembly of metal salts with multifunctional ligands. [4] CTV itself is not a good molecular component for such assemblies as it lacks predictable coordination sites for transition metals, hence we have synthesized a number of CTV analogues with pyridyl groups at the upper rim for use as ligands for these types of assemblies as well as coordination networks.[5] Notably, a number of crystalline hydrogenbonded assemblies feature tetrahedral clusters of CTV hosts arranged in a back-to-back fashion, [6] which excludes the possibility of forming an assembly with internal space. Likewise a 3D coordination polymer with a C 3 -substituted CTV derivative also shows this back-to-back stacking.[7] Self-assembled complexes of CTV-related host molecules that do show an internal space would result from the head-to-head association of host molecules. Examples are currently restricted to a dimeric [Pd 3 L 2 ] capsule that has been characterized in solution by Shinkai and co-workers.[8] This capsule results from the assembly of a trisubstituted CTV derivative possessing rigid pyridyl arms with cis-protected Pd salts. Covalently linked cryptophanes, where two CTV fragments are linked in a head-to-head fashion by organic spacers, are well established.[9]Other bowl-shaped host molecules have been shown to form both capsule assemblies and more complicated polyhedral structures. Capsule structures assembled from hydrogen bonds or coordination chemistry are known with calixarenes and calixresorcinarenes.[10] Considerably more complicated assemblies such as a cyclic trimer, [11] cyclic hexamers, [12] a tetrahedron, [11b] a snub cube, [13] icosahedron, [14] and a cuboctahedron [15] have also been reported from similar tetrameric molecular hosts.We report herein a new type of [M 2 L 2 ] dimeric capsule with a CTV-related host molecule in the complex [Ag 2 (tris(3-pyridylmethylamino)cyclotriguaiacylene) 2 (CH 3 CN) 2 ]·2 PF 6 · 4 CH 3 CN, along with the first example of a discrete polyhedral cluster larger than a dimeric capsule assembled with a CTVrelated host molecule. This latter complex [Ag 4 (tris(4-pyridylmethylamino)cyclotriguaiacylene) 4 (CH 3 CN) 4 ]·4 BF 4 · 7 CH 3 CN·2.8 H 2 O features a tetrahe...

Metallo-supramolecular capsules

“…When considering some of the largest supramolecular container assemblies formed to date [16,19,23,[47][48][49][50][68][69][70][84][85][86], hydrogen-bonded pyrogallol- [4]arene capsules (either dimeric or hexameric) are useful for the encapsulation of reasonably large non-solvent molecules, as identified by X-ray diffraction studies [96][97][98][99][100][101][102][103]116,[118][119][120]. From the work reviewed in this article, we have also shown that they are also useful templates for metal-organic analogues [116,[118][119][120][121]124,127].…”

“…10B) or tetrameric (Fig. 10C) capsular architectures with the 3-pyridyl or 4-pyridyl derivatives, respectively [84]. The tetrameric capsule has a host at each vertex of a tetrahedron, and the pyridyl moieties are bound to silver centres that in some cases also have bound acetonitrile molecules on the exterior of the capsule, resulting in a star-burst tetrahedron.…”

“…Recent work in the Hardie group has focused on using selectively modified cyclotriveratrylene (CTV) to assemble capsular architectures [84,85]. The basic CTV framework is bowl-shaped, has a shallow cavity suitable for hosting guest species, and the molecule has a roughly pyramidal shape that is suitable for the formation of polyhedral shapes.…”

Tuning the coordination chemistry of cyclotriveratrylene ligand pairs through alkyl chain aggregation

“…Tripodal derivatives such as those shown in Scheme 1 are chiral. The self-assembly of derivatised CTVs with metal cations is well established, and coordination polymers, [35][36][37] discrete metallocages, [38][39][40][41][42] and mechanically interlocked architectures [43][44][45] are known. However, the chemistry of mixed ligand systems is generally limited to the solution-phase, and we have recently demonstrated how the formation and manipulation of homo-and heteroleptic [Pd 6 L 8 ] 12+ octahedral cages can be controlled with high fidelity.…”

Lanthanide coordination polymers with pyridyl-N-oxide or carboxylate functionalised host ligands

“…The self-assembly of derivatised CTVs with metal cations is well established, and coordination polymers, [35][36][37] discrete metallo-cages, [38][39][40][41][42] and mechanically interlocked architectures [43][44][45] are known. However, the chemistry of mixed ligand systems is generally limited to the solution-phase, and we have recently demonstrated how the formation and manipulation of homo-and heteroleptic [Pd 6 L 8 ] 12+ octahedral cages can be controlled with high fidelity.…”