Symmetry breaking in self-assembled M ₄ L ₆ cage complexes

Abstract

Here we describe the phenomenon of symmetry breaking within a series of M 4 L 6 container molecules. These containers were synthesized using planar rigid bis-bidentate ligands based on 2,6-substituted naphthalene, anthracene, or anthraquinone spacers and Fe II ions. The planarity of the ligand spacer favors a stereochemical configuration in which each cage contains two metal centers of opposite handedness to the other two, which would ordinarily result in an S 4 -symmetric, achiral configuration. Reduction of symmetry from S 4 to C 1 is achieved by the spatial offset between each ligand's pair of binding sites, which breaks the S 4 symmetry axis. Using larger Cd II or Co II ions instead of Fe II resulted, in some cases, in the observation of dynamic motion of the symmetry-breaking ligands in solution. NMR spectra of these dynamic complexes thus reflected apparent S 4 symmetry owing to rapid interconversion between energetically degenerate, enantiomeric C 1 -symmetric conformations.coordination chemistry | metal-organic capsules | self-assembly | supramolecular chemistry | stereochemistry S ymmetry breaking must occur before complexity can develop (1, 2). In cosmology, the perfect symmetry of the singularity at the inception of time (3) evolved under the direction of physical laws to produce the present-day universe filled anisotropically with asymmetrical pieces of matter. In biology, a zygote must break its symmetry before the functional specialization involved with cell differentiation and tissue architecture development can occur (1, 4). Studies on model systems for biological symmetry breaking reveal that the complexity on larger scales is often underpinned by asymmetry on smaller scales (4, 5), which is a consequence of dynamic interactions at the molecular level (4). Investigations of symmetry breaking during complex molecular self-assembly phenomena thus present an opportunity to shed light upon the foundations of the evolution of matter toward complexity.To contribute to the understanding of symmetry breaking, in the present study we demonstrate a rational method of systematic symmetry breaking within a series of M 4 L 6 metal-organic tetrahedra through the control of linker geometry.Metal-organic polyhedra (6-13) have attracted significant attention due to their host-guest behavior that can be applied in molecular storage (14-17), separation (18), and catalysis (19-22). Platonic or Archimedean metal-organic polyhedra can be constructed by a careful choice of ligands and metal ions (8,(23)(24)(25)(26)(27), where the geometries of the individual building blocks define the symmetry axes of the polyhedron. Although high-symmetry architectures represent a great achievement in terms of logical molecular design, lower-symmetry ones are possibly of still greater interest. Besides the fundamental interest in symmetry breaking mentioned above (1), an asymmetric capsule could achieve recognition of asymmetric substrates (28) and possibly catalyze asymmetric transformations (29). So far all reported metal-organic c...

Stereochemistry in Subcomponent Self-Assembly

“…40 The use of rigid diamine 20 was predicted to form an S 4 -symmetric framework because the enforced coplanarity of the two terminal phenyl rings of the spacer favored the syn ligand arrangement that predominates for S 4 -symmetric M 4 L 6 structures. The offset geometry of the naphthyl spacers of 20 caused them to adopt an arrangement within cage 21 whereby within each ligand one ring is orientated inward, roughly toward the center of the cage, and one ring points outward; the S 4 symmetry axis is thus broken with an offset introduced between the two coordination sites within the same linear bis-bidentate ligand, and the cage is therefore asymmetrical, albeit racemic.…”

Encapsulation of Organic Dyes within an Electron‐Deficient Redox Metal‐Organic Tetrahedron for Photocatalytic Proton Reduction

“…Vapour diffusion of diethyl ether into the solution of the sample in CH 3 CN combined with other attempts only afforded microcrystals of Ni‐AFP , which were not suitable for single‐crystal X‐ray diffraction analysis. However, when the Ni(BF 4 ) 2 was instead with Fe(OTf) 2 among the assembly process, the iso‐structure tetrahedra Fe‐AFP was obtained, which was confirmed by ESI‐MS spectrum and has been well‐characterized by J. R. Nitschke group . Fortunately, good quality crystals of Fe‐AFP were prepared and the single‐crystal analysis clearly showed the formation of a Fe 4 L 4 tetrahedron with a C 3 symmetry (Figure ).…”

“…Preparation of Fe‐AFP . The metal‐organic tetrahedron Fe‐AFP was synthesized according to the reported literature method with slight modification . A mixture solution of 2,6‐diaminoanthraquinone(36 mg, 0.15 mmol, 3.0 equiv.…”

Mapping the Assembly of Metal–Organic Cages into Complex Coordination Networks

“…These consisted of discrete cages and one‐ as well as two‐dimensional cage‐connected frameworks. This enabled us to map the process of the conversion reactions and show that they proceed with sequential change of symmetries, which is a rare phenomenon in the chemistry of supramolecular cages …”