Precise Molecular Fission and Fusion: Quantitative Self-Assembly and Chemistry of a Metallo-Cuboctahedron

The quest for new materials in many cases, hinges on developments of the past. And, while discoveries are numerous, some open avenues for all to exploit. Such are the revelations in materials construction provided by polymers and dendrimers. The broadly defined supramolecular- and nanotechnology-based arenas have afforded vastly expanded abilities to instill desired physical properties. Utilitarian macromolecular engineering provides an unparalleled potential to bridge the macro- and nano-molecular interface.

Thus, our team’s interests (led by Profs. George R. Newkome and Chrys Wesdemiotis at the University of Akron) in crafting high molecular weight, structurally precise architectures has led to the quantitative, single-step, self-assembly of the largest and highest carbon content (53 %) molecular sphere (a cuboctahedron) that has been unequivocally characterized by synchrotron X-ray analysis, to date. The physicochemical properties of this unique 6 nm diameter molecular cage combine to facilitate a phenomenon whereby the cuboctahedron sphere undergoes a dilution-based transformation into two identical, smaller spheres (octahedrons) each possessing a molecular weight of exactly ½ the molecular weight of the parent cage molecule; the remarkable reversibility of this process was followed and confirmed by standard spectroscopic methods (see Figure). This phenomena can be considered to be the synthetic chemistry equivalent of prokaryotic-based fission. Upon concentration, the octahedrons undergo an apparent molecular fusion to quantitatively regenerate the original cuboctahedron. Other than in biological systems, this is the first such reversible, 3D, supramolecular transformation in the non-biological regime.

The novel cuboctahedron was conceived from the well-known architectural gallery of Archimedean polyhedra that comprise a group of symmetrical solids characterized by regular polygon faces and equal angles at the vertices.  Examples in Nature include the rigid outer virus shells or capsids and some subcellular transport vesicles.  Mimicking these shapes by chemists is a logical endeavor for nanotechnology exploration.

In our case, the molecular cuboctahedron possessing a surface of 8 triangles and 6 squares was derived by assembly of 12 corner units that each contribute two 90° and two 60° angles along with a 125° dihedral bend between coordinating groups.  These unique vertex units were perfectly provided by a centrally bridged anthracene substituted on the four ends with terpyridine ligands capable of connecting adjacent terpyridine ligands through metal coordination.  Mixing a precise 1:2 ratio of the 12 tetraterpyridine corners with 24 coordinating metal ions [e.g., Zn(NO₃)₂] quantitatively generated the desired sphere that was easily isolated and characterized.  Dilution of the cuboctahedron sphere to generate the two new octahedra was demonstrated by spectroscopic methods including nuclear magnetic resonance (NMR) and traveling wave ion mobility mass spectrometry (TWIM-MS).

Ramifications of this phenomenon provides support for the ability to synthetically craft vary large nanomolecular structures with precise shapes, sizes and molecular weights with a new level of functional unit positional certainty.  New routes to the engineering of previously challenging materials can be accessed via the interplay of assembly-disassembly-reassembly processes and are envisioned.  As we approach and pass the limits of established characterization methods due to size and molecular complexity constraints, such as with current X-ray crystallography parameters, new analytical methods will be undoubtedly be developed in the wake of this new paradigm.