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Mimicking biological structures, such as the interesting shapes and functions of fruits and seeds, using molecules and molecular assemblies is a great challenge for many Synthetic Chemists.
There have been many different types of fascinating chemical structures appearing in literature recently, and a team of Scientists from Japan and India have synthesized a peanut-shaped nanostructure which is comprised of two fullerene molecules surrounded by a dumbbell-like polyaromatic shell.
The ability to mimic biological architectures is something that both fascinates and challenges Chemists, especially those in the supramolecular field. Many complex architectures using interlocked molecules, such as rotaxanes, can produce complex mechanisms such as molecular switches and shuttles. Yet, the multi-layer, multicomponent bio-structures of fruits and seeds are so complex that they remain a challenge to mimic.
Peanuts, in the grand scheme of fruit and nut architecture, are actually relatively simple and adopt a core-shell like structure where two beans are encased by a pod. There have been attempts to mimic its structure before, by synthesizing nanostructures containing a few open cavities that allow binding of small ions, such as Cl-, PF6- and cisplatin. However, realizing the characteristic core-shell structure on the nanoscale remained a challenge.
The synthesis of the peanut-shape molecule started by designing W-shaped tripyridine ligands, which contained four anthracene rings and two metaphenylene spacers, through a Suzuki-Miyaura cross-coupling reaction. These ligands were then complexed with square planar metal ions, to produce a molecular double capsule, which acted as the ‘shell of the peanut’.
This was then followed by demetallation of the central pyridine rings using a mixture of fullerene molecules- C60, C70 and Sc3[email protected]80. The fullerenes acted as the ‘beans’ inside of the shell and the internal rearrangement allowed the structure to look like that of an everyday peanut.
The first two stages occurred simultaneously through controlled and well designed, coordinative and pi-stacking interactions and metal-ligand coordination bonds. The aromatic–aromatic pi-stacking interactions acted as an orthogonal chemical ‘glue’ that connected the multiple molecular components together into a dumbbell, or peanut, shape.
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The W-shaped ligand created actually exists as a mixture of ten different stereoisomers. This is due to the restricted rotation around the sterically hindered, pyridyl–anthryl and anthryl–phenyl bonds. However, despite possessing so many potential structural conformations, the peanut-shell shape is thermodynamically favored over the other isomers.
During the formation of the molecular beans, the releasing of the sterically hindered central metallic hinge, i.e. the central metal ion, allowed the two closed cavities to doubly encapsulate the fullerene molecules- both of which were different, but were both medium-sized molecules in a heterolytic manner. During this process, it was found that the volume of the second cavity decreased (by about 4%) upon encapsulation of the fullerene into the first cavity.
These unique architectures were found to be around 3 nm in length, with a molecular weight of up to 8,820 Da.
To identify and characterize the architecture and bonding within, the Researchers used a combination of nuclear magnetic resonance spectroscopy (Bruker AVANCE-HD500) including diffusion-ordered NMR spectroscopy (DOSY), electrospray ionisation time-of-flight mass spectrometry (ESI-TOF MS, Bruker micrOTOF II), ultraviolet-visible (UV-Vis) spectroscopy (JASCO V-670DS), Fourier-transform infrared spectroscopy (FTIR, JASCO FT/IR-420), X-ray diffraction (XRD, Rigaku XtaLAB Pro P200) and elemental analysis (LECO CHNS-932 VTF-900). Molecular force-field calculations were preformed using Materials Studio (version 5.5.3, Accelrys Software Inc.).
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The combination of ligand-metal interactions, are what makes this process so unique and achievable, and is predicted to have a far-reaching potential for the facile preparation of advanced artificial nanoarchitectures inspired by complex natural systems.
Sources and Further Reading
- “Polyaromatic molecular peanuts”- Yazaki K., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15914