The tetrahedral cage with Xenon inside it. Taken from (1).
The paper I want to share with you this time concerns a topic I am always fascinated with – Supramolecular Chemistry. We all know that organic chemistry is governed by covalent bonds, which in a sense hold the atoms together to give stable and interesting compounds. At the frontier of supramolecular chemistry, the forces are inter-molecular in nature. They are transient forces powerful enough to hold different molecules together. This has led to a lot of monumental achievements in the fields of nanotechnology, molecular recognition, sensor chemistry and so on.
A key concept in Supramolecular Chemistry is ‘sub-component
assembly’. Imagine this concept as some sort of a molecular Lego Game. You have
different chemical building blocks (termed the sub-components) and they form
transient interactions which in turn hold them together to give a final
structure. The final chemical structure is usually governed by thermodynamic
factors, which means the pathway towards the final product is usually the one
that gives the lowest energy state. It is possible to control the final result
at will - for example, the inclusion of metal ions will lead to some
metal-ligand interactions that can ‘narrow down’ the possible chemical pathways
to the ones that are desirable. In this paper from Journal of American Chemical
Society (JACS), this is indeed one of those cases. (1)
Telkki et. Al. has reported the synthesis of a
chemical ‘cage’ – which means that certain chemical compounds can act as guests
and reside inside this chemical structure. The shape of this cage is
tetrahedral. Each of the 6 sides of the tetrahedron is made up from 2 simple
chemical building blocks. Since there are heteroatoms (nitrogen in this case)
on these 4 sides, they all have the potential to bond to a metal ion. And, the
4 vertices of the tetrahedron are Fe (II) ion. Thus the compound is
self-assembled in an aqueous media (as the building blocks for the sides
contain water-solubilizing groups) and this observation implies that the
chemical properties of the ligands and the metal ions dictate the final course
of the product formation.
Exotic aside, how can we use this chemical cage? The
researchers have found that they can indeed encapsulate Xenon, a noble gas
element. Since Xenon has a NMR-active isotope, therefore they can observe the
difference in the chemical shifts when Xenon has been introduced into their
system. Not only this can find use in NMR, the chemical cage can be used to ‘tame’
Xenon and allow the element to exhibit more controllable behavior, and this
should contribute to chemical sensor technology.
The novel cage from Sanders / Nitschke et. Al. Taken from (2).
Professors Sanders and Nitschke from Cambridge have
also contributed tremendously to this field. They have a recent paper in Angew.
Chem. , where they have made an interesting metallosupramolecular complex
involving fullerene. (2) Check that out too if you are interested!
By Ed Law
13/2/2015
If you would like to explore more about the concept of molecular encapsulation, see:
Reference:
1. Encapsulation
of Xenon by a Self-Assembled Fe4L6 Metallosupramolecular
Cage.
J. Roukala, J. Zhu, C. Giri, K. Rissanen, P. Lantto,
V.-V. Telkki, J. Am. Chem. Soc., Article ASAP. DOI: 10.1021/ja5130176,
Publication Date (Web): February 5, 2015.
2. Guest-Induced
Transformation of a Porphyrin-Edged FeII4L6 Capsule
into a CuIFeII2L4 Fullerene
Receptor.
D. M. Wood, W. Meng, T. K. Ronson, A. R. Stefankiewicz, J. K. M. Sanders and J. R. Nitschke, published
online: 5 Feb 2015, DOI: 10.1002/anie.201411985.
