Modern materials discovered in nature – the rhubarb molecule did the job

​A research group at Chalmers University of Technology and University of Yaoundé in Cameroon has discovered natural varieties of materials which, until now, have only been synthesised in laboratories. In a new study, the “rhubarb molecule” oxalic acid and its ability to bind in different directions have been shown to play a crucial role. The research is published in the journal CrystEngComm by the Royal Society of Chemistry in the United Kingdom.
"It is fascinating to see how we in nature discover variations of materials, that has been made in the laboratory only the past twenty years, and how our mathematical description of them is at once informative and beautiful," says Françoise Noa, postdoctoral research fellow at Chalmers University of Technology.
 
Can pave the way for understanding and producing new materials
The researchers investigated published crystal structures of a large number of chemical compounds with metal ions and oxalate ions. The basis for these materials is two-headed carboxylic acids that can bind a metal ion at each end and thus build up network structures. The simplest of these carboxylic acids, oxalic acid, is found for example in rhubarb. When the oxalic acid, HOOC-COOH, loses two positive hydrogen ions and becomes the oxalate ion, -OOC-COO-, it can instead bind metal ions at each end and form networks as several oxalate ions can bind to one metal ion. The team systematized these networks based on their topological properties.

"The network-forming function of the oxalate ion is technically very important and arise because it binds individual metal ions well using two oxygen atoms at the same time (the so called chelate effect) and is able to do so in two directions," says Professor Lars Öhrström at the Department of Chemistry and Chemical Engineering, Chalmers University of Technology and co-author of the study.
 
The materials are described as symmetric and periodically repeating networks with the metal ion at the junctions. A simple version in two dimensions is a regular square grid pattern. However, square grid patterns were not found in these oxalate materials. Instead, the team found four brand new three-dimensional networks, which can pave the way for understanding and producing new materials.

Important applications for other scientific studies   
Metal-Organic Frameworks or MOFs have many possible applications, from drug delivery to biogas storage. Several materials have already been commercialized. In addition, it was discovered in this study that a component of kidney stones, weddellite, is a MOF with calcium ions and oxalate ions. The weddellite network has water-filled channels, and the water molecules' interactions with the network have been found to have important applications for other scientific studies. At the University of California – Berkeley researchers have optimized this interaction and thus developed a method for extracting water from desert air with the help of another MOF. Professor Omar Yaghi at Berkeley comments the Chalmers and Yaoundé researchers work by saying that it is an important part of learning the grammar and taxonomy of Metal-Organic Frameworks, how they are constructed and how they can be designed.
 
Facts Oxalic acid
Oxalic acid is found in a variety of plants and is named after the wood sorrel, Oxalis acetosella. It is a relatively strong acid and its salts, compounds where it has released two hydrogen ions, are called oxalates. Oxalic acid and oxalates have many practical uses, several of these related to its ability to bind metal ions with a "pinch manoeuvre" using two of the oxygen atoms.

Facts Metal-Organic Frameworks
Materials where metal ions are bridged by organic molecules and form network structures, often with relatively large channels and cavities. The commercial applications that exist today are based on the materials' ability to adsorb and store gases. Another interesting use is to extract water from air even in very dry climates.

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Published: Mon 04 Nov 2019.