Young scientists have made a metal sandwich with its inner layer on the outside

Scientists from St Petersburg University have created and described metal compounds that resemble “reverse sandwiches” in structure.

These can help to create new materials with controlled properties. The work was carried out under the scope of a project supported by the Presidential Research Projects Program, conducted by the Russian Science Foundation. Its results were published in the Angewandte Chemie International Edition magazine.

In most metal compounds atoms are positively charged. A typical example is sodium chloride NaCl, in which sodium, being a metal, is positively charged, whereas chlorine has a negative charge. Sodium chloride is salt, and its crystals can be found in any kitchen. They exist due to the interaction of oppositely charged ions of sodium and chlorine.

In the second half of the 20^th century, English chemists Ernst Otto Fischer and Geoffrey Wilkinson were awarded the Nobel prize for revealing so called sandwich compounds. One such compound is, for example, ferrocene.  Its molecules consist of two components. One of them is planar negatively charged organic fragments comprising carbon and hydrogen. The other component is a positively charged ion of iron, sandwiched between organic fragments, like a filler between two slices of bread.

Although all metals in most of their compounds have positive charges, there are some exceptions to the rule. For example, platinic and palladium in some of their compounds become negatively charged. The authors of the article showed in a number of earlier works that negatively charged ions of metals attract positively charged ions of halogens, elements of Group 7 in the Periodic Table. The researchers decided to use it in their work, but with some alterations: they replaced the positively charged fragment of halogen with a planar structure where the positive charge is on the atoms of carbon.

We managed to put together a system where negatively charged ions of metal are attracted to positive charges on organic planar fragments. We decided to name these systems “reverse sandwich” because of their reverse layout.

Daniil Ivanov, one of the authors of the article, Assistant Lecturer of the Department of Physical Organic Chemistry of the Institute of Chemistry, St Petersburg University

In order to attain such structure, the scientists took a compound of platinum and palladium and added to it organic compounds with planar molecules. Then the researchers slowly boiled the solutions down to dispose of a solute and increase the concentration of matter. Reverse sandwiches assembled themselves in the course of the boiling down process during the solid phase. 

During the solid phase crystals are formed. In order to identify their structure, the researchers used x-ray diffraction analysis that allows to “take a photo” of the relative disposition of atoms and molecules in the solid phase.  The data obtained on the structure was supplemented with theoretical predictions.

“We are going to continue studying interactions involving atoms of metals carrying negative charge. Specifically, many metal compounds have useful photophysical properties, for example light emission and luminescence. We want to figure out, how the inclusion of metal compounds into reverse sandwiches effects such properties”, Daniil Ivanov added.

The work was carried out in collaboration with scientists from the Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences.

The researchers also point out that they are now studying the properties of metal compounds both free and as part of reverse sandwiches. If in the future scientists discover their useful properties, they will be able to use them for creating new materials and for managing these properties. For example, there is a chance that the colour of luminescence could differ if the same metal compound is used in two different pieces of lighting equipment, with one of them free but in the other one within a reverse sandwich.