Researchers at the Institute of Physical Chemistry and Electrochemistry suggested that anion-anion intermolecular interactions in rhenium salt crystals hinder phase transitions.
In crystals, solutions, and gases, molecules are interconnected through various non-covalent bonds, such as hydrogen, electrostatic, pi-, sigma-, and others. These relatively weak interactions determine many physical, chemical, and biological properties of substances. For instance, the way hydrogen and other non-covalent interactions form in compounds used for medicinal purposes influences both their three-dimensional structure and how and with which receptor the molecule will bind. Non-covalent interactions play a significant role in self-assembly processes, crystal growth, and the synthesis of molecules, particularly biological ones.
Although non-covalent interactions in seventh group metal compounds have been studied for a long time, their role in the physical properties of these compounds has remained unclear. The aim of this study was to investigate the influence of anion-anion interactions on possible phase transitions in crystalline substances.
Scientists at the Institute of Chemical Physics, Russian Academy of Sciences, synthesized 10 salts of perrhenic acid with organic cations, which were then analyzed using MALDI spectroscopy, X-ray structural analysis, X-ray phase analysis, and thermal gravimetric analysis. Non-covalent interactions were examined using the Hirshfeld surfaces method, which revealed that hydrogen bonds make the most significant contribution to crystal formation. In addition to hydrogen bonds, other supramolecular interactions were present in the crystals.
“The crystal lattice of perrhenates with organic cations forms due to weak non-covalent interactions. Short anion-anion interactions between ReO4 (-1) anions are among the strongest. Although there may be more hydrogen interactions, and their total contribution is greater, a single anion-anion interaction can outweigh a dozen hydrogen bonds,” noted one of the authors of the study, Anton Novikov, a junior researcher at the Laboratory of Radioactive Materials Analysis, Institute of Chemical Physics, Russian Academy of Sciences.
The researchers proposed a classification method for anion-anion interactions in perrhenates and introduced the terms “single lock” (the oxygen atom of one perrhenate ion connects with the rhenium atom of another) and “double lock” (the oxygen atom and the rhenium atom of one anion bind respectively to the rhenium atom and the oxygen atom of another perrhenate ion). New types of infinite quasi-polymeric two-dimensional networks with varying cell shapes and binding strengths were discovered in the synthesized crystals.
“By heating various perrhenates, we observed how the properties of the crystals changed until the molecule broke down. It turned out that the more anion-anion interactions there are in the crystal and the better they are ordered, the less likely it is that phase transitions of the second kind will occur in the substance before the chemical structure of the molecule breaks down,” said Mikhail Volkov, a research associate at the Technetium Chemistry Laboratory, Institute of Chemical Physics, Russian Academy of Sciences, PhD in Chemistry.
Phase transitions of the first kind occur when significant parameters of a substance, such as specific volume, component concentration, and stored internal energy, change abruptly with temperature or pressure variations. The most well-known first-order phase transitions are changes in the state of matter (melting, crystallization, boiling, condensation). Typically, these phase transitions are reversible.
Phase transitions of the second kind are significant in the synthesis of materials with specific magnetic or optical properties. They manifest as changes in parameters such as heat capacity, compressibility, optical or magnetic properties, and others. Changes in the crystalline structure without altering the state of matter also represent a second-order phase transition. They can be accompanied by changes in color, but often, specialized instruments are required to capture a second-order phase transition.
“The synthesis of substances that undergo a second-order phase transition under certain conditions has great industrial significance. For example, substances with an irreversible second-order phase transition can be used to label products that require specific storage conditions. If a small tag made from a substance that irreversibly changes color at, say, minus five degrees is placed on a container, it will be possible to clearly know whether the container was stored at the prescribed minus 18 or, conversely, zero degrees, or if the storage rules were violated. This is crucial for food products, certain medications, vaccines, and so on,” emphasized Mikhail Volkov.
Currently, approximately 600 salts of perrhenic acid with organic cations are described in the Cambridge Structural Database. Among them, about 140 structures exhibit “short” anion-anion intermolecular bonds. “This work is just the beginning of extensive research,” said Anton Novikov. “We hypothesize that anion-anion interactions significantly impact the stability of the crystal. Our conclusion needs to be confirmed with a larger sample that will include different compounds, including salts of other metals. Our ideas pave the way for the practical application of knowledge about non-covalent interactions in crystals.”
The work was supported by the Russian Science Foundation and published in the high-ranking journal of the Royal Society of Chemistry, CrystEngComm.