Skip to main content

Discovery of novel chiral magnetic molecular materials for the study of magnetochiral effects

Periodic Reporting for period 1 - CHIMMM (Discovery of novel chiral magnetic molecular materials for the study of magnetochiral effects)

Reporting period: 2016-04-01 to 2018-03-31

Chirality is a property found in everyday objects (e.g. screws and springs), in the human body (e.g. hands and feet), or in molecules (e.g. the DNA helix) that originates from the lack of an inversion center. Chiral objects are distinct from their mirror images, which, on the molecular level, are called enantiomers (Fig. 1). An enantiomeric pair has identical physical properties in non-chiral media, and can only be distinguished by their interaction with other chiral media.
Molecular chirality was discovered by Louis Pasteur in the 19th century. During his experiments, he observed that a solution of natural tartaric acid obtained from wine yeast rotates polarized light, while a solution of the same product synthesized in the lab has no such effect. This is because the natural tartaric acid is a homochiral product, consisting of only one enantiomer that rotates the polarization plane of light, while synthetic tartaric acid contains a 50/50 mixture of enantiomers, or a racemic mixture, with each component rotating polarized light in opposite sense and thus cancelling the effect.
Because two enantiomers are chemically identical, a chemical reaction always produces a mixture of enantiomers, unless a chiral agent is present. However, the building blocks of life, such as amino acids, DNA, and carbohydrates are not found as racemic mixtures, but are instead homochiral. This raises the question of what was the external chiral influence that generated the preference for one handedness at the origin life. One explanation is based on the fact that enantiomers react differently to polarized electromagnetic radiation. This phenomenon is called natural optical activity and some authors have hypothesized that it could be responsible for biological homochirality based on studies of enantioselective reactions using circularly polarized light. Note that this type of polarized light is very rare on earth and thus homochirality according to this hypothesis would probably have an extraterrestrial origin. Another source could be the magnetochiral effect, which is the different interaction of enantiomers with nonpolarized light in the presence of a magnetic field. Although the magnetochiral effect was predicted in the 1980s, to date, there have only been a few investigations of this phenomenon.
Because the magnetochiral effect is very small, we wanted to try to maximize it by using strongly chiral compounds with a high spin, (e.g. many unpaired electrons). Such compounds are quite rare, as although molecular magnetism has been an active topic of research for the last 30 years, homochiral compounds have been scarcely explored mainly because of the absence of rational preparation procedures. This is a undoubtedly a subject that deserves more attention as the combination of chirality and magnetism can provide important information about the origin of homochirality in life and give rise to advanced materials including piezoelectrics, pyroelectrics, non-linear optical devices or molecular multiferroics.
As previously mentioned, chemical reactions in achiral media always give racemic mixtures of enantiomers. Therefore, the first step in obtaining homochiral metal complexes is determining a strategy for separating the two enantiomers. We decided to use chiral anions to selectively crystallize one or the other enantiomer, leaving the undesired enantiomer in solution. The first family of anions is based on enantiopure arsenic tartrate ions (AsT), while the second family is formed by helicochiral arsenic tris-catecholate ions (TRISCAS). Both of the selected salts were synthesized in multigram scale.
Chiral AsT ions were used to separate racemic mixtures of helico-chiral tricobalt complexes (Fig. 2). These compounds were selected particularly for their strong optical chirality and stability in solution. We were able to separately crystallize both enantiomers and study their optical properties and absorption of left and right-handed circularly polarized light. As expected for two enantiomeric species, we obtained mirror-image circular dichroism spectra, not only using standard UV/visible circular dichroism techniques, but also using infrared and X-rays (Fig. 3). Although the signals were very intense, X-ray magnetochiral dichroism signals could not be detected, most likely because the spin state was too low, with only one unpaired electron shared over the three cobalt ions. We then decided to work with trinickel analogues, which were likewise separated by crystallization withchiral AsT, and are currently under study.
We then turned our attention to building chiral extended structures from these trimetal units. We had previously found a way to synthesize one-dimensional polymers using fluorine-based linkers, and thought that we could use a similar strategy to synthesize chiral polymers, using the homochiral trimetal building units. We were interested in these complexes because some of them showed a high spin state at low temperature, suggesting they would be promising candidates for magnetochiral studies. Surprisingly, we found that samples of homochiral tricobalt complexes converted into racemic mixtures upon polymer formation. Moreover, this unexpected isomerization occurred whenever we attempted to replace the axial ligands, limiting the chemistry that can be done withthem . However, we found that using chiral linkers prevented racemization and yielded chiral polymers. For example, we discovered two different chiral one-dimensional polymers made from alternating tricobalt or trinickel compounds bridged by chiral AsT ions (Fig. 4).
We also investigated a variety of mononuclear chiral complexes (Fig. 5). For example, we synthesized homochiral iron complexes with a propeller-like structure using chiral AsT and TRISCAS anions. In this case, spectroscopic methods evidenced the racemization of these compounds in solution. Nonetheless, in the solid state, the complexes demonstrated strong non-linear optical properties. A series of similar compounds with a variety of metals was prepared in order to study the influence of the spin state on the chiroptical properties. We were excited to observe magnetochiral effects in the UV-visible range for some of these complexes (unpublished work).
Some of these results have been already published in the following per-reviewed articles: 1) Eur. J. Inorg. Chem. 2018, 320. 2) Chem. Sci., 2018, 9, 1136. 3) Polymers 2018, 10, 311. These results have been presented in the following conferences: 42nd ICCC (Brest, 2016), 2nd and 3rd BOOK-D (Pessac, 2016 and 2017), 2016 and 2017 GdR MCM-2 (Dourdan), 6th ECMM (Bucharest, 2018) and 2018 JCC (Brest). The results have been presented in two formal group meetings to the teams involved in the project and several visiting researchers. The fellow attended to 2016 AIC International Crystallography School (Rimini) and a workshop on chirality intended for young students was developed in the 2017 Fête de la science (Pessac).
We have developed a robust procedure based on anion exchange, facilitating the synthesis of a variety of homochiral metal complexes. The combination of chirality and magnetism is quite rare in the literature and most of the examples reported to date were obtained by serendipity. Our rational approach will allow the in-depth study of the chirality in systems like those described above displaying interesting magnetic, electrical and optical properties.
Representation of two enantiomers, showing their handedness. (Wikimedia Commons).
Chiral iron complex with propeller -like structure.
Chiral polymers formed from tricobalt or trinickel complexes and AsT linkers.
Mirror-image circular dichroism signals for the two tricobalt enantiomers using X-rays.
Mirror-image circular dichroism signals for the two tricobalt enantiomers using UV-visible light
a) Side-on view of a chiral tricobalt complex. b) View along the metal axis of the two enantiomers.