If one electron is removed from a closed-shell aromatic molecule, one might imagine the delocalization of positive charge and/or radical over the entire π-system. It is correct in many cases, but there can be other fate for oxidized aromatic molecules to follow if they contain heavy elements, such as tellurium.
In our latest paper, we have reported the comprehensive study on the redox behavior of tellurium(II)-containing five-membered aromatic ring, tellurophene. This type of motif has attracted significant attention in both organic chemistry and material science. Our collaborator, Prof. Seferos and his co-workers (University of Tronto) have previously reported unique reactions of tellurophene with halogene-based oxidants, such as Br2, ICl and XeF2, tellurophene to form hypervalent Te(IV) state such as PT-X2 (X = F, Cl, Br) as stable compounds. This reaction was also applicable for tellrophenes in conjugated polymeric backbone. However, the electrochemical aspect of redox chemistry of tellurophene has been unexplored.
At first, we were interested in the electrochemical equivalent reaction of PT to PT-X2 formation. If we use halides as a nucleophilein the electrolyte solution, the reaction should follow 2-electron oxidation concomitant with the attacks of two halide atoms to give PT-X2 (Figure). To monitor this reaction, we employed spectroelectrochemistry technique, which enables real-time monitoring of electrochemical reaction via UV-vis absorption spectrum measurement. Indeed, the transformation proceeded to give desired PT-X2, judged by the appearance of the diagnostic peak derived from intramolecular charge-transition (ICT) (Figure 2 in Manuscript).
The most exciting observation was made when we run the spectroelectrochemistry in the absence of halides. We used 0.1 M Bu4NBF4/MeCN as an electrolyte, but we still exclusively observed the ICT band (Figure 3 in Manuscript). In general, the oxidized conjugated molecules show polaron or bipolaron band, but these types of bands did not appear. The appearance of ICT band indicated the coordination of BF4 or MeCN onto Te atom to localize the charge to give PT-L2 (L = BF4 or MeCN), instead of generating delocalized charge over the entire π-system. In addition, this result indicated that two-electron oxidation proceeded, probably by the effect of ligation of BF4 or MeCN. This unprecedented phenomenon caught our attention, thus we were prompted to acquire the big picture of the redox chemistry of tellurophenes. We found that PT-L2 could be electrochemically generated in preparative scale (see picture in bottom Figure), and this hemilabile species was amenable to react with halides to give PT-X2. Electrochemical oxidation of PT in weakly-coordinating electrolyte generated delocalized radical cation species. Significant effect of π-expansion was also discussed.
All these experiments tell us a highly media-dependent nature of redox-chemistry of tellurophenes, which has been overlooked for a long time. We believe that these lessons will give a direction to design a new class of small molecules and polymeric materials containing tellurophenes-motifs.
For the details, check out our article "Redox Chemistry of π-extended Tellurophene" in Communications Chemistry!!
- Carrera, E. I. & Seferos, D. S. Semiconducting polymers containing tellurium: perspectives toward obtaining high-performance materials. Macromolecules 48, 297–308 (2015).