A Comprehensive Study on Redox Behavior and Halogen Exchange in Telluropyran Derivatives

Incorporation of tellurium into polycyclic compounds may endow them with unique chemical and optoelectronic properties which are not observed in their lighter chalcogen analogues. Herein, a telluropyran-containing polycyclic compound (T1) synthesized through a ring-expansion reaction from the corresponding tellurophene analogue can be reversibly oxidized into halogen adducts T1 center dot X-2 (X=Cl, Br, I) with the formation of two Te-X bonds. Their chemical structures have been verified by two-dimensional H-1-H-1 correlation spectroscopy and single crystal X-ray diffraction analysis. The halogen oxidations of T1 and the reverse thermal eliminations as well as the halogen exchange in halogen adducts T1 center dot X-2 have been systematically investigated and compared by UV-vis absorption titration, electrochemical measurements, thermogravimetric analysis, and density functional calculations (DFT). The oxidation of Te(II) in T1 to Te(IV) in T1 center dot X-2 results in the switch from aromaticity to nonaromaticity for the six-membered telluropyran ring, as revealed by nucleus-independent chemical shift calculations. It is also found that the halides in the halogen adducts can be exchanged by lighter ones, but not vice versa. The stabilities of the oxidized products are in the order of T1 center dot Cl-2>T1 center dot Br-2>T1 center dot I-2, which are consistent with the calculated rate constants and energy barriers of the elimination reactions.