On the Electron-Donating Properties of Oxygen vs. Sulfur. Redox Potentials for Some Pyrylium and Thiapyrylium Salts

Article abstract of DOI:10.1021/ja00521a047

The redox levels for two series of pyrylium and thiapyrylium salts were measured by cyclic voltammetry.This information was utilized to provide a quantitative comparison of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) which is a measure of the thermodynamic stabilities to electron transfer of oxygen vs. sulfur adjacent to carbon cation, radical, and anionic centers in a single system.In one of the series dicationic, cationic, radical, and anionic centers were compared.The ionic and radical species were found to be thermodynamically more stable in the sulfur analogue (less stable in oxygen analogue) in every case except when the HOMO and/or LUMO level is localized on the diethylaminophenyl moiety.The order of the greatest sulfur preference or least oxygen preference is the following: anion>radical and radical>cation for the HOMO and LOMO levels, respectively.The wavelength of the intramolecular charge transfer band and the electrochemical reduction potentials indicate that the thiapyrylium moiety is more electron withdrawing than pyrylium.