XPS and ¹²⁵Te NMR Studies of Organotellurium Compounds. 1. Tellurapyrans, Tellurapyranones, Tellurapyrylium Salts, and Their Benzo Analogues in Both the Tellurium(II) and Tellurium(IV) Oxidation States

¹²⁵Te NMR and XPS (X-ray photoelectron spectroscopy or ESCA, electron specroscopy for chemical analysis) analyses of several series of tellurapyran, tellurapyranone, and tellurapyrylium compounds were carried out. In the tellurium(II) oxidation state of these compounds, the chemical shifts in the ¹²⁵Te NMR of these compounds were sensitive to the presence of a formal positive charge. 2,6-Di-tert-butyltellurapyrylium hexafluorophosphate (23) had the furthest downfield chemical shift in this study at δ 1304 ppm (dimethyl telluride, 6 0.0 ppm) with a Te(3d_5/2) binding energy of 575.2 eV. Introduction of a methyl substituent at the 4-position resulted in a more electron-rich tellurium center as evidenced by an upfield shift to δ 1185 ppm and a Te(3d₅/₂) binding energy of 574.9 eV. In tellurapyrylium dye chromophores where the positive charge could be shared by more that one heteroatom, the ¹²⁵Te NMR chemical shifts were even further upfield in the range δ 784–934 ppm with Te(3d_5/2) binding energies from 573.1 to 574.1 eV. A linear correlation was found between the ¹²⁵Te NMR chemical shift and the Te(3d_5/2) binding energies on comparison of seven closely related cationic Te(II) compounds with a least-squares correlation coefficient of 0.98. A linear correlation was also found on the comparison of the ¹²⁵Te NMR chemical shift and the Te(3d_5/2) binding energies of six closely related neutral Te(II) compounds with a least-squares correlation coefficient of 0.91. The neutral Te(II) tellurapyranones have ¹²⁵Te NMR chemical shifts that are downfield of their neutral Te(II) tellurapyran analogues by 139–188 ppm. Contributions from resonance forms that give a tellurapyrylium ring in the tellurapyranones, placing a partial positive charge on the tellurium center, could be responsible, in part, for the downfield shifts. The examination of a series of Te(IV) compounds, with Te(3d_5/2) binding energies from 575.8 to 576.4 eV, did not find a correlation with the ¹²⁵Te NMR chemical shift. As expected, the ¹²⁵Te NMR chemical shift was found to be sensitive to changes which affect the paramagnetic screening constant in these compounds.