XPS and ¹²⁵Te NMR Studies of Organotellurium Compounds. 2. Oxatellurolylium Halides and Dioxatellurapentalenes and Their Products of Oxidative Halogen Addition

XPS (X-ray photoelectron spectroscopy or ESCA, electron spectroscopy for chemical analysis) analyses of oxatellurolylium halides 13 and dioxatellurapentalenes 14 have shown them to be tellurium(II) compounds when their Te(3d_5/2) binding energies are compared to those of tellurium(II) models. The oxatellurolylium trihalides 15 (and, by extension, the dioxatellurapentalene dihalides 16) have Te(3d_5/2) binding energies consistent with a tellurium(IV) oxidation state when compared to tellurium(IV) models. Changes in the XPS binding energies, which result from substituent changes in the carbon π-framework, are similar in magnitude to changes from substitution in the electronegative ligands attached to tellurium. This makes quantitative assessment of the effect of higher order bonding on the electronic environment at tellurium difficult using the Te(3d₅/₂) binding energies. The O(ls) electrons of 13–15 all have higher binding energies than model compounds 9, 11, and 12, suggesting that partial bonding (linear, three-center, four-electron bonding) exists between the tellurium halide, the tellurium, and the enone oxygen (X-Te****O). While the XPS data show the oxygen atoms to be donating electrons in the three-center, four-electron bonds, the ¹²⁵Te NMR chemical shifts show deshielding as three-center, four-electron bonding increases. With dioxatellurapentalene 14a as one limit with a symmetrical, strong three-center, four-electron bond and tellurapyranone 9 as the other limit with no bond between tellurium and oxygen, the ¹²⁶Te NMR chemical shift moves sequentially downfield from 9 to β-(phenyltelluro)butenoate ester 10 to the oxatellurolylium iodides to the oxatellurolylium bromides to the oxatellurolylium chlorides in the tellurium(II) series. This is exactly the trend to be expected if increasingly strong linear three-center, four-electron bonding, on net, deshields tellurium through increased ionic bonding. The compounds of this study are of AX₃E₂ and AX₅E types for tellurium(II) and tellurium(IV), respectively. The XPS and ¹²⁵Te NMR studies of these compounds are the first systematic studies of compounds of this type. The data suggest that ¹²⁵Te NMR chemical shifts are sensitive to the geometry of the complexes, the oxidation state of tellurium, and the electronic environment of tellurium.