LIGAND-EXCHANGE REACTIONS IN ORGANOTELLURIDES BY TE-125 NMR-SPECTROSCOPY

Te-125 NMR spectra for a series of symmetric and unsymmetric tellurides containing allyl and 2-methylallyl groups are reported. The effect of allyl substitution on the chemical shift was found to be additive, consistent with earlier observations for alkyl substitution. Alkyl and allyl ligand exchange was observed in solutions of two symmetrical telluriders (R2Te + R'2Te --> 2RTeR' for R, R' = allyl, 2-methylallyl, isopropyl, and tert-butyl). The relative rates of these reactions were found to be in the order R = allyl, R' = 2-methylallyl > tert-butyl, isopropyl > tert-butyl, 2-methylallyl approximately tert-butyl, allyl > isopropyl, 2-methylallyl approximately isopropyl, allyl. Equilibrium constants for the forward reaction were determined for each combination of symmetrical tellurides and for the reverse reaction in the case of (t)BuTeR (R = allyl, 2-methylallyl) over the temperature range 25-80-degrees-C. DELTA-Gs for the reactions were calculated to be in the range -0.89 kcal/mol for R = allyl, R' = 2-methyallyl to -2.1 kcal/mol for R = tert-butyl, R' = allyl. The formation of symmetrical ditellurides ((t)BuTe(t)Bu and (i)PrTeTe(i)Pr) was observed after prolonged heating in reactions of (t)BuTeR and (i)PrTeR, respectively (R = allyl, 2-methylallyl). Spectroscopic evidence for the formation of unsymmetrical ditellurides (t)BuTeTeR (R = allyl, 2-methylallyl) was observed.