Bis(2,6-dimethoxyphenyl)tellurium dihalides (Cl, Br or I) and dithiocyanate: crystal structure and temperature-dependent NMR spectra

Tris(2,6-dimethoxyphenyl)telluronium chloride hydrate, [R3Te]Cl . nH(2)O 1a [R = 2,6-(MeO)(2)C6H3, n = 2-2.5] was prepared by the reaction of RLi and TeCl4. It decomposed in hot 0.1 M hydrochloric acid to give R2TeCl2 3a, exc lusively, from which R2TeX2 (X = Br 3b, I 3c or SCN 3d) were derived by hal ogen exchange. The X-ray crystallographic analyses of 3a-3d showed that the se compounds have a twofold axis (except for 3d) with essentially pseudo-tr igonal bypyramidal co-ordination with two R groups and a lone pair of elect rons occupying the equatorial sites and two halogen atoms the apical sites. The thiocyanate groups in 3d bind to tellurium atom via sulfur. No intermo lecular Te ... X secondary bond was observed for 3a-d. The Te-C bond distan ces of 3a-c [2.09 +/- 0.01 Angstrom] are somewhat shorter than those report ed for phenyl derivatives, and those of 3d [2.042(3) and 2.073(2) Angstrom] are the shortest ever reported. The C-Te-C bond angle is much larger [107. 6(2)-104.37(9)degrees] than those reported. The X-Te-X bond angles are very close to 180 degrees. The Te ... O distances of 3a-d [2.880-3.323 Angstrom ] are significantly shorter than the sum of the O and Te van der Waals radi i [3.60 Angstrom]. The H-1 NMR spectra of 3a-3c were halogen-, solvent-, an d temperature-dependent showing that the rotation of R-Te bonds was restric ted due to the barrier between R groups and halogen atoms. The activation e nergies Delta G double dagger decreased in the order 3a (90 kJ mol(-1) in D MSO-d(6)) > 3b (80 kJ mol(-1) in DMSO-d(6)) > 3d (greater than or equal to 65 kJ mol(-1) in CDCl3) > 3c (60 kJ mol(-1) in CDCl3) greater than or equal to 3d (59 kJ mol(-1) in CD3CN).