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, exclusively, from which R2TeX2 (X = Br 3b, I 3c or SCN 3d) were derived by halogen exchange. The X-ray crystallographic analyses of 3a-3d showed that these compounds have a twofold axis (except for 3d) with essentially pseudo-trigonal bypyramidal co-ordination with two R groups and a lone pair of electrons occupying the equatorial sites and two halogen atoms the apical sites. The thiocyanate groups in 3d bind to tellurium atom via sulfur. No intermolecular Te ... X secondary bond was observed for 3a-d. The Te-C bond distances of 3a-c [2.09 +/- 0.01 Angstrom] are somewhat shorter than those reported 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 radii [3.60 Angstrom]. The H-1 NMR spectra of 3a-3c were halogen-, solvent-, and temperature-dependent showing that the rotation of R-Te bonds was restricted due to the barrier between R groups and halogen atoms. The activation energies Delta G double dagger decreased in the order 3a (90 kJ mol(-1) in DMSO-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).