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·nH₂O 1a [R = 2,6-(MeO)₂C₆H₃, 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 Å] are somewhat shorter than those reported for phenyl derivatives, and those of 3d [2.042(3) and 2.073(2) Å] are the shortest ever reported. The C–Te–C bond angle is much larger [107.6(2)–104.37(9)°] than those reported. The X–Te–X bond angles are very close to 180°. The Te  · · ·  O distances of 3a–d [2.880–3.323 Å] are significantly shorter than the sum of the O and Te van der Waals radii [3.60 Å]. The ¹H 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 ΔG^‡ decreased in the order 3a (90 kJ mol⁻¹ in DMSO-d₆) > 3b (80 kJ mol⁻¹ in DMSO-d₆) > 3d (≥65 kJ mol⁻¹ in CDCl₃) > 3c (60 kJ mol⁻¹ in CDCl₃) ≥ 3d (59 kJ mol⁻¹ in CD₃CN).