Dependence of the rotational barrier of the Ar-group in RArTeX2 on the R-group [Ar = 2,6-(MeO)2C6H3; R = Me, Et, i-Pr; X = Cl, Br, I]

Abstract

Alkyl(2,6-dimethoxyphenyl)tellurium dihalides, RArTeX₂ [Ar = 2,6-(MeO)₂C₆H₃; X = Cl 2a–c, Br 3a–c, I 4a–c; R = Me a, Et b, i-Pr c] were prepared by the reactions of alkyl 2,6-dimethoxyphenyl telluride, RArTe 1, with SOCl₂, Br₂ or I₂, respectively. The rotational barrier ΔG^‡ of the Ar-group around the Te–C bond in 2a–c, 3a–c and 4a–c estimated by variable temperature ¹H NMR spectra was dependent on the alkyl (R) group as well as on the halogen atoms. It decreased in the order R = Me > Et > i-Pr as well as X = Cl > Br > I. The ¹²⁵Te resonances of 1 were observed at higher magnetic fields than those of RPhTe, and those of 1a–c, 2a–c, 3a–c and 4a–c shifted to lower magnetic field in the order R = Me > Et > i-Pr. The X-ray crystallographic analyses of 2a–c, 3a, 3b and 4a showed that the geometry around tellurium was pseudo-trigonal bipyramidal with the alkyl group, the Ar group and a lone pair of electrons in the equatorial positions and with two halogen atoms in the apical positions. Whereas each of the Te–C(Ar) bond distances were very similar [2.10 ± 0.01 Å], the Te–C(R) bonds of 2a–c were longer than Te–C(Ar) and increased in length in the order R = Me < Et < i-Pr. The C(Ar)–Te–C(R) bond angles also increased in the order R = Me < Et < i-Pr. These molecules were bridged by intermolecular Te⋯X bonding to form dimers or polymers. Based on these results and VSEPR theory, the dependence of the rotational barrier ΔG^‡ of the Ar-group in RArTeX₂ on the R-group is discussed.