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]

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 G 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 ], 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 TeX bonding to form dimers or polymers. Based on these results and VSEPR theory, the dependence of the rotational barrier G of the Ar-group in RArTeX2 on the R-group is discussed.