Synthesis and structure of arylselenium(ii) and aryltellurium(ii) cations based on rigid 5-tert-butyl-1,3-bis-(N-pentylbenzimidazol-2'-yl)benzenes.

The transmetalation reactions of a mercury precursor, [Pentyl(N^C^N)HgCl] (19), with selenium halides (SeCl4, SeBr4, and SeCl2) were attempted to obtain the corresponding organoselenium trichloride [Pentyl(N^C^N)SeCl3], tribromide [Pentyl(N^C^N)SeBr3], and monochloride [Pentyl(N^C^N)SeCl], respectively [(N^C^N) = 5-tert-butyl-1,3-bis-(N-pentyl-benzimidazol-2'-yl)phenyl]. However, in all the cases, a very facile ionization of the Se-halogen bond was observed leading to the isolation of a new class of air stable arylselenium(ii) complexes: [Pentyl(N^C^N)Se+]2[HgCl4]2- (20) and [Pentyl(N^C^N)Se+]2[HgBr4]2- (21). This is the first report on the formation of NCN pincer-based arylselenium(ii) cations via the transmetalation route. Similar reactions were further investigated with several tellurium precursors: {TeCl4, TeBr4 and TeI2} which resulted in the formation of analogous aryltellurium(ii) complexes: [Pentyl(N^C^N)Te+]2[HgCl4]2- (22), [Pentyl(N^C^N)Te+][Cl]- (23), [Pentyl(N^C^N)Te+]2[HgBr4]2- (24), [Pentyl(N^C^N)Te+][Br]- (25) and [Pentyl(N^C^N)Te+]4[Hg2Cl4.72I3.28]4- (26). These are only the second set of examples of aryltellurium cations (hypervalent 10-Te-3 species) with the NCN pincer-based ligand, characterised by X-ray crystallographic studies. The crystallographic studies show a strong SeN/TeN intramolecular interaction, which is confirmed by NBO calculations suggesting the donation of a lone pair of electrons on nitrogen to a lone p-vacancy on selenium/tellurium atoms. The analysis based on NPA derived charges indicates that the contribution of SeN interactions to the electrostatic stabilization energy is in the range of 40-60%, whereas TeN interactions have a contribution of about 84% and more, attributed to the differences in the electronegativity of selenium and tellurium. Furthermore, the formation of arylselenium(ii) and aryltellurium(ii) complexes was favoured due to the presence of the σ-hole on the Se/Te centres.