Organotellurium ligands – designing and complexation reactions

A variety of tellurium ligands has been designed and studied for their complexation reactions in the last decade. Of these hybrid telluroethers, halotellurium ligands and polytellurides are the most notable ones. RTe– and polytelluride ions have also been used to design clusters. Ligation of ditelluroethers and several hybrid telluroethers is extensively studied in our laboratories. The ditelluroether ligand RTeCH2TeR (where R = 4-MeOC6H4) (1), similar to dppm [1,2-bis(diphenylphosphino) methane], has been synthesized in good yield (~80%) by reacting CHCl3 with RTe– (generated in situ by borohydride reduction of R2Te2). Iodine reacts with 1 to give tetra-iodo derivative, which has intermolecular TeLI interactions resulting in a macro structure containing rectangular Te-ILTe bridges. 1 readily forms four membered rings with Pd(II) and Ru(II). On the formation of this chelate ring, the signal in 125Te NMR spectra shifts significantly upfield (50–60 ppm). The bridging mode of 1 has been shown in [Ru(p-cymene)Cl2](m-1)[Ru(p-cymene)Cl2]. The hybrid telluroether ligands explored are of the types (Tex, Sy), (Tex, Ny) and (Tex, Oy). The tellurium donor site has strong trans influence, which is manifested more strongly in square planar complexes of palladium(II). The morpholine N-donor site has been found to have weaker donor characteristics in (Tex, Ny) ligands than pyridine and alkylamine donor sites of analogous ligands. The singlet oxygen readily oxidises the coordinated Te. This oxidation follows first order kinetics. The complexation reaction of RuCl3.xH2O with N-[2-(4-methoxyphenyltelluro)ethyl]phthalimide (2) results in a novel (Te, N, O)-heterocycle, Te-chloro,Te-anisyl-1a-aza-4-oxa-3-tellura-1H, 2H, 4aH-9 fluorenone. The (Te, O) ligands can be used as hemilabile ligands, the oxygen atom temporarily protects the vacant coordination site before the arrival of the substrate. The chelate shifts observed in 125Te NMR spectra of metal complexes of Te-ligands have a close parallel to those of 31P NMR. For the formation of fivemembered rings, the value is positive and of the order of 130 ppm whereas for sixmembered rings it is negative and ~30 ppm only.