Organotellurium ligands — designing and complexation reactions

Abstract

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^-andpolytelluride 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^- (generatedin situ by borohydride reduction of R₂Te₂). Iodine reacts with1 to give tetra-iodo derivative, which has intermolecular Te.I interactions resulting in a macro structure containing rectangular Te-I.Te bridges.1 readily forms four membered rings with Pd(II) and Ru(II). On the formation of this chelate ring, the signal in¹²⁵Te NMR spectra shifts significantly upfield (50-60 ppm). The bridging mode of1 has been shown in [Ru(p-cymene)Cl₂]](μ-l)[Ru(p-cymene)Cl₂]. The hybrid telluroether ligands explored are of the types (Te_x, S_y ), (Te_x, N_y) and ( Te_x,O_y ). The tellurium donor site has strongtrans 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 (Te_x, N_y) 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 in¹²⁵Te NMR spectra of metal complexes of Te-ligands have a close parallel to those of³¹P NMR. For the formation of five-membered rings, the value is positive and of the order of 130 ppm whereas for six-membered rings it is negative and ∼30 ppm only.