Complexes of metals with organotellurium compounds and nanosized metal tellurides for catalysis, electrocatalysis and photocatalysis

Tellurium chemistry has not evolved significantly and there are only a handful of complexes of metals with organotellurium ligands and nanosized metal tellurides that have been explored in catalysis. Though this chemistry is much less explored than that of sulfur and selenium counterparts, the recent work on the applications of metal complexes of tellurium ligands, and nanosized metal tellurides in catalysis indicate that there is a lot of scope for new discoveries. In this review, we have summarized complexes of different transition metals with organotellurium compounds and nanosized metal tellurides, their synthesis and applications in different catalytic transformations with critical analysis of present status of the subject. Such metal complexes have been explored in various organic transformations, but the examples are very limited. Telluroether ligands have been used to prepare several distinct metal complexes. The geometry around the metal centre in the complexes depends on metal. The geometry is square planar or neraly square planar in the complexes of palladium. In case of other metals, the complexes have pseudo-octahedral half sandwich 'piano-stool' geometry. Though the metal complexes show the potential to catalyse different types of reactions (including Suzuki coupling, Heck coupling, C–N coupling, amidation reaction, oxidation of alcohols, transfer hydrogenation of carbonyl compounds, reduction of 4-nitrophenol, hydrosilylation reaction), extensive studies have been conducted on their applications in catalysis of C–C coupling reactions. Nanosized metal tellurides have been synthesized and characterized as catalytically active materials. However, they remain at a back seat with respect to their exploration in catalysis. These materials have been developed with different morphologies (e.g., in the form of nanoparticles, nanotubes and nanowires, etc). The architecture of nanosized crystals mainly reflects the morphology, complexity and the symmetry of its crystal structures. Their synthetic strategies include a variety of methods such as hydrothermal methods, solvothermal methods, ultrasonification, etc. Their catalytic properties have been explored in electrocatalysis, electro-oxidation of alcohols, photocatalysis, dye-sensitized solar cells. Among them, the applications in hydrogen evolution reactions (HER), oxygen reduction reactions (ORR) and oxidation of alcohols are quite interesting.