Synthesis and Characterization of Tellurium Catecholates and Their N-Oxide Adducts

Tellurium catecholate complexes were investigated to probe the redox chemistry of tellurium, whose oxidation state can span from -2 to +6. Treating TeO₂ with catechols resulted in tellurium coordination complexes in high yields within minutes to hours at room temperature or with extended heating, depending on the ligand substituents, giving Te(IV) complexes of the form Te(C)₂, where C = 3,5-di-tert-butylcatecholate, o-catecholate, or tetrachlorocatecholate. The redox behavior of these complexes was investigated through addition of organic oxidants, giving nearly quantitative adducts of pyridine N-oxide or N-methylmorpholine N-oxide with each tellurium complex, the latter set leading to ligand oxidation upon heating. Each compound was characterized crystallographically and computationally, providing data consistent with a mostly electrostatic interaction and very little covalent character between the N-oxide and Te complex. The Te N-oxide bond orders are consistently lower than those with the catechol derivatives, as characterized with the Mayer, Gopinathan-Jug (G-J), and first Nalewajski-Mrozek (N-M1) bond indices. The tellurium lone pair is energetically buried by 1.93-2.81 eV, correlating with the observation that the ligands are more reactive than the tellurium center toward oxidation. This combined experimental and theoretical study finds structure-property relationships between ligand design and reactivity that will aid in future efforts for the recovery of tellurium.