A Sterically Hindered Derivative of 2,1,3-Benzotelluradiazole: A Way to the First Structurally Characterised Monomeric Tellurium–Nitrogen Radical Anion

Interaction of the tetradentate redox-active 6,6′-[1,2-phenylenebis(azanediyl)]bis(2,4-di-tert-butylphenol) (H₄L) with TeCl₄ leads to neutral diamagnetic compound TeL (1) in high yield. The molecule of 1 has a nearly planar TeN₂O₂ fragment, which suggests the formulation of 1 as Te^IIL²⁻, in agreement with the results of DFT calculations and QTAIM and NBO analyses. Reduction of 1 with one equivalent of [CoCp₂] leads to quantitative formation of the paramagnetic salt [CoCp₂]⁺[1]^.−, which was characterised by single-crystal XRD. The solution EPR spectrum of [CoCp₂]⁺[1]^.− at room temperature features a quintet due to splitting on two equivalent ¹⁴N nuclei. Below 150 K it turns into a broad singlet line with two weak satellites due to the splitting on the ¹²⁵Te nucleus. Two-component relativistic DFT calculations perfectly reproduce the a(¹⁴N) HFI constants and A_∥(¹²⁵Te) value responsible for the low-temperature satellite splitting. Calculations predict that the additional electron in 1^.− is localised mainly on L, while the spin density is delocalised over the whole molecule with significant localisation on the Te atom (≥30 %). All these data suggest that 1^.− can be regarded as the first example of a structurally characterised monomeric tellurium–nitrogen radical anion.