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 TeIIL2−, 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. Radical route: Reduction of sterically hindered 2,1,3‐benzotelluradiazole derivative 1 opened the way to its reduced radical anion form 1.−, and both were isolated and characterised by XRD. The solution EPR spectrum of 1.− was recorded in a wide temperature range (77–300 K), and the values of a(14N) and A∥(125Te) were determined. Two‐component relativistic DFT calculations perfectly reproduce these values and predict that the spin density is delocalised over the whole molecule with high spin population of Te and N atoms.