Tellurium-Nitrogen π-Heterocyclic Chemistry - Synthesis, Structure, and Reactivity Toward Halides and Pyridine of 3,4-Dicyano-1,2,5-telluradiazole

The reaction of 2,3-diaminomaleonitrile with TeX4 (X = Cl, Br) in the presence of pyridine (Py) and/or triethylamine (Et3N) provided 3,4-dicyano-1,2,5-telluradiazole (1), which was isolated neat and as stable adducts with pyridine, chloride, and bromide, namely, 1·2Py, (PyH)(1·Cl), (PyH)2(1·2Cl), (Et3NH)(1·Cl), (PyH)(1·Br), and (PyH)2(1·2Br). The molecular and supramolecular structures of these compounds were investigated by X-ray crystallography. In the solid state, intermolecular associations through secondary Te···N interactions as well as N–H···X and N–H···N hydrogen bonding (X = Cl, Br) were observed. For (PyH)(1·Br), two polymorphs were found. The bonding situation of 1 and its pyridine and chloride adducts were investigated by MP2 calculations supplemented with the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. The π symmetry of the frontier molecular orbitals (MOs) of 1 are preserved in the 1·2Py, (1·Cl–), and (1·2Cl–) adducts. In the chloride adducts, the highest occupied molecular orbital (HOMO) can be described as an antibonding combination of the HOMO of 1 with the 3p atomic orbitals (AOs) of the chloride ions, whereas the lowest occupied molecular orbital (LUMO) resembles that of the parent 1. The charge transfer onto the heterocycle in the adducts increases in the order 1·2Py, (1·2Cl–), and (1·Cl–). QTAIM analyses of the adducts in the gas phase reveal closed-shell interactions, whereas NBO analyses indicate negative hyperconjugation as the main formation pathway in these complexes. This description agrees with the Alcock model suggested for secondary bonding interactions between atoms of heavy p-block elements and atoms with lone pairs.