Polytellurides of Mn, Fe, and Zn from mild solvothermal reactions in liquid ammonia

The reaction of elemental Mn, Fe, and Zn with Te in liquid ammonia at 50 °C leads to the polytellurides [Mn(NH3)6]Te4 (1), [Fe(NH3)6]Te4·NH3 (2), and [Zn(NH3)4]2Te15 (3) in quantitative yield for 1 and 3, and in 30-50% yield for 2. The compounds form black crystals, which are air sensitive and easily lose ammonia without a protective atmosphere of NH3. Compound 3 is semiconducting with a thermal activation energy of 1.2 eV. In the crystal structures of 1 and 2, tetratelluride anions Te4(2-) in gauche conformation with dihedral angles around 90° are present, which are linked to form infinite spiral chains. Compound 3 contains an unusual Te15(4-) polyanion in the form of a bent chain Te7-Te-Te7. The connection between the Te4 groups in 1 and 2 and the two Te7 groups in 3 is achieved via linear Te3 entities, which are strongly asymmetric in 1, almost symmetric in 2, and symmetric in 3 (for 1, Te-Te···Te 174.0°, d1 = 2.87, d2 = 3.25 Å; for 2, Te-Te-Te 178.8°, d1 = 3.01, d2 = 3.09 Å; for 3, Te-Te-Te 180°, d1 = d2 = 3.06 Å). Periodic DFT calculations show that interaction between the Te4(2-) units is negligible in 1 and weak but undoubtedly present in 2. The overlap population amounts to 0.09 in the linear Te3 group of 3. The band structure calculation of 3 gives semiconducting behavior with a band gap of 1.5 eV in fair agreement with experimental data.