Ditechnetium Heptoxide Revisited: Solid-State, Gas-Phase, and Theoretical Studies

Ditechnetium heptoxide was synthesized from the oxidation of TcO₂ with O₂ at 450 °C and characterized by single-crystal X-ray diffraction, electron-impact mass spectrometry (EI-MS), and theoretical methods. Refinement of the structure at 100 K indicates that Tc₂O₇ crystallizes as a molecular solid in the orthorhombic space group Pbca [a = 7.312(3) Å, b = 5.562(2) Å, c = 13.707(5) Å, and V = 557.5(3) ų]. The Tc₂O₇ molecule can be described as corner-sharing TcO₄ tetrahedron [Tc---Tc = 3.698(1) Å and Tc-O_Bri-Tc = 180.0°]. The EI-MS spectrum of Tc₂O₇ consists of both mononuclear and dinuclear species. The main dinuclear species in the gas-phase are Tc₂O₇ (100%) and Tc₂O₅ (56%), while the main mononuclear species are TcO₃ (33.9%) and TcO₂ (42.8%). The difference in the relative intensities of the M₂O₅ (M = Tc, Re) fragments (1.7% for Re) indicates that these group 7 elements exhibit different gas-phase chemistry. The solid-state structure of Tc₂O₇ was investigated by density functional theory methods. The optimized structure of the Tc₂O₇ molecule is in good agreement with the experimental one. Simulations indicate that the more favorable geometry for the Tc₂O₇ molecule in the gas-phase is bent (Tc-O_Bri-Tc = 156.5°), while a linear geometry (Tc-O_Bri-Tc = 180.0°) is favored in the solid-state.