Ditechnetium heptoxide was synthesized from the oxidation of TcO with O 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 TcO 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) Å]. The TcO molecule can be described as corner-sharing TcO tetrahedron [Tc---Tc = 3.698(1) Å and Tc-O-Tc = 180.0°]. The EI-MS spectrum of TcO consists of both mononuclear and dinuclear species. The main dinuclear species in the gas-phase are TcO (100%) and TcO (56%), while the main mononuclear species are TcO (33.9%) and TcO (42.8%). The difference in the relative intensities of the MO (M = Tc, Re) fragments (1.7% for Re) indicates that these group 7 elements exhibit different gas-phase chemistry. The solid-state structure of TcO was investigated by density functional theory methods. The optimized structure of the TcO molecule is in good agreement with the experimental one. Simulations indicate that the more favorable geometry for the TcO molecule in the gas-phase is bent (Tc-O-Tc = 156.5°), while a linear geometry (Tc-O-Tc = 180.0°) is favored in the solid-state.
Multifunctional cyclopentadiene ligands are used to prepare difunctional and monofunctional, PSMA-targeting Re(i) and 99mTc(i) complexes. The difunctional rhenium complex is shown to bind the PSMA with an order of magnitude lower KD.
The molecular and electronic structures of the group 7 heptoxides were investigated by computational methods as both isolated molecules and in the solid-state. The metal-oxygen-metal bending angle of the single molecule increased with increasing atomic number, with ReO preferring a linear structure. Natural bond orbital and localized orbital bonding analyses indicate that there is a three-center covalent bond between the metal atoms and the bridging oxygen, and the increasing ionic character of the bonds favors larger bond angles. The calculations accurately reproduce the experimental crystal structures within a few percent. Analysis of the band structures and density of states shows similar bonding for all of the solid-state heptoxides, including the presence of the three-center covalent bond. DFT+U simulations show that PBE-D3 underpredicts the band gap by ∼0.2 eV due to an undercorrelation of the metal d conducting states. Homologue and compression studies show that ReO adopts a polymeric structure because the Re-oxide tetrahedra are easily distorted by packing stresses to form additional three-center covalent bonds.
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