Recent advances in technetium halide chemistry

Transition metal binary halides are fundamental compounds, and the study of their structure, bonding, and other properties gives chemists a better understanding of physicochemical trends across the periodic table. One transition metal whose halide chemistry is underdeveloped is technetium, the lightest radioelement. For half a century, the halide chemistry of technetium has been defined by three compounds: TcF6, TcF5, and TcCl4. The absence of Tc binary bromides and iodides in the literature was surprising considering the existence of such compounds for all of the elements surrounding technetium. The common synthetic routes that scientists use to obtain binary halides of the neighboring elements, such as sealed tube reactions between elements and flowing gas reactions between a molecular complex and HX gas (X = Cl, Br, or I), had not been reported for technetium. In this Account, we discuss how we used these routes to revisit the halide chemistry of technetium. We report seven new phases: TcBr4, TcBr3, α/β-TcCl3, α/β-TcCl2, and TcI3. Technetium tetrachloride and tetrabromide are isostructural to PtX4 (X = Cl or Br) and consist of infinite chains of edge-sharing TcX6 octahedra. Trivalent technetium halides are isostructural to ruthenium and molybdenum (β-TcCl3, TcBr3, and TcI3) and to rhenium (α-TcCl3). Technetium tribromide and triiodide exhibit the TiI3 structure-type and consist of infinite chains of face-sharing TcX6 (X = Br or I) octahedra. Concerning the trichlorides, β-TcCl3 crystallizes with the AlCl3 structure-type and consists of infinite layers of edge-sharing TcCl6 octahedra, while α-TcCl3 consists of infinite layers of Tc3Cl9 units. Both phases of technetium dichloride exhibit new structure-types that consist of infinite chains of [Tc2Cl8] units. For the technetium binary halides, we studied the metal-metal interaction by theoretical methods and magnetic measurements. The change of the electronic configuration of the metal atom from d(3) (Tc(IV)) to d(5) (Tc(II)) is accompanied by the formation of metal-metal bonds in the coordination polyhedra. There is no metal-metal interaction in TcX4, a Tc═Tc double bond is present in α/β-TcCl3, and a Tc≡Tc triple bond is present in α/β-TcCl2. We investigated the thermal behavior of these binary halides in sealed tubes under vacuum at elevated temperature. Technetium tetrachloride decomposes stepwise to α-TcCl3 and β-TcCl2 at 450 °C, while β-TcCl3 converts to α-TcCl3 at 280 °C. The technetium dichlorides disproportionate to Tc metal and TcCl4 above ∼600 °C. At 450 °C in a sealed Pyrex tube, TcBr3 decomposes to Na{[Tc6Br12]2Br}, while TcI3 decomposes to Tc metal. We have used technetium tribromide in the preparation of new divalent complexes; we expect that the other halides will also serve as starting materials for the synthesis of new compounds (e.g., complexes with a Tc3(9+) core, divalent iodide complexes, binary carbides, nitrides, and phosphides, etc.). Technetium halides may also find applications in the nuclear fuel cycle; their thermal properties could be utilized in separation processes using halide volatility. In summary, we hope that these new insights on technetium binary halides will contribute to a better understanding of the chemistry of this fascinating element.

β-Technetium dichloride: solid-state modulated structure, electronic structure, and physical properties

A second polymorph of technetium dichloride, β-TcCl2, has been synthesized from the reaction of Tc metal and chlorine in a sealed tube at 450 °C. The crystallographic structure and physical properties of β-TcCl2 have been investigated. The structure of β-TcCl2 consists of infinite chains of face sharing [Tc2Cl8] units; within a chain, the Tc≡Tc vectors of two adjacent [Tc2Cl8] units are ordered in the long-range where perpendicular and/or parallel arrangement of Tc≡Tc vectors yields a modulated structure. Resistivity and Seebeck measurements performed on a β-TcCl2 single crystal indicate the compound to be a p-type semiconductor while a magnetic susceptibility measurement shows technetium dichloride to be diamagnetic. A band gap of 0.12(2) eV was determined by reflectance spectroscopy measurements. Theoretical calculations at the density functional level were utilized for the investigation of other possible stable forms of TcCl2.