Recent Advances in Technetium Halide Chemistry

Abstract

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...

Crystal and Magnetic Structures in Layered, Transition Metal Dihalides and Trihalides

“…The two exceptions are the heavier transition metal compounds MoCl 3 and TcCl 3 , in which the net is broken into dimers that are separated from one another by a distance about one Angstrom longer than their intradimer distance. Metal-metal bonding in Tc halides including layered (β) TcCl 3 is discussed in [86].…”

Fusion-Based Neutron Generator Production of Tc-99m and Tc-101: A Prospective Avenue to Technetium Theranostics

“…Although, Re and Tc share many chemical similarities, it is well-known that some pertinent physicochemical properties, such as redox potentials and predominant oxidation states, do differ [ 77 ], making the exchange of one for the other not so straightforward [ 78 ]. Even in simple systems, such as the binary halides, divergence between the chemistry of the two elements has often been observed [ 79 ]. Thus, the availability of a radiotherapeutic/theranostic isotope of Tc would serve as a powerful tool and provide better certainty under these scenarios.…”

Hydrothermal synthesis and solid-state structures of polynuclear technetium iodide compounds

“…[3] As part of an effort to expand our knowledge of technetium metal-metal bond chemistry, we prepared seven new binary halides (TcBr 4 , TcBr 3 , -TcCl 3 , -TcCl 2, and TcI 3 ). [4][5][6][7][8][9][10] The oxidation state of the Tc atom likely determines the extent to which Tc-Tc bonding occurs. In the tetrahalide TcBr 4 , there is no discernable metal-metal interaction.…”

“…[7,8] Thus, new technetium metal-metal bonded dimers with carboxylate ligands are of particular interest as precursors for the preparation of new technetium binary halides, TcX y (X = F, Cl, Br, I; y = 2, 3, 4). [9,10] The study of Tc 2 (O 2 CCH 3 ) 2 Cl 4 also permitted to better understand the influence of acetate ligand on the Tc-Tc bonding. [11] Currently, only five compounds containing a Tc 2 n+ (n = 5, 6) units coordinated to acetate and halogen ligands are structurally characterized: Tc 2 (O 2 CCH 3 ) 4 X [12,13] , K[Tc 2 (O 2 CCH 3 ) 4 Cl 2 ] and Tc 2 (O 2 CCH 3 ) 4 X 2 (X = Cl, Br), [14,15] and no complexes with multiple metal-metal bonds coordinated to iodide ligands have been reported yet.…”