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Keywords:

  • Technetium;
  • Polyoxometalates;
  • Synthesis design;
  • Ethylene glycol;
  • Transfer ligand;
  • Photoreduction

Abstract

Synthesis of technetium-99 (99Tc; t1/2: 2.1 × 105 years, βmax: 253 keV) materials is of importance in studies of the nuclear fuel cycle where Tc is a major fission product (6 % thermal yield from 235U and 239Pu), in understanding radioactive tank waste composition, and in identifying 99mTc compounds for nuclear medicine imaging. One of the most useful synthetic starting materials, (NBu4)TcOCl4, is susceptible to disproportionation in water to form TcO4 and TcIV species, especially TcO2·2H2O. This unwanted reaction is especially problematic when working with ligands bearing “hard” donor atoms, such as oxygen, where the stability with the “soft” TcV=O3+ core may be low. Polyoxometalates (POMs) are such ligands. They possess defect sites with four hard oxygen atoms and show low (ca. 108) stability constants with transition metals. Tc complexes of POMs are molecular-level models for Tc metal oxide solid-state materials and can provide information on coordination and redox environments of metal oxides that stabilize low-valent Tc. In order to synthesize pure Tc POM complexes [TcVO(α1-P2W17O61)]7– (TcVO-α1) and [TcVO(α2-P2W17O61)]7– (TcVO-α2) from (NBu4)TcOCl4, we have identified strategies that minimize formation of TcIV species and optimize the formation of pure TcV species. The parameters that we consider are the amount of ethylene glycol, which is employed as a “transfer ligand” to prevent hydrolysis of (NBu4)TcOCl4, and the precipitating agent. The TcIV species that contaminates the non-optimized syntheses is likely a TcIV μ-oxido-bridged dimer [TcIV-(μ-O)2-TcIV]. We also employ a novel procedure where the α2 ligand is photoactivated and reduced (in the presence of a sacrificial electron donor) to subsequently reduce TcVIIO4 to an isolatable TcVO-α2 product that is remarkably free of TcIV.