Nuclear scientists have recently investigated thorium for the prospective conversion of fertile 232Th to 233U in breeder reactors. The advantages of using breeder fuel, such as 232Th, over fissile materials (i.e., 235U) arises from its greater abundance, reduced waste, better safeguarding, and improved reactor life. (1) Moreover, unlike other early actinides (An), thorium has a unique stable tetravalent oxidation state, exhibited by most of its compounds. Therefore, it is used as a surrogate to probe the chemistries of less abundant, more radioactive, but critical transuranic actinides including Np(IV) and Pu(IV). The last 2 decades has yielded structural insights into Th(IV) coordination and speciation. (2) Specifically relevant to the current study includes inorganic Th-oxoanion complexes with halogens (3−6) borate, (7) sulfate, (4,8−11) molybdate, (12,13) chromate, (14) nitrate, (4) selenate, (15,16) and arsenate; (17) assembled as oxoclusters, chains, sheets, and frameworks. (2,8) In addition to the specific knowledge of aqueous thorium complexation, structure, and reactivity; studies are carried out to expand our knowledge of trends across the f-block; and for example comparing structure and bonding of Ce(IV), Th(IV), U(IV), Np(IV), and Pu(IV). (10)

99Tc is one of the highest yields and longest-lived radioactive decay products of 235U fission. Its stable oxoanion TcVIIO4– can likewise complex hard metal cations, similar to phosphate and sulfate, despite the monovalent charge. It was estimated in the late 1990s that upward of 2000 kg of 99Tc was contained in the alkaline Hanford tank wastes. (18) The majority of 99Tc can be removed from alkaline nuclear wastes via solvent extraction as pertechnetate. However, a fraction exists in lower oxidation states that resists extraction, presumably reduced and stabilized in water by organic complexants. (19,20) Technetium is also problematic in the reprocessing of nuclear fuel. The plutonium uranium redox extraction (PUREX) process involves dissolution of spent nuclear fuel (U and/or Pu oxides) in nitric acid, followed by extraction of the fuel elements in their highest oxidation state (i.e., UVIO22+, uranyl) into an organic phase, leaving behind the fission products and other non-radioactive components of fuel. Co-extraction of Tc with actinides and Zr(IV) (both as a radioactive fission product and non-radioactive fuel cladding) is a considerable challenge of the PUREX process, presumably by forming coordination complexes between hard metal cations and pertechnetate. (21) For example, it was prior-observed that TcO4–/ReO4– can co-ligate Th(IV), U(IV), and U(VI) along with organophosphorus ligands that are representative of the extractants used in the PUREX process. (22−24) These include tributyl phosphate (TBP), triethyl phosphate (TEP), tri-iso-butyl phosphate (TiBP), and tri-n-butyl phosphine oxide (TBPO).

Equally important but less-studied is the An–TcO4– heterometallic species that form in aqueous acid prior to extraction into the organic phase. Formation of complexes could enhance extraction, prevent extraction, promote precipitation, or prevent precipitation. For example, hard metal cations by themselves polymerize, (25,26) even in highly acidic solution. However, complexation by oxoanions, such as TcO4–, could hinder polymerization. Several earlier examples of inorganic actinide complexes with TcO4–/ReO4– have been reported. For example, Fedosseev and colleagues (27) shown by IR that aqueous phase interactions between Np(V/VI) and Pu(V) with TcO4– are weak in solution, but strong in the solid state. Only one structure was obtained from this study, featuring NpO22+ cations in pentagonal bipyramidal coordination, linked into layers via bridging TcO4–. (27) Karimova and Burns crystallized several complexes containing uranyl monomers, dimers, and trimers complexed with perrhenate. (28) These complexes of higher oxidation state actinides, such as uranyl, neptunyl, and plutonyl, with TcO4–/ReO4– are indeed relevant to both nuclear fuel reprocessing and legacy nuclear wastes. The PUREX process exploits the linear actinyl oxocations for selective extraction, while the highly alkaline conditions of tank wastes stabilize high oxidation states in solution
.
Lower oxidation state actinides (i.e., tetravalent) are also relevant in nuclear fuel reprocessing and the environment. AnIV is generally insoluble in the environment, and it is the dominant oxidation state in oxide nuclear materials. To our knowledge, the only crystal structure evidencing coordination of TcO4–/ReO4– with tetravalent actinides, such as Th(IV), is a monomeric complex, also ligated with triphenylphosphine oxide as a surrogate for the TPB ligand used in the PUREX process. (23) Thus, atomic level knowledge of An(IV)–TcO4–/ReO4–, including Th(IV), without inclusion of organic ligands, is lacking. This is important for both fundamental understanding of trends in M(IV)–TcO4–/ReO4– bonding (M = Zr, Hf, Ce, Th, U, Np, and Pu), as well as nuclear materials management in all stages of the nuclear fuel cycle. The relevance of Th–ReO4/TcO4 phases inspired earlier researchers to attempt crystallization of representative species, but only amorphous materials were obtained. (29,30) Sutton et al. 2006 described synthesizing a thorium perrhenate/pertechnetate compound formulated as [Th(MO4)4]·4H2O (M = Re/Tc). The formulae were determined by elemental analysis, but no crystal structures were reported. (23) This inorganic compound was then used to synthesize a series of isostructural complexes of the general formula [Th(MO4)4(L)4], where M = Re/Tc and L = TEP/TiBP/TBPO, which helped to elucidate possible key steps for the coextraction of actinides with pertechnetate in the PUREX process. (23)

Using a very simple synthetic approach of dissolution of freshly precipitated amorphous thorium hydroxide in pertechnic acid or perrhenic acid, we have isolated and structurally described nine extended compounds by direct complexation of perrhenate/pertechnetate with Th(IV). Structures are described by single-crystal X-ray diffraction (SCXRD), and further characterization includes Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD).

2.6. Synthesis of [Th2(OH)2(TcO4)6(H2O)6]·2H2O-Chain

Thorium hydroxide (0.25 mmol) was suspended in ∼1 mL of deionized water inside a 20 mL glass scintillation vial. Then, HTcO4 (2 mL, ∼1 mmol) was added dropwise in the suspension using a 1 mL plastic transfer pipette. The suspension was kept in constant stirring at 1000 rpm during the addition of acid. The resulting clear solution inside the vial was allowed to evaporate at room temperature. Plate-shaped crystals of [Th2(OH)2(TcO4)6(H2O)6]·2H2O-chain (ThTc3-chain-4) formed after 14 days when all the mother liquor had evaporated.