Synthetic Chemistry of f-Elements and Their Coordination Chemistry with Organic Ligands
Synthetic chemistry of actinides (An) is still an unexplored research field, because not only of their high radioactivity but also of their limited amount as a material source. It is also well known that An can possess a wide range of oxidation states from II to VII, making the synthetic chemistry of An trickier and more complicate than that of other transition elements. Amongst the wide range of oxidation states of An, our current research activities are focusing particularly on their tetravalent state (An(IV)), which is predominant primarily under anaerobic conditions such as the geochemical conditions expected in the deep geological repositories for radioactive wastes. The synthetic and coordination chemistry of An(IV) has been much less explored as compared with that of An(VI) and –(V), mainly due to their instability under normal atmospheric conditions.
One of our first attempts to initiate the synthetic chemistry of An(IV) are to establish a research capability to synthesize starting compounds of An(IV) in a routine manner. Here is one example to prepare anhydrous chloride salts with different An(IV), which are versatile compounds for the subsequent research on coordination chemistry, using Schlenk technique and an inert glove box.
Figure 1: Anhydrous chloride compounds of different tetravalent actinides (From left; Th(IV), U(IV) and Np(IV), respectively).
For the successful syntheses of An(IV) complexes, the selection of ligand systems and the optimisation of the synthetic conditions are the significant factors to be considered to maintain the tetravalent state during the synthesis. Amongst a wide variety of selection of organic ligands, our research activities are focusing particularly on naturally relevant organic ligands with O- and/or N-donor atoms, such as carboxylic acids, amides or Schiff bases (see Figure 2). The properties of the complex can be manipulated specifically by modifying the ligand structure and by changing the substituents. Furthermore, a series of complexes with different An(IV) and different ligands allows us to perform systematic investigations to reveal the correlations between the complex structure and the physical/chemical properties. The obtained results will further help develop model systems for more complex systems, such as biological- and geological systems relevant to the geological waste repositories.
Figure 2: Typical organic ligands used in our research activities.
Figure 3: Inert glove box for synthesis and characterisation of redox- and water-sensitive actinide compounds.
Core techniques involved
- Single-crystal X-ray diffraction
- Powder X-ray diffraction
- UV-visible-NIR absorption spectroscopy
- FT-IR spectroscopy
- Quantum chemical calculations
- FENABIUM, Project No. 02NUK046B