Crystal structure versus solution for two new lutetium thiocyanato complexes

Abstract

The comparative study of chemical reactivity between trivalent late actinides (An) and the lanthanides (Ln) has always been a challenging issue. For that purpose, soft donor ligands (containing for example sulfur atoms or more borderline N atoms) are known to have a relative tendency for higher interaction with the An(III) than with the Ln(III) metal cations. Consequently, thiocyanates have been envisioned as possible ligands for the selective complexation of heavy actinides (namely, Am and Cm) over lanthanides. This paper is an illustration of this approach and reports on the thiocyanate chemistry of lutetium. Three new complexes, 1, [n-(C₄H₉)₄N]₃[Lu(NCS)₄(NO₃)₂], 2, K₄[Lu(NCS)₄(H₂O)₄](NCS)₃(H₂O)₂ and for comparison 3, K₄[Nd(NCS)₄(H₂O)₄](NCS)₃(H₂O)₂ have been characterized by X-ray single crystal diffraction. In addition, dissolution of the nitrato lutetium adduct (1) in wet ethanol solvent brings some valuable information about the structure of the cation coordination sphere in solution compared to the solid crystalline state. Another point of comparison comes from the dissolution of lutetium nitrate also in wet ethanol. In both cases, the cation's coordination sphere has been probed by IR and EXAFS at the Lu L₃ edge. Additional comparison with molecular dynamic simulations of the lutetium-nitrate–ethanol (wet) system has been performed and coupled to the EXAFS data fitting. Upon dissolution of 1 as well as of lutetium nitrate, a decrease of the number of nitrate ligands has been observed. In the case of 1, a clear decrease of the number of thiocyanate ligand coordination has also been observed, leading to a strong rearrangement of the cation polyhedron from solid state to solution.