Actinide(IV) Complexes Containing 2-Mercaptopyridyl

Actinides (An) play an important role in chemical research and environmental science related to the nuclear industry or nuclear waste repositories. Investigating their coordination chemistry can function as a tool to obtain fundamental understanding of actinide binding. Due to the radiotoxicity of actinide complexes, special care in handling those material is needed in form of working in a controlled area lab. Therefore, the understanding of complexation properties of the actinides, in particular the transuranium (TRU) elements, is lacking behind those of the d- or 4f-elements.
For the early actinides possible oxidation states typically range from +II to +VII. A suitable approach to explore fundamental physico-chemical properties of the actinides is to study series of isostructural An compounds in which the An is in the same oxidation state. Therefore, our investigations are directed towards the synthesis of actinide complexes (An = Th, U, Np, and Pu) with the f-element in the oxidation state IV, the dominant oxidation state particularly under anoxic conditions. Observed changes in e.g. the binding situation or magnetic effects along such series deliver insight into the elements’ unique electronic properties mainly originating from the f-electrons. One important question in the field of An chemistry is the degree of “covalency” in compounds across the An series, which may be addressed by systematic studies on series of An compounds, including transuranium (TRU) elements.
An complexes using ambidentate ligand systems can give valuable information on covalency differences between soft and hard donor atoms in one complex system. We therefore chose the 2-mercaptopyridyl ligand system (PyS) as monoanionic ambidentate S,N-moiety. Due to possible resonance structures in the ligand backbone, the negative charge can be distributed over both donor atoms. This leaves sufficient flexibility for PyS to coordinate to the actinide atom in a S- or N-monodentate or S,N-chelating manner. We synthesized a series of PyS-containing Th, U, Np, and Pu complexes and compare their structural and spectroscopic characteristics along the early actinides. These results are used as a basis to further analyze bonding trends along the actinide series by means of quantum chemical calculations.
From the results, trendlines along the actinides were obtained, which shed some light in the ongoing debate of covalency in actinide bonding.