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How 5f electron polarizability drives covalency and selectivity in actinide N-donor complexes

Köhler, L.; Patzschke, M.; Schmidt, M.; Stumpf, T.; März, J.

We report a series of isostructural tetravalent actinide (Th, U-Pu) complexes with the N-donor ligand N,N'-ethylene-bis((pyrrole-2-yl)methanimine) (H₂ L, H₂ pyren). Structural data from SC-XRD analysis reveal [An(pyren)₂ ] complexes with different An–N(imine) vs. An–N(pyrrolide) bond lengths, respectively. Quantum chemical calculations elucidate the bonding situation, including differences in covalent character of the coordinative bonds. A comparison to the intensely studied analogous N,N′‐ethylene‐bis(salicylideneimine) (H₂ salen) based complexes [An(salen)₂ ] displays on average almost equal electron sharing of pyren or salen with the An(IV), pointing to a potential ligand-cage-driven complex stabilization. This is shown in the fixed ligand arrangement of pyren and salen in the respective An(IV) complexes. The overall bond strength of the pure N-donor ligand pyren to An(IV) (An=Th, U, Np, Pu) is slightly weaker compared to salen, with the exception of the Pa(IV) complex, which exhibits extraordinarily high electron sharing of pyren with Pa(IV). Such an altered ligand preference within the early An(IV) series points to a specificity of the 5f¹ configuration, which can be explained by polarization effects of the 5f electrons, allowing strongest f electron backbonding from Pa(IV) (5f¹ ) to the N-donors of pyren.

Keywords: actinides; N donor ligands; bonding analysis; f electrons; pyrrole

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