Abstract
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) (H(2) L, H(2) pyren). Structural data from SC-XRD analysis reveal [An(pyren)(2) ] complexes with different An-N(imine) versus An-N(pyrrolide) bond lengths. Quantum chemical calculations elucidated the bonding situation, including differences in the covalent character of the coordinative bonds. A comparison to the intensely studied analogous N,N'-ethylene-bis(salicylideneimine) (H(2) 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 stabilisation. 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 than 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 5f(1) configuration, which can be explained by polarisation effects of the 5 f electrons, allowing the strongest f electron backbonding from Pa(IV) (5f(1) ) to the N donors of pyren.