How 5 f Electron Polarisability Drives Covalency and Selectivity in Actinide N-Donor Complexes

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) (H2L, H2pyren). Structural data from SC-XRD analysis reveal [An(pyren)2] complexes with different An−Nimine versus An−Npyrrolide 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) (H2salen)-based complexes [An(salen)2] displays, on average, almost equal electron sharing of pyren or salen with the AnIV, pointing to a potential ligand-cage-driven complex stabilisation. This is shown in the fixed ligand arrangement of pyren and salen in the respective AnIV complexes. The overall bond strength of the pure N-donor ligand pyren to AnIV (An=Th, U, Np, Pu) is slightly weaker than to salen, with the exception of the PaIV complex, which exhibits extraordinarily high electron sharing of pyren with PaIV. Such an altered ligand preference within the early AnIV series points to a specificity of the 5f1 configuration, which can be explained by polarisation effects of the 5 f electrons, allowing the strongest f electron backbonding from PaIV (5f1) to the N donors of pyren.