A series of tetravalent Pu, Np, U and Th complexes of a salen type ligand

Fundamental actinide (An) coordination chemistry is still rather scarcely explored though it can provide a deep insight into the bonding situation and bonding trends across the An series. Characteristic of actinides is their huge variety of possible oxidation states, typically ranging from +II to +VII for early An. A suitable approach to explore fundamental physical-chemical properties of actinides is to study series of isostructural An compounds in which the An possesses the same oxidation state. Changes in e.g. the binding situation or magnetic effects among the An series allow insights into their unique electronic properties mainly originating from the 5f-electrons. The tetravalent actinides (An(IV)) are particularly suitable for this kind of systematic studies, as this is the largest accessible series within the early actinides.
Against this background, we performed the current study focusing on a systematic comparison of isostructural An(IV) complexes of Th, U, Np and Pu with a salen type ligand (H2L).
All syntheses and characterizations are conducted under inert, water-free nitrogen atmosphere. SC-XRD results prove that an isostructural complex series was achieved with a molecular unit where two ligands coordinate tetradentately to the An with all oxygen and nitrogen donor atoms. The resulting eightfold coordination environment exhibits a distorted square antiprismatic coordination geometry around the An center. Moreover, the relevant complexes are characterized in solution by NMR spectroscopy displaying characteristic paramagnetic effects according to the unpaired f-electrons. Interestingly, the paramagnetic contributions to the 1H and 13C NMR chemical shifts reach their maximum with [NpL2], and are drastically lower for [PuL2]. The acquired experimental results are further supported by quantum chemical calculations to study the electronic structure of the complexes.