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.

Comparative Analysis of Mononuclear 1:1 and 2:1 Tetravalent Actinide (U, Th, Np) Complexes: Crystal Structure, Spectroscopy, and Electrochemistry

“…This is in agreement with the observed slight differences between the solid-state structures, which may be affected by packing effects and weak intermolecular interactions, and the more symmetrical solution structure, which was suggested by our NMR results. In the end, we believe that An–ligand coordination is comparatively relaxed and does not show considerable changes in the structural properties while changing the An 4+ , whereas coordination in the 2:1 complexes is comparatively strained and consequently exhibits greater structural rearrangements with small changes in the ionic radii (0.02 Å from U 4+ to Np 4+ ) of An 4+ …”

“…Typically, only sizeable PCS contributions are detected on nuclei remote from the metal center. From previous studies, a prolate density of unpaired electrons is expected at the An 4+ centers. − ,,− This results in a PCS field that leads to shielding PCS shifts in the direction of the z -axis of the magnetic tensor and to deshielding PCS shifts in the x , y -plane. Thus, nuclei with resonances found in the negative scale are expected to be located in the z -direction relative to the metal center and those found shifted to positive ppm values are expected close to the x , y -plane.…”

“…In the end, we believe that An−ligand coordination is comparatively relaxed and does not show considerable changes in the structural properties while changing the An 4+ , whereas coordination in the 2:1 complexes is comparatively strained and consequently exhibits greater structural rearrangements with small changes in the ionic radii (0.02 Å from U 4+ to Np 4+ ) of An 4+ . 28 Electrochemistry. To understand the redox properties of all synthesized compounds, cyclic voltammetry experiments were performed in an acetonitrile solvent (Figures 4 and S14 and Table 4), and the values are calculated vs Fc 0/+ .…”

“…In recent years, coordination chemistry of actinide elements has gained widespread attention due to their underexplored coordination properties as well as interesting redox and catalytic properties. − Importantly, studies on actinide chemistry mostly focus on uranyl U­(VI)­O 2 2+ , with significantly fewer studies for any other actinide or oxidation state. − Nowadays, efforts are underway to synthesize and isolate highly reactive low-valent actinide compounds, after the realization of their potential in small-molecule activation and catalysis. ,− Meanwhile, the low-valent actinide complexes are dominated by cyclopentadienyl and related ligands, emphasizing a demand to design and synthesize other organic ligand(s) to stabilize low-valent actinide complexes. ,,, In this context, di- and monoanionic Schiff base ligands are frequently explored due to the presence of a strongly coordinating aryl-oxide donor along with a chelating motif. While multiple U­(VI)­O 2 2+ , U­(IV), and Th­(IV) and fewer Np­(IV) complexes have been reported with dianionic Schiff base ligands, e.g., of salen type, ,,− uranium complexes with Schiff base ligands have been shown to promote interesting electrochemical processes owing to the noninnocent redox behavior of the ligand(s). − Therefore, there is a need for the in-depth characterization of structure, bond properties, and redox behavior of actinide complexes with nonredox innocent ligands to achieve actinide complexes of desired properties.…”

Proton-Coupled Electron Transfer at the Pu5+/4+ Couple

“…The delineation of periodic trends is critical to mapping the unique bonding − and reactivity − of the actinides. The recent renewed interest in plutonium chemistry has greatly expanded the number of well-characterized complexes. ,,− However, comparative reactivity studies across the mid-actinides are rare. ,,− Recently, we reported the redox properties of a valence series of four-coordinate neptunium complexes supported by the NPC ligand (NPC = [NP t Bu(pyrr) 2 ] − ; t Bu = C(CH 3 ) 3 ; pyrr = pyrrolidinyl). , These studies established the first Np 5+/4+ couple in a complex without a metal–ligand multiple bond (MLMB), the isolation of a 4-coordinate Np 5+ complex, and the characterization of the proton-coupled electron transfer (PCET) reactivity of this Np 5+ complex in tetrahydrofuran (THF) .…”