Accessibility of Uranyl–Plutonium Complex Supported by a Polypyrrolic Macrocycle: An Implication for Experimental Synthesis

Abstract

The reaction of (THF)(H 2 L)(U VI O 2 ) (L is a tetra-anion of polypyrrolic macrocycle) with An III Cp 3 (Cp = cyclopentadienyl) afforded two intriguing cation−cation interaction (CCI) complexes (i.e., uranyl-Np and -U), but did not yield the uranyl−Pu analogue. To complement and extend experimental results, a scalar relativistic density functional theory has been performed on the formation reactions and various relevant properties of (THF)(A 2 L)(OUO)−An(CpX) 3 (A = Li and H; An = Pu, Np, and U; X = Me, H, Cl, and SiMe 3 ). Inspired by a strategy that improves uranyl precursor reactivity, we utilized (THF)-(Li 2 L)(U VI O 2 ) instead to gain a uranyl−Pu complex. Reaction free energy is reduced even to be negative (i.e., undergoing an exergonic process), which provides the thermodynamic possibility for experimental synthesis. This manner is further rationalized by the lithiated precursor showing the increased Li−O endo bond, uranium oxidation ability (VI → V), and exo−oxo basicity, as well as the lithiated uranyl−Pu product having more amount of electron transfer and a stronger O exo −Pu bond (i.e., representing the CCI). Electronic structures and electron-transfer analyses reveal a U V −Pu IV oxidation state for the new complex. Applying the more reactive lithiated precursor also decreases the formation reaction energies of uranyl−An (An = Np and U) complexes. The second strategy via exploiting substituted Cp to raise the reactivity of the plutonium reactant does not work well.

Relativistic DFT Probe for Reaction Energies and Electronic/Bonding Properties of Polypyrrolic Hetero-Bimetallic Actinide Complexes: Effects of Uranylendo-Oxo Functionalization

“…The QTAIM parameters have been found to be reliable and accurate for the characterization of actinide and ligand/actinide bonds. ,,,,, Generally, the type of chemical bond could be classified according to QTAIM values (au.) at corresponding BCPs. ,, The values of the electron density ρ­( r ) and the energy density H ( r ) at the BCP were used to identify the character of the bonding interactions according to the Cremer–Kraka criterion. , When H ( r ) < 0, ρ­( r ) > 0.15, and ∇ 2 ρ­( r ) < 0, the bond is classified as a typical covalent one (shared interactions); ,, in this case, if ∇ 2 ρ­( r ) > 0, it belongs to a strong but polarized covalent bond. In contrast, the ionic (closed shell) interactions have parameters of H ( r ) > 0, ρ­( r ) > 0, and ∇ 2 ρ­( r ) > 0.…”

“…For instance, the CCI complexes have been constructed previously in cases of uranyl­(VI) with either actinyl or actinide ion in solution and the solid state. − Theoretically, fabricating homo- and heterovalent bimetallic actinide CCI complexes that contain multiple oxidation states facilitates the exploration of structures, bonding properties, and reactivity of actinides. More importantly, this is of great importance to simulate the unique but extremely complicated phenomenon of co-existence of multiple oxidation states in the nuclear fuel cycle and moreover offers some rationalizations. − …”

“…The versatile Schiff-base polypyrrolic macrocycle H 4 L prepared by the groups of Sessler and Love is able to accommodate actinide, lanthanide, and transition metal atoms. − Due to the high Lewis basicity of -yl oxygen atoms of AnO 2 + (An = U, Np, and Pu), pentavalent actinyl ions more commonly form CCI complexes. ,,− Differential uranyl­(V) complexes of a Schiff-base polypyrrolic macrocycle in which there is oxo-group bonding between uranium and the metal cations from groups 1, 2, 4, and 12 have been obtained . In comparison with U­(VI)-U­(V), the Lewis basicity of uranyl­(VI) is higher in U­(VI)-U­(IV) hetero-binuclear uranium one, which makes it quite an appropriate candidate to construct CCI complexes.…”

“…For example, bimetallic actinide CCI complexes K 2 [(U VI O 2 ) 2 (μ-O 2 )­(L)], [{(Me 3 Si)­OU V (μ-O)} 2 (L)], and (THF)­(H 2 L)­(OU VI O)­(An′) III Cp 3 (An′ = U and Np) have been experimentally obtained. Moreover, the actinyl–actinide complexes (THF)­(H 2 L)­(OAnO)­(An′)­Cp 3 (An and An′ = U, Np and Pu) where the exo -oxo atom was functionalized by the actinide ion have been extensively investigated using relativistic density functional theory (DFT). , However, uranyl–actinide CCI complexes for encapsulating either the actinide ion or actinyl in the empty pocket of the experimental known (THF)­(U VI O 2 )­(H 2 L) ( M – U VI ), i.e., the uranyl endo -oxo functionalization mode, could not give satisfactory results. , Furthermore, despite the successfully predicted uranium-modified endo -oxo binuclear uranyl [(THF)­(OU VI OU IV )­(THF)­(L)] 2+ complex by our previous DFT calculations, the calculated formation reaction energies Δ r G (sol) were very high . It was shown that 77.9 kcal/mol Δ r G (sol) was required to obtain [(THF)­(OU VI OU IV )­(THF)­(L)] 2+ , which is far higher than 3.2 kcal/mol of the experimentally synthesized M – U VI .…”

“…Moreover, the actinyl−actinide complexes (THF)(H 2 L)(OAnO)(An′)Cp 3 (An and An′ = U, Np and Pu) where the exo-oxo atom was functionalized by the actinide ion have been extensively investigated using relativistic density functional theory (DFT). 18,19 However, uranyl− actinide CCI complexes for encapsulating either the actinide ion or actinyl in the empty pocket of the experimental known (THF)(U VI O 2 )(H 2 L) (M−U VI ), i.e., the uranyl endo-oxo functionalization mode, could not give satisfactory results. 32,33 Furthermore, despite the successfully predicted uraniummodified endo-oxo binuclear uranyl [(THF)(OU VI OU IV )-(THF)(L)] 2+ complex by our previous DFT calculations, the calculated formation reaction energies Δ r G(sol) were very high.…”