Bonding Trends Traversing the Tetravalent Actinide Series: Synthesis, Structural, and Computational Analysis of An^IV(^Aracnac)₄Complexes (An = Th, U, Np, Pu;^Aracnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-^tBu₂C₆H₃)

Abstract

A series of tetravalent An(IV) complexes with a bis-phenyl β-ketoiminate N,O donor ligand has been synthesized with the aim of identifying bonding trends and changes across the actinide series. The neutral molecules are homoleptic with the formula An((Ar)acnac)(4) (An = Th (1), U (2), Np (3), Pu (4); (Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)) and were synthesized through salt metathesis reactions with actinide chloride precursors. NMR and electronic absorption spectroscopy confirm the purity of all four new compounds and demonstrate stability in both solution and the solid state. The Th, U, and Pu complexes were structurally elucidated by single-crystal X-ray diffraction and shown to be isostructural in space group C2/c. Analysis of the bond lengths reveals shortening of the An-O and An-N distances arising from the actinide contraction upon moving from 1 to 2. The shortening is more pronounced upon moving from 2 to 4, and the steric constraints of the tetrakis complexes appear to prevent the enhanced U-O versus Pu-O orbital interactions previously observed in the comparison of UI(2)((Ar)acnac)(2) and PuI(2)((Ar)acnac)(2) bis-complexes. Computational analysis of models for 1, 2, and 4 (1a, 2a, and 4a, respectively) concludes that both the An-O and the An-N bonds are predominantly ionic for all three molecules, with the An-O bonds being slightly more covalent. Molecular orbital energy level diagrams indicate the largest 5f-ligand orbital mixing for 4a (Pu), but spatial overlap considerations do not lead to the conclusion that this implies significantly greater covalency in the Pu-ligand bonding. QTAIM bond critical point data suggest that both U-O/U-N and Pu-O/Pu-N are marginally more covalent than the Th analogues.

Theoretical Study of Plutonium(IV) Complexes Formed within the PUREX Process: A Proposal of a Plutonium Surrogate in Fire Conditions

“…All these quantities are collected in Table 5. For all complexes the metal−TBP and metal−nitrate bonds the values of ρ b range from 0.051 to 0.060 e − /bohr 3 , below the value 0.08 e − /bohr 3 calculated for the ionic LiF molecule, suggesting only minor 52 From an electron density viewpoint there is a strong similarity between the electronic densities in the Pu(IV) and Ce(IV) complexes, as there is between the metal−ligand bond distances and metal−ligand binding energies, suggesting that Ce(IV) can be used as a potential surrogate for Pu(IV).…”

Should environmental effects be included when performing QTAIM calculations on actinide systems? A comparison of QTAIM metrics for Cs2UO2Cl4, U(Se2PPh2)4 and Np(Se2PPh2)4 in gas phase, COSMO and PEECM

“…This is due partly to the experimental difficulties associated with obtaining high quality experimental electron densities on radioactive systems featuring very heavy elements, and partly a function of the QTAIM only recently being extended to quantum chemical studies of the actinide elements. Indeed, we were the first group to extensively employ the QTAIM in this capacity, and have used it to study both covalency [6][7][8][9][10][11][12][13][14][15][16][17][18] and bond strength [19][20][21] in a range of molecular f element systems.…”

Computational study of An–X bonding (An = Th, U; X = p-block-based ligands) in pyrrolic macrocycle-supported complexes from the quantum theory of atoms in molecules and bond energy decomposition analysis

“…31 Finally for our pruposes, the delocalisation index between atoms A and B δ(A,B) gives the number of electrons shared between two atomic basins and is a measure of the bond order between two atoms. 32 The QTAIM has been previously applied to a range of actinide-ligand and other metal-ligand bonds 18,[33][34][35][36][37][38][39] and it has been concluded that while An bonding is predominantly ionic, covalency differences across the 5f series can be distinguished.…”