Paramagnetic Properties of [AnIV(NO3)6]2- Complexes (An = U, Np, Pu) Probed by NMR Spectroscopy and Quantum Chemical Calculations

Actinide +IV complexes with six nitrates [AnIV(NO3)6]2- (An = Th, U, Np, and Pu) have been studied by 15N and 17O NMR spectroscopy in solution and first-principles calculations. Magnetic susceptibilities were evaluated experimentally using the Evans method and are in good agreement with the ab initio values. The evolution in the series of the crystal field parameters deduced from ab initio calculations is discussed. The NMR paramagnetic shifts are analyzed based on ab initio calculations. Because the cubic symmetry of the complex quenches the dipolar contribution, they are only of Fermi contact origin. They are evaluated from first-principles based on a complete active space/density functional theory (DFT) strategy, in good accordance with the experimental one. The ligand hyperfine coupling constants are deduced from paramagnetic shifts and calculated using unrestricted DFT. The latter are decomposed in terms of the contribution of molecular orbitals. It highlights two pathways for the delocalization of the spin density from the metallic open-shell 5f orbitals to the NMR active nuclei, either through the valence 5f hybridized with 6d to the valence 2p molecular orbitals of the ligands, or by spin polarization of the metallic 6p orbitals which interact with the 2s-based molecular orbitals of the ligands.

Contrasting impact of coordination polyhedra and site symmetry on the electronic energy levels in nine-coordinated Eu(III) and Sm(III) crystals structures determined from single crystal luminescence spectra

Lanthanide luminescence is characterised by "forbidden" 4f-4f transitions and a complicated electronic structure. Our understanding of trivalent lanthanide(III) ion luminescence is centered on Eu3+ because absorbing and emitting transitions in Eu3+ occur from a single electronic energy level. In Sm3+ both absorbing and emitting multiplets have a larger multiplicity. A band arising in transitions from the first emitting state multiplet to the ground state multiplet will have nine lines for a Sm3+ complex. In this study, high-resolution emission and excitation spectra were used to determine the electronic energy levels for the lowest multiplet and first emitting multiplet in four Sm3+ compounds with either tricapped trigonal prismatic TTP or capped square antiprismatic cSAP coordination polyhedra but different site symmetry. This was achieved by the use of Boltzmann distribution population analysis and experimentally determined transition probabilities from emission and excitation spectra. Using this analysis it was possible to show the effect of changing three oxygen atoms with three nitrogen atoms in the donor set for two compounds with the same coordination polyhedra and site symmetry. This work celebrates the 40th anniversary of Kirby and Richardson's first report of [Eu(ODA)3]3- luminescence.

Bonding and Magnetic Trends in the [AnIII(DPA)3]3- Series Compared to the Ln(III) and An(IV) Analogues

The crystal field parameters are determined from first-principles calculations in the [AnIII(DPA)3]3- series, completing previous work on the [LnIII(DPA)3]3- and [AnIV(DPA)3]2- series. The crystal field strength parameter follows the Ln(III) < An(III) < An(IV) trend. The parameters deduced at the orbital level decrease along the series, while J-mixing strongly impacts the many-electron parameters, especially for the Pu(III) complex. We further compile the available data for the three series. In some aspects, An(III) complexes are closer to Ln(III) than to An(IV) complexes with regard to the geometrical structure and bonding descriptors. At the beginning of the series, up to Pu(III), there is a quantitative departure from the free ion, especially for the Pa(III) complex. The magnetic properties of the actinides keep the trends of the lanthanides; in particular, the axial magnetic susceptibility follows Bleaney's theory qualitatively.

Structural and Bonding Analysis in Monomeric Actinide(IV) Oxalate from Th(IV) to Pu(IV): Comparison with the An(IV) Nitrate Series

Single-crystal X-ray diffraction (SC-XRD) structures and Raman spectra of a series of new isomorphous molecular An(IV)-oxalate compounds (Th, U, Np, and Pu) are reported. These complexes are crystallized with cobalt(III) hexamine ([Co(NH₃)₆]³⁺) as the counter cations, [Co(NH₃)₆]₂[An(C₂O₄)₅]·4H₂O, revealing five bidentate nonbridging oxalate ligands in the first coordination sphere (CN = 10). The nonbridging oxalate is rather uncommon for An(IV)-oxalate systems, which are widely characterized as polymeric compounds. Density functional theory (DFT) calculations were performed to examine the bonding between An(IV) cations and oxalate ligands. For comparison, we also report results obtained for the An(IV)-hexanitrate series, [(C₂H₅)₄N]₂[An(NO₃)₆] (with An = Th, U, Np, Pu, and Ce), which consists of O-donor ligands as well but with a larger coordination number (CN = 12). The bonding analysis confirms that the actinide-oxygen bond is predominantly ionic with a minor increase in covalency from Th to U and slight variations from U to Pu. Further comparison showed that the charge transfer increases slightly when increasing the number of anions in the coordination sphere (C₂O₄^2-: CN = 10; NO₃^-: CN = 12), but covalent effects as indicated by the amount of internuclear electron density accumulation are small and similar for oxalate and nitrate.

Dipolar and Contact Paramagnetic NMR Chemical Shifts in AnIV Complexes with Dipicolinic Acid Derivatives

Actinide +IV complexes (An^IV = Th^IV, U^IV, Np^IV, and Pu^IV) with two dipicolinic acid derivatives (DPA and Et-DPA) have been studied by ¹H and ¹³C NMR spectroscopies and first-principles calculations. The Fermi contact and dipolar contributions to the actinide-induced shifts (AIS) are evaluated from a temperature dependence analysis, combined with ab initio results. It allows an experimental estimation of the axial anisotropy of the magnetic susceptibility Δχ_ax and of the hyperfine coupling constants of the NMR-active nuclei. Due to the compactness of the coordination sphere, the magnetic anisotropy of the paramagnetic center is small, and this makes the contact contribution to be the dominant one, even on the remote atoms. The sign of the hyperfine coupling constants and related spin densities is alternating on the nuclei of the ligand cycle, denoting a preponderant spin polarization mechanism. This is well reproduced by unrestricted density functional theory (DFT) calculations. Those values are furthermore slightly decreasing in the actinide series, which indicates a small decrease of the covalency from U^IV to Pu^IV.

Deciphering the Role of Symmetry and Ligand Field in Designing Three-Coordinate Uranium and Plutonium Single-Molecule Magnets

Actinide single-molecule magnets (SMMs) have gained paramount interest in molecular magnetism as they offer a larger barrier height of magnetization (U_eff) reversal compared to the lanthanide analogue, thanks to their greater metal-ligand covalency. However, the reported actinide SMMs to date yield a relatively smaller U_eff as there is no established design principle to enhance U_eff values. To address this issue, we have employed ab initio CASSCF/CASPT2/NEVPT2 calculations to study a series of three-coordinate U³⁺ and Pu³⁺ SMMs. To begin with, we have studied two experimentally characterized U³⁺ ion-field-induced SMMs, namely, planar [U{N(SiMe₂^tBu)₂}₃] (1) and pyramidal [U{N(SiMe₃)₂}₃] (2) complexes reported earlier. Both the complexes were found to stabilize m_J = |±1/2⟩ as the ground state with a very strong quantum tunneling of magnetization (QTM), rendering them unsuitable for SMMs. Our calculations reveal that in the pyramidal geometry (such as in 2), the energy of the 5f²6d¹ state is lowered compared to the planar geometry (as in 1), resulting in a slightly better SMM characteristic in the former. To unravel the effect of symmetry in magnetic properties, ab initio calculations were performed on two reported T-shaped complexes [U(NSiⁱPr₂)₂(I)] (3) and [U(NHAr^iPr6)₂I] (4, Ar^iPr6 = 2,6-(2,4,6-iPr₃C₆H₂)₂C₆H₃). Quite interestingly, m_J = |±9/2⟩ is found to be the ground state for both the complexes with a blocking barrier exceeding 900 cm^-1. Furthermore, to decipher the effect of the transuranic element in magnetic anisotropy, ab initio calculations were extended to the Pu analogue of 2, [Pu{N(SiMe₃)₂}₃] (5), which yields a record-breaking blocking barrier of ∼1933 cm^-1. Among the three-coordinate geometries studied, the pyramidal geometry was found to offer substantial magnetic anisotropy for Pu³⁺ ions, while a T-shaped geometry is best suited for U³⁺ ions. While the chosen theoretical protocols' overestimation of barrier height cannot be avoided, these values are still several orders of magnitude larger than the U_eff values reported for any actinide SMMs and unveil a design principle for superior three-coordinate actinide-based SMMs.

Gas-phase structure, bonding, and fragmentation chemistry of the An (IV)-TMPDCAM complexes studied using mass spectrometry and theoretical calculation (An = Th and U)

RATIONALE

Pyridine-2,6-dicarboxamides (PDCAMs) exhibit a certain extraction ability for tetravalent actinide ions, but quite limited information regarding the structures and reactivities of the corresponding An⁴⁺ -PDCAMs complexes is available. Neutral diamides can form multiply charged complexes with lanthanide and actinide cations, which are well suited for gas-phase investigations using electrospray ionization (ESI) mass spectrometry in conjunction with theoretical calculation.

METHODS

Binary Th (TMPDCAM)₃ ⁴⁺ /U (TMPDCAM)₃ ⁴⁺ (TMPDCAM = N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide) complexes were generated in the gas phase via ES) of Th (ClO₄ )₄ /U (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile; collision-induced dissociation (CID) was employed to reveal their fragmentation behaviors; the structure and bonding were investigated by density functional theory (DFT) calculation.

RESULTS

An (TMPDCAM)₃ ⁴⁺ (An = Th and U) tetracations dominated the ESI mass spectra of An (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile. DFT calculations indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes have C₃ geometry, and the bonding analyses demonstrate that the thorium or uranium center interacts with both O_carbonyl and N_pyridine , but the latter is weaker. CID of Th (TMPDCAM)₃ ⁴⁺ generated a series of multiply charged thorium-containing products via bond cleavages of the TMPDCAM ligand, whereas U (TMPDCAM)₃ ⁴⁺ yielded only oxygen extraction product UO (TMPDCAM)²⁺ and hydrolysis product UO (OH)⁺ .

CONCLUSION

An⁴⁺ (An = Th and U) can form stable tetrapositive complexes in the gas phase on coordination of three neutral TMPDCAM ligands. The structure and bonding analyses indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes possess twisted tricapped trigonal prismatic geometry and the An⁴⁺ centers are coordinated by six O_carbonyl and three N_pyridine atoms while the interactions between An⁴⁺ and O_carbonyl are stronger. The fragmentation chemistry of Th (TMPDCAM)₃ ⁴⁺ and U (TMPDCAM)₃ ⁴⁺ is quite different from each other, which reveals that the gas-phase chemistry of quadruply charged actinide-diamide complexes is affected by the metal centers with distinct properties.