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.

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.

Are Actinyl Cations Good Probes for Structure Determination in Solution by NMR?

We report on NMR spectroscopy, CAS-based method calculations, and X-ray diffraction of AnV and AnVI complexes with a neutral and slightly flexible TEDGA ligand. After checking that pNMR shifts mainly arise from pseudocontact interactions, we analyze pNMR shifts considering the axial and rhombic anisotropy of the actinyl magnetic susceptibilities. The results are compared to those of a previous study performed on [AnVIO2]2+ complexes with dipicolinic acid. It is shown that 5f2 cations (PuVI and NpV) make very good candidates for determining the structure of actinyl complexes in solution by 1H NMR spectroscopy as shown by the invariance of the magnetic properties to the equatorial ligands, as opposed to the NpVI complexes with a 5f1 configuration.

Contribution of Molecular Dynamics in pNMR for the Structural Determination of AnV and AnVI Complexes in Solution.

In this study, we propose to use classical molecular dynamics (MD) coupled with 1H NMR spectroscopy to study the conformations of different actinyl AnVI (An = U, Np, and Pu) and AnV (An = Np) complexes with tetra-ethyl dyglicolamide (TEDGA) ligands in order to have a better representation of such complexes in solution. Molecular dynamics simulations showed its effectiveness in interpreting the experiments by the calculation of geometric factors needed for the determination of magnetic properties of these complexes. We demonstrated that different conformations of the AnV and AnVI complexes with TEDGA exist in solution with different coordination modes, which is experimentally confirmed by 1H NMR and EXAFS spectroscopies. Furthermore, MD simulations provide additional insights into the structures of complexes in solution since conformations with fast exchanges, which are not accessible from NMR experiments, have been observed by MD simulations.

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 (C2O42-: 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 (AnIV = ThIV, UIV, NpIV, and PuIV) 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 Δχax 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 UIV to PuIV.

Oligomers Intermediates in Between Two New Distinct Homonuclear Uranium(IV) DOTP Complexes*.

Two new aqueous UIV complexes were synthesized by the interaction between the tetravalent uranium cation and the (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) (DOTP) macrocyclic ligand. Two distinct homonuclear complexes were identified; the first was characterized by X-ray crystallography as a unique "out-of-cage", [U(DOTPH6 )2 ] complex, in which the UIV cation is octa-coordinated to 4 phosphonic arms from each ligand in a square anti-prism geometry, with a C4 symmetry. The second is the "in-cage" [U(DOTPH4 )] complex, in which the tetravalent cation is located between the macrocycle O4 and N4 planes. With the help of UV-Vis absorption, 1 H/31 P NMR, ATR-IR, and MALDI-TOFMS analytical techniques, the chemical interchange between both species is presented. It is shown that the one-way transition is governed by the formation of a multiple number of soluble oligomeric species consisting of varied stoichiometric ratios of both characterized homonuclear complexes.

Temperature Dependence of 1 H Paramagnetic Chemical Shifts in Actinide Complexes, Beyond Bleaney's Theory: The AnVI O2 2+ -Dipicolinic Acid Complexes (An=Np, Pu) as an Example.

Actinide +VI complexes ( A n V I = U V I , N p V I and P u V I ) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, N p V I and P u V I complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the 1 H paramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans' method. The SO-RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the N p V I complex. The temperature dependence of the pNMR chemical shifts has a strong 1 / T contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two-Kramers-doublet model for the N p V I complex and a non-Kramers-doublet model for the P u V I complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.

On the Aqueous Chemistry of the UIV -DOTA Complex.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of UIV with H2 O, OH- , and F- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The UIV -DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The UIV /UIII redox couple was found to be quasi-reversible for the complex with both axially bonded H2 O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron UV /UIV oxidation of the [UIV (DOTA)(H2 O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated UV cation.

Crystallographic structure and crystal field parameters in the [AnIV(DPA)3]2- series, An = Th, U, Np, Pu.

The [AnIV(DPA)3]2- series with An = Th, U, Np, Pu has been synthesized and characterized using SC-XRD and vibrational spectroscopy. First principles calculations were performed, the total electron density is analyzed using the Quantum Theory of Atoms in Molecules. Crystal field parameters and strength parameters are deduced following a previous work on the LnIII analog series e.g. [J. Jung et al., Chem. - Eur. J., 2019, 25, 15112]. The trends in the parameters along the series are compared to the LnIII complexes. They evidence larger covalent interactions and larger J mixing.

Derivation of Lanthanide Series Crystal Field Parameters From First Principles.

Two series of lanthanide complexes have been chosen to analyze trends in the magnetic properties and crystal field parameters (CFPs) along the two series: The highly symmetric LnZn16 (picHA)16 series (Ln=Tb, Dy, Ho, Er, Yb; picHA=picolinohydroxamic acid) and the [Ln(dpa)3 ](C3 H5 N2 )3 ⋅3H2 O series (Ln=Ce-Yb; dpa=2,6-dipicolinic acid) with approximate three-fold symmetry. The first series presents a compressed coordination sphere of eight oxygen atoms whereas in the second series, the coordination sphere consists of an elongated coordination sphere formed of six oxygen atoms. The CFPs have been deduced from ab initio calculations using two methods: The AILFT (ab initio ligand field theory) method, in which the parameters are determined at the orbital level, and the ITO (irreducible tensor operator) decomposition, in which the problems are treated at the many-electron level. It has been found that the CFPs are transferable from one derivative to another, within a given series, as a first approximation. The sign of the second-order parameter B 0 2 differs in the two series, reflecting the different environments. It has been found that the use of the strength parameter S allows for an easy comparison between complexes. Furthermore, in both series, the parameters have been found to decrease in magnitude along the series, and this decrease is attributed to covalent effects.

Insight of the Metal-Ligand Interaction in f-Element Complexes by Paramagnetic NMR Spectroscopy.

The magnetic properties of LnIII and AnIII complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometries of these complexes, a crystallographic study by single-crystal X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) in solution was performed. Several methods to separate the paramagnetic shifts observed in the NMR spectra were applied to these complexes. Methods using a number of nuclei of the dipicolinate ligands revealed an abrupt change in the geometries of the complexes and a metal-ligand interaction in the middle of the lanthanide series. A study of the variation of the paramagnetic shifts with temperature demonstrated that higher-order terms of the dipolar and contact contributions are required, especially for the lightest LnIII and almost all the studied AnIII . Bleaney's parameters a and C a D relating to the contact and dipolar terms, respectively, were deduced from experimental data and compared with the results of ab initio calculations. Quite a good agreement was found for the temperature dependencies of a and C a D . However, the C a D values obtained from cation magnetic anisotropy calculations showed some discrepancies with the values derived from Bleaney's equation defined for LnIII . Other parameters, such as the crystal field parameter and the hyperfine constants Fi obtained from the experimental data of the [An(ethyl-dpa)3 ]3- complexes (ethyl-dpa=4-ethyl-2,6-dipicolinic acid), are at odds with the assumptions underlying Bleaney's theory.

Exploring the Coordination of Plutonium and Mixed Plutonyl-Uranyl Complexes of Imidodiphosphinates.

The coordination chemistry of plutonium(IV) and plutonium(VI) with the complexing agents tetraphenyl and tetra-isopropyl imidodiphosphinate (TPIP- and TIPIP-) is reported. Treatment of sodium tetraphenylimidodiphosphinate (NaTPIP) and its related counterpart with peripheral isopropyl groups (NaTIPIP) with [NBu4]2[PuIV(NO3)6] yields the respective PuIV complexes [Pu(TPIP)3(NO3)] and [Pu(TIPIP)2(NO3)2] + [PuIV(TIPIP)3(NO3)]. Similarly, the reactions of NaTPIP and NaTIPIP with a Pu(VI) nitrate solution lead to the formation of [PuO2(HTIPIP)2(H2O)][NO3]2, which incorporates a protonated bidentate TIPIP- ligand, and [PuO2(TPIP)(HTPIP)(NO3)], where the protonated HTPIP ligand is bound in a monodentate fashion. Finally, a mixed U(VI)/Pu(VI) compound, [(UO2/PuO2)(TPIP)(HTPIP)(NO3)], is reported. All these actinyl complexes remain in the +VI oxidation state in solution over several weeks. The resultant complexes have been characterized using a combination of X-ray structural studies, NMR, optical, vibrational spectroscopies, and electrospray ionization mass spectrometry. The influence of the R-group (R = phenyl or iPr) on the nature of the complex is discussed with the help of DFT studies.

Structural Characterization of Am(III)- and Pu(III)-DOTA Complexes.

The complexation of 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) ligand with two trivalent actinides (Am3+ and Pu3+) was investigated by UV-visible spectrophotometry, NMR spectroscopy, and extended X-ray absorption fine structure in conjunction with computational methods. The complexation process of these two cations is similar to what has been previously observed with lanthanides(III) of similar ionic radius. The complexation takes place in different steps and ends with the formation of a (1:1) complex [(An(III)DOTA)(H2O)]-, where the cation is bonded to the nitrogen atoms of the ring, the four carboxylate arms, and a water molecule to complete the coordination sphere. The formation of An(III)-DOTA complexes is faster than the Ln(III)-DOTA systems of equivalent ionic radius. Furthermore, it is found that An-N distances are slightly shorter than Ln-N distances. Theoretical calculations showed that the slightly higher affinity of DOTA toward Am over Nd is correlated with slightly enhanced ligand-to-metal charge donation arising from oxygen and nitrogen atoms.

Paramagnetism of Aqueous Actinide Cations. Part II: Theoretical Aspects and New Measurements on An(IV).

The magnetic properties of actinide(IV) (An(IV)) cations are investigated in various solutions (HClO4, HCl, and HNO3) by the Evans NMR method. The magnetic susceptibilities measured in noncomplexing medium are compared with the previous studies, and the influence of the medium is verified with new measurements in complexing solutions. To rationalize these results, spin-orbit complete active space perturbation theory at second order calculations are performed on the free ions and on the aquo complexes to determine the nature of electronic states, the magnetic susceptibility, and the UV-visible-near-IR spectra. The different factors contributing to the An(IV) magnetic properties were identified. The ligand field effect on the magnetic behavior (Curie constant and temperature-independent susceptibilities) was analyzed by considering different solvation environments. These results indicate a significant effect of the zero-field splitting of the ground J manifold on the An(IV) magnetic susceptibility.

Magnetic susceptibility of actinide(iii) cations: an experimental and theoretical study.

In a previous paper, the influence of radioactive decay (α and ß(-)) on magnetic susceptibility measurements by the Evans method has been demonstrated by the study of two americium isotopes. To characterize more accurately this phenomenon and particularly its influence on the Curie law, a new study has been performed on two uranium isotopes ((238)U and (233)U) and on tritiated water ((3)H2O). The results on the influence of α emissions have established a relationship between changes in the temperature dependence and the radioactivity in solution. Regarding the ß(-) emissions, less influence was observed while no temperature dependence linked to this kind of radioactive emission could be identified. Once magnetic susceptibility measurements of actinide(iii) cations were corrected from radioactivity effects, methods of quantum chemistry have been used on free ions and aquo complexes to calculate the electronic structure explaining the magnetic properties of Pu(iii), Am(iii) and Cm(iii). The ligand field effect on the magnetic behavior (the Curie constant and temperature-independent susceptibilities) was analyzed by considering different solvation environments.

Paramagnetism of aqueous actinide cations. Part I: Perchloric acid media.

A comprehensive study of actinide cation paramagnetism in acidic aqueous solution has been completed in perchlorate media. Employing the Evans method, all the readily accessible actinide cations have been studied using our specially outfitted NMR spectrometer equipped for use with radioactive samples. The effective magnetic moments observed, ranging from 0 to 13 µB, differ from the isoelectronic lanthanides, previous solid actinide studies, and older solution studies. Actinide (IV) and (V) ions show less paramagnetic character, while some actinide (III) ions exhibit greater paramagnetic behavior than predicted from free-ion calculation. Temperature dependence of actinide magnetic susceptibilities from 5 to 80 °C are in good agreement with a Curie-like law except for U(VI), which appears to be temperature-independent. Diamagnetic behavior of Th(IV) exhibits a very low temperature dependence of the magnetic susceptibility. Some explanations for the observations are offered, and the 5f electron behavior is compared to the 4f analogues.

The influence of radioactive decay on actinide magnetic susceptibility measurements obtained using the Evans method.

In order to explain the higher magnetic susceptibility of some aquo actinide ions than predicted by Hund's rules, the molar magnetic susceptibilities of two americium isotopes ((241)Am and (243)Am) were measured using the Evans method. The results obtained show a growing change in the magnetic susceptibility with α and also a ß(-) activity increase in solution. ß(-) particle effects appear to be stronger than radicals formed by α particles on the experimental values. The temperature dependence of Am(iii) magnetic susceptibility has been observed but from experiments carried out here, it appears to be difficult to prove whether this effect arises from radicals or ß(-). Finally, magnetic susceptibilities of americium recorded in different media (HClO4, HCl, and HNO3) have been compared to alpha and beta emissions' impact.

Effect of metal complexation on the radiolytic stability of DOTA.

Radiometals are increasingly used in nuclear medicine for both diagnostic and therapeutic purposes. The DOTA ligand (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) is widely used as a chelating agent for various radionuclides, including 89Zr, with high thermodynamic stability constants and great in vivo stability. However, in contact with radioisotopes, chelating molecules are subjected to the effects of radiation, which can lead to structural degradation and induce alteration of their complexing properties. For the first time, the radiolytic stability of the Zr-DOTA complex in aqueous solution was studied and compared to the stability of the DOTA ligand. The identification of the major degradation products allows us to propose two different degradation schemes for the DOTA ligand and Zr-DOTA complex. DOTA is degraded preferentially by decarboxylation and cleavage of an acetate arm CH2-COOH, whereas in Zr-DOTA, DOTA tends to oxidize by the addition of the OH group in its structure. In addition, the degradation of the ligand, when involved in a Zr complex, is significantly less than when the ligand is free in solution, indicating that the metal protects the ligand from degradation. DFT calculations were performed to supplement the experimental data and give an improved understanding of the behaviour of DOTA and Zr-DOTA solutions after irradiation: the increase in stability upon complexation is attributed to the strengthening of the bonds in the presence of metal cations, which become less vulnerable to radical attack. Bond dissociation energies and Fukui indices are shown to be useful descriptors to estimate the most vulnerable sites of the ligand and to predict the protective effect of the complexation.

Complexation of lanthanides(III), americium(III), and uranium(VI) with bitopic N,O ligands: an experimental and theoretical study.

New functionalized terpyridine-diamide ligands were recently developed for the group actinide separation by solvent extraction. In order to acquire a better understanding of their coordination mode in solution, protonation and complexation of lanthanides(III), americium(III), and uranium(VI) with these bitopic N,O-bearing ligands were studied in homogeneous methanol/water conditions by experimental and theoretical approaches. UV-visible spectrophotometry was used to determine the protonation and stability constants of te-tpyda and dedp-tpyda. The conformations of free and protonated forms of te-tpyda were investigated using NMR and theoretical calculations. The introduction of amide functional groups on the terpyridine moiety improved the extracting properties of these new ligands by lowering their basicity and enhancing the stability of the corresponding 1:1 complexes with lanthanides(III). Coordination of these ligands was studied by density functional theory and molecular dynamics calculations, especially to evaluate potential participation of hard oxygen and soft nitrogen atoms in actinide coordination and to correlate with their affinity and selectivity. Two predominant inner-sphere coordination modes were found from the calculations: one mode where the cation is coordinated by the nitrogen atoms of the cavity and by the amide oxygen atoms and the other mode where the cation is only coordinated by the two amide oxygen atoms and by solvent molecules. Further simulations and analysis of UV-visible spectra using both coordination modes indicate that inner-sphere coordination with direct complexation of the three nitrogen and two oxygen atoms to the cation leads to the most likely species in a methanol/water solution.