Frédéric Gendron

Magnetic properties and electronic structure of neptunyl(VI) complexes: Wavefunctions, orbitals, and crystal-field models

The electronic structure and magnetic properties of neptunyl(VI), NpO2(2+), and two neptunyl complexes, [NpO2(NO3)3](-) and [NpO2Cl4](2-), were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal-field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin-orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g-factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g-factors were calculated for the ground and excited states. For [NpO2Cl4](2-), a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn-Sham DFT with standard functionals can produce reasonable g-factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state

Bonding to berkelium A geographical theme prevailed in the recent naming of the heaviest chemical elements. The choices brought to mind berkelium (Bk) and californium (Cf), the names chosen for elements 97 and 98 over half a century ago. Silver et al. now revisit the chemistry of Bk, which has proven fiercely challenging to study over the years on account of its vigorous radioactive decay. Synthetic crystallized Bk borate and dipicolinate compounds structurally resembled Cf analogs in the solid state but manifested distinct electronic and magnetic characteristics stemming from spin-orbit coupling effects. Science, this issue p. 888 Experiments and theory probe the coordination chemistry of a highly radioactive heavy element. INTRODUCTION Developing the chemistry of late actinides is hindered by the lack of availability of isotopes, the need for specialized research facilities, and the nuclear instability of the elements. Berkelium represents one of the last elements that can be prepared on a milligram scale in nuclear reactors. However, its only available isotope, 249Bk, has a half-life of only 320 days, which has greatly curtailed the expansion of its chemistry and fundamental exploration of how large relativistic and spin-orbit coupling effects alter its electronic structure. Furthermore, data gathered from Bk(III) in aqueous media suggest that its coordination may be different from that of earlier actinides. However, a single-crystal structure of a berkelium compound has remained elusive, leaving unanswered whether these structural changes occur in the solid state. RATIONALE This work focuses on characterizing two distinct berkelium compounds on the milligram scale. In particular, the goal was to obtain crystals of these compounds that could be used in structure determinations and physical property measurements. Two compounds were selected: a coordination complex of dipicolinate and a borate. Dipicolinate complexation occurs with most other lanthanides and actinides in the +3 oxidation state, facilitating comparisons across the series to discern periodic trends. In the borate family, the structural frameworks are hypersensitive to the nature of the bonding at the metal center and are rearranged accordingly. Modeling the experimental data using a variety of computational techniques allows us to deconvolute the role of covalent bonding and spin-orbit coupling in determining the electronic properties of berkelium. RESULTS Experiments with milligram quantities of 249Bk were choreographed for 6 months before the arrival of the isotope because the total quantity used in the studies was 13 mg, which corresponds to a specific activity of 21 Ci. Although this isotope is a low-energy β emitter, it decays to 249Cf at a rate of about 1.2% per week, and the latter produces hard γ radiation that represents a serious external hazard. In addition, the samples described in this work undergo about 1012 decays per second. This rapid decomposition necessitated the development of techniques for swiftly preparing and encapsulating samples and for collecting all structural and spectroscopic data within 24 hours of crystal formation. After this preparation, the single-crystal structures of Bk(III)tris(dipicolinate) and Bk(III) borate were determined. The latter compound has the same topology as that of californium(III) (Cf) and contains an eight-coordinate BkO8 unit. This reduction in coordination number is consistent with previous solution-phase x-ray absorption measurements and indicates that a drop in coordination number in the actinide series from nine to eight begins at berkelium. The magnetic and optical properties of these samples were also measured. The red luminescence from Bk(III) was similar in nature to that of curium(III) and is primarily based on an f-f transition. The ingrowth of the broad green luminescence from Cf(III), which is caused by a ligand-to-metal charge transfer, was shown to be distinct in nature from that originating from Bk(III). Ligand-field, density functional theory, and wave-function calculations were used to understand the spectroscopic features and revealed that the single largest contributor to the unexpected electronic properties of Bk(III) is spin-orbit coupling. This effect mixes the first excited state with the ground state and causes a large deviation from a pure Russell-Saunders state. The reduction in the measured magnetic moment for these samples from that calculated for an f8 electron configuration is primarily attributable to this multiconfigurational ground state. CONCLUSION The crystallographic data indicate that Bk(III) shares more structural similarities with Cf(III) than with Cm(III). However, ligand-field effects are more similar between Bk(III) and Cm(III). Terbium (Tb), in the lanthanide series, represents the closest analog of Bk because the trivalent cations possess 4f8 and 5f8 configurations, respectively. Spin-orbit coupling in Bk(III) creates mixing of the first excited state (5G6) with the ground state. In contrast, the ground state of the Tb(III)tris(dipicolinate) contains negligible contributions of this type. An overall conclusion from this study is that spin-orbit coupling plays a large role in determining the ground state of late actinide compounds. Crystal structure of a berkelium coordination compound. The central Bk(III) ion is coordinated by three monoprotonated dipicolinate ligands in tridentate O,N,O fashion. Bk, yellow; C, gray; N, blue; O, red; H, white. Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f7 configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium’s only available isotope, 249Bk, has hindered in-depth studies of the element’s coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium’s complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium’s multiconfigurational ground state.

Calculating NMR Chemical Shifts for Paramagnetic Metal Complexes from First-Principles

Paramagnetic effects on NMR shifts (pNMR) for paramagnetic metal complexes are calculated from first-principles, without recourse to spin Hamiltonian parameters. A newly developed code based on complete active space (CAS) and restricted active space (RAS) techniques in conjunction with treating spin-orbit (SO) coupling via state interaction is applied to (13)C NMR shifts of actinyl tris-carbonate complexes, specifically [UO2(CO3)3](5-) and [NpO2(CO3)3](4-). The experimental pNMR shifts as well as the sizable difference of the (13)C NMR shift for these iso-electronic species are well reproduced by the calculations. Approximations to the pNMR shift equations using spin Hamiltonian parameters or the magnetic susceptibility are calculated for the same systems at the same level of theory, and it is shown how the approximations relate to the ab initio data.

Magnetic resonance properties of actinyl carbonate complexes and plutonyl(VI)-tris-nitrate.

Electronic structures and magnetic properties of actinyl ions AnO2(n+) (An = U, Np, and Pu) and the equatorially coordinated carbonate complexes [UO2(CO3)3](5–), [NpO2(CO3)3](4–), and [PuO2(CO3)3](4–) are investigated by ab initio quantum chemical calculations. The complex [PuO2(NO3)3](−) is also included because of experimentally available g-factors and for comparison with a previous study of [NpO2(NO3)3](−) (Chem.—Eur. J. 2014, 20, 7994-8011). The results are rationalized with the help of crystal-field (CF)-type models with parameters extracted from the ab initio calculations, and with the help of natural orbitals and natural spin orbitals contributing to the magnetic properties and the unpaired spin distribution, generated from the spin–orbit wave functions. These orbitals resemble textbooklike representations of the actinide 5f orbitals. Calculated paramagnetic susceptibilities are used to estimate dipolar 13C chemical shifts for the carbonate ligands. Their signs and order of magnitude are compared to paramagnetic effects observed experimentally in NMR spectra. The results indicate that the experimental spectra are also influenced by contact shifts.

Electronic Structure and Properties of Berkelium Iodates

The reaction of 249Bk(OH)4 with iodate under hydrothermal conditions results in the formation of Bk(IO3)3 as the major product with trace amounts of Bk(IO3)4 also crystallizing from the reaction mixture. The structure of Bk(IO3)3 consists of nine-coordinate BkIII cations that are bridged by iodate anions to yield layers that are isomorphous with those found for AmIII, CfIII, and with lanthanides that possess similar ionic radii. Bk(IO3)4 was expected to adopt the same structure as M(IO3)4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller ZrIV cation. BkIII-O and BkIV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO3)4 show evidence for doping with BkIII in these crystals. In addition to luminescence from BkIII in the Bk(IO3)4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249Bk (t1/2 = 320 d) causes oxidation of BkIII and only BkIV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO3)3 and Bk(IO3)4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in BkIV that does not strictly adhere to Russel-Saunders coupling and Hunds Rule even though it possesses a half-filled 5f 7 shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the BkIV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f7 ions such as GdIII or CmIII.

Puzzling Lack of Temperature Dependence of the PuO2 Magnetic Susceptibility Explained According to Ab Initio Wave Function Calculations

The electronic structure and the magnetic properties of solid PuO2 are investigated by wave function theory calculations, using a relativistic complete active space (CAS) approach including spin-orbit coupling. The experimental magnetic susceptibility is well reproduced by calculations for an embedded PuO812- cluster model. The calculations indicate that the surprising lack of temperature dependence of the magnetic susceptibility χ of solid PuO2 can be rationalized based on the properties of a single Pu4+ ion in the cubic ligand field of the surrounding oxygen ions. Below ∼300 K, the only populated state is the nonmagnetic ground state, leading to standard temperature-independent paramagnetism (TIP). Above 300 K, there is an almost perfect cancellation of temperature-dependent contributions to χ that depends delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling between them.

Structure and Bonding Investigation of Plutonium Peroxocarbonate Complexes Using Cerium Surrogates and Electronic Structure Modeling

Herein, we report the synthesis and structural characterization of K8[(CO3)3Pu]2(μ-η2-η2-O2)2·12H2O. This is the second Pu-containing addition to the previously studied alkali-metal peroxocarbonate series M8[(CO3)3A]2(μ-η2-η2-O2)2·xH2O (M = alkali metal; A = Ce or Pu; x = 8, 10, 12, or 18), for which only the M = Na analogue has been previously reported when A = Pu. The previously reported crystal structure for Na8[(CO3)3Pu]2(μ-η2-η2-O2)2·12H2O is not isomorphous with its known Ce analogue. However, a new synthetic route to these M8[(CO3)3A]2(μ-η2-η2-O2)2·12H2O complexes, described below, has produced crystals of Na8[(CO3)3Ce]2(μ-η2-η2-O2)2·12H2O that are isomorphous with the previously reported Pu analogue. Via this synthetic method, the M = Na, K, Rb, and Cs salts of M8[(CO3)3Ce]2(μ-η2-η2-O2)2·xH2O have also been synthesized for a systematic structural comparison with each other and the available Pu analogues using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory calculations. The Ce salts, in particular, demonstrate subtle differences in the peroxide bond lengths, which correlate with Raman shifts for the peroxide Op-Op stretch (Op = O atoms of the peroxide bridges) with each of the cations studied: Na+ [1.492(3) Å/847 cm-1], Rb+ [1.471(1) Å/854 cm-1], Cs+ [1.474(1) Å/859 cm-1], and K+ [1.468(6) Å/870 cm-1]. The trends observed in the Op-Op bond distances appear to relate to supermolecular interactions between the neighboring cations.

Nuclear Magnetic Resonance Measurements and Electronic Structure of Pu(IV) in [(Me)4N]2PuCl6

The synthesis, electronic structure, and characterization via single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and magnetic susceptibility of (Me4N)2PuCl6 are reported. NMR measurements were performed to both search for the direct (239)Pu resonance and to obtain local magnetic and electronic information at the Cl site through (35)Cl and (37)Cl spectra. No signature of (239)Pu NMR was observed. The temperature dependence of the Cl spectra was simulated by diagonalizing the Zeeman and quadrupolar Hamiltonians for (35)Cl, (37)Cl, and (14)N isotopes. Electronic structure calculations predict a magnetic Γ5 triplet ground state of Pu(IV) in the crystalline electric field of the undistorted PuCl6 octahedron. A tetragonal distortion would result in a very small splitting (∼20 cm(-1)) of the triplet ground state into a nonmagnetic singlet and a doublet state. The Cl shifts have an inflection point at T ≈ 15 K, differing from the bulk susceptibility, indicating a nonmagnetic crystal field ground state. The Cl spin-lattice relaxation time is constant to T = 15 K, below which it rapidly increases, also supporting the nonmagnetic crystal field ground state.

Low-Spin Fe(III) Macrocyclic Complexes of Imidazole-Appended 1,4,7-Triazacyclononane as Paramagnetic Probes

Two macrocyclic complexes of 1,4,7-triazacyclononane (TACN), one with N-methyl imidazole pendants, [Fe(Mim)]3+, and one with unsubstituted NH imidazole pendants, [Fe(Tim)]3+, were prepared with a view toward biomedical imaging applications. These low-spin Fe3+ complexes produce moderately paramagnetically shifted and relatively sharp 1H NMR resonances for paraSHIFT and paraCEST applications. The [Fe(Tim)]3+ complex undergoes pH-dependent changes in NMR spectra in solution that are consistent with the consecutive deprotonation of all three imidazole pendant groups at high pH values. N-Methylation of the imidazole pendants in [Fe(Mim)]3+ produces a complex that dissociates more readily at high pH in comparison to [Fe(Tim)]3+, which contains ionizable donor groups. Cyclic voltammetry studies show that the redox potential of [Fe(Mim)]3+ is invariant with pH ( E1/2 = 328 ± 3 mV vs NHE) between pH 3.2 and 8.4, unlike the Fe(III) complex of Tim which shows a 590 mV change in redox potential over the pH range of 3.3-12.8. Magnetic susceptibility studies in solution give magnetic moments of 0.91-1.3 cm3 K mol-1 (μeff value = 2.7-3.2) for both complexes. Solid-state measurements show that the susceptibility is consistent with a S = 1/2 state over the temperature range of 0 to 300 K, with no crossover to a high-spin state under these conditions. The crystal structure of [Fe(Mim)](OTf)3 shows a six-coordinate all-nitrogen bound Fe(III) in a distorted octahedral environment. Relativistic ab initio wave function and density functional theory (DFT) calculations on [Fe(Mim)]3+, some with spin orbit coupling, were used to predict the ground spin state. Relative energies of the doublet, quartet, and sextet spin states were consistent with the doublet S = 1/2 state being the lowest in energy and suggested that excited states with higher spin multiplicities are not thermally accessible. Calculations were consistent with the magnetic susceptibility determined in the solid state.

Complete Active Space Wavefunction-Based Analysis of Magnetization and Electronic Structure

A theoretical framework for the generation of natural orbitals, natural spin orbitals, as well as orbital- and spin-magnetizations, from multi-configurational ab initio wavefunction calculations including spin-orbit coupling is presented. Selected case studies show how these computational orbital and magnetization tools can be used to interpret and rationalize the magnetic properties of complexes containing transition metals, lanthanides, and actinides.