Origin of Selectivity of a Triazinyl Ligand for Americium(III) over Neodymium(III)

[M(EtBTP)₃][BPh₄]₃ ⋅ 3 CH₃CN (M=Nd, Am; EtBTP=2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine) have been synthesized from reactions of MCl₃ ⋅ n H₂O with EtBTP in acetonitrile followed by anion metathesis. Structural analysis reveals that these compounds contain M³⁺ cations bound by tridentate EtBTP ligands to create a tricapped trigonal prismatic geometry around the metal centers. Collection of high-resolution, single-crystal X-ray diffraction data also allowed reduction in bond lengths esd's, such that a slight contraction of Δ=0.0158(18) Å in the Am−N versus Nd−N bond lengths was observed, even though these cations ostensibly have matching ionic radii. Theoretical evaluation revealed enhanced metal–ligand bonding through back donation in the [Am(EtBTP)₃]³⁺ complex that is absent in [Nd(EtBTP)₃]³⁺.     

Evidence of Alpha Radiolysis in the Formation of a Californium Nitrate Complex

Well-characterized complexes of transplutonium elements are scarce because of the experimental challenges of working with these elements and the rarity of the isotopes. This leads to a lack of structural and spectroscopic data needed to understand the nature of chemical bonds in these compounds. In this work, the synthesis of Cf(DOPO^q)₂(NO₃)(py) (DOPO^q=2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate; py=pyridine) is reported, in which the nitrate anion is hypothesized to form through the α-radiolysis-induced reaction of pyridine and/or the ligand. Computational analysis of the electronic structure of the complex reveals that the Cf^III–ligand interactions are largely ionic.     

Contemporary Chemistry of Berkelium and Californium

The merging of small-scale syntheses and rapid crystallization methods have provided access to crystalline samples of berkelium (Z=97) and californium (Z=98) coordination complexes and compounds that can be interrogated with a suite of spectroscopic tools and structural elucidation approaches that have come online over the last 20 years. The combination of this experimental data with relativistic theoretical methods that capture the effects of spin-orbit coupling and scalar relativistic effects have allowed us to understand the electronic structure of berkelium and californium compounds at a level of detail that was not previously possible. The harbinger of this new era of post-curium chemistry was the synthesis and characterization of [Cf{B₆O₈(OH)₅}]. This compound possesses a structure type that is distinct from earlier actinide borates, a reduction in coordination number for californium, contracted Cf−O bond lengths, a substantially reduced magnetic moment with respect to the calculated free-ion moment and, most importantly, vibronically coupled broadband photoluminescence. Ligand-field analysis also showed that the splitting of the ground state was larger than typically found in the f-block elements, and when taken together places its overall electronic structure as a hybrid of d- and f-block components. The discovery of the unusual properties of this compound has led to the development of large families of 4f and 5f coordination complexes, in an effort to uncover the underlying origin of the electronic structure oddities, and whether there really is a sharp onset of these changes at californium. This in turn pushed the development of far more challenging berkelium chemistry (from a radiologic standpoint) because the half-life of the isotopes decreases from 351 years for ²⁴⁹Cf to 330 days for ²⁴⁹Bk. This short review details some of the chemistry that has been reported over the last 15 years, and its consequences for understanding the periodic table.     

Differentiating between trivalent lanthanides and actinides

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.     

Syntheses, Structures, and Spectroscopic Properties of Plutonium and Americium Phosphites and the Redetermination of the Ionic Radii of Pu(III) and Am(III)

A series of isotypic rare earth phosphites (RE = Ce(III), Pr(III), Nd(III), Pu(III), or Am(III)) with the general formulas RE(2)(HPO(3))(3)(H(2)O) along with a Pu(IV) phosphite, Pu[(HPO(3))(2)(H(2)O)(2)], have been prepared hydrothermally via reactions of RECl(3) with phosphorous acid. The structure of RE(2)(HPO(3))(3)(H(2)O) features a face-sharing interaction of eight- and nine-coordinate rare earth polyhedra. By use of the crystallographic data from the isotypic series along with data from previously reported isotypic series, the ionic radii for higher coordinate Pu(III) and Am(III) were calculated. The (VIII)Pu(III) radius was calculated as 1.112 ± 0.004 ?, and the (IX)Pu(III) radius was calculated to be 1.165 ± 0.002 ?. The (VIII)Am(III) radius was calculated as 1.108 ± 0.004 ?, and the (IX)Am(III) radius was calculated as 1.162 ± 0.002 ?.     

Systematic evolution from uranyl(VI) phosphites to uranium(IV) phosphates

Six new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds can contain U(VI) or U(IV), and two are mixed-valent U(VI)/U(IV) compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from U(VI) to U(IV) happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.     

Periodic trends in hexanuclear actinide clusters

Four new Th(IV), U(IV), and Np(IV) hexanuclear clusters with 1,2-phenylenediphosphonate as the bridging ligand have been prepared by self-assembly at room temperature. The structures of Th(6)Tl(3)[C(6)H(4)(PO(3))(PO(3)H)](6)(NO(3))(7)(H(2)O)(6)·(NO(3))(2)·4H(2)O (Th6-3), (NH(4))(8.11)Np(12)Rb(3.89)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(24)·15H(2)O (Np6-1), (NH(4))(4)U(12)Cs(8)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(24)·18H(2)O (U6-1), and (NH(4))(4)U(12)Cs(2)[C(6)H(4)(PO(3))(PO(3)H)](12)(NO(3))(18)·40H(2)O (U6-2) are described and compared with other clusters of containing An(IV) or Ce(IV). All of the clusters share the common formula M(6)(H(2)O)(m)[C(6)H(3)(PO(3))(PO(3)H)](6)(NO(3))(n)((6-n)) (M = Ce, Th, U, Np, Pu). The metal centers are normally nine-coordinate, with five oxygen atoms from the ligand and an additional four either occupied by NO(3)(-) or H(2)O. It was found that the Ce, U, and Pu clusters favor both C(3i) and C(i) point groups, while Th only yields in C(i), and Np only C(3i). In the C(3i) clusters, there are two NO(3)(-) anions bonded to the metal centers. In the C(i) clusters, the number of NO(3)(-) anions varies from 0 to 2. The change in the ionic radius of the actinide ions tunes the cavity size of the clusters. The thorium clusters were found to accept larger ions including Cs(+) and Tl(+), whereas with uranium and later elements, only NH(4)(+) and/or Rb(+) reside in the center of the clusters.     

Cation-cation interactions between neptunyl(VI) units

The boric acid flux reaction of NpO(2)(ClO(4))(2) with NaClO(4) affords Na[(NpO(2))(4)B(15)O(24)(OH)(5)(H(2)O)](ClO(4))·0.75H(2)O (NaNpBO-1). NaNpBO-1 possesses a layered structure consisting of double neptunyl(VI) borate sheets bridged by another Np(VI) site through cation-cation interactions. The sole presence of Np(VI) in NaNpBO-1 is supported by absorption and vibrational spectroscopy.     

Role of anions and reaction conditions in the preparation of uranium(VI), neptunium(VI), and plutonium(VI) borates

U(VI), Np(VI), and Pu(VI) borates with the formula AnO(2)[B(8)O(11)(OH)(4)] (An = U, Np, Pu) have been prepared via the reactions of U(VI) nitrate, Np(VI) perchlorate, or Pu(IV) or Pu(VI) nitrate with molten boric acid. These compounds are all isotypic and consist of a linear actinyl(VI) cation, AnO(2)(2+), surrounded by BO(3) triangles and BO(4) tetrahedra to create an AnO(8) hexagonal bipyramidal environment. The actinyl bond lengths are consistent with actinide contraction across this series. The borate anions bridge between actinyl units to create sheets. Additional BO(3) triangles and BO(4) tetrahedra extend from the polyborate layers and connect these sheets together to form a three-dimensional chiral framework structure. UV-vis-NIR absorption and fluorescence spectroscopy confirms the hexavalent oxidation state in all three compounds. Bond-valence parameters are developed for Np(VI).