Spectroscopic and Computational Analysis of Rare Earth and Actinide Complexes in Unusual Coordination Environments and Oxidation States

This dissertation describes the use of spectroscopic and computational methods to understand new classes of rare earth and actinide coordination complexes. In Chapters 1 and 2, the use of UV-vis spectroscopy and density functional theory (DFT) to understand a rare form of photochemical activation of rare earth mixed-ligand tris(cyclopentadienyl) complexes, (C5Me5)3-x(C5Me4H)xLn, and metallocene allyl complexes, (C5Me5)2Ln(C3H5) (Ln = Lu, Y) is described. The photochemistry involves a ligand-based reduction in a trivalent rare earth complex that generates a reducing system powerful enough to reduce dinitrogen. Chapter 3 describes the use of Raman spectroscopy to understand bond lengths in reduced dinitrogen rare earth complexes, [(C5Me5)2Ln](μ-η2:η2-N2), and analyze the degree of dinitrogen reduction based on the ancillary ligands. Chapter 4 describes the power of NMR spectroscopy to characterize complicated mixtures of heterobimetallic bridging hydride complexes, (C5Me5)2Ln(H)2Ln′(C5Me5)2, and tuckover hydride complexes, (C5Me5)2Ln(μ-H)(μ-η1:η5-CH2C5Me4)Ln′(C5Me5) (Ln, Ln′ = La, Y, Lu). DFT was used to investigate the metal site preferences in these complexes. Chapters 5 through 10 describe different techniques to understand the first examples of molecular rare earth and actinide tris(cyclopentadienyl) complexes in the formal +2 oxidation state, [K(2.2.2-cryptant)][(C5R5)3Ln] (Ln = Y, lanthanides, Th, U). DFT is used to describe the configuration of these complexes as 4d1 for Y, [Ln3+]5d1 for 10 lanthanides and [An3+]6d1 for Th and U (Chapter 5 and 6). UV-vis spectroscopy was used to distinguish between different electron configurations of Ln2+ complexes (Chapter 7). Magnetic susceptibility measurements characterize two Ln2+ complexes to have record high single-ion magnetic moments (Chapter 8). Ligand and metal edge X-ray absorption spectroscopy were used to analyze the oxidation state of the metals (Chapter 9). Reactivity of cyclooctatetraene with Ln2+ complexes is described in Chapter 10. Chapter 11 presents the synthesis of new Ln3+ and Ln2+ complexes, using a tris(aryloxide)arene coordination environment. Chapter 12 describes the use of DFT to predict new coordination environments that could allow the stabilization of the +2 oxidation state for the rare earths and actinides

Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C5H3(SiMe3)2]3Th}1- anion containing thorium in the formal +2 oxidation state.

Reduction of the Th3+ complex Cp''3Th, 1 [Cp'' = C5H3(SiMe3)2], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp''3Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC8 in the presence of 18-crown-6 generates the analogous Th2+ compound, [K(18-crown-6)(THF)2][Cp''3Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M-1 cm-1, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 Å Th-(Cp'' ring centroid) distances, respectively, equivalent to the 2.520 Å distance measured in 1. Density functional theory analysis of (Cp''3Th)1- is consistent with a 6d2 ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp''2Th(C8H8), 4, and [K(18-crown-6)]Cp''

Record High Single-Ion Magnetic Moments Through 4f(n)5d(1) Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]⁻.

The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 μB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets

Record High Single-Ion Magnetic Moments Through 4f(n)5d(1) Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]⁻.

The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 μB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets

Reactivity of Complexes of 4f^<i>n</i>5d¹ and 4f^<i>n</i>+1 Ln²⁺ Ions with Cyclooctatetraene

The Ln²⁺ complexes [K­(2.2.2-cryptand)]­[Cp′₃Ln] (Ln = La, Ce, Pr, Nd, Sm, Eu, Dy, Tm, Yb; Cp′ = C₅H₄SiMe₃) were reacted with 1,3,5,7-cyclooctatetraene, C₈H₈, to determine if the reactivity of the complexes of 4f^<i>n</i>+1 ions differed from that of 4f^<i>n</i>5d¹ ions. Crystallographically characterizable (C₈H₈)^2– complexes were obtained only for the larger metals in the lanthanide series, and two types of products were obtained: [K­(2.2.2-cryptand)]­[Cp′₂Ln­(C₈H₈)] (Ln = La, Ce) and [K­(2.2.2-cryptand)]­[Ln­(C₈H₈)₂] (Ln = Ce, Pr, Nd, Sm). The expected co-products of the two-electron reduction of C₈H₈ by 2 equiv of [K­(2.2.2-cryptand)]­[Cp′₃Ln], namely, the tetrakis­(cyclopentadienyl) complexes, [K­(2.2.2-cryptand)]­[Cp′₄Ln], were crystallographically characterized for six metals (Ln = Ce, Pr, Nd, Sm, Dy, Tm)

Reactivity of the Y³⁺ Tuck-Over Hydride Complex, (C₅Me₅)₂Y(μ-H)(μ-CH₂C₅Me₄)Y(C₅Me₅)

The trivalent yttrium tuck-over hydride complex, (C₅Me₅)₂Y­(μ-H)­(μ-η¹:η⁵-CH₂C₅Me₄)­Y­(C₅Me₅), <b>1</b>, acts as a reductant in reactions in which the (μ-H)⁻ hydride ligand and the bridging Y–C alkyl anion linkage in the (μ-η¹:η⁵-CH₂C₅Me₄)^2– ligand combine to form a C–H bond in (C₅Me₅)⁻ and deliver two electrons to a substrate. Complex <b>1</b> reacts with PhSSPh, AgOTf (OTf = OSO₂CF₃), and Et₃NHBPh₄ to form [(C₅Me₅)₂Y­(μ-SPh)]₂, [(C₅Me₅)₂Y­(μ-OTf)]₂, and (C₅Me₅)₂Y­(μ-Ph)₂BPh₂, respectively. The reactivity of the Y–H and Y–CH₂C₅Me₄ linkages in <b>1</b> was probed via carbodiimide insertion reactions. ^<i>i</i>PrNCN^<i>i</i>Pr inserts into both Y–H and Y–C bonds to yield (C₅Me₅)­[^<i>i</i>PrNC­(H)­N^<i>i</i>Pr]­Y­{μ-η⁵<i>-</i>C₅Me₄CH₂[^<i>i</i>PrNCN^<i>i</i>Pr]}­Y­(C₅Me₅)₂. Carbodiimide insertion with [(C₅Me₅)₂YH]₂, <b>2</b>, was also examined for comparison, and (C₅Me₅)₂Y­[^<i>i</i>PrNC­(H)­N^<i>i</i>Pr-κ²<i>N</i>,<i>N</i>′] was isolated and structurally characterized. To examine the possibility of selective reactivity of the bridging ligands, μ-H versus μ-CH₂C₅Me₄, trimethylsilylchloride was reacted with <b>1</b>, and the tuck-over chloride complex, (C₅Me₅)₂Y­(μ-Cl)­(μ-η¹:η⁵-CH₂C₅Me₄)­Y­(C₅Me₅), was isolated

Dinitrogen Reduction via Photochemical Activation of Heteroleptic Tris(cyclopentadienyl) Rare-Earth Complexes

Dinitrogen can be reduced by photochemical activation of the Ln³⁺ mixed-ligand tris­(cyclopentadienyl) rare-earth complexes (η⁵-C₅Me₅)_3–<i>x</i>(C₅Me₄H)_<i>x</i>Ln (Ln = Y, Lu, Dy; <i>x</i> = 1, 2). [(C₅Me₄R)₂Ln]₂(μ-η²:η²-N₂) products (R = H, Me) are formed in reactions in which N₂ is reduced to (NN)^2– and (C₅Me₄H)⁻ is oxidized to (C₅Me₄H)₂. Density functional theory indicates that this unusual example of rare-earth photochemistry can be rationalized by absorptions involving the (η³-C₅Me₄H)⁻ ligands

Dinitrogen Reduction via Photochemical Activation of Heteroleptic Tris(cyclopentadienyl) Rare-Earth Complexes

Dinitrogen can be reduced by photochemical activation of the Ln³⁺ mixed-ligand tris­(cyclopentadienyl) rare-earth complexes (η⁵-C₅Me₅)_3–<i>x</i>(C₅Me₄H)_<i>x</i>Ln (Ln = Y, Lu, Dy; <i>x</i> = 1, 2). [(C₅Me₄R)₂Ln]₂(μ-η²:η²-N₂) products (R = H, Me) are formed in reactions in which N₂ is reduced to (NN)^2– and (C₅Me₄H)⁻ is oxidized to (C₅Me₄H)₂. Density functional theory indicates that this unusual example of rare-earth photochemistry can be rationalized by absorptions involving the (η³-C₅Me₄H)⁻ ligands