UC Irvine

This dissertation describes strategies for the synthesis of low valent f element complexes, isolation of f element cryptate complexes using 2.2.2-cryptand, and the analysis of the electronic structures of (C5H4SiMe3)3Ln and [K(2.2.2-cryptand)][(C5H4SiMe3)3Ln] complexes (Ln = Sm, Eu, Gd, Tb) using X-ray photoelectron spectroscopy (XPS). The primary motivation of this research sought to expand the number of f element complexes in the +2 oxidation state by modifying counterions and ligand environments to investigate the chemical properties that help enable the isolation of these low valent ions. Several new complexes were isolated and strategies for the isolation of these ions will be discussed throughout this dissertation. In addition to developing synthetic strategies, the electronic structure of Ln(II) ions were examined using XPS to probe the electronic structure of both core electrons and valence electrons of these unusual ions with mixed-electron configurations.

In Chapter 1, lithium reduction of Cp′3Ln (Cp′ = C5H4SiMe3; Ln = Y, Tb, Dy, Ho) under Ar in the presence of 2.2.2-cryptand (crypt) is discussed. Examples of crystallographically-characterizable Ln(II) complexes of these metals are isolable, [Li(crypt)][Cp′3Ln]. In each complex, lithium is found in an N2O4 donor atom coordination geometry that is unusual for the cryptand ligand. Lithium reduction of Cp′3Y under N2 at −35 °C forms the Y(II) complex (Cp′3Y)1−, which reduces dinitrogen upon warming to room temperature to generate the (N2)2− complex [Cp′2Y(THF)]2(µ-ƞ2:ƞ2-N2).

In Chapter 2, the synthetic options for generating complexes of the actinide metals in the +2 oxidation state are discussed. Reduction of Cp″3U [Cp″ = C5H3(SiMe3)2] and the lanthanide analogs, Cp″3La and Cp″3Ce with lithium in the absence of crown ether and cryptand chelates is described. In each case, crystallographically-characterizable [Li(THF)4][Cp″3M] (M = La, Ce, U) complexes were obtainable. Reductions using Cs were also explored and X-ray crystallography revealed the formation of an oligomeric structure, [Cp″U(μ-Cp″)2Cs(THF)2]n, involving Cp″ ligands that bridge "(Cp″UII)1+" moieties to "[Cp"2Cs(THF)2]1−" units.

In Chapter 3, the synthesis of crystallographically-characterizable Ln(II) complexes of Tb and Ho by reducing CpMe3Ln(THF) (CpMe = C5H4Me) with KC8 in THF in the presence of 18-crown-6 (18-c-6) is described. X-ray crystallography revealed that these complexes are isolated with a methylcyclopentadienide inverse sandwich countercation: [(18-c-6)K(µ-CpMe)K(18-c-6)][CpMe3Ln] (Ln = Tb, Ho).

In Chapter 4, the reactivity of Cp′2Ln(THF)2 metallocenes with crypt to form Ln(II)-in-crypt complexes, [Ln(crypt)(THF)][Cp′3Ln]2 (Ln = Sm, Eu) and [Yb(crypt)][Cp′3Yb]2, is discussed. In each of the complexes, a ligand rearrangement occurs to form a Ln(II) dication with two [Cp′3Ln]1– counteranions.

In Chapter 5, the facile encapsulation of U(III) and La(III) by crypt using simple starting materials is described. Addition of crypt to UI3 and LaCl3 forms the crystallographically-characterizable complexes, [U(crypt)I2]I and [La(crypt)Cl2]Cl. In the presence of water, the U(III)-aquo adducts, [U(crypt)I(OH2)][I]2 and [Ucrypt)I(OH2)][I][BPh4], can be isolated.

In Chapter 6, the reactivity of LnI2(THF)2 (Ln = Sm, Eu, Yb) with crypt is discussed to examine if these readily accessible precursors could provide new examples of lanthanide-in-crypt complexes. The crystallographically-characterized Ln(II)-in-crypt complexes [Ln(crypt)(DMF)2][I]2 (Ln = Sm, Eu) and [Yb(crypt)(DMF)][I]2 were synthesized by reacting LnI2(THF)2 (Ln = Sm, Eu, Yb) with crypt in THF and recrystallizing from DMF. Crystallographic data were also obtained on the Ln(II)-in-crypt complex [Ln(crypt)(DMF)2][BPh4]2 which was synthesized by addition of two equivalents of NaBPh4 to [Ln(crypt)(DMF)2][I]2.

In Chapter 7, the synthesis of Ln(III)-in-crypt complexes using Ln(OTf)3 (Ln = Nd, Dy) starting materials is discussed. In MeCN, the Dy(III)-in-crypt complex formed is [Dy(crypt)(OTf)][OTf]2 and in DMF, the Nd(III)-in-crypt complex formed is [Nd(crypt)(DMF)2][OTf]3. A Nd(III)-in-crypt complex, [Nd(crypt)(OTf)2][OTf], can also be formed in THF and subsequently reduced using KC8 to form the Nd(II)-in-crypt complex [Nd(crypt)(OTf)2].

In Chapter 8, the electrochemical properties of U(III)-in-crypt complex [U(crypt)I2]I were examined in DMF and MeCN to determine the oxidative stability offered by crypt as a ligand. Cyclic-voltammetry revealed a U(IV)/U(III) quasi-reversible redox couple of –0.55 V (vs Fc+/0). In the presence of [CoCp2][PF6] in MeCN, a reversible U(III)/U(II) redox couple of –1.84 V (vs Fc+/0) was observed. U(III)-in-crypt complexes were also and was found to be robust to water. Additional examples of a U(III)-in-crypt complex with a DMF, MeCN and water adducts have also been crystallographically-characterized.

In Chapter 9, reduction of Th(OC6H2tBu2-2,6-Me-4)4 using either KC8 or Li in THF forming crystallographically-characterizable Th(III) complexes in the salts [K(THF)5(Et2O)][Th(OC6H2tBu2-2,6-Me-4)4] and [Li(THF)4][Th(OC6H2tBu2-2,6-Me-4)4] is discussed. In each structure the four aryloxide ligands are arranged in a square planar geometry, the first example of this coordination mode for an f element complex. The Th(III) ion and four oxygen donor atoms are coplanar to within 0.05 Å. EPR spectroscopy reveals an axial signal consistent with a metal-based radical in a planar complex. DFT calculations yield a C4-symmetric structure which accommodates a low-lying SOMO of 6dz2 character with 7s Rydberg admixture.

In Chapter 10, using X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations are discussed to evaluate the electronic structure of molecular Ln(II) complexes [Cp′3Ln]1– (Cp′ = C5H4SiMe3; Ln = Sm, Eu, Gd, Tb) formed by reduction of the Ln(III) precursors, Cp′3Ln. DFT calculations suggest that Eu(II) has a 4f7 and Gd(II) has a 4f75d1 electron configuration, whereas Tb(II) could not be unambiguously assigned due to its possible multiconfigurational ground state.