UC Berkeley

Chapter 1 describes the synthesis of tri- and tetravalent lanthanide metallocene complexes supported by a sterically demanding 1,4-bis(triphenylsilyl)cyclooctatetraene ligand, COTbig. The kinetic stabilization conferred by the COTbig ligand allows for the isolation of the formally tetravalent praseodymocene, PrIV(COTbig)2. The praseodymocene was extensively characterized via cyclic voltammetry, x-ray absorption spectroscopy, high-field electron paramagnetic resonance spectroscopy and far-infrared magnetospectroscopy. Taken together and combined with empirical crystal field Hamiltonian modelling these data support the presence of a multiconfigurational ground state dominated by the ligand-metal charge-transferred 4f2π3 configuration with a minor contribution from the 4f1π4 configuration. This chapter emphasizes the ability of sterically demanding ligands to kinetically stabilize otherwise inaccessible molecules with unusual electronic structures.Chapter 2 extends the extremely bulky MIV(COTbig)2 architecture used in Chapter 1 to the early actinides. The steric bulk and crystallinity of the ligand allows for isolation of an isostructural series of tetravalent actinocenes, AnIV(COTbig)2. This new series of actinocenes displays a pronounced bend between the two COTbig ligands providing a contrast to previously reported, coplanar actinocene sandwich series. Combined structural, spectroscopic and computational investigations give evidence for allowed 5f-6d mixing driven by the bent geometry and evidence of increased energy driven covalency in the plutonocene not observed for other cyclooctatetraene ligands. Additionally, preliminary evidence for the oxidation of the uranocene to form [UV(COTbig)2] + is presented, encouraging further exploration of the COTbig ligand across a range of oxidation states. Chapter 3 reports the investigation of the rare-earth coordination chemistry of a new 1,3,4,6-tetraphenylpentalenide (PnPh4) ligand. Homoleptic sandwich complexes, [LnIIIPnPh42]-, were synthesized for a range of rare-earths (Ln = Y, La, Ce, Tb, Yb) as well as the oxidized CeIVPnPh42, highlighting the adaptability of the ligand to a range of metal sizes. A combination of experiment and theory show that the PnPh4 ligands are more weakly donating than cyclooctatetraenide ([C8H8]2-) or the parent pentalenide ([C8H6]2-). Additionally, the lanthanum half-sandwich complex LaPnPh4I(THF)3 was synthesized as a platform for future study of heteroleptic PnPh4 complexes.Chapter 4 details efforts to extend the coordination chemistry of the PnPh4 ligand developed in Chapter 3 to the tri- and tetravalent actinides. Reactions targeting the tetravalent actinide complexes AnIVPnPh42 (An = Th, U) were complicated by the limited solubility of the target complexes but results for uranium and plutonium demonstrate the efficacy of the PhPh4 ligand in furnishing crystallographically characterizable complexes of the trivalent actinides. Structural comparison emphasizes the increased covalency of actinide organometallics compared to their lanthanide congeners. Additionally, reactions to synthesize the trivalent berkelium analogue are presented and show promise warranting further investigation.Chapter 5 describes the synthesis of a series of strontium bis- and tris-cyclopentadienyl complexes with three different cyclopentadienyl (CpR) ligands, Cptet (C5Me4H), Cp' (C5H4(SiMe3)), and Cp" ((1,3-SiMe3)2C5H3). The anionic [K(2.2.2-cryptand)][SrCpR3] complexes are isostructural to the analogous divalent lanthanide compounds and are promising candidates as diamagnetic diluents for studying the magnetic properties of these species. To this effect, the europium complex [K(2.2.2-cryptand)][EuIICptet3] was prepared and investigated via solid-state electron paramagnetic resonance spectroscopy both diluted and undiluted. The diluted spectrum displays significantly decreased linewidths and allows resolution of the weak hyperfine coupling between the europium nucleus and the electron spins, proving the efficacy of strontium as a diamagnetic diluent for the divalent lanthanides.