Synthesis and characterization of uranium(IV) compounds: From mononuclear complexes to multinuclear assemblies

This dissertation describes the synthesis of multinuclear compounds that possess magnetically-coupled actinide, namely uranium-238, clusters. These assemblies are supported by both acetylide-type ligands as well as triamidoamine or softer phosphine ligands. Synthetic inorganic chemists have been able to synthesize molecules and clusters with increased spin, S, or axial anisotropy, D, in an effort to augment the spin-reversal barriers and create better single-molecule magnets (SMMs). However, efforts to simultaneously increase these parameters are complicated. One potential route utilizes heavy atoms as a result of their larger single-ion anisotropy and believed ability to modulate the magnetism of other systems. My research is placed in this context in Chapter 1, where recent efforts to incorporate heavy atoms into expanded clusters are discussed. In Chapter 2, the preparation and magnetic property investigations of a structurally related family of mono-, di- and trinuclear U(IV) aryl acetylide complexes are presented. The reaction between [(NN&feet;3)UCl] and lithiated aryl acetylides leads to the formation of hexacoordinate compounds. In contrast, combining the uranacycle [(bit-NN&feet; 3)U] (bit-NN&feet;3 = [N(CH2CH 2NSitBuMe2)2(CH2CH 2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords pentacoordinate arylacetylide complexes, where NN&feet;3 = [N(CH2CH2NSi tBuMe2)3]. The measured magnetic susceptibilities for these compounds trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the DC magnetic susceptibility data, using the spin Hamiltonian Ĥ = –2J(Ŝi·Ŝ j). Geometry-optimized Stuttgart/6-31g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes and the electrochemistry of the monomeric phenylacetylide complex exhibits a reversible redox couple at -1.02 V versus [Cp2Fe]+/0, assignable to an oxidation of U(IV) to U(V). Efforts to study the magnetic correlations as a result of cubic ligands fields are presented in Chapter 3, whereby a neutral bidentate phosphine ligand was utilized. In the course of structurally characterizing previously reported complexes based on the 1,2-bis(dimethylphosphino)ethane)) (dmpe) ligand ([(dmpe)2UCl4] (3.1) and [(dmpe) 2UMe4] (3.2)), we found that adjusting the U:dmpe ratio leads to an unprecedented species. Whereas the use of two or three equivalents of dmpe relative to UCl4 produces 3.1 as a blue-green solid, use of a 1:1 dmpe:UCl4 stoichiometry yields [(dmpe) 4U4Cl16]˙2CH2Cl2˙( 3.3˙2CH2Cl2) as a green solid. In turn, 3.3 is used to prepare a mixed-chelating ligand complex featuring the bidentate ligand 4,4'-dimethyl-2,2'-bipyridine (dmbpy), [(dmpe)(dmbpy)UCl 4] (3.4). The measured magnetic susceptibilities for 3.1-3.4 trend toward non-magnetic ground states at low temperatures. In Chapter 4, we hypothesized that preparing complexes that contain U(IV) in a cubic ligand field environment, using acetylide ligands, might allow for the isolation of compounds exhibiting enhanced magnetic coupling. In that vein, we report the synthesis and characterization of [(dmpe)2U(CCPh) 4] (4.1) (CCPh = phenylacetylide) and [(dmpe)2U(CCPh) 5(Li·Et2O)] (4.2). No reproducible magnetic susceptibility data were obtained and a discussion about these difficulties is presented. In the course of studying the crystal structure of the mixed-chelating ligand complex [(dmpe)(dmbpy)UCl4] (3.4) an interesting effect on the U-Cl···H was observed. Several computation methods were utilized to determine that the M-Cl···HC distance based on approach angles is suggestive that Cl is acting more like chlorine and less like chloride. This provides a route to study U–L bonding and is presented in Chapter 5. Finally, in Chapter 6, efforts to synthesize a mixed-metal complex are discussed and preliminary characterization of a dinuclear ethynylbenzene 5f-3d complex (6.3) is presented. While an unambiguously paramagnetic metal-complex was not isolated, initial electrochemical studies indicate a redox process takes place. A short discussion about the temperature dependence of the magnetic susceptibility is given