SYNTHESIS AND CHARACTERIZATION OF A SERIES OF URANIUM(IV) SPECIES: Investigating Coordination With a Redox Innocent Triamine Ligand Student Author Selena L. Staun earned a BS complexes containing redox-active ligands, and studies 2+ degree in chemistry with a minor in the reductive silylation of the UO2 cation. in sociology in May 2016; she graduated with department honors. She will start graduate school Suzanne Bart graduated from the to pursue a PhD in chemistry University of Delaware with a BS beginning September 2016 at the in chemistry (2001) and earned her University of California, Santa PhD from Cornell University with Barbara. While at Purdue, Staun was an undergraduate Professor Paul J. Chirik (2006). researcher in the Bart Laboratory for over 2 years and Subsequently, she was an Alexander received Purdue’s Department of Chemistry 2015 von Humboldt Postdoctoral Summer Research Award. She also found time to Fellow at the Friedrich-Alexander facilitate student learning as a Supplemental Instruction University Erlangen-Nuremberg under the direction Leader for a math course and as a teaching assistant for a of Professor Karsten Meyer. In 2008 she became an biology lab. assistant professor at Purdue University, then in 2014 she was promoted to associate professor. Her research Mentors interests include organometallic transformations mediated by organoactinide species. Bart recently won John J. Kiernicki earned AB an NSF CAREER award, and has been named a 2012 degrees in chemistry and history Cottrell Scholar and 2014 Organometallics Young from Ripon College in 2011 and Investigator Fellow. is currently working on his PhD in inorganic chemistry at Purdue University. His current research project involves the synthesis and reactivity of low-valent uranium 56 Journal of Purdue Undergraduate Research: Volume 6, Fall 2016 http://dx.doi.org/10.5703/1288284316161 INTRODUCTION Abstract Transition metals, those in the center of the periodic Investigation of uranium(IV) complexes table, are at the heart of organometallic chemistry, a chelated by a tridentate amine, H3RITA fi eld that studies the bond breaking and bond forming (H3RITA = (MesNHCH2CH2)2NH, Mes = of organic molecules by metal species. These 2,4,6-trimethylphenyl), has afforded novel important manipulations are responsible for essential new compounds with the potential for processes, such as olefi n polymerization, which small molecule activation. Deprotonation makes materials and other consumer goods, as well of H3RITA with two equivalents of benzyl as asymmetric hydrogenation, which is used to make potassium affords K2HRITA, which upon pharmaceuticals and commodity chemicals. reaction with uranium tetrachloride forms The metals of the f-block, those at the bottom of (THF)2UCl2(HRITA) (THF = tetrahydrofuran). The labile THF molecules are easily substituted the periodic table, have their own unique chemical properties. For instance, the lanthanides in the top for triphenylphosphine oxide (OPPh3), a much row, often called rare earth elements, are used for stronger ligand, forming (Ph3PO)UCl2(HRITA). Installation of a cyclopentadienyl ligand, Cp* making strong magnets, batteries for hybrid cars, (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl), and lasers. Those in the bottom row, the actinides, was achieved by the metathesis of are known for their radioactivity, and have important applications in nuclear energy, medicine, and other (THF)2UCl2(HRITA) with KCp* to form Cp*UCl(HRITA). For Cp*UCl(HRITA), full power sources. In light of these important uses for deprotonation of the RITA ligand was possible the f-block elements, they are far less explored for by using the strong base, methyllithium. The organometallic chemistry than are their d-block reaction of methyllithium with Cp*UCl(HRITA) counterparts. This is due to several reasons, including their redox properties, radioactivity, and in affords Cp*U(RITA)(LiCl(THF)2)—a uranium species containing a weakly bound lithium some cases, availability. chloride ligand. Replacement of the lithium chloride with triphenylphosphine oxide forms In our laboratory, we have been studying the organometallic chemistry of uranium. This element the neutral species, Cp*U(RITA)(OPPh3), which contains a rare trianionic RITA ligand. All is naturally occurring and inexpensive, making it complexes have been characterized by 1H NMR ideal for exploratory studies of the basic chemistry and IR spectroscopies, and where possible, their of the actinides. Its large atomic and ionic radii electronics were probed by electronic absorption allow a high ligand (an organic molecule bonded to spectroscopy. The molecular structures of a metal) coordination number and many accessible oxidation states (+3, +4, +5, +6) as compared to its (THF)2UCl2(HRITA), (Ph3PO)UCl2(HRITA), Cp*UCl(HRITA), and Cp*U(RITA) transition metal counterparts. A major difference between uranium and transition metals lies in their (LiCl(THF)2) were determined by single X-ray diffraction studies. redox chemistry. Whereas many transition metals undergo two- or multi-electron chemistry, uranium Staun, S. L. (2016). Synthesis and is generally unable to perform such processes due and Characterization of a Synthesis characterization of a series of uranium(IV) to low redox potentials between oxidation states. Series of Uranium(IV) Species species: Investigating coordination with a redox Thus, one-electron chemistry is generally observed. innocent triamine ligand. Journal of Purdue This is problematic, as two-electron chemical Undergraduate Research, 6, 56–63. http://dx.doi. processes are desirable and commonly encountered org/10.5703/1288284316161 in organometallic transformations performed by transition metals. Keywords Our studies are focused on the synthesis of organometallic, uranium, redox-chemistry, uranium(IV) compounds, which are chelated by inorganic, coordination chemistry, actinides, an organic amine ligand. For our experiments, we synthesis, crystallography, ligand, spectroscopy made compounds that have a central uranium atom that is chelated to a tridentate amine, one that has three nitrogen atoms that are each bonded to the metal center. This ligand supports the uranium center, keeping it soluble in organic solvents. This 57 ligand is known as one that is redox innocent, as it that were commonly used for the air- and moisture- simply “hangs out” on the uranium, not involving sensitive reactions described. itself in redox-chemistry with the metal center. In order to study how these uranium complexes behave, To perform this reaction, both H3RITA and benzyl and if they can mediate bond breaking and bond potassium were weighed into separate vials with forming reactions, we formed and characterized tetrahydrofuran solvent (THF) and frozen using liquid these compounds to make sure they were what we nitrogen. Upon thawing, the two solutions were mixed hypothesized. via pipette and stirred for 15 minutes. After this time, the volatiles of the reaction were removed using a RESULTS AND DISCUSSION strong vacuum (in vacuo) and the crude product was washed with pentane to afford the pure product, To begin, the redox innocent ligand, H3RITA, was K2HRITA, as a green solid (Scheme 1). Successful easily synthesized following literature methods. deprotonation was assessed by IR spectroscopy, This ligand is a desirable starting point because which showed a significant decrease in the intensity it is easily synthesized on large scales and easily of the N-H absorption. This is consistent with removal modified. Furthermore, variations of this ligand have of two out of the three N-H moieties. Attempts at full been extensively used on transition metals and will deprotonation by removing the third proton to form allow for comparison of uranium compounds to their K3RITA were unsuccessful, but this was not viewed transition metal counterparts (Schrock, Lee, Liang, as a problem since it is likely that deprotonation of & Davis, 1998). The synthetic procedure follows a this third nitrogen could be successfully performed in generic Buchwald-Hartwig cross-coupling reaction a subsequent reaction. (Wolfe, Wagaw, & Buchwald, 1996). Infrared spectroscopy was used to detect the presences Next, the ligand, K2HRITA, was metallated using of an N-H bond, which shows an absorption at a common uranium(IV) starting material, UCl . To -1 4 3368 cm due to the stretching of the three N-H a THF solution of UCl in the glovebox, K HRITA 1 4 2 bonds. Additional characterization by H NMR was added, resulting in a color change to dark green/ spectroscopy, which analyzes the presence and brown. After stirring for one hour, the volatiles were location of hydrogen atoms in a molecule, was removed in vacuo. The product was extracted into also possible, and the data obtained matched that diethyl ether and filtered over Celite (an inert filtering reported in the literature. The next step, which was agent) to remove the insoluble byproduct, KCl. performed in a glovebox, was the deprotonation Washing the residue with pentane and drying afforded of H3RITA using two equivalents of the strong a green powder, assigned as (THF)2UCl2(HRITA) base benzyl potassium (KCH2Ph). In this reaction, (Scheme 1). the H’s (protons) are removed by a strong base; since this reagent is very sensitive to moisture, this Characterization of (THF)2UCl2(HRITA) was manipulation was performed in an inert dinitrogen accomplished using both IR and 1H NMR atmosphere with dry solvents. Figure 1 displays spectroscopy. The IR spectrum again showed a the MBraun glovebox (left)