Inorganic and Environmental Chemistry of the f-block Metals (Actinides/Lanthanides)

The primary theme in our research encompasses accessing coordination environments among the f-block metals that either:

a) emphasize the chemical properties that distinguish the lanthanides from the actinides

b) bridge to the chemical attributes of the d-block.

As detailed below the intent of these research lines is to obtain fundamental information on the role of the valence orbitals in f-element redox chemistry and metal-ligand multiple bonding. For the actinides the insights gleaned from this research can be applied towards better understanding the chemical behavior of these radioactive metals under environmental conditions, or in developing new separations techniques for processing and remediation. For the lanthanides, the non-traditional coordination environments and oxidation states are targeted to extend the utility of these reactive metals in organic synthesis and catalysis.

As members of the f-block, the electronic structure and chemical properties of both the lanthanides and actinides are governed by the occupancy of valence electrons residing in 4f or 5f orbitals. For the lanthanides, poor shielding of the compact 4f orbitals makes them inaccessible for covalent bonding, which leads to overwhelmingly ionic interactions between these large electropositive metals with the surrounding ligands. Conversely, a combination of relativistic effects and an additional radial node increases both the energy and spatial extent of the valence 5f/6d orbitals of the actinides. These factors enhance the covalency in metal-ligand bonding and permit the actinides to access higher oxidation states and engage in metal-ligand multiple bonding, which most clearly differentiates the chemistry of these metals from those of the lanthanides. This is perhaps best exemplified by the linear dioxo actinyl ions [O=M=O]ⁿ⁺ (M = U, Np, Pu, Am; n = 1, 2), a series whose structural and electronic properties are unique to the actinides.

Our research entails handling radioactive materials utilizing a variety of synthetic techniques, including the manipulation of air-sensitive compounds under inert conditions using dual vacuum/Schlenk lines and a glovebox system. We also incorporate a diverse array of characterization techniques (multinuclear NMR spectroscopy, X-ray crystallography, vibrational (IR/Raman) spectroscopy, UV-VIS-NIR spectrophotometry, electrochemistry, EPR spectroscopy, luminescence, magnetism, EXAFS/XANES, photochemistry, and theoretical studies), to elucidate the unique chemical complexities of the actinides. This research fosters multidisciplinary interactions with groups in radiochemistry, biochemistry, and nuclear engineering at UMC, and additional collaborations with DOE National Laboratories (i.e., Los Alamos).

Current topics of interest:

1) Non-aqueous uranyl(VI) coordination chemistry. We have recently synthesized and structurally characterized new uranyl(VI) chloride and bromide complexes of the general formula UO₂Cl₂L₂ (1) that possess the following desirable characteristics: 1) high-yield benchtop synthesis from readily available starting materials; 2) stability with respect to hydrolysis; 3) good solubility in common organic solvents (including hydrocarbons); and 4) tunability through modification of the steric and/or electronic properties of L. The new derivatives meet all of the above criteria through the use of aliphatic N,N-dialkyl amides (R’C(O)NR₂) as the neutral donor L.

By employing the neutral tridentate chelating ligand (ONO) we can extend this methodology to uranyl(VI) cis-dihalide complexes UO₂X₂(ONO) (2). The enforced stereochemistry confines reactivity to cis coordination sites, providing additional synthetic control (i.e., for reaction pathways that are unavailable to a trans geometry).

These versatile starting compounds allow us to test the resiliency of the linear uranyl(VI) O=U=O unit in the presence of strong-donor equatorial ligands, or provide coordination environments with suitable electronic and steric properties to stabilize the uranium center during electrochemical and photochemical uranyl(VI) reduction.

2) Anhydrous photochemical reduction of uranyl(VI). The uranyl(VI) ion possesses the rare combination of luminescence and photochemical reactivity. The strong oxidizing ability of the *UO₂²⁺ excited state has been utilized in the catalytic aerobic photo-oxidation of organic substrates, and applications have also been directed towards improved uranium reprocessing. We are examining the photochemistry of two general types of uranyl complexes:

i) uranyl(VI) complexes coordinated by chromophoric (i.e., Schiff base) ligands that possess their own distinct photochemical properties. In this case the excited state properties of the ligand and the uranyl(VI) center can combine, as in the energy-transfer process below that activates a coordinated THF molecule:

ii) cationic uranyl(VI) precursors coordinated by bulkyelectron-withdrawing (i.e., phosphine-oxide) equatorial ligands. A combination of steric bulk and a lowering of the energy of the uranium valence orbitals (with the cationic charge) renders the uranyl(V) intermediate less prone to disproportionation. This has allowed a dramatic reversal of the normal pattern of observed reactivity in uranyl chemistry, including an unprecedented reversal of the normal trend in uranyl ligand reactivity whereby the normally labile equatorial coordination sphere remains intact while the usually robust axial site occupancy cleanly and reversibly interchanges between oxo and methoxide ligands.

3) Actinide(III)/lanthanide(III) separations. Current interest in expanding nuclear energy in this country is tied to efforts aimed at developing the next generation (GenIV) of nuclear reactors, with a transition to an Advanced Fuel Cycle of recycling and reprocessing to minimize (and ultimately reduce) the growing radioactive waste. Much of this goal hinges on sound separation techniques under harsh, highly acidic and radiolytic conditions.

One key aspect is the separation of the minor transuranic isotopes (Am, Cm) from spent nuclear fuel. However, as a result of the periodic trend that confirms the true identity of the actinide elements as members of the f-block and becomes more pronounced as the series is progressed, Am and Cm possess the same favored trivalent oxidation state that is so prevalent throughout the whole lanthanide series, making separation of these minor transuranic radionuclides from the more plentiful lanthanide fission products unquestionably the most challenging aspect of closing the nuclear fuel cycle. Separation strategies have relied on the modest degree of covalency in metal-ligand bonding displayed by the actinides relative to the lanthanides. Outlined below are two different approaches to this problem that accentuate this subtle contrast in coordination chemistry:

ii) diglycolamide ligands. It is well established that the hard oxygen donors in organic amides are effective in coordinating the highly oxophilic members of the f-block, and a number of multidentate derivatives have been employed in separations strategies. Among these extractants, the diglycolamides (A) possess advantages of easier synthetic access and gentler stripping processing following extraction in comparison to related counterparts such as malonamides and CMPO. Additionally, coordination of the central ether oxygen permits three of these potentially tridentate ligands to encapsulate a nine-coordinate geometry that is particularly favored for the trivalent metal ions. Accordingly we have structurally characterized both La(III) and uranyl(VI) nitrate derivatives.

ii) redox-active ligands. One factor that could significantly aid An/Ln separations is the capacity for Am and (to a lesser extent) Cm to access higher oxidation states that are chemically distinct and generally unavailable to the lanthanides. This trait correlates with the greater tendency of actinides to engage in covalent bonding interactions and arises from the greater spatial extent and chemical accessibility of the valence 5f orbitals. Redox-active ligands provide a means to coax oxidation to the Am⁴⁺ ion.

U and Ce serve as ideal surrogates for Am/Cm and Eu/Nd, respectively, as these two metals possess comparable ionic radii and a fortuitous match in redox potentials. For example, although U(III) is far easier to oxidize to U(IV) than later actinides such as Am, Ce can grudgingly access a tetravalent state as well, so that the difference in redox potentials between U and Ce roughly coincides with that encountered between Am and Nd. Accordingly, we are examining the coordination chemistry and redox properties of a suite of 1,2-disubstitued redox-active ligands (i.e., catecholates, dithiolenes) with U and Ce.

Among the results obtained thus far include the first example of a lanthanide tetrakis (dithiolene) complex, [Na₅(THF)₁₀Ce(mnt)₄]_¥ (9),which forms a 2-D honeycomb network in the solid state due to unusual dative N-Na bridging interactions from the nitrile termini of the mnt ligands. These dative bonds are readily broken in donor solvents to give soluble [Ce(mnt)₄]^5- units that are intimately associated with solvated sodium counterions.