A stable complex of Terbium +4.

[NNadir]

The paper to which I'll refer in this post is this one: Design, Isolation, and Spectroscopic Analysis of a Tetravalent Terbium Complex Natalie T. Rice, Ivan A. Popov, Dominic R. Russo, John Bacsa, Enrique R. Batista, Ping Yang, Joshua Telser, and Henry S. La Pierre, Journal of the American Chemical Society 2019 141 (33), 13222-13233.

It had not occurred to me that lanthanides, which generally exhibit a +3 oxidation state, other than cerium exhibit a +4 oxidation state, but apparently one has been known for terbium for some time. One is trivial, TbF₄, which is obtained by the treatment of TbF₃ with fluorine gas. Terbium also has a +2 oxidation state, one that is discussed in the same issue of JACS as the above cited paper is listed.

I found myself suddenly interested in terbium this morning when I came across a paper on self healing polymers reliant on a terbium complex, and wandered around some issues in terbium chemistry. The element, which is fairly valuable, running at around $2200/kg as of this writing, has a number of important uses in electronics.

I am generally interested in the Ce⁺⁴/Ce⁺³ couple since it can be used in thermochemical cycles wherein, with high thermodynamic efficiency (in particular in process intensification settings) Ce⁺⁴ can oxidize water, yielding oxygen and hydrogen, i.e. a water splitting cycle. Ce⁺³ can reduce (again at elevated temperatures) carbon dioxide to carbon monoxide for captive use as a key synthetic intermediate useful for replacing applications of petroleum.

Anyway, from the paper:

Introduction

In molecular complexes, the lanthanides are predominantly trivalent (Ln⁺³). In fact, of the 15 lanthanide ions, only cerium is known to have significant solution chemistry in its tetravalent oxidation state (Ce⁺⁴). (1) While examples of low-valent lanthanide complexes have been reported, (2−7) high-valent ions in lanthanide chemistry remain a challenge. Concomitant with the preparation of this article, Mazzanti and co-workers published an example of a molecular Tb⁺⁴ complex in a low-symmetry, oxygen coordination environment in the complex [Tb(OSi(OtBu)₃)₃(κ2-OSi(OtBu)₃]. (8) The accessibility of high-valent lanthanide chemistry could transform two key industrial chemical processes: (1) the beneficiation and purification of lanthanide ores (9) and (2) the separation of the minor actinides from lanthanide fission products in spent nuclear fuel reprocessing. (10) As a result, efforts to extend the aqueous solution chemistry of tetravalent lanthanides to the next two most readily oxidized lanthanides, praseodymium and terbium (Pr4+/Pr⁺³ = +3.2; Tb⁺⁴/Tb⁺³ = +3.1 V vs NHE), (11) have been pursued, but with little success beyond the in situ spectroscopic and potentiometric identification of redox processes. (12−14) From the most fundamental perspective, the isolation of a tetravalent terbium complex is important since it puts a new paramagnetic, isotropic ion on the periodic table and allows the crystal field effects in the tetravalent lanthanides to be benchmarked against the transition metals (Mn2+) and trivalent lanthanides (Gd⁺³).

Historically, in the absence of isolable molecular, tetravalent praseodymium or terbium complexes, cerium has been used as a surrogate to examine the ligand and solvent dependence of the Ln⁺⁴/Ln⁺³ redox couple. (15−17) Depending on the ligand, supporting cation, and solvent, the redox potential can be shifted up to 4 V. Given the large change in ionic radii on oxidation from Ce⁺³ to Ce⁺⁴ (∼0.14 Å), (18) the coordination sphere has also been demonstrated to have kinetic control of the redox process. (19) This ligand control has led to rapid growth of tetravalent cerium coordination chemistry. (20) The use of cerium as a surrogate is validated by the demonstration that tetravalent late actinides, such as berkelium, can be preferentially stabilized by using similar ligand design principles. (21,22) These cerium coordination studies have also inspired the attempted oxidation of molecular terbium complexes in anaerobic and anhydrous conditions, but these efforts have failed in the isolation of a tetravalent terbium complex. (23−25) The limited data on the physical properties of tetravalent terbium are derived from studies on solid-state materials including doped oxides (e.g., ThO₂:Tb⁺⁴), (26,27) bulk binary terbium oxides and fluorides (TbO₂ and TbF₄), (28,29) and other extended solids. (30−34) Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) studies of these solid-state terbium complexes and related tetravalent lanthanide and actinide complexes indicate that the increased covalent bonding present in tetravalent f-block metal–ligand bonds can be employed to stabilize reactive tetravalent lanthanide ions. (26,28,33−36) Herein, we independently report the synthesis and structural characterization of a molecular tetravalent terbium complex that is stable in both solution and the solid state. (37−39) Most importantly, the 4f⁷, 8S^7/2 ground state is validated and probed through EPR, Tb L3-edge XAS, magnetic susceptibility, and density functional theory (DFT) studies...

The oxidant in this case is relatively mild, silver iodide which is reduced to silver metal.

The application discussed, the removal of terbium from the lanthanides obtained from used nuclear fuel is not likely to be significant or meaningful. In the fast fission of plutonium, the yield of terbium (as ¹⁵⁹Tb) is rather low, 0.05% of fissions, with some short lived neutron heavy isotopes, with the longest living of these would be ¹⁶⁰Tb with t_1/2¹⁶⁰Tb being about 72 days, decaying into the stable isotope ¹⁶⁰Dy of the valuable element dysprosium. Neither terbium nor dysprosium are likely to be commercially important products of nuclear fuel reprocessing, at least until enough americium exists to make it available as a reactor fuel.

TbO₂ has been mentioned as a possible constituent of thermochemical water or carbon dioxide splitting cycles, but I don't think that will amount to much either, except perhaps as a dopant.

Naghavi, S.S., Emery, A.A., Hansen, H. et al. Giant onsite electronic entropy enhances the performance of ceria for water splitting. Nat Commun 8, 285 (2017)

From a process standpoint I'm not sure that I favor lanthanide thermochemical cycle catalysts owing to their relative rarity and the fact that they are not subject to flow chemistry as is the much more widely studied SI (sulfur iodine) thermochemical cycle.

Some years ago, in this space, I engaged in a thought experiment about how much cerium would be required to split 1 billion tons of carbon dioxide into oxygen and carbon monoxide: Cerium Requirements to Split One Billion Tons of Carbon Dioxide, the Nuclear v Solar Thermal cases.

I very much doubt that terbium, a far rarer element, could add very much.

The use of thermochemical cycles driven by nuclear heat is a key to the elimination of dependence on dangerous fossil fuels. It is already too late to do this, but with whatever time is left, we should do our best.

Happy New Year.