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Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry
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Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry
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Inorganic Chemistry

Cite this: Inorg. Chem. 2024, 63, 21, 9355–9362
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https://doi.org/10.1021/acs.inorgchem.4c01504
Published May 27, 2024

Copyright © Published 2024 by American Chemical Society. This publication is available under these Terms of Use.

This publication is licensed for personal use by The American Chemical Society.

Copyright © Published 2024 by American Chemical Society

SPECIAL ISSUE

This article is part of the Ligand-Metal Complementarity in Rare Earth and Actinide Chemistry special issue.

The rare-earths are comprised of scandium, yttrium, and the lanthanides (atomic numbers 21, 39, and 57–71). (1) While these elements are essential to modern life, they were historically characterized as simple, perhaps dull, compared to the rich multielectron chemistry of d-block metals in catalysts and enzymes. These descriptions were quite fair and largely true at the time that these elements were being discovered and the first compounds containing them were being synthesized, although much progress has been made to overturn this view of the rare earths. Similarly, discussions about actinide chemistry are sometimes limited to research pertinent to the nuclear industry, which, while comprising technologies vital to modern society, does not celebrate the fascinating chemical space that much of the actinide series occupies. These historic characterizations of most of the rare-earth and actinide elements persist in some textbooks, and we acknowledge that it is a helpful place to start; however, altogether these series comprise 32 elements─27% of the 118 known by 2024─and we suggest it would be an unfortunate twist of physics if such a large proportion of the Periodic Table were genuinely dull. Indeed, as this Inorganic Chemistry Forum Issue “Ligand-Metal Complementarity in Rare-Earth and Actinide Chemistry” highlights, the chemistry─let alone the physics, that is, spectroscopy and magnetism─of these elements is rich and depends on ligands!

The “earth” part of “rare-earth elements” historically refers to ores that could be dissolved in mineral acids and the subsequent solid oxides obtained after processing. The “rare” part is often attributed to the rarity of gadolinite, the first ore from which individual rare-earth elements were isolated, which is found only in one location─Ytterby, Sweden. (2−4) Readers of Inorganic Chemistry will undoubtedly know that many rare-earth elements are not particularly rare, with the most abundant, cerium at 66.5 ppm in the Earth’s crust, being as common as copper at 60 ppm. (5) The least abundant nonradioactive member, thulium at 0.52 ppm, is roughly as prevalent as iodine at 0.45 ppm and more than 10 times as abundant as selenium at 0.05 ppm─the latter two of which are essential to life. (5) However, ores with economically viable concentrations of rare-earths are not common; they are unevenly dispersed around the world, and the separation of neighboring rare-earth elements from each other can be complex and tedious. By way of example, we mention the heroic work of James, who produced the first high-purity sample of thulium as various salts, a summary of which appeared in 1911. (6) Working in gallon-sized cast-iron vessels on multikilogram scales and relying upon fractional crystallization of bromate salts, James verified that “after about 15,000 operations the absorption spectrum underwent no change.” (6) Clearly, fractional crystallization of bromates works, but modern processes use ion-exchange resins that exploit the very small size differences between rare-earth cations to separate them more efficiently. (4)

The history of rare-earth element separation and the quest to obtain pure samples is littered with frustrating incremental advances, such as “pure” samples later being found to be mixtures by spectroscopic or chemical tests. Berzelius and Hisinger, along with Klaproth working independently, isolated cerium oxides or “ceria” from samples of gadolinite. (2) Mosander, a former student of Berzelius, found that upon further treatment, samples of ceria could be separated into at least three further fractions, one of which was dubbed “lanthan”, or lanthanum in English, derived from the Greek “λανθάνω” meaning hidden or to escape notice. The other two fractions were didymium, which was subsequently shown to be comprised of samarium, praseodymium, and neodymium─and also true ceria. (7) Thus, the lanthanide series derives its name from the hidden nature of its first member, lanthanum. Many of the fascinating developments in rare-earth and actinide chemistry over the past few decades have come about by challenging conventions and through advancements in ligand design to bring out the “hidden nature” of these elements.

In the modern era, samples of high-purity rare-earth (and to some extent early-actinide) materials are available in many research laboratories that wish to explore their properties in coordination complexes. Previous issues of Inorganic Chemistry have focused on ligand design strategies for f-element chelation (8) or trends of current exploration in lanthanide chemistry. (9) The impetus for this Forum Issue stems partly from our experiences of working either briefly or for an entire Ph.D. with one of the pioneers in f-element coordination chemistry, William J. Evans. Evans’ work in low-oxidation-state molecular chemistry, along with that of Michael Lappert, Geoffrey Cloke, and others, has redefined the limits of redox chemistry for all of the rare earths (save promethium due to its high radioactivity) and also thorium, and uranium through to plutonium; see refs (10−22) for just a small fraction of these works, with no implied preferences on our part. A defining feature of Evans’ career, which extends to the very first reactions with rare earths in his group, is the idea that if you put the right ligand on the right rare earth or actinide, exciting things will happen and that one should set assumptions, and the rulebook, aside when doing so. (23−25)

Throughout this Forum Issue there is an overarching theme of using specific ligands to tame the coordination environments of rare-earth (RE or Ln) and actinide (An) ions. Some of the research focuses on using different ligand systems to minimize oligomerization, or prevent the coordination of undesirable Lewis basic moieties, while others seek to characterize small or large multimetallic complexes that arise due to fine control of the ligand properties or solvent conditions. In the following sections, a brief overview of each paper is provided to tempt the reader into exploring some of the fascinating research being undertaken in this area.

While complexes of rare-earth cations are not known to engage in single-electron or multielectron catalytic reactions, they are all potent Lewis acids with similar coordination chemistry, which means that Lewis acid catalyst systems can be fine-tuned based on the metal size. Fieser and co-workers have examined the influence of axial Lewis-base donor properties in [RE(CH2SiMe3)3(THF)2] (RE = Y, Sm, Gd, Tm) precatalysts on the stereoselectivity and rate of isoprene polymerization. (26) Further variables, such as the rate and order of precursor addition and the stoichiometry of the catalyst activator, were all found to influence the reaction significantly, but under the right conditions, a living polymerization reaction was achievable. Zhu and Wang’s team further explored the Lewis acid chemistry of rare earths by demonstrating the insertion of a range of nitrile and isonitrile molecules into metal–alkyl bonds in [RE(L)(CH2SiMe3)(THF)2] (Ln = Y, Yb, Lu; L = {2,5-(2-C4H3N)CPh2)2(C4H2NMe)}) , forming unusual β-diketiminato, imidazolyl, and indolyl moieties. (27) For some of these, a dependence on both the reaction temperature and reaction stoichiometry is found, which shows that both kinetic and thermodynamic products can be accessed under appropriate conditions. (27) Continuing the theme of using rare-earth cations to assemble p-block atoms, Dehnen and co-workers shift our focus from carbon, to the heavy Group 13 and 15 members indium and bismuth. (28) While numerous examples of d-block ions encapsulated by Zintl phases are known, analogous rare-earth and actinide examples are rare. Treatment of the solid Zintl phase K10Ga3Bi6.65 with [In(Mes)3] (Mes = {C6H2-2,4,6-Me3}) and 2.2.2-cryptand in the presence of [La(C5Me4H)3] gives either the [La@In2Bi11]4– anion or the larger [{La@In2Bi11}2(μ-Bi)2]6–, where longer reaction times produce the latter, and hence, it is likely the thermodynamic product. In both, [K(2.2.2-cryptand)] cations provide charge balance. Bridging bismuth groups in the dimeric complex show interactions with one potassium cation each, somewhat “softening” the charge buildup on the indium sites, which have formal –2 charges, stabilizing this complex relative to the monomeric example. When the same reaction was explored without [La(C5Me4H)3], the researchers isolated a smaller cage in [Bi6(InMes)(InMes2)]3–, and also [{InMes3}2]2– from the same reaction, both as their [K(2.2.2-cryptand)] salts. This work demonstrates that the LaIII cation provides a significant stabilizing role in assembling larger cages. (28)

While the potent Lewis acidity and large coordination spheres of rare-earth cations are useful in building bigger bismuth cages, it makes the isolation of high-performance single-molecule magnets (SMMs) challenging because well-defined and specific coordination environments are required to maximize their properties. Several papers in this issue relate to the synthesis of different types of sandwich complexes; however, if you have ever wished to know “What is a sandwich complex?”, then an Editorial by Goodwin has this covered. (29) In their contribution, Chilton, Mills, and colleagues demonstrated that the sought-after SMM candidate [Dy(Cp*)2]+ (Cp* = {C5Me5}) cannot be isolated free of equatorial donor coordination, even when extremely weakly donating haloarenes are used, resulting in the first f-block bis-haloarene adducts [Dy(Cp*)2(PhX-κ-X)2][Al{OC(CF3)3}4] (X = F, Cl, Br). (30) Even when the size of the supporting ligands is increased, as in a second paper by Chilton and Mills that features [Dy(Cp*)(Cpttt)(XnC6H6–n)]+ (Cpttt = {C5H2-1,2,4-tBu3}), equatorial haloarene adducts could be isolated, although this class has better SMM characteristics than the previous due to the reduced equatorial donation. (31) Roesky and co-workers have explored the influence of arsolyl ligands, {η5-AsC4R4}, on the coordination environment and magnetic properties of several LnIII ions (Sm, Dy, and Er), finding that the nuclearity of [RE(η8-COT)(η5n-AsC4R4)]n (Ln = Sm, Dy, Er; n = 1, 2; COT = {C8H8}) complexes depends on the steric protection at the 2,5 positions of the {AsC4R4} ring with large SmIII ions, but with small DyIII and ErIII ions, monomeric complexes are obtained even with small R groups such as in {AsC4Me4}. The softer {AsC4R4} ligand gives magnetic properties at ErIII that are similar to those of the phosphorus analog, but slightly worse than those of the corresponding CpR (CpR = substituted Cp ligand) congeners. (32) Complexes of ErIII using the COT ligand, and 1,4-(trialkyl)silylated variants thereof, have been known for some time, and many have SMM properties. While regioisomers of silylated COT rings have been reported previously, methods for their purposeful isolation have remained elusive; therefore, the influence of multiple substitution patterns on the SMM properties of ErIII has not been systematically explored. In their work, Ding, Zhu, and Zheng’s team presented a unique twist on the area by exploring the differences between 1,3-, 1,4-, and also 1,5-substituted analogues of the {C8H6-(SiMe3)2} ligand on the SMM properties of ErIII complexes. (33) Rogachev, Shatruk, Petrukhina, and their team continued the COT sandwich complex theme but instead have focused on a benzannulated analogue, dibenzo[a,e]cyclooctatetraene (DBCOT), which forms charge-separated ion pairs, [RE(DBCOT)(THF)4][RE(DBCOT)2] (RE = Y, La, Gd, Tb, Dy, Er). (34) The SMM properties of the DyIII analogue were explored and showed a complicated relaxation pathway due to the presence of structurally dissimilar anionic and cationic DyIII fragments within the complex. (34)

Poor valence orbital overlap between trivalent rare-earth ions and ligand orbitals makes the isolation of multiply bonded species difficult because the linkages are highly polarized and very reactive. Thus, while alkylidenes and alkylidynes are well-represented with d-block ions, examples with rare-earth cations are much rarer or non-existent in the case of terminal unsupported variants. Nevertheless, in their contribution, Anwander’s group revisits and vastly expands the chemistry of [La(TptBu,Me)(μ3-CH2){μ-CH3(AlMe2)}2] (TptBu,Me = {HB(N2C3-2-Me-4-tBu)3}), previously reported by some of the same authors as the first lanthanide analogue of Tebbe’s reagent─[Ti(Cp)2{(μ-Cl)(μ-CH2)AlMe2]. (35) In this new work, routes to rare-earth methylidene complexes supported by TpR,R-type ligands were exhaustively explored, revealing a delicate balance between the ligand steric bulk, rare-earth ion radius, and ligand delivery route. For example, while [La(TptBu,Me)(μ3-CH2){μ-CH3(AlMe2)}2] can be isolated from the reaction of KTptBu,Me with [La(AlMe4)3], the reaction mixture contains additional products that have now been elucidated and result from the cleavage of N–B bonds in the TptBu,Me ligand and another that results from cyclometalation of pendant tBu groups on TptBu,Me along with a 1,2-borotropic shift of one of the pyrazolyl ligand arms. (35) An alternative route to methylidene complexes was explored via protonolysis between [RE(AlMe4)3] (RE = La, Nd) and HTptBu,Me, which produced dimeric [{RE(AlMe42Me)2}2(N2C3-2-tBu-4-Me)2] as the sole product. By switching to the smaller TpMe,Me ligand (TpMe,Me = {HB(N2C3-2,4-Me)3} and returning to salt-metathesis routes, the researchers were able to isolate methylaluminate complexes [RE(TpMe,Me)(AlMe42Me)(AlMe4Me)] with small Y and Lu, while methylidene complexes, [RE(TpMe,Me)(μ3-CH2){μ-CH3(AlMe2)}2], could be isolated with larger Nd and Sm. Their reactivity with ketones was also examined.

Contributions in the previous three sections have leveraged the Lewis acidity of rare-earth ions for chemical transformations or as part of designing molecular structures with desirable stereochemical properties to tune their electronic structures. Separate works by the groups of Knope (36) and also Vlaisavljevich and Daly (37) show another side of the Lewis acidity, where taming it requires judicious choice of reaction conditions and where seemingly minor ligand features can drive discrete product formation or distributions. The construction of high-nuclearity rare-earth clusters of defined composition has applications in catalysis, optical devices, and potentially fuel storage. However, a deeper understanding of the ligand and reaction conditions that produce specific clusters is challenging. For example, Knope and co-workers showed that using ethanol solvent in the reaction of Hacac (acac = {HC([C═O]CH3)2}) with Ce(NO3)3·(H2O)6 produces [Ce(acac)4], but changing the solvent to methanol leads to an oxo cluster with a {Ce6IVCe4IIIO8(OMe)4}16+ core; the subsequent addition of acetonitrile to this cluster further changes the core to {Ce10IVCe2IIIO12(OH)4}18+. This substantial change in the redox states of core cerium atoms from a simple change in the solvent conditions demonstrates the oxidative flexibility of these large oxo clusters. (36) Separate work by Vlaisavljevich’s and Daly’s groups showed that the solid-state speciation of lanthanide(III) (Nd, Sm, Tb, and Er) and uranium(III) phosphinodiboronates, {PR2(H3B)2} (R = iPr, Et, Me), has a large dependence on the identity of R groups at the phosphorus atom. Moreover, the authors showed that the degree of oligomerization in solution depends strongly on the identity of the R groups, which can be correlated to measurements of the B–P–B angles from the solid-state structures. (37) Allen and co-workers described their work on tris[2-(2-methoxyethoxy)ethyl]amine, an acyclic analogue to 2.2.2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; or 2.2.2-crypt henceforth) designed to tame large and small rare-earth and actinide ions. With softer lanthanide(II) ions (Sm, Eu, and Yb) and an array of different counteranions, a huge variety of coordination modes and geometries can be realized, including monomers and dimers, neutral species, and separated ion pairs. (38) Alongside this, LaIII, CeIII, and UIII were successfully encapsulated in this new framework, and ultraviolet–visible-near-infrared absorption and luminescence spectroscopy of these, along with the X-ray photoelectron spectroscopy data of the LaIII and CeIII complexes, adds valuable insight into the coordination chemistry and photophysics of these ions across diverse environments. While LaIII cations in isolation are somewhat spectroscopically uninteresting, at least at visible wavelengths of light, due to the [Xe] closed-shell electron configuration, the solution chemistry of LaIII is often comparable to that of CeIII and UIII, due to similarities in their ionic radii, and so it acts as a useful diamagnetic surrogate for the exploration of new ligand frameworks for members at the start of the 4f and 5f rows. (39) In their contribution, La Pierre and co-workers explored a novel bulky phosphoramide ligand, {OP(NHtBu)(tBuNC2H4NtBu)}, with LaIII and showed that a variety of heteroleptic motifs can be isolated depending on the reaction stoichiometry. (40) For example, the reaction of 2 equivalents of K{OP(NHtBu)(tBuNC2H4NtBu)} with [LaI3(THF)4] in the presence of 4-dimethylaminopyridine (DMAP) afforded [La{OP(NHtBu)(tBuNC2H4NtBu)}2(I)(DMAP)], while the use of 3 equivalents of the K salt gave [La{OP(NHtBu)(tBuNC2H4NtBu)}3(THF)], where two of the ligands bind in a κO,N fashion and the third through the oxygen atom alone. The authors extended the work to show that complexes featuring a mixture of both anionic and neutral ligands could be isolated, as well as a benzyl derivative, [La{OP(NHtBu)(tBuNC2H4NtBu)}2(CH2Ph)(THF)]. The isolation of a heteroleptic fluoro-anilide complex, [La{OP(NHtBu)(tBuNC2H4NtBu)}3{N(H)C6H4-4-F}], was successful, but attempted deprotonation of the anilido N(H) group with benzylpotassium instead led to elimination of K{OP(NHtBu)(tBuNC2H4NtBu)}. (40) Extension of this flexible ligand set to other rare-earth cations is sure to open up interesting new chemical space.

The redox chemistry of rare-earth metals and their compounds is somewhat limited, although advances continue in this area in both high- and low-oxidation states. In the case of the elements samarium, europium, and ytterbium, their behavior often differs from the rest of the series, particularly so in the metallic state. (4) They are often able to act in oxidative addition chemistry to give products in the MII oxidation state under mild reaction conditions, much like the Group 2 elements. Junk, Jaroschik, and co-workers exploited this in their synthesis of ethylene-linked ansa-octaphenyllanthanocenes, [RE(C5Ph4CH2)2(THF)2] (RE = Sm, Eu, Yb), from 1,2,3,4-tetraphenylfulvene─itself reported via a novel route by these authors. (41) The EuII complex is luminescent and has been compared to related complexes [Eu(C5Ph4H)2(DME)] and [Eu(C5Ph5)2], revealing a complicated relationship between the degree of solvation, M···C5–centroid distance, and fluorescence emission maximum. (41) In contrast to the tendency of samarium, europium, and ytterbium to form divalent compounds, the team of Anderson, Klamm, Mason, and Tondreau report the oxidative dissolution of cerium and holmium metals in molten GaCl3, which affords trivalent [Ga][RE(GaCl42Cl)4] (RE = Ce, Ho) from the melt along with cocrystallized [Ga][GaCl4], (42) the former of which was purified by recrystallization from fluorobenzene to give [Ga(η6-FC6H5)2][RE(GaCl42Cl)4]. The oxidizing molten GaCl3 solvent conditions offer a rare route to soluble trivalent rare-earth complexes under mild conditions that do not require hydrocarbon solvents, except in a final purification step. Cerium is unique among rare earths in being able to access the tetravalent oxidation state across a wide range of ligand types and under both aerobic and anhydrous conditions. In their contribution, the groups of Bart and Schelter explored the redox chemistry of cerium with the noninnocent amidophenolate ligand “Clamp” (H6Clamp = {N(C6H4-2-NHC6H2-2-OH-3,5-tBu)3}), which is heptadentate and anchored with a basal amine group, and also the simple bidentate ligand, “ap” (H2ap = {HOC6H2-2-(NHDipp)-4,6-tBu}; Dipp = 2,6-di-iso-propylphenyl). (43) The groups found that CeIII complexes with both ligands exhibit metal- and ligand-centered redox events, which may help in the design of molecules that can engage in multielectron reactivity using metals, such as cerium, that are not conventionally able to do so. In their contribution, Walensky and co-workers explored the photophysics and electrochemistry of a mononuclear organometallic CeIII complex, [Ce(Cp*)2(ODipp)] (ODipp = OC6H3-2,6-iPr2), toward a better understanding of the ground- and excited-state electronic structures, and to determine the available redox chemistry. (44) As is typical of CeIII, they observed one intense dipole-allowed 4f → 5d excitation in the absorbance spectrum at 401 nm (ε = 3653 M–1 cm–1) along with a weaker shoulder at 347 nm (ε = 2000 M–1 cm–1), due relaxation to the two spin–orbit ground states (2F5/2 and 2F7/2). They found that this CeIII complex is readily oxidized with simple cuprous halides (CuCl, CuBr, CuI) to give structurally characterized mononuclear CeIV complexes [Ce(Cp*)2(ODipp)(X)] (X = Cl, Br, I). Transmetalation with AgF using either the bromo or iodo complexes gave [Ce(Cp*)2(ODipp)(F)], although the molecular structure was not determined. 1H NMR spectroscopy of these CeIV complexes reveals that as the halide series is descended, δ1H for the Cp* Me groups moves progressively downfield (i.e., Cl < Br < I), and electrochemistry shows that the CeIV/III couple becomes increasingly more anodic─that is, the complexes become more easily reduced with the heavier halides. (44) Curiously, the fluoride congener, [Ce(Cp*)2(ODipp)(F)], was found to be highly unstable, even in the solid state. The recalcitrant nature of rare-earth redox chemistry can instead be of benefit to certain areas of research. Multinuclear lanthanide complexes with radial-anion bridges have historically been record-setting SMMs, (45,46) and so a better understanding of the ligand (both bridging and ancillary at the metal) features that maximize desirable magnetic properties is patently important. In their contribution, Demir’s group used variable-temperature electron paramagnetic resonance spectroscopy combined with density functional theory (DFT) calculations to experimentally and computationally determine the ligand and metal spin density in mononuclear [Y{C(NiPr)2(N[SiMe3]2)}2(2,2′-bipy)] (2,2′-bipy = 2,2′-bipyridine radical anion) and dinuclear [Y{C(NiPr)2(N[SiMe3]2)}2]2(Bbim)] (Bbim = bis-benzimidizoyl radical anion) complexes. (47) The guanidinate ligand systems used in this study are highly customizable and represent tremendous scope for tuning the properties of future radical-bridged complexes using ligand design.

The actinides thorium, uranium, neptunium, and plutonium appear early in the 5f series and have isotopes with half-lives long enough to explore their chemistry in detail. They differ from rare-earth elements and transition metals in ways that sometimes give rise to divergent coordination chemistry and reactivity. The energetic proximity of valence 5f and 6d orbital sets means that highly covalent metal–element multiple bonding is possible under suitable conditions, most notably in the actinyl {AnO2}n+ linear dioxo species. (48) One powerful yet accessible technique for studying metal–element bonding in diamagnetic complexes is the combination of NMR spectroscopy alongside DFT calculations. This provides an experimental grounding to calculated results, and it has allowed Autschbach, Hayton, and co-workers to explore U–Caryl bonding in rare UVI organometallic species [Cat][UO2{C6H2-2,4,6-(CF3)3}3] (Cat = [Li(THF)3] or [Li(Et2O)3(THF)]). (49) They find appreciable covalent character in the U–Caryl bond and were able to excellently reproduce the experimental data, allowing them to describe trends in the 5f contribution to the bonding in these complexes and to compare this to other UVI organometallic complexes. (49) Continuing the theme of actinide-ligand covalency, the groups of Zi, Ding, and Walter reported a comparative tour de force case study between thorium(IV) and uranium(IV) imido complexes, [An(Cpttt)2(=NC6H4-4-Me)] (An = Th, U). (50) In this work, small structural differences, such as the uranium complex being slightly more sterically congested due to increased covalency, and hence tighter ligand binding, and the smaller size of UIV versus ThIV, are used to rationalize a substantial body of divergent reactivity. For example, the thorium complex reacts with internal alkynes to give [Th(Cpttt)2{N(C6H4-4-Me)C2R2N,C}] (R = Me, Ph), whereas the uranium analogue is unreactive. Conversely, because the redox chemistry of thorium is limited, the reaction with Ph2Se2 results in the Cpttt anion acting as a sacrificial reductant, producing a half-sandwich complex product, [Th(Cpttt)(SePh){μ-N(C6H4-4-Me)}]2, whereas the uranium analogue undergoes one-electron oxidation to give [U(Cpttt)2(═NC6H4-4-Me)(SePh)]. (50) Eisen and co-workers continued the theme of small-molecule reactivity with ThIV bent-metallocenes by exploring the reactivity of [Th(Cp*)2(Cl)2] with substituted cyclopropenyl imines, finding an unexpected temperature dependence. (51) The addition of {HN═C(CNR2)2} (R = iPr, Cy) to [Th(Cp*)2(Cl)2] at −20 °C affords simple Lewis-base adducts [Th(Cp*)2(Cl)2{HN═C(CNR2)2}] (R = iPr, Cy), where donation from the imine nitrogen atom to the metal increases aromatic stabilization in the cyclopropene ring. However, performing these reactions at room temperature causes the cyclopropene ring to be cleaved and gives [Th(Cp*)2(Cl)2{N≡C(CNR2)C(H)(NR2)}] (R = iPr, Cy), presumably via a 1,3-hydride shift. These studies were extended to the use of the methyl and mixed methyl/chloride complexes [Th(Cp*)2(Me)(R′)] (R′ = Cl, Me) under the same sets of contrasting conditions. Under kinetic control (i.e., at −20 °C), the methyl anion deprotonates these imines, but under the thermodynamic regime at room temperature, the cyclopropene rings are again cleaved. (51) In another show of how versatile the {M(Cp*)2}n+ framework is in rare-earth and actinide chemistry, Fortier and co-workers reported the isolation of a uranium cis-bis-phosphoranocarbene complex, [U(Cp*)2{C(H)PPh3}2], and explored its reactivity with alcohols. (52) This complex is competent in protonolysis reactions with, for example, HODipp, to give the monoaryloxide complex [U(Cp*)2{C(H)PPh3}(ODipp)], which itself can be reacted with stoichiometric or adventitious H2O to afford the mono-oxo complex [Ph3PCH3][UO(Cp*)2(ODipp)]. During the synthesis of [U(Cp*)2{C(H)PPh3}2] from [U(Cp*)2(Me)2], the authors identified an intermediate species, [U(Cp*)2{C(H)PPh3}(Me)], by 1H and 31P{1H} NMR spectroscopy and also found that thermolysis of the reaction mixture affords the metalated complex [U(Cp*)2{C(H)PPh2(C6H5C)}]. (52)

The single-electron and multielectron redox chemistry of uranium is well-known, as is the ability for UIII to engage in challenging reductions. (53,54) However, the formal UIV/III E1/2 for the archetypal uranium(III) silylamide complex [U(N″)3] (N″ = {N(SiMe3)2}) is only −1.4 V vs Fc+/0, which puts it comfortably outside the range of the reduction potential of processes such as the dearomatization of pyridine (ca. −2.9 V vs Fc+/0, though this will depend on the solvent used). While the UII complex [K(2.2.2-crypt)][U(N″)3] (E1/2 = −2.8 V vs Fc+/0 for UIII/II) is capable of this, it is a challenging compound to isolate. (55,56) Mazzanti and co-workers reported the isolation of a UIII cyclometalated complex, [K(2.2.2-cryptand)][U(N″)2{N(SiMe3)(SiMe2CH2N,C)], with an extremely negative UIV/III E1/2 of −2.6 V that is capable of reductively dimerizing pyridine. (57) This UIII complex was isolated in high yield from the room-temperature KC8 reduction of [U(N″)3] in the presence of 2.2.2-cryptand, liberating 1/2 equiv of H2 in the process. The ability to access UII-like reduction potentials from such a readily available precursor is sure to drive further work in this area. A vast array of exotic UIV element linkages are known, but in many instances, the oxidation of these to UV or UVI is not successful, so new and more strongly donating frameworks are attractive for this. In their work, Liddle reports the first use of a strongly donating and coordinatively flexible boryloxide ligand, {OB(NDippCH)2}, on an actinide. (58) By installation of the ligand on UIII using protonolysis with [U(N″)3] in toluene (i.e., free of coordinating solvents), the homoleptic [U{OB(NDippCH)2}3] complex was isolated, which features an η6-arene interaction between the uranium center and a single Dipp group. Salt metathesis between UCl4 and 3 equiv of K{OB(NDippCH)2} in THF gave pseudo-octahedral [U{OB(NDippCH)2}2(Cl)2(THF)2], where the two boryloxide groups are trans-disposed, along with unreacted K{OB(NDippCH)2}. When a two-step procedure was employed in which the previous reaction mixture was dried and then refluxed in toluene, the pseudo-trigonal-bipyramidal [U{OB(NDippCH)2}3(Cl)(THF)] complex could be isolated. The reaction of the UIII complex with adamantyl (Ad) azide afforded the uranium(IV) imido complex [U(NAd){OB(NDippCH)2}3], not only demonstrating the use of a new ligand class in actinide chemistry but also showing that it is capable of supporting uranium cations over at least three oxidation states.

The separation of actinide elements uranium, neptunium, and plutonium from lanthanide fission products in spent nuclear fuel is, while logistically demanding, somewhat readily achieved by their substantially different redox chemistries─accepting that neptunium is more complicated due to the propensity for the readily accessible{NpVO2}+ cation to disproportionate into NpIII and {NpVIO2}2+ and contaminate both the liquid and solid product streams. (48) Conversely, the transplutonium elements americium and curium predominantly exist as AnIII ions under aerobic conditions and have ionic radii similar to those of several LnIII ions that appear early in the series. (39) Therefore, separating these minor actinides from trivalent lanthanides is an ongoing challenge requiring the design of specialty ligand systems that exploit small differences in metal–ligand covalency to engender larger separation factors. (59−62) This experimentally demanding synthetic work sometimes shows that minute changes to the ligand frameworks can drive large differences in behavior. In their work, Wu and Shi’s team applied DFT calculations to rationalize the separation factors in two known phenanthroline-based extractants, alongside two novel frameworks, and found that the degree of nitrogen versus oxygen binding in these systems strongly influences their behavior and that softer ligands, which overall bind both AmIII and EuIII more weakly than harder ligands, offer better selectivity for AmIII over EuIII. (63) The divergence between the ligand binding in lanthanides versus actinides is often attributed to differences in the 4f vs 5f orbital contributions to bond formation. For actinides early in the series, thorium and uranium in particular, the availability of both 5f and 6d orbitals to accept donation from ligands often results in complex bond descriptions. Experimental methods to probe these properties exist, such as X-ray absorption spectroscopy, but luminescence spectroscopy combined with DFT calculations is a convenient laboratory-scale method to probe the nature of ligand-to-metal or metal-to-ligand charge-transfer events and hence begin to deconvolute the nature of the metal–ligand interaction. In their contribution, Milsmann’s and Matson’s groups explored the use of pyridine dipyrrolide ligands on both ThIV and UIV. (64) While both heteroleptic [Th(MesPDPPh)(Cl)2(THF)] (MesPDPPh = {NC6H3-2,6-(NC4H-3-Ph-5-Mes)2}) and homoleptic [Th(MesPDPPh)2] are highly luminescent, the UIV analogues are not, which is attributed to self-quenching due to ligand-to-metal charge transfer and transitions within the 5f manifold by analogy with d-block analogues with this ligand system. (64)

In conclusion, the collection of works in this Forum Issue “Ligand-Metal Complementarity in Rare-Earth and Actinide Chemistry” illuminates the expansive application space available to these elements that reside mainly near the bottom of the Periodic Table, at the nexus between catalysis, separation science, nuclear fuel research, molecular magnetism along with fundamental coordination/redox chemistries, and spectroscopy. Looking forward, we will not be surprised to see the pioneers in the field of rare-earth and actinide chemistry continue to push the limits of what is considered possible, especially into the heavier actinides, with the slow but steady addition of transuranium facility capabilities coming online in research institutions around the world.

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      Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.

    Biographies

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    Jordan F. Corbey earned her B.S. in Chemistry (2010) from Eastern Washington University and her Ph.D. in Chemistry (2015) from the University of California, Irvine, where she studied air-sensitive rare-earth and f-element coordination complexes under Prof. William J. Evans. Dr. Corbey is currently a staff scientist at Pacific Northwest National Laboratory in Richland, WA, where her research focuses on the synthesis and analysis of radioactive materials for national security missions.

    Conrad A. P. Goodwin undertook his Ph.D. (2013–2017) at the University of Manchester, studying low-coordinate rare-earth and actinide coordination chemistry using sterically demanding bis-silylamide ligands under Prof. David P. Mills. He followed his graduate work with a brief postdoctoral position at the University of Manchester working on organometallic SMMs and then a J. Robert Oppenheimer Distinguished Postdoctoral Research Fellowship (2018–2021) at Los Alamos National Laboratory under Dr Andrew J. Gaunt. Dr. Goodwin is now a Royal Society University Research Fellow at the University of Manchester, where his group researches the physicochemical properties of rare-earth and actinide complexes with low and high oxidation states.

    References

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      Modder, D. K.; Scopelliti, R.; Mazzanti, M. Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation. Inorg. Chem. 2024,  DOI: 10.1021/acs.inorgchem.3c03668
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      Dan, X.; Du, J.; Zhang, S.; Seed, J. A.; Perfetti, M.; Tuna, F.; Wooles, A. J.; Liddle, S. T. Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes. Inorg. Chem. 2024,  DOI: 10.1021/acs.inorgchem.3c04275
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      Su, L.-L.; Wu, Q.-Y.; Wang, C.-Z.; Lan, J.-H.; Shi, W.-Q. Heterocyclic Ligands with Different N/O Donor Modes for Am(III)/Eu(III) Separation: A Theoretical Perspective. Inorg. Chem. 2023,  DOI: 10.1021/acs.inorgchem.3c03229
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      Valerio, L. R.; Hakey, B. M.; Leary, D. C.; Stockdale, E.; Brennessel, W. W.; Milsmann, C.; Matson, E. M. Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes. Inorg. Chem. 2024,  DOI: 10.1021/acs.inorgchem.3c04391

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    • Jordan F. Corbey

      Jordan F. Corbey earned her B.S. in Chemistry (2010) from Eastern Washington University and her Ph.D. in Chemistry (2015) from the University of California, Irvine, where she studied air-sensitive rare-earth and f-element coordination complexes under Prof. William J. Evans. Dr. Corbey is currently a staff scientist at Pacific Northwest National Laboratory in Richland, WA, where her research focuses on the synthesis and analysis of radioactive materials for national security missions.

      Conrad A. P. Goodwin

      Conrad A. P. Goodwin undertook his Ph.D. (2013–2017) at the University of Manchester, studying low-coordinate rare-earth and actinide coordination chemistry using sterically demanding bis-silylamide ligands under Prof. David P. Mills. He followed his graduate work with a brief postdoctoral position at the University of Manchester working on organometallic SMMs and then a J. Robert Oppenheimer Distinguished Postdoctoral Research Fellowship (2018–2021) at Los Alamos National Laboratory under Dr Andrew J. Gaunt. Dr. Goodwin is now a Royal Society University Research Fellow at the University of Manchester, where his group researches the physicochemical properties of rare-earth and actinide complexes with low and high oxidation states.

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