Lanthanide Photocatalysis

High-efficiency dysprosium-ion extraction enabled by a biomimetic nanofluidic channel

Biological ion channels exhibit high selectivity and permeability of ions because of their asymmetrical pore structures and surface chemistries. Here, we demonstrate a biomimetic nanofluidic channel (BNC) with an asymmetrical structure and glycyl-L-proline (GLP) -functionalization for ultrafast, selective, and unidirectional Dy3+ extraction over other lanthanide (Ln3+) ions with very similar electronic configurations. The selective extraction mainly depends on the amplified chemical affinity differences between the Ln3+ ions and GLPs in nanoconfinement. In particular, the conductivities of Ln3+ ions across the BNC even reach up to two orders of magnitude higher than in a bulk solution, and a high Dy3+/Nd3+ selectivity of approximately 60 could be achieved. The designed BNC can effectively extract Dy3+ ions with ultralow concentrations and thereby purify Nd3+ ions to an ultimate content of 99.8 wt.%, which contribute to the recycling of rare earth resources and environmental protection. Theoretical simulations reveal that the BNC preferentially binds to Dy3+ ion due to its highest affinity among Ln3+ ions in nanoconfinement, which attributes to the coupling of ion radius and coordination matching. These findings suggest that BNC-based ion selectivity system provides alternative routes to achieving highly efficient lanthanide separation.

Photon-Modulated Bond Covalency of [Sm(II)(η9-C9H9)2]

Lanthanides are widely assumed not to form covalent bonds due to the localized nature of their 4f valence electrons. This work demonstrates that the ionic bond of Sm(II) with cyclononatetraenyl (η9-C9H9-) in [Sm(η9-C9H9)2] can be modulated and becomes more covalent by photon-induced transfer of Sm 4f electrons to Sm 5d orbitals. This photon-induced change in bonding properties facilitates a subsequent reconfiguration of [Sm(η9-C9H9)2]. As a result, Sm-C bond length contraction is detected and the local Sm coordination environment exhibits more extensive disorder. Both Sm 4f and 5d electrons have increased participation in covalent Sm-ligand interactions. The Sm L3-edge valence band resonant inelastic X-ray scattering (VB-RIXS), high-resolution X-ray absorption near-edge structure (HR-XANES), and quantum chemical computations showcase a spectroscopic methodology for in-depth studies of bond covalency of lanthanide atoms.

Assembly of a trapped valent CeIII/IV-TCNQ complex through metal-ligand redox cooperativity

Complex (Cp3CeIV)2(TCNQ)(CeIIICp3)2 (1) was prepared by reducing neutral TCNQ0 (tetracyanoquinodimethane) with Cp3Ce(THF). Two types of cerium centres with a dianionic TCNQ2- moiety are present in 1, wherein each of the four cyano-groups are bound by a cation. Formation of this trapped-valent organocerium compound 1 is facilitated by metal-ligand redox cooperativity. Characterization of 1 was carried out using structural-, magnetometry-, and IR-spectroscopic analyses. Photophysical studies on this compound reveal CeIII luminescence, and opens up avenues for promising multifunctional, mixed-valent lanthanide materials.

Rare earth elements in biology: From biochemical curiosity to solutions for extractive industries

Rare earth elements (REEs) are critical for our modern lifestyles and the transition to a low-carbon economy. Recent advances in our understanding of the role of REEs in biology, particularly methylotrophy, have provided opportunities to explore biotechnological innovations to improve REE mining and recycling. In addition to bacterial accumulation and concentration of REEs, biological REE binders, including proteins (lanmodulin, lanpepsy) and small molecules (metallophores and cofactors) have been identified that enable REE concentration and separation. REE-binding proteins have also been used in several mechanistically distinct REE biosensors, which have potential application in mining and medicine. Notably, the role of REEs in biology has only been known for a decade, suggesting their considerable scope for developing new understanding and novel applications.

Metal-Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-phenyl) amine (H6Clamp) proligand was synthesized for comparison of its cerium complex with a potassium-cerium heterobimetallic complex of the 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where CeIII(CH3CN)1.5(H3Clamp) (1-Ce(H3Clamp)) exhibits shorter Ce-O distances and longer Ce-N bond distances compared to the analogous distances in K3(THF)3CeIII(dippap)3 (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H3Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand- and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H3Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H3Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium-ligand bonds upon oxidation.

Guanidinate Yttrium Complexes Containing Bipyridyl and Bis(benzimidazolyl) Radicals

Ancillary ligand scaffolds that sufficiently stabilize a metal ion to allow its coordination to an open-shell ligand are scarce, yet their development is essential for next-generation spin-based materials with topical applications in quantum information science. To this end, a synthetic challenge must be met: devising molecules that enable the binding of a redox-active ligand through facile displacement and clean removal of a weakly coordinating anion. Here, we probe the accessibility of unprecedented radical-containing rare-earth guanidinate complexes by combining our recently discovered yttrium tetraphenylborate complex [{(Me3Si)2NC(NiPr)2}2Y][(μ-η6-Ph)(BPh3)] with the redox-active ligands 2,2'-bipyridine (bpy) and 2,2'-bis(benzimidazole) (Bbim), respectively, under reductive conditions. Our endeavor resulted in the first evidence of guanidinate complexes that contain radicals, namely, a mononuclear bipyridyl radical complex, {(Me3Si)2NC(NiPr)2}2Y(bpy•) (1), and a dinuclear bis(benzimidazolyl) radical-bridged complex, [K(crypt-222)][{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim•) (2'). The latter was achieved by an in situ reduction of [{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim) (2), which was isolated from a salt metathesis reaction. 1 and 2 were characterized by X-ray crystallography and IR and UV-vis spectroscopy. Variable-temperature electron paramagnetic resonance spectroscopy was applied to gain insight into the distribution of unpaired spin density on 1 and 2'. Density functional theory calculations were conducted on 1 and 2' to elucidate further their electronic structures. The redox activity of 1 and 2' was also probed by electrochemical methods.

Siloxide tripodal ligands as a scaffold for stabilizing lanthanides in the +4 oxidation state

Synthetic strategies to isolate molecular complexes of lanthanides, other than cerium, in the +4 oxidation state remain elusive, with only four complexes of Tb(iv) isolated so far. Herein, we present a new approach for the stabilization of Tb(iv) using a siloxide tripodal trianionic ligand, which allows the control of unwanted ligand rearrangements, while tuning the Ln(iii)/Ln(iv) redox-couple. The Ln(iii) complexes, [LnIII((OSiPh2Ar)3-arene)(THF)3] (1-LnPh) and [K(toluene){LnIII((OSiPh2Ar)3-arene)(OSiPh3)}] (2-LnPh) (Ln = Ce, Tb, Pr), of the (HOSiPh2Ar)3-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(iii) analogue complexes, [LnIII((OSi(OtBu)2Ar)3-arene)(THF)] (1-LnOtBu) and [K(THF)6][LnIII((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (2-LnOtBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OtBu)2Ar)3-arene. The cyclic voltammetry studies showed a cathodic shift in the oxidation potential for the cerium and terbium complexes of the more electron-donating phenyl substituted scaffold (1-LnPh) compared to those of the tert-butoxy (1-LnOtBu) ligand. Furthermore, the addition of the -OSiPh3 ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [CeIV((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (3-CeOtBu) and [CeIV((OSiPh2Ar)3-arene)(OSiPh3)(THF)2] (3-CePh), were prepared by chemical oxidation of the Ce(iii) analogues. Chemical oxidation of the Tb(iii) and Pr(iii) complexes (2-LnPh) was also possible, in which the Tb(iv) complex, [TbIV((OSiPh2Ar)3-arene)(OSiPh3)(MeCN)2] (3-TbPh), was isolated and crystallographically characterized, yielding the first example of a Tb(iv) supported by a polydentate ligand. The versatility and robustness of these siloxide arene-anchored platforms will allow further development in the isolation of more oxidizing Ln(iv) ions, widening the breadth of high-valent Ln chemistry.

Redox-Neutral Multicatalytic Cerium Photoredox-Enabled Cleavage of O-H Bearing Substrates

The need for synthetic methodologies capable of rapidly altering molecular structure are in high demand. Most existing methods to modify scaffolds rely on net exothermicity to drive the desired transformation. We sought to develop a general strategy for the cleavage of C-C bonds β to hydroxyl groups independent of inherent substrate strain. To this end we have applied a multicatalytic cerium photoredox-based system capable of activating O-H bonds in lactols to deliver formate esters. The same system is also capable of effecting hydrodecarboxylation and hydrodecarbonylation reactions. Initial mechanistic probes demonstrate atomic chlorine (Cl⋅) is generated under the reaction conditions, but substrate activation through cerium-alkoxides or -carboxylates cannot be ruled out.

Photochemical Mn-Mediated Generation of Azide Radicals for Improvement of Alkene Hydroxyazidation

State-of-the-art strategies for alkene-hydroxyazidation, which yield a mixture of β-azido alcohol and β-azido peroxide, must rely on phosphine reagents to improve the chemoselectivity. To overcome the above problems, we present a photochemical hydroxyazidation of alkenes via Mn-mediated ligand-to-metal charge transfer (LMCT) in O2, which activates N3- to •N3 and incorporates O2 to be used as an oxygen source in the hydroxyazidation products. Broad alkene range and step-economy chemistry for the hydroxyazidation transformation were also demonstrated.

Photoionization cross sections measurements of the excited states of lutetium and ytterbium in the near threshold region

In this work, the threshold photoionization cross sections from the excited states of lutetium and ytterbium atoms were investigated by the laser pump-probe scheme under the condition of saturated resonant excitation. We obtained the resonance enhanced multiphoton ionization spectra of the lutetium and ytterbium atoms of the lanthanide metals in the range of 307.50-312.50 nm and 265.00-269.00 nm, respectively; the photoionization cross sections of the 5d6s(1D)6p(2D05/2) and 5d6s(3D)6p(2P01/2) states of lutetium and the 4f13(2F0)5d6s2(J = 1) states of ytterbium above threshold regions (0.4-1.6 eV) were measured, and measured values ranged from 2.3 ± 0.2 to 17.7 ± 1.5 Mb.

Triple Inverse Sandwich versus End-On Diazenido: Bonding Motifs across a Series of Rhenium-Lanthanide and -Actinide Complexes

While synthesizing a series of rhenium-lanthanide triple inverse sandwich complexes, we unexpectedly uncovered evidence for rare examples of end-on lanthanide dinitrogen coordination for certain heavy lanthanide elements as well as for uranium. We begin our report with the synthesis and characterization of a series of trirhenium triple inverse sandwich complexes with the early lanthanides, Ln[(μ-η5:η5-Cp)Re(BDI)]3(THF) (1-Ln, Ln = La, Ce, Pr, Nd, Sm; Cp = cyclopentadienide, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). However, as we moved across the lanthanide series, we ran into an unexpected result for gadolinium in which we structurally characterized two products for gadolinium, namely, 1-Gd (analogous to 1-Ln) and a diazenido dirhenium double inverse sandwich complex Gd[(μ-η1:η1-N2)Re(η5-Cp)(BDI)][(μ-η5:η5-Cp)Re(BDI)]2(THF)2 (2-Gd). Evidence for analogues of 2-Gd was spectroscopically observed for other heavy lanthanides (2-Ln, Ln = Tb, Dy, Er), and, in the case of 2-Er, structurally authenticated. These complexes represent the first observed examples of heterobimetallic end-on lanthanide dinitrogen coordination. Density functional theory (DFT) calculations were utilized to probe relevant bonding interactions and reveal energetic differences between both the experimental and putative 1-Ln and 2-Ln complexes. We also present additional examples of novel end-on heterobimetallic lanthanide and actinide diazenido moieties in the erbium-rhenium complex (η8-COT)Er[(μ-η1:η1-N2)Re(η5-Cp)(BDI)](THF)(Et2O) (3-Er) and uranium-rhenium complex [Na(2.2.2-cryptand)][(η5-C5H4SiMe3)3U(μ-η1:η1-N2)Re(η5-Cp)(BDI)] (4-U). Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing divalent double inverse sandwich complex Yb[(μ-η5:η5-Cp)Re(BDI)]2(THF)2 (5-Yb), as well as base-free, homoleptic rhenium-rare earth triple inverse sandwich complex Y[(μ-η5:η5-Cp)Re(BDI)]3 (6-Y).

Mechanism of the Visible-Light-Promoted C(sp3)-H Oxidation via Uranyl Photocatalysis

Uranyl cation, as an emerging photocatalyst, has been successfully applied to synthetic chemistry in recent years and displayed remarkable catalytic ability under visible light. However, the molecular-level reaction mechanisms of uranyl photocatalysis are unclear. Here, we explore the mechanism of the stepwise benzylic C-H oxygenation of typical alkyl-substituted aromatics (i.e., toluene, ethylbenzene, and cumene) via uranyl photocatalysis using theoretical and experimental methods. Theoretical calculation results show that the most favorable reaction path for uranyl photocatalytic oxidation is as follows: first, hydrogen atom transfer (HAT) from the benzyl position to form a carbon radical ([R•]), then oxygen addition ([R•] + O2 → [ROO•]), then radical-radical combination ([ROO•] + [R•] → [ROOR] → 2[RO•]), and eventually [RO•] reduction to produce alcohols, of which 2° alcohol would further be oxidized to ketones and 1° would be stepwise-oxygenated to acids. The results of the designed verification experiments and the capture of reactive intermediates were consistent with those of theoretical calculations and the previously reported research that the active benzylic C-H would be stepwise-oxygenated in the presence of uranyl. This work deepens our understanding of the HAT mechanism of uranyl photocatalysis and provides important theoretical support for the relevant application of uranyl photocatalysts in organic transformation.

Rare Earths-The Answer to Everything

Rare earths, scandium, yttrium, and the fifteen lanthanoids from lanthanum to lutetium, are classified as critical metals because of their ubiquity in daily life. They are present in magnets in cars, especially electric cars; green electricity generating systems and computers; in steel manufacturing; in glass and light emission materials especially for safety lighting and lasers; in exhaust emission catalysts and supports; catalysts in artificial rubber production; in agriculture and animal husbandry; in health and especially cancer diagnosis and treatment; and in a variety of materials and electronic products essential to modern living. They have the potential to replace toxic chromates for corrosion inhibition, in magnetic refrigeration, a variety of new materials, and their role in agriculture may expand. This review examines their role in sustainability, the environment, recycling, corrosion inhibition, crop production, animal feedstocks, catalysis, health, and materials, as well as considering future uses.

Photoinduced cerium-catalyzed C-H acylation of unactivated alkanes

Ketones are ubiquitous motifs in the realm of pharmaceuticals and natural products. Traditional approaches to accessing these species involve the addition of metal reagents to carboxyl compounds under harsh conditions. Herein, we report a cerium-catalyzed acylation of unactivated C(sp3)-H bonds using bench-stable acyl azolium reagents under mild and operationally-friendly conditions. This reaction exhibits excellent generality, accommodating a wide range of feedstock chemicals such as cycloalkanes and acyclic compounds as well as facilitating the late-stage functionalization of pharmaceuticals. We demonstrate further applications of our strategy with a three-component radical relay reaction and an enantioselective N-heterocyclic carbene (NHC) and cerium dual-catalyzed reaction.

Capturing Rare-Earth Elements by Synthetic Aluminosilicate MCM-22: Mechanistic Understanding of Yb(III) Capture

We studied the mechanism underlying the solid-phase adsorption of a heavy rare-earth element (HREE, Yb) from acidic solutions employing MCM-22 zeolite, serving as both a layered synthetic clay mimic and a new platform for the mechanistic study of HREE adsorption on aluminosilicate materials. Mechanistic studies revealed that the adsorption of Yb(III) at the surface adsorption site occurs primarily through the electrostatic interaction between the site and Yb(III) species. The dependence of Yb adsorption on the pH of the solution indicated the role of surface charge, and the content of framework Al suggested that the Brønsted acid sites (BAS) are involved in the adsorption of Yb(III) ions, which was further scrutinized by spectroscopic analysis and theoretical calculations. Our findings have illuminated the roles of surface sites in the solid-phase adsorption of HREEs from acidic solutions.

Organolanthanide Complexes Containing Ln-CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

We report the gas-phase preparation, isolation, and reactivity of a series of organolanthanides featuring the Ln-CH3 bond. The complexes are formed by decarboxylating anionic lanthanide acetates to form trivalent [LnIII(CH3)(CH3CO2)3]- (Ln = La, Ce, Pr, Nd, Sm, Tb, Tm, Yb, Lu), divalent [EuII(CH3)(CH3CO2)2]-, and the first examples of tetravalent organocerium complexes featuring CeIV-Calkyl σ-bonds: [CeIV(O)(CH3)(CH3CO2)2]- and [CeIV(O)(CH3)(NO3)2]-. Attempts to isolate PrIV-CH3 and TbIV-CH3 were unsuccessful; however, fragmentation patterns reveal that the oxidation of LnIII to a LnIV-oxo-acetate complex is more favorable for Ln = Pr than for Ln = Tb. The rate of Ln-CH3 hydrolysis is a measure of bond stability, and it decreases from LaIII-CH3 to LuIII-CH3, with increasing steric crowding for smaller Ln stabilizing the harder Ln-CH3 bond against hydrolysis. [EuII(CH3)(CH3CO2)2]- engages in a much faster hydrolysis versus LnIII-CH3. The surprising observation of similar hydrolysis rates for CeIV-CH3 and CeIII-CH3 is discussed with respect to sterics, the oxo ligand, and bond covalency in σ-bonded organolanthanides.

Visible-light induced C(sp3)-H arylation of glycine derivatives by cerium catalysis

A Ce(III)-catalyzed, visible-light induced aerobic oxidative dehydrogenative coupling reaction between glycine derivatives and electron-rich arenes is disclosed. The protocol proceeds efficiently under mild conditions, providing an efficient method for the rapid synthesis of α-arylglycine derivatives without the need for an external photosensitizer and additional oxidant. Moreover, this protocol could be performed on a 5 mmol scale, without obvious reduction of the efficiency.

[Ba4X][In19S32] (X = Cl, Br): two quaternary metal chalcohalides exhibiting remarkable photocurrent responses

Inorganic metal chalcohalides, as significant semiconductor materials, have emerged as promising candidates for photoelectric applications. Herein, a new type of quaternary chalcohalide, [Ba4X][In19S32] (X = Cl, Br), has been discovered using the high-temperature halide salt flux method. Single-crystal X-ray diffraction analysis reveals that they are isostructural and crystallize in the tetragonal space group I41/amd (no. 141) featuring the octahedral hole formed by six [InS4]5- tetrahedra filled with a [ClBa4]7+ polycation, surrounded by a three-dimensional covalent framework formed by interconnecting [InS6]9- octahedra through corner-sharing and edge-sharing. Moreover, [Ba4Cl][In19S32] and [Ba4Br][In19S32] exhibit wide optical bandgaps of 2.70 eV and 2.46 eV, respectively, and moderate birefringences (0.044 @ 2100 nm and 0.042 @ 2100 nm, respectively). Specifically, [Ba4X][In19S32] (X = Cl, Br) display remarkable photocurrent responses under simulated solar-light illumination, implying their potential for photocatalytic applications. Theoretical calculations were employed to understand the interrelationship between the optical properties and electronic structure. The study on the synthesis and structure-property relationship analysis of inorganic metal chalcohalides provides new insight into the exploration of promising photoelectric materials.

Photocatalytic Generation of Divalent Lanthanide Reducing Agents

Divalent lanthanide (Ln) compounds are excellent reducing agents with unique reactivity profiles. These reagents are typically used in superstoichiometric amounts, often in combination with harmful additives. Reactions catalytic in Ln(II) reagents that retain the reactivity and selectivity of the stoichiometric transformations are currently lacking due to the absence of effective and selective methods to form reactive Ln(II) species from stable precursors. Here, active Ln(II) is generated from a Ln(III) precursor through reduction by a photoexcited coumarin or carbostyril chromophore, which, in turn, is regenerated by a sacrificial reductant. The reductant can be metallic (Zn) or organic (amines) and can be used in strictly stoichiometric amounts. A broad range of reactions, including C-halogen, C═C, C═X (X = O, N), P═O, and N═N reductions, as well as C-C, C-X (X = N, S, P), and N-N couplings were readily carried out in yields and selectivities comparable to or better than those afforded by the analogous stoichiometric transformations. The reaction outcomes could be altered by changing the ligand or the lanthanide or through the addition of environmentally benign additives (e.g., water). EPR spectroscopy supported the formation of both Ln(II) and oxidized chromophore intermediates. Taken together, these results establish photochemical Ln(II) generation as a powerful strategy for rendering Ln(II)-mediated reactions catalytic.

Heterojunction Photocatalyst Loaded on Electrospun Nanofibers for Synergistic Enhanced Photocatalysis and Real-Time Temperature Monitoring

Bi2WO6:Ho3+, Yb3+/g-C3N4 (BHY/CN) photocatalysts are successfully loaded on polyacrylonitrile (PAN) nanofibers by electrospinning technology, which combines an upconversion effect and heterojunctions to achieve dual-functional characteristics. Polymer-modified photocatalytic materials offer a large specific surface area of 24.1 m2/g and a pore volume of 0.1 cm3/g, promoting the utility of solar energy. The introduction of rare earth ions and g-C3N4 optimizes the structural band gap, which broadens the light absorption range and promotes electron transfer. Moreover, the heterojunction between Bi2WO6 and g-C3N4 has suppressed the complexation of photoinduced carriers, further improving catalytic performance. The optimized photocatalysts have higher photocatalytic activity with degrading 92.6% tetracycline-hydrochloride (120 min) under simulated sunlight irradiation. The optical thermometry has also been achieved based on the fluorescence intensity ratio technique, where the maximum absolute and relative sensitivity values of BHY/CN-1:6@PAN are 3.322% K-1 and 0.842% K-1, respectively. This dual-functional nanofibers with excellent mechanical properties provide noncontact temperature feedback and efficient catalytic performance for better wastewater treatment and ecological restoration in extreme harsh environments.

Controlling Extraction of Rare Earth Elements Using Functionalized Aryl-vinyl Phosphonic Acid Esters

Ligands that can discriminate between individual rare earth elements are important for production of these critical elements. A set of aryl-vinyl phosphonic acid ligands for extracting rare earth elements were designed and synthesized under the hypothesis that the strength of the rare earth-ligand interactions could be tuned by changing the dipole moment of the ligand. The ligands were synthesized via a two-step reaction procedure using a Heck coupling reaction to functionalize vinyl phosphonic acid, followed by Steglich esterification to obtain high-purity styryl phosphonic acid monoesters with varying dipole moments along the P-C bond. The metal binding strength and composition of the rare earth complexes formed with these styryl phosphonic acid monoesters were experimentally studied by liquid-liquid extraction techniques, while DFT calculations were performed to determine the dipole moments of the free and complexed ligands and the electronic structure of the complexes formed. All three prepared ligands were much stronger extracting agents for europium(III) than the dialkylphosphonic acids usually used for this separation. However, the order of increasing extraction strength was found to match the order of the decreasing calculated dipole moment along the P-C bond of the three styryl-based ligands, rather than correlating with increasing ligand basicity, as reflected by the pKa of the ligands. These findings suggest that this approach can be used to systematically alter the extraction strength of aromatic phosphonic monoesters for rare earth element purification.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.

Photocatalytic dechlorination of unactivated chlorocarbons including PVC using organolanthanide complexes

Simple lanthanide cyclopentadienyl (Cp) complexes can photochemically cleave the sp3 carbon-chlorine bond of unactivated chlorinated hydrocarbons including polyvinyl chloride (PVC). The excited state lifetimes of these simple complexes are among the longest observed for cerium complexes (175 ns for [(CpMe4)2Ce(μ-Cl)]2) and the light absorption by the Cp ligand is efficient, so photocatalytic reactivity is enhanced for cerium and now also made possible for neighboring, normally photoinactive, lanthanide congeners.

Exploring Differences in Lanthanide Excited State Reactivity Using a Simple Example: The Photophysics of La and Ce Thenoyltrifluoroacetone Complexes

A series of four lanthanide thenoyltrifluoroacetone (TTA) complexes consisting of two f0 (La3+ and Ce4+) and two f1 (Ce3+) complexes was examined using steady-state and time-resolved spectroscopic techniques. The wide range of spectroscopic techniques presented herein have enabled us to discern the nature of the excited states (charge transfer, CT vs ligand localized, LL) as well as construct a Jablonski diagram for detailing the excited state reactivity within the series of molecules. The wavelength and excitation power dependence for these series of complexes are the first direct verification for the presence of simultaneous competing, noninteracting CT and LL excited states. Additionally, a computational framework is described that can be used to support spectroscopic assignments as a guide for future studies. Finally, the relationship between the obtained photophysics and possible photochemical separation mechanisms is described.

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides

ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)₆ complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W⁺/*W⁰) = -2.2 to -3.0 V vs Fc^[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)₆ photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)₆-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.

Co-function Mechanisms of Chlorine and Alkoxy Radicals in Cerium-Catalyzed C-H Functionalization of Alkane Mediated by Visible Light

Identification of radical intermediates for the catalytic functionalization of alkanes offers a number of unique challenges and has recently raised a controversial issue concerning the subtle role of chlorine versus alkoxy radicals in cerium photocatalysis. This study is an attempt to settle the controversy within the theoretical frameworks of Marcus electron transfer and transition state theory. Co-function mechanisms were proposed together with a scheme of kinetic evaluations to account for ternary dynamic competition among photolysis, back electron transfer, and hydrogen atom transfer (HAT). Cl•-based HAT has been proven to initially control the early dynamics of the photocatalytic transformation on the picosecond to nanosecond time scale, which is subsequently taken over by a postnanosecond event of alkoxy radical-mediated HAT. The theoretical models developed herein provide a uniform understanding of the continuous time dynamics of photogenerated radicals to address some paradoxical arguments in lanthanide photocatalysis.

Low-melting multicharge ionic liquids with [Ln(NO3)5]2- (Ln = Ho-Lu): structural, electrostatic, thermochemical, and fluorescence properties

A series of green and safe heavy-rare-earth ionic liquids were obtained using a straightforward method. The stable structures of these ionic liquids, characterized by high-coordinating anions, were confirmed by nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and single crystal X-ray diffraction (XRD). These ionic liquids exhibited wide liquid phase intervals and excellent thermal stability. The bidentate nitrato ligands occupied a sufficient number of coordination sites on the lanthanide ions, resulting in the formation of water-free 10-coordination structures. To explain the anomalous melting points observed in these multi-charged ionic liquids, a combination of experimental data and theoretical studies was employed to investigate the relationship between the electrostatic properties and the melting point. The electrostatic potential density per unit ion surface and volume were proposed and utilized for melting point prediction, demonstrating good linearity. Furthermore, the coordinating spheres of the lanthanide ions in these ionic liquids were devoid of luminescence quenchers such as O-H and N-H groups. Notably, the ionic liquids containing Ho³⁺, Er³⁺, and Tm³⁺ exhibited long lifetime near-infrared (NIR) and blue emissions, respectively. The UV-vis-NIR spectra revealed numerous electronic transitions of the lanthanide ions, which were attributed to their unique optical properties.

A pillar[5]arene noncovalent assembly boosts a full-color lanthanide supramolecular light switch

A photo-responsive full-color lanthanide supramolecular switch was constructed from a synthetic 2,6-pyridine dicarboxylic acid (DPA)-modified pillar[5]arene (H) complexing with lanthanide ion (Ln3+ = Tb3+ and Eu3+) and dicationic diarylethene derivative (G1) through a noncovalent supramolecular assembly. Benefiting from the strong complexation between DPA and Ln3+ with a 3 : 1 stoichiometric ratio, the supramolecular complex H/Ln3+ presented an emerging lanthanide emission in the aqueous and organic phase. Subsequently, a network supramolecular polymer was formed by H/Ln3+ further encapsulating dicationic G1via the hydrophobic cavity of pillar[5]arene, which greatly contributed to the increased emission intensity and lifetime, and also resulted in the formation of a lanthanide supramolecular light switch. Moreover, full-color luminescence, especially white light emission, was achieved in aqueous (CIE: 0.31, 0.32) and dichloromethane (CIE: 0.31, 0.33) solutions by the adjustment of different ratios of Tb3+ and Eu3+. Notably, the photo-reversible luminescence properties of the assembly were tuned via alternant UV/vis light irradiation due to the conformation-dependent photochromic energy transfer between the lanthanide and the open/closed-ring of diarylethene. Ultimately, the prepared lanthanide supramolecular switch was successfully applied to anti-counterfeiting through the use of intelligent multicolored writing inks, and presents new opportunities for the design of advanced stimuli-responsive on-demand color tuning with lanthanide luminescent materials.

Solution Structures of Europium Terpyridyl Complexes with Nitrate and Triflate Counterions in Acetonitrile

Lanthanide-ligand complexes are key components of technological applications, and their properties depend on their structures in the solution phase, which are challenging to resolve experimentally or computationally. The coordination structure of the Eu³⁺ ion in different coordination environments in acetonitrile is examined using ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. AIMD simulations are conducted for the solvated Eu³⁺ ion in acetonitrile, both with or without a terpyridyl ligand, and in the presence of either triflate or nitrate counterions. EXAFS spectra are calculated directly from AIMD simulations and then compared to experimentally measured EXAFS spectra. In acetonitrile solution, both nitrate and triflate anions are shown to coordinate directly to the Eu³⁺ ion forming either ten- or eight-coordinate solvent complexes where the counterions are binding as bidentate or monodentate structures, respectively. Coordination of a terpyridyl ligand to the Eu³⁺ ion limits the available binding sites for the solvent and anions. In certain cases, the terpyridyl ligand excludes any solvent binding and limits the number of coordinated anions. The solution structure of the Eu-terpyridyl complex with nitrate counterions is shown to have a similar arrangement of Eu³⁺ coordinating molecules as the crystal structure. This study illustrates how a combination of AIMD and EXAFS can be used to determine how ligands, solvent, and counterions coordinate with the lanthanide ions in solution.

Visible-light-induced Csp3-H functionalization of glycine derivatives by cerium catalysis

A Ce(III)-catalyzed, visible-light-induced aerobic oxidative dehydrogenative coupling/aromatization reaction between glycine derivatives and alkenes has been developed, which provides an efficient approach for the synthesis of quinoline derivatives and post-modification of oligopeptides containing glycine residues under mild conditions without the need for external photosensitizers.

Back to the future of organolanthanide chemistry

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp₃ complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

Norm-Conserving 4f-in-Core Pseudopotentials and Basis Sets Optimized for Trivalent Lanthanides (Ln = Ce-Lu)

We present here a set of scalar-relativistic norm-conserving 4f-in-core pseudopotentials, together with complementary valence-shell Gaussian basis sets, for the lanthanide (Ln) series (Ce-Lu). The Goedecker, Teter, and Hutter (GTH) formalism is adopted with the generalized gradient approximation (GGA) for the exchange-correlation Perdew-Burke-Ernzerhof (PBE) functional. The 4f-in-core pseudopotentials are built through attributing 4f-subconfiguration 4fⁿ (n = 1-14) for Ln (Ln = Ce-Lu) into the atomic core region, making it possible to circumvent the difficulty of the description of the open 4fⁿ valence shell. A wide variety of computational benchmarks and tests have been carried out on lanthanide systems including Ln³⁺-containing molecular complexes, aqueous solutions, and bulk solids to validate the accuracy, reliability, and efficiency of the optimized 4f-in-core GTH pseudopotentials and basis sets. The 4f-in-core GTH pseudopotentials successfully replicate the main features of lanthanide structural chemistry and reaction energetics, particularly for nonredox reactions. The chemical bonding features and solvation shells, hydrolysis energetics, acidity constants, and solid-state properties of selected lanthanide systems are also discussed in detail by utilizing these new 4f-in-core GTH pseudopotentials. This work bridges the idea of keeping highly localized 4f electrons in the atomic core and efficient pseudopotential formalism of GTH, thus providing a highly efficient approach for studying lanthanide chemistry in multi-scale modeling of constituent-wise and structurally complicated systems, including electronic structures of the condensed phase and first-principles molecular dynamics simulations.

Controlled monodefluorination and alkylation of C(sp3)-F bonds by lanthanide photocatalysts: importance of metal-ligand cooperativity

The controlled functionalization of a single fluorine in a CF3 group is difficult and rare. Photochemical C-F bond functionalization of the sp3-C-H bond in trifluorotoluene, PhCF3, is achieved using catalysts made from earth-abundant lanthanides, (CpMe4)2Ln(2-O-3,5- t Bu2-C6H2)(1-C{N(CH)2N(iPr)}) (Ln = La, Ce, Nd and Sm, CpMe4 = C5Me4H). The Ce complex is the most effective at mediating hydrodefluorination and defluoroalkylative coupling of PhCF3 with alkenes; addition of magnesium dialkyls enables catalytic C-F bond cleavage and C-C bond formation by all the complexes. Mechanistic experiments confirm the essential role of the Lewis acidic metal and support an inner-sphere mechanism of C-F activation. Computational studies agree that coordination of the C-F substrate is essential for C-F bond cleavage. The unexpected catalytic activity for all members is made possible by the light-absorbing ability of the redox non-innocent ligands. The results described herein underscore the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.

Lanthanide-based MOFs: synthesis approaches and applications in cancer diagnosis and therapy

Metal-organic frameworks (MOFs) have attracted considerable attention as emerging nanomaterials. Based on their tunable size, high porosity, and large specific surface area, MOFs have a wide range of applications in the fields of chemistry, energy, and biomedicine. However, the MOF materials obtained from lanthanides with a unique electronic configuration as inorganic building units have unique properties such as optics, magnetism, and radioactivity. In this study, various synthetic methods for preparing MOF materials using lanthanides as inorganic building units are described. Combined with the characteristics of lanthanides, their application prospects of lanthanide-based MOFs in tumor diagnosis and treatment are emphasized. The authors hope to provide methodological reference for the construction of MOF materials of rare-earth elements, and to provide ideas and inspiration for their practical applications in the field of biomedicine.

A synergistic chemodynamic-photodynamic-photothermal therapy platform based on biodegradable Ce-doped MoOx nanoparticles

Near-infrared light-induced catalysts are considered to be potential nanoagents for tumor therapy. Cerium (Ce) is a non-biotoxic lanthanide element and exhibits variable valence states for catalytic reactions. In this work, we report a one-step hydrothermal synthesis for Ce-doped MoO_x (CMO) nanomaterials. The obtained CMO nanomaterials show high absorption in the NIR II regime and a high photothermal conversion efficiency of 67.7% (1064 nm). Moreover, due to the doping of Ce element, the consumption of hydrogen peroxide (H₂O₂) and glutathione (GSH) is boosted which enhances the chemodynamic and photodynamic therapy simultaneously. Under NIR II laser irradiation, the designed CMO nanocatalysts induce metabolism disruption and mitochondrial damage in the tumor cells. As-prepared CMO nanomaterials also show good biocompatibility and pH-responsive degradation behavior, which can be degraded rapidly under alkaline conditions (pH = 7.4) and remain stable in acidic solution (pH = 5.6). These properties make CMO nanomaterials ideal biodegradable nanotheranostic agents for synergistic chemodynamic-photodynamic-photothermal antitumor therapy.

Coordination Chemistry-Driven Approaches to Rare Earth Element Separations

Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth's crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent-solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals.The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2-^tBuNO)C₆H₄CH₂}₃N]^3- (TriNOx^3-) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SF_Nd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations.Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx^3- ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SF_Eu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-defined speciation to afford purified RE materials. Recently, we determined that magnetic properties can be used to enhance both thermodynamic and kinetic RE separations processes to give an approximately 100% boost for pairs of paramagnetic/diamagnetic REs. These results have shown that both thermodynamic and kinetic RE separations were efficient for different selected RE binary pairs through coordination chemistry. The focus of this Account will detail the differences that are observed for RE separations when promoted by thermodynamic or kinetic factors. Overall, the development of rationally adjusted speciation of REs provides a basis for future industrial separations processes for technologies applied to ECEs derived from wind turbines, batteries for electric vehicles, and LEDs.

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes

Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI₂ are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li⁰ and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.

Upcycling to Sustainably Reuse Plastics

Plastics are now indispensable in daily lives. However, the pollution from plastics is also increasingly becoming a serious environmental issue. Recent years have seen more sustainable approaches and technologies, commonly known as upcycling, to transform plastics into value-added materials and chemical feedstocks. In this review, the latest research on upcycling is presented, with a greater focus on the use of renewable energy as well as the more selective methods to repurpose synthetic polymers. First, thermal upcycling approaches are briefly introduced, including the redeployment of plastics for construction uses, 3D printing precursors, and lightweight materials. Then, some of the latest novel strategies to deconstruct condensation polymers to monomers for repolymerization or introduce vulnerable linkers to make the plastics more degradable are discussed. Subsequently, the review will explore the breakthroughs in plastics upcycling by heterogeneous and homogeneous photocatalysis, as well as electrocatalysis, which transform plastics into more versatile fine chemicals and materials while simultaneously mitigating global climate change. In addition, some of the biotechnological advances in the discovery and engineering of microbes that can decompose plastics are also presented. Finally, the current challenges and outlook for future plastics upcycling are discussed to stimulate global cooperation in this field.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d⁶ valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)₂](PF₆), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L^CNC)₂](PF₆) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L^CNC)₂](PF₆), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides in 1,2-Difluorobenzene

We have studied the photochemical cyclization of 1-(2-iodobenzyl)-pyrrole (IBP) and 1-(2-bromobenzyl)-pyrrole (BBP) to 5H-pyrrolo[2,1-a]isoindol catalyzed by W(CNDipp)₆ (CNDipp = 2,6-diisopropylphenylisocyanide) in 1,2-difluorobenzene (DFB). Irradiation (445 nm) of W(CNDipp)₆ (5 mol %) in DFB solution converted 78% of IBP (50 mM) to product after 1 h (16 turnovers). Addition of tetra-n-butyl ammonium hexafluorophosphate (TBAPF₆) (0.2 M) to the DFB solution led to rapid photoinduced disappearance of W(CNDipp)₆ but, remarkably, did not inhibit photochemical cyclization of IBP, indicating that IBP cyclization could be driven by a nonluminescent photocatalyst.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)₃(THF)₃] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P2₁/c space group with a slight compression of the unit cell from 3396.4(2) ų to 3373.2(4) ų along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT²⁻ ligands with an end-bound {Ca²⁺(THF)₃} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT²⁻ ligands act in a η⁸-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)₃(THF)₃] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)₃(THF)₃] in contrast to [ErKCa(COT)₃(THF)₃], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

A new class of heterotrimetallic COT-based self-assemblies accommodates metals from groups I–III in three different oxidation states and enables tuning of electronic and magnetic properties.