Expanding Rare-Earth Oxidation State Chemistry to Molecular Complexes of Holmium(II) and Erbium(II)

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).

Stabilization of Pr4+ in Silicates─High-Pressure Synthesis of PrSi3O8 and Pr2Si7O18

The reaction between PrO2 and SiO2 was investigated at various pressure points up to 29 GPa in a diamond anvil cell using laser heating and in situ single-crystal structure analysis. The pressure points at 5 and 10 GPa produced Pr2III(Si2O7), whereas Pr4IIISi3O12 and Pr2IV(O2)O3 were obtained at 15 GPa. Pr4IIISi3O12 can be interpreted as a high-pressure modification of the still unknown orthosilicate Pr4III(SiO4)3. PrIVSi3O8 and Pr2IVSi7O18 that contain praseodymium in its rare + IV oxidation state were identified at 29 GPa. After the pressure was released from the reaction chamber, the Pr(IV) silicates could be recovered, indicating that they are metastable at ambient pressure. Density functional theory calculations of the electronic structure corroborate the oxidation state of praseodymium in both PrIVSi3O8 and Pr2IVSi7O18. Both silicates are the first structurally characterized representatives of Pr4+-containing salts with oxoanions. All three silicates contain condensed networks of [SiO6] octahedra which is unprecedented in the rich chemistry of lanthanoid silicates.

Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(μ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(μ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(μ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η5:η5-C5H5)Ti4(η5-C5Me5)3(μ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(μ4-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ3-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

Perplexing EPR Signals from 5f36d1 U(II) Complexes

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band electron paramagnetic resonance (EPR) data were collected at <10 and 77 K on 10 U(II) complexes with 5f36d1 electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K. In contrast, U(II) complexes with 5f4 configurations are EPR-silent. Unlike U(II), the congenic 4f35d1 Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f15d1 configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. Since the coupling in U(II) systems is expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration, the results are viewed as unexplained phenomena. However, they do show that 5f36d1 U(II) samples can be differentiated from 5f4 U(II) complexes by EPR spectroscopy.

Cyclopentadienyl capped thorium(IV) porphyrinoid complex

The reaction between Th(IV) dipyriamethyrin dichloride and sodium cyclopentadienyl (Cp) results in the formation of a cyclopentadienyl capped thorium dipyriamethyrin complex, which to our knowledge represents the first expanded porphyrin f-element Cp complex.

A DFT perspective on organometallic lanthanide chemistry

Computational studies of the coordination chemistry and bonding of lanthanides have grown in recent decades as the need for understanding the distinct physical, optical, and magnetic properties of these compounds increased. Density functional theory (DFT) methods offer a favorable balance of computational cost and accuracy in lanthanide chemistry and have helped to advance the discovery of novel oxidation states and electronic configurations. This Frontier article examines the scope and limitations of DFT in interpreting structural and spectroscopic data of low-valent lanthanide complexes, elucidating periodic trends, and predicting their properties and reactivity, presented through selected examples.

Thermally Stable Terbium(II) and Dysprosium(II) Bis-amidinate Complexes

The thermostable four-coordinate divalent lanthanide (Ln) bis-amidinate complexes [Ln(Piso)2] (Ln = Tb, Dy; Piso = {(NDipp)2CtBu}, Dipp = C6H3iPr2-2,6) were prepared by the reduction of parent five-coordinate Ln(III) precursors [Ln(Piso)2I] (Ln = Tb, Dy) with KC8; halide abstraction of [Ln(Piso)2I] with [H(SiEt3)2][B(C6F5)] gave the respective Ln(III) complexes [Ln(Piso)2][B(C6F5)]. All complexes were characterized by X-ray diffraction, ICP-MS, elemental analysis, SQUID magnetometry, UV-vis-NIR, ATR-IR, NMR, and EPR spectroscopy and ab initio CASSCF-SO calculations. These data consistently show that [Ln(Piso)2] formally exhibit Ln(II) centers with 4fn5dz21 (Ln = Tb, n = 8; Dy, n = 9) valence electron configurations. We show that simple assignments of the f-d coupling to either L-S or J-s schemes are an oversimplification, especially in the presence of significant crystal field splitting. The coordination geometry of [Ln(Piso)2] is intermediate between square planar and tetrahedral. Projecting from the quaternary carbon atoms of the CN2 ligand backbones shows near-linear C···Ln···C arrangements. This results in strong axial ligand fields to give effective energy barriers to magnetic reversal of 1920(91) K for the Tb(II) analogue and 1964(48) K for Dy(II), the highest values observed for mononuclear Ln(II) single-molecule magnets, eclipsing 1738 K for [Tb(C5iPr5)2]. We tentatively attribute the fast zero-field magnetic relaxation for these complexes at low temperatures to transverse fields, resulting in considerable mixing of mJ states.

Structural distortion by alkali metal cations modulates the redox and electronic properties of Ce3+ imidophosphorane complexes

A series of Ce3+ complexes with counter cations ranging from Li to Cs are presented. Cyclic voltammetry data indicate a significant dependence of the oxidation potential on the alkali metal identity. Analysis of the single-crystal X-ray diffraction data indicates that the degree of structural distortion of the secondary coordination sphere is linearly correlated with the measured oxidation potential. Solution electronic absorption spectroscopy confirms that the structural distortion is reflected in the solution structure. Computational studies further validate this analysis, deciphering the impact of alkali metal cations on the Ce atomic orbital contributions, differences in energies of Ce-dominant molecular orbitals, energy shift of the 4f-5d electronic transitions, and degree of structural distortions. In sum, the structural impact of the alkali metal cation is demonstrated to modulate the redox and electronic properties of the Ce3+ complexes, and provides insight into the rational tuning of the Ce3+ imidophosphorane complex oxidation potential through alkali metal identity.

TURBOMOLE: Today and Tomorrow

TURBOMOLE is a highly optimized software suite for large-scale quantum-chemical and materials science simulations of molecules, clusters, extended systems, and periodic solids. TURBOMOLE uses Gaussian basis sets and has been designed with robust and fast quantum-chemical applications in mind, ranging from homogeneous and heterogeneous catalysis to inorganic and organic chemistry and various types of spectroscopy, light-matter interactions, and biochemistry. This Perspective briefly surveys TURBOMOLE's functionality and highlights recent developments that have taken place between 2020 and 2023, comprising new electronic structure methods for molecules and solids, previously unavailable molecular properties, embedding, and molecular dynamics approaches. Select features under development are reviewed to illustrate the continuous growth of the program suite, including nuclear electronic orbital methods, Hartree-Fock-based adiabatic connection models, simplified time-dependent density functional theory, relativistic effects and magnetic properties, and multiscale modeling of optical properties.

Two-Electron Oxidations at a Single Cerium Center

Two-electron oxidations are ubiquitous and play a key role in the synthesis and catalysis. For transition metals and actinides, two-electron oxidation often takes place at a single-metal site. However, redox reactions at rare-earth metals have been limited to one-electron processes due to the lack of accessible oxidation states. Despite recent advancements in nontraditional oxidation state chemistry, the low stability of low-valent compounds and large disparity among different oxidation states prevented the implementation of two-electron processes at a single rare-earth metal center. Here we report two-electron oxidations at a cerium(II) center to yield cerium(IV) terminal oxo and imido complexes. A series of cerium(II-IV) complexes supported by a tripodal tris(amido)arene ligand were synthesized and characterized. Experimental and theoretical studies revealed that the cerium(II) complex is best described as a 4f2 ion stabilized by δ-backdonation to the anchoring arene, while the cerium(IV) oxo and imido complexes exhibit multiple bonding characters. The accomplishment of two-electron oxidations at a single cerium center brings a new facet to molecular rare-earth metal chemistry.

Altering the spectroscopy, electronic structure, and bonding of organometallic curium(III) upon coordination of 4,4'-bipyridine

Structural and electronic characterization of (Cp'₃Cm)₂(μ-4,4'-bpy) (Cp' = trimethylsilylcyclopentadienyl, 4,4'-bpy = 4,4'-bipyridine) is reported and provides a rare example of curium-carbon bonding. Cp'₃Cm displays unexpectedly low energy emission that is quenched upon coordination by 4,4'-bipyridine. Electronic structure calculations on Cp'₃Cm and (Cp'₃Cm)₂(μ-4,4'-bpy) rule out significant differences in the emissive state, rendering 4,4'-bipyridine as the primary quenching agent. Comparisons of (Cp'₃Cm)₂(μ-4,4'-bpy) with its samarium and gadolinium analogues reveal atypical bonding patterns and electronic features that offer insights into bonding between carbon with f-block metal ions. Here we show the structural characterization of a curium-carbon bond, in addition to the unique electronic properties never before observed in a curium compound.

Isolation and redox reactivity of cerium complexes in four redox states

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides with an arene ring in a tripodal ligand can stabilize cerium complexes in four different redox states and can promote multielectron redox reactivity in cerium complexes. Ce(iii) and Ce(iv) complexes [(LO₃)Ce(THF)] (1) and [(LO₃)CeCl] (2) (LO₃ = 1,3,5-(2-OSi(O^tBu)₂C₆H₄)₃C₆H₃) were synthesized and fully characterized. Remarkably the one-electron reduction and the unprecedented two-electron reduction of the tripodal Ce(iii) complex are easily achieved to yield reduced complexes [K(2.2.2-cryptand)][(LO₃)Ce(THF)] (3) and [K₂{(LO₃)Ce(Et₂O)₃}] (5) that are formally "Ce(ii)" and "Ce(i)" analogues. Structural analysis, UV and EPR spectroscopy and computational studies indicate that in 3 the cerium oxidation state is in between +II and +III with a partially reduced arene. In 5 the arene is doubly reduced, but the removal of potassium results in a redistribution of electrons on the metal. The electrons in both 3 and 5 are stored onto δ-bonds allowing the reduced complexes to be described as masked "Ce(ii)" and "Ce(i)". Preliminary reactivity studies show that these complexes act as masked Ce(ii) and Ce(i) in redox reactions with oxidizing substrates such as Ag⁺, CO₂, I₂ and S₈ effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(μ-CpAn)Y(μ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance

The reduction of a bimetallic yttrium ansa-metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d¹ Y(II) ions. The precursor [Cp^AnY(μ-H)(THF)]₂ (Cp^An = Me₂Si[C₅H₃(SiMe₃)-3]₂) was synthesized by hydrogenolysis of the allyl complex Cp^AnY(η³-C₃H₅)(THF), which was prepared from (C₃H₅)MgCl and [Cp^AnY(μ-Cl)]₂. Treatment of [Cp^AnY(μ-H)(THF)]₂ with excess KC₈ in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(μ-Cp^An)Y(μ-H)]₂. The two rings of each Cp^An ligand in the reduced anion [(μ-Cp^An)Y(μ-H)]₂^1- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF)₂][(μ-Cp^An)Dy(μ-H)]₂, was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f⁹ Dy(III) center and one 4f⁹(5d_z²)¹ Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

Expanding Transuranium Organoactinide Chemistry: Synthesis and Characterization of (Cp'3M)2(μ-4,4'-bpy) (M = Ce, Np, Pu)

Dinuclear, organometallic, transuranium compounds, (Cp'₃M)₂(μ-4,4'-bpy) (Cp'^- = trimethylsilylcyclopentadienide, 4,4'-bpy = 4,4'-bipyridine, M = Ce, Np, Pu), reported herein provide a rare opportunity to probe the nature of actinide-carbon bonding. Significant splitting of the f-f transitions results from the unusual coordination environment in these complexes and leads to electronic properties that are currently restricted to organoactinide systems. Structural and spectroscopic characterization in the solid state and in solution for (Cp'₃M)₂(μ-4,4'-bpy) (M = Np, Pu) are reported, and their structural metrics are compared to a cerium analogue.

Room-Temperature Stable Ln(II) Complexes Supported by 2,6-Diadamantyl Aryloxide Ligands

The sterically bulky aryloxide ligand OAr* (OAr* = ^-OC₆H₂-Ad₂-2,6^tBu-4; Ad = 1-adamantyl) has been used to generate Ln(II) complexes across the lanthanide series that are more thermally stable than complexes of any other ligand system reported to date for 4fⁿd¹ Ln(II) ions. The Ln(III) precursors Ln(OAr*)₃ (1-Ln) were synthesized by reacting 1.2 equiv of Ln(NR₂)₃ (R = SiMe₃) with 3 equiv of HOAr* for Ln = La, Ce, Nd, Gd, Dy, Yb, and Lu. 1-Ce, 1-Nd, 1-Gd, 1-Dy, and 1-Lu were identified by single-crystal X-ray diffraction studies. Reductions of 1-Ln with potassium graphite (KC₈) in tetrahydrofuran in the presence of 2.2.2-cryptand (crypt) yielded the Ln(II) complexes [K(crypt)][Ln(OAr*)₃] (2-Ln). The 2-Ln complexes for Ln = Nd, Gd, Dy, and Lu were characterized by X-ray crystallography and found to have Ln-O bond distances 0.038-0.087 Å longer than those of their 1-Ln analogues; this is consistent with 4fⁿ5d¹ electron configurations. The structure of 2-Yb has Yb-O distances 0.167 Å longer than those predicted for 1-Yb, which is consistent with a 4f¹⁴ electron configuration. Although 2-La and 2-Ce proved to be challenging to isolate, with 18-crown-6 (18-c-6) as the potassium chelator, La(II) and Ce(II) complexes with OAr* could be isolated and crystallographically characterized: [K(18-c-6)][Ln(OAr*)₃] (3-Ln). The Ln(II) complexes decompose at room temperature more slowly than other previously reported 4fⁿ5d¹ Ln(II) complexes. For example, only 30% decomposition of 2-Dy was observed after 30 h at room temperature compared to complete decomposition of [Dy(OAr')₃]^- and [DyCp'₃]^- under similar conditions (OAr' = OC₆H₂-2,6-^tBu₂-4-Me; Cp' = C₅H₄SiMe₃).

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.

Identification of Oxidation State +1 in a Molecular Uranium Complex

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η⁵-C₅ⁱPr₅)₂] and the uranium(III) metallocene [IU(η⁵-C₅ⁱPr₅)₂] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η⁵-C₅ⁱPr₅)₂]⁻ (1) (C₅ⁱPr₅ = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]⁺. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f³(7s/6d_z²)¹(6d_x²–y²/6d_xy)¹ configuration. The metal–ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

X-ray absorption near edge structure (XANES) spectroscopy, coupled with ab initio calculations, has emerged as the state-of-the-art tool for elucidating the metal-ligand bonding in f-element complexes. This highlight presents recent efforts in calculating XANES spectra of lanthanide and actinide compounds with relativistic multiconfiguration wavefunction approaches that account for differences in donation bonding in the ground state (GS) versus a core-excited state (ES), multiplet effects, and spin-orbit-coupling. With the GS and ES wavefunctions available, including spin-orbit effects, an arsenal of chemical bonding tools that are popular among chemists can be applied to rationalize the observed intensities in terms of covalent bonding.

Cyclopentadienyl coordination induces unexpected ionic Am-N bonding in an americium bipyridyl complex

Variations in bonding between trivalent lanthanides and actinides is critical for reprocessing spent nuclear fuel. The ability to tune bonding and the coordination environment in these trivalent systems is a key factor in identifying a solution for separating lanthanides and actinides. Coordination of 4,4'-bipyridine (4,4'-bpy) and trimethylsilylcyclopentadienide (Cp') to americium introduces unexpectedly ionic Am-N bonding character and unique spectroscopic properties. Here we report the structural characterization of (Cp'₃Am)₂(μ - 4,4'-bpy) and its lanthanide analogue, (Cp'₃Nd)₂(μ - 4,4'-bpy), by single-crystal X-ray diffraction. Spectroscopic techniques in both solid and solution phase are performed in conjunction with theoretical calculations to probe the effects the unique coordination environment has on the electronic structure.

Metal–metal bond in lanthanide single-molecule magnets

This review surveys recent critical advances in lanthanide SMMs, highlighting the influences of metal–metal bonds on the magnetization dynamics.

A Rare-Earth Metal Retrospective to Stimulate All Fields

Formulating insightful questions and experiments is crucial to the advancement of science. The purpose of this Perspective is to encourage scientists in all areas of chemistry to ask more "What if?" questions: What if we tried this experiment? What if we used these conditions? What if that idea is not correct? To stimulate this thinking, a retrospective analysis of a specific field, in this case rare-earth metal chemistry, is presented that describes the "What if?" questions that could have and should have been asked earlier based on our current knowledge. The goal is to provide scientists with a historical perspective of discovery that exemplifies how previous views in chemistry were often narrowed by predominant beliefs in principles that were incorrect. The same situation is likely to exist today, but we do not realize the limitations! Hopefully, this analysis can be used as a springboard for posing important "What if?" questions that should be asked right now in every area of chemical research.

Monovalent lanthanide(I) in borozene complexes

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻ species changes from C_s to C_7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C_7v-LnB₈⁻ clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻ doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻ series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

The most common oxidation state for lanthanides is +3. Here the authors use photoelectron spectroscopy and theoretical calculations to study half-sandwich complexes where a lanthanide center in the oxidation state +1 is bound to an aromatic wheel-like B₈^2- ligand.

Density Functional Theory Analysis of the Importance of Coordination Geometry for 5f36d1 versus 5f4 Electron Configurations in U(II) Complexes

Density functional theory (DFT) calculations on four known and seven hypothetical U(II) complexes indicate the importance of coordination geometry in favoring 5f³6d¹ versus 5f⁴ electronic ground states. The known [Cp″₃U]^-, [Cp^tet₃U]^-, and [U(NR₂)₃]^- [Cp″ = C₅H₃(SiMe₃)₂, Cp^tet = C₅Me₄H, and R = SiMe₃] anions were found to have 5f³6d¹ ground states, while a 5f⁴ ground state was found for the known compound (NHAr^iPr₆)₂U. The UV-visible spectra of the known 5f³6d¹ compounds were simulated via time-dependent DFT and are in qualitative agreement with the experimental spectra. For the hypothetical U(II) compounds, the 5f³6d¹ configuration is predicted for [U(CHR₂)₃]^-, [U(H₃BH)₃]^-, [U(OAr')₄]^2-, and [(C₈H₈)U]^2- (OAr' = O-C₆H₂^tBu₂-2,6-Me-4). In the case of [U(bnz')₄]^2- (bnz' = CH₂-C₆H₄^tBu-4), a 5f³ configuration with a ligand-based radical was found as the ground state.

Evaluating electrochemical accessibility of 4fn5d1 and 4fn+1 Ln(II) ions in (C5H4SiMe3)3Ln and (C5Me4H)3Ln complexes

The reduction potentials (reported vs. Fc⁺/Fc) for a series of Cp'₃Ln complexes (Cp' = C₅H₄SiMe₃, Ln = lanthanide) were determined via electrochemistry in THF with [ⁿBu₄N][BPh₄] as the supporting electrolyte. The Ln(III)/Ln(II) reduction potentials for Ln = Eu, Yb, Sm, and Tm (-1.07 to -2.83 V) follow the expected trend for stability of 4f⁷, 4f¹⁴, 4f⁶, and 4f¹³ Ln(II) ions, respectively. The reduction potentials for Ln = Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu, that form 4fⁿ5d¹ Ln(II) ions (n = 2-14), fall in a narrow range of -2.95 V to -3.14 V. Only cathodic events were observed for La and Ce at -3.36 V and -3.43 V, respectively. The reduction potentials of the Ln(II) compounds [K(2.2.2-cryptand)][Cp'₃Ln] (Ln = Pr, Sm, Eu) match those of the Cp'₃Ln complexes. The reduction potentials of nine (C₅Me₄H)₃Ln complexes were also studied and found to be 0.05-0.24 V more negative than those of the Cp'₃Ln compounds.

Reductive Reactivity of the 4f75d1 Gd(II) Ion in {GdII[N(SiMe3)2]3}-: Structural Characterization of Products of Coupling, Bond Cleavage, Insertion, and Radical Reactions

The reductive reactivity of a Ln(II) ion with a nontraditional 4fⁿ5d¹ electron configuration has been investigated by studying reactions of the {Gd^II(N(SiMe₃)₂)₃]}^- anion with a variety of reagents that survey the many reaction pathways available to this ion. The chemistry of both [K(18-c-6)₂]⁺ and [K(crypt)]⁺ salts (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand) was examined to study the effect of the countercation. CS₂ reacts with the crown salt [K(18-c-6)₂][Gd(NR₂)₃] (1) to generate the bimetallic (CS₃)^2- complex {[K(18-c-6)](μ₃-CS₃-κS,κ²S',S'')Gd(NR₂)₂]}₂, which contains two trithiocarbonate dianions that bridge Gd(III) centers and a potassium ion coordinated by 18-c-6. In contrast, the only crystalline product isolated from the reaction of CS₂ with the crypt salt [K(crypt)][Gd(NR₂)₃] (2) is [K(crypt)]{(R₂N)₂Gd[SCS(CH₂)Si(Me₂)N(SiMe₃)-κN,κS]}, which has a CS₂ unit inserted into a cyclometalated amide ligand. Complexes 1 and 2 reductively couple pyridine to form bridging dipyridyl moieties, (NC₅H₄-C₅H₄N)^2-, that generate bimetallic complexes differing only in the countercation, {[K(18-c-6)(C₅H₅N)₂]}₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]} and [K(crypt)]₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]}. Complexes 1 and 2 also show similar reactivity with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the (TEMPO)^- complexes [K(18-c-6)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)] and [K(crypt)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)], respectively. The first example of a bimetallic coordination complex containing a Bi-Gd bond, [K(crypt)][(R₂N)₃Gd(BiPh₂)], was obtained by treating 2 with BiPh₃.

Correlation between Electronic Configuration and Magnetic Stability in Dysprosium Single Atom Magnets

Single atom magnets offer the possibility of magnetic information storage in the most fundamental unit of matter. Identifying the parameters that control the stability of their magnetic states is crucial to design novel quantum magnets with tailored properties. Here, we use X-ray absorption spectroscopy to show that the electronic configuration of dysprosium atoms on MgO(100) thin films can be tuned by the proximity of the metal Ag(100) substrate onto which the MgO films are grown. Increasing the MgO thickness from 2.5 to 9 monolayers induces a change in the dysprosium electronic configuration from 4f⁹ to 4f¹⁰. Hysteresis loops indicate long magnetic lifetimes for both configurations, however, with a different field-dependent magnetic stability. Combining these measurements with scanning tunneling microscopy, density functional theory, and multiplet calculations unveils the role of the adsorption site and charge transfer to the substrate in determining the stability of quantum states in dysprosium single atom magnets.

High-Resolution X-ray Photoelectron Spectroscopy of Organometallic (C5H4SiMe3)3LnIII and [(C5H4SiMe3)3LnII]1- Complexes (Ln = Sm, Eu, Gd, Tb)

The capacity of X-ray photoelectron spectroscopy (XPS) to provide information on the electronic structure of molecular organometallic complexes of Ln(II) ions (Ln = lanthanide) has been examined for the first time. XPS spectra were obtained on the air-sensitive molecular trivalent 4fⁿ Cp'₃Ln^III complexes (Ln = Sm, Eu, Gd, Tb; Cp' = C₅H₄SiMe₃) and compared to those of the highly reactive divalent complexes, [K(crypt)][Cp'₃Ln^II] (crypt = 2.2.2-cryptand), which have either 4fⁿ⁺¹ (Sm, Eu) or 4fⁿ5d¹ electron configurations (Gd, Tb). The Ln 4d, Si 2p, and C 1s regions of the Ln(III) and Ln(II) complexes were identified and compared. The metal 4d peaks of these molecular lanthanide complexes were used diagnostically to compare oxidation states. The valence region of the Gd(III) and Gd(II) complexes was also examined with XPS and density function theory/random phase approximation (DFT/RPA) calculations, and this led to the tentative assignment of a signal from the 5d¹ electron consistent with a 4f⁷5d¹ electron configuration for Gd(II).

Property-optimized Gaussian basis sets for lanthanides

Property-optimized Gaussian basis sets of split-valence, triple-zeta valence, and quadruple-zeta valence quality are developed for the lanthanides Ce-Lu for use with small-core relativistic effective core potentials. They are constructed in a systematic fashion by augmenting def2 orbital basis sets with diffuse basis functions and minimizing negative static isotropic polarizabilities of lanthanide atoms with respect to basis set exponents within the unrestricted Hartree-Fock method. The basis set quality is assessed using a test set of 70 molecules containing the lanthanides in their common oxidation states and f electron occupations. 5d orbital occupation turns out to be the determining factor for the basis set convergence of polarizabilities in lanthanide atoms and the molecular test set. Therefore, two series of property-optimized basis sets are defined. The augmented def2-SVPD, def2-TZVPPD, and def2-QZVPPD basis sets balance the accuracy of polarizabilities across lanthanide oxidation states. The relative errors in atomic and molecular polarizability calculations are ≤8% for augmented split-valence basis sets, ≤ 2.5% for augmented triple-zeta valence basis sets, and ≤1% for augmented quadruple-zeta valence basis sets. In addition, extended def2-TZVPPDD and def2-QZVPPDD are provided for accurate calculations of lanthanide atoms and neutral clusters. The property-optimized basis sets developed in this work are shown to accurately reproduce electronic absorption spectra of a series of LnCp₃ ^'- complexes (Cp' = C₅H₄SiMe₃, Ln = Ce-Nd, Sm) with time-dependent density functional theory.

Ytterbium (II) Hydride as a Powerful Multielectron Reductant

A dimeric β-diketiminato ytterbium(II) hydride affects both the two-electron aromatization of 1,3,5,7-cyclooctatetraene (COT) and the more challenging two-electron reduction of polyaromatic hydrocarbons, including naphthalene (E⁰ =-2.60 V). Confirmed by Density Functional Theory calculations, these reactions proceed via consecutive polarized Yb-H/C=C insertion and deprotonation steps to provide the respective ytterbium (II) inverse sandwich complexes and hydrogen gas. These observations highlight the ability of a simple ytterbium(II) hydride to act as a powerful two-electron reductant at room temperature without the necessity of an external electron to initiate the reaction and avoiding radicaloid intermediates.

High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopic Analysis of Metal-Ligand Covalency in a 4f7 Valence Series (Eu2+, Gd3+, and Tb4+)

The recent isolation of molecular tetravalent lanthanide complexes has enabled renewed exploration of the effect of oxidation state on the single-ion properties of the lanthanide ions. Despite the isotropic nature of the ⁸S ground state in a tetravalent terbium complex, [Tb(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄], preliminary X-band electron paramagnetic resonance (EPR) measurements on tetravalent terbium complexes show rich spectra with broad resonances. The complexity of these spectra highlights the limits of conventional X-band EPR for even qualitative determination of zero-field splitting (ZFS) in these complexes. Therefore, we report the synthesis and characterization of a novel valence series of 4f⁷ molecular complexes spanning three oxidation states (Eu²⁺, Gd³⁺, and Tb⁴⁺) featuring a weak-field imidophosphorane ligand system, and employ high-frequency and -field electron paramagnetic resonance (HFEPR) to obtain quantitative values for ZFS across this valence series. The series was designed to minimize deviation in the first coordination sphere from the pseudotetrahedral geometry in order to directly interrogate the role of metal identity and charge on the complexes' electronic structures. These HFEPR studies are supported by crystallographic analysis and quantum-chemical calculations to assess the relative covalent interactions in each member of this valence series and the effect of the oxidation state on the splitting of the ground state and first excited state.

Intermediate Valence States in Lanthanide Compounds

Over more than 50 years, intermediate valence states in lanthanide compounds have often resulted in unexpected or puzzling spectroscopic and magnetic properties. Such experimental singularities could not be rationalised until new theoretical models involving multiconfigurational electronic ground states were established. In this minireview, the different singularities that have been observed among lanthanide complexes are highlighted, the models used to rationalise them are detailed and how such electronic effects may be adjusted depending on energy and symmetry considerations is considered. Understanding and tuning the ground-state multiconfigurational behaviour in lanthanide complexes may open new doors to modular and unusual reactivities.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Distinct electronic structures and bonding interactions in inverse-sandwich samarium and ytterbium biphenyl complexes

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized by the reduction of their trivalent halide precursors with potassium graphite in the presence of biphenyl. While the samarium complex had a similar structure as previously reported rare earth metal biphenyl complexes, with the two samarium ions bound to the same phenyl ring, the ytterbium counterpart adopted a different structure, with the two ytterbium ions bound to different phenyl rings. Upon the addition of crown ether to encapsulate the potassium ions, the inverse-sandwich samarium biphenyl structure remained intact; however, the ytterbium biphenyl structure fell apart with the concomitant formation of a divalent ytterbium crown ether complex and potassium biphenylide. Spectroscopic and computational studies were performed to gain insight into the electronic structures and bonding interactions of these samarium and ytterbium biphenyl complexes. While the ytterbium ions were found to be divalent with a 4f¹⁴ electron configuration and form a primarily ionic bonding interaction with biphenyl dianion, the samarium ions were in the trivalent state with a 4f⁵ electron configuration and mainly utilized the 5d orbitals to form a δ-type bonding interaction with the π* orbitals of the biphenyl tetraanion, showing covalent character.

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized and characterized by X-ray crystallography. Combined experimental and computational studies indicated that they have distinct electronic structures and bonding interactions.

Electrically tuned hyperfine spectrum in neutral Tb(II)(CpiPr5)2 single-molecule magnet

Molecular spin qubits with long spin coherence time as well as non-invasive operation methods on such qubits are in high demand. It was shown that both molecular electronic and nuclear spin levels can be used as qubits. In solid state systems with dopants, an electric field was shown to effectively change the spacing between the nuclear spin qubit levels when the electron spin density is high at the nucleus of the dopant. Inspired by such solid-state systems, we propose that divalent lanthanide (Ln) complexes with an unusual electronic configuration of Ln2+ have a strong interaction between the Ln nuclear spin and the electronic degrees of freedom, which renders electrical tuning of the interaction. As an example, we study electronic structure and hyperfine interaction of the 159Tb nucleus in a neutral Tb(ii)(CpiPr5)2 single-molecule magnet (SMM), which exhibits unusually long magnetization relaxation time, using the complete active space self-consistent field (CASSCF) method with spin-orbit interaction included within the restricted active space state interaction (RASSI). Our calculations show that the low-energy states arise from 4f8(6s,5dz2)1, 4f8(5dx2-y2)1, and 4f8(5dxy)1 configurations. We compute the hyperfine interaction parameters and the electronic-nuclear spectrum within our multiconfigurational approach. We find that the hyperfine interaction is about one order of magnitude greater than that for Tb(iii)Pc2 SMMs. This stems from the strong Fermi contact interaction between the Tb nuclear spin and the electron spin density at the nucleus that originates from the occupation of the (6s,5d) orbitals. We also uncover that the response of the Fermi contact term to electric field results in electrical tuning of the electronic-nuclear level separations. This hyperfine Stark effect may be useful for applications of molecular nuclear spins for quantum computing.

Synthesis of LnII -in-Cryptand Complexes by Chemical Reduction of LnIII -in-Cryptand Precursors: Isolation of a NdII -in-Cryptand Complex

Lanthanide triflates have been used to incorporate Nd^III and Sm^III ions into the 2.2.2-cryptand ligand (crypt) to explore their reductive chemistry. The Ln(OTf)₃ complexes (Ln=Nd, Sm; OTf=SO₃ CF₃ ) react with crypt in THF to form the THF-soluble complexes [Ln^III (crypt)(OTf)₂ ][OTf] with two triflates bound to the metal encapsulated in the crypt. Reduction of these Ln^III -in-crypt complexes using KC₈ in THF forms the neutral Ln^II -in-crypt triflate complexes [Ln^II (crypt)(OTf)₂ ]. DFT calculations on [Nd^II (crypt)]²⁺ ], the first Nd^II cryptand complex, assign a 4f⁴ electron configuration to this ion.

Comparison of tetravalent cerium and terbium ions in a conserved, homoleptic imidophosphorane ligand field

A redox pair of Ce4+ and Ce3+ complexes has been prepared that is stabilized by the [(NP(1,2-bis- t Bu-diamidoethane)(NEt2))]1- ligand. Since these complexes are isostructural to the recently reported isovalent terbium analogs, a detailed structural and spectroscopic comparative analysis was pursued via Voronoi-Dirichlet polyhedra analysis, UV-vis-NIR, L3-edge X-ray absorption near edge spectroscopy (XANES), cyclic voltammetry, and natural transitions orbital (NTO) analysis and natural bond orbital (NBO) analysis. The electrochemical studies confirm previous theoretical studies of the redox properties of the related complex [K][Ce3+(NP(pip)3)4] (pip = piperidinyl), 1-Ce(PN). Complex 1-Ce(PN*) presents the most negative E pc of -2.88 V vs. Fc/Fc+ in THF of any cerium complex studied electrochemically. Likewise 1-Tb(PN*) has the most negative E pc for electrochemically interrogated terbium complexes at -1.79 V vs. Fc/Fc+ in THF. Complexes 1-Ce(PN*) and 2-Ce(PN*) were also studied by L3-edge X-ray absorption near edges spectroscopy (XANES) and a comparison to previously reported spectra for 1-Tb(PN*), 2-Tb(PN*), 1-Ce(PN), and, [Ce4+(NP(pip)3)4], 2-Ce(PN), demonstrates similar n f values for all the tetravalent lanthanide complexes. According to the natural bond orbital analysis, a greater covalent character of the M-L bonds is found in 2-Ce(PN*) than in 1-Ce(PN*), in agreement with the shorter Ce-N bonds in the tetravalent counterpart. The greater contribution of Ce orbitals in the Ce-N bonding and, specifically, the higher participation of 4f electrons accounts for the stronger covalent interactions in 2-Ce(PN*) as compared to 2-Tb(PN*).