Overdestabilization vs Overstabilization in the Theoretical Analysis of f-Orbital Covalency.

The complex nature of the f-orbital electronic structures and their interaction with the chemical environment pose significant computational challenges. Advanced computational techniques that variationally include scalar relativities and spin-orbit coupling directly at the molecular orbital level have been developed to address this complexity. Among these, variational relativistic multiconfigurational multireference methods stand out for their high accuracy and systematic improvement in studies of f-block complexes. Additionally, these advanced methods offer the potential for calibrating low-scaling electronic structure methods such as density functional theory. However, studies on the Cl K-edge X-ray absorption spectra of the [Ce(III)Cl6]3- and [Ce(IV)Cl6]2- complexes show that time-dependent density functional theory with approximate exchange-correlation kernels can lead to inaccuracies, resulting in an overstabilization of 4f orbitals and incorrect assessments of covalency. In contrast, approaches utilizing small active space wave function methods may understate the stability of these orbitals. The results herein demonstrate the need for large active space, multireference, and variational relativistic methods in studying f-block complexes.

Tetravalent Cerium Alkyl and Benzyl Complexes.

High-valent cerium complexes of alkyl and benzyl ligands are unprecedented due to the incompatibility of the typically highly oxidizing Ce4+ ion and the reducing alkyl or benzyl ligand. Herein we report the synthesis and isolation of the first tetravalent cerium alkyl and benzyl complexes supported by the tri-tert-butyl imidophosphorane ligand, [NP(tBu)3]1-. The Ce4+ monoiodide complex, [Ce4+I(NP(tert-butyl)3)3] (1-CeI), serves as a precursor to the alkyl and benzyl complexes, [Ce4+(Npt)(NP(tert-butyl)3)3] (2-CeNpt) (Npt = neopentyl, CH2C(CH3)3) and [Ce4+(Bn)(NP(tert-butyl)3)3] (2-CeBn) (Bn = benzyl, CH2Ph). The bonding and structure of these complexes are characterized by single-crystal XRD, NMR and UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT studies.

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes.

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system.

Coordination Modes and Binding Patterns in Lanthanum Phosphoramide Complexes.

A monoanionic phosphoramide ligand is introduced, which forms a series of lanthanum complexes with the ligand in both anionic and neutral forms. Stoichiometric control alone provides monometallic complexes with either two or three phosphoramide ligands. Alternatively, a combination of anionic and neutral proteo ligands featuring intramolecular hydrogen bonding can be obtained. The anionic form of the ligand binds lanthanum as a bi- or monodentate ligand, depending on the steric demand at the metal center, while the protonated ligand binds exclusively through the phosphoramide oxygen donor.

Increased Crystal Field Drives Intermediate Coupling and Minimizes Decoherence in Tetravalent Praseodymium Qubits.

Crystal field (CF) control of rare-earth (RE) ions has been employed to minimize decoherence in qubits and to enhance the effective barrier of single-molecule magnets. The CF approach has been focused on the effects of symmetry on dynamic magnetic properties. Herein, the magnitude of the CF is increased via control of the RE oxidation state. The enhanced 4f metal-ligand covalency in Pr4+ gives rise to CF energy scales that compete with the spin-orbit coupling of Pr4+ and thereby shifts the paradigm from the ionic ζSOC ≫ VCF limit, used to describe trivalent RE-ion, to an intermediate coupling (IC) regime. We examine Pr4+-doped perovskite oxide lattices (BaSnO3 and BaZrO3). These systems are defined by IC which quenches orbital angular momentum. Therefore, the single-ion spin-orbit coupled states in Pr4+ can be chemically tuned. We demonstrate a relatively large hyperfine interaction of Aiso = 1800 MHz for Pr4+, coherent manipulation of the spin with QM = 2ΩRTm, reaching up to ∼400 for 0.1Pr:BSO at T = 5 K, and significant improvement of the temperature at which Tm is limited by T1 (T* = 60 K) compared to other RE ion qubits.

Ground-State Spin Dynamics in d1 Kagome-Lattice Titanium Fluorides.

Many quantum magnetic materials suffer from structural imperfections. The effects of structural disorder on bulk properties are difficult to assess systematically from a chemical perspective due to the complexities of chemical synthesis. The recently reported S = 1/2 kagome lattice antiferromagnet, (CH3NH3)2NaTi3F12, 1-Ti, with highly symmetric kagome layers and disordered interlayer methylammonium cations, shows no magnetic ordering down to 0.1 K. To study the impact of structural disorder in the titanium fluoride kagome compounds, (CH3NH3)2KTi3F12, 2-Ti, was prepared. It presents no detectable structural disorder and only a small degree of distortion of the kagome lattice. The methylammonium disorder model of 1-Ti and order in 2-Ti were confirmed by atomic-resolution transmission electron microscopy. The antiferromagnetic interactions and band structures of both compounds were calculated based on spin-polarized density functional theory and support the magnetic structure analysis. Three spin-glass-like (SGL) transitions were observed in 2-Ti at 0.5, 1.4, and 2.3 K, while a single SGL transition can be observed in 1-Ti at 0.8 K. The absolute values of the Curie-Weiss temperatures of both 1-Ti (-139.5(7) K) and 2-Ti (-83.5(7) K) are larger than the SGL transition temperatures, which is indicative of geometrically frustrated spin glass (GFSG) states. All the SGL transitions are quenched with an applied field >0.1 T, which indicates novel magnetic phases emerge under small applied magnetic fields. The well-defined structure and the lack of structural disorder in 2-Ti suggest that 2-Ti is an ideal model compound for studying GFSG states and the potential transitions between spin liquid and GFSG states.

Ligand Control of Oxidation and Crystallographic Disorder in the Isolation of Hexavalent Uranium Mono-Oxo Complexes.

The development of high-valent transuranic chemistry requires robust methodologies to access and fully characterize reactive species. We have recently demonstrated that the reducing nature of imidophosphorane ligands supports the two-electron oxidation of U4+ to U6+ and established the use of this ligand to evaluate the inverse-trans-influence (ITI) in actinide metal-ligand multiple bond (MLMB) complexes. To extend this methodology and analysis to transuranic complexes, new small-scale synthetic strategies and lower-symmetry ligand derivatives are necessary to improve crystallinity and reduce crystallographic disorder. To this end, the synthesis of two new imidophosphorane ligands, [N═PtBu(pip)2]- (NPC1) and [N═PtBu(pyrr)2]- (NPC2) (pip = piperidinyl; pyrr = pyrrolidinyl), is presented, which break pseudo-C3 axes in the tetravalent complexes, U[NPC1]4 and U[NPC2]4. The reaction of these complexes with two-electron oxygen-atom-transfer reagents (N2O, trimethylamine N-oxide (TMAO) and 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene (dbabhNO)) yields the U6+ mono-oxo complexes U(O)[NPC1]4 and U(O)[NPC2]4. This methodology is optimized for direct translation to transuranic elements. Of the two ligands, the NPC2 framework is most suitable for facilitating detailed bonding analysis and assessment of the ITI. Theoretical evaluation of the U-(NPC) bonding confirms a substantial difference between axially and equatorially bonded N atoms, revealing markedly more covalent U-Nax interactions. The U 6d + 5f combined contribution for U-Nax is nearly double that of U-Neq, accounting for ITI shortening and increased bond order of the axial bond. Two distinct N-atom hybridizations in the pyrrolidine/piperidine rings are noted across the complexes, with approximate sp2 and sp3 configurations describing the slightly shorter P-N"planar" and slightly longer P-N"pyramidal" bonds, respectively. In all complexes, the NPC2 ligands feature more planar N atoms than NPC1, in accordance with a higher electron-donating capacity of the former.

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes.

The study of the redox chemistry of mid-actinides (U-Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=Pt Bu(pyrr)2 ]- ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1-M, 2-M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U5+/4+ , U6+/5+ , and to an unprecedented, well-behaved Np5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

Photoluminescence of Pentavalent Uranyl Amide Complexes.

Pentavalent uranyl species are crucial intermediates in transformations that play a key role for the nuclear industry and have recently been demonstrated to persist in reducing biotic and abiotic aqueous environments. However, due to the inherent instability of pentavalent uranyl, little is known about its electronic structure. Herein, we report the synthesis and characterization of a series of monomeric and dimeric, pentavalent uranyl amide complexes. These synthetic efforts enable the acquisition of emission spectra of well-defined pentavalent uranyl complexes using photoluminescence techniques, which establish a unique signature to characterize its electronic structure and, potentially, its role in biological and engineered environments via emission spectroscopy.

In-Plane Cation Ordering and Sodium Displacements in Layered Honeycomb Oxides with Tetravalent Lanthanides: Na2LnO3 (Ln = Ce, Pr, and Tb).

The detailed structural characterization of "213" honeycomb systems is a key concern in a wide range of fundamental areas, such as frustrated magnetism, and technical applications, such as cathode materials, catalysts, and thermoelectric materials. Na2LnO3 (Ln = Ce, Pr, and Tb) are an intriguing series of "213" honeycomb systems because they host tetravalent lanthanides. "213" honeycomb materials have been reported to adopt either a cation-disordered R3̅m subcell, a cation-ordered trigonal (P3112), or monoclinic (C2/c or C2/m) supercell. On the basis of analysis of the average (synchrotron diffraction) and local [pair distribution function (PDF) and solid-state NMR] structure probes, cation ordering in the honeycomb layer of Na2LnO3 materials has been confirmed. Through rationalization of the 23Na chemical shifts and quadrupolar coupling constants, the local environment of Na atoms was probed with no observed evidence of cation disorder. Through these studies, it is shown that the Na2LnO3 materials adopt a C2/c supercell derived from symmetry-breaking displacements of intralayered Na atoms from the ideal crystallographic position (in C2/m). The Na displacement is validated using distortion index parameters from diffraction data and atomic displacement parameters from PDF data. The C2/c supercell is faulted, as evidenced by the increased breadth of the superstructure diffraction peaks. DIFFaX simulations and structural considerations with a two-phase approach were employed to derive a suitable faulting model.

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.

Two-Electron Oxidative Atom Transfer at a Homoleptic, Tetravalent Uranium Complex.

A tetrahomoleptic, pseudotetrahedral U4+ imidophosphorane complex, [U(NP(pip)3)4], 1-U(PN), is reported. This complex can be oxidized by two electrons with either mesityl azide or nitrous oxide. This two-electron atom/group transfer oxidation is the first example observed at a homoleptic, tetravalent uranium complex. The mesityl imido compound [U(NMes)(NP(pip)3)4], 2-U(PN)NMes, exhibits a unique square pyramidal geometry in contrast to the expected trigonal bipyramidal geometry of the oxo complex [U(O)(NP(pip)3)4], 2-U(PN)O. The bonding driving the structural dichotomy of these structures and the absence of a structurally observable inverse trans-influence in 2-U(PN)NMes were examined by DFT and natural bonding orbital analysis.

Design, Isolation, and Spectroscopic Analysis of a Tetravalent Terbium Complex.

Synthetic strategies to yield molecular complexes of high-valent lanthanides, other than the ubiquitous Ce4+ ion, are exceptionally rare, and thorough, detailed characterization in these systems is limited by complex lifetime and reaction and isolation conditions. The synthesis of high-symmetry complexes in high purity with significant lifetimes in solution and the solid state is essential for determining the role of ligand-field splitting, multiconfigurational behavior, and covalency in governing the reactivity and physical properties of these potentially technologically transformative tetravalent ions. We report the synthesis and physical characterization of an S4 symmetric, four-coordinate tetravalent terbium complex, [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] (where Et is ethyl and tBu is tert-butyl). The ligand field in this complex is weak and the metal-ligand bonds sufficiently covalent so that the tetravalent terbium ion is stable and accessible via a mild oxidant from the anionic, trivalent, terbium precursor, [(Et2O)K][Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4]. The significant stability of the tetravalent complex enables its thorough characterization. The stepwise development of the supporting ligand points to key ligand control elements for further extending the known tetravalent lanthanide ions in molecular complexes. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), and density functional theory studies indicate a 4f7 ground state for [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] with considerable zero-field splitting, demonstrating that magnetic, tetravalent lanthanide ions engage in covalent metal-ligand bonds. This result has significant implications for the use of tetravalent lanthanide ions in magnetic applications since the observed zero-field splitting is intermediate between that observed for the trivalent lanthanides and for the transition metals. The similarity of the multiconfigurational behavior in the ground state of [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] (measured by Tb L3-edge XAS) to that observed in TbO2 implicates ligand control of multiconfigurational behavior as a key component of the stability of the complex.

Frustrated Magnetism in a 2-D Ytterbium Fluoride.

Geometric frustration of magnetic ions combines with spin-orbit coupling and structural disorder to give rise to potentially, highly entangled magnetic states such as quantum spin liquids. While fluoride-based frustrated magnets are common in the d-block, in the f-block, fluoride-based frustrated magnets are extremely rare. Herein, we report the synthesis of KYb2F5SO4, a fluoride-based distorted triangular lattice antiferromagnet with no structural disorder but significant geometric distortion from an ideal triangular lattice. In KYb2F5SO4, no long-range ordering can be observed down to 0.10 K, and the low-temperature Curie-Weiss temperature θcw = -0.46(2) K yields a frustration parameter of greater than 4.6. The magnetic entropy released at low temperature indicates an effective spin-1/2 Kramer's doublet ground state. However, the low saturation field and incomplete recovery of magnetic entropy down to 0.10 K under zero-field imply weak quantum entanglement.

Synthesis and Magneto-Structural Characterization of Yb3(OH)7SO4·H2O: a Frustrated Quantum Magnet with Tunable Stacking Disorder.

The quenched structural disorder in frustrated magnets can lead to apparent quantum spin liquid (QSL) behavior or to a valence bond glass state: the transition between these thermodynamic states has not been demonstrated experimentally. Herein, we report the synthesis of a novel layered rare earth hydroxide Yb3(OH)7SO4·H2O as single crystals. The interplay between the strong distortion of the triangular lattice and low point group symmetry of the three distinct Yb3+ sites leads to quenched disorder. The variable stacking disorder in Yb3(OH)7SO4·H2O is elucidated by comparison to the lutetium analogue, Lu3(OH)7SO4·H2O. The degree of disorder in Yb3(OH)7SO4·H2O is controlled by the chemical form of the starting material and solution pH. In a low magnetic field, Yb3(OH)7SO4·H2O displays QSL behavior, while, under a high field, a valence bond glass state is observed. The degree of stacking fault disorder in Yb3(OH)7SO4·H2O modulates the observed magnetic properties and the transition between QSL and valence bond glass states.

Homoleptic Imidophosphorane Stabilization of Tetravalent Cerium.

The homoleptic complexes of cerium with the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]-, present the most negative Ce3+/4+ redox couple known (<-2.64 V vs Fc/Fc+). This dramatic stabilization of the cerium tetravalent oxidation state [>4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV-vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal-ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.

Energy-Degeneracy-Driven Covalency in Actinide Bonding.

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2 g*/eg *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M-Cl orbital mixing, such that AnIV 6d - and Cl 3p-mixing decreased and metal 5f - and Cl 3p-orbital mixing increased across this series.

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species.

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [CpPU(MesPDIMe)]2 (1-CpP), Cp*U(MesPDIMe)(THF) (1-Cp*) (CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(tBu-MesPDIMe) (THF) (1-tBu) (2,6-((Mes)N═CMe)2-p-R-C5H2N, Mes = 2,4,6-trimethylphenyl; R = H, MesPDIMe; R = C(CH3)3, tBu-MesPDIMe), has been investigated. While 1-Cp* and 1-CpP readily reduce N3R (R = Ph, p-tolyl) to form trans-bis(imido) species, CpPU(NAr)2(MesPDIMe) (Ar = Ph, 2-CpP; Ar = p-Tol, 3-CpP) and Cp*U(NPh)2(MesPDIMe) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-CpP, and 3-CpP are uranium(V) trans-bis(imido) species supported by neutral [MesPDIMe]0 ligands formed by complete oxidation of [MesPDIMe]3- ligands of 1-CpP and 1-Cp*. Variation of the arylimido substituent in 2-Cp* from phenyl to p-tolyl, forming Cp*U(NTol)2(MesPDIMe) (3-Cp*), changes the electronic structure, generating a uranium(VI) ion with a monoanionic pyridine(diimine) radical. The tert-butyl-substituted analogue, Cp*U(NTol)2(tBu-MesPDIMe) (3-tBu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3-tBu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol)2(MesPDIMe)][SbF6] (4-Cp*) and [Cp*U(NTol)2(tBu-MesPDIMe)][SbF6] (4-tBu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF6 affords cationic [Cp*U(NPh)2(MesPDIMe)][SbF6] (5-Cp*) from a metal-based U(V)/U(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of 1-CpP, 3-Cp*, 3-tBu, 4-Cp*, 4-tBu, and 5-Cp* have been elucidated by X-ray crystallography.

Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates.

The reaction of Ce(III) or Pu(III) with 1,10-phenanthroline-2,9-dicarboxylic acid (PDAH2) results in the formation of new f-element coordination complexes. In the case of cerium, Ce(PDA)(H2O)2Cl·H2O (1) or [Ce(PDAH)(PDA)]2[Ce(PDAH)(PDA)] (2) was isolated depending on the Ce/ligand ratio in the reaction. The structure of 2 is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a Ce(III) cation bound by one PDA(2-) dianionic ligand and one PDAH(-) monoanionic ligand, both of which are tetradentate. Bridging by the carboxylate moieties leads to either [Ce(PDAH)(PDA)]2 dimers or [Ce(PDAH)(PDA)]1∞ helical chains. For plutonium, Pu(PDA)2 (3) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH2, Pu(III) is oxidized to Pu(IV), generating 3. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in 1 and 2 from UV-vis-near-IR spectra invoked the use of Ce L3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of Ce(III) in both compounds. The bond distances and coordination numbers are also consistent with these assignments. 3 contains 8-coordinate Pu(IV), whereas the cerium centers in 1 and 2 are 9- and/or 10-coordinate, which correlates with the increased size of Ce(III) versus Pu(IV). Taken together, these data provide an example of a system where the differences in the redox behavior between these f elements creates more complex chemistry with cerium than with plutonium.