Superexchange coupling and slow magnetic relaxation in a transuranium polymetallic complex

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

Chemical design of electronic and magnetic energy scales of tetravalent praseodymium materials

Lanthanides in the trivalent oxidation state are typically described using an ionic picture that leads to localized magnetic moments. The hierarchical energy scales associated with trivalent lanthanides produce desirable properties for e.g., molecular magnetism, quantum materials, and quantum transduction. Here, we show that this traditional ionic paradigm breaks down for praseodymium in the tetravalent oxidation state. Synthetic, spectroscopic, and theoretical tools deployed on several solid-state Pr4+-oxides uncover the unusual participation of 4f orbitals in bonding and the anomalous hybridization of the 4f1 configuration with ligand valence electrons, analogous to transition metals. The competition between crystal-field and spin-orbit-coupling interactions fundamentally transforms the spin-orbital magnetism of Pr4+, which departs from the Jeff = 1/2 limit and resembles that of high-valent actinides. Our results show that Pr4+ ions are in a class on their own, where the hierarchy of single-ion energy scales can be tailored to explore new correlated phenomena in quantum materials.

Theoretical Investigation of Single-Molecule-Magnet Behavior in Mononuclear Dysprosium and Californium Complexes

Early-actinide-based (U, Np, and Pu) single-molecule magnets (SMMs) have yet to show magnetic properties similar to those of highly anisotropic lanthanide-based ones. However, there are not many studies exploring the late-actinides (more than half-filled f shells) as potential candidates for SMM applications. We computationally explored the electronic structure and magnetic properties of a hypothetical Cf(III) complex isostructural to the experimentally synthesized Dy(dbm)₃(bpy) complex (bpy = 2,2'-bipyridine; dbm = dibenzoylmethanoate) via multireference methods and compared them to those of the Dy(III) analogue. This study shows that the Cf(III) complex can behave as a SMM and has a greater magnetic susceptibility compared to other experimentally and computationally studied early-actinide-based (U, Np, and Pu) magnetic complexes. However, Cf spontaneously undergoes α-decay and converts to Cm. Thus, we also explored the isostructural Cm(III)-based complex. The computed magnetic susceptibility and g-tensor values show that the Cm(III) complex has poor SMM behavior in comparison to both the Dy(III) and Cf(III) complexes, suggesting that the performance of Cf(III)-based magnets may be affected by α-decay and can explain the poor performance of experimentally studied Cf(III)-based molecular magnets in the literature. Further, this study suggests that the ligand field is dominant in Cf(III), which helps to increase the magnetization blocking barrier by nearly 3 times that of its 4f congener.

Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)

Reaction of 3 equiv of NaNR₂ (R = SiMe₃) with NpCl₄(DME)₂ in THF afforded the Np(IV) silylamide complex, [Np(NR₂)₃Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC₈ in THF, in the presence of 1 equiv of dibenzo-18-crown-6, resulted in formation of [{K(DB-18-C-6)(THF)}₃(μ₃-Cl)][Np(NR₂)₃Cl]₂ (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl₄(DME)₂ with 5 equiv of NaNR₂ and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et₂O)_0.62(κ¹-DME)_0.38}₂(μ-DME)][Np{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (8), in moderate yield. Complex 8 was characterized by ¹H NMR spectroscopy and X-ray crystallography and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, we also synthesized the uranium analogues of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR₂)₃Cl] (2), [K(DB-18-C-6)(THF)₂][U(NR₂)₃Cl] (3), [Na(DME)₃][U{N(R)(SiMe₂CH₂)}₂(NR₂)] (6), and [{Na(DB-18-C-6)(Et₂O)_0.5(κ¹-DME)_0.5}₂(μ-DME)][U{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (7). Complexes 2, 3, 6, and 7 were characterized by a number of techniques, including NMR spectroscopy and X-ray crystallography.

Decisive Role of 5f-Orbital Covalence in the Structure and Stability of Pentavalent Transuranic Oxo [M6O8] Clusters

Actinide metal oxo clusters are of vital importance in actinide chemistry, as well as in environmental and materials sciences. They are ubiquitous in both aqueous and nonaqueous phases and play key roles in nuclear materials (e.g., nuclear fuel) and nuclear waste management. Despite their importance, our structural understanding of the actinide metal oxo clusters, particularly the transuranic ones, is very limited because of experimental challenges such as high radioactivity. Herein we report a systematic theoretical study on the structures and stabilities of seven actinide metal oxo-hydroxo clusters [An^IV₆O₄(OH)₄L₁₂] (1-An; An = Th-Cm; L = O₂CH^-) along with their group 4 (Ti, Zr, Hf, Rf) and lanthanide (Ce) counterparts [M^IV₆O₄(OH)₄L₁₂] (1-M). The work shows the T_d-symmetric structures of all of the 1-An/M clusters and suggests the positions of the -OH functional groups, which are experimentally challenging to determine. Furthermore, by removing six electrons from 1-An, we found that oxidation could happen on the An^IV metal ions, producing [An^V₆O₄(OH)₄L₁₂]⁶⁺ (2-An; An = Pa, U, Np), or on the O^2- and OH^- ligands, producing [An^IV₆(O^•-)₄(OH^•)₂(OH)₂L₁₂]⁶⁺ (3-An; An = Pu, Am, Cm). On the basis of 2-An, we constructed a series of tetravalent and pentavalent actinide metal oxo clusters [An^IV₆O₁₄]^4- (4-An) and [An^V₆O₁₄]²⁺ (5-An), which proves the feasibility of the highly important pentavalent actinyl clusters, demonstrates the f orbital's structure-directing role in the formation of linear [O≡An^V═O]⁺ actinyl ions, and expands the concept of actinyl-actinyl interaction into pentavalent transuranic actinyl clusters.

Origins of Slow Magnetic Relaxation in Single-Molecule Magnets

Exponential and power law temperature dependences are widely used to fit experimental data of magnetic relaxation time in single molecular magnets. We derived a theory to show how these rules arise from the underling relaxation mechanisms and to clarify the conditions for their occurrence. The theory solves the puzzle of lower-than-expected Orbach barriers found in recent experiments, and elucidates it as a result of the Raman process in disguise. Our results highlight the importance of reducing the rate of direct tunneling between the ground state doublet so as to achieve longtime coherence in magnetic molecules. To this end, large spin and small transverse magnetic anisotropy can reduce magnitude of the transition operator, and rigid ligands may weaken the spin-phonon coupling in that they raise the energy of vibrational modes and better screen the acoustic phonons.

Correlating Electronic Structure and Magnetic Anisotropy in Actinide Complexes [An(COT)2], AnIII/IV = U, Np, and Pu

The electronic structures and magnetic anisotropies for compounds [An(COT)₂] (An = U^III/U^IV, Np^III/Np^IV and Pu^III/Pu^IV, COT = cyclooctatetraene) are characterized using scalar relativistic density functional theory calculations and second-order perturbation theory based on a complete active space self-consistent field reference including spin-orbit coupling. The degree of participation of 5f orbitals in actinide-ligand bonding and the associated metal-ligand covalency is found to trend as U > Np ≥ Pu for both the tetra-positive and tripositive An complexes. A spin-Hamiltonian analysis indicates only weak single-molecule magnet (SMM) characteristics for [U(COT)₂]^- and [Np(COT)₂] complexes and no significant SMM behavior for the other complexes. The weak SMM behavior in [U(COT)₂]^- and [Np(COT)₂] is attributed to a subtle interplay between local symmetry and ligand-field splitting. Such a result suggests that magnetic anisotropy in 5f³ ions can be modulated in general by electrostatic ligand field design. In particular, σ-donor ligands oriented 180 degrees relative to one another will have a maximal influence on the 5f-orbital ligand field splitting, while π donors like cyclopentadiene and COT generate ligand field influences that have more acute angles associated with corresponding atoms on the individual ligands. These observations rationalize the differences in SMM characteristics for [U(Bc^Me)₃] (Bc^Me- = dihydrobis(methylimidazolyl)borate) and [U(Bp^Me)₃] (Bp^Me- = dihydrobis(methylpyrazolyl)borate) and indicate strategies to design new actinide-based SMMs with high magnetic relaxation barriers.

DFT Investigations of the Magnetic Properties of Actinide Complexes

Over the past 25 years, magnetic actinide complexes have been the object of considerable attention, not only at the experimental level, but also at the theoretical one. Such systems are of great interest, owing to the well-known larger spin–orbit coupling for actinide ions, and could exhibit slow relaxation of the magnetization, arising from a large anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Furthermore, more diffuse 5f orbitals than lanthanide 4f ones (more covalency) could lead to stronger magnetic super-exchange. On the other hand, the extraordinary experimental challenges of actinide complexes chemistry, because of their rarity and toxicity, afford computational chemistry a particularly valuable role. However, for such a purpose, the use of a multiconfigurational post-Hartree-Fock approach is required, but such an approach is computationally demanding for polymetallic systems—notably for actinide ones—and usually simplified models are considered instead of the actual systems. Thus, Density Functional Theory (DFT) appears as an alternative tool to compute magnetic exchange coupling and to explore the electronic structure and magnetic properties of actinide-containing molecules, especially when the considered systems are very large. In this paper, relevant achievements regarding DFT investigations of the magnetic properties of actinide complexes are surveyed, with particular emphasis on some representative examples that illustrate the subject, including actinides in Single Molecular Magnets (SMMs) and systems featuring metal-metal super-exchange coupling interactions. Examples are drawn from studies that are either entirely computational or are combined experimental/computational investigations in which the latter play a significant role.

Oxidation of uranium(iv) thiocyanate complexes: cation–cation interactions in mixed-valent uranium coordination chains

Oxidation of Cs₄[U(NCS)₈] in different solvents results in two mixed-valent uranium compounds. Spectroscopic, magnetic and computational data support a unique [U^IVU^VU^IV][U^VI] oxidation state assignment in [U(DMF)₈(μ-O)U(NCS)₅(μ-O)U(DMF)₇(NCS)][UO₂(NCS)₅].

Structure and properties of [(4,6-tBu2C6H2O)2Se]2An(THF)2, An = U, Np, and their reaction with p-benzoquinone

The synthesis and characterization of U(iv) and Np(iv) selenium bis(phenolate) complexes are reported. The reaction of two equivalents of the U(iv) complex with p-benzoquinone results in the formation of a U(v)-U(v) species with a bridging reduced quinone. This represents a rare example of high-valent uranium chemistry as well as a rare example of a neptunium aryloxide complex.

Future Directions for Transuranic Single Molecule Magnets

Single Molecule Magnets (SMMs) based on transition metals and rare earths have been the object of considerable attention for the past 25 years. These systems exhibit slow relaxation of the magnetization, arising from a sizeable anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Despite initial predictions that SMMs based on 5f-block elements could outperform most others, the results obtained so far have not met expectations. Exploiting the versatile chemistry of actinides and their favorable intrinsic magnetic properties proved, indeed, to be more difficult than assumed. However, the large majority of studies reported so far have been dedicated to uranium molecules, thus leaving the largest part of the 5f-block practically unexplored. Here, we present a short review of the progress achieved up to now and discuss some options for a possible way forward.

Organometallic Neptunium Chemistry

Fifty years have passed since the foundation of organometallic neptunium chemistry, and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and separations of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a critical assessment of the successes and difficulties in their analysis and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorganic chemistry laboratories, the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochemical synthesis of one organometallic compound), and the available anhydrous starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chemical behavior between Np and its closest neighbors, uranium and plutonium, are found.

Axially Symmetric U-O-Ln- and U-O-U-Containing Molecules from the Control of Uranyl Reduction with Simple f-Block Halides

The reduction of U^VI uranyl halides or amides with simple Ln^II or U^III salts forms highly symmetric, linear, oxo-bridged trinuclear U^V /Ln^III /U^V , Ln^III /U^IV /Ln^III , and U^IV /U^IV /U^IV complexes or linear Ln^III /U^V polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

Assessing crystal field and magnetic interactions in diuranium-μ-chalcogenide triamidoamine complexes with UIV-E-UIV cores (E = S, Se, Te): implications for determining the presence or absence of actinide-actinide magnetic exchange

We report the synthesis and characterisation of a family of diuranium(iv)-μ-chalcogenide complexes including a detailed examination of their electronic structures and magnetic behaviours. Treatment of [U(Tren^TIPS)] [1, Tren^TIPS = N(CH₂CH₂NSiPrⁱ₃)₃] with Ph₃PS, selenium or tellurium affords the diuranium(iv)-sulfide, selenide, and telluride complexes [{U(Tren^TIPS)}₂(μ-E)] (E = S, 2; Se, 5; Te, 6). Complex 2 is also formed by treatment of [U(Tren^TIPS){OP(NMe₂)₃}] (3) with Ph₃PS, whereas treatment of 3 with elemental sulfur gives the diuranium(iv)-persulfido complex [{U(Tren^TIPS)}₂(μ-η²:η²-S₂)] (4). Complexes 2-6 have been variously characterised by single crystal X-ray diffraction, NMR, IR, and optical spectroscopies, room temperature Evans and variable temperature SQUID magnetometry, elemental analyses, and complete active space self consistent field spin orbit calculations. The combined characterisation data present a self-consistent picture of the electronic structure and magnetism of 2, 5, and 6, leading to the conclusion that single-ion crystal field effects, and not diuranium magnetic coupling, are responsible for features in their variable-temperature magnetisation data. The presence of magnetic coupling is often implied and sometimes quantified by such data, and so this study highlights the importance of evaluating other factors, such as crystal field effects, that can produce similar magnetic observables, and to thus avoid misassignments of such phenomena.

Synthesis and SMM behaviour of trinuclear versus dinuclear 3d–5f uranyl(v)–cobalt(ii) cation–cation complexes

Trinuclear versus dinuclear heterodimetallic U^VO₂⁺⋯Co²⁺ complexes were selectively assembled via a cation–cation interaction by tuning the ligand. The trimeric complex, exhibits magnetic exchange and slow relaxation providing the first example of a U–Co exchange-coupled SMM.

Molecular and electronic structure of terminal and alkali metal-capped uranium(V) nitride complexes

Determining the electronic structure of actinide complexes is intrinsically challenging because inter-electronic repulsion, crystal field, and spin-orbit coupling effects can be of similar magnitude. Moreover, such efforts have been hampered by the lack of structurally analogous families of complexes to study. Here we report an improved method to U≡N triple bonds, and assemble a family of uranium(V) nitrides. Along with an isoelectronic oxo, we quantify the electronic structure of this 5f1 family by magnetometry, optical and electron paramagnetic resonance (EPR) spectroscopies and modelling. Thus, we define the relative importance of the spin-orbit and crystal field interactions, and explain the experimentally observed different ground states. We find optical absorption linewidths give a potential tool to identify spin-orbit coupled states, and show measurement of UV···UV super-exchange coupling in dimers by EPR. We show that observed slow magnetic relaxation occurs via two-phonon processes, with no obvious correlation to the crystal field.

Three-Dimensional Network of Cation-Cation-Bound Neptunyl(V) Squares: Synthesis and in Situ Raman Spectroscopy Studies

Cation-cation interactions (CCIs) are an essential feature of actinyl chemistry, particularly neptunyl(V). To better understand the formation mechanisms of CCIs, the crystallization process of Np(V) CCI compounds has been explored during the evaporation of acidic Np(V) stock solutions using X-ray diffraction and both ex situ and in situ Raman spectroscopy. At least four Np solid products have been isolated from evaporation of the same Np(V) acidic solution. In situ evaporation using a continuous wave laser (532 nm) as a local heat source produced similar solid products to ex situ experiments with matching Raman signatures. The formation of these products is highly dependent on the evaporation conditions. Slower evaporation appears to favor the formation of a new neptunyl(V) compound, (NpO2)Cl(H2O)2 (1), over other solid products. The structure of 1 features a three-dimensional network of NpO2(+) cations, where neighboring Np(V) ions are only connected to each other through CCIs in a square arrangement. The O═Np═O stretching region shows similar Raman bands in both the solids and solution suggesting that CCIs between Np(V) cations exist prior to crystallization. These results provide new insight into the formation mechanism of Np(V) CCI compounds from solutions.

Understanding the magnetism of {Fe2Ln} dimers, step-by-step

A hierarchical, comparative approach to the complex magnetochemistry of {Fe₄M₂}-type coordination clusters (M = Y, Eu, Gd, Dy) allows determination of intramolecular exchange energies.

Influence of the coordination environment on slow magnetic relaxation and photoluminescence behavior in two mononuclear dysprosium(iii) based single molecule magnets

Two luminescent mononuclear Dy(iii) based complexes with SMM behaviour have been reported. The SMM behaviours differ due to the slight variation of local symmetry around the Dy(iii) centers and photoluminescence study showed quantum yields of 0.98 and 1.44%.

A zig-zag uranyl(v)–Mn(ii) single chain magnet with a high relaxation barrier

A 1D zig-zag coordination polymer based on a cation–cation [(U^VO₂)Mn^II] repeated unit shows single chain magnet (SCM) behaviour with a high energy barrier of 122 K.

Actinide-based single-molecule magnets

Actinide single-molecule magnets compose a small yet complex consortium, presenting new design and characterization challenges for magnetochemists and physicists.

A uranium-based UO2(+)-Mn2+ single-chain magnet assembled trough cation-cation interactions

Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [{[UO2(salen)(py)][M(py)4](NO3)}]n, (M=Cd (1) and M=Mn (2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp*2Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm(-1)), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K.

Magnetic properties and chiral states of a trimetallic uranium complex

The magnetic properties of the triangular molecular nanomagnet [UO2L]3 (L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate) have been investigated through electron paramagnetic resonance spectroscopy, high-field magnetization and susceptibility measurements. The experimental findings are well reproduced by a microscopic model including exchange interactions and local crystal fields. These results show that [UO2L]3 is characterized by a non-magnetic ground doublet corresponding to two oppositely twisted chiral arrangements of the uranium moments. The non-axial character of single-ion crystal fields leads to quantum tunneling of the noncollinear magnetization in the presence of a magnetic field applied perpendicularly to the triangle plane.

Oxo-functionalization and reduction of the uranyl ion through lanthanide-element bond homolysis: synthetic, structural, and bonding analysis of a series of singly reduced uranyl-rare earth 5f1-4f(n) complexes

The heterobimetallic complexes [{UO2Ln(py)2(L)}2], combining a singly reduced uranyl cation and a rare-earth trication in a binucleating polypyrrole Schiff-base macrocycle (Pacman) and bridged through a uranyl oxo-group, have been prepared for Ln = Sc, Y, Ce, Sm, Eu, Gd, Dy, Er, Yb, and Lu. These compounds are formed by the single-electron reduction of the Pacman uranyl complex [UO2(py)(H2L)] by the rare-earth complexes Ln(III)(A)3 (A = N(SiMe3)2, OC6H3Bu(t)2-2,6) via homolysis of a Ln-A bond. The complexes are dimeric through mutual uranyl exo-oxo coordination but can be cleaved to form the trimetallic, monouranyl "ate" complexes [(py)3LiOUO(μ-X)Ln(py)(L)] by the addition of lithium halides. X-ray crystallographic structural characterization of many examples reveals very similar features for monomeric and dimeric series, the dimers containing an asymmetric U2O2 diamond core with shorter uranyl U═O distances than in the monomeric complexes. The synthesis by Ln(III)-A homolysis allows [5f(1)-4f(n)]2 and Li[5f(1)-4f(n)] complexes with oxo-bridged metal cations to be made for all possible 4f(n) configurations. Variable-temperature SQUID magnetometry and IR, NIR, and EPR spectroscopies on the complexes are utilized to provide a basis for the better understanding of the electronic structure of f-block complexes and their f-electron exchange interactions. Furthermore, the structures, calculated by restricted-core or all-electron methods, are compared along with the proposed mechanism of formation of the complexes. A strong antiferromagnetic coupling between the metal centers, mediated by the oxo groups, exists in the U(V)Sm(III) monomer, whereas the dimeric U(V)Dy(III) complex was found to show magnetic bistability at 3 K, a property required for the development of single-molecule magnets.