Chemical structure and bonding in a thorium(iii)-aluminum heterobimetallic complex

We describe the syntheses of [Th(iii)]–[Al] and [U(iii)]–[Al] bimetallics that demonstrate An→Al interactions where the actinide behaves as an electron donor.

Thorium sits at a unique position on the periodic table. On one hand, there is little evidence that its 5f orbitals engage in bonding as they do in other early actinides; on the other hand, its chemistry is distinct from Lewis acidic transition metals. To gain insight into the underlying electronic structure of Th and develop trends across the actinide series, it is useful to study Th(iii) and Th(ii) systems with valence electrons that may engage in non-electrostatic metal–ligand interactions, although only a handful of such systems are known. To expand the range of low-valent compounds and gain deeper insight into Th electronic structure, we targeted actinide bimetallic complexes containing metal–metal bonds. Herein, we report the syntheses of Th–Al bimetallics from reactions between a di-tert-butylcyclopentadienyl supported Th(iv) dihalide (Cp^‡₂ThCl₂) and an anionic aluminum hydride salt [K(H₃AlC(SiMe₃)₃) (1)]. Reduction of the [Th(iv)](Cl)–[Al] product resulted in a [Th(iii)]–[Al] complex [Cp^‡₂Th(μ-H₃)AlC(SiMe₃)₃ (4)]. The U(iii) analogue [Cp^‡₂U(μ-H₃)AlC(SiMe₃)₃ (5)] could be synthesized directly from a U(iii) halide starting material. Electron paramagnetic resonance studies on 4 demonstrate hyperfine interactions between the unpaired electron and the Al atom indicative of spin density delocalization from the Th metal center to the Al. Density functional theory and atom in molecules calculations confirmed the presence of An→Al interactions in 4 and 5, which represents the first examples of An→M interactions where the actinide behaves as an electron donor.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

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.

In search of molecules displaying ferromagnetic exchange: multiple-decker Ni12 and Ni16 complexes from the use of pyridine-2-amidoxime

The use of pyridine-2-amidoxime (pyaoxH₂) in Ni chemistry has provided access to a dodecanuclear complex and a hexadecanuclear Ni cluster, namely [Ni₁₂(pyaox)₆(pyaoxH)₆(MeOH)₂Cl₂]Cl₄·5MeOH (1·5MeOH) and [Ni₁₆(pyaox)₈(pyaoxH)₈(MeOH)₄](SO₄)₄·10H₂O·26MeOH (2·10H₂O·26MeOH). Complex 1·5MeOH was isolated by the reaction of NiCl₂·6H₂O, pyaoxH₂ and NaOMe in a 1 : 1 : 2 molar ratio in MeOH in 60% yield. Treatment of NiSO₄·6H₂O with pyaoxH₂ and NEt₃ in a 1 : 1 : 2 molar ratio in MeOH afforded 2·10H₂O·26MeOH in good yield (65%). The two compounds display a multi-decker configuration based on stacked Ni₄ layers, {Ni₄(pyaox)₂(pyaoxH)₂}²⁺_x (x = 3, 1·5MeOH; x = 4, 2·10H₂O·26MeOH); each deck consists of two square planar and two octahedral Ni^II centres. The number of decks observed in 1·5MeOH and 2·10H₂O·26MeOH depends on the nature of the inorganic anion that is present in the reaction system, which provides elements of synthetic control towards new high nuclearity Ni^II species. 2·10H₂O·26MeOH is the first structurally characterized complex of any metal displaying a quadruple-decker configuration, being also the highest nuclearity metal cluster bearing pyaoxH₂ and the highest nuclearity Ni^II cluster with any type of 2-pyridyl oxime. Each cluster cation displays ferromagnetic exchange between the octahedral Ni^II ions resulting in a spin ground state of S = 6 for 1 and S = 8 for 2. Magnetothermal studies have been performed and discussed for both clusters.

Binding of oxime group to uranyl ion

Currently, the leading approach for extraction of uranium from seawater is selective sorption of UO2(2+) ions onto a poly(acrylamidoxime) fiber. Amidoxime functional groups are the most studied extractant moieties for this application, but are not perfectly selective, and understanding how these groups (and more generally the oxime groups) interact with UO2(2+) and competing ions in seawater is an important step in designing better extractants. We have started a new research programme aiming at in-depth studies of the uranyl-oxime/amidoxime interactions and we report here our first results which cover aspects of the coordination chemistry of 2-pyridyl ketoximes towards UO2(2+). Detailed synthetic investigations of various UO2(2+)/mepaoH and UO2(2+)/phpaoH reaction systems (mepaoH is methyl 2-pyridyl ketoxime and phpaoH is phenyl 2-pyridyl ketoxime) have provided access to the complexes [UO2(mepao)2(MeOH)2]{[UO2(NO3)(mepao)(MeOH)2]}2 (), [UO2(mepao)2(MeOH)2] (), [(UO2)2(O2)(O2CMe)2(mepaoH)2] () and [UO2(phpao)2(MeOH)2] (). The peroxido group in , which was isolated without the addition of external peroxide sources, probably arises from a bis(aquo)- and/or bis(hydroxido)-bridged diuranyl precursor in solution followed by photochemical oxidation of the bridging groups. The U(VI) atom in the [UO2(NO3)(mepao)(MeOH)2] molecules of () is surrounded by one nitrogen and seven oxygen atoms in a very distorted hexagonal bipyramidal geometry; two oxygen atoms from the terminal MeOH ligands, two oxygen atoms from the bidentate chelating nitrato group, and the oxygen and nitrogen atoms from the η(2) oximate group of the 1.110 (Harris notation) mepao(-) ligand define the equatorial plane. This plane consists of two terminal MeOH ligands and two η(2) oximate groups in the [UO2(mepao)2(MeOH)2] molecule () of . The structure of the [UO2(mepao)2(MeOH)2] molecule that is present in is very similar to the structure of the corresponding molecule in . The structure of the dinuclear molecule that is present in consists of two {UO2(O2CMe)(mepaoH)}(+) units bridged by a η(2):η(2):μ O2(2-) group. The equatorial plane of each uranyl site is composed of the pyridyl and oxime nitrogen atoms of a 1.011 mepaoH ligand, the oxygen atoms of an almost symmetrically coordinated bidentate chelating MeCO2(-) group and the two oxygen atoms of the peroxido groups. The core molecular structure of is similar to that of , the only difference being the presence of 1.110 phpao(-) ligands in the former instead of mepao(-) groups in the latter. The free pyridyl nitrogen atoms of mepao(-) and phpao(-) ligands of , and are acceptors of intramolecular H bonds from the ligated MeOH oxygen atoms. H-bonding and π-π stacking interactions build interesting supramolecular networks in the crystal structures of the four complexes. Compounds are the first structurally characterized uranyl complexes with 2-pyridyl aldoximes or ketoximes as ligands. IR data are discussed in terms of the coordination modes of the ligands in the complexes. (1)H NMR data in DMSO-d6 suggest that the complexes decompose in solution. The ESI(-) MS spectrum of dissolved in the NH4(O2CMe) buffer is indicative of the presence of [UO2(O2CMe)3](-), [UO2(O2CMe)2(phpao)](-), [UO2(O2CMe)(phpao)2](-) and [UO2(phpao)3](-) species. A common structural motif of the complexes containing the anionic mepao(-) (, ) and phpao(-) () ligands is that the deprotonated oximate group prefers to bind in the η(2) fashion forming a 3-membered chelating ring in spite of the presence of a pyridyl nitrogen atom, whose coordination would be expected to lead to 5- or 6-membered chelating rings.

Some new trends in the design of single molecule magnets

In this review we briefly discuss some new trends in the design of single molecule magnets based on transition (3d, 4d, 5d) and rare-earth (4f) metal ions. Within this broad theme the emphasis of the present review is placed on the molecules which exhibit strong magnetic anisotropy originating from the unquenched orbital angular momenta in the ground orbitally degenerate (or quasi-degenerate) states. Along with the general concepts we consider selected examples of the systems comprising orbitally-degenerate metal ions and demonstrate how one can benefit from strong single-ion anisotropy arising from the first-order orbital angular momentum. The role of crystal fields, spin-orbit coupling and structural factors is discussed. Some observation stemming from the analysis of the isotropic exchange interactions, magnetic anisotropy and strongly anisotropic orbitally-dependent superexchange are summarized as guiding rules for the controlled design of single molecule magnets exhibiting high barriers for magnetization reversal and, consequently, high blocking temperatures.

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.

Proof of Principle: Immobilisation of Robust CuII3 TbIII -Macrocycles on Small, Suitably Pre-functionalised Gold Nanoparticles

In a proof-of-principle study, a soluble macrocyclic single-molecule magnet (SMM) containing a Cu^II₃ Tb^III magnetic core was covalently grafted onto small gold nanoparticles pre-functionalised with carboxylate-terminated tethers. A modified microemulsion method allowed production of the small and monodisperse nanoparticles (approximately 3.5 nm in diameter) for the chemisorption of a large amount of intact macrocyclic complexes in the hybrid system.

Embedding 1D or 2D cobalt–carboxylate substrates in 3D coordination polymers exhibiting slow magnetic relaxation behaviors: crystal structures, high-field EPR, and magnetic studies

A novel 3D coordination polymer containing 2D cobalt–carboxylate layer had been constructed, exhibiting slow magnetic relaxation.

NMR analysis of an Fe(i)–carbene complex with strong magnetic anisotropy

A paramagnetic, easy-plane anisotropic Fe^I complex, bearing cyclic-alkyl(amino) carbene (cAAC) ligands, is studied by means of NMR and DFT.

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.

Benzoquinonoid-bridged dinuclear actinide complexes

We report the coordination chemistry of benzoquinonoid-bridged dinluclear thorium(iv) and uranium(iv) complexes with the tripodal ligand tris[2-amido(2-pyridyl)ethyl]amine ligand,L.

Temperature-induced reversible single-crystal-to-single-crystal isomerisation of uranyl polyrotaxanes: an exquisite case of coordination variability of the uranyl center

A reversible solid-state isomerisation mediated by the change of uranyl-ligand coordination modes has been achieved in uranyl polyrotaxanes by a temperature-induced strategy.

Modeling the electronic states and magnetic properties derived from the f1 configuration in lanthanocene and actinocene compounds

The electronic structure and magnetic properties of a series of Kramers ions with f¹ configuration in axial symmetry have been analyzed with a combination of theoretical methods: ab initio relativistic wavefunction methods as well as a crystal-field (CF) model with parameters extracted from the ab initio calculations.

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.

Polymorphism in a Cobalt-Based Single-Ion Magnet Tuning Its Barrier to Magnetization Relaxation

A large barrier to magnetization reversal, a signature of a good single-molecule magnet (SMM), strongly depends on the structural environment of a paramagnetic metal ion. In a crystalline state, where SMM properties are usually measured, this environment is influenced by crystal packing, which may be different for the same chemical compound, as in polymorphs. Here we show that polymorphism can dramatically change the magnetic behavior of an SMM even with a very rigid coordination geometry. For a cobalt(II) clathrochelate, it results in an increase of the effective barrier from 109 to 180 cm^-1, the latter value being the largest one reported to date for cobalt-based SMMs. Our finding thus highlights the importance of identifying possible polymorphic phases in search of new, even more efficient SMMs.

Strategies toward High-Temperature Lanthanide-Based Single-Molecule Magnets

Lanthanide-based single-molecule magnets are leading materials for achieving magnetization blocking at the level of one molecule. In this paper, we examine the physical requirements for efficient magnetization blocking in single-ion complexes and identify the design principles for achieving very high magnetization blocking barriers in lanthanide-based compounds. The key condition is the preponderant covalent binding of the Ln ion to one of the ligand atoms, tremendously enhancing the axial crystal field. We also make an overview of practical schemes for the implementation of this principle. These are (1) the effective lowering of the coordination number via displacement of the Ln ion to one of the atoms in the coordination polyhedron, (2) the design of two-coordinated complexes, and (3) the stabilization of diatomic compounds in cages and on surfaces. The last proposal is appealing in connection to spintronics applications, especially via the exploration of robust and highly anisotropic [LnX] units displaying multilevel blocking barriers of thousands of Kelvin and prospects for room-temperature magnetization blocking.

Main Group Metal-Actinide Magnetic Coupling and Structural Response Upon U(4+) Inclusion Into Bi, Tl/Bi, or Pb/Bi Cages

The encapsulation of actinide ions in intermetalloid clusters has long been proposed but was never realized synthetically. We report the isolation and experimental, as well as quantum chemical, characterization of the uranium-centered clusters [U@Bi12](3-), [U@Tl2Bi11](3-), [U@Pb7Bi7](3-), and [U@Pb4Bi9](3-), upon reaction of (EE'Bi2)(2-) (E = Ga, Tl, E' = Bi; E = E' = Pb) and [U(C5Me4H)3] or [U(C5Me4H)3Cl] in 1,2-diaminoethane. For [U@Bi12](3-), magnetic susceptibility measurements rationalize an unprecedented antiferromagnetic coupling between a magnetic U(4+) site and a unique radical Bi12(7-) shell.

Organometallic neptunium(III) complexes

Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chemistry of uranium, which has focused strongly on U(III) and has seen some spectacular advances, that of the transuranics is significantly technically more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(IV). Here we report the synthesis of three new Np(III) organometallic compounds and the characterization of their molecular and electronic structures. These studies suggest that Np(III) complexes could act as single-molecule magnets, and that the lower oxidation state of Np(II) is chemically accessible. In comparison with lanthanide analogues, significant d- and f-electron contributions to key Np(III) orbitals are observed, which shows that fundamental neptunium organometallic chemistry can provide new insights into the behaviour of f-elements.

Electronic Structure and Magnetic Properties of Dioxo-Bridged Diuranium Complexes with Diamond-Core Structural Motifs: A Relativistic DFT Study

Electronic structures and magnetic properties of the binuclear bis(μ-oxo) U(IV)/U(IV) K2[{(((nP,Me)ArO)3tacn)U(IV)}2(μ-O)2] and U(V)/U(V) [{(((nP,Me)ArO)3tacn)U(V)}2(μ-O)2] (tacn = triazacyclononane, nP = neopentyl) complexes, exhibiting [U(μ-O)2U] diamond-core structural motifs, have been investigated computationally using scalar relativistic Density Functional Theory (DFT) combined with the Broken Symmetry (BS) approach for their magnetic properties. Using the B3LYP hybrid functional, the BS ground state of the pentavalent [U(V)(μ-O)2U(V)] 5f(1)-5f(1) complex has been found of lower energy than the high spin (HS) triplet state, thus confirming the antiferromagnetic character in agreement with experimental magnetic susceptibility measurements. The nonmagnetic character observed for the tetravalent K2[U(IV)(μ-O)2U(IV)] 5f(2)-5f(2) species is also predicted by our DFT calculations, which led practically to the same energy for the HS and BS states. As reported for related dioxo diuranium(V) systems, superexchange is likely to be responsible for the antiferromagnetic coupling through the π-network orbital pathway within the (μ-O)2 bridge, the dissymmetrical structure of the U2O2 core playing a determining role. In the case of the U(IV) species, our computations indicate that the K(+) counterions are likely to play a role for the observed magnetic property. Finally, the MO analysis, in conjunction with NPA and QTAIM analyses, clarify the electronic structures of the studied complexes. In particular, the fact that the experimentally attempted chemical oxidation of the U(V) species does not lead straightforwardly to binuclear complexes U(VI) is clarified by the MO analysis.

A Mononuclear Uranium(IV) Single-Molecule Magnet with an Azobenzene Radical Ligand

A tetravalent uranium compound with a radical azobenzene ligand, namely, [{(SiMe2 NPh)3 -tacn}U(IV) (η(2) -N2 Ph2 (.) )] (2), was obtained by one-electron reduction of azobenzene by the trivalent uranium compound [U(III) {(SiMe2 NPh)3 -tacn}] (1). Compound 2 was characterized by single-crystal X-ray diffraction and (1) H NMR, IR, and UV/Vis/NIR spectroscopy. The magnetic properties of 2 and precursor 1 were studied by static magnetization and ac susceptibility measurements, which for the former revealed single-molecule magnet behaviour for the first time in a mononuclear U(IV) compound, whereas trivalent uranium compound 1 does not exhibit slow relaxation of the magnetization at low temperatures. A first approximation to the magnetic behaviour of these compounds was attempted by combining an effective electrostatic model with a phenomenological approach using the full single-ion Hamiltonian.

Direct Observation of Very Large Zero-Field Splitting in a Tetrahedral Ni(II)Se4 Coordination Complex

The high-spin (S = 1) tetrahedral Ni(II) complex [Ni{(i)Pr2P(Se)NP(Se)(i)Pr2}2] was investigated by magnetometry, spectroscopic, and quantum chemical methods. Angle-resolved magnetometry studies revealed the orientation of the magnetization principal axes. The very large zero-field splitting (zfs), D = 45.40(2) cm(-1), E = 1.91(2) cm(-1), of the complex was accurately determined by far-infrared magnetic spectroscopy, directly observing transitions between the spin sublevels of the triplet ground state. These are the largest zfs values ever determined--directly--for a high-spin Ni(II) complex. Ab initio calculations further probed the electronic structure of the system, elucidating the factors controlling the sign and magnitude of D. The latter is dominated by spin-orbit coupling contributions of the Ni ions, whereas the corresponding effects of the Se atoms are remarkably smaller.

The Renaissance of Non-Aqueous Uranium Chemistry

Prior to the year 2000, non-aqueous uranium chemistry mainly involved metallocene and classical alkyl, amide, or alkoxide compounds as well as established carbene, imido, and oxo derivatives. Since then, there has been a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-molecule activation, and magnetism have been reported. This Review 1) introduces the reader to some of the specialist theories of the area, 2) covers all-important starting materials, 3) surveys contemporary ligand classes installed at uranium, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compounds, 4) describes advances in the area of single-molecule magnetism, and 5) summarizes the coordination and activation of small molecules, including carbon monoxide, carbon dioxide, nitric oxide, dinitrogen, white phosphorus, and alkanes.

Improving f-element single molecule magnets

Historical developments, trends, pitfalls and strategies in improving f-element single molecule magnets are described.

A family of fourteen soluble stable macrocyclic [NiII3LnIII] heterometallic 3d–4f complexes

Fourteen macrocyclic NiII3Ln^III (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) complexes were made, and checked for Single-Molecule Magnet behaviour; two acyclic analogues were structurally characterised.

Observation of unusual slow-relaxation of the magnetisation in a Gd-EDTA chelate

A Gadolinium EDTA chelate displays characteristic isotropic behaviour common of Gd^III complexes under zero applied magnetic field, and anisotropic behaviour arising from dipolar coupling and weak spin–phonon coupling under an applied magnetic field.

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.