A trinuclear metallasilsesquioxane of uranium(III)

The silsesquioxane ligand (iBu)7Si7O9(OH)3 (iBuPOSSH3) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U3(iBuPOSS)3]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

Self-assembly of fish-bone and grid-like CoII-based single-molecule magnets using dihydrazone ligands with NNN and NNO pockets

Three Co^II complexes, [Co₂(H₂L¹)₂](ClO₄)₄·4CH₃OH (1), [Co₂(H₄L²)₂](ClO₄)₄ (2) and [Co₄(H₄L²)₄](ClO₄)₈ (3), were constructed by the self-assembly of the symmetrical dihydrazone ligands H2L1 and H4L2 with Co^II ions under different synthetic conditions. The fish-bone-like complex 1 was obtained using the ligand H2L1 in methanol via the solvothermal method, while the self-assembly of H4L2 with Co^II ions is solvent-dependent, producing the fish-bone-like complex 2 and [2 × 2] grid-like complex 3. Magnetic susceptibility measurements and theoretical calculations reveal that the large negative D values for the three complexes stem from their easy-axis magnetic anisotropy. Ac magnetic susceptibility measurements disclosed field-induced slow magnetic relaxation behaviors and the presence of Raman and/or direct processes of the three complexes at various applied dc fields.

Effects of Substituents on the Molecular Structure and Redox Behavior of Uranyl(V/VI) Complexes with N3O2-Donating Schiff Base Ligands

Uranyl(VI) complexes with pentadentate N₃O₂-donating Schiff base ligands having various substituents at the ortho (R₁) and/or para (R₂) positions on phenolate moieties, R₁,R₂-^Mesaldien^2-, were synthesized and thoroughly characterized by ¹H nuclear magnetic resonance, infrared, elemental analysis, and single-crystal X-ray diffraction. Molecular structures of UO₂(R₁,R₂-^Mesaldien) are more or less affected by the electron-donating or -withdrawing nature of the substituents. The redox behavior of all UO₂(R₁,R₂-^Mesaldien) complexes was investigated to understand how substituents introduced onto the ligand affect the redox behavior of these uranyl(VI) complexes. As a result, the redox potentials of UO₂(R₁,R₂-^Mesaldien) in dimethyl sulfoxide increased from -1.590 to -1.213 V with an increase in the electron-withdrawing nature of the substituents at the R₁ and R₂ positions. The spectroelectrochemical measurements and theoretical calculation [density functional theory (DFT) and time-dependent DFT calculations] revealed that the center U⁶⁺ of each UO₂(R₁,R₂-^Mesaldien) complex undergoes one-electron reduction to afford the corresponding uranyl(V) complex, [UO₂(R₁,R₂-^Mesaldien)]^-, regardless of the difference in the substituents. Consequently, the redox active center of uranyl(VI) complexes seems not to be governed by the redox potentials but to be determined by whether the LUMO is centered on a U 5f orbital or on one π* orbital of a surrounding ligand.

Pentagonal Bipyramidal Ln(III) Complexes Containing an Axial Phosphine Oxide Ligand: Field-induced Single-ion Magnetism Behavior of the Dy(III) Analogues

A series of neutral homologous complexes [(L)Ln(Cy₃PO)Cl] {where Ln = Gd (1), Tb (2), Dy (3), and Er (5)} and [(L)Dy(Ph₃PO)Cl] (4) [H₂L = 2,6-diacetylpyridine bis-benzoylhydrazone] were isolated. In these complexes, the central lanthanide ion possesses a pentagonal bipyramidal geometry with an overall pseudo D_5h symmetry. The coordination environment around the lanthanide ion comprises of three nitrogen and two oxygen donors in an equatorial plane. The axial positions are taken up by a phosphine oxide (O donor) and a chloride ion. Among these compounds, the Dy(III) (3 and 4) analogues were found to be field-induced single-ion magnets.

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe^II₂ complexes [(Me₃TPyA)₂Fe₂(^RL)]ⁿ⁺ (Me₃TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: ^OMeLH₂ = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ^ClLH₂ = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na₂[^NO₂L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: ^SMe₂L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe^II centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO₂, SMe₂) cm^-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. The minimal effects of substituents in the 3- and 6-positions of ^RL^x-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U_eff = 50(1), 41(1), 38(1), and 33(1) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

An aromatic selenite bridged Mn(iii) chain compound showing the coexistence of single chain magnet and metamagnet behaviour

A new chain compound consisting of Mn₂(salen)₂ building blocks bridged by aromatic selenite was synthesized and characterized.

Theoretical Investigation of the Electronic Structure and Magnetic Properties of Oxo-Bridged Uranyl(V) Dinuclear and Trinuclear Complexes

The uranyl(V) complexes [UO₂(dbm)₂K(18C6)]₂ (dbm = dibenzoylmethanate) and [UO₂(L)]₃(L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate), exhibiting diamond-shaped U₂O₂ and triangular-shaped U₃O₃ cores respectively with 5f¹-5f¹ and 5f¹-5f¹-5f¹ configurations, have been investigated using relativistic density functional theory (DFT). The bond order and QTAIM analyses reveal that the covalent contribution to the bonding within the oxo cores is slightly more important for U₃O₃ than for U₂O₂, in line with the shorter U-O distances existing in the trinuclear complex in comparison to those in the binuclear complex. Using the broken symmetry (BS) approach combined with the B3LYP functional for the calculation of the magnetic exchange coupling constants (J) between the magnetic centers, the antiferromagnetic (AF) character of these complexes was confirmed, the estimated J values being respectively equal to -24.1 and -7.2 cm^-1 for the dioxo and trioxo species. It was found that the magnetic exchange is more sensitive to small variations of the core geometry of the dioxo species in comparison to the trioxo species. Although the robust AF exchange coupling within the U_xO_x cores is generally maintained when small variations of the UOU angle are applied, a weak ferromagnetic character appears in the dioxo species when this angle is higher than 114°, its value for the actual structure being equal to 105.9°. The electronic factors driving the magnetic coupling are discussed.

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].

Magnetic dimensionality and the crystal structure of two copper(ii) coordination polymers containing Cu6 and Cu2 building units

The preparation, structural characterization, EPR spectroscopy and magnetic structural correlation of two Cu(ii) coordination polymers are reported.

The effect of iron binding on uranyl(v) stability

The tripodal heptadentate Schiff base trensal^3– ligand allowed the synthesis and characterization of stable uranyl(v) complexes presenting UO₂⁺···K⁺ or UO₂⁺···Fe²⁺ cation–cation interactions. The presence of Fe²⁺ bound to the uranyl(v) oxygen leads to increased stability with respect to proton induced disproportionation and to an increased range of stability of the uranyl(v) species with respect both to oxidation and reduction reactions.

Here we report the effect of UO₂⁺···Fe²⁺ cation–cation interactions on the redox properties of uranyl(v) complexes and on their stability with respect to proton induced disproportionation. The tripodal heptadentate Schiff base trensal^3– ligand allowed the synthesis and characterization of the uranyl(vi) complexes [UO₂(trensal)K], 1 and [UO₂(Htrensal)], 2 and of uranyl(v) complexes presenting UO₂⁺···K⁺ or UO₂⁺···Fe²⁺ cation–cation interactions ([UO₂(trensal)K]K, 3, [UO₂(trensal)] [K(2.2.2crypt)][K(2.2.2crypt)], 4, [UO₂(trensal)Fe(py)₃], 6). The uranyl(v) complexes show similar stability in pyridine solution, but the presence of Fe²⁺ bound to the uranyl(v) oxygen leads to increased stability with respect to proton induced disproportionation through the formation of a stable Fe²⁺–UO₂⁺–U⁴⁺ intermediate ([UO₂(trensal)Fe(py)₃U(trensal)]I, 7) upon addition of 2 eq. of PyHCl to 6. The addition of 2 eq. of PyHCl to 3 results in the immediate formation of U(iv) and UO₂²⁺ compounds. The presence of an additional UO₂⁺ bound Fe²⁺ in [(UO₂(trensal)Fe(py)₃)₂Fe(py)₃]I₂, 8, does not lead to increased stability. Redox reactivity and cyclic voltammetry studies also show an increased range of stability of the uranyl(v) species in the presence of Fe²⁺ with respect both to oxidation and reduction reactions, while the presence of a proton in complex 2 results in a smaller stability range for the uranyl(v) species. Cyclic voltammetry studies also show that the presence of a Fe²⁺ cation bound through one trensal^3– arm in the trinuclear complex [{UO₂(trensal)}₂Fe], 5 does not lead to increased redox stability of the uranyl(v) showing the important role of UO₂⁺···Fe²⁺ cation–cation interactions in increasing the stability of uranyl(v). These results provide an important insight into the role that iron binding may play in stabilizing uranyl(v) compounds in the environmental mineral-mediated reduction of uranium(vi).

Synthesis and Characterization of a Water Stable Uranyl(V) Complex

We have identified a polydentate aminocarboxylate ligand that stabilizes uranyl(V) in water. The mononuclear [UO₂(dpaea)]X, (dpaeaH₂ = Bis(pyridyl-6-methyl-2-carboxylate)-ethylamine; X = CoCp₂^*+ or X = K(2.2.2.cryptand) complexes have been isolated from anaerobic organic solution, crystallographically and spectroscopically characterized both in water and organic solution. These complexes disproportionate at pH ≤ 6, but are stable in anaerobic water at pH 7-10 for several days.

Mixed metal CoII1−xZnIIx–organic frameworks based on chains with mixed carboxylate and azide bridges: magnetic coupling and slow relaxation

A series of isomorphous three-dimensional metal–organic frameworks [Co^II_1−xZn^II_x(L)(N₃)]·H₂O (x = 0.26, 0.56 and 0.85) based on bimetallic Co^II_1−xZn^II_x (x = 0.26, 0.56 and 0.85) chains with random metal sites have been synthesized and magnetically characterized. The Co^II_1−xZn^II_x series, which intrinsically feature random anisotropic/diamagnetic sites, shows complex magnetic interactions. By gradually introducing the diamagnetic Zn^II ions into the pure anisotropic Co^II single-chain magnets system, the ferromagnetic interactions between Co^II ions are gradually diluted. Moreover, the slow magnetic relaxation behaviour of the mixed metal Co^II_1−xZn^II_x systems also changes. In this bimetallic series Co^II_1−xZn^II_x, the Co-rich materials exhibit slow relaxation processes that may arise from SCM mechanism, while the Zn^II-rich materials show significantly low slow magnetic relaxation. A general trend is that the activation energy and the blocking temperature decrease with the increase in diamagnetic Zn^II content, emphasizing the importance of anisotropy for slow relaxation of magnetization.

A series of mixed-metal systems consisting of Co^II_1−xZn^II_x chains with (μ-COO)₂(μ-EO-N₃) bridges were prepared, exhibiting interesting magnetic coupling and slow magnetic relaxation.

Random mixed-metal Co1−xNix single-chain magnets with simultaneous azide, carboxylate and tetrazolate bridges

A series of MOFs based on random ferromagnetic Co_1−xNi_x chains with simultaneous azide, carboxylate and tetrazolate bridges show metamagnetism and slow magnetic relaxation with complicated composition dependence and synergistic effects on single-chain slow dynamics.

A [Cr2Ni] coordination polymer: slow relaxation of magnetization in quasi-one-dimensional ferromagnetic chains

The first example of a Ni^II–Cr^III chain to exhibit slow relaxation of magnetization is reported.

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.

Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species

A survey is given of recent original structural results on heterometallic species incorporating uranyl ions, particularly with carboxylate ligands.

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.