Synthesis of bimetallic uranium and neptunium complexes of a binucleating macrocycle and determination of the solid-state structure by magnetic analysis

A Phosphorus-Based Pacman Dication Generated by Cooperative Self-Activation of a Pacman Phosphane

Formal coordination of phosphorus(III) by a calix[4]pyrrole Schiff base ligand was achieved through the reaction of this ligand with PCl₃ under basic conditions. The reaction product adopts a Pacman conformation with two P-Cl moieties, one in exo and one in endo position. It represents the first non-metal compound of calix[4]pyrrole Schiff base ligands and of Pacman ligands in general. The spatial neighborhood of the two phosphorus atoms enables cooperative reactions. As a first example, the chloride abstraction with AgOTf is presented, yielding a macrocyclic dication with two embedded phosphorus(III) monocations, which both undergo a cooperative, internal activation reaction with an adjacent C=N double bond. This intramolecular redox process affords two pentacoordinated phosphorus(V) centers within the Pacman dication. All reaction products were fully characterized and all results are supported by computations.

New zinc complexes derived from "self-adaptable" acyclic diiminodipyrromethanes as potent catalysts for the reduction of curing temperature of bisphenol-A/F benzoxazines

The simple modification of the Schiff-base ligands often brings significant changes in the coordination properties of the metal-complexes, providing newer prospects for their unexplored applications. In this context, the present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of benzoxazines. The two zinc complexes of composition [Zn{(Ph)(CH₃)C(2,6-ⁱPr₂C₆H₃–N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH–C₄H₂N)(2,6-ⁱPr₂C₆H₃–NCH–C₄H₂NH)}₂] (3) and [ZnCl₂{(Ph)(CH₃)C(Ph₃C–NHCH–C₄H₂N)₂}] (4) were synthesized in good yields, and the structures were confirmed by single crystal X-ray diffraction (XRD). Later, zinc complexes (3 & 4) were used as catalysts to reduce the curing (ring-opening polymerization) temperature of benzoxazine monomers such as Bisphenol-A (BA-a) and Bisphenol-F (BF-a) benzoxazines. Dynamic scanning calorimetry (DSC) studies revealed that the on-set curing (T_p) temperatures were reasonably decreased upto 20% for the benzoxazines. Furthermore, the thermal stabilities of the polybenzoxazines (PBzs) derived in the presence of zinc catalysts (3 and 4) were compared with PBz obtained in the absence of catalyst under similar conditions. The thermal studies reveled that there is no significant changes in the initial degradation of polymers. However, the thermal stability in terms of char yields at 800 °C improved upto 10–21% for the bisphenol-A/F benzoxazines.

The present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of BA-a and BF-a benzoxazines.

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).

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.

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.

Theoretical investigation of low-valent uranium and transuranium complexes of a flexible small-cavity macrocycle: structural, formation reaction and redox properties

Size matching of a flexible macrocycle with low-valent actinide(III/IV) ions as well as their bonding determines different coordination modes.

Metal complexes containing natural and and artificial radioactive elements and their applications

Recent advances (during the 2007–2014 period) in the coordination and organometallic chemistry of compounds containing natural and artificially prepared radionuclides (actinides and technetium), are reviewed. Radioactive isotopes of naturally stable elements are not included for discussion in this work. Actinide and technetium complexes with O-, N-, N,O, N,S-, P-containing ligands, as well π-organometallics are discussed from the view point of their synthesis, properties, and main applications. On the basis of their properties, several mono-, bi-, tri-, tetra- or polydentate ligands have been designed for specific recognition of some particular radionuclides, and can be used in the processes of nuclear waste remediation, i.e., recycling of nuclear fuel and the separation of actinides and fission products from waste solutions or for analytical determination of actinides in solutions; actinide metal complexes are also usefulas catalysts forcoupling gaseous carbon monoxide, as well as antimicrobial and anti-fungi agents due to their biological activity. Radioactive labeling based on the short-lived metastable nuclide technetium-99m (^99mTc) for biomedical use as heart, lung, kidney, bone, brain, liver or cancer imaging agents is also discussed. Finally, the promising applications of technetium labeling of nanomaterials, with potential applications as drug transport and delivery vehicles, radiotherapeutic agents or radiotracers for monitoring metabolic pathways, are also described.

Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes

Actinide complexes of a small-cavity, dipyrrolide macrocycle exhibit unusual bent metallocene-type binding, or bis(arene)-type binding, or both at once in a di-uranium adduct.

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.

A delocalized arene-bridged diuranium single-molecule magnet

Single-molecule magnets (SMMs) are compounds that, below a blocking temperature, exhibit stable magnetization purely of molecular origin, and not caused by long-range ordering of magnetic moments in the bulk. They thus show promise for applications such as data storage of ultra-high density. The stability of the magnetization increases with increasing ground-state spin and magnetic anisotropy. Transition-metal SMMs typically possess high-spin ground states, but insufficient magnetic anisotropies. Lanthanide SMMs exhibit large magnetic anisotropies, but building high-spin ground states is difficult because they tend to form ionic bonds that limit magnetic exchange coupling. In contrast, the significant covalent bonding and large spin-orbit contributions associated with uranium are particularly attractive for the development of improved SMMs. Here we report a delocalized arene-bridged diuranium SMM. This study demonstrates that arene-bridged polyuranium clusters can exhibit SMM behaviour without relying on the superexchange coupling of spins. This approach may lead to increased blocking temperatures.