Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering.

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

Tunable valence tautomerism in lanthanide-organic alloys.

The inimitable electronic structures of the lanthanide (Ln) ions are key to advanced materials and technologies involving these elements. The trivalent ions are ubiquitous and are used much more widely than the divalent and tetravalent analogues, which possess vastly different optical and magnetic properties. Hence, alteration of the valence electron count by external stimuli can lead to dramatic changes in materials properties. Compounds exhibiting a temperature-induced complete Ln(III) ⇄ Ln(II) switch, referred to as a valence tautomeric (VT) transition, are rare. Here we present an abrupt and hysteretic VT transition in a lanthanide-based coordination polymer, SmI2(pyrazine)3, driven by the interconversion of Sm(II)-pyrazine(0) and Sm(III)-pyrazine(·-) redox pairs. Alloying SmI2(pyrazine)3 with Yb(II) yields isomorphous Sm1-xYbxI2(pyrazine)3 solid solutions with VT transition critical temperatures ranging widely from 200 K to ∼50 K at ambient pressure. These findings demonstrate a simple strategy to realize thermally switchable magnetic materials with chemically tunable transition temperatures.

Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering.

Magnetic exchange coupling can modulate the slow magnetic relaxation in single-molecule magnets. Despite this, elucidation of exchange coupling remains a significant challenge for the lanthanoid(iii) ions, both experimentally and computationally. In this work, the crystal field splitting and 4f-π exchange coupling in the erbium-semiquinonate complex [ErTp2dbsq] (Er-dbsq; Tp- = hydro-tris(1-pyrazolyl)borate, dbsqH2 = 3,5-di-tert-butyl-1,2-semiquinone) have been determined by inelastic neutron scattering (INS), magnetometry, and CASSCF-SO ab initio calculations. A related complex with a diamagnetic ligand, [ErTp2trop] (Er-trop; tropH = tropolone), has been used as a model for the crystal field splitting in the absence of coupling. Magnetic and INS data indicate antiferromagnetic exchange for Er-dbsq with a coupling constant of Jex = -0.23 meV (-1.8 cm-1) (-2Jex formalism) and good agreement is found between theory and experiment, with the low energy magnetic and spectroscopic properties well modelled. Most notable is the ability of the ab initio modelling to reproduce the signature of interference between localised 4f states and delocalised π-radical states that is evident in the Q-dependence of the exchange excitation. This work highlights the power of combining INS with EPR and magnetometry for determination of ground state properties, as well as the enhanced capability of CASSCF-SO ab initio calculations and purposely developed ab initio-based theoretical models. We deliver an unprecedentedly detailed representation of the entangled character of 4f-π exchange states, which is obtained via an accurate image of the spin-orbital transition density between the 4f-π exchange coupled wavefunctions.

Direct observation of magnetoelastic coupling in a molecular spin qubit: new insights from crystal field neutron scattering data.

Single-molecule magnets are promising candidates for data storage and quantum computing applications. A major barrier to their use is rapid magnetic relaxation and quantum decoherence due to thermal vibrations. Here we report a reanalysis of inelastic neutron scattering (INS) data of the candidate qubit Na9[Ho(W5O18)2]·35D2O, wherein we demonstrate for the first time that magnetic relaxation times and mechanisms can be directly observed as crystal field (CF) peak broadening in INS spectra of a lanthanoid molecular system. The magnetoelastic coupling between the lower energy CF states and phonons (lattice vibrations) is determined by the simultaneous measurement of CF excitations and the phonon density of states, encoded within the same INS experiment. This directly results in the determination of relaxation coupling pathways that occur in this molecule. Such information is invaluable for the further advancement of SMMs and to date has only been obtained from techniques performed in external magnetic fields. Additionally, we determine a relaxation rate of quantum-tunnelling of magnetisation that is consistent with previously measured EPR spectroscopy data.

Towards frustration in Eu(II) Archimedean tessellations.

Self-assembly of trans-{EuI2} nodes and ditopic ligands leads to isoreticular 2D frameworks featuring a rare, non-kagome Archimedean tessellation. The topology and intra-layer Eu(II)-Eu(II) antiferromagnetic interactions provide the prerequisites for geometrical spin frustration, which, due to the spin state degeneracy, is key for novel phenomena such as enhanced magnetic refrigeration.

Modulation of Slow Magnetic Relaxation in Gd(III)-Tetrahalosemiquinonate Complexes.

Incorporating lanthanoid(III)-radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single-molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln-radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)-radical compounds of formula [Gd(18-c-6)X4 SQ(NO3 )].I3 (18-c-6=18-crown-6, X4 SQ⋅- =tetrahalo-1,2-semiquinonate, 1: X=Cl, 2: X=Br) have been synthesized, and the presence of the dioxolene ligand in its semiquinonate form confirmed by X-ray crystallography, UV-Visible-NIR spectroscopy and voltammetry. Static magnetometry and EPR spectroscopy indicate differences in the low temperature magnetic properties of the two compounds, with antiferromagnetic exchange coupling of JGd-SQ ∼-2.0 cm-1 (Hex =-2JGd-SQ (SGd SSQ )) determined by data fitting. Interestingly, compound 1 exhibits slow magnetic relaxation in applied magnetic fields while 2 relaxes much faster, pointing to the major role of packing effects in modulating slow relaxation of the magnetization.

Polytypism in mcalpineite: a study of natural and synthetic Cu3TeO6.

Synthetic and naturally occurring forms of tricopper orthotellurate, CuII3TeVIO6 (the mineral mcalpineite) have been investigated by 3D electron diffraction (3D ED), X-ray powder diffraction (XRPD), Raman and infrared (IR) spectroscopic measurements. As a result of the diffraction analyses, CuII3TeVIO6 is shown to occur in two polytypes. The higher-symmetric CuII3TeVIO6-1C polytype is cubic, space group Ia3, with a = 9.537 (1) Šand V = 867.4 (3) Å3 as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of CuIIO6 octahedra and both CuIIO6 and TeVIO6 octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic CuII3TeVIO6-2O polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2O polytype crystallizes in space group Pcca, with a = 9.745 (3) Å, b = 9.749 (2) Å, c = 9.771 (2) Šand V = 928.3 (4) Å3. High-precision XRPD data were also collected on CuII3TeVIO6-2O to verify the lower-symmetric structure by performing a Rietveld refinement. The resultant structure is identical to that determined by 3D ED, with unit-cell parameters a = 9.56157 (19) Å, b = 9.55853 (11) Å, c = 9.62891 (15) Šand V = 880.03 (2) Å3. The lower symmetry of the 2O polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second CuIIO6 and TeVIO6 octahedral layer by (1/4, 1/4, 0), leading to an offset of TeVIO6 and CuIIO6 octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of CuII3TeVIO6 showed that low-temperature (473 K) hydrothermal conditions generally produce the 2O polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2O polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of CuII3TeVIO6. In Raman spectroscopy, CuII3TeVIO6-1C has a single strong band around 730 cm-1, whereas CuII3TeVIO6-2O shows a broad double maximum with bands centred around 692 and 742 cm-1.

Tetraoxolene-bridged rare-earth complexes: a radical-bridged dinuclear Dy single-molecule magnet.

Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(µ-tetraoxolene)] (RE = Y and Dy; HBpz3- = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2-; 1-RE) or bromanilate (ba2-; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3- ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(µ-ba˙)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(iii) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes.

Slow magnetisation relaxation in tetraoxolene-bridged rare earth complexes.

Three families of tetraoxolene-bridged dinuclear rare earth (RE) complexes have been synthesised and characterised, with general formula [((HB(pz)3)2RE)2(µ-tetraoxolene)] (HB(pz)3- = hydrotris(pyrazolyl)borate; tetraoxolene = chloranilate (1-RE), the dianionic form of 2,5-dihydroxy-1,4-benzoquinone (2-RE), or its 3,6-dimethyl analogue (3-RE)). In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and species with selected lanthanoid(iii) ions from Eu(iii) to Yb(iii) have been obtained, as well as the diamagnetic Y(iii) analogues. Use of the 3,6-dimethyl substituted tetraoxolene ligand (Me2-dhbq2-) has also afforded the two byproducts [((HB(pz)3)(MeOH)(B(OMe)4)Y)2(µ-Me2dhbq)] (4-Y) and [{((HB(pz)3)(MeOH)Y)2(µ-B(OMe)4)}2(µ-Me2dhbq)2]Cl2 (5-Y), with the B(OMe)4- ligands arising from partial decomposition of HB(pz)3-. Electrochemical studies on the soluble 1-RE and 3-RE families indicate multiple tetraoxolene-based redox processes. Magnetochemical and EPR studies of 3-Gd indicate the negligible magnetic coupling between the two Gd(iii) centres through the diamagnetic tetraoxolene bridge. Alternating current magnetic susceptibility studies of 1-Dy and 3-Dy reveal slow magnetic relaxation, with quantum tunnelling of the magnetisation (QTM) dominant in the absence of an applied dc field. The application of a dc field suppresses the QTM and relaxation data are consistent with an Orbach relaxation mechanism playing a major role in both cases, with effective energy barriers to magnetisation reversal determined as 47 and 24 K for 1-Dy and 3-Dy, respectively. The different dynamic magnetic behaviour evident for 1-Dy and 3-Dy arises from small differences in the local Dy(iii) coordination environments, highlighting the subtle structural effects responsible for the electronic structure and resulting magnetic behaviour.

YCu(TeO3)2(NO3)(H2O)3: a novel layered tellurite.

A new hydrated yttrium copper tellurite nitrate, yttrium(III) copper(II) bis-[trioxidotellurate(IV)] nitrate trihydrate, has been synthesized hydro-thermally in a Teflon-lined autoclave and structurally determined using synchrotron radiation. The new phase is the first example containing yttrium, copper and tellurium in one structure. Its crystal structure is unique, with relatively strongly bound layers extending parallel to (020), defined by YO8, CuO4 and TeO3 polyhedra, while the NO3 (-) anions and one third of the water mol-ecules lie between those layers. The structural unit consists of [Cu2(TeO3)4](4-) loop-branched chains of {Cu⋯Te⋯Cu⋯Te} squares running parallel to [001], which are linked further into layers only through Y(O,H2O)8 polyhedra. Weak 'secondary' Te bonds and O-H⋯O hydrogen-bonding inter-actions, involving water mol-ecules and layer O atoms, link the layers and inter-layer species. IR spectroscopic data are also presented.