The Heat of Formation of the Uranyl Dication: Theoretical Evaluation Based on Relativistic Density Functional Calculations

By using a set of model reactions, we estimated the heat of formation of gaseous UO₂²⁺ from quantum‐chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO₂²⁺, UO₂, UF₆, and UF₅. We used two gradient‐corrected exchange‐correlation functionals (revised Perdew–Burke–Ernzerhof (PBEN) and Becke–Perdew (BP)) and we accounted for spin‐orbit interaction in a self‐consistent fashion. Indeed, spin‐orbit interaction notably affects the energies of the model reactions, especially if compounds of U^IV are involved. Our resulting theoretical estimates for Δ_f (UO₂²⁺), 365±10 kcal mol^?1 (PBEN) and 370±12 kcal mol^?1 (BP), are in quantitative agreement with a recent experimental result, 364±15 kcal mol^?1. Agreement between the results of the two different exchange‐correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well‐established data for other compounds of the element in question.     

A Strongly Spin‐Frustrated FeIII7 Complex with a Canted Intermediate Spin Ground State of S=7/2 or 9/2

A disk‐shaped [Fe^III₇(Cl)(MeOH)₆(μ₃‐O)₃(μ‐OMe)₆ (PhCO₂)₆]Cl₂ complex with C₃ symmetry has been synthesised and characterised. The central tetrahedral Fe^III is 0.733 Å above the almost co‐planar Fe^III₆ wheel, to which it is connected through three μ₃‐oxide bridges. For this iron‐oxo core, the magnetic susceptibility analysis proposed a Heisenberg–Dirac–van Vleck (HDvV) mechanism that leads to an intermediate spin ground state of S=7/2 or 9/2. Within either of these ground state manifolds it is reasonable to expect spin frustration effects. The ⁵⁷Fe Mössbauer (MS) analysis verifies that the central Fe^III ion easily aligns its magnetic moment antiparallel to the externally applied field direction, whereas the other six peripheral Fe^III ions keep their moments almost perpendicular to the field at stronger fields. This unusual canted spin structure reflects spin frustration. The small linewidths in the magnetic Mössbauer spectra of polycrystalline samples clearly suggest an isotropic exchange mechanism for realisation of this peculiar spin topology.     

First principles investigations of Fe,Co, Ni in model honeycomb carbon networks

The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC₆. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μ_B for FeC₆ and 1 μ_B for CoC₆ while no magnetic solution could be identified for NiC₆. In the D_6h point group of the P6/mmm space group l_m decomposition of the d states results with increasing energy into doublet state E_1g with d(x²-y²) and d(xy); singlet state A_1g d(z²) and doublet state E_2g d(xz) and d(yz) lying on E_F and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC₆ compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications.     

Finding all‐nonmetal transition‐metal‐like superatom and its magnetic building block

In novel superatom chemistry, it is very attractive that all‐metal clusters can mimic the behaviors of nonmetal atoms and simple nonmetal molecules. Wizardly all‐metal halogen‐like superatom Al₁₃ with 2P⁵ sub shell (corresponding to the 3p⁵ of chlorine) is the most typical example. In contrast, how to mimic the behaviors of magnetic transition‐metal atom using all‐nonmetal cluster is an intriguing challenge for superatom chemistry. In response to this based on human intuition, using quantum chemistry methods and extending jellium model from metal cluster to all‐nonmetal cluster, we have found out that all‐nonmetal octahedral B₆ cluster with characteristic jellium electron configuration 1S²1P⁶2S²1D⁸ in the triplet ground state can mimic the behaviors of transition‐metal Ni atom with electron configuration 3s²3p⁶4s²3d⁸ in electronic configuration, physics and chemistry. Interestingly, the characteristic order of 1S1P2S1D for the B₆ nonmetal cluster with short B‐B lengths is different from that of the traditional jellium model—1S1P1D2S for metal clusters with long M‐M lengths, which exhibits a novel size effect of nonmetal cluster on jellium orbital ordering. Based on the jellium electron configuration, the B₆ with the spin moment value of 2μ_B is a new all‐nonmetal transition‐metal nickel‐like superatom exhibiting a new kind of all‐nonmetal magnetic superatom. Finding the application of the all‐nonmetal magnetic superatom, we encapsulate the magnetic superatom B₆ inside fully hydrogenated fullerene forming a clathrate B₆@C₆₀H₆₀ with the spin moment value of 2μ_B. As the C₆₀H₆₀ cage as a polymerization unit can conserve the spin moment of endohedral B₆, the clathrate B₆@C₆₀H₆₀ is a new all‐nonmetal magnetic superatom building block. Naturally, magnetic superatom structures of the B₆ and B₆@C₆₀H₆₀ may be metastable.     

Noncollinear magnetization density in VAu4

The spin and orbital magnetic moments of VAu₄ have been calculated using a first principles method that allows for noncollinear magnetic ordering. The large spin–orbit coupling of the Au atom is argued to induce large noncollinear components of the magnetization density as well as a parallel coupling between spin and orbital moments of the V atom, in contrast to expectations from Hund's third rule. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Ferromagnetism Induced by Substitution of the Iron(IV) Ion by an Unusual High‐Valence Nickel(IV) Ion in Antiferromagnetic SrFeO3

Novel cubic perovskites SrFe_1?xNi_xO₃ (0≤x≤0.5) with unusual high‐valence iron(IV) and nickel(IV) ions were obtained by high‐pressure and high‐temperature synthesis. Substantial magnetic moments of Ni^IV, which is intrinsically nonmagnetic with a nominal d⁶ electron configuration, were induced by the large magnetic moments of Fe^IV through orbital hybridization with oxygen. As a result, ferromagnetism with the transition temperature (T_c) above room temperature could be induced.     

UF6 and UF4 in Liquid Ammonia: [UF7(NH3)]3− and [UF4(NH3)4]

From the reaction of uranium hexafluoride UF₆ with dry liquid ammonia, the [UF₇(NH₃)]^3? anion and the [UF₄(NH₃)₄] molecule were isolated and identified for the first time. They are found in signal‐green crystals of trisammonium monoammine heptafluorouranate(IV) ammonia (1:1; [NH₄]₃[UF₇(NH₃)] ? NH₃) and emerald‐green crystals of tetraammine tetrafluorouranium(IV) ammonia (1:1; [UF₄(NH₃)₄] ? NH₃). [NH₄]₃[UF₇(NH₃)] ? NH₃ features discrete [UF₇(NH₃)]^3? anions with a coordination geometry similar to a bicapped trigonal prism, hitherto unknown for U^IV compounds. The emerald‐green [UF₄(NH₃)₄] ? NH₃ contains discrete tetraammine tetrafluorouranium(IV) [UF₄(NH₃)₄] molecules. [UF₄(NH₃)₄] ? NH₃ is not stable at room temperature and forms pastel‐green [UF₄(NH₃)₄] as a powder that is surprisingly stable up to 147 °C. The compounds are the first structurally characterized ammonia complexes of uranium fluorides.     

Redox Switches for Single‐Molecule Magnet Activity: An Ab Initio Insight

A dinuclear Co^II complex ( 1 ) featuring unprecedented anodic and cathodic switches for single‐molecule magnet (SMM) activity has been recently investigated (J. Am. Chem. Soc. 2013 , 135, 14670). The presence of sandwiched radicals in different oxidation states of this compound mediates magnetic coupling between the high‐spin (S=3/2) cobalt ions, which gives rise to SMM activity in both the oxidized ([ 1 (OEt₂)]⁺) and reduced ([ 1 ]^?) states. This feature represents the first example of a SMM exhibiting fully reversible, dual ON/OFF switchability. Here we apply ab initio and broken‐symmetry DFT calculations to elucidate the mechanisms responsible for magnetic properties and magnetization blocking in these compounds. It is found that due to the strong delocalization of the magnetic molecular orbital, there is a strong antiferromagnetic interaction between the radical and cobalt ions. The lack of high axiality of the cobalt centres explains why these compounds possess slow relaxation of magnetization only in an applied dc magnetic field.     

Theoretical approach to the magnetic properties of Co(II) ion in CoL2·5H2O compound (L is 5-chloro-2-methoxybenzoate)

The magnetic moments of a Co(II) complex in a recently reported CoL₂·5H₂O compound (L is 5-chloro-2-methoxybenzoate, ClC₆H₃(OCH₃)COO^?) were studied by a semi-theoretical approach in the vicinity of tetragonal distortion. It was found that in order to obtain the observed temperature-dependent magnetic moments, some parameters in the magnetic Hamiltonian which are related to the geometry of the complex should be varied. Comparing the calculated results of this study with experimental results from the literature, a distorted octahedral environment of Co(II) ion in the compound is predicted. Also, rather high probability of large orbital contribution to the magnetic moment is expected. Some abnormalities in previously reported data might be reduced by taking the experimental data with VSM (Vibrational Susceptibility Magnetometer).     

Synthesis,characterization and magnetism of copper(II)‐lanthanide(III) heterobimetalic complexes with N,N'‐oxamidobis‐(benzoato)cuprate (II)

Seven new μ‐oxamido copper(II)‐lanthanide(III) heterobimetalic complexes described by the formula Cu(obbz) Ln‐(Ph‐phen)₂NO₃(Ln = La, Nd, Eu, Gd, Tb, Ho, Er), where obbz denotes the oxamidobis(benzoato) and Ph‐phen represents 5‐phenyl‐1, 10‐phenanthroline, have been synthesized and characterized by the elemental analyses, spectroscopic (IR, UV, ESR) studies, magnetic moments (at room temperature) and molar conductivity measurement. The temperature dependence of the magnetic susceptibility of Cu(obbz)Gd(Ph‐phen)₂NO₃ complex has been measured over the range 4.2–300 K. The least‐squares fit of the experimental susceptibilities based on the spin Hamiltonian operator, ? = ?2 J?₁·?₂, yielded J= +1.28 cm^?1, a weak ferromagnetic coupling, A plausible mechanism for a ferromagnetic coupling between Gd(III)‐Cu(II) is discussed in terms of spin‐polarization.     

β‐Ag3RuO4, a Ruthenate(V) Featuring Spin Tetramers on a Two‐Dimensional Trigonal Lattice

Open‐shell solids exhibit a plethora of intriguing physical phenomena that arise from a complex interplay of charge, spin, orbital, and spin‐state degrees of freedom. Comprehending these phenomena is an indispensable prerequisite for developing improved functional materials. This type of understanding can be achieved, on the one hand, by experimental and theoretical investigations into known systems, or by synthesizing new solids displaying unprecedented structural and/or electronic features. β‐Ag₃RuO₄ may serve as such a model system because it possesses a remarkable anionic structure, consisting of tetrameric polyoxoanions (Ru₄O₁₆)^12?, and is an embedded fragment of a 2D trigonal MO₂ lattice. The notorious frustration of antiferromagnetic (AF) exchange couplings on such lattices is thus lifted, and instead strong AF occurs within the oligomeric anion, where only one exchange path remains frustrated among the relevant six. The strong magnetic anisotropy of the [Ru₄O₁₆]^12? ion, and the effectively orbital nature of its net magnetic moment, implies that this anion may reveal the properties of a single‐molecule magnet if well‐diluted in a diamagnetic matrix.     

Slow Magnetic Relaxation,Antiferromagnetic Ordering,and Metamagnetism in MnII(H2dapsc)-FeIII(CN)6 Chain Complex with Highly Anisotropic Fe-CN-Mn Spin Coupling

Reactions of [Mn(H₂dapsc)Cl₂] ⋅ H₂O (dapsc=2,6- diacetylpyridine bis(semicarbazone)) with K₃[Fe(CN)₆] and (PPh₄)₃[Fe(CN)₆] lead to the formation of the chain polymeric complex {[Mn(H₂dapsc)][Fe(CN)₆][K(H₂O)_3.5]}_n ⋅ 1.5n H₂O ( 1 ) and the discrete pentanuclear complex {[Mn(H₂dapsc)]₃[Fe(CN)₆]₂(H₂O)₂} ⋅ 4 CH₃OH ⋅ 3.4 H₂O ( 2 ), respectively. In the crystal structure of 1 the high-spin [Mn^II(H₂dapsc)]²⁺ cations and low-spin hexacyanoferrate(III) anions are assembled into alternating heterometallic cyano-bridged chains. The K⁺ ions are located between the chains and are coordinated by oxygen atoms of the H₂dapsc ligand and water molecules. The magnetic structure of 1 is built from ferrimagnetic chains, which are antiferromagnetically coupled. The complex exhibits metamagnetism and frequency-dependent ac magnetic susceptibility, indicating single-chain magnetic behavior with a Mydosh-parameter φ=0.12 and an effective energy barrier (U_eff/k_B) of 36.0 K with τ₀=2.34×10⁻¹¹ s for the spin relaxation. Detailed theoretical analysis showed highly anisotropic intra-chain spin coupling between [Fe^III(CN)₆]³⁻ and [Mn^II(H₂dapsc)]²⁺ units resulting from orbital degeneracy and unquenched orbital momentum of [Fe^III(CN)₆]³⁻ complexes. The origin of the metamagnetic transition is discussed in terms of strong magnetic anisotropy and weak AF interchain spin coupling.     

MnTaO2N: Polar LiNbO3‐type Oxynitride with a Helical Spin Order

The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO₂N are reported. High‐pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high‐density structure containing middle‐to‐late transition metals. Synchrotron X‐ray and neutron diffraction studies revealed that MnTaO₂N adopts the LiNbO₃‐type structure, with a random distribution of O^2? and N^3? anions. MnTaO₂N with an “orbital‐inactive” Mn²⁺ ion (d⁵; S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G‐type order observed in other orbital‐inactive perovskite oxides and LiNbO₃‐type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO₄N₂ octahedral network combined with the mixed O^2?/N^3? species that results in a distribution of (super)‐superexchange interactions.     

The [U2F12]2− Anion of Sr[U2F12]

The D_2h‐symmetric dinuclear complex anion [U₂F₁₂]^2? of pastel green Sr[U₂F₁₂] shows a hitherto unknown structural feature: The coordination polyhedra around the U atoms are edge‐linked monocapped trigonal prisms, the U^V atoms are therefore seven‐coordinated. This leads to a U–U distance of 3.8913(6) Å. A weak U^V–U^V interaction is observed for the dinuclear [U₂F₁₂]^2? complex and described by the antiferromagnetic exchange J_exp of circa ?29.9 cm^?1. The crystalline compound can be easily prepared from SrF₂ and β‐UF₅ in anhydrous hydrogen fluoride (aHF) at room temperature. It was studied by means of single crystal X‐ray diffraction, IR, Raman and UV/VIS spectroscopy, magnetic measurements, and by molecular as well as by solid‐state quantum chemical calculations.     

A Redox‐Active Bridging Ligand to Promote Spin Delocalization,High‐Spin Complexes,and Magnetic Multi‐Switchability

A dinuclear Co^II complex, [Co₂(tphz)(tpy)₂]ⁿ⁺ (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox‐active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin‐crossover with T_1/2≈470 K for the pristine compound (n=4) to single‐molecule magnet with an S_T=5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high‐spin species even at room temperature.     

A Redox‐Active Bridging Ligand to Promote Spin Delocalization,High‐Spin Complexes,and Magnetic Multi‐Switchability

A dinuclear Co^II complex, [Co₂(tphz)(tpy)₂]ⁿ⁺ (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox‐active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin‐crossover with T_1/2≈470 K for the pristine compound (n=4) to single‐molecule magnet with an S_T=5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high‐spin species even at room temperature.     

Unraveling σ and π Effects on Magnetic Anisotropy in cis‐NiA4B2 Complexes: Magnetization,HF‐HFEPR Studies,First‐Principles Calculations,and Orbital Modeling

By using complementary experimental techniques and first‐principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C_2v, has been investigated. Four complexes have the general formula [Ni(bpy)X₂]ⁿ⁺ (bpy=2,2′‐bipyridine; X₂=bpy ( 1 ), (NCS^?)₂ ( 2 ), C₂O₄^2? ( 3 ), NO₃^? ( 4 )). In the fifth complex, [Ni(HIM₂‐py)₂(NO₃)]⁺ ( 5 ; HIM₂‐py=2‐(2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM₂‐py. Analysis of the high‐field, high‐frequency electronic paramagnetic resonance (HF‐HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from ?1 to ?10 cm^?1. First‐principles SO‐CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3 , on one hand, and 4 and 5 , on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and g_i) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the e_g‐like orbitals and is due to the difference in the σ‐donor strength of NO₃^? and bpy or HIM₂‐py, whereas the difference in anisotropy between the two compounds is due to splitting of the t_2g‐like orbitals; and 2) the anisotropy of complexes 1 – 3 arises from the small splitting of the t_2g‐like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.     

Intramolecular and Intermolecular Magnetic Coupling Mechanism of a Dinuclear Copper(II) Complex

Abstract. A new dinuclear complex, [Cu₂(μ_{1, 3}‐NCS)₂(Ophen)₂(OH₂)₂], (HOphen = 1, 10‐phenanthrolin‐2‐ol) was synthesized and its crystal structure was determined by X‐ray crystallography. In the complex, the Cu^II ion assumes a distorted square pyramidal arrangement and the thiocyanate anion functions as bridged ligand and Ophen as capped ligand. The analysis of the crystal structure shows that there exists a π–π stacking interaction between the adjacent complexes. The theoretical calculations reveal that the magnetic coupling pathways from the thiocyanate anions bridge ligand and the π–π stacking magnetic coupling pathway resulted in the weak ferromagnetic interactions with 2J = 18.46 cm^–1 and 2J = 10.46 cm^–1, respectively. The calculations also display that the spin delocalization and the spin polarization occur in the bridge magnetic coupling system and the π–π stacking magnetic coupling system, and the magnetic coupling mechanism of the π–π stacking can be explained with McConnell I spin‐polarization mechanism. The fitting for the data of the variable‐temperature magnetic susceptibility with dinuclear Cu^II formula gave the magnetic coupling constant 2J = 2.84 cm^–1 and zJ′ = 0.03 cm^–1, in which the 2J = 2.84 cm^–1 is attributed to the magnetic coupling from the bridge dinuclear Cu^II unit and the zJ′ = 0.03 cm^–1 is ascribed to the π–π stacking magnetic coupling system. The study may benefit to understand the magnetic coupling mechanism of π–π stacking system.     

Spin State in (Porphinato)iron(III) Complex. A Spin‐Crossover Complex with Two Magnetically Distinct Sites

The results of low temperature X‐ray determination, Mössbauer and magnetic measurement of spin‐crossover (isothiocyanato)(porphinato)iron(III) hemipyridine are reported. The features in 77 K Mössbauer spectrum include two doublets, one with a quadrupole splitting (ΔE_Q) of 1.961 mm s^?1 (low‐spin site) and the other with ΔE_Q = 0.792 mm s^?1 (high‐spin site). As the temperature of the sample is increased to 300 K, the signal intensity of the high‐spin site grows to 92% at the expense of the low‐spin signal. The variable‐temperature magnetic susceptibility data also support that the tetraphenyl complex is a spin‐crossover complex.     

Relativity and the chemistry of UF6: A molecular Dirac—Hartree—Fock—CI study

The electronic structure and bonding of UF₆ and UF₆⁻ are studied within a relativistic framework using the MOLFDIR program package. A stronger bonding but more ionic molecule is found if one compares the relativistic with the nonrelativistic results. The first peak in the photoelectron spectrum of Karlsson et al. is assigned to the 12γ_8u component of the 4t_1u orbital, in agreement with other theoretical and experimental results. Good agreement is found between the experimental and theoretical 5f spectrum UF₆⁻. Some properties, like the dissociation energy and electron affinity, are calculated and the necessity of a fully relativistic framework is shown. The Breit interaction has an effect on the core spinors and the spin-orbit splitting of these spinors but the influence on the valence spectrum is negligible. © 1996 John Wiley & Sons, Inc.