Ab initio study of the magnetic behavior of four dithiadiazolyl radical compounds

We have studied, by means of ab initio calculations, the magnetic interaction mechanisms in four radical crystals, X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br), which has allowed us to explain their different magnetic behaviour (ferromagnetism, antiferromagnetism, paramagnetism, spin frustration, etc.). First, we have identified the magnetic exchange pathways considering those with distances between two atoms of different dithiadiazolyl rings shorter than 7 Å and those with an intermolecular distance between an atom of the heterocyclic ring and an atom in a neighbouring radical shorter than 4 Å. Second, the calculations have been carried out in the framework of the DFT Broken Symmetry. Following this procedure we have determined the magnitude and the sign of the relevant coupling constants for the X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br) radicals. In the cases where the radicals order magnetically, ordering temperatures determined with our ab initio calculations agree very well with the experimental ones. Thus, in the case of the O₂N derivative ferromagnetic ordering is observed below 1.3 K, in very good agreement with an ordering temperature around 1.6 K predicted from our calculated exchange constants and using a mean field approximation.     

Controlling the dimensionality of oxalate-based bimetallic complexes: The ferromagnetic chain {[K(18-crown-6)][Mn(bpy)Cr(ox)3]}∞(18-crown-6 = C12H24O6, ox=C2O42-, bpy = C10H8N2)

The bimetallic ferromagnetic chain {[K(18-crown-6)][Mn(bpy)Cr(ox)₃]}_∞ (1) has been synthesized and characterized. It crystallizes in the orthorhombic chiral space group P2₁2₁2₁ [a = 9.0510(2) Å, b = 14.4710(3) Å, c = 26.8660(8) Å, V = 3510.97(1) Å³, Z = 2]. Compound 1 is made up by anionic [Mn(bpy)Cr(ox)₃]⁻ 1D chains and cationic [K(18-crown-6)]⁺ complexes. The magnetic exchange within the chain is ferromagnetic [J = +7.8(7) cm⁻¹]. In the solid state, the ferromagnetic chains are well isolated magnetically and no long range magnetic ordering has been observed above 2 K.     

Dinuclear manganese,iron, chromium,and cobalt complexes derived from aroylhydrazone ligands: Synthetic strategies,crystal structures,and magnetic properties

Coordination clusters of 3d metals continue to attract the intense interest of the scientists from the synthetic inorganic chemistry, bioinorganic chemistry and molecular magnetism communities. We review here the synthetic strategies employed in a continuous effort to obtain new and potentially magnetically interesting dinuclear molecules based on iron, manganese, chromium, and cobalt metal ions. The reported systems are pure homometallic 3d materials. We have focused on describing aspects of the synthesis, the crystal structures and the magnetic behaviour of these coordination compounds with low nuclearity. A deep solid-state and magnetic characterization of these systems has allowed us to gain evidence regarding the role played by weak exchange interactions and geometrical factors on the slow dynamics of the magnetization. In addition, the analysis through ab initio calculations has provided a valuable insight into the influence of organic periphery, bridging ligands, and remote substituents on the exchange coupling constant (J). In the case of a dinuclear complex based on manganese, the largest ferromagnetic interaction between two Mn^III has been observed (J = 19.7 cm⁻¹).     

Theoretical study of magnetic interaction between C60 anion radicals

The magnetic interactions between two C₆₀ anions are investigated by using unrestricted B3LYP (UB3LYP) calculations. Among four types of interactions, only one type of SOMO–SOMO interaction shows a week ferromagnetic interaction (J_ab = 4.6 cm^?1) whilst other interactions show week anti-ferromagnetic interactions. In order to explain a mechanism of the ferromagnetic and the anti-ferromagnetic interactions, a natural orbital (NO) analysis and a spin density analysis are carried out. The results of the analyses suggest that orbital orthogonality between SOMOs of each C₆₀ anions is the origin of the ferromagnetic interaction. On the other hand, a spin polarization effect does not appear in a spin density map in the ferromagnetic coupling state.     

Ferromagnetic interaction between [Ni(bdt)2]− anions in [Mn2(Saloph)2(μ-OH)][Ni(bdt)2](CH3CN)2

A new molecule-based magnetic material [Mn₂(Saloph)₂(μ-OH)][Ni(bdt)₂](CH₃CN)₂ was prepared by the metathesis of [Mn(Saloph)(H₂O)(ClO₄)] (S = 2) and TBA[Ni(bdt)₂] (S = 1/2). In the crystal, [Ni(bdt)₂]^? anions form square lattices which are separated from each other by the layers of antiferromagnetically coupled binuclear cations [Mn₂(Saloph)₂(μ-OH)]⁺. The magnetic susceptibility of the material coincides with the sum of the S = 2 van Vleck dimer model and S = 1/2 Heisenberg ferromagnetic square lattice model with 2J = ?92.4 and +4.5 K, respectively. The origin of the ferromagnetic interaction can be explained by the T-shaped intermolecular overlap mode of SOMOs which spreads to the ends of [Ni(bdt)₂]^? molecules.     

Magnetic and theoretical characterization of a ferromagnetic Mn(III) dimer

Reaction of 1,1,1-tris(hydroxymethyl)ethane (H₃thme) with the complex [Mn₂O₂(bpy)₄](ClO₄)₃ produces the dimeric species [Mn₂(Hthme)₂(bpy)₂](ClO₄)₂ in high yield. Magnetic measurements in the temperature range 1.8–300 K and in fields up to 7 T reveal weak ferromagnetic exchange between the metal centres with J = +2.13 cm⁻¹. A fit of the magnetization data, assuming only the ground state is populated, gives S = 4, g = 1.71 and D = −0.65 cm⁻¹. Low temperature single crystal measurements suggest the co-existence of SMM behaviour and strong intermolecular interactions. Density functional calculations also support a weak exchange interaction between the Mn ions.     

Simple versus composite nature of the magnetic coupling in copper and nickel-based metallic chains. Over- and underestimation of J

Extended metal atom chains (EMACs) are molecular linear arrangements of metal atoms featuring magnetic properties. By means of the density functional theory (DFT), we have studied the magnetic coupling constants for [Cu₃(dpa)₄Cl₂]⁺, Ni₃(dpa)₄Cl₂ and Ni₅(tpda)₄Cl₂ to understand which is the origin of the previously reported theoretical underestimation of J for nickel complexes. We have decomposed J = J_σ + J_δ, finding that the former contribution is underestimated and the latter part is overestimated at the DFT/B3LYP level of computation. Varying the amount of Hartree-Fock exchange, we show that the B3LYP functional fails to describe the σ interaction properly, whereas the δ coupling is exaggerated.     

Magnetic interactions in p-phenylene-bis(nitronyl nitroxide) biradicals with large torsion angles

We have designed and synthesized new biradicals of p-phenylene-bis(nitronyl nitroxide) substituted with two methoxy groups at 2,3- (2) and 2,5-positions (3). A parent biradical p-phenylene-bis(nitronyl nitroxide) (1) has intramolecular antiferromagnetic exchange interaction of 2J/k_B = −104 K ∼ −106 K with a torsion angle of 28.5° between the phenyl and the imidazole rings of nitronyl nitroxide. X-ray crystal structure analysis shows that the bulky substituents in 2 and 3 give large torsion angles of 65–70°. The larger torsion angles should weaken the magnitude of intramolecular exchange interactions, which is attributed to a decrease in π-conjugation over the p-phenylene and the radical groups. Magnetic susceptibility measurements indicate that the intramolecular exchange interactions in 2 and 3 are severely weakened to about 6% of that in 1, 2J/k_B = −6 K ∼ −8 K. The relation between the torsion angle and the intramolecular exchange interaction is consistent with DFT calculations. The ground-state singlet biradicals with suppressed intramolecular exchange interactions can be a building block for exotic exchange-coupled spin systems as predicted in our theoretical studies.     

Structural investigation of the Zn1−xCdxSb solid solution by density-functional theory approach

A peculiar behaviour was already reported in literature from experimental investigations in the Zn_1?xCd_xSb solid solution around x = 0.5. This behaviour was assumed to be linked to an ordering in the phase; however this assumption was never confirmed. The aim of this work was to understand this behaviour from a theoretical point of view. DFT calculations were performed to calculate the energy and the lattice parameters for all the possible structures of Zn_1?xCd_xSb with several compositions (x = 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, 1.0). The results show that an ordering appears in the most stable structure for x = 0.5. This ordering consists in stacked planes each containing only one chemical species.     

Preparation and magnetic properties of 4,6-bis(imino nitroxide)-substituted resorcinol and its Cu-complex

A novel bis(imino nitroxide)-substituted resorcinol 3H, that has two metal-binding sites with two pairs of the phenolate anion and the imino nitrogen atom, was prepared. The powdered sample of 3H showed an intramolecular ferromagnetic interaction (J/k_B = +5 K) between two (imino nitroxide)s through a m-phenylene bridge and a weak intermolecular antiferromagnetic interaction (J/k_B = − 0.9 K). The reaction of 3H with copper acetate in methanolic ammonia was examined to give a hardly soluble Cu-complex that exhibited ferromagnetic behavior in relatively high temperatures (298–55 K).     

Ferromagnetic coupling induced by spin-orbit coupling in dipyridylamide linear trinuclear Cu-Pd-Cu and Cu-Pt-Cu complexes

The mechanism of the magnetic coupling in the [CuMCu(pda)₄Cl₂] complexes with M = Pd, Pt is analyzed. First principles calculations based on the wave function theory are performed (DDCI + SO-RASSI): it is shown that the interaction without spin-orbit coupling is antiferromagnetic while the contribution due to the spin-orbit is ferromagnetic. The agreement between calculations and experiment is very good for the CuPdCu complex but is lesser for the CuPtCu complex. A Hubbard model is used in order to rationalize these results: this shows that the two d frontier orbitals of the central atom play a key role in these two interactions. Although the model parameters extracted from CASCI calculations do not permit to recover the order of magnitude of the interactions, it allows understanding the main interactions that give physical insight.     

Dimethyl sulfoxide oxidation mediated by a copper(II) diamine complex: A possible source of problem in the synthesis of molecular magnetic compounds

In the present work we report a reaction in which dimethyl sulfoxide, initially used as solvent, undergoes oxidation to form sulfate, which then participates to the formation of a linear one-dimensional copper chain. Indeed, using [Cu(bipy)Cl₂], a building block largely applied in synthesis of molecular magnetic compounds, the coordination compound [Cu(bipy)(H₂O)₂(SO₄)]_n, where bipy = 2,2′-bipyridine was obtained. Magnetic characterization of complex shows a weak antiferromagnetic interaction between the copper(II) ions with J = ?0.53 cm^?1. DFT calculations demonstrate that the pathway for the weak antiferromagnetic interaction is through the sulfate bridge.     

Cooperative Jahn–Teller distortion leading to the spin-1/2 uniform antiferromagnetic chains in triclinic perovskites AgCuF3 and NaCuF3

Polycrystalline samples of AgCuF₃, isostructural with NaCuF₃, were synthesized by solid state reaction and characterized by powder X-ray diffraction. The magnetic properties of AgCuF₃ and NaCuF₃ were examined by measuring their magnetic susceptibilities and evaluating their spin exchange interactions. The three-dimensional CuF₃ network of corner-sharing CuF₆ octahedra present in AgCuF₃ and NaCuF₃ shows a cooperative Jahn–Teller distortion such that their magnetic susceptibilities above 50 K are well described by an S = 1/2 Heisenberg uniform antiferromagnetic chain model with average spin exchange of J/k_B ≈ ?300 and ?180 K, respectively. The relative strengths of these interactions are well reproduced by spin dimer analysis based on tight-binding calculations, but not by mapping analysis based on first principles density functional calculations.     

Molecular spin ladders self-assembly from [Ni(dmit)2]− building blocks: Syntheses,structures and magnetic properties

Two ion-pair compounds, consisting of 1-(4′-R-benzyl)pyridinium ([RBzPy]⁺, R = NO₂ (1) and Br (2)) and [Ni(dmit)₂]⁻ (dmit²⁻ = 2-thioxo-1,3-dithion-4,5-dithiolato), have been synthesized and structurally characterized. The anions of [Ni(dmit)₂]⁻ stack into dimers, which further construct into two-leg ladder through terminal S⋯S interactions in 1, lateral S⋯S interactions in 2. The weak H-bonding interactions of C–H⋯S were observed in 2, while only weak van de Waals interactions between anion and cations in 1. The magnetic susceptibilities measured in 2–300 K indicate AFM exchange interaction domination both two compounds. A peculiar magnetic transition at ∼100 K was observed in 1. An AFM ordering below ∼11 K was found in 2, and the best fit to magnetic susceptibility above 45 K in this compound, using a dimer model with s = 1/2, give rise to Δ/k_B = 36.1 K, zJ = −0.91 K, C = 3.2 × 10⁻³ emu K mol⁻¹ and χ₀ = −4.0 × 10⁻⁶ emu mol⁻¹ with g of 2.0 fixed.     

Relative permittivity data of binary mixtures containing 2-butanol, 2-butanone,and cyclohexane

Relative permittivity measurements were made on binary mixtures of (2-butanol + 2-butanone) and (2-butanol or 2-butanone + cyclohexane) for various concentrations at T = (298.2, 308.2, and 318.2) K. Some experimental results are compared with those obtained from theoretical calculations and interpreted in terms of homo- and heterogeneous interactions and structural effects. The molecular dipole moments were determined using Guggenheim–Debye method within the temperature range of (298.2 to 318.2) K. The variations of effective dipole moment and correlation factor, g, with the mole fraction in these materials were investigated using Kirkwood–Frohlich equation. The pure compounds showed a negative and small temperature coefficient of effective dipole moment. In order to obtain valuable information about heterogeneous interaction (interactions between the unlike molecules), the Kirkwood correlation factor, the Bruggeman dielectric factor and the excess permittivity were calculated. In order to predict the permittivity data of polar–apolar binary mixtures, five mixing rules were applied.     

Solubility of tridecanedioic acid in pure solvent systems: An experimental and computational study

Solubility has been extensively investigated by the phase equilibria approach at the mesoscale level, but its origin on the molecular and electronic levels is poorly understood. This study explored the solubility behaviour of crystalline solid in selected pure solvents with various functional groups by using both phase equilibria and molecular modelling methods. The model compound tridecanedioic acid (TDDA) solubility in methanol, ethanol, acetic acid, acetone, and ethyl acetate was determined from T = (283.15 to 323.15) K by a static method. It was found that almost all solutions studied exhibit non-ideal behaviour and deviate positively from Raoult’s law indicating the important role of homo-molecules interactions. Thermodynamic analyses of solution suggest that both enthalpy and entropy of solution govern the dissolution process. Computational studies on solubility behaviour were performed by using both density functional theory (DFT) calculations and molecular dynamic (MD) simulations. The results conclude that the (solute + solvent) interaction is not the only factor determining solubility, and (solvent + solvent) interaction also plays an important role. The simulated results are found to be qualitatively consistent with experimental values. Finally, solubility values were correlated by the empirically modified Apelblat equation and two local composition models of Wilson and NRTL.     

DFT study of group 8 catalysts for the hydrophenylation of ethylene: Influence of ancillary ligands and metal identity

Computational methods are used to investigate catalytic hydrophenylation of ethylene using complexes of the type [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ [Y = Mp, n = 1; Y = Tp, n = 0; M = Ru or Os; L = PMe₃, PF₃, or CO; Mp = tris(pyrazolyl)methane; Tp = hydrido-tris(pyrazolyl)borate]. The conversion of ethylene and benzene to ethylbenzene with [(Y)M(L)(Ph)]ⁿ⁺ as catalyst involves four steps: (1) ethylene coordination, (2) ethylene insertion into the M–Ph bond, (3) benzene coordination, and (4) benzene C–H activation. DFT calculations form the basis to compare stoichiometric benzene C–H activation by [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ complexes to yield methane and [(Y)M(L)(Ph)(NCMe)]ⁿ⁺. In addition, starting from the 16-electron species [(Y)M(L)(Ph)]ⁿ⁺, potential energy surfaces for the formation of ethylbenzene are calculated to reveal the impact of modifications to the scorpionate ligand (Mp or Tp), co-ligand (L) and metal center (M).     

Excess molar enthalpies of (acetonitrile + butan-2-one) and (methanol + acetonitrile + butan-2-one) atT = 298.15 K

The excess molar enthalpies of (acetonitrile  +  butan-2-one) and (methanol  +  acetonitrile  +  butan-2-one) were measured atT =  298.15 K and atmospheric pressure using a flow microcalorimeter. The experimental results are correlated with polynomial equations and compared with those calculated from associated solution models taking account into self-association of methanol and acetonitrile as well as solvation between unlike molecules and a non-polar interaction term.