Joan Cano

Broken symmetry approach to calculation of exchange coupling constants for homobinuclear and heterobinuclear transition metal complexes

The application of broken symmetry density functional calculations to homobinuclear and heterobinuclear transition metal complexes produces good estimates of the exchange coupling constants as compared to experimental data. The accuracy of different hybrid density functional theory methods was tested. A discussion is presented of the different methodological approaches that apply when a broken symmetry wave function is used with either Hartree–Fock or density functional calculations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1391–1400, 1999

About the calculation of exchange coupling constants in polynuclear transition metal complexes

The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H · · · He and CH2 · · · He units to compare with Hartree–Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data.

About the calculation of exchange coupling constants using density-functional theory: the role of the self-interaction error.

The effect of the correction of the self-interaction error on the calculation of exchange coupling constants with methods based on density-functional theory has been tested in simple model systems. The inclusion of the self-interaction correction cancels the nondynamical correlation energy contributions simulated by the commonly used functionals. Hence, such correction should be important in the accurate determination of exchange coupling constants. We have also tested several recent functionals to calculate exchange coupling constants in transition-metal complexes, such as meta-GGA functionals or new formulations of hybrid functionals. The influence of the basis set and of the use of pseudopotentials on the calculated J values has also been evaluated for a Fe(III) dinuclear complex in which the paramagnetic centers bear several unpaired electrons.

Spin Density Distribution in Transition Metal Complexes: Some Thoughts and Hints

Abstract The spin density distribution in transition metal complexes is discussed in qualitative terms, taking into account the coexistence of spin delocalization and spin polarization mechanisms, with the help of numerical results for several complexes obtained from density functional calculations. The covalent character of the metal-ligand bonds as well as the σ- or π-characteristics of the partially filled d orbitals must be taken into account to qualitatively predict the sign of the spin density at a particular atom within a ligand. The same patterns can be applied to binuclear complexes and can be helpful in determining the ferro- or antiferromagnetic character of the exchange coupling between two paramagnetic ions when the energy gap between the partially occupied molecular orbitals is small. An attempt is made to establish a link between the qualitative-Hay-Thibeault-Hoffmann model of exchange coupling and the of spin polarization model.

Can large magnetic anisotropy and high spin really coexist

This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn6 complexes and suggests that large magnetic anisotropy is not favoured by a high spin state of the ground state.

Exchange Coupling in Oxalato-Bridged Copper(II) Binuclear Compounds: A Density Functional Study

A recently developed computational strategy is applied to examine the influence of several factors on the exchange coupling constants of oxalato-bridged copper(II) binuclear complexes (shown schematically here); molecular topology, the nature of terminal ligands and selected structural parameters are discussed.

Field‐Induced Hysteresis and Quantum Tunneling of the Magnetization in a Mononuclear Manganese(III) Complex

High-nuclearity complexes of transition-metal ions have been of special interest during the last two decades owing to the possibility of observing slow magnetic relaxation effects at the molecular level. These molecular nanomagnets have potential applications as new high-density magnetic memories and quantum-computing devices in the field of molecular spintronics. The first example of a discrete molecule exhibiting hysteresis and quantum tunneling of the magnetization was the mixed-valent dodecanuclear manganese(III,IV) complex [Mn12O12(CH3CO2)16(H2O)4]. [3] Since then, a plethora of both homoand heterovalent, manganese-based molecular nanomagnets of varying metal oxidation states (i.e., Mn, Mn and/or Mn) have been reported, with nuclearities from up to [Mn84] down to the smaller [Mn III 2] species. [4] However, to our knowledge, there are no examples of mononuclear manganese complexes exhibiting the slow magnetic relaxation effects typical of molecular nanomagnets, referred to as single-ion magnet (SIMs). This is somewhat puzzling, since several SIMs of other highly anisotropic first-row transition metals (i.e., Co and Fe) have been recently reported which has rekindled the debate in the field of singlemolecular magnetism. The six-coordinated octahedral high-spin d Mn ion (S = 2) has an orbitally degenerate Eg ground electronic term that is split by the Jahn–Teller effect into A1g and B1g orbital singlet low-lying states. Owing to the large mixing between them, second-order spin-orbit coupling (SOC) effects are ultimately responsible for the occurrence of a large axial magnetic anisotropy whose sign depends on the ground state, that is, on the nature of the axial tetragonal distortion. For an axially elongated octahedral Mn environment, negative D values are expected that can potentially lead to a large energy barrier for the magnetization reversal between the two lowest MS = 2 states. To provide this type of geometry and obtain manganese(III)-based SIMs, planar tetradentate chelating ligands with strong donor groups are a well-suited choice. Herein, we report a complete study on the synthesis, structural characterization, spectroscopic and magnetic properties, and theoretical calculations of Ph4P[Mn(opbaCl2)(py)2] (1) [H4opbaCl2 = N,N’-3,4-dichloro-o-phenylenebis(oxamic acid), py = pyridine, and Ph4P + = tetraphenylphosphonium cation]. Complex 1 is the first example of a mononuclear manganese(III) complex exhibiting a field-induced slow magnetic relaxation behavior, thus increasing the number of first-row transition-metal-ion SIMs. Complex 1 was obtained as well-formed deep brown cubic prisms by slow evaporation of a methanol/pyridine (1:4 v/v) solution of its tetramethylammonium salt in the presence of an excess of Ph4PCl (see Supporting Information). It crystallizes in the P21/c space group of the monoclinic system (Table S1, Supporting Information). The crystal structure of 1 consists of mononuclear manganese(III) complex anions, [Mn(opbaCl2)(py)2] (Figure 1), which are well separated from each other due to the presence of the bulky tetraphenylphosphonium countercations (Figure S1, Supporting Information). The manganese atom of 1 has a tetragonally elongated octahedral coordination geometry which is typical of the Jahn–Teller distorted d Mn ion. The equatorial plane is formed by two amidate nitrogen and two carboxylate oxygen atoms from the opbaCl2 ligand, while the axial positions are occupied by two pyridine nitrogen atoms (Figure 1a). The planar opbaCl2 ligand adopts a tetradentate coordination

Synthesis, Structural Characterisation, and Monte Carlo Simulation of the Magnetic Properties of the 3D-Stacked Honeycomb Csn[{Mn(N3)3}n] and the Irregular Double Chain [{N(C2H5)4}n][{Mn2(N3)5(H2O)}n]

Two new polymeric manganese-azido systems with formula Cs(n)-[[Mn(N3)3]n] (1) and [[N(C2H5)4]n][[Mn2-(N3)5(H2O)]n] (2) were synthesised and structurally characterised. Compound 1 crystallises in the P2(1)/n group and consists of a three-dimensional system with end-to-end and end-on azido bridges with the caesium atoms in the holes of the net. Magnetically, compound 1 is a rare case of a three-dimensional network with alternate ferro-antiferromagnetic interactions. Compound 2 crystallises in the P1 group and consists of double chains of manganese atoms bridged by end-on and, the exceptional, (mu-1,1,1)-azido bridges. Magnetically, compound 2 shows net ferromagnetic behaviour. Exact fit of the magnetic data was performed for the two compounds by means of Monte Carlo simulations based on the Metropolis algorithm on sets of 10 x 10 x 10 (1) and 1 x 1 x 320 (2) S = 5/2 classical spin centres.

Magnetic structure of the large-spin Mn10 and Mn19 complexes: a theoretical complement to an experimental milestone.

High-spin molecules have been proposed as candidates for the storage of information at the molecular level. The electronic structure of two complex magnetic molecular systems, Mn 10 and Mn 19, is characterized by means of a computational study based on density functional theory. All the exchange interactions in the recently reported Mn 19 complex with the highest known spin value of 83/2, and in its highly symmetric Mn 10 parent compound, are ferromagnetic. In these complexes, there are two kinds of ferromagnetic coupling: the first one corresponds to Mn (II)-Mn (III) interactions through a double mu 2-alkoxo-mu 4-oxo bridge where the high coordination number of the Mn (II) cations results in long Mn (II)-O bond distances, while the second one involves Mn (III)-Mn (III) interactions through mu 2-alkoxo-mu 3-eta (1):eta (1):eta (1) azido bridging ligands with long Mn (III)-N distances due to a Jahn-Teller effect.

Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets

The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) centers (S(Re) = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)](2-) and [ReBr4(ox)](2-) centers, originating from a combination of spin-orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.