Theoretical Modeling of the Ligand-Tuning Effect over the Transition Temperature in Four-Coordinated Fe-II Molecules

Spin-crossover molecules are systems of great interest due to their behavior as molecular level switches, which makes them promising candidates for nanoscale memory devices, among other applications. In this paper, we report a computational study for the calculation of the transition temperature (T-1/2), a key physical quantity in the characterization of spin-crossover systems, for the family of tetracoordinated Feu transition-metal complexes of generic formula [PhB(MesIm)(FeNPR1R2R3)-Fe-3]. Our calculations correctly reproduce the experimentally reported decrease in the T-1/2 with an increasing size of the phosphine and allow for the prediction of the T-1/2 in new members of the family that are not reported so far. More importantly, further insight into the factors that control the fine-tuning of the T-1/2 can be obtained by direct analysis of the underlying electronic structure in terms of the relevant molecular orbitals

Electronic and steric control of the spin-crossover behavior in [(Cp-R)(2)Mn] manganocenes

A computational study of the spin-crossover behavior in the family [(Cp-R)(2)Mn] (R = Me, Pr-i, Bu-t) is presented. Using the OPBE functional, the different electronic and steric effects over the metal's ligand field are studied, and trends in the spin-crossover-temperature (T-1/2) behavior are presented in terms of the cyclopentadienyl (Cp) ligand functionalization. Our calculations outlined a delicate balance between both electronic and steric effects. While an increase in the number of electron donating groups increases the spin-crossover temperature (T-1/2) to the point that the transition is suppressed and only the low-spin state is observed, steric effects play an opposite role, increasing the distance between the Cp rings, which in turns shifts T-1/2 to lower values, eventually stabilizing the high-spin state. Both effects can be rationalized by exploring the electronic structure of such systems in terms of the relevant d-based molecular orbitals

Computational Modeling of Transition Temperatures in Spin-Crossover Systems

A survey of different computational approaches to compute thermochemical properties and, in particular, transition temperatures (T1/2) in spin-crossover (SCO) systems is presented. Asides from the possibility of computing accurate values, this work centers its efforts in the use of such computational tools to explain trends in different families of SCO systems, aiming to understand the impact that chemical modifications (both electronic and steric) have over the ligand-field around the metal center, and how such effects can tune the corresponding T1/2. By using concepts from molecular orbital theory combined with the results from the calculations, a simple yet accurate depiction of the shift in T1/2 can be explai

Theoretical modeling of two-step spin-crossover transitions in FeII dinuclear systems

A computational methodology to model the spin-transition in the dinuclear iron(II) systems [Fe(bt)(NCX)2]2(μ-bpym) and [Fe(pypzH)(NCX)]2(μ-pypz)2 (X = S, Se or BH3) is presented. Using the hybrid meta-GGA exchange-correlation functional TPSSh, accurate values for the thermochemical quantities associated with the different spin-states can be computed, and subsequently used to calculate the corresponding transition temperatures. This results also allow for the correct modeling of the spin-crossover curve, in agreement with the two-step or single-step nature experimentally reported for the transition. Our results indicate that the presence or absence of a two-step transition is mostly dominated by electronic effects and cooperativity between binding pockets plays a minor role. Insight in the electronic structure effects that enhance or suppress this behavior and its origins can be outlined from direct analysis of the relevant d-based molecular orbitals, which allows for a quantitative computational prediction to screen for new dinuclear systems with selected properties

Assessment of the SCAN Functional for Spin-State Energies in Spin-Crossover Systems

The Strongly-Constrained and Appropriately Normed (SCAN) functional has been tested towards the calculation of spin-state energy differences in a dataset of 20 spin-crossover (SCO) systems, ranging from d4 to d7. The results shown that SCAN functional is able to correctly predict the low-spin state as the ground state for all systems, and the energy window provided by the calculations falls in the approximately range of energies that will allow for SCO to occur. Moreover, because SCAN is a pure meta-GGA functional, one can use such method in periodic calculations, accounting for the effect of collective crystal vibrations and counterions in the thermochemistry of the spin-transition. Our results validate this functional as a potential method for in silico screening of new SCO systems at both, molecular and crystal packed levels

Spin dynamics in single-molecule magnets and molecular qubits

Over recent decades, much effort has been made to lengthen spin relaxation/decoherence times of single-molecule magnets and molecular qubits by following different chemical design rules as maximizing the total spin value, controlling symmetry, enhancing the ligand field or inhibiting key vibrational modes. Simultaneously, electronic structure calculations have been employed to provide an understanding of the processes involved in the spin dynamics of molecular systems and served to refine or introduce new design rules. This review focuses on contemporary theoretical approaches focused on the calculation of spin relaxation/decoherence times, highlighting their main features and scope. Fundamental aspects of experimental techniques for the determination of key Single Molecule Magnet/Spin Qubit properties are also reviewed

Single-molecule magnet properties of transition-metal ions encapsulated in lacunary polyoxometalates: a theoretical study

Single-molecule magnet (SMM) properties of transition-metal complexes coordinated to lacunary polyoxo-metalates (POM) are studied by means of state of the art ab initio methodology. Three [M(gamma-SiW10O36)(2)] (M =Mn-III Fe-III, Co-II) complexes synthesized by Sato et al. (Chem. Commun. 2015, 51, 4081-4084) are analyzed in detail. SMM properties for the Coll and Mninnsystems can be rationalized due to the presence of low-energy excitations in the case of Co-II, which are much higher in energy in the case of Mn-II. The magnetic behavior of both cases is consistent with simple d-orbital splitting considerations. The case of the Fe-III complex is special, as it presents a sizable demagnetization barrier for a high-spin d(5) configuration, which should be magnetically isotropic. We conclude that a plausible explanation for this behavior is related to the presence of low-lying quartet and doublet states from the iron(III) center. This scenario is supported by ab initio ligand field analysis based on complete active space self-consistent field results, which picture a d-orbital splitting that resembles more a square-planar geometry than an octahedral one, stabilizing lower multiplicity states. This coordination environment is sustained by the rigidity of the POM ligand, which imposes a longer axial bond distance to the inner oxygen atom in comparison to the more external, equatorial donor atoms

Magnetic and transport properties of Fe-4 single-molecule magnets: a theoretical insight

Here, methods of density functional theory (DFT) were employed to study the magnetic and transport properties of a star-shaped single-molecule magnet Fe-4 S = 5 complex deposited on a gold surface. The study devoted to the magnetic properties focused on changes in the exchange coupling constants and magnetic anisotropy (zero-field splitting parameters) of the isolated and deposited molecules. Molecule surface interactions induced significant changes in the antiferromagnetic exchange coupling constants because these depend closely on the geometry of the metal complex. Meanwhile, the magnetic anisotropy remained almost constant. Transport properties were analysed using two different approaches. First, we studied the change in magnetic anisotropy by reducing and oxidizing the Fe-4 complex as in a Coulomb blockade mechanism. Then we studied the coherent tunnelling using DFT methods combined with Green functions. Spin filter behaviour was found because of the different numbers of alpha and beta electrons, due to the S = 5 ground state

Benchmarking density functional methods for calculation of state energies of first row spin-crossover molecules

A systematic study of the performance of several density functional methodologies to study spin-crossover (SCO) on first row transition metal complexes is reported. All functionals have been tested against several mononuclear systems containing first row transition metal complexes and exhibiting spin-crossover. Among the tested functionals, the hybrid meta-GGA functional TPSSh with a triple-ζ basis set including polarization functions on all atoms provides the best results across different metals and oxidation states, and its performance in both predicting the correct ground state and the right energy window for SCO to occur is quite satisfactory. The effect of some additional contributions,such as zero-point energies, relativistic effects, and intra-molecular dispersion interactions, has been analyzed. The reported strategy thus expands the use of the TPSSh functional to other metals and oxidation states other than FeII, making it the method of choice to study SCO in first row transition metal complexes. Additionally, the presented results validate the potential use of the TPSSh functional for virtual screening of new molecules with SCO, or its use in the study of the electronic structure of such systems

Mononuclear lanthanide complexes with 18-crown-6 ether: synthesis, characterization, magnetic properties, and theoretical studies

A family of lanthanide metal complexes with general formula [Ln(H2O)3(18-crown-6)](ClO4)3 (Ln: TbIII, DyIII, ErIII and YbIII) has been synthesized. Their magnetic properties have been characterized by DC and AC SQUID measurements and analyzed with the help of CASSCF-type calculations. The DyIII and YbIII compounds show slow relaxation of the magnetization under an external magnetic field. The analysis of the dependence of the relaxation time with the temperature and external magnetic field reveals that the main contributions are respectively the quantum tunneling and the Raman term, respectively. The analysis of the beta electron density and electrostatic potentials indicate that the axial ligands (three water molecules) generate a relatively small repulsion with the lanthanide electron density being the reason of the moderate magnetic anisotropy found in these systems