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

Fine-tuning of the spin-crossover properties of Fe(III) complexes via ligand design

Exploring the chemical space of a given ligand aiming to modulate its ligand field strength is a versatile strategy for the fine-tuning of physical properties such as the transition temperature (T1/2) of spin- crossover (SCO) complexes. The computational study presented herein aims at systematically exploring the extent to which the ligand substituent effects can modulate T1/2 in two families of Fe(III) SCO systems with a N4O2 coordination environment and at identifying the best descriptors for fast and accurate prediction of changes in T1/2 upon ligand functionalization. B3LYP* calculations show that the attachment of substituents to b-ketoiminato fragments (L1) leads to drastic changes in T1/2, while functionalization of phenolato moieties (L2) allows for a finer degree of control over T1/2. Natural Bond Orbital (NBO) charges of the donor atoms, Hammett parameters for both para and meta- functionalization of L2, and Swain–Lupton parameters for L1 and para-functionalization of L2 have been found to be the suitable descriptors for predicting the changes in T1/2. Further analysis of the ligand-field splitting in such systems rationalizes the observed trends and shows that ligand substituents modify both the s and p bonds between the Fe(III) center and the ligands. Thus, we provide simple yet reliable guide- lines for the rational design of new SCO systems with specific values of T1/2 based on their ligand design

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

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 assessment on the Tolman cone angles for P-ligands

The Tolman cone angle (θ), the par excellence descriptor of the steric measure of a phosphine, has been recomputed for a set of 119 P-ligands, including simple phosphines and phosphites, as well as bulky biaryl species often employed in catalytic processes. The computed cone angles have been obtained from three different transition metal coordination environments: linear [AuCl(P)] (θL), tetrahedral [Ni(CO)3(P)] (θT) and octahedral [IrCl3(CO)2(P)] (θO), allowing us to observe the steric behavior of the ligand when increasing the steric hindrance around the metal center. The computed cone angles have been extracted from the lowest-energy conformer geometry obtained with a combined MM/DFT methodology. A conformational screening has been done using MM, which allows us to identify the lowest energy structure of each ligand in each coordination environment. These low energy conformers have been subsequently reoptimized at the DFT theory level, from which the cone angle value can be extracted. The computed cone angles have been compared with the original Tolman cone angles, and with other steric parameters such as solid angles (Θ), percent buried volumes (%Vbur), and angular symmetric deformation coordinate (S′4). This new set of values correlates with phosphine ligand dissociation enthalpies in titanocene complexes of the general formula [Ti(2,4-C7H11)2(PR3)], and with reaction barriers in the Suzuki-Miyaura reaction between [Pd-PR3] and bromobenzene, proving that this newly proposed set of cone angles can be employed to establish linear correlations between different experimental and calculated properties for systems in which phosphine ligands play a significant role

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

Mesures de forma, estereoquímica i estructura electrònica de compostos de metalls de transició

[cat] Els objectius d'aquest treball són donar una perspectiva general de la metodologia de les mesures continues de forma, que permeten una descripció senzilla, però acurada, de l'entorn de coordinació en compostos de metalls de transició. S'introduirà una nova eina associada a la metodologia de les mesures de forma pel seguiment d'estructures que es troben en camins d'interconversió entre poliedres. Les mesures de forma seran emprades per analitzar l'esteroquímica de compostos organometàl·lics, comparant aquests compostos i els seus lligands típics amb els anàlegs de química de coordinació. S'estudiarà l'estereoquímica de compostos tetracoordinats i pentacoordinats en funció de la configuració electrònica del metall i de les restriccions estereoquímiques imposades per lligands amb diferent denticitat. Pels compostos tetracoordinats, es farà un estudi de les corbes d'energia per totes les configuracions electròniques i pels diferents estats de spin, per correlacionar la distribució de les estructures experimentals amb les geometries i estats de spin predits a nivell teòric. Es mostrarà com es poden obtenir un conjunt de regles senzilles per la predicció de l'entom de coordinació en compostos tetracoordinats en funció de l'estructura electrònica del metall, i quins són els factors que poden modificar l'estat de spin del metall. Finalment, i donada la relació intima que hi ha entre estructura i propietats, es farà un estudi de la correlació entre el paràmetre de desdoblarnent a camp zero, calculat teòricament, i l'entom de coordinació del metall

Thermal spin crossover in Fe(ii) and Fe(iii). Accurate spin state energetics at the solid state

The thermal spin crossover (SCO) phenomenon refers to an entropy-driven spin transition in some materials based on d6-d9 transition metal complexes. While its molecular origin is well known, intricate SCO behaviours are increasingly common, in which the spin transition occurs concomitantly to e.g. phase transformations, solvent absorption/desorption, or order-disorder processes. The computational modelling of such cases is challenging, as it requires accurate spin state energies in the solid state. Density Functional Theory (DFT) is the best framework, but most DFT functionals are unable to balance the spin state energies. While a few hybrid functionals perform better, they are still too expensive for solid-state minima searches in moderate-size systems. The best alternative is to dress cheap local (LDA) or semi-local (GGA) DFT functionals with a Hubbard-type correction (DFT+U). However, the parametrization of U is not straightforward due to the lack of reference values, and because ab initio parametrization methods perform poorly. Moreover, SCO complexes undergo notable structural changes upon transition, so intra- and inter-molecular interactions might play an important role in stabilizing either spin state. As a consequence, the U parameter depends strongly on the dispersion correction scheme that is used. In this paper, we parametrize U for nine reported SCO compounds (five based on FeII, 1-5 and four based on FeIII, 6-9) when using the D3 and D3-BJ dispersion corrections. We analyze the impact of the dispersion correction treatments on the SCO energetics, structure, and the unit cell dimensions. The average U values are different for each type of metal ion (FeIIvs. FeIII), and dispersion correction scheme (D3 vs. D3-BJ) but they all show excellent transferability, with mean absolute errors (MAE) below chemical accuracy (i.e. MAE <4 kJ mol−1). This enables a better description of SCO processes and, more generally, of spin state energetics, in materials containing FeII and FeIII ions

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