Exploring the Ligand Chemical Space with Computational Tools and its Effect on the Electronic Structure of Spin-Crossover of Different Complexity

Abstract

[eng] Spin-crossover (SCO) compounds are a fascinating class of molecular systems containing first-row transition metal ions with d4 to d7 electronic configurations, capable of alternate between two electronic states that are close in energy. Originally described by Cambi and co- workers in 1931 on some tris(N,N-dialkyldithiocarbamato)iron(III) complexes, this phenomenon has since grown into a vibrant field of research due to its potential applications as sensors, molecular level based switches, nanodevices, energy storage, and self-healing materials, among others. The SCO transition is triggered by an external stimulus, commonly temperature but also pressure or electromagnetic radiation, and is accompanied by significant changes in the physical properties of the system since the spin state of the metal centre is changed, including changes in the magnetic moment, colour, or structural distortions. The temperature at which the two spin states are equally populated, is defined as the transition temperature (T1/2), and is a key parameter in the physical characterization of such systems. Research into SCO compounds has predominantly focused on d6-Fe(II) coordination complexes, with numerous examples spanning mononuclear, polynuclear, and extended systems, including coordination polymers. While these systems dominate the field, other 3d metal ions capable of exhibiting SCO behaviour remain comparatively underexplored. In this thesis, some unusual systems exhibiting SCO have been studied computationally, such as Cr(II)-based, or anionic Fe(II)-based complexes. The level of complexity has also been increased, studying dinuclear Fe(II)-based metal-organic cages (MOCs) that can undergo SCO showing different types of transitions depending on the nature of the guest molecules that the cages can encapsulate. In all cases, ligand functionalization has been studied to understand how these changes affect the transitions, with the aim of design new materials with tailored properties. Different levels of computation have been used, from density functional theory (DFT) to higher-level multireference methods, like Complete Active Space Self-Consistent Field (CASSCF)/N-Electron Valence Perturbation Theory (NEVPT2) and Ab initio Ligand Field Theory (AILFT). In this thesis also a new methodology that develops density functional theories that use multiconfigurational reference wave functions, like Multiconfiguration Pair- Density Functional Theory (MC-PDFT) was used, for a Fe(II)-based SCO system that is well described both experimentally and computationally. MC-PDFT, in principle, allows a

description of static and dynamic correlation with affordable computational costs and with an accuracy comparable to multireference perturbation theory methods. The results outlined in this thesis help in understanding the versatility and unique properties of SCO systems, which continue to drive interest in their applications expanding the scope of functional materials in advanced technologies.

[cat] Spin-crossover (SCO) compounds are a fascinating class of molecular systems containing first-row transition metal ions with d4 to d7 electronic configurations, capable of alternate between two electronic states that are close in energy. Originally described by Cambi and co- workers in 1931 on some tris(N,N-dialkyldithiocarbamato)iron(III) complexes, this phenomenon has since grown into a vibrant field of research due to its potential applications as sensors, molecular level based switches, nanodevices, energy storage, and self-healing materials, among others. The SCO transition is triggered by an external stimulus, commonly temperature but also pressure or electromagnetic radiation, and is accompanied by significant changes in the physical properties of the system since the spin state of the metal centre is changed, including changes in the magnetic moment, colour, or structural distortions. The temperature at which the two spin states are equally populated, is defined as the transition temperature (T1/2), and is a key parameter in the physical characterization of such systems. Research into SCO compounds has predominantly focused on d6-Fe(II) coordination complexes, with numerous examples spanning mononuclear, polynuclear, and extended systems, including coordination polymers. While these systems dominate the field, other 3d metal ions capable of exhibiting SCO behaviour remain comparatively underexplored. In this thesis, some unusual systems exhibiting SCO have been studied computationally, such as Cr(II)-based, or anionic Fe(II)-based complexes. The level of complexity has also been increased, studying dinuclear Fe(II)-based metal-organic cages (MOCs) that can undergo SCO showing different types of transitions depending on the nature of the guest molecules that the cages can encapsulate. In all cases, ligand functionalization has been studied to understand how these changes affect the transitions, with the aim of design new materials with tailored properties. Different levels of computation have been used, from density functional theory (DFT) to higher-level multireference methods, like Complete Active Space Self-Consistent Field (CASSCF)/N-Electron Valence Perturbation Theory (NEVPT2) and Ab initio Ligand Field Theory (AILFT). In this thesis also a new methodology that develops density functional theories that use multiconfigurational reference wave functions, like Multiconfiguration Pair- Density Functional Theory (MC-PDFT) was used, for a Fe(II)-based SCO system that is well described both experimentally and computationally. MC-PDFT, in principle, allows a

description of static and dynamic correlation with affordable computational costs and with an accuracy comparable to multireference perturbation theory methods. The results outlined in this thesis help in understanding the versatility and unique properties of SCO systems, which continue to drive interest in their applications expanding the scope of functional materials in advanced technologies.