Advanced Study of Switchable Spin Crossover Compounds

Abstract

Systems based on the organic ligand 3-bpp display a wide variety of magnetic behaviour, and have been studied in depth in the field of spin crossover (SCO). However, despite the apparent richness of SCO behaviour observed for 3-bpp systems, at the time of undertaking this thesis there were no published examples of the use of synthetic derivatives of 3-bpp in this field of research. This was remarked upon by Olguín and Brooker, who attributed this lack of 3-bpp analogues not to a lack of interest, but to “synthetic issues”. Therefore, the initial challenge set out was to develop a methodology that would allow the synthesis of functionalised 3-bpp ligands. Once obtained, these ligands would then be used for the generation of mononuclear Fe(II) compounds, with a view to study their macroscopic properties. Provided that this strategy was successful, and depending on the availability of structural data, an attempt would then be made to outline correlations between the observed magnetic properties and the relationship between the lattice entities, both spin-active and –inactive. The development of this work is detailed over the course of the ten Chapters. The first is an Introduction, primarily to the general field of SCO, the theory underlying its occurrence and its potential utility, but also describes the latest advances in the field before focussing on the family of 3-bpp systems.

Chapter 2 contains the synthetic procedures and experimental details for the physical characterisation of the compounds obtained. Chapter 3 then describes the development of the novel polypyrazolyl ligands that have been designed for the subsequent investigation of their coordination chemistry. The magneto-structural properties of a new [Fe(3-bpp)2]2+ salt are described, and the compound shown to display a gradual spin crossover.

Chapters 4 and 5 provide an in-depth study of a mononuclear compound, [Fe(H4L)2](ClO4)2•H2O•2(CH3)2CO (1), containing one of the polypyrazolyl ligands. A strongly cooperative spin transition is found to be intimately related to the structure and level of disorder within the lattice. This conclusion is supported by a description of the thermally trapped crystal structure. The photo-physical properties of 1 are then described, through the study of its meta-stable HS phase, which is distinct to the thermally trapped meta-stable HS phase. A Raman spectroscopy study of the system is then used to observe photo-switching within the hysteresis loop, and as a means of following the pressure-induced spin switch.

Chapters 6 and 7 use compound 1 as the basis of an investigation into the effect of varying the anions and solvents within the lattice on the magnetic properties and crystal packing. A case where the spin crossover becomes more cooperative on ageing the compound is described. Chapter 8 details the compounds obtained using another 3-bpp derivative, which results in the formation of distorted structures that do not display SCO.

In Chapter 9, the structural data provided by this thesis is collated with that found in the CSD for mononuclear Fe(II) systems with 3-bpp ligands. The data is then analysed from two points of view: a first, which involves previously defined parameters for the distortion of the cations, both in terms of their shape and in terms of the coordination sphere using Continuous Symmetry Measures. And a second, which uses Hirshfeld surface analysis to look at the bonding motifs and intermolecular interactions that are observed in the novel compounds obtained in this thesis. Chapter 10 offers conclusions based on the results obtained in this thesis.

A pesar de la riqueza de los comportamientos de la transición de espín de los sistemas basados en el ligando 3-bpp, no se ha estudiado ningún derivado del ligando. Así pues, el reto inicial de esta tesis fue el de desarrollar ligandos derivados del 3-bpp. Estos se utilizarían en la síntesis de compuestos mononucleares de Fe(II), para estudiar sus propiedades macroscópicas. Este trabajo se detalla a lo largo de nueve capítulos.

El capítulo 2 contiene los procedimientos experimentales para la síntesis y caracterización de los compuestos obtenidos. El capítulo 3 describe el desarrollo de los ligandos que fueron utilizados en la química de coordinación. Se detallan las propiedades magneto-estructurales de una nueva sal [Fe(3-bpp)2](ClO4)2 que presenta una transición de espín gradual.

Los capítulos 4 y 5 estudian el compuesto [Fe(H4L)2](ClO4)2•H2O•2(CH3)2CO (1). Se observa que una transición de alta cooperatividad se asocia al desorden cristalográfico. Se procede a describir las propiedades foto-físicas del compuesto, mediante el estudio de su fase meta-estable de espín alto inducido por irradiación. Un estudio de espectroscopia de Raman se llevó a cabo, permitiendo así la observación de un proceso de “photo-switch” dentro del ciclo de histéresis. Los espectros obtenidos sirvieron para seguir la transición al aplicar una presión externa.

Los capítulos 6 y 7 investigan el efecto de los aniones y disolventes sobre las propiedades magnéticas y el empaquetamiento estructural del compuesto 1. Se describe un caso dónde un proceso de envejecimiento del compuesto lleva a que se aumente su grado de cooperatividad. En el capítulo 8, se utiliza otro derivado del 3-bpp para preparar unos compuestos mononucleares de Fe(II). Debido a la deformación estructural presentada por estos compuestos, no se observa una transición de espín.

En el capítulo 9, se recogen los datos estructurales obtenidos a lo largo de la tesis, y se combinan con los datos disponibles en el CSD para compuestos mononucleares de Fe(II). Se analizan los datos para medir la forma y distorsión de los cationes, y para estudiar los tipos de interacción intermolecular mediante los análisis de superficies de Hirshfeld. El capítulo 10 saca unas conclusiones basadas en los resultados descritos en los capítulos anteriores.