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Understanding and predicting magnetic coupling in complex systems: from inorganic complexes to organic polyradicals

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Universitat de Barcelona. Departament de Química Física

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[spa]  Esta tesis presenta un estudio teórico y computacional sobre la predicción precisa de constantes de acoplamiento magnético en una serie de sistemas complejos. Estos incluyen dos familias de compuestos principales. La primera familia está formada por complejos de coordinación inorgánicos, los cuales presentan centros magnéticos localizados sobre los átomos metálicos y estructuras cristalinas bien definidas. El segundo grupo se trata de radicales puramente orgánicos, con conjugación del sistema π, los cuales presentan una flexibilidad estructural mucho mayor y una deslocalización de los electrones desapareados más extensa. La tesis tiene dos partes principales. La primera se dedica a establecer una estrategia para la extracción de constantes de acoplamiento magnético, lo cual se lleva a cabo en una serie de complejos de coordinación heterodinucleares y homotrinucleares. La estrategia se basa la propuesta de una formulación alternativa del mapping approach, que evita el uso de un proyector de espín y con ello las deficiencias derivadas de ello, en comparación con la formulación inicial propuesta por Noodleman. Esta propuesta es aplicada al problema de tres-electrones tres-centros, y validada por comparación con el experimento y a través de le teoría del Hamiltoniano efectivo. La segunda parte de la tesis se centra en compuestos radicalarios puramente orgánicos π—conjugados, que presentan interacción entre los electrones desapareados a través de enlace. Con el objetivo de promover la estabilidad química de los centros radicalarios, diferentes unidades básicas son consideradas. En base a diferentes esquemas de acoplamiento de estas unidades, y del papel de la flexibilidad estructural, el principal objetivo es establecer los principales factores electrónicos y estructurales para aumentar la estabilidad del radical y promover una interacción ferromagnética robusta entre ellos. Las principales conclusiones de la tesis son dos: primero, la propuesta para extraer constantes de acoplamiento en sistemas complejos resulta en valores consistentes; segundo, esta tesis propone utilizar poliradicales lineales π-conjugados, basados en unidades moleculares derivados de poliarylmethyl radicales, para lograr propiedades ferromagnéticas robustas en sistemas puramente orgánicos estables.

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[eng]  This thesis presents a theoretical and computational approach to the accurate description of magnetic exchange interactions in a variety of complex systems. These include two main families of compounds. The first family is formed by inorganic coordination complexes, presenting localized magnetic centres and well-defined crystal structures. The second family consists of purely organic, π−conjugated odd alternant neutral polyradicals, which display a much larger structural flexibility and greater delocalization of the unpaired electrons over the π system. The thesis has two main parts. The first one refers to the adopted strategy for the accurate extraction of magnetic exchange interactions. The systems used to investigate this issue are coordination compounds of increasing complexity, including heterobinuclear and homotrinuclear complexes, for which experimental crystal structures and magnetic data are available. The adopted strategy is based on the mapping approach, which relies on a one to one correspondence between the non-relativistic, time-independent exact Hamiltonian and two model spin Hamiltonians, the so-called HDVV and Ising. Ultimately, the mapping approach consists on describing both the energy and the wave function of the pure spin states by means of broken symmetry functions, using a spin projector to establish a univocal relation. In this thesis, a detailed analysis of the mapping approach has enabled establishing an alternative and accurate manner for extracting magnetic interactions in complex systems. By pointing out two main deficiencies that make the standard mapping approach proposed by Noodleman not appropriate to certain systems, and following previous work in our group, we propose an alternative approach. This is based on a direct use of the energy of the broken symmetry solutions which are mapped into the energy expectation values of the corresponding broken symmetry solutions of the HDVV Hamiltonian. This strategy relies on the one-to-one correspondence of the diagonal terms of the HDVV and Ising matrix representations. This proposal has been applied to the three-centre three-electron problem, and is further verified by comparison of the calculated coupling constants with the available experimental data and by means of effective Hamiltonian theory. The exchange coupling constant values obtained with this approach are consistent. Additionally, effective Hamiltonian theory offers the possibility to check whether the system can be described as a Heisenberg system. The second part of the thesis deals with purely organic π−conjugated neutral radicals interacting through-bond, and applies the strategy developed for inorganic molecules to extract the magnetic coupling constants between the unpaired electrons in these compounds. On the basis of chemical stability, different building blocks are investigated. Then, by considering different coupling schemes (strategies to assemble them leading to different dimensionalities) and the role of structural flexibility, the main goal of the investigation is to establish the main electronic and structural factors to enhance the stability of the radical centres and promote a robust ferromagnetic interaction among them. Thus, Chapter 1 compares explicitly through-space and through-bond interacting organic radicals, providing experimental arguments for the choice of the latter as building blocks. Chapter 2 introduces the theoretical foundations on which all computational methodologies used in this thesis are based. Chapter 3 provides an analysis of electronic structure methods to establish an accurate scheme to compute magnetic coupling constants in complex systems with well-defined structural parameters. Based on different studies on organic polyradicals, Chapter 4 offers reliable arguments to design purely π−conjugated organic polyradical interacting through-bond, with large S value, high-spin ground state, robust ferromagnetic properties, strong magnetic anisotropy and chemical stability. The appearance of a secondary structure in these flexible molecules is found to be crucial for stabilizing the polyradical high-spin ground state. Altogether, the main conclusions of this thesis are that the proposed strategy for the extraction of magnetic exchange interactions provides consistent results and the proposal for using linear π-conjugated polyradicals, based on molecular units derived from triarylmethyl radicals, to achieve robust ferromagnetic properties in stable purely organic systems.

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Tesis Doctorals - Departament - Química Física

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Understanding and predicting magnetic coupling in complex systems: from inorganic complexes to organic polyradicals