Theoretical studies of metal-metal cooperativity in pyrazolate-bridged complexes
by Anton Römer
Date of Examination:2023-06-20
Date of issue:2024-04-11
Advisor:Prof. Dr. Ricardo A. Mata
Referee:Prof. Dr. Peter E. Blöchl
Persistent Address:
http://dx.doi.org/10.53846/goediss-10436
Files in this item
Name:aroemer_thesis.pdf
Size:37.7Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
Transition metal complexes can play a variety of important roles in regard to catalysis, materials synthesis, photochemistry, and biological systems through use of their unique electronic structure, and the resulting chemical versatility. These complexes can be tuned towards a multitude of different applications through careful design of their ligand structure. Electronic structure methods can provide great insight into these aspects, being able to predict compound properties even before any synthetic efforts have taken place, or in elucidating reaction mechanisms and unexplained compound behaviour afterwards. The results from one such project are detailed in the first chapter of this work, describing the oxidative splitting of water molecules by a bimetallic Ni II complex, with special attention toward the metal-metal cooperativity. Furthermore, transition metal complexes often exhibit spin transitions in response to external stimuli such as changes in temperature and pressure, a phenomenon known as spin crossover (SCO). This phenomenon goes hand in hand with changes in physical properties, and the attainable bi- or multistabilities grant these types of complexes wide potential application as molecular switches in display, memory and sensing devices. It is no surprise that they have gathered significant interest for decades, and many attempts are made to design SCO complexes with specific characteristics. Especially of interest currently are complexes containing two or even more transition metal centers. However, it is crucial to balance ligand field stabilization and spin pairing energies to reach magnetic multistability conditions. Again, electronic structure methods should be able to further the understanding of these aspects. However, the widely used general gradient approximation density functional calculations provide only qualitatively correct results, being known to overestimate the stability of low-spin (LS) states. Higher level methods on the other hand are often not available for systems of any significant size. The aim of this work then is to firstly demonstrate the application of a range of techniques involving density functional theory (DFT), which are currently in everyday use for the validation and explanation of experimental observations. From there on, this work extrapolates from these commonly used methods to develop a methodology to improve DFT techniques both in accuracy and in the range of applicable systems. Specifically, it is attempted to describe the spin states of a [2x2] Fe II grid complex in a self-consistent, ab initio approach. Wave function methods are applied to the investigation of smaller model systems, with the result being used to parameterize a local hybrid functional.
Keywords: Computational Chemistry; Density Functional Theory; Coupled Cluster; Bayesian Optimization; Machine Learning; Wave Function Theory; CP-PAW; Spin States; Transition Metal Complexes; Spin Crossover; Benchmarking; Cooperativity; Functional Parameterization
