Density functional theory to the rescue of transition-metal chemistry

Martins, Frederico F.
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Density functional theory (DFT) was postulated almost 60 years ago and equipped chemists with a powerful framework to simulate, in silico, the behavior of chemical systems. Despite its widespread utility, DFT methods have encountered difficulties in accurately modeling the reactivity of transition-metal complexes, due to, e.g., their unique open-shell electronic structures, multireference character and associated consequences. However, these complexes play a crucial role as essential constituents of materials with exceptional functionality, enabling the execution of complex reactions that would otherwise be exceedingly challenging, akin to those facilitated by enzymatic cofactors. Remarkably, this work unintentionally demonstrates the capabilities of DFT to overcome the existing obstacles posed by transition-metal chemistry. In Chapter 4, we explore the relationship between vibrational frequencies, structure, and magnetic properties in oxo-bridged diiron complexes reminiscent of the cofactor found in the soluble methane monooxygenase enzyme. Chapter 5 employs DFT-based techniques to locate electrons in highly delocalized -systems of metalloporphyrins, shedding light on their influence on the Soret band of these complexes. Chapter 6 emphasizes the significance of the initial guess in studying reactivity, as we encountered challenges in achieving the desired C-H activation reactivity within a Nickel-halide complex, likely due to an erroneous potential energy surface minimum obtained. In Chapter 7, we demonstrate the usefulness of time-dependent DFT calculations in accurately predicting the UV-Visible spectra of high-valent iron-oxo DFT ∩ Transition-metal chemistry complexes, enabling their identification. Lastly, Chapter 8 investigates cooperative molecular nitrogen activation using a transition-metal Rhenium complex and Lewis acids, providing insights into the observed phenomena through the lens of molecular orbital theory. Overall, this thesis aims to showcase the enduring relevance and practicality of DFT methods in contemporary research. While employing DFT to elucidate chemical problems, we remain aware of its limitations and often employ alternative approaches to mitigate these challenges. ​
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