Computational studies of metal complexes and organic transformations

Emily J Campbell

doi:10.25820/etd.007753

Density functional theory (DFT) is a computational method that can help understand chemical systems at an atomistic level. DFT is a quantum chemical method that is used to calculate the electronic structure of a given system. These calculations have been used in many studies to support and add to experimental understanding. Density functional theory is used to support findings from experiment, disambiguate findings, or elucidate details that are not always visible to experiment. In this thesis, there are four chapters that focus on density functional theory calculations conducted in collaboration with experimentalists. The first of these chapters (Chapter 2) investigates metal-ligand cooperative (MLC) binding of substrates to transition metal complexes. MLC binding is when a metal and ligand work together to activate small molecules that are bound to a metal complex. The first series of complexes studied contained a triaryl tetradentate ligand derived from o-phenylenediamine. This ligand is referred to here as N4 because it binds to metals via four nitrogen donor groups. Complexes containing N4 were modeled with six different first-row transition metals: vanadium, chromium, manganese, iron, cobalt and nickel. DFT was used to calculate the thermodynamics of carbon dioxide binding and investigate the underlying spin densities of various metal complexes containing these transition metals. It was found that high spin complexes favored MLC binding of CO2 while low spin complexes favored single atom binding of CO2 at the metal. The second series focused on PNP pincer complexes that were modeled with seven different transition metals: chromium, manganese, iron, molybdenum, ruthenium, tungsten and rhenium. The calculations identified that high spin vanadium, manganese, iron, cobalt and nickel pincer complexes are the most likely to bind CO2 in an MLC fashion, thereby guiding the efforts of our experimental collaborators. The manganese complexes were unique in this series because they could have antiferromagnetic or ferromagnetic coupling between the metal and bipyridine ancillary ligand; this was not present in the other transition metal complexes. The calculations revealed that changes in the Mn–P bond lengths correlate to the degree of antiferromagnetic and ferromagnetic coupling. Work described in the third chapter was aimed at understanding iron-iron interactions in diiron complexes synthesized using the N4 ligand described in Chapter 2 with flanking NMe2 groups, as well as a related N2S2 ligand containing flanking thioether donors (SMe). DFT geometry optimizations and molecular orbital analysis were performed to determine the presence of metal-metal interactions. Notably, the calculations predicted a lower energy structural isomer of the N4 diiron complex that was not observed initially but was discovered in follow-on experiments. The DFT calculations suggested that there may be a relationship between the field strength of the N4 and N2S2 ligands that lead to different iron-iron distances. Molecular orbital analysis revealed little overlap between the iron d-orbitals, suggesting weak (if any) metal-metal bonding. Additional calculations were conducted with CASSCF/CASPT2 which is known to do better than DFT in modeling metal-metal bonding interactions. CASSCF/CASPT2 calculations revealed that there was no overlap between the metal d-orbitals and no significant metal-metal bonding present. Additionally, a study on hypothetical diiron complexes containing alternative ligands with a range of donor strength for the ligands was completed to determine if there is a correlation between the donor strength and the iron-iron distance. One of the hypothetical diiron complexes had a N2O2 ligand with OMe flanking donors (vs. NMe2 and SMe for N4 and N2S2) and it was found to shorten the iron-iron distance. Overall, the study revealed that the iron-iron distance is significantly influenced by the identity of the flanking groups, which could lead more effective means to control metal-metal interactions and associated magnetic properties in bimetallic complexes. Chapter 4 investigates the exchange of two halogens between a halide from a boron trihalide and a fluorine from trifluorotoluene with the assistance of an iron(III) halide catalyst. DFT calculations were performed to model reaction energies and catalyst-substrate interactions to help elaborate the possible mechanism for halogen exchange. This work with the support of DFT calculations, indicate another route for fluorine substitution by determining possible pathways to functionalize trifluoromethyl groups further for uses in organic transformations. Chapter 5 describes DFT calculations aimed at determining the origin conjugation present in polythiazyl derived polymers (polyNAS). Polythiazyl is one of the first known polymer of sulfur-nitrogen with metallic-like properties.properties, like being electrically conductive. Geometry optimizations for oligomers and periodic systems of polythiazyl and polyNAS polymers were performed to analyze the geometries and optical properties. The structures between polythiazyl and polyNAS were similar, and it is believed that they have similar conjugation across the sulfur-nitrogen backbone. Time-dependent DFT was able to reproduce similar UV-vis spectroscopy data on polyNAS that was collected experimentally. Characterization of these new polymers indicate that they could be used for chemical sensing and is support by DFT calculations. The final chapter in this work summarizes the findings of each chapter as well as the future work for each project.

Computational studies of metal complexes and organic transformations

Files and links (1)

Abstract

Details

Metrics