Structure-Property Relationships of Iron N-Heterocyclic Carbene Complexes : Synthesis, Characterization and Application

Abstract: Resulting from the general availability of first-row transition metals, compared to their second- and third-row counterparts, photosensitizers (PSs) based on metals like iron have gained interest as candidates to enable photochemistry from solar energy, a largely underutilized energy source. Iron complexes equivalent to other metal complexes, currently being used as PS, do lack some of the advantageous properties required to challenge the established PSs.In this thesis the investigations on the reactivity and properties of heteroleptic Fe-NHC (N-heterocyclic carbene) complexes is described. A group of new complexes have been synthesized and characterized in an attempt to better understand their structure-property relationships and how synthetic design can aid in the search of complexes with properties desired for photochemical applications.In chapters 2-4 the synthesis of iron complexes, mostly based on previously reported synthesis of heteroleptic Fe-NHC complex, is described. The effects of structural changes in NHC ligands as well as functionalization of 2,2’-bipyridine (bpy) ligands are investigated. Introducing methylene spacers between the NHC units of a bidentate ligand influences the geometry of the complex in an unexpected manner, while functional groups on bpy affect complex properties in a subtle, predictable fashion. For a few of the complexes ground state characterizations reveal that at least two low-energy structural conformations exist in a dynamic equilibrium, slowed down by lowering the temperature. Many of the complexes can be oxidized from oxidation state Fe(II) to Fe(III) by atmospheric oxygen which changes their optical properties.In the last part of the thesis, application of an Fe-NHC complex as a photoredox catalyst to drive a chemical reaction with green light is presented. While irradiation of lower energy and milder conditions than for conventional methods to catalytically perform the reaction are needed, this photoredox catalyst shows that Fe-NHC complexes can offer value as unconventional photocatalysts. With complexes featuring carboxylic acid anchor groups, photovoltaic performance can now be investigated as well. Its broad absorbance over most of the visible part of the electromagnetic spectrum is promising but rather short excited state lifetime might limit applicability.