Transition Metal Complexes with Fluorinated Ligand Systems: Investigation of the magnetic and (spectro-)electrochemical properties

Abstract

This thesis presents the synthesis metal complexes with a variety of fluorinated ligand systems, featuring tripodal, terpiridine and azobenzene ligands. The resulting compounds thereof show various characteristics and therefore were investigated in terms of their geometric and electronic structures by single crystal X-ray diffraction, cyclic voltammetry, superconducting quantum interference device (SQUID) magnetometry and/or (spectro- )electrochemical methods. The first part of this thesis addresses new compounds of the form [M(L)2](BF4)2 (M = cobalt(II), iron(II); L = fluorinated tripodal ligand) and the influence of fluorine specific interactions in the secondary coordination sphere on the spin state of the central metal ion. For this purpose, a combined study by single X-ray diffraction analysis and SQUID magnetometry measurements was performed to investigate the spin states of the metal centers. It could be shown, that for the ligand, which contains either a pentrafluoro-benzyl or a 4-fluorobenzyl substituents, an effect of the fluorine specific interactions was observed. The complexes bearing the pentafluorobenzyl substituents do not display the expected!⋯! interactions, but an edge-to-face interaction. However, this leads to a change of the spin state for the iron(II) complex, while the corresponding cobalt(II) complexes remain in the low spin state. For the other substituent a partial spin crossover (SCO) behavior was observed for one cobalt(II) complex, depending on the co-crystallizing solvent, while the iron(II) complex displays a complete SCO. These results were obtained by a combination of X-ray diffraction analysis, SQUID magnetometry, electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. In the second part terpyridine ligands bearing different fluorinated backbones were implemented with cobalt(II) and iron(II) and the differences within the complexes were investigated primarily by SQUID magnetometry and EPR studies. The cobalt(II) complexes, where the tpy ligands bear a long fluorinated alkyl chain, show fluorine specific interactions that have an impact on the EPR spectra. Furthermore, ruthenium(II) complexes combining a mesoionic carbene and a terpyridine ligand were employed for electrocatalytic reduction of CO2. The complexes show a high selectivity towards CO and a faradaic efficiency of 92%. The final part deals with a series of platinum(II) donor-acceptor complexes bearing different azobenzene and bridging quinone ligands. The complexes exhibit strong electrochromic behavior and were studied towards the influence of different (perfluorinated) alkyl chains on the azobenzene ligands on the electrochemical behavior. In the present thesis the impact of fluorine on different ligand systems and its influence on the physical and electrochemical properties of the resulting metal complexes are highlighted, which can be useful for estimating the properties of potential target compounds.