Authors:	Köhler, Christian
Title:	Spin crossover behavior of 1,3,4-oxadiazole based dinuclear iron(II) complexes und functionalized phosphonates as bridging ligands in inorganic-organic hybrid-materials
Online publication date:	24-Jun-2016
Year of first publication:	2016
Language:	english
Abstract:	The change between the electronic states high-spin and low-spin is called spin crossover or spin transition. The transition can be induced by a change in temperature, pressure, or the irradiation with light. The requirement for such properties is a 3d transition metal ion with the electronic configuration 3d4 to 3d7. Crucial for the switchability is a coordination environment that can stabilize both, the High-Spin and Low-Spin state. Therefore, the ligand field has to have the appropriate strength to split the d-orbitals up to the point, where the preferred state is determined by the external stimuli. One of the most studied metal ions in this matter is probably iron(II) in an octahedral coordination geometry. Especially dinuclear compounds are of great interest because both, intermolecular and intramolecular interaction can be observed. Furthermore, a third mixed [HS-LS] state can occur. Herein, several new compounds based on a symmetric 2,5-disubstituted 1,3,4-oxadiazole ligand are reported, that were synthesized using different iron(II) metal salts. With single crystal X-Ray diffraction, Mössbauer-spectroscopy and magnetic measurements, the spin crossover properties of the novel complexes were investigated. The measurements revealed a strong dependence on classical and non-classical hydrogen bonds between the complex cation and the counter ions: a change of the hydrogen bond acceptor at the counter ions results in a different spin crossover behavior. Additionally, the number of contacts between a cation and its next neighbors via hydrogen bonds mediated by the counterions has a strong influence on the nature of the spin transition. The steric hindrance and flexibility of the ligand has to be considered as well. Compared to analog complexes based on thiadiazole and triazole ligands, they differ not only on a structural level, but also significantly in their spin crossover characteristics. The origin for these changes is found in the substitution of the hetero atom in the five membered ring. This new family of oxadiazole based dinuclear iron(II) compounds is an ideal model system to illustrate the diversity and complexity of spin crossover compounds and pronounce the important part of intermolecular contacts. The utilization of magnetic materials has become an essential task in modern materials science. A great deal of applications is based on the unique properties which come along with unpaired electrons. The most relevant application, and probably one of the most influential discoveries during the last century, is the addressability of ferromagnetic materials, which enabled the storage of data in the information technology. Even though the employed materials and methods have experienced a massive improvement during the last decades, the basic concept is to date the standard in regard of data storage. Among magnetic materials, the addressability of ferromagnetic compounds is only one of many properties, which hold the potential for new groundbreaking applications. The requirement to observe such phenomena is the presence of unpaired electrons. Especially the 3d transition metal ions and the 4f rare earth metal ions satisfy these requirements. Compounds, which incorporate several different or both types of elements, can consequently benefit from both advantages they bear: large spin ground states and magnetic anisotropy of the 4f metal ions and strong magnetic coupling of the 3d transition metal ions. In this part of the work at hand, the preparation of novel heterometallic coordination complexes based on multi functionalized ligands forming extended more-dimensional networks is investigated. Offering a high diversity in coordination modes and a broad spectrum of metal ions which can be coordinated, the phosphonate group is the central motive in all presented ligand system. In order to increase the probability of the formation of heterometallic compounds, a second functionality is part of the ligand as well, which provides a different coordinative environment. Using these ligands in combination with various metal salts, several homometallic and heterometallic extended networks incorporating 3d transition metal ions as well as 4f rare earth elements could be obtained. The reactions outcomes were controllable by the employment of various crystallization techniques and the utilization of the pH value. The complexes were structurally characterized by single crystal X-ray diffraction. To investigate their magnetic properties, temperature dependent measurements of the magnetic molar susceptibility and the magnetization were conducted.
DDC:	540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-4064
URN:	urn:nbn:de:hebis:77-diss-1000005407
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Extent:	164 Seiten
Appears in collections:	JGU-Publikationen