Multifunctional Spin Crossover Complexes
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Abstract
The spin crossover phenomenon (SCO) describes the transition between the high-spin and low-spin state of octahedral 3dn (n = 4 – 7) transition metal complexes by temperature or pressure change as well as light irradiation. Highly cooperative systems with abrupt or even hysteretic transitions are discussed for the application as key elements in molecular switches, storage devices or sensors. The combination of the SCO phenomenon with further relevant aspects for such applications as for example electric conductivity, luminescence or other magnetic properties leads to highly valuable structures, that can be adjusted easily by changing the spin state. The research in the field of spin crossover has therefore focused on the investigation of the interplay between the spin transition and the named properties. In this context, this thesis provides an insight into the synergy between the spin crossover phenomenon and the magnetic exchange coupling as well as slow relaxation of magnetization of single-ion magnets (SIMs).
The influence of magnetic exchange interactions on the spin crossover behavior was analyzed in the first part of the thesis. Simple multinuclear copper(II) model complexes were synthesized and characterized by X-ray structure analysis as well as magnetic measurements to investigate the structural and electronic aspects of the magnetic exchange coupling and to transfer the information to the appropriate iron(II) and cobalt(II) spin crossover complexes. Therefore, two ligand systems based on 1,3,4-oxadiazole and bis-1,3,4-thiadiazole moieties were developed. The different denticity and number of donor atoms enabled the targeted formation of mono-, di- and polynuclear complexes with various magnetic exchange pathways.
In the second part, the synergy between the spin crossover phenomenon and the slow relaxation of magnetization was investigated on discrete trinuclear complexes. Therefore, two different synthetic methods were developed to combine spin crossover complexes with single-ion magnets. In both cases, a central cobalt(II) bis-terpyridine moiety serves as spin crossover complex which was functionalized according to the requirements of the synthetic approach.
The first method describes the linkage by using the well-known copper(I)-catalyzed azide-alkyne cycloaddition. Therefore, an azide substituent was introduced to the central cobalt(II) bis-terpyridine complex and an alkyne functionality to the cobalt(II)-based single-ion magnet. The obtained trinuclear cobalt(II) complex was characterized by IR and UV-Vis spectroscopy and the magnetic behavior was investigated by susceptibility and magnetization measurements.
In the second approach, the linkage of spin crossover complexes and single-ion magnets was performed by the usage of a ligand with two different coordination pockets. The central cobalt(II) bis-terpyridine spin crossover complex is formed by two ligands via the terpyridine moiety. Cobalt(II) or dysprosium(III) ions should be further coordinated by the 15 crown-5 coordination pocket to form the single-ion magnets.