Amphiphilic Spin Crossover Complexes

The aim of the present thesis was the synthesis and characterization of amphiphilic iron(II) spin crossover complexes. The prime attention was on the self-assembly ability of the synthesized compounds and the influence of the supramolecular arrangement in the solid on the SCO behaviour. The used ligand system is based on Schiff base-like Jäger ligands that are suitable to realize iron(II) SCO compounds. The ligands were modified such that alkyl chains with different lengths were attached to the outer periphery. The newly established eight-step synthesis of the complexes comprises alkylation of catechol, subsequent nitration in para-positions and reduction of the nitro functionalities resulting in the diamino compound. Further reactions with different keto-enol ethers gave the ready ligand that reacts with iron(II) acetate to iron(II) complexes with methanol in axial positions. In order to shift the iron centre into the right energy region to enable thermally induced SCO, in the last step the methanol was replaced by N-donor ligands like pyridine and its derivatives such as apy and dmap, or by bpea, bpee and bpey which are bridging the iron centres, to provide the aimed [N4O2] coordination sphere. Structural investigations using single crystal X-ray structure analysis gave a detailed insight into structure-property relationship of the synthesized complexes. The mechanism of the SCO of [FeLa(C16)(py)2], a complex with the chain length of 16 carbon atoms and pyridine in axial positions was elucidated. The molecules organize in lipid layer-like arrangement where the iron centres point to each other. By this, a hydrogen bonding network between the polar heads was formed with co-crystallized H2O, what is partially responsible for the 47 K wide hysteresis. It was demonstrated that, despite of long alkyl chains, abrupt ST with hysteresis is possible and that the cooperativity of SCO depends on the self-assembly of the amphiphiles. This concept was confirmed by comparison with complexes bearing short C8 alkyl chains. The self-assembly is unpredictable resulting in different coordination geometries and crystal packings without lipid layer-like arrangement. The STs of the octahedrally coordinated modifications of [FeLc(C8)(dmap)2] in the solid were found to be almost identical to that of the same compound in solution what proved the absence of cooperative effects in the crystal. Investigations on pyridine complexes with different chain lengths (8, 12 and 16 carbon atoms) and substituents at R1 and R2 showed that the substituents influence the ST temperature T1/2 but not the different chain lengths. Temperature-dependent paramagnetic 1H NMR investigations confirm that the SCO behaviour in the solid is dominated by packing effects. Attempts to synthesize coordination polymers or to coordinate bigger axial ligands with a lipid layer-like structure resulted in crystallization of dinuclear, penta-coordinated or other SCO inactive complexes. Extensive investigations on the compounds using X-ray structure analysis led to a concept enabling the prediction of crystallization behaviour depending on the chain length and the dimensions of the polar head group, named self-assembly parameter, sap = (H+B)/L. H and B denote the height and the broadness of the polar part and L the entire length of the complex. When sap ≈ 1, lipid layer-like arrangement can be expected. This concept was also applied to other amphiphilic systems. In order to coordinate sterical more demanding axial ligands, the chain length was elongated from 16 to 22 carbon atoms and the synthesis pathway of the complexes was adjusted. This resulted on the one hand in the crystallization of the complex [FeLd(C22)(dmap)2] which crystallized despite of sterical demanding substituents octahedrally with dmap in the layered structure motif. On the other hand, coordination polymers with bpea, bpee and bpey were synthesized that show all abrupt SCOs above room temperature. It could be demonstrated that cooperativity as well as T1/2 can be increased choosing more rigid axial ligands. The three compounds organize in spherulites after warming which can be observed between crossed polarizers.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Schlamp, Stephan

Contributors dc:contributor

• Weber, Birgit

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1718/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1718