title: Transition metal complexes with tridentate ligands – a variety of properties
Creators: name: Dankhoff, Katja
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: magnetism
subject: iron
subject: copper
subject: spin crossover
subject: coordination compounds
subject: tridentate ligands
subject: 540

description: The aim of this thesis was the synthesis and characterisation of new Cu(II), Zn(II), Fe(II) and Fe(III) complexes with tridentate, Schiff base-like ligands. Their magnetic behaviour (Cu and Fe) was investigated, as well as their catalytic activity (Zn), and cytotoxicity (Cu). The ligands are derived from the Jäger type; those are normally rigid, tetradentate, and provide an N2O22− or N42− coordination sphere around the metal centre. The tridentate N2O ligands on the other hand are more flexible due a methylene group. The coordination geometries (e.g. square planar/pyramidal, trigonal bipyramidal, octahedral) are similar to those realised by tetradentate ligands, but the coordination of additional co-ligands (anions or solvent molecules) in cis position is possible. Another advantage of these ligands is the enhanced stability of octahedral, mononuclear complexes compared to those derived from the tetradentate ligands. The synthesis of the ligands was established and carried out in one step by condensation of 2-picolylamine and the corresponding keto-enol ether. Fe(II) and Fe(III) complexes were synthesised and characterised with regard to their potential spin crossover behaviour. The coordination geometry is octahedral and in case of Fe(III) as central metal atom varying anions were used to determine their influence on the spin transition. The single crystal X-ray structures of five Fe(III) and one Fe(II) complex could be obtained. The Fe(II) compounds stay mostly high spin, the majority of Fe(III) complexes on the other hand show SCO behaviour. The transition from HS to LS is mostly rather gradual over a large temperature range, indicating low cooperativity between the metal centres. In the case of [Fe(L1)2]ClO4 a parallel fourfold aryl embrace interaction was found in the crystal structure of the complex. Therefore the packing is very dense and the volume change required for a SCO is prevented. The isostructural pair [Fe(L2)2]ClO4 and [Fe(L2)2]BF4 allowed the direct evaluation of the size of the anion on the transition temperature. Both complexes show an abrupt ST which is shifted to lower temperatures for the larger perchlorate anion. Strong hydrogen bonds from a methyl group of one ligand to the keto group of another ligand explain the abrupt SCO. No direct influence of the anion on the SCO behaviour was seen in the other cases. The electrochemical properties of the Fe complexes were measured, quasi-reversible processes between −0.40 and −0.51 V (vs. Ag/AgNO3) take place, corresponding to the redox process Fe(II) ↔ Fe(III). The values are independent of the oxidation state of the starting material. The Cu(II) complexes with varying anions were synthesised as well. Single crystal X-ray structures revealed that most of the compounds crystallised as dimers, with the Cu(II) centres coordinated by one tridentate ligand and connected via the anions. This resulted in a square pyramidal coordination sphere. It was found that anions with more than one donor atom (such as acetate or nitrate) coordinate mostly with only one of those. The magnetism of the compounds were investigated as dimeric Cu(II) complexes can show magnetic exchange interactions like superexchange. In almost all cases either weak ferro- or antiferromagnetic interactions were found and no direct relation between the structure and the magnetism could be established. The complex [(μ-1,1-NO3)(μ-1,3-NO3)(CuL1)2] showed a rather strong superexchange, which can be explained with the slightly different structure of the compound. One of the two nitrate anions is bridging the metal centres with two instead of one oxygen atom. This results in a larger bridging angle for the other anion and therefore a better overlap of the p orbital of the oxygen and the magnetic orbital of the Cu(II) centres. Not only the magnetic properties of the compounds were of interest, the potential of 18 Cu(II) substances as anticancer agents was investigated as well. Complexes with different side chains were chosen and additional substituents at the pyridine ring were introduced. Most compounds showed moderate activity against the tested cancer cell lines with IC50 values between 10 and 50 µM. Two complexes with methoxy or methyl groups in 4-position on the pyridine ring and only ester groups on the chelate cycle were very active with IC50 values below 10 µM. The closely related compounds with a cyanide side chain on the other hand showed no activity, pointing towards a combination of steric and electronic effects. The possible mechanism of action of those complexes was investigated. No correlation with the formation of reactive oxygen species could be detected, but the inhibition of the enzyme topoisomerase I, which plays a crucial part in the supercoiling of the DNA, was found. It was found that the Zn(II) complexes are capable of catalysing the ring opening polymerisation of lactide. The dimeric compounds were obtained by the reaction of zincacetate and the tridentate ligands. The metal centre is coordinated by one tridentate ligand and two acetates are bridging the two zinc atoms. The complexes were tested with regard to their catalytic behaviour in the ROP of non-purified rac-lactide in melt at 150 °C. A coordination-insertion mechanism was proposed and the resulting molecular weight of the polymer in combination with end group analysis revealed that the monomeric species is the catalytically active one. This also explains an induction phase in the beginning of the polymerisation. The cytotoxicity of one complex against five different cell lines was investigated. With IC50 values &gt; 100 µM the compound can be considered non-hazardous to health.