Transition metal complexes with tridentate ligands : a variety of properties

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 Jager 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…