Synthesis and Investigation of Transition Metal Complexes With Small Pendant Arm Macrocycles

[摘要] This work is centred upon the first row transition metal complexes formed with pendant arm macrocycles. The pendant arms in question are alkyl chains containing hydroxyl functions, they are substituted at the 2 position with either methyl or isopropyl groups there by creating a chiral centre on each arm. The ligand L1H3 has three tertiary nitrogen and three alcohol functions, it is thus potentially hexadentate (N3O3). The chromophores of the resulting complexes exhibit varying degrees of trigonal distortion and are highly suitable for investigation using absorption and circular dichroism spectroscopy. The donor properties of the pendant hydroxyl groups are pH dependant. At low pH values they remain protonated acting as alcohol ligators; increasing the pH leads to deprotonation forming alkoxide donors. L1H3 is able to stabilise many different oxidation states. It forms extremely stable complexes with V(IV), Cr(III) and Mn(IV). In the presence of low (divalent) oxidation levels the pendant groups, in neutral solution, remain protonated and monomeric complexes of the form [ML1H3]2+ predominate. With higher oxidation states, e.g. Mn(IV) the superior Lewis acid properties of the metal leads to an increase in the acidity of the hydroxyl protons. Even in neutral conditions, deprotonation occurs yielding a tris alkoxide species [ML1]+. This unit forms strong hydrogen bonds with protonated species, e.g. resulting in dimer formation: [ML1H3L1M]3+ where the metal ions are in +2/+4 or +3/+3 oxidation states. Dimer formation is prevented by increasing the steric bulk about the O,O',O" face. This is achieved by replacing the pendant arm methyl substituents as on L1H3 for isopropyl groups i.e. L2H3. A monomeric complex [Mn(IV)L2]+ is formed under aerobic conditions. Modified "parent" macrocycles are discussed where one nitrogen donor is replaced by a neutral oxygen group -O-, creating an N2O ring system. Placing pendant alcohol groups on the remaining nitrogen donors creates a potentially pentadentate N2O3 ligand L4H2. Spectroscopic studies on complexes with this ligand are presented.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry, Inorganic chemistry, Molecular chemistry