In several oxidoreductase metalloenzymes, organic cofactors are transiently converted to radicals in order to achieve efficient catalytic turnover. This is, for example, the case of a tyrosine residue in galactose oxidase. Similarly, small-molecule catalysis can be improved when a transient radical ligand is involved. The present work aims at tailoring the structure of three classic ligands to make them redox-active, i.e. active participants in the electronic structure and reactivity of their complexes upon oxidation. The approach is
to render an aromatic moiety on the ligand electron-rich by addition of NMe2 substituents.
The first chapter describes
a neutral Ni(II) complex
with 1. This complex
undergoes two reversible
oxidations that were
characterized by electrochemistry, electron paramagnetic resonance spectroscopy, optical spectroscopy and density functional theory. These oxidations are ligand-based, producing a radical ligand and a diiminoquinone successively.
The second chapter investigates Cu(II) complexes of 1 and 2, for the bio-relevance of Cu in several oxidoreductases. Both ligands oxidize to a radical that is ferromagnetically coupled to the unpaired electron of Cu(II). Spectroscopic characterization and theoretical calculations provide a description of the electronic structure of the oxidized complexes. The third chapter looks at a Co(III) complex of 3, which is electron-rich and undergoes redox processes at low potentials. The optical absorption behaviour of the oxidized products is similar to that of the free ligand, suggesting ligand-based oxidation.
As a whole, this thesis describes the effect of strongly electron-donating ligand substituents on the redox and electronic properties of a metal complex. The design leads to an easy strategy by which to tune the electronic structure of a complex by controlling the redox properties of its ligand.