Mononuclear non-heme iron enzymes featuring a 2-His-1-carboxylate facial triad active site have received particular attention primarily due the stunningly diverse oxidative catalytic transformations that are catalyzed by these enzymes. Many efforts by synthetic inorganic chemists have been devoted to biomimetic modeling studies of the facial triad enzymes in order to understand the immense oxidative power of these enzymes and to develop new synthetic catalysts. Initial studies have focused on the development of functional models of the different subfamilies and employed mostly polydentate all-N donor ligands. While impressive results have been obtained, limited effort has been devoted to the design of models that more faithfully reproduce the coordination environment of the 2-His-1-carboxylate facial triad, i.e. that contain combined N,N,O donor ligands.
In this project, we developed a new class of facial triad N,N,O ligands based on the bis(alkyl-imidazolyl)propionate (BAIP) architecture for the synthesis of bio-inspired mono-nuclear non-heme iron(II) complexes. The reactivities of the synthesized iron complexes towards the oxidative transformation of bio-relevant bidentate substrates have been examined.
Beside that, we have investigated the ligand-based redox chemistry of homoleptic cobalt and zinc complexes derived from a b-diiminate ligand (bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane anion; an intermediate in the synthesis of the bulky BAIP ligand) that is structurally very similar to widely studied b-diketiminate ligands. These complexes show four reversible/quasi-reversible single-electron oxidations that are all ligand based. The anionic and oxidized pi-radical ligand containing cobalt complexes were synthesized and characterized by a number of spectroscopic and computational techniques that corroborated their ligand-based redox chemistry.
The design and development of highly efficient as well selective catalysts is one of the major goals of research in chemistry. During the last decades, transition metal-catalysed transformations have been dominated by the use of precious metals, for example, Pt, Pd, Rh, and Ir, which are expensive, toxic and scarce. The desire to replace these noble metals by base metals such as iron and cobalt, which are earth-abundant and have no toxicity issues, as catalytic materials represent one on the biggest challenges in the fields of catalysis and synthesis. As part of the general endeavors of creating sustainable chemical production methods, the development and use of catalysts and catalytic reaction procedures is vital. The search for new homogeneous catalysts based on non-noble metals also includes the design of new organic ligands that allow the properties of the catalysts to be tuned and optimized.
Protocols for the design of new sterically encumbered BAIP-ligands for the generation of highly reactive 1:1 Fe/ligand complexes that mimic the bio-inspired mono-nuclear non-heme iron(II) enzymes are the first objective of this project. Next, the physical properties of the resulting iron complexes are an important objective, as these provide insight in the nature of the overall complex and help to correlate their chemical reactivity to their structural design. Another important objective is the study of the chemical reactivity of the iron complexes towards organic substrates. Insight in this reactivity provides a means to link the properties of the iron complexes to those of the non-heme iron enzymes, which they are meant to mimic, and also to investigate their properties as man-made catalysts for important oxidation reactions.
