The Rybak-Akimova Group

Biomimetic Catalysis

Non-heme iron-containing metalloproteins are crucial for many forms of aerobic life, as they are responsible for oxygen binding and transport as well as oxygen activation and functionalization. We seek to mimic not only the structures of these efficient enzymes but also their functions. An ongoing theme in our group’s research involves the study of redox active iron centers supported by non-heme/macrocyclic ligands and their ability to activate small molecules, namely oxygen and hydrogen peroxide, for “green” oxidative coupling with organic molecules. The biological significance of iron in oxygen activating enzymatic systems makes it a prime target for biomimetic studies and from our knowledge of these natural systems we can potentially design novel synthetic complexes that exhibit enhanced reactivity, efficiency and substrate specificity. Utilizing the principles of metalloenzymes allows for manipulation of their catalytic activities in the laboratory to achieve selective catalytic transformations of organic molecules with the benefit of producing nonhazardous by-products. Discovering and understanding such biomimetic systems would not only further scientific knowledge but also broaden the scope of cheap, efficient, selective, and environmentally friendly oxygenation catalysts that are available. Some of our ongoing work in this field is highlighted below.

Aromatic Hydroxylation by a Non-Heme Fe(II) Complex

In this work we employ a biomimetic non-heme iron(II) complex supported by a tetradentate aminopyridine ligand containing two cis-oriented labile sites allowing for oxidant and substrate to bind and react. This is similar to the structure of the active site of Phenylalanine Hydroxylase (PheH), an iron-containing metalloenzyme responsible for converting phenylalanine into tyrosine. Our model complex closely mimics the function of PheH as well. Our main interest is to understand the mechanism of the reaction, with emphasis on generation and characterization of the iron-based intermediate(s) that are responsible for aromatic hydroxylation.

We rely on stopped-flow methodology with UV-visible spectrophotometric detection as well as other spectroscopic methods to study and identify transient colored intermediates that form over the course of the reactions.

Enantioselective Epoxidations with Chiral Non-Heme Fe(II) & Mn(II) Complexes

A great number of bio-inspired mononuclear Fe(II) complexes have been reported as efficient catalysts for olefin oxidation, especially those supported by PYBP ligands. Catalytic and kinetic studies indicate that the Fe(II)PYBP complex is quite promising for selective olefin epoxidation, affording up to 200 turnovers of cyclooctene into cyclooctene oxide exclusively within only 5 minutes at room temperature. We are currently extending this work to include enantioselective epoxidations by preparing and isolating the enantiomeric forms of the PYBP ligand as well as introducing other bio-friendly, redox active transition metals.

Peroxide Activation & Catalytic Epoxidation with Fe(II) Aminopyridine Macrocyclic Complexes

Aminopyridine macrocycles with functionalized pendant arms represent an interesting class of ligands that offer exquisite control over the equatorial coordination environment of the redox center while simultaneously providing an additional donor atom and/or an intramolecular proton delivery pathway. The study of hydrogen peroxide activation by our iron(II) complexes is essential to understanding the mechanism of catalyzed epoxidation and enables the development of green oxidation catalysts. The nature of the added functional group can dramatically alter the geometric and electronic properties of the metal center and we seek to understand what structural features will help tune the catalytic activity of metallomacrocycles. Subtle changes in the functionalized pendant arms has led to drastic differences in the catalytic abilities of these complexes!