Metalloenzymes, as natural catalysts, can break a reaction with high activation energy to multiple small reactions with lower activation energies; in these small reactions, multiple bonds form and dissociate rapidly, some metalloenzymes use redox-active ligands in their active site to provide the needed electrons for these multiple small reactions. Additionally, in the secondary coordination sphere in the vicinity of the metalloenzymes, the hydrogen bonding interactions facilitate the transfer and interaction of the substrates and the stabilization of the active intermediates. These unique features in metalloenzymes inspired chemists to design molecular models that can partially mimic the structure and reactivity of the metalloenzymes. This approach leads to the development of more efficient catalysts and opens the way for beyond enzymatic reactivity in synthetic chemistry. On the other hand, efforts on the study, recognition and, quantification of biological systems are limited by their complex structures that were evaluated during years; the bio-inspired molecular design is beneficial in this regard too because of the simple and rational design in their structure.
Inspired by the work from the Borovik lab and extensive reports on redox-active molecular systems, a family of bidentate, first-row transition metal complexes have been synthesized and fully characterized. The combination of the redox-active ligand scaffold, first-row transition metal center, and intramolecular, multi-center hydrogen bonding interaction made the metal complexes a novel case study in structure and chemical reactivity. The effect of the different factors, including metal center, ligand scaffold, the solvent of the crystallization, and counter cation on the geometry of the metal complexes in the solid state, has been studied. Based on the results from X-ray crystallography, an intramolecular multicenter H-bonding interaction has been recognized in the structure of the metal complexes that are directly dependent on the adopted geometry of the metal complex, so by the control of the factors involving in the reaction progress, the ability to tune the secondary coordination sphere is achievable. A series of spectroscopic methods, including UV-vis, electron paramagnetic resonance, and cyclic voltammetry, have been done to generate and quantify the various oxidation states of the copper complexes. Inspired by the structure and function of the galactose oxidase, some of the copper complexes' reactivity was tested and showed an excellent reactivity in catalytic oxidation of alcohols to corresponding aldehydes. The mechanistic studies revealed that the reaction selects a pathway different from the known mechanism of the galactose-like alcohol oxidations. By the change in the electron-donating properties of the ligand scaffold, isolation, and crystallization of a higher oxidation state of the copper complex were successfully done at room temperature and an aerobic condition. The fully oxidized copper complex applied as a catalyst in hydrogen atom abstraction from 2,6-di-tert-buthylphenols.
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Rajabimoghadam, Khashayar, "Structure and Reactivity of The First Row Transition Metals Bearing Redox-Active Ligands and Tunable H-Bonding Interactions" (2021). Chemistry Theses and Dissertations. 27.