The Stabilization of Reactive Copper Complexes Using 1,4,7-Triazacyclononane Derivatives Open Access

Karahalis, Gregory (2016)

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Copper dioxygen complexes are the subject of immense study due to their role in a wide array of enzymatic processes, from dioxygen transport to O-atom insertion into both aromatic and aliphatic C--H bonds. However, due to the highly reactive nature of many copper dioxygen systems, study outside of proteins significantly departs from the aqueous solvent and ambient temperatures in which enzymes operate, often requiring low temperatures and organic solvents to avoid complex decay. This report describes a method using the oxidatively robust ligand, 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane (tBu3tacn), to support a copper complex capable of forming a copper dioxygen adduct under ambient conditions. The complex is well-characterized as a dinuclear copper μ-η2:η2-peroxo complex, with support for this structure deriving from x-ray crystallography, electronic absorption spectroscopy, and resonance Raman spectroscopy. The dioxygen complex displays a room temperature half-life of 9.6 days in water with dibasic sodium phosphate, representing the highest solution stability outside of a protein. The dioxygen complex is capable of catalytic aerobic reactivity with 3,5-di-tert-butylcatechol, 2,4-di-tert-butylphenol, and benzoin, but is incapable of phenol oxidation in aqueous conditions. Therefore, to expand the study of reactive copper complexes, the synthesis of derivatives of 1,4,7-triazacyclononane is detailed, including modification of the synthesis of tBu3tacn.

Table of Contents

I. Introduction 1

A. Modeling Copper Dioxygen Chemistry in Metalloenzymes 1

B. Investigation of the Synthesis of 1,4,7-Triazacyclononane Derivatives 11

II. Results and Discussion 14

A. Synthesis and Characterization of a Copper Dioxygen Complex 14

B. Reactivity of Copper Dioxygen Complexes 22

C. Modification of the Synthesis of 1,4,7-Tri-tert-butyl-1,4,7-triazacyclononane 32

D. Exploring Modularity in the Synthesis of Derivatives of 1,4,7-Triazacyclononane 37

III. Conclusions 45

IV. Future Directions 46

A. Synthesis and Utility of Tethered Tacn Derivatives 46

B. Ambient Electrochemical and Chemical Reduction of Peroxo Complexes Toward the Active Site of Particulate Methane Monooxygenase 47

C. Investigation of Steric Modularity of Tacn on Copper Dioxygen Cluster Formation, Stability, and Reactivity 54

D. Stabilization of a Mononuclear Copper Nitrene 61

V. Supplementary Information 68

A. General Considerations 68

B. Experimental Procedures 69

C. Resonance Raman Spectra 89

D. UV-Vis Spectra 92

E. NMR Spectra 101

VI. References 122

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