Synthesis and investigation of copper(I) complexes with a thioether ligand system

Abstract

The selective oxidation of organic molecules plays an important role in the chemical industry for many years. Most reactions that use molecular dioxygen or alternative oxidants require a catalyst for the oxidation of organic substrates. Although many processes are long established in the chemical industry, catalysts are far from achieving 100% selectivity of the desired target products. Harsh reaction conditions, high resource consumption, and the formation of numerous by-products reflect several disadvantages that can arise during most reactions. Inspired by the reaction behavior of copper-containing enzymes, which can activate atmospheric dioxygen and selectively transfer it to organic molecules under mild reaction conditions, the focus is to reproduce the reaction behavior of copper-containing enzymes with oxidants, such as dioxygen, by synthesizing model complexes and stabilizing the oxygen species by appropriate ligand systems. A particular focus was placed on the PHM enzyme and its active site (end-on copper(II) superoxido complex). In the first part of this research, based on the TMPA and TMG3tren ligands, different sulfur-containing N3S ligands were synthesized, and the corresponding copper(I) complexes were investigated in regard to their reactivity towards dioxygen using low-temperature stopped-flow technique. While the reaction of the copper(I) complexes with the TMPA derivatives showed reversible dioxygen binding with the formation of a trans-μ-1,2-peroxido species at −80 °C, specific ligand modification of the copper(I) complex with the TMG3tren derivatives stabilized the end-on copper(II) superoxido complex at temperatures above −90 °C and allowed it to be kinetically studied using the stopped-flow technique. Similar systems known from the literature showed the formation of the end-on copper(II) superoxido complex at temperatures below −130 °C. The results show that even minimal changes to the ligand system can have a large effect on the reaction behavior and stability of the complex, and that oxygen species can be stabilized by specific ligand modification. In the second part of this research, the sulfur-containing macrocyclic ligand systems N2S2 and S4 were synthesized based on the cyclam ligand. This time, the corresponding copper(I) complexes showed no reactivity towards oxygen due to the increased stability of the copper(I) complexes by sulfur donor atoms. Due to this fact, ozone was investigated as another oxidant that is more reactive compared to dioxygen. Using the stopped-flow technique, it was shown that the copper(I) complexes form oxygen adducts with ozone at low temperatures. While the copper(I) complex forms a side-on peroxido copper(II) complex with the N2S2 ligand and ozone, the copper(I) complex with the S4 ligand reacts with ozone at −90 °C to form an end-on copper(II) superoxido complex, thus providing a model complex for the active site of the PHM enzyme. A possible mechanism for the reaction of the copper(I) complex with ozone was also established. The use of ozone as an oxidant offers the possibility of activating copper(I) complexes, which are inert to dioxygen, and form oxygen adducts.