The synthesis and characterization of new synthetic copper dioxygen adduct complexes had gained a lot of interest during the last years. The reason for the increased interest is based on the fact that these compounds can be regarded as model complexes for the reactivity of redox active copper enzymes. These enzymes had demonstrated that selective oxidations of organic substrates with air under ambient conditions are possible. These reactions are of high interest in the laboratory and in industry and therefore special attention has been focused upon the synthesis, characterization and the reactivity of copper dioxygen adduct complexes. This is also the topic of the research described herein and the results of the different projects have been separated in two parts: copper superoxo and peroxo complexes.
The amine tris(tetramethylguanidino)tren (TMG3tren) has been synthesized previously as an optimized ligand system for the preparation of end-on coordinated copper superoxo complexes. TMG3tren consists of an aliphatic tren backbone that shows sterical crowding due to the guanidine groups and additionally has electronic properties of N-donor atoms between imine and amine groups. After a large number of experiments, it was finally possible, in the course of this work, to obtain the reactive intermediate [Cu(TMG3tren)O2]+ in form of single crystals at -80 °C. Therefore, the structural characterization of this compound is the first X-ray proof for a synthetic copper superoxo complex with an end-on coordination. Studies of the reactivity showed ligand hydroxylation analogous to the copper enzyme PHM. This type of ligand hydroxylation was observed for the first time for synthetic model compounds. During the reactions of the superoxo complex with a number of mono- and diphenolic substrates the corresponding oxygenated and oxidized products were formedFor the synthesis of end-on coordinated peroxo complexes tripodal amine ligands (based on substituted tren) have been successfully used in the past. Tris(2-dimethylaminoethyl)amine (Me6tren), bis(2-dimethyl-aminoethyl)(2-pyridylmethyl)amine (Me4apme), 2-dimethyl-aminoethyl)bis(2-pyridylmethyl)amine (Me2uns-penp) and tris(2-pyridyl-methyl)amine (tmpa) are ligands with different types of N-donor atoms. If the aliphatic N-donor atoms of Me6tren are substituted in a systematic way by introducing pyridine N-donor atoms finally the ligand tmpa is obtained.
Based on these four ligands, the influence of the N-donor atoms on the reactivity of dioxygen binding of the according copper(I) complexes was investigated using stopped-flow techniques. For these studies, the corresponding copper(I)-complexes were synthesized and reacted with dioxygen at low temperatures. Furthermore, the complex [Cu2(Me6tren)(O2)](BPh4)2 could be structurally characterized.
Isolation of the four peroxo complexes in the solid state without using solvents led for the first time to room temperature stable compounds. This is quite remarkable, because in the past these complexes have only been known as extremely sensitive complexes which only persist at low temperatures. Using tetraphenylborate as anion, we could increase the decomposition temperature to more than 70 °C. The influence of the ligands was demonstrated in the different decomposition temperatures. The complex with tmpa has a decomposition temperature above 110 °C and therefore is significantly more stable than the according Me6tren complex with a decomposition temperature around 70 °C.
Preliminary reactivity studies of these complexes in the solid state showed successful oxidations of organic compounds. For example, the oxidation of toluene at room temperature to benzaldehyde and benzyl alcohol was performed.
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