Activation of small molecules with iron(II)- and copper(I)-complexes

Abstract

The studies on the reactivity of dioxygen and nitrogen monoxide with transition metal model complexes provide important insights for a better understanding of similar processes in biological environments. Due to its short-lived nature in the human body the physiological significance of nitrogen monoxide compared to dioxygen has only been known for a relatively short time. Accordingly, there are still many unanswered questions regarding the reaction mechanisms of nitrogen monoxide in biological processes, as well as its potential as a drug and for treating various diseases. Dioxygen, on the other hand, is already well studied due to its importance as an oxidizing and oxygenating agent in the human body. Therefore, the focus is to mimic the reactivities of enzymes by model systems to perform selective and catalytic/stoichiometric oxygenation reactions under mild reaction conditions in the laboratory or for an application in industry. During this research, various iron complexes were kinetically investigated for their reactivity towards nitrogen monoxide in methanol using low temperature stopped-flow technique to gain a better understanding of the mechanisms of such reactions. The used systems had different complex coordination environments, starting with the "simple" iron(II) chloride, via the mononuclear complex iron-bztpen to the dinuclear iron-HPTB. It was shown that in the MNIC/DNIC system the reaction to the mononitrosyl complex (MNIC) is, as expected, very fast, independent of the nitrogen monoxide concentration and proceeds via a dissociative interchange mechanism. The further reaction to the dinitrosyl complex (DNIC) proceeds very slowly, also in comparison to the other complexes studied (reaction rate to MNIC differs by a factor of 109). This is most likely related to the fact that not simply a second nitrogen monoxide molecule coordinates, but at the same time electron transfers take place and a conversion from {FeNO}7- to {Fe(NO)2}9-species occurs. In the study of the iron(II) complexes with the ligands bztpen and HPTB, it was shown that the reaction proceeds via an associative mechanism. In the case of the iron-HPTB complex, the activation parameters are in accordance with data from a similar complex in the literature. The activation of dioxygen by means of copper(I) complexes modelled on natural systems offers the possibility of replacing time-consuming or expensive synthetic routes by simple methods. The clip-and-cleave concept, which was already introduced by Becker et al., provide a pathway for a selective and intramolecular ligand hydroxylation. During these studies, this concept was examined and optimized by modifications on the ligand scaffold, changes in reaction conditions, and investigations of the hydroxylation mechanism. This investigation could show that even small changes on the ligand scaffold have a great influence on the reactivity of the complex and can lead to an increase, but also to a complete suppression of the hydroxylation reactivity. It was shown that hydroxylation of the different ligands (even with small differences) leads to the formation of different copper-dioxygen intermediates or even shows no intermediate at all despite successful ligand hydroxylation. Using cyclohexanecarbaldehyde as a substrate, it could be shown that in addition to the two known mechanisms for copper-mediated hydroxylation (via a bis(μ-oxido) or hydroperoxido complex), there is a third possibility for ligand hydroxylation starting from a copper(II) complex, which represents a promising, alternative and facile pathway for further oxygenation reactions. Hydroxylation of these substrates and also the various hydroxylation methods provide the opportunity for a stoichiometric and easy preparation of specialty chemicals or functionalization of difficult to access sites of substrates.