Ferrocene-phenol conjugates – secondary structures and reactivity
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Abstract
The investigation of electron transfer reactions, especially in combination with proton transfers − so called proton coupled electron transfers − is of considerable interest from a chemical and biological perspective. For example, in photosystem II, the electron transfer from the oxygen evolving complex is achieved via a proton coupled electron transfer to a tyrosyl residue. Successful operation of photosynthesis is influenced by many factors. The presence of highly reactive intermediates and the complicated environment, however, render the investigation of such processes in vitro quite difficult, despite of their utmost importance. Accordingly, especially proton coupled electron transfer reactions between organic and organometallic redox centers lack experimental studies.
The successful syntheses of several amide-linked conjugates of ferrocene and phenol moieties allows a facile insight into the influences of intramolecular hydrogen bonding on the redox potentials of both redox centers (ferrocene vs. phenol(ate)) and the stability of the respective valence isomers and tautomers. Intramolecular proton coupled electron transfer via light excitation is evidenced by the presence of a characteristic NIR intervalence charge transfer band for a mixed-valent ferrocenium-phenolate involving proton movement in the intramolecular hydrogen bond according to DFT calculations. The influence of the lack of intramolecular hydrogen bonds is illustrated by an appropriate ferrocene-phenol conjugate, showing rapid follow-up reactions under alkaline and oxidative conditions. Cooperative hydrogen bonding is furthermore presented for a series of conjugates varying in substitution pattern and hydrogen bond donating/accepting capabilities, respectively. The mutual influence of one or more intramolecular hydrogen bonds on the stability of secondary structures and on the redox behavior is presented.
In a second project, the general behavior of ferrocenium compounds in an alkaline medium is investigated. Possible degradation pathways of ferrocenyl compounds are in focus of pharmaceutical research, due to the high efficiency of ferrocene-containing (pro-)drugs in the treatment of malaria and some types of cancer. The increased efficiency of such pharmaceuticals in comparison with similar ferrocene-free compounds is at least partially assigned to the enhanced redox behavior of ferrocene and its derivatives in contrast to purely organic substrates. Besides the formation of reactive oxygen species, the presence of carbon-centered ferrocenyl radicals was proposed for some ferrocene-containing drugs. Experimental evidence of such highly reactive species for simple ferrocene derivatives is illustrated in this work by rapid-freeze EPR and spin-trapping techniques, providing references for the effectiveness of ferrocene-containing drugs.
The last project deals with the investigation of the influence of dipole moments on electron transfer reactions via a hopping mechanism. In this theoretical approach, synthetic access to immobilizable wires, available from literature known ferrocenyl precursors by established amide coupling procedures, is presented. The formation of stable macrodipole moments of oligoferrocenyl amides is evidenced by literature references. DFT calculations corroborate these results for the wires. Therefore, these wires could act as a first test system for the investigation of intramolecular electron transfer rates with respect to a macrodipole moment within the wire.