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RESEARCH PRODUCT

How Hydrogen Bonds Affect Reactivity and Intervalence Charge Transfer in Ferrocenium‐Phenolate Radicals

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The ferrocenyl-phenol 2,4-di-tert-butyl-6-(ferrocenylcarbamoyl)phenol (H-1) forms intramolecular hydrogen bonds which are absent in its constitutional isomer 2,6-di-tert-butyl-4-(ferrocenylcarbamoyl)phenol (H-2). Their corresponding bases 1– and 2– show intra- and intermolecular NH···O hydrogen bonds, respectively. The phenolate 1– is reversibly oxidized to 1·, whereas 2– only undergoes a quasi-reversible oxidation to 2·, which suggests a higher reactivity. The radical pools of 1· and 2· formed by the oxidation/deprotonation of H-1 and H-2 have been probed by (rapid-freeze) electron paramagnetic resonance (EPR) spectroscopy and by spin-trapping techniques to elucidate the types of radicals present. Ferrocenium phenolate [1a]· featuring an NH···O intramolecular hydrogen bond is the most stable radical and undergoes thermal and photoinduced valence isomerization to the phenoxyl radical valence isomer [1b]· with participation of the NH stretching mode (proton-coupled electron transfer). A ferrocenium iminolate radical [1c]· is present as well and equilibrates with the carbon-centered ferrocenyl radicals [1Cp]· and [1β]·. The latter radicals are intercepted by nitrobenzene to give the corresponding stable nitroxyl radicals [6Cp]· and [6β]·. All the radicals 2·, which lack intramolecular hydrogen bonds, are transient in nature due to rapid follow-up reactions. However, rapid-freeze EPR spectroscopy indicated the presence of ferrocenium iminolate [2c]·, the phenoxyl radical [2b]·, and/or carbon-centered radicals [2Cp]· and [2β]·. The carbon-centered radicals [2Cp]· and [2β]· are selectively trapped as the corresponding nitroxide radicals [7Cp]· and [7β]·. These diverse reactivity patterns are relevant for cytostatic ferrocenyl-phenols such as ferrocifen.