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

Abstract

AbstractThe 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.