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

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 [ 1 Cp ] · and [ 1 β ] · . The latter radicals are intercepted by nitrobenzene to give the corresponding stable nitroxyl radicals [ 6 Cp ] · and [ 6 β ] · . 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 [ 2 Cp ] · and [ 2 β ] · . The carbon‐centered radicals [ 2 Cp ] · and [ 2 β ] · are selectively trapped as the corresponding nitroxide radicals [ 7 Cp ] · and [ 7 β ] · . These diverse reactivity patterns are relevant for cytostatic ferrocenyl‐phenols such as ferrocifen.