Nitric oxide induces decomposition of electron-deficient aminoferrocene drugs 4 with formation of oxime 7, which is derived from the upper cyclopentadienyl ligand. Drugs 4 are not responsive to hydrogen peroxide. Based on the latter reaction, the fluorogenic probes are developed for detecting NO in cells and inflammation in vivo.

Electron-deficient aminoferrocenes (edAFs) exhibit anticancer activity both in vitro and in vivo. However, their mechanism of action remains unclear. Studies using fluorogenic edAF derivatives suggest that the ferrocenyl moiety undergoes oxidation or decomposition within cells, resulting in the formation of unknown products. Interestingly, this process is not significantly facilitated by H₂O₂, indicating that this intracellular oxidant does not alter edAFs in the cellular environment. To identify alternative endogenous oxidants, NO is investigated as a potential candidate. Under aerobic conditions, NO is found to efficiently induce the oxidation and decomposition of edAFs. This transformation is mediated by an electrophilic nitrosation reaction, followed by nitroso-oxime tautomerism and subsequent degradation of the ferrocenyl moiety with the release of ligand-derived oxime 7 and iron ions. These findings suggest that NO may play a key role in the intracellular modification of edAFs, potentially contributing to their anticancer activity or their metabolism or both. Building on this mechanism, an effective probe is developed for detecting NO in living cells and identifying sites of inflammation in vivo. These probes are based on a modular design that enables facile substitution of the fluorescent dye, allowing straightforward customization for diverse applications both in cellulo and in vivo.