Spring‐Loaded Iron(II) Complexes as Magnetogenic Probes Reporting on a Chemical Analyte in Water

A paramagnetic quality may be established in an aqueous sample if it contains a molecular probe that responds to the addition of a chemical analyte by turning from a diamagnetic state into a paramagnetic one (off–on). We explore here a stable, low‐spin, binary iron(II) complex that stores so much potential energy that its transformation by the target analyte leads to its fragmentation into a high‐spin complex despite the imposing strength of the chelate effect. The underlying ligand is a mixed aminal, the components of which can be freely varied to optimize response kinetics with the initial probe stability preserved. With decreasing leaving‐group character of the azole component, the probe stability improves, and the response kinetics diminish. An optimal arrangement can be found with a pyrazole paired with an electron‐deficient aromatic carbaldehyde component (nitro substitution). The drastic electronic reversion associated with the reduction of the nitro group to an amino group is the principal reason for the observation of an initially stable probe that suffers swift fragmentation when reacted with a reductive analyte.