Proton‐induced spin state switching in an FE^III complex

Reversible proton‐induced spin state switching of an FeIII complex in solution is observed at room temperature. A reversible magnetic response was detected in the complex, [FeIII(sal2323)]ClO4 (1), using Evans’ method 1H NMR spectroscopy which indicated cumulative switching from low‐spin to high‐spin upon addition of one and two equivalents of acid. Infrared spectroscopy suggests a coordination‐induced spin state switching (CISSS) effect, whereby protonation displaces the metal‐phenoxo donors. The analogous complex, [FeIII(4‐NEt2‐sal2323)]ClO4 (2), with a diethylamino group on the ligand, was used to combine the magnetic change with a colorimetric response. Comparison of the protonation responses of 1 and 2 reveals that the magnetic switching is caused by perturbation of the immediate coordination sphere of the complex. These complexes constitute a new class of analyte sensor which operate by magneto‐modulation, and in the case of 2, also yield a colorimetric response.