Proton‐Induced Spin State Switching in an FeIII Complex

Abstract Reversible proton‐induced spin state switching of an Fe III complex in solution is observed at room temperature. A reversible magnetic response was detected in the complex, [Fe III (sal 2 323)]ClO 4 ( 1 ), using Evans’ method 1 H 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, [Fe III (4‐NEt 2 ‐sal 2 323)]ClO 4 ( 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.