Proton-Induced Spin State Switching in an Fe III Complex

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