Proton-Induced Switchingof Paramagnetism: ReversibleConversion between a Low and High Spin Co^III Center withina Heterobimetallic Core

The development of molecular species with switchable magnetic properties has been a long-standing challenge in chemistry. One approach involves binding an analyte, such as protons, to a compound to trigger a change in magnetism. Transition metal complexes have been targeted for this type of magnetic modulation because they can undergo changes in their spin states. However, heterobimetallic complexes have had limited utility because of a lack of ligands that create differentiated structures around each metal center that are often necessary to regulate the electronic and magnetic properties. To circumvent this problem, we have used a tripodal ligand with phosphinic amido groups to prepare a complex with a discrete [Co^III(μ-OH)Fe^III] core and an overall system spin of <i>S</i>_T = 5/2. Deprotonation readily produces a species with a unique [Co^III(μ-O)Fe^III] core and an <i>S</i>_T = 1/2 system spin. X-ray diffraction studies, electron paramagnetic resonance spectroscopy, and Mössbauer spectroscopy pinpoint the hexacoordinate Co^III center as the cause of this spin change: the typical <i>S</i>_Co = 0 spin state of the Co^III center in the [Co^III(μ-OH)Fe^III] complex switches to a rare <i>S</i>_Co = 2 spin state in the [Co^III(μ-O)Fe^III] analogue; this change turns on antiferromagnetic coupling between the two metal centers. Computational studies link an increase in π bonding within the Co–oxido unit to the change in the Co^III spin state. The conversion is reversible and provides a blueprint for using oxido/hydroxido ligands within a heterobimetallic core to regulate the spin state of a metal site and thus modulate the paramagnetism of a system.