Benefitting from Magnetic Field-Induced Torquing in Terahertz EPR of a MnIII Coordination Complex

Manganese(III) (3d(4), S = 2) coordination complexes have been widely studied by high-frequency and -field EPR (HFEPR) for their own inherent chemical interest and for providing information for the burgeoning area of molecular magnetism. In the present study, we demonstrate how a stable, easily handled complex of Mn-III, [MnLKNO3], where L3- is a hexadentate tripodal ligand, trianion of 1,1,1-tris[(3- methoxysalicylideneamino)methyl]ethane, can be used for another purpose entirely. This purpose is as a field and frequency standard for HFEPR that is superior to a "traditional" standard such as an organic radical (e.g., DPPH) with its single, g = 2.00 signal, or to atomic hydrogen, which is less readily available than DPPH and provides only two signals for calibration purposes (Stoll et al. in J Magn Reson 207:158-163, 2010). By contrast, polycrystalline [MnLKNO3] (1) orients in the external magnetic field of an HFEPR spectrometer (three different spectrometers were employed in this study). The crystal structure of 1 allows determination of the exact, reproducible molecular orientation of 1 in the applied field. This phenomenon provides multiple, well-defined resonances over a broad field sweep range (0-36 T) at any of a wide range of frequencies (tested up to 1 THz so far) allowing accurate calibration of magnetic field in a multi-frequency HFEPR study.