High frequency and field EPR spectroscopy of Mn(III) complexes in frozen solutions

We have performed high-frequency and -field electron paramagnetic resonance (HFEPR) experiments on two complexes of high-spin Mn(III) (3d(4),S=2): mesotetrasulfonato-porphyrinatomanganese(III) (Mn(TSP)) and [(R,R)-(-)-N,N(')-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)] (Mn(salen)). The main aim of this work was to qualitatively and quantitatively characterize the conditions suitable for HFEPR of high-spin transition metal complexes in frozen solutions, and compare them with experiments performed on solid samples. Mn(TSP) is a porphyrin complex soluble in water, in contrast to most metalloporphyrins. Mn(salen), often referred to as Jacobsen's catalyst, is a complex widely used in organic synthesis for alkene epoxidation, and is soluble in organic solvents. High-quality HFEPR signals were observed for solid state Mn(TSP), as has been previously shown for many Mn(III) complexes. The present study is, however, the first to report high-quality HFEPR spectra of a Mn(III) complex in frozen aqueous solution. Analysis of the data yielded the following spin Hamiltonian parameters: S=2; D=-3.16+/-0.02 cm(-1), E=0, and isotropic g=2.00(2). No X-band EPR signals were observed for Mn(TSP), which is a consequence of this being a rigorously axial spin system. Mn(salen), in contrast, did not give good quality HFEPR spectra in the solid state, but high-quality HFEPR spectra were recorded in frozen organic solutions. Analysis of the data yielded the following spin Hamiltonian parameters: S=2; D=-2.47+/-0.02 cm(-1), |E|=0.17+/-0.01 cm(-1), and isotropic g=2.00(2). These values differ from those reported using X-band parallel mode EPR [J. Am. Chem. Soc. 123 (2001) 5710], as discussed in the text. Therefore, a comparison between HFEPR and parallel-mode X-band spectroscopy is made. Finally, the concentration sensitivity aspect of HFEPR spectroscopy is also discussed.