High-Field EPR Study of the Spin-Crossover Transitions in a Mn(III) Schiff-Base Complex

Spin crossover (SCO) transitions from a high-spin (HS) to a low-spin (LS) state occur in certain molecular complexes of octahedrally coordinated 3d$^{\mathrm{4}}$ to 3d$^{\mathrm{7}}$ transition metals and can be induced using temperature, pressure, or optical perturbations.$^{\mathrm{[1]}}$ It has been observed that not all SCO complexes exhibit a complete transition, resulting in mixed LS/HS phases.$^{\mathrm{[2]}}$ Such situations are difficult to characterize experimentally due to the inhomogeneous nature of the mixed phase.$^{\mathrm{[2]}}$ The Mn(III) Schiff-base complex considered in this investigation, [Mn((3-MeO-5-NO$_{\mathrm{2}}$-sal)$_{\mathrm{2}}$323)]PF$_{\mathrm{6}}$, exhibits a complete transition from a pure HS ($S$~$=$~2) to a pure LS ($S$~$=$~1) state below a relatively sharp transition (T$_{\mathrm{1/2}} \quad =$ 51~K, with \textless 10~K hysteresis). Using continuous-wave high-field (0 to 14.5~T) EPR spectroscopy on a powder sample, the zero-field splitting parameters were characterized for both the LS ($D \quad =$ $+$20.8~cm$^{\mathrm{-1}})$ and HS states ($D \quad =$ -4.80 cm$^{\mathrm{-1}}$, $E \quad =$ 2.175 cm$^{\mathrm{-1}})$. References: 1. Kahn, O., and C. Jay Martinez. ``Spin-Transition Polymers: From Molecular Materials Toward Memory Devices.'' \textit{Science}, vol. 279, no. 5347, 2 Jan. 1998, pp. 44--48., doi:10.1126/science.279.5347.44. 2. Capel Berdiell, Izar, et al. ``Frontispiece: An Incomplete Spin Transition Associated with a Z$\prime =$1$\to $Z$\prime =$24 Crystallographic Symmetry Breaking.'' \textit{Chemistry - A European Journal}, vol. 24, no. 20, 7 Nov. 2017, pp. 1--5., doi:10.1002/chem.201882065.