Tracking Structural and Electron Spin Density Changes in a Cooperative Mn³⁺ Spin Crossover Complex at Atomic Scale...

Electron spin-state changes in transition-metal (TM) complexes underpin many biochemical processes and molecular spin-control technologies. Such transitions, triggered by external stimuli like temperature, light, or pressure, alter both the molecular structure and electron spin density (ESD) distribution. Paramagnetic NMR offers atomic-scale insights into these changes, yet traditional solution-state measurements bear limitations due to solvent effects, unaccounted lattice cooperativity, and inaccessibility at cryogenic temperatures. We overcome these limitations by extending the approach to spinning solids at cryogenic temperatures. Specifically, we report high-resolution ¹³C and ¹H magic-angle spinning (MAS) NMR spectra of a mononuclear spin-crossover (SCO) Mn(III) complex across the SCO transition at 130 K. Such low-temperature experiments are particularly challenging because paramagnetic shift and shift anisotropy are inversely proportional to the temperature. The experimental findings are supported by advanced quantum chemical calculations of the NMR and EPR parameters to assign and rationalize the observed paramagnetic shifts. Additionally, monitoring selected ¹H resonances upon heating and cooling through the transition provides access to the order parameter ( $q=\frac{N_{\mathrm{HS}}-N_{\mathrm{LS}}}{N_{\mathrm{HS}}+N_{\mathrm{LS}}}$ ), revealing hysteresis behavior similar to the magnetic susceptibility measurements. This work demonstrates that paramagnetic NMR combined with quantum chemical calculations provides a unique route to probing SCO at the atomic level.