Crystallographic Detection of the Spin State in Fe^III Complexes

We report a single example of thermal spin crossover in a series of Fe^III complexes, [Fe^III(R-sal₂323)]⁺, which typically stabilize the low-spin (S = 1/2) state. Single-crystal X-ray diffraction analysis of 53 such complexes with varying “R” groups, charge-balancing anions, and/or lattice solvation confirms bond lengths in line with an S = 1/2 ground state, with only the [Fe^III(4-OMe-sal₂323)]NO₃ complex (1a) exhibiting longer bond lengths associated with a percentage of the spin sextet form at room temperature. Structural distortion parameters are investigated for the series. A magnetic susceptibility measurement of 1a reveals a gradual, incomplete transition, with T_1/2 = 265 K in the solid state, while Evans method NMR reveals that the sample persists in the low-spin form in solution at room temperature. Computational analysis of the spin state preferences for the cations [Fe^III(4-OMe-sal₂323)]⁺ and [Fe^III(sal₂323)]⁺ confirmed the energetic preference for the spin doublet form in both, and the thermal spin crossover in complex 1a is therefore attributed to perturbation of the crystal packing on warming.