Crystallographic Detection of the Spin State in FeIII 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.

Celebrating Professor Len Lindoy’s 80th Birthday

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