Crystallographic Detection of the Spin State in Fe Complexes.

We report a single example of thermal spin crossover in a series of Fe complexes, [Fe(R-sal323)], which typically stabilize the low-spin ( = 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 = 1/2 ground state, with only the [Fe(4-OMe-sal323)]NO complex () 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 reveals a gradual, incomplete transition, with = 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(4-OMe-sal323)] and [Fe(sal323)] confirmed the energetic preference for the spin doublet form in both, and the thermal spin crossover in complex is therefore attributed to perturbation of the crystal packing on warming.