Symmetric and asymmetric ligands for Fe^III spin crossover - the influence of the C₂ axis

Modulation of the local strain and geometry in Fe^III Schiff base complexes has been shown to allow the stabilisation of both the high spin (HS) and low spin (LS) states, along with thermal spin crossover (SCO). Complexes with hexadentate Schiff base ligands can be readily modified by changing the length of the tetraamine backbone linker. We report here 34 complexes of the symmetric [Fe^III(R-sal₂232)]⁺ and asymmetric [Fe^III(R-sal₂223)]⁺ families, where the former typically support the HS state, along with a handful of SCO examples, and the latter only supports the HS state. Magnetic measurements reveal that one symmetric example, [Fe^III(5-I-sal₂232)]ClO₄1.5, undergoes thermal SCO close to room temperature. We compare the structural distortion and spectroscopic properties in these examples, to indentify the factors that influence spin state choice. This reveals the importance of molecular symmetry, by way of a C₂ axis bisecting the complex which is present in the samples which stabilise the LS state so far. The aforementioned example and three others, one reported previously, have short metal-ligand bond lengths suggesting adoption of the LS state coupled with the presence of a C₂ axis. The additional strain in the [Fe^III(R-sal₂223)]⁺ complexes due to the asymmetric nature of the backbone results in significantly greater distortion around the Fe^III centre which inhibits the stabilisation of the less distorted LS state. Computational analysis of the [Fe^III(5-I-sal₂232)]⁺ and [Fe^III(5-I-sal₂223)]⁺ isomers reveals that the HS state is more stable in the asymmetric [Fe^III(5-I-sal₂223)]⁺ species, whereas the energy difference between the HS and LS state for the [Fe^III(5-I-sal₂232)]⁺ cation is sufficiently small to allow for SCO to occur.