Homoleptic complexes of the Fe(II) ion with bidentate chelating diimine ligands are one of the most widespread representatives of spin-crossover complexes. The rationalization of the equilibrium betweent the low-spin and high-spin states of such compounds typically invokes arguments about π-accepting and π-donating ability of the ligands, which often are not straightforward to analyze. Even the electronic-structure methods at the DFT level of theory are usually unreliable or too expensive to predict the possibility of spin-crossover in such Fe(II) complexes. Driven by this challenge, we performed an extensive analysis of metric parameters in the known tris-homoleptic Fe(II) complexes. Remarkably, we find that the spin state of these complexes can be reliably predicted based on a single parameter within a free (non-coordinated) ligand – the interatomic separation between the N-donor metal-binding sites. Our analysis revealed that there exist critical N---N distances which dictate the regions of stability for the high-spin and low-spin complexes, as well as the intermediate range in which the spin crossover can be observed. We demonstrated the validity of this prediction for several specific examples of Fe(II) complexes.

Simple method to predict the electronic spin configuration of Fe(II) tris-diimine complexes

254th National Meeting (2017)

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