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Molecules that can reversibly switch between electronic states under an external stimulus are of interest to numerous applications. Complexes of open shell metal ions with redox active ligands undergo valence tautomerism, resulting in magnetic, colour and structural switching, relevant to data storage and actuators. However, the precise structural changes occurring during valence tautomerism in the solid state are unclear due to the lack of atomic-resolution characterization. Here, variable temperature and high-pressure single crystal X-ray diffraction is used to characterize valence tautomerism in two isostructural cobalt complexes, [Co(bis(6-methyl-2-pyridylmethyl) amine)(3,5-di-tert-butyl-1,2-dioxolene)] + and [Co(tris(6-methyl-2-pyridylmethyl)amine)(3,5-di-tert-butyl-1,2-dioxolene)] + to atomic resolution. The less sterically hindered dimethylated complex exhibits two-step thermally-induced interconversion between the high-spin Co II -seminquinonate and low-spin Co III -catecholate forms (valence tautomerism) at 155 and 95 K due to the presence of two symmetry-independent complexes. In contrast, the more sterically hindered trimethylated complex does not display thermal valence tautomerism. Both complexes exhibit unique behaviour under high pressure. The dimethylated species undergoes gradual, one-step valence tautomerism in both symmetry-independent complexes concurrently between 0.43 GPa and 1.30 GPa. In the trimethylated species, pressure is sufficient to overcome steric hindrance, leading to one-step valence tautomerism between 2.60 GPa and 3.10 GPa, demonstrating pressure-triggered valence tautomerism in a thermally inactive complex. This study is among the few investigations using in situ high-pressure single crystal X-ray diffraction to achieve atomic-level structural characterization of valence tautomerism, aiding the development of robust structure-property relationships in these types of complexes., (© 2024. The Author(s).)

The ability to identify promising candidate switchable molecules computationally, prior to synthesis, represents a considerable advance in the development of switchable molecular materials. Even more useful would be the possibility of predicting the switching temperature. Cobalt-dioxolene complexes can exhibit thermally induced valence tautomeric switching between low-spin Co III -catecholate and high-spin Co II -semiquinonate forms, where the half-temperature ( T 1/2 ) is the temperature at which there are equal amounts of the two tautomers. We report the first simple computational strategy for accurately predicting T 1/2 values for valence tautomeric complexes. Dispersion-corrected density functional theory (DFT) methods have been applied to the [Co(dbdiox)(dbsq)(N 2 L)] (dbdiox/dbsq •- = 3,5-di- tert -butyldioxolene/semiquinonate; N 2 L = diimine) family of valence tautomeric complexes, including the newly reported [Co(dbdiox)(dbsq)(MeO-bpy)] ( 1 ) (MeO-bpy = 4,4'-dimethoxy-2,2'-bipyridine). The DFT strategy has been thoroughly benchmarked to experimental data, affording highly accurate spin-distributions and an excellent energy match between experimental and calculated spin-states. Detailed orbital analysis of the [Co(dbdiox)(dbsq)(N 2 L)] complexes has revealed that the diimine ligand tunes the T 1/2 value primarily through π-acceptance. We have established an excellent correlation between experimental T 1/2 (toluene) values for [Co(dbdiox)(dbsq)(N 2 L)] complexes and the calculated lowest unoccupied molecular orbital energy of the corresponding diimine ligand. The model affords accurate T 1/2 (toluene) values for [Co(dbdiox)(dbsq)(N 2 L)] complexes, with an average error of only 3.7%. This quantitative and simple DFT strategy allows experimentalists to not only rapidly identify proposed VT complexes but also predict the transition temperature. This study lays the groundwork for future in silico screening of candidate switchable molecules prior to experimental investigation, with associated time, cost, and environmental benefits.