Uncovering Correlations between Structure and Valence Tautomerism in Cobalt-o-Dioxolene Crystals.

Bistability in transition-metal complexes can originate from several phenomena, including mixed valence (MV), spin-crossover (SCO), and valence tautomerism (VT). In the latter case, valence-tautomeric interconversion (VTI) generates pairs of switchable redox isomers whose physical properties can be controlled by external stimuli, such as light, temperature, and pressure, making these systems promising candidates for molecular devices, spintronics, and chemosensing. Although several strategies exist to modulate VTI in crystalline materials, tuning intermolecular interactions has emerged as a particularly effective route. In cobalt-o-dioxolene complexes of the type [Co-(dioxolene)2(PyL)2] (PyL = pyridyl-like ancillary ligands), VTI is extremely sensitive to crystal packing and the local chemical environment, and its occurrence can be promoted or suppressed by solvation. Here, we show that, in addition to the intrinsic metal-ligand distance changes across VTI, torsion of the pyridyl ancillary ligands in [Co-(dioxolene)2(Py)2] stabilizes specific electrostatic interactions and acts as a structural trigger for VTI, establishing a direct geometric-electronic coupling mechanism. This conclusion is supported by single-crystal X-ray diffraction (SCXRD) data collected on distinct solvated [Co-(dioxolene)2(Py)2] crystals, complemented by DFT calculations performed with the SIESTA package and benchmarked against experimental magnetic susceptibility data using the local density approximation (LDA) functional. DFT spin-polarization analyses reveal that the torsion angle of the PyL plane in [Co-(dioxolene)2(Py)2] correlates directly with the spin state of the system. This behavior extends to all trans-[Co-(dioxolene)2(PyL)2] derivatives with 3D structures deposited in the Cambridge Structural Database (CSD). Our findings establish a coherent picture of the redox and spin-state dynamics in VT complexes and provide concrete design guidelines for their use in practical solid-state applications.