Force-Activated Spin-Crossover in Fe<sup>2+</sup> and\nCo<sup>2+</sup> Transition Metal Mechanophores

Transition metal mechanophores exhibiting\nforce-activated spin-crossover\nare attractive design targets, yet large-scale discovery of them has\nnot been pursued due in large part to the time-consuming nature of\ntrial-and-error experiments. Instead, we leverage density functional\ntheory (DFT) and external force explicitly included (EFEI) modeling\nto study a set of 395 feasible Fe<sup>2+</sup> and Co<sup>2+</sup> mechanophore candidates with tridentate ligands that we curate from\nthe Cambridge Structural Database. Among nitrogen-coordinating low-spin\ncomplexes, we observe the prevalence of spin crossover at moderate\nforce, and we identify 155 Fe<sup>2+</sup> and Co<sup>2+</sup> spin-crossover\nmechanophores and derive their threshold force for low-spin to high-spin\ntransition (<i>F</i><sub>SCO</sub>). The calculations reveal\nstrong correlations of <i>F</i><sub>SCO</sub> with spin-splitting\nenergies and coordination bond lengths, facilitating rapid prediction\nof <i>F</i><sub>SCO</sub> using force-free DFT calculations.\nThen, among all Fe<sup>2+</sup> and Co<sup>2+</sup> spin-crossover\nmechanophores, we further identity 11 mechanophores that combine labile\nspin-crossover and good mechanical robustness that are thus predicted\nto be the most versatile for force-probing applications. We discover\ntwo classes of <i>mer-</i>symmetric complexes comprising\nspecific heteroaromatic rings within extended π-conjugation\nthat give rise to Fe<sup>2+</sup> mechanophores with these characteristics.\nWe expect the set of spin-crossover mechanophores, the design principles,\nand the computational approach to be useful in guiding the high-throughput\ndiscovery of transition metal mechanophores with diverse functionalities\nand broad applications, including mechanically activated catalysis.