Force-Activated Spin-Crossover in Fe²⁺ and Co²⁺ Transition Metal Mechanophores

Transition metal mechanophores exhibiting force-activated spin-crossover are attractive design targets, yet large-scale discovery of them has not been pursued due in large part to the time-consuming nature of trial-and-error experiments. Instead, we leverage density functional theory (DFT) and external force explicitly included (EFEI) modeling to study a set of 395 feasible Fe²⁺ and Co²⁺ mechanophore candidates with tridentate ligands that we curate from the Cambridge Structural Database. Among nitrogen-coordinating low-spin complexes, we observe the prevalence of spin crossover at moderate force, and we identify 155 Fe²⁺ and Co²⁺ spin-crossover mechanophores and derive their threshold force for low-spin to high-spin transition (F_SCO). The calculations reveal strong correlations of F_SCO with spin-splitting energies and coordination bond lengths, facilitating rapid prediction of F_SCO using force-free DFT calculations. Then, among all Fe²⁺ and Co²⁺ spin-crossover mechanophores, we further identity 11 mechanophores that combine labile spin-crossover and good mechanical robustness that are thus predicted to be the most versatile for force-probing applications. We discover two classes of mer-symmetric complexes comprising specific heteroaromatic rings within extended π-conjugation that give rise to Fe²⁺ mechanophores with these characteristics. We expect the set of spin-crossover mechanophores, the design principles, and the computational approach to be useful in guiding the high-throughput discovery of transition metal mechanophores with diverse functionalities and broad applications, including mechanically activated catalysis.