Fine-tuning of the spin-crossover properties of Fe(iii) complexes via ligand design

Exploring the chemical space of a given ligand aiming to modulate its ligand field strength is a versatile strategy for the fine-tuning of physical properties such as the transition temperature (T-1/2) of spin-crossover (SCO) complexes. The computational study presented herein aims at systematically exploring the extent to which the ligand substituent effects can modulate T-1/2 in two families of Fe(iii) SCO systems with a N4O2 coordination environment and at identifying the best descriptors for fast and accurate prediction of changes in T-1/2 upon ligand functionalization. B3LYP* calculations show that the attachment of substituents to beta-ketoiminato fragments (L-1) leads to drastic changes in T-1/2, while functionalization of phenolato moieties (L-2) allows for a finer degree of control over T-1/2. Natural Bond Orbital (NBO) charges of the donor atoms, Hammett parameters for both para and meta-functionalization of L-2, and Swain-Lupton parameters for L-1 and para-functionalization of L-2 have been found to be the suitable descriptors for predicting the changes in T-1/2. Further analysis of the ligand-field splitting in such systems rationalizes the observed trends and shows that ligand substituents modify both the sigma and pi bonds between the Fe(iii) center and the ligands. Thus, we provide simple yet reliable guidelines for the rational design of new SCO systems with specific values of T-1/2 based on their ligand design.