Constant no more: reevaluating Hammett constants through spin-crossover

Abstract

Hammett constants, a popular tool for predicting substitution effects on chemical reactivity and catalytic activity, the binding affinity of drugs and the spin-crossover (SCO) behavior of metal complexes, are frequently criticized. Among others, they define the acetoxy group as electron-withdrawing (EWG), although it activates electrophilic substitution in the benzene ring as an electron donor (EDG). We argue that SCO compounds provide an excellent platform for reevaluating Hammett constants, with the SCO midpoint temperature as a quantitative measure of even subtle changes in organic ligands. For a new series of iron(II) complexes of bis(pyrazol-3-yl)pyridines (3-bpp), this measure showed a contradictory increase induced by both EWGs and EDGs. Yet, it correlated linearly with NMR chemical shifts, metal–ligand bond lengths and energies. Together, these correlations yielded Hammett constants that correctly identified the acetoxy group as an EDG, among others. The new constants allowed us not only to explain the SCO behaviour of metal complexes with 3-bpp and other popular families of N-donor ligands but also to predict the SCO midpoint temperature with high accuracy. Beyond the SCO realm, they were extended to rationalize the outcomes of Diels–Alder and polymerization reactions. By using metal–ligand bond lengths routinely measured for coordination compounds, Hammett constants can be further ‘refined’ to help design new, more efficient catalysts, drugs and materials for advanced applications in quantum technologies and (bio)sensing, energy conversion and smart/self-healing coatings.