AtomAccess: A Predictive Tool for Molecular Design and Its Application to the Targeted Synthesis of Dysprosium Single-Molecule Magnets

Isolated dysprosocenium cations, [Dy(Cp^R)₂]⁺ (Cp^R = substituted cyclopentadienyl), have recently been shown to exhibit superior single-molecule magnet (SMM) properties over closely related complexes with equatorially bound ligands. However, gauging the crossover point at which the Cp^R substituents are large enough to prevent equatorial ligand binding, but small enough to approach the metal closely and generate strong crystal field splitting has required laborious synthetic optimization. We therefore created the computer program AtomAccess to predict the accessibility of a metal binding site and its ability to accommodate additional ligands. Here, we apply AtomAccess to identify the crossover point for equatorial coordination in [Dy(Cp^R)₂]⁺ cations in silico and hence predict a cation that is at the cusp of stability without equatorial interactions, viz., [Dy(Cp^ttt)(Cp*)]⁺ (Cp^ttt = C₅H₂^tBu₃-1,2,4, Cp* = C₅Me₅). Upon synthesizing this cation, we found that it crystallizes as either a contact ion-pair, [Dy(Cp^ttt)(Cp*){Al[OC(CF₃)₃]₄-κ-F}], or separated ion-pair polymorph, [Dy(Cp^ttt)(Cp*)][Al{OC(CF₃)₃}₄]·C₆H₆. Upon characterizing these complexes, together with their precursors, yttrium and yttrium-doped analogues, we find that the contact ion-pair shows inferior SMM properties to the separated ion-pair, as expected, due to faster Raman and quantum tunneling of magnetization relaxation processes, while the Orbach region is relatively unaffected. The experimental verification of the predicted crossover point for equatorial coordination in this work tests the limitations of the use of AtomAccess as a predictive tool and also indicates that the application of this type of program shows considerable potential to boost efficiency in exploratory synthetic chemistry.