Aufbau vs. non-Aufbau ground states in two-coordinate d7 single-molecule magnets

Abstract

Single-molecule magnets (SMMs) with d⁷ electronic configurations often require designer ligands to satisfy the metals electronic conditions to achieve large angular momentum. Herein, the slow relaxation of the magnetization in two d⁷ metal complexes in near identical ligand fields is achieved from divergent origins. The two compounds, [Co^II{N(SiMePh₂)₂}₂] and [K(2,2,2-crypt)][Fe^I{N(SiMePh₂)₂}₂] (2,2,2-crypt = 2,2,2-cryptand), display unusual electronic configurations giving rise to SMM behavior originating either from 3d–4s orbital mixing or a non-Aufbau ground state. The chracteristics contributing to the rare non-Aufbau ground state configurations are illuminated by the use of a highly donating amido-ligand, which would be expected to significantly split the respective orbitals. Magnetic circular dichroism provides experimental support for ab initio determined electronic structures. Moreover, computational models reveal that the relative electronic configurations are largely retained independently of coordination geometry, provided that some degree of pseudo-linearity is retained. Thus, providing generalized design principles in the pursuit of linear d⁷ SMMs.