Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)–Iron Complexes Containing Redox-Active α‑Diimine Ligands

Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal–radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ2-PhTt tBu]­Fe­(AdNCHCHNAd) (1), and its analogue [κ2-PhTt tBu]­Fe­(CyNCHCHNCy) (2) (PhTt tBu = phenyltris­(tert-butylthiomethyl)­borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron­(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron­(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < −250 cm–1 (J = −657 cm–1, DFT). Calibrated CAS calculations revealed that the ground-state Fe­(II)−α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe­(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from −4 to–1 cm–1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.