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, [κ-PhTt]Fe(AdNCHCHNAd) (), and its analogue [κ-PhTt]Fe(CyNCHCHNCy) () (PhTt = phenyltris(butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). and 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 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. and 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 , with an estimated coupling constant < -250 cm ( = -657 cm, 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 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter in the range from -4 to-1 cm. Finally, dynamic magnetic studies show that exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.