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 alpha-diiminato radicalcontaining mononuclear transition metal SMM, namely, [kappa(2)-PhTt(tBu)]Fe-(AdNCHCHNAd) (1), and its analogue [kappa(2)- 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 Mossbauer 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 alpha-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)-alpha-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral alpha-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 vertical bar D vertical bar. These results demonstrate that incorporating strongly coupled alpha-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.