On the Single-Molecule Magnetic Behavior of Linear Iron(I) Arylsilylamides

The rational design of 3d-metal-based single-molecule magnets (SMM) requires a fundamental understanding of their intrinsic electronic and structural properties and how they translate into experimentally observable features. Here, we determined the magnetic properties of the linear iron­(I) silylamides K­{crypt}­[FeL₂] and [KFeL₂] (L = −N­(Dipp)­SiMe₃; crypt = 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]­hexacosan). For the former, slow-relaxation of the magnetization with a spin reversal barrier of U_eff = 152 cm^–1 as well as a closed-waist magnetic hysteresis and magnetic blocking below 2.5 K are observed. For the more linear [KFeL₂], in which the potassium cation is encapsulated by the aryl substituents of the amide ligands, the relaxation barrier and the blocking temperature increase to U_eff = 184 cm^–1 and T_B = 4.5 K, respectively. The increase is rationalized by a more pronounced axial anisotropy in [KFeL₂] determined by dc-SQUID magnetometry. The effective relaxation barrier of [KFeL₂] is in agreement with the energy spacing between the ground and first-excited magnetic states, as obtained by field-dependent IR-spectroscopy (178 cm^–1), magnetic measurements (208 cm^–1), as well as theoretical analysis (212 cm^–1). In comparison with the literature, the results show that magnetic coercivity in linear iron­(I) silylamides is driven by the degree of linearity in conjunction with steric encumbrance, whereas the ligand symmetry is a marginal factor.