Single Molecule Magnetism in Linear Fe(I) Complexes with Aufbau and non-Aufbau ground-state

With the ongoing efforts on synthesizing mono-nuclear single-ion magnets (SIMs) with promising applications in high-density data storage and spintronics devices, the linear Fe(I) complexes emerge as the enticing candidates possessing large unquenched angular momentum. Herein, we have studied five experimentally synthesized linear Fe(I) complexes to uncover the origin of single-molecule magnetic behavior of these complexes. To begin with, we benchmarked our methodology on the experimentally and theoretically well-studied complex, [Fe{C(SiMe3)}3]−1] (1) (SiMe3 = trimethylsilyl) which is characterized with large spin-reversal barrier of 226 cm−1 [Nat. Chem. 2013, 5, 577–581]. Further, the two Fe(I) complexes, i.e., [Fe(cyIDep)2]+1 (2) ((cyIDep= 1,3-bis(20,60-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene) and [Fe(sIDep)2]+1] (3) (sIDep = 1,3-bis(20,60-diethylphenyl)-imidazolin-2-ylidene) are studied that do not possess SIM behavior under ac or dc magnetic fields, however, they are reported to exhibit large opposite axial zero field splitting (-62.4 and +34.0 cm−1 respectively) from ab initio calculations. Employing state-of-the-art ab initio calculations, we have unwrapped the origin of this contrasting observation between experiment and theory by probing their magnetic relaxation pathways and the pattern of d-orbitals splitting. Additionally, the two experimentally synthesized Fe(I) complexes, i.e., [(η6-C6H6)FeAr*-3,5-Pri 2] (4) (Ar*-3,5-Pri 2 = C6H-2,6-(C6H2-2,4,6-Pri 3)2-3,5-Pri 2) and [(CAAC)2Fe]+1 (5) (CAAC = cyclic (alkyl)(amino)carbene) are investigated for SIM behavior, since there is no report on their magnetic properties. To this end, complex 4 presents itself as the potential candidate for SIM.