Single-Molecule Magnetism in Linear Fe(I) Complexes with Aufbau and Non-Aufbau Ground States

With
the ongoing efforts on synthesizing mononuclear single-ion
magnets (SIMs) with promising applications in high-density data storage
and spintronics devices, the linear or quasi-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 the methodology
on the experimentally and theoretically well-studied complex [Fe­(C­(SiMe3)3)2]−1 (1) (SiMe3 = trimethylsilyl), which is characterized with
a large spin-reversal barrier of 226 cm–1. Subsequently,
the spin-phonon coupling coefficients are calculated for the low-frequency
vibrational modes to understand the relaxation mechanism of the complex.
Furthermore, the two Fe­(I) complexes, that is, [Fe­(cyIDep)2]+1 (2) (cyIDep = 1,3-bis­(2′,6′-diethylphenyl)-4,5-(CH2)4-imidazole-2-ylidene) and [Fe­(sIDep)2]+1 (3) (sIDep = 1,3-bis­(2′,6′-diethylphenyl)-imidazolin-2-ylidene),
are studied that are experimentally reported with no SIM behavior
under ac or dc magnetic fields; however, they exhibit large opposite
axial zero field splitting (−62.4 and +34.0 cm–1, respectively) from 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
orbital splitting. Additionally, the two experimentally synthesized
Fe­(I) complexes, that is, [(η6-C6H6)­FeAr*-3,5-Pr2i] (4) (Ar*-3,5-Pr2i = C6H-2,6-(C6H2-2,4,6-Pr3i)2-3,5-Pr2i) and [(CAAC)2Fe]+1 (5) (CAAC = cyclic (alkyl) (amino)­carbene), are investigated
for SIM behavior, since there is no report on their magnetic anisotropy.
To this end, complex 4 presents itself as the possible
candidate for SIM.