Magnetospectroscopic Studies of a Series of Fe(II) Scorpionate Complexes: Assessing the Relationship between Halide Identity and Zero-Field Splitting.

Ferrous ions in four-coordinate environments are common in protein structures, synthetic catalysts, and molecular magnets. The 3d6 configuration of high-spin Fe(II) imparts an S = 2 ground state, whose analysis using conventional spectroscopic methods is often hindered by substantial zero-field splitting (ZFS). Herein, we provide detailed electronic-structure descriptions for [FeIIX(TptBu,Me)] (1-X; X = F, Cl, Br, I), where (TptBu,Me)- is hydrotris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate. The three pyrazolyl N-donors of the "scorpionate" ligand facially coordinate to Fe(II), giving idealized C3v symmetry with the halide occupying the axial position. Although originally reported by Theopold and co-workers, this series is revisited herein using advanced experimental and theoretical tools. Ground-state transitions were probed by high-frequency and -field electron paramagnetic resonance (HFEPR) and far-infrared magnetic spectroscopy (FIRMS). Variable-temperature/-field (VTVH) 57Fe Mössbauer spectroscopy, paramagnetic susceptibility, and VTVH reduced magnetization were also utilized. This combined approach provided complete sets of spin-Hamiltonian parameters. Interpretation using ab initio multiconfigurational calculations enabled quantification of halide-dependent magnetoelectronic effects. Jahn-Teller distortions induce a descent in symmetry from C3v to Cs in both solution and solid state. Finally, we demonstrate that the 1-X series is ionic, with the ZFS arising from combined Jahn-Teller and ligand field effects, rather than intrinsic spin-orbit coupling from the halides.