Engineering first-order spin–orbit coupling in a pentagonal bipyramidal Fe(II) complex and subsequent SMM behavior

Abstract

Pentagonal bipyramidal (PBP) complexes with a first-order spin–orbit coupling contribution can be readily obtained, mainly through chemical design optimization ensuring minimum structural distortion and a more symmetrical ligand field. This conclusion follows from the investigation of a series of five Fe(II) complexes: [FeL^N5(H₂O)Cl]Cl·4.5H₂O, 1; [FeL^N5Cl₂]·3H₂O, 2; [FeL^N5Br₂], 3; [FeL^N5I₂], 4; and [Fe_0.12Zn_0.88L^N5I₂], 5 (L^N5 stands for the pentadentate macrocyclic ligand formed by the condensation of 2,6-diacetylpyridine and 2,9-di(α-methylhydrazino)-1,10-phenanthroline). Theoretical calculations revealed quasi-degeneracy of the d_xz and d_yz orbitals for the complexes with halide ligands at the apical positions (ΔE = 91, 134, and 142 cm⁻¹, respectively, for 2–4). This small energy gap leads to SO states with very strong mixing of the ground and first excited quintet states. Therefore, the ZFS Hamiltonian is not suitable for modelling the magnetic properties of complexes 2–5. This does not apply for 1 with ΔE = 412 cm⁻¹. The recorded magnetic behaviors indicated strong magnetic anisotropy; for 1 D = −24 cm⁻¹ was obtained. The Br and I derivatives were found to behave as SMMs (with a U/k_B of about 90 K), the latter even in the absence of a static field.