Substituent-Guided Cluster Nuclearity for Tetranuclear Iron(III) Compounds with Flat {Fe₄(μ₃-O)₂} Butterfly Core

The tetranuclear iron(III) compounds [Fe₄(μ₃-O)₂(μ-L^Z)₄] (1-3) were obtained by reaction of FeCl₃ with the shortened salen-type N₂O₂ tetradentate Schiff bases N,N'-bis(salicylidene)-o-Z-phenylmethanediamine H₂L^Z (Z = NO₂, Cl and OMe, respectively), where the one-carbon bridge between the two iminic nitrogen donor atoms guide preferentially to the formation of oligonuclear species, and the ortho position of the substituent Z on the central phenyl ring selectively drives towards Fe₄ bis-oxido clusters. All compounds show a flat almost-symmetric butterfly-like conformation of the {Fe₄(μ₃-O)₂} core, surrounded by the four Schiff base ligands, as depicted by both the X-ray molecular structures of 1 and 2 and the optimized geometries of all derivatives as obtained by UM06/6-311G(d) DFT calculations. The strength of the antiferromagnetic exchange coupling constants between the iron(III) ions varies among the three derivatives, despite their magnetic cores remain structurally almost unvaried, as well as the coordination of the metal ions, with a distorted octahedral environment for the two-body iron ions, Fe_b, and a pentacoordination with trigonal bipyramidal geometry for the two-wing iron ions, Fe_w. The different magnetic behavior within the series of examined compounds may be ascribed to the influence of the electronic features of Z on the electron density distribution (EDD) of the central {Fe₄(μ₃-O)₂} core, substantiated by a Quantum Theory of Atoms In Molecules (QTAIM) topological analysis of the EDD, as obtained by UM06 calculations 1-3.