Experimental and Theoretical Electron Density Distribution and Magnetic Properties of the Butterfly-like Complex [Fe₄O₂(O₂CCMe₃)₈(NC₅H₄Me)₂]·2CH₃CN

The structure and electron density distribution (EDD) of the carboxylate-bridge iron complex [FeIII4(μ3-O)2(O2CCMe3)8(NC5H4Me)2]·2CH3CN, 1, has been determined from synchrotron X-ray diffraction data (Rint = 0.025) collected with the crystal cooled to 16(5) K. At this temperature complex 1 crystallized in the triclinic space group P1̄ with cell parameters a = 12.6926(7) Å, b = 12.9134(8) Å, c = 13.4732(8) Å, α = 115.372(2)°, β = 107.702(3)°, and γ = 102.731(2)°. The theoretical EDD determined from a density functional theory (DFT) single point calculation of an entire molecule of 1 at the experimental geometry has been analyzed and compared to the experimental EDD. The latter is expressed in the framework of a multipolar model with parameters determined by least-squares refinement (Rw(F2) = 0.024) based on the X-ray diffraction data. The central μ3-oxygen atom in 1 is significantly out of the plane spanned by the three Fe atoms coordinated to this oxygen. Comparison of measures for the bonding geometry around the iron atoms in 1 with the corresponding values for the iron atoms in relevant trinuclear complexes suggests that there are significant differences in the Fe−(μ3-O) bonds in the two cases. Analyses of both the experimental and theoretical EDDs reveal very significant differences between the two Fe−(μ3-O) bonds in 1, with one bond being much more directed and stronger than the other bond. A topological analysis of the EDDs using the atoms in molecules approach also reveals very distinct differences between the properties of the two FeIII atoms. A clear exponential relationship is found between the Laplacian of the experimental density at the bond critical points in the Fe−ligand bonds and their bond lengths. Mössbauer spectroscopy of 1 shows two easily separable doublets corresponding to the two different iron sites. Magnetic susceptibility measurements between 4.2 and 300 K indicate antiferromagnetically coupled FeIII atoms constituting an S = 0 ground state.