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((mu)3-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 P with cell parameters a = 12.6926(7) (angstrom), b = 12.9134(8) (angstrom), c = 13.4732(8) (angstrom), (alpha)= 115.372(2) °, (beta)= 107.702(3) °, and (gamma)= 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 (mu)3-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-((mu)3-O) bonds in the two cases. Analyses of both the experimental and theoretical EDDs reveal very significant differences between the two Fe-((mu)3-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. Mossbauer 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.