The Competition for Electrons: Aromatic Stabilization in a Six-Membered Ring vs. Cyclobutadiene-Iron Complex. The Molecular Structure of Tris(tricarbonylironcyclobutadieno)benzene.
The title compd. (shown as I) was prepd. from hexabromotricyclobutabenzene and diiron nonacarbonyl, and its x-ray structure was measured. The C-C bonds in the six-membered ring are longer than in an typical arom. moiety (1.443-1.470 .ANG.), suggesting that the system is best regarded as three C(sp2)-C(sp2) single bonds connecting three (CO)3Fe-cyclobutadiene complexes. The effect of complexing tricarbonyliron to a benzocyclobutadiene moiety is not additive. Ab initio calcns. (Gaussian 94, Revision D.1 at HF/3-21G, HF/6-31G*, HF/6-311G**, MP2/6-31G*, B3-LYP/6-31G* levels) and NRT analyses were used to understand the geometrical, electronic, and energetic properties of the systems. Complexation of (CO)3Fe to benzocyclobutadiene aromatizes the systems, whereas the opposite is happening upon complexation of three Fe moieties to tris(cyclobutadieno)benzene. The study of the energy required for the deformation of the ligands to their geometry in the complex suggests that the Fe-cyclobutadiene bond is stronger in the title complex than in (CO)3Fe-cyclobutadiene. The title complex shows a small but significant Mills-Nixon effect.
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