Alternative modes of bonding of C4F8 units in mononuclear and binuclear iron carbonyl complexes

Abstract

Density functional theory studies show that the lowest energy C₄F₈Fe(CO)₄ structure is not the very stable experimentally known ferracyclopentane isomer (CF₂CF₂CF₂CF₂)Fe(CO)₄ obtained from Fe(CO)₁₂ and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO)₄ structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol⁻¹. However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO)₄ complexes from tetrafluoroethylene are very high (e.g., ∼70 kcal mol⁻¹). Therefore the ferracyclopentane isomer (CF₂CF₂CF₂CF₂)Fe(CO)₄, which does not require a fluorine shift to form from Fe₃(CO)₁₂ and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C₄F₈)₂Fe₂(CO)_n (n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands.