Fluorine Migration from Carbon to Iron and Fluorine–Iron Dative Bonds in Octafluorocyclohexadiene Iron Carbonyl Chemistry

Abstract

The geometries and energies of mononuclear and binuclear octafluorocyclohexadiene iron carbonyl derivatives have been studied using density functional theory. The lowest energy C6F8Fe(CO)3 tricarbonyl structure is the octafluoro-1,3-cyclohexadiene derivative known experimentally. Isomeric structures containing the other two possible C6F8 hexagon ligands lie at higher energies. Unprecedented oxidative addition involving fluorine migration from carbon to iron is theoretically predicted in the C6F8Fe(CO)4 tetracarbonyl systems to give low-energy C6F7Fe(CO)4F structures. A similar energetically favored fluorine migration, also from carbon to iron, converts the coordinatively unsaturated (η4-C6F8)Fe(CO)2 dicarbonyl complex to the coordinatively saturated heptafluorocyclohexadienyl complex (η5-C6F7)Fe(CO)2F. For C6F8Fe2(CO)8, trans isomers with two Fe(CO)4 moieties on opposite sides of the C6F8 ring coordinated to C═C bonds are the lowest energy structures. These are similar to the experimentally known hexafluorocyclopentadiene complex C5F6[Fe(CO)4]2. The corresponding cis isomers with the Fe(CO)4 moieties on the same size of the C6F8 ring are high-energy structures, possibly a consequence of the steric hindrance between the two closely spaced Fe(CO)4 moieties. The low-energy C6F8Fe2(CO)7 structures may be viewed as the substitution products of the experimentally known Fe2(CO)9, with the two carbonyl groups replaced by a terminal or bridging C6F8 ligand donating four electrons to one or both iron atoms of the central Fe2 unit. The low-energy singlet structures in the unsaturated C6F8Fe2(CO)6 system have C6F8 ligands donating electrons to the central Fe2 system not only through Fe–C bonds but also through F→Fe dative bonds involving fluorine lone pairs.