Alternative modes of bonding of C4F8 units in mononuclear and binuclear iron carbonyl complexes. (12th April 2019) - Huang, Liping
Abstract : The lowest energy C4 F8 Fe(CO)4 structure is not the experimentally known ferracyclopentane complex but instead isomeric (perfluorobutene)iron tetracarbonyls. However, activation energies for the fluorine shifts required to form the latter isomers are very high. The lowest energy (C4 F8 )2 Fe2 (CO) n ( n = 7, 6) structures have bridging perfluorocarbene and terminal perfluoroolefin ligands. Abstract : Density functional theory studies show that the lowest energy C4 F8 Fe(CO)4 structure is not the very stable experimentally known ferracyclopentane isomer (CF2 CF2 CF2 CF2 )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 (CF2 CF2 CF2 CF2 )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 (C4 F8 )2 Fe2 (CO) n ( n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands.