Tetrafluoroethylene versus trifluoromethylfluorocarbene complexes of cobalt carbonyl

Abstract

The reaction of tetrafluoroethylene with Co 2 (CO) 8 under mild conditions is reported to give (OC) 4 CoCF 2 CF 2 Co(CO) 4. This compound readily undergoes decarbonylation with accompanying fluorine migration to give the trifluoromethylfluorocarbene complex (µ-CF 3 CF)Co 2 (CO) 6 (µ-CO) and eventually the cluster CF 3 CCo 3 (CO) 9. In order to understand the chemistry of these cobalt carbonyl complexes obtained from tetrafluoroethylene, the structures and thermochemistry of the (C 2 F 4)Co 2 (CO) n (n = 8, 7, 6, 5) systems have been investigated by density functional theory. The lowest energy (C 2 F 4)Co 2 (CO) 8 isomer is the experimentally observed (OC) 4 CoCF 2 CF 2 Co(CO) 4 , lying ~6 kcal/mol in energy below the isomeric (CF 3 CF)[Co(CO) 4 ] 2. Loss of CO from (OC) 4 CoCF 2 CF 2 Co(CO) 4 with accompanying fluorine migration to give (µ-CF 3 CF)Co 2 (CO) 6 (µ-CO) is essentially thermoneutral within ~1 kcal/mol. The higher energy of ~20 kcal/mol for the isomeric (µ-CF 2 CF 2)Co 2 (CO) 6 (µ-CO) structure, where fluorine migration has not occurred, suggests a significant activation energy for this process. Further loss of CO from (µ-CF 3 CF)Co 2 (CO) 6 (µ-CO) gives low-energy (µ-CF 3 CF)Co 2 (CO) n (µ-CO) isomers (n = 5, 4) containing Co=Co multiple bonds and/or vacant coordination sites. Such structures are possible intermediates to form CF 3 CCo 3 (CO) 9 by reaction with excess Co 2 (CO) 8 followed by Co(CO) n F elimination.