Cooperative and competitive effects associated with Fe(CO)3 binding to annelated benzenes

Abstract

Density functional theory (DFT) calculations have been carried out on the sequential transfer of Fe(CO)₃ groups from the Fe(CO)₃ complex of cyclobutadiene [CBD–Fe(CO)₃, 5] to tris(etheno)-annelated benzene (3) and to 1,2;4,5-bis(etheno)-annelated benzene (9). The changes in the electronic structure of the central benzene ring in 3, brought about by each Fe(CO)₃ transfer reaction, make the transfer of Fe(CO)₃ groups to 3 from 1–3 molecules of 5 cooperative. Our calculations find that, as the number of Fe(CO)₃ groups transferred to 3 increases, the bond lengths between the benzo carbons and each Fe atom decrease; and the energetic favorability of each Fe(CO)₃ transfer reaction increases. In contrast, the transfer of Fe(CO)₃ groups to 9 from two molecules of 5 is competitive. Transfer of a second Fe(CO)₃ group from 5 to 9 increases the bond lengths between the benzo carbons and the Fe atoms and makes the second Fe(CO)₃ transfer reaction more energetically unfavorable than the first. Our calculations also indicate that there is little, if any, π bonding between the three CBD–Fe(CO)₃ rings in 8, the tris-Fe(CO)₃ adduct of 3. Therefore, π bonding between the four-membered rings in 8 cannot explain the comparatively favorable energy for its formation.