Bonding of Iron Tricarbonyl Units to Heptafulvene: Trimethylenemethane, Butadiene, and Allylic Coordination Modes

Complexation of the very unstable free heptafulvene with iron carbonyls is known experimentally to lead to stable complexes, including two isomers of (η⁴-C₇H₆CH₂)­Fe­(CO)₃ as well as the binuclear (C₇H₆CH₂)₂Fe₂(CO)₆. Density functional theory shows that structures of the mononuclear (C₇H₆CH₂)­Fe­(CO)₃ with trimethylenemethane and butadiene subunits of the heptafulvene ligand bonded to the Fe­(CO)₃ moiety have nearly equal energies within ∼3 kcal/mol, consistent with the experimental observation of two isomers depending upon the synthetic method. For (C₇H₆CH₂)­Fe₂(CO)₆ a <i>trans</i>-η⁴:η⁴ structure with no iron–iron bond and a <i>cis</i>-η³:η³ allylic structure with an iron–iron bond are predicted to have energies within ∼3.0 kcal/mol. Comparison of the predicted ν­(CO) frequencies for these two structures with the experimental ν­(CO) frequencies for the structurally uncharacterized (C₇H₆CH₂)­Fe₂(CO)₆ suggests the <i>cis</i>-η³:η³ allylic structure for the latter