Bonding of Iron Tricarbonyl Units to Heptafulvene:
Trimethylenemethane, Butadiene, and Allylic Coordination Modes
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Abstract
Complexation
of the very unstable free heptafulvene with iron carbonyls
is known experimentally to lead to stable complexes, including two
isomers of (η<sup>4</sup>-C<sub>7</sub>H<sub>6</sub>CH<sub>2</sub>)Fe(CO)<sub>3</sub> as well as the binuclear (C<sub>7</sub>H<sub>6</sub>CH<sub>2</sub>)<sub>2</sub>Fe<sub>2</sub>(CO)<sub>6</sub>.
Density functional theory shows that structures of the mononuclear
(C<sub>7</sub>H<sub>6</sub>CH<sub>2</sub>)Fe(CO)<sub>3</sub> with
trimethylenemethane and butadiene subunits of the heptafulvene ligand
bonded to the Fe(CO)<sub>3</sub> moiety have nearly equal energies
within ∼3 kcal/mol, consistent with the experimental observation
of two isomers depending upon the synthetic method. For (C<sub>7</sub>H<sub>6</sub>CH<sub>2</sub>)Fe<sub>2</sub>(CO)<sub>6</sub> a <i>trans</i>-η<sup>4</sup>:η<sup>4</sup> structure
with no iron–iron bond and a <i>cis</i>-η<sup>3</sup>:η<sup>3</sup> 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 (C<sub>7</sub>H<sub>6</sub>CH<sub>2</sub>)Fe<sub>2</sub>(CO)<sub>6</sub> suggests the <i>cis</i>-η<sup>3</sup>:η<sup>3</sup> allylic structure for the latter
