The rigidity of the central C4Fe2 unit in binuclear ferrole iron carbonyl derivatives upon decarbonylation

Reactions of acetylene, thiophene, or even tellurophene with iron carbonyls have been found to give the tricarbonylferrole iron tricarbonyl, [η4-C4H4Fe(CO)3]Fe(CO)3, which has been shown by X-ray crystallography to contain a metallacyclic FeC4 ring with an Fe–Fe bonding distance of ∼2.5 Å to the exocyclic iron atom. This structure has been shown by density functional theory to be the lowest energy C4H4Fe2(CO)6 structure by more than 30 kcal/mol relative to the lowest energy isomeric triplet structure. Dissociation of carbonyl groups from this structure requires ∼40 kcal/mol per carbonyl group. The central C4H4Fe2 unit in the resulting unsaturated C4H4Fe2(CO)n (n = 5, 4, 3) structures is relatively rigid upon carbonyl loss with the Fe–Fe distance decreasing only to ∼2.4 Å in even the highly unsaturated C4H4Fe2(CO)3. Thus 16-electron metal configurations with iron–iron single or double bonds rather than the normally favored 18-electron metal configurations are preferred over structures with higher order iron–iron multiple bonds. For the carbonyl-rich species the lowest energy C4H4Fe2(CO)8 structure has one CC double bond of a tetracarbonylferrole ring bonded to an exocyclic Fe(CO)4 moiety with a long Fe⋯Fe non-bonding distance of ∼3.8 Å. The low-energy structures for C4H4Fe2(CO)7 either have a dihapto ferrole ligand with an iron–iron bond or a tetrahapto ferrole ligand without an iron–iron bond. However, C4H4Fe2(CO)7 does not appear to be viable since carbonyl dissociation from the lowest energy C4H4Fe2(CO)7 structure to give C4H4Fe2(CO)6 is predicted to be an exothermic process.

Graphical abstractThe experimental C4H4Fe2(CO)6 structure is the lowest energy structure by more than 30 kcal/mol. Dissociation of carbonyl groups from this C4H4Fe2(CO)6 structure requires ∼40 kcal/mol per CO group. The central C4H4Fe2 unit in the unsaturated C4H4Fe2(CO)n (n = 5, 4, 3) structures remains relatively rigid upon such CO loss.Figure optionsDownload full-size imageDownload as PowerPoint slide