Abstract
The highly unsaturated binuclear butadiene iron carbonyls (C4H6)2Fe2(CO)n (n = 2, 1) have been examined using density functional theory. For (C4H6)2Fe2(CO)n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C4H6)2Fe2(CO)n (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C4H6)2Fe2(CO)2 structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C4H6)2Fe2(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C4H6)2Fe2(CO)n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH2 groups. Singlet (C4H6)2Fe2(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.
Highlights
The chemistry of metal carbonyl complexes of acyclic hydrocarbons dates back to the 1930 discovery by Reihlen et al [1] of the mononuclear butadiene iron tricarbonyl complex, C4H6Fe(CO)3 by the reaction of butadiene with iron pentacarbonyl at elevated temperatures
In order to assess the possibilities for binuclear iron carbonyl derivatives with iron-iron bonds we have performed a density functional theory (DFT) study on possible structures for (η4-C4H6)2Fe2(CO)n (n = 5, 4, 3), predicted to have structures with formal Fe–Fe single bonds, Fe=Fe double bonds, and Fe Fe triple bonds, respectively [4]
The lowest energy structures for these (η4-C4H6)2Fe2(CO)n derivatives were found to be coaxial structures in which each metal atom is bonded to a single butadiene ligand (Figure 1B)
Summary
The chemistry of metal carbonyl complexes of acyclic hydrocarbons dates back to the 1930 discovery by Reihlen et al [1] of the mononuclear butadiene iron tricarbonyl complex, C4H6Fe(CO) by the reaction of butadiene with iron pentacarbonyl at elevated temperatures. In 1962 Murdoch and Weiss [3] used the reaction of butadiene with Fe2(CO) at room temperature to synthesize the tetracarbonyl (η2-C4H6)Fe(CO) in which only one of the two C=C double bonds of the butadiene ligand is bonded to the iron atom. An additional product from the latter reaction was the binuclear complex C4H6[Fe(CO)4]2 in which each C=C double bond of the butadiene ligand is bonded to a separate Fe(CO) unit with the iron atoms much too far apart for any kind of direct iron-iron bond. The lowest energy structures for these (η4-C4H6)2Fe2(CO)n derivatives were found to be coaxial structures in which each metal atom is bonded to a single butadiene ligand (Figure 1B)
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