Diverse bonding modes of the tetramethyleneethane ligand in binuclear iron carbonyl derivatives

Abstract

The tetramethyleneethane (TME) iron carbonyl derivative cis-(η3,η3-TME)Fe2(CO)6 has long been known as a product from the reaction of allene with iron carbonyls. Theoretical studies on the (TME)Fe2(CO)6 system show this experimentally observed singlet cis bis(allyl) structure with an Fe–Fe bond length of ∼2.9 A to be the lowest energy structure. However, three other singlet (TME)Fe2(CO)6 structures lying in energy within 10 kcal mol−1 of this global minimum exhibit different modes of bonding of the TME ligand to the Fe2(CO)6 unit including cis and trans bis(butadiene) structures and a trans bis(trimethylenemethane) structure. The lowest energy structure for the heptacarbonyl (TME)Fe2(CO)7 has four TME carbon atoms bonded to an Fe(CO)3 unit as a butadiene ligand with the remaining two TME carbon atoms forming a five-membered FeC4 chelate ring. The lowest energy (TME)Fe2(CO)8 structure has one Fe(CO)4 unit bonded to a TME CC double bond and the other Fe(CO)4 unit bonded to two external TME carbon atoms to form a five-membered FeC4 chelate ring thereby leaving an uncomplexed CC double bond. The octacarbonyl (TME)Fe2(CO)8 is disfavored relative to CO loss to give the heptacarbonyl (TME)Fe2(CO)7. The heptacarbonyl is marginally viable with a low but endothermic CO dissociation energy of ∼11 kcal mol−1 to give the hexacarbonyl (TME)Fe2(CO)6. This thermochemistry can account for the experimental observation of the hexacarbonyl (TME)Fe2(CO)6 rather than carbonyl richer species as products from the reactions of allene with iron carbonyls.