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 Å 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.