Synthesis, crystal structure, and stereochemical non-rigidity of µ-[1-σ:1–2-η-2-carboxylato-1-ethylbut-1-enyl-O(2Fe)]-bis(tricarbonyl-iron)(Fe–Fe), a new complex obtained from iron carbonyls under apolar conditions

Abstract

The title complex [Fe₂(CO)₆{CEtCEt(CO₂)}] has been obtained from [Fe₃(CO)₁₂] and hex-3-yne in hydrocarbon solvents and characterized by i.r., mass, and ¹H and ¹³C n.m.r. spectra. The crystal structure has been determined by X-ray diffraction methods. The crystals are triclinic, space group P, with Z= 2, in a unit cell of dimensions a= 9.500(9), b= 10.627(11), c= 9.140(9)Å, α= 109.61(8), β= 117.27(8), and γ= 79.71(7)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.027 for 3 295 observed reflections. The complex is formed by two non-equivalent iron atoms (Fe–Fe 2.438 Å), both co-ordinated by three terminal CO, and by a diethyl-substituted ‘acrylate’ ligand bound to both irons via Fe–C π and Fe–C and Fe–O σ bonds. This ligand is derived by insertion of a CO₂ group between the alkyne and the metal atoms. In the complex an unusual five atom carbon–oxygen–iron heterocycle is present and an oxygen of the carboxyl group symmetrically bridges both iron atoms (Fe–O 1.950 and 1.990 Å). Variable-temperature ¹³C n.m.r. studies in solution show localized exchange of the CO groups at lower temperatures, whereas at room temperature a bonding interchange within the organic moiety and the iron atoms is proposed. Some comments on the factors influencing the formation of this complex and on the possible intermediates in the reaction are made.