Crystal structure and stereochemical non-rigidity of octacarbonyl-(η-1,3-dimethyl-2-vinylcyclopentadienyl)-µ3-propylidyne-triangulo-tri-iron

Abstract

The title compound has been obtained by treating [Fe₃(CO)₁₂] with methylacetylene and is characterized by i.r., mass, ¹H, and ¹³C n.m.r. spectra. The crystal structure has been determined by X-ray methods. Crystals are monoclinic, space group P2₁/c, with Z= 4, in a unit cell of dimensions a= 7.584(7), b= 15.851(9), c= 18.535(11)Å, and β= 103.3(1)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by block-diagonal least squares to R 0.032 for 3145 observed reflections. The molecule is formed by a Fe₃C cluster very similar to the Co₃C core in [Co₃(CY)(CO)₉](Y = alkyl, aryl, or other group). Trimerization of the alkyne leads to the formation of a cyclopentadienyl unit (bearing one vinyl and two methyl groups)η-bonded to one iron atom. Of the eight carbonyls co-ordinated to the iron atoms, six are terminal and two are bridging the shortest sides of the cluster. Variable-temperature ¹³C n.m.r. spectra show that a two-stage exchange process occurs.