Dimolybdenum complexes derived from cyclo-octa-1,5-diene and penta-1,3-diene. Crystal and molecular structure of [Mo2(CO)3(µ-C8H10)(η-C5H5)2]

Abstract

In boiling octane, cyclo-octa-1,5-diene and [Mo₂(CO)₆(η-C₅H₅)₂] react to give the compounds [Mo₂(CO)₃(µ-C₈H₁₀)(η-C₅H₅)₂] and [Mo(CO)₂(η³-C₈H₁₃)(η-C₅H₅)]. An X-ray diffraction study has been carried out on the dimolybdenum species, crystals of which are monoclinic, a= 16.852(10), b= 7.427(2), c= 17.810(12)Å, β= 124.94(4)°, space group P2₁/c(no. 14). The structure was solved by heavy-atom methods and has been refined to R 0.061 (R′ 0.059) for 3 613 independent reflections. The alkyne group of a cyclo-oct-1-en-5-yne ligand forms a symmetrical transverse bridge across a Mo–Mo single bond [2.986(1)Å], with the ring adopting a ‘tub’ conformation, concave to the metal–metal bond. This allows the alkene group to form a second ring-to-metal attachment involving only one of the metal atoms. This metal atom also carries one cyclopentadienyl group and one terminal carbonyl ligand. The other metal atom carries one cyclopentadienyl group and two carbonyl ligands, one of which, however, shows the characteristic geometry of a ‘weakly semibridging’ carbonyl (Mo–C 1.94 and 2.89 Å, Mo–Mo–C 68°, Mo–C–O 170°). The molecule possesses no symmetry; the C₈ ring is twisted so that the alkene and alkyne moieties are not parallel to one another, and the other ligands are not symmetrically related to the metal–metal bond. The i.r. spectrum of the compound in solution indicates the existence of isomers. However, the ¹H and ¹³C n.m.r. spectra are invariant down to –80 °C and therefore the isomers are interconverting sufficiently rapidly to provide averaged nuclear environments on the n.m.r. time-scale, although the nature of the spectra reveal the species present to lack symmetry. It is proposed that rotamers exist, produced by rotation of the Mo(CO)₂(η-C₅H₅) group about an axis through the midpoint of the µ-alkyne bond, with low barriers to their inter-conversion. The compound [Mo₂(CO)₃(µ-C₈H₁₀)(η-C₅H₅)₂] reacts with CO to give [Mo₂(CO)₄(µ-C₈H₁₀)(η-C₅H₅)₂], which shows dynamic n.m.r. properties similar to other [Mo₂(CO)₄(µ-alkyne)(η-C₅H₅)₂] species. Treatment of [Mo₂(CO)₆(η-C₅H₅)₂] with buta-1,3-diene,2-methylbuta-1,3-diene or 2,3-dimethylbuta-1,3-diene in boiling heptane or octane failed to give isolable dimolybdenum compounds, but a cis/trans mixture of penta-1,3-diene afforded the complex [Mo₂(CO)₂(µ-C₅H₈)(η-C₅H₅)₂], from the ¹H n.m.r. spectrum of which it is deduced that the µ-C₅H₈ ligand is cis-MeCHCH–CHCH₂.