Reactions of [Fe2(CO)6(μ-CO)(μ-dppm)] with alkynes: stepwise synthesis of tropone at a dinuclear metal centre

Article abstract of DOI:10.1016/0277-5387(95)00145-I

The di-iron complex [Fe2(CO)6(μ-CO)(μ-dppm)] [1; dppm=bis(diphenylphosphino)methane] reacts with alkynes R(1)C.ident.CR(2) under UV irradiation to produce dimetallacyclopentenone complexes [Fe2(CO)5{μ-σ:η3-CR(1)CR(2)C(O)}(μ-dppm)] (2a-e; R(1)=R(2)=H, Me or Ph; R(1)=H, R(2)=Me or Ph). The complexes exist in two isomeric forms in which the diphosphine ligand is either cis or trans with respect to the organic ligand. In the cis isomers of complexes 2 the R(1) site is occupied by hydrogen, while in the trans isomers the R(1) site is occupied by Me or Ph, suggesting that steric factors control the geometry. The molecular structure of cis-[Fe2(CO)5{μ-σ:η3-CHCHC(O)}(μ-dppm)] (2a) has been elucidated by X-ray diffraction. The complex exists in the solid state as a mixture of enantiomers, shown by variable temperature NMR spectroscopy to interconvert in solution via ethyne migration from one COligand to another. Further reaction of 2a with ethyne affords [Fe2(CO)4 {μ-σ,σ':η2,η2'-CHCHC(O)CHCH}(μ-dppm)] (3) and [Fe2(CO)4{μ-η3:η3'-C6H6C(O)}(μ-dppm)] (4) stepwise, containing two and three molecules of ethyne, respectively. X-ray diffraction studies show that 3 is a pentadienone fly-over bridge complex and that4 is a complex of tropone, stabilized by a weak intramolecular C-H...O hydrogen bond. Addition of diphenylacetylene to trans-[Fe2(CO)5{μ-.si gma.:η3-CPhCPhC(O)}(μ-dppm)] (2e) affords the ferrole complex trans-[Fe2(CO)5{μ-σ,sigma.':η4-C4Ph4}(μ-dppm)] (5), also crystallographically charcterized. The different pathways for alkyne linking with 2a and 2e are traced to steric factors, arising from the different orientations of the diphosphine and organic ligands in the two species.