REACTIONS OF [FE-2(CO)(6)(MU-CO)(MU-DPPM)] WITH ALKYNES - STEPWISE SYNTHESIS OF TROPONE AT A DINUCLEAR METAL CENTER

The di-iron complex [Fe-2(CO)(6)(mu-CO)(mu-dppm)] [1; dppm = bis(diphenylphosphino)methane] reacts with alkynes R(1)C drop CR(2) under UV irradiation to produce dimetallacyclopentenone complexes [Fe-2(CO)(5){mu-sigma:eta(3)-CR(1)CR(2)C(O)}(mu-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-[Fe-2(CO)(5){mu-sigma:eta(3)-CHCHC(O)}(mu-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 CO ligand to another. Further reaction of 2a with ethyne affords [Fe-2(CO)(4){mu-sigma,sigma':eta(2),eta(2)'-CHCHC(O)CHCH}(mu-dppm)] (3) and [Fe-2(CO)(4){mu-eta(3):(eta(3)'-C6H6C(O)}(mu-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 that 4 is a complex of tropone, stabilized by a weak intramolecular C-H ... O hydrogen bond. Addition of diphenylacetylene to trans-[Fe-2(CO)(5){mu-sigma:eta(3)--CPhCPhC(O)}(mu-dppm)] (2e) affords the ''ferrole'' complex trans-[Fe-2(CO)(5){mu-sigma,sigma:eta(4)-C(4)Ph(4)}(mu-dppm)] (5), also crystallographically characterized. 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.