METALLACYCLE FORMATION THROUGH THE LINKING OF AN ALKYNE WITH PHOSPHIDO AND ACETYL GROUPS AT AN IRON CENTER - X-RAY STRUCTURE OF [CPFE(CO)(PH2PCH(CO2ME)C(CO2ME)=C(ME)O)] (CP = ETA-5-C5H5)

The diphenylphosphine ligand of [CpFe(CO)(PPh2H)(COMe)] (Cp = eta5-C5H5) can be deprotonated with DBU (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) or n-butyllithium; subsequent alkylation with RI (R = Me, Et) gives the complexes [CpFe(CO)(PPh2R)(COMe)]. Reaction of the anionic phosphido complex with the electrophilic alkyne methyl propiolate (HC=CCO2Me) followed by reprotonation with CH3COOH gives the vinylphosphine complex [CpFe(CO)(PPh2CH=CHCO2Me) (COMe)], whereas a similar reaction sequence with dimethyl acetylenedicarboxylate (DMAD, MeO2CC=CCO2Me) produces two isomers of the complex [CpFe(CO)(Ph2PCH(CO2Me)C(CO2Me)=C(Me)O}], in which linking of the alkyne with both the phosphido and acetyl ligands has occurred to form a six-membered metallacycle. The structure of one of the two isomers has been determined by X-ray diffraction and shows that the metallacyclic ring is bound to the iron atom through the phosphorus and the carbonyl oxygen of the acetyl group, and adopts a boat conformation in the solid state.