SYNTHESIS AND REACTIVITY TOWARDS UNSATURATED ORGANICS OF THE DIIRON HYDRIDE COMPLEX [FE2(CO)4(MU-CO)(MU-H)-(MU-PPH2)(MU-PH2PCH2PPH2)]

Ultraviolet irradiation of [Fe2(CO)6(mu-CO)(mu-dppm)] 1 (dppm = Ph2PCH2PPh2) with diphenylphosphine results in phosphorus-hydrogen bond cleavage to give [Fe2(CO)4(mu-CO)(mu-H)(mu-PPh2)(mu-dppm)] 2.The hydride functionality in 2 readily inserts unsaturated organics. Thus reaction with ethyne yields the mu-ethenyl complex [Fe2(CO)4(mu-CH=CH2)(mu-PPh2)(mu-dppm)] 3. Both allene and propyne insert into 2 to give [Fe2(CO)4(mu-MeC=CH2)(mu-PPh2)(mu-dppm)] which exists in two isomeric forms 4a, 4b. Both contain the 1-methylvinyl ligand but differ in the relative orientations of diphosphine and vinyl moieties. A third isomer [Fe2(CO)4(mu-HC=CHMe)(mu-PPh2)(mu-dppm)] 4c containing a 2-methylvinyl ligand has been isolated from the reaction of 1 with allyldiphenylphosphine via phosphorus-carbon bond cleavage at the diiron centre. Reaction of 2 with phenylethyne gives two isomeric forms of [Fe2(CO)4(mu-PhC=CH2)(mu-PPh2)(mu-dppm)] 5a, 5b. The nature of these isomers is deduced from P-31 NMR data and an X-ray crystallographic study of one isomer of the propyne insertion product. Compound 4a crystallises in the monoclinic space group P2(1)/c with a = 10.749(8), b = 18.122(15), c = 19.854(9) angstrom, beta = 94.32(5)-degrees and Z = 4. On the basis of 3492 unique reflections and 478 parameters the structure has been refined to R = 0.055 and R' = 0.054. The structure confirms the formation of a 1-methylvinyl ligand which lies trans to the diphosphine moiety but cis to the diphenylphosphido bridge. A variable-temperature P-31 NMR study allows monitoring of the sigma-eta vinyl fluxionality in each isomer. Reaction of 2 with ethyl diazoacetate results in insertion of the nitrogen-nitrogen bond into the hydride to give the unusual zwitterionic product [Fe2(CO)4{mu-HN-N=CH(CO2Et)}(mu-PPh2)(mu-dppm)] 6 which is stable to prolonged thermolysis in toluene.