Synthesis and reactivity towards unsaturated organics of the diiron hydride complex [Fe2(CO)4(µ-CO)(µ-H)-(µ-PPh2)(µ-Ph2PCH2PPh2)

Abstract

Ultraviolet irradiation of [Fe2(CO)6(µ-CO)(µ-dppm)]1(dppm = Ph2PCH2PPh2) with diphenylphosphine results in phosphorus–hydrogen bond cleavage to give [Fe2(CO)4(µ-CO)(µ-H)(µ-PPh2)(µ-dppm)]2. The hydride functionality in 2 readily inserts unsaturated organics. Thus reaction with ethyne yields the µ-ethenyl complex [Fe2(CO)4(µ-CHCH2)(µ-PPh2)(µ-dppm)]3. Both allene and propyne insert into 2 to give [Fe2(CO)4(µ-MeCCH2)(µ-PPh2)(µ-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(µ-HCCHMe)(µ-PPh2)(µ-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(µ-PhCCH2)(µ-PPh2)(µ-dppm)]5a, 5b. The nature of these isomers is deduced from 31P NMR data and an X-ray crystallographic study of one isomer of the propyne insertion product. Compound 4a crystallises in the monoclinic space group P21/c with a= 10.749(8), b= 18.122(15), c= 19.854(9)Å, β= 94.32(5)° 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 31P NMR study allows monitoring of the σ–η 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{µ-HN–NCH(CO2Et)}(µ-PPh2)(µ-dppm)]6 which is stable to prolonged thermolysis in toluene.