Abstract
Addition of the activated alkyne dimethyl acetylenedicarboxylate (dmad) to [Fe2(CO)4(µ-H)(µ-CO)(µ-PPh2)(µ-dppm)]1(dppm = Ph2PCH2PPh2) afforded the σ-η vinyl complex cis-[Fe2(CO)4{µ-C(CO2Me)CH(CO2Me)}(µ-PPh2)µ-dppm)]2. Crystallography showed that the insertion is stereo-selectively cis, with the phosphorus-containing ligands also adopting a cis configuration. Ultraviolet irradiation of 2 resulted in slow conversion into a second isomer, trans-[Fe2(CO)4{µ-C(CO2Me)CH-(CO2Me)}(µ-PPh2)(µ-dppm)]3, which still retains the cis arrangement of phosphines as shown by 31P NMR spectroscopy. This transformation is reversible, 3 slowly converting back into 2 at room temperature, a process which is accelerated upon heating. At temperatures of 80 °C, 2(and 3) was converted irreversibly into a third isomer [Fe2(CO4{C(CO2Me)CHC(OMe)O}(µ-PPh2)(µ-dppm)]4, containing a metallacyclic ring by virtue of metal co-ordination of an ester carbonyl. X-Ray crystallography revealed a trans vinyl orientation and a trans disposition of the phosphorus-containing groups.
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