Stereoselective alkyne insertion into a bridging hydride at a diiron centre: cis–trans isomerisation and metallacycle formation

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.