Synthesis of the cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)C[tbnd]C(CO2Et)} and structural characterization of trimethylsilyl substituted cobaltacyclopentadiene complexes derived therefrom

Abstract

The reaction of (η5-C5H5)Co(PPh3)2 with TMSCC(CO2Et) leads to the formation of the cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)CC(CO2Et)} (6). In benzene-d6 solution containing PPh3, 6 undergoes slow conversion to the cobaltacyclopentadiene complex (η5-C5H5)(PPh3)Co{κ2-(TMS)CC(CO2Et)C(TMS)C(CO2Et)} (7, TMS = SiMe3). Alternatively, reaction of 6 and PhCCPh forms the metallacyclopentadiene regioisomers (η5-C5H5)(PPh3)Co{κ2-(R1)CC(R2)C(Ph)=C(Ph)} [9-major, R1 = SiMe3, R2 = CO2Et; 9-minor, R1 = CO2Et, R2 = SiMe3). The metallacycle substitution pattern in 9-major and 9-minor is readily deduced from the 1H NMR spectral resonances of the diastereotopic ethoxycarbonyl hydrogens. When the diastereotopic hydrogens of the ethoxycarbonyl have similar chemical shifts the ester is situated on the β-carbon of the metallacycle. When the methylene hydrogens give rise to well-separated resonances the ethoxycarbonyl is situated on the α-carbon of the metallacycle. The solid state structures of 7 and 9-major were determined by X-ray crystallography.