Synthesis of the cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)Ctriple bond; length of mdashC(CO2Et)} and structural characterization of trimethylsilyl substituted cobaltacyclopentadiene complexes derived therefrom

The reaction of (η5-C5H5)Co(PPh3)2 with TMSCtriple bond; length of mdashC(CO2Et) leads to the formation of the cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)Ctriple bond; length of mdashC(CO2Et)} (6). In benzene-d6 solution containing PPh3, 6 undergoes slow conversion to the cobaltacyclopentadiene complex (η5-C5H5)(PPh3)Co{κ2-(TMS)Cdouble bond; length as m-dashC(CO2Et)C(TMS)double bond; length as m-dashC(CO2Et)} (7, TMS = SiMe3). Alternatively, reaction of 6 and PhCtriple bond; length of mdashCPh forms the metallacyclopentadiene regioisomers (η5-C5H5)(PPh3)Co{κ2-(R1)Cdouble bond; length as m-dashC(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.