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

• The cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)CC(CO2Et)} (6) undergoes reaction with TMSCC(CO2Et), and with diphenylacetylene, to give cobaltacyclopentadiene complexes bearing trimethylsilyl substituents on the metallacycle ring.
• In the case of diphenylacetylene, two 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)] are formed and readily distinguished on the basis of 1H NMR spectral data.
• The first solid state structures of cobaltacyclopentadienes bearing trimethylsilyl substituents are reported.

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.

The reaction of (η5-C5H5)Co(PPh3)2 with TMSCCCO2Et leads to the formation of the cobalt–alkyne complex (η5-C5H5)(PPh3)Co{η2-(Me3Si)CC(CO2Et)} which in turn serves as a precursor to structurally characterized trimethylsilyl-substituted cobaltacyclopentadiene complexes. The metallacycle substitution pattern is readily deduced from the 1H NMR spectral resonances of the diastereotopic ethoxycarbonyl hydrogens.Figure optionsDownload as PowerPoint slide