Synthesis of linear oligomers of [1,3-diethynyl-2,4-bis(trimethylsilyl)cyclobutadiene]cyclopentadienylcobalt: Dimer to nonamer

Coupling of [1,3-diethynyl-2,4-bis(trimethylsilyl)cyclobutadiene]cyclopentadienylcobalt (1) under Hay conditions in boiling butanone gives rise to the isolation of the oligomers (dimer 2 to nonamer 9) after column chromatography (2-4) and preparative HPLC over a cyanophenyl phase (5-9) in moderate yields. Depending on the reaction time either favorably 2 (45 min), a mixture of 2-4 (90 min), or higher oligomers 3-9 (3 h) are formed. Quite interesting are the UV-vis spectra. They show from 3 up to 9 the occurrence of a band at lambda = 312-348 nn which intensifies during the series. The highest wavelength absorption at lambda = 435 nm is almost independent of the oligomer length. An X-ray crystal structure analysis of 2 was undertaken. Deprotection of 2 and 3 with tetramethylammonium fluoride in DMSO yielded the parent dimer 11 and the trimer 12 in almost-quantitative yield. Attempts to determine the rotation barrier around the butadiyne units in 2, 3, 10, and 11 showed that the complexes are torsionally mobile on the NMR time scale down to -100 degrees C and no rotational barrier could be determined.