Add time:08/16/2019         Source:sciencedirect.com

Four different chromocenes, in which two hydrogen atoms in the 1 and 1' positions have been substituted by ethyl, t-butyl, trimethylsilyl and phenyl groups, have been reacted with silica gel. As in the case with chromocene (IR bands of adsorbed CO at 2047.1, 1972.2, 1917.2, 1824.1 and 1621.4 cm−1) five strong CO bands are observed, of which the two bands at higher wavenumbers are shifted to lower wavenumbers for the ethyl (2039.7 and 1962.4 cm−1), t-butyl (2042.0 and 1965.2 cm−1) and trimethylsilyl (2041.3 and 1965.9 cm−1) compounds and shifted very little for the phenyl one (2045.2 and 1971.9 cm−1). These two CO bands are interpreted as the asymmetric and symmetric CO stretching vibrations of two CO molecules bonded to one chromium atom. A shift of these bands to lower wavenumbers indicates a higher electron density, and a shift to higher wavenumbers a lower electron density on the chromium. A similar trend is also observed for the three CO bands at lower wavenumbers. On reaction of butyllithium with chromocene and addition of ethylbromide or trimethysilylchloride mixtures of the 1,1'-disubstituted, the monosubstituted and the unsubstituted chromocene were obtained. These mixtures, after reaction with silica gel and CO, showed that in the case of the ethyl-substituted chromocene no higher reactivity for the unsubstituted ligand exists, while for the mixed trimethylsilyl chromocene only the cyclopentadienyl ligand reacts with the silica gel.

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