OBSERVATION OF SLOWED ROTATION ABOUT THE ETA-6-ARENE CHROMIUM BOND IN THE TRIPODAL CHROMIUM COMPLEXES OF THE TRIMERS OF BICYCLO[2.2.1]HEPT-2-YNE - INTRAMOLECULAR ROTATIONAL BARRIERS IN ORGANOMETALLIC COMPLEXES AND THEIR CORRELATION WITH INTERNAL NONBONDING INTERACTIONS AND STRUCTURAL-CHANGES

The all-syn 1 and syn-anti-syn 2 trimers of bicyclo[2.2.1]hept-2-yne have been considered as realistic role models for some of the intramolecular rotational behaviour of hexaethylbenzene in its metal complexes. Both 1 and 2 complex to chromium as arenes rather than as cyclohexatrienes. The variable-temperature 75.5 MHz C-13-{1 H} NMR spectra of a series of complexes [Cr(eta-6-arene) (CO)2L] (L = CO, CS, or PPh3) have been observed and restricted rotation about the chromium-arene bond detected at very low temperatures only for the tricarbonyl complex of 2 and the dicarbonyl(triphenylphosphine) complex of 1. A detailed structural analysis covering twenty-one metal complexes of 1, 2, hexaethylbenzene, 1,3,5-triethyl-2,4,6-tris(trimethylsilylmethyl)benzene, hexamethylbenzene, hexa-n-propylbenzene, and pentaethylacetophenone is reported and conclusions presented concerning the effects of intramolecular strain on ground-state structures and on internal mobility.