Observation of slowed rotation about the η 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 non-bonding interactions and structural changes

The all-syn1 and syn-anti-syn2 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 13C-{1H} NMR spectra of a series of complexes [Cr(η6-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.