EFFECT OF TRANSITION-METAL COMPLEXATION ON THE STEREODYNAMICS OF PERSUBSTITUTED ARENES - EVIDENCE FOR STERIC COMPLEMENTARITY BETWEEN ARENE AND METAL TRIPOD

The stereodynamics in 1,4-dimethoxy-2,3,5,6-tetraethylbenzene (5), 1,4-bis(methoxymethyl)-2,3,5,6-tetraethylbenzene (6), and 1,4-dineohexyl-2,3,5,6-tetraethylbenzene (7) and their respective tricarbonylchromium complexes, 5(Cr), 6(Cr), and 7(Cr), have been studied by variable-temperature NMR techniques. Barriers to rotation about the sp2-sp3 bonds for 5-7 and 5(Cr)-7(Cr) have been determined using the Gutowsky-Holm approximation to be 7.7, 9.4, 11.2, 6.6, 8.9, and 11.8 kcal/mol, respectively. Unlike previous studies in this area, the stereodynamics of the arene do not change demonstrably upon metal complexation. This observation is attributed to a lock-and-key complementarity between the metal tripod and the arene. The possibility of correlated dynamics between the metal tripod rotation and the ethyl group rotation is discussed.