Regarding the Structures and Fluxionality of Tricyclopentadienylaluminum Compounds

X-ray crystallographic studies, low-temperature 1 H and 13 C NMR studies, and 27 Al NMR studies of a series of homoleptic tricyclopentadienylaluminum compounds [(C 5 H 5 ) 3 Al ( 1 ), (MeC 5 H 4 ) 3 Al ( 2 ), (1,2,4-Me 3 C 5 H 2 ) 3 Al ( 3 ), (1,2,3,4-Me 4 C 5 H) 3 Al ( 4 )] are reported along with ab initio calculations on model cyclopentadienylaluminum compounds. The ring-coordination geometries exhibited by the tricyclopentadienylaluminum compounds in the solid state vary with the number of methyl substituents on the cyclopentadienyl rings. The X-ray crystal structure of compound 1 revealed two unique molecules in the unit cell, one with an {η 2, η 1.5, η 1.5 } combination of ring geometries and the other with an {η 2, η 1.5, η 1 } combination of ring-coordination geometries. In the crystal structure of compound 3, one cyclopentadienyl ring is coordinated η 5 to the aluminum while the other two rings are η 1 . Compound 4 exhibits monohapto coordination of all three tetramethyl-substituted cyclopentadienyl rings in the solid state. These compounds are highly fluxional in solution and exhibit averaged 1 H and 13 C NMR spectra in the fast-exchange limit at temperatures as low as −110 °C. This behavior is explained by the ab initio calculations on model cyclopentadienylaluminum compounds which reveal almost negligible (1−2 kcal/mol) energy differences between different ring hapticities (η 1, η 2, η 3, η 5 ).