Regarding the structures and fluxionality of tricyclopentadienylaluminum compounds

X-ray crystallographic studies, low-temperature H-1 and C-13 NMR studies, and Al-27 NMR studies of a series of homoleptic tricyclopentadienylaluminum compounds [(C5H5)(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 {eta(2), eta(1.5), eta(1.5)} combination of ring geometries and the other with an {eta(2), eta(1.5), eta(1)} combination of ring-coordination geometries. In the crystal structure of compound 3, one cyclopentadienyl ring is coordinated eta(5) to the aluminum while the other two rings are eta(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 1H and C-13 NMR spectra in the fast-exchange Limit at temperatures as low as -110 degrees 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 (eta(1), eta(2), eta(3), eta(5)).