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The ansa-aluminocene compound [Me4C2(C5H4)2Al-μ-Cl]2 (2) was prepared by reacting [Me4C2(η5-C5H4)2Mg] with AlCl3. Addition of THF and t-butyl isocyanide to 2 produced the corresponding monomeric Lewis base adducts [Me4C2(η5-C5H4)2Al(L)] (L = THF (3), t-BuNC (4)). X-ray crystal structure determinations revealed (η1,η5), (η1,η3), and (η1,η1) combinations of cyclopentadienyl ring hapticities for 2, 3, and 4, respectively. The solution structures and fluxional behavior of the compounds were characterized using a battery of dynamic and 2-D NMR techniques. Line shape analysis of variable temperature 1H and 13C{1H} NMR spectra of 2 in toluene-d8 yielded an activation barrier (Ea) of 8 kcal mol−1 for the dynamic averaging of proton and carbon environments of the bridged dicyclopentadienyl ligand framework. Circumambulatory migration of aluminum about its cyclopentadienyl rings could be ruled out as the source of this averaging. The cyclopentadienyl ring coordination modes that were identified in the solution spectra of 2 and 4 at slow exchange (η1,η1 and η2,η3, respectively) are considerably different from the ones found in the crystal structures of the compounds. Besides rearrangements in the bonding between aluminum and its cyclopentadienyl rings, there is another dynamic process, possibly inversion at aluminum, that leads to additional signal averaging in the 1H and 13C{1H} NMR spectra of compound 4. |
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