摘要：

Cyclopentadienylaluminum compounds have been a curiosity due to the ring-slipped structures they exhibit and due to their highly fluxional nature, which has prevented the freezing out of discrete solution structures on the NMR time scale. Lately, increased attention toward these compounds in the form of molecular structure determinations, theoretical calculations, and 27Al-NMR spectroscopic measurements is providing a clearer picture of the bonding between the aluminum and its cyclopentadienyl ring(s). A variety of ring coordination geometries (η1, η2, η3, η5) have been identified in molecular structures of these compounds, and although theoretical calculations reveal very small energy differences between different haptotropes, consistent with their highly fluxional behavior in solution, there is close agreement between the calculated ground state geometries for the compounds and the geometries they exhibit in the solid state and in solution. Theory and experiment reveal a delicate balance of covalent π-interactions and ionic interactions between the aluminum and its cyclopentadienyl ring. Although the balance is easily tipped by steric effects, some structural trends are emerging. The solution 27Al-NMR chemical shifts of cyclopentadienylaluminum compounds are surprisingly diagnostic of their geometries, exhibiting a strong correlation with coordination numbers that take into account the hapticity of the cyclopentadienyl ring.

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