A "lithium-bonded" cyclopropyl edge: The X-ray crystal structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6 and computational studies

Abstract

The short Li-C distances (Li1-C2 = 2.615(3) Å, Li1-C3 = 2.644(3) Å) in the X-ray crystal structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6 (7)6 characterize Li-cyclopropane edge coordinations. The Li-cyclopropane interactions increase the C2-C3 distances (1.519(3) Å) relative to those of the free cyclopropyl edges (C2-C4 = C6-C7 = 1.499(2) Å) by 0.02 Å. The bent bonds of cyclopropane give rise to an electrostatic potential pattern, which strongly favors edge coordination as is observed experimentally in (7)6, but also permits a metastable Li+ face complex. The cyclopropane edge also is the favored site for hydrogen bonding, but not for protonation. The C-C bond length elongations, the coordination energies EcOord, and the charge redistributions upon metal cation edge interactions all are related to the distances between the cyclopropane C-C bond centers and the cations. This is evaluated for the alkali metal cation-cyclopropane complexes (cation = Li+ to Cs+). More generally, the cyclopropane C-C bond length variations can be employed as a structural measure for the magnitudes of electrostatic interactions. © 1996 American Chemical Society.