Alkali Metal Cation π-Interactions in Metalated and Nonmetalated Acetylenes: π-Bonded Lithiums in the X-ray Crystal Structures of [Li−C⋮C−SiMe2−C6H4−OMe]6and [Li−O−CMe2−C⋮C−H]6and Computational Studies

The X-ray crystal structure of [Li−C⋮C−SiMe 2 −C 6 H 4 −OMe] 6 ( 14 ) 6 features nearly symmetric π-interactions between the lithium ions and the acetylide anions (Li 1 −C β = 2.353(9) Å, Li 1 −C α = 2.292(9) Å). These π-contacts are facilitated by the chelating o -anisyl methoxy groups (Li 1 −C α −C β = 77.6(4)°, Li 1 −O 1 = 2.169(9) Å). The Li−C α distances in the (LiC α ) 6 core of ( 14 ) 6 differ significantly (Li 1α −C α = 2.132(9) Å, Li 1b −C α = 2.205(11) Å). This Li−C α distance differentiation is unique in organolithium hexamers, and is due to Li(C⋮C−R) “side-on-π” and “end-on-σ” contacts, as is shown computationally in H−C⋮C−Li(LiH) 2 ( 20 ). A second X-ray crystal structure, [Li−O−CMe 2 −C⋮C−H] 6 ( 22 ) 6, reveals electrostatic π-interactions between the lithiums in the (LiO) 6 core and the nonmetalated acetylene groups (Li 1 −C 2 = 2.443(5) Å, Li 1 −C 3 = 2.749(6) Å). These Li−C π-contacts shorten upon acetylene lithiation, as is shown computationally in Li−O−CH 2 −C⋮C−(H/Li) ( 24-H/Li ). Additional computations reveal that the π-interactions in (HC⋮C)M 2 H ( 26-Li-Cs ) complexes (modelling oligo- and polymeric M−C⋮C−R) are weak (only 0.7 kcal/mol for Li), but substantial in M + (H−C⋮C−H) ( 27-Li-Cs ) species (20.2 kcal/mol for Li + ). In 26-Li-Cs, the π-contacts increase the C⋮C bond lengths slightly (0.005 Å for Li) and lower the C⋮C stretching frequencies (33 cm -1 for Li), they polarize charge density from C α toward C β and hence result in counterion-induced charge delocalizations. The degrees of π-interactions both in ( 26-Li-Cs ) and in ( 27-Li-Cs ) decrease with increasing size of the alkali cations.