Alkali metal cation pi-interactions in metalated and nonmetalated acetylenes: pi-bonded lithiums in the X-ray crystal structures of [Li-C C-SiMe(2)-C6H4-OMe](6) and [Li-O-CMe(2)-C C-H](6) and computational studies

The X-ray crystal structure of [Li-C=C-SiMe(2)-C6H4-OMe](6) (14)(6) features nearly symmetric pi-interactions between the lithium ions and the acetylide anions (Li-1-C beta=2.353(9) Angstrom, Li-1-C-alpha=2.292(9) Angstrom). These pi-contacts are facilitated by the chelating o-anisyl methoxy groups (Li-1-C-alpha-C-beta=77.6(4)degrees, Li-1-O-1=2.169(9) Angstrom). The L(1)-C-alpha distances in the (LiCalpha)(6) core of (14)(6) differ significantly (Li-1 alpha-C-alpha=2.132(9) Angstrom, Li-1b-C-alpha=2.205(11) Angstrom). This Li-C-alpha distance differentiation is unique in organolithium hexamers, and is due to Li(C=C-R) ''side-on-pi'' and ''end-on-sigma'' 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 pi-interactions between the lithiums in the (LiO)(6) core and the nonmetalated acetylene groups (Li-1-C-2=2.443(5) Angstrom, Li-1-C-3=2.749(6) Angstrom). These Li-C pi-contacts shorten upon acetylene Lithiation, as is shown computationally in Li-O-CH2-C=C-(H/Li) (24-H/Li). Additional computations reveal that the pi-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 pi-contacts increase the C=C bond lengths slightly (0.005 Angstrom for Li) and lower the C=C stretching frequencies (33 cm(-1) for Li), they polarize charge density from C-alpha toward C-beta and hence result in counterion-induced charge delocalizations. The degrees of pi-interactions both in (26-Li-Cs) and in (27-Li-Cs) decrease with increasing size of the alkali cations.