LITHIO-AVERSION OF THIOPHENE SULFUR-ATOMS IN THE X-RAY CRYSTAL-STRUCTURES OF [LI-O-SIME2(2-C4H3S)](6) AND [LI-O-CH(I-PR)(2-C4H3S)(])6 - MODELS FOR ELECTROSTATIC METAL-THIOPHENE INTERACTIONS

Although lithium-sulfur interactions should result if the thienyl grou ps in [Li-O-SiMe2(2-C4H3S)](6) ((6)(6)) and in [Li-O-CH(i-Pr)(2-C4H3S) ](6) ((9)(6)) were rotated, no short distances between the lithiums of the (LiO)(6) cores and the thiophene S atoms (Li-S > 3 A) are apparen t in their X-ray crystal structures. Instead, the thienyl conformation s in (9)(6) benefit from Li (C=C) pi-interactions (Li-1-C-2 = 2.631(7) Angstrom, Li-1-C-3 = 2.845(7) Angstrom). DFT(B3LYP) computations show Li-S contacts to be only slightly favored over Li-C binding in Li-O-S iH2(2-C4H3S) (7) (1.4 kcal/mol) and in Li-O-CH2(2-C4H3S) (8) (1.7 kcal /mol). Semiempirical PM3 conformational analyses of thienyl groups on the (LiO)(6) cores of the model hexamers [Li-O-SiH2-(2-C4H3S)](6) (7)( 6) and [Li-O-CH2(2-C4H3S)](6) ((8)(6)) show preferences for pyramidal Li-S(thiophene) contacts, whereas planar Li-O arrangements are favored for the furanyl analogues. Due to the higher aromaticity of thiophene , the sigma ''in-plane'' Li+-S(thiophene) coordination energy (Li+-SC4 H4, 16.9 kcal/mol) is reduced relative to that of the Li+-SMe2 referen ce (29.5 kcal/mol) more than is the less aromatic furan (Li+-OC4H4, 29 .2 kcal/mol) relative to Li+-OMe2 (39.1 kcal/mol). Consistently, the L i+ pi-coordination affinity of thiophene (32.1 kcal/mol) is higher tha n that of furan (29.6 kcal/mol). The electrostatic potential (EP) of t hiophene is only slightly negative in the ring plane at sulfur but con siderably more negative in the ''out-of-plane'' pi-region. This ration alizes the ''lithio-aversion'' of thienyl sulfur atoms in the X-ray cr ystal structures of (6)(6) and of (9)(6): electrostatic metal-thiophen e interactions favor the thiophene pi-system rather than the ''in-plan e'' sulfur region.