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
B. Goldfuss et al., 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, Organometallics, 16(23), 1997, pp. 5032-5041
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.