Abstract
Tetrathia-12-crown-4 (12S4) in the solid state adopts a square conformation with the sulfur atoms at the corners, to yield a structure derived from fusion at the terminal S atoms of two “bracket” units. This macrocycle crystallizes in the
S—CH2—ch2—S
CH2
CH?
I S
13.028 (7) Á, b = 12.884 (5) A, c = 14.493 (7) A, ß = 108.18 (4)°, and Z = 8. Pentathia-15-crown-5 (15S5) assumes an irregular conformation generated from two bracket units by fusion at one S atom and connection of the remaining two terminal S atoms by a -CH2CH2- linkage. This crown thioether also crystallizes in the
monoclinic system, space group P2fn, with a = 16.444 (3) A, b = 5.432 (1) A, c = 18.255 (3) A, (3=115.58 (1)°, and Z = 4. Hexathia-18-crown-6 (18S6) adopts a conformation produced by connection of two bracket units by two -CH2CH2- linkages. It crystallizes in the orthorhombic system, space group Fddl, with a = 20.466 (1) A, b = 33.222 (3) A, c = 5.213 (4) A, and Z = 8. Analysis of these structures reveals a pronounced preference for gauche placement at C-S bonds. This preference causes the ubiquity of bracket units and contrasts with the antipathy to gauche placement of the C-0 bonds in oxa-crown ethers. This marked difference derives from the difference in C-E bond lengths, which changes nonbonded 1,4-interactions in both C-C-E-C and E-C-C-E fragments.