Crown Thioether Chemistry: Structural and Conformational Studies of Tetrathia-12-crown-4, Pentathia-15-crown-5, and Hexathia-18-crown-6. Implications for Ligand Design

Article abstract of DOI:10.1021/ja00248a031

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 monoclinic system, space group Cc, with a = 13.028(7) Angstroem, b = 12.884(5) Angstroem, c = 14.493(7), (β = 108.18(4) deg, 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 P2₁/n, with a = 16.444(3) Angstroem, b = 5.432(1) Angstroem, c = 18.255(3) Angstroem, β = 115.58(1) deg, 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 Fdd/2, with a = 20.466(1) Angstroem, b = 32.222(3) Angstroem, c = 5.213(4) Angstroem, 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-O bonds in oxa-crown ethers.This marked difference derives from the difference in C-E bond lenghts, which changes nonbonded 1,4-interactions in both C-C-E-C and E-C-C-E fragments.