In connection with studies
on possible mesoionic systems, syntheses of
1,3-bis(4'-bromophenyl)-propane-1,2,3-trione (5), 3,5-diphenyl-1,2-dithiolylium-4-olate
(9) and 4-hydroxy-3,5-diphenyl-1,2-dithiolium perchlorate (10) have been
reexamined. ��� The crystal structures of (5), (9) and (10)
have been determined by X-ray diffraction from diffractometer data at 295 K and
were refined by full-matrix least squares to residuals of 0.050 (1118
'observed' reflections), 0.072 (2388) and 0.087 (2000), respectively. Crystals
of (5) are monoclinic, C2/c, a 18.535(4), b 7.734(2), c 12.184(3) Ǻ,
β 126.60(3)°, Z 4. Crystals of (9) are orthorhombic, Pmcn,
a 26.047(6), b 7.170(2), c 6.431(2) Ǻ, Z 4. Crystals of (10) are
monoclinic, P21/a, a 18.601(7), b 10.636(4), c 8.152(2) Ǻ,
β 102.60(3)°, Z 4. ��� In (5) π-character is localized in the
phenyl rings and carbonyl groups [<C=O> 1.22(1) Ǻ] and the substance
is best represented as a normal vicinal triketone. In (9) and (10) S-S bonds
are present and (9) is correctly regarded as a heterocyclic system. The
geometry of the heterocyclic ring in (10) agrees well with that previously
found for the 1,2-dithiolium cation; <C-O> is 1.35(1) Ǻ. In (9) the
heterocyclic C-C distances [1.442(2)Ǻ] are longer than in (10); this
indicates diminished aromaticity on deprotonation. The C-C-C angle [111.2(2)°]
in (9) is also smaller than that [118.3(8)°] in (10) and <C-O> in (9) is
1.250(3) Ǻ. ��� The behaviour of
1,3-diphenylpropane-1,2,3-trione (4) and (9) with acetylenic dipolarophiles has
been studied but no evidence has been obtained that either substance undergoes
a 1,3-dipolar cycloaddition reaction.