Neutral and Dianionic Organoantimony(III) Dithiolene Complexes: Syntheses, X-ray Crystal Structures, and Unexpected Reactivity

The new neutral PhSb(dmid) (dmid2- = 4,5-dithiolato-1,3-dithiol-2-one) complex has been
synthesized upon reaction of deprotected 1,3,4,6-tetrathiapentalene-2,5-dione with dichlorophenylstibine. Single-crystal X-ray structure determination shows an original three-dimensional network in the solid state, via Sb- - -O and Sb- - -S secondary interactions, based
on an octagonal “Sb4S4” motif. Not only PhSb(dmid) but also PhSb(dmit) (dmit2- = 4,5-dithiolato-1,3-dithiole-2-thione) reacts with a second 1 equiv of dithiolate to give a series of
highly reactive dianionic complexes, unknown so far. One of these complexes, [Me4N]2[PhSb(dmid)2], was structurally characterized by single-crystal X-ray measurements. Both
dithiolene ligands, lying in the equatorial plane of a distorted octahedron, are unsymmetrically coordinated, with two Sb−S distances much longer than the other two. These dianionic
species reacted with 1 equiv of PhSbCl2 to give, unexpectedly, a series of Sb(V)
[Ph2Sb(dithiolene)2]- complexes. The same compounds were obtained directly upon slow
addition of 1 equiv of PhSbCl2 on the corresponding dithiolate suspensions. This demonstrates
that dianionic [PhSbIII(dithiolene)2]2- species are key intermediates in the synthesis of
[Ph2SbV(dithiolene)2]- salts. The single-crystal X-ray structure determination of the tetraethylammonium salts of [Ph2Sb(dmid)2]-, [Ph2Sb(dmit)2]-, and [Ph2Sb(dmid)(dmit)]- show
a distorted-octahedral coordination geometry for the Sb center with a cis configuration of
the phenyl groups. This is preserved also in solution, as established by 13C NMR. Using the
same technique, combined with 1H NMR, in a scrambling experiment, a fast dithiolene
exchange has been emphasized. In the case of the [Ph2Sb(dmid)2]- salt, two polymorphs,
with different folding angles along the S- - -S hinge, were crystallized.