Auration of Thiophene and Furan: Structures of the 2-Mono- and 2,2-Diaurated Products

Thiophene undergoes electrophilic substitution by (triphenylphosphine)gold(I) tetrafluoroborate in tetrahydrofuran under very mild conditions to give directly the 2,2-diaurated
cation {2,2-[(Ph3P)Au]2C4H3S}+ as the BF4- salt. The same product is obtained in a two-step process via the monoaurated 2-[(Ph3P)Au]C4H3S molecule, which adds 1 equiv of [(Ph3P)Au]BF4. Together with the mono- and diaurated furan analogues, which were also prepared
via this route, the thiophene compounds have been structurally characterized in single-crystal X-ray studies. The mononuclear complexes of the general formula 2-[(L)Au]C4H3E,
with L = Ph3P or Ph3As and E = O or S, were all found to have conventional structures
with parameters representing classical types of metalated aromatic heterocycles. Crystals
of the arsenic analogue, 2-[(Ph3As)Au]C4H3O, are isomorphous with the Ph3P complex. The
products of the second auration step, with the general formula {2,2-[(L)Au]2C4H3E}+BF4-
(L = Ph3P; E = O, S), have the two gold atoms sharing one of the two α-carbon atoms with
short intramolecular Au···Au contacts of 2.8134(4) and 2.8461(2) Å and small Au−C−Au
angles of 82.5(2)° and 84.5(1)°, respectively. The geometry of the five-membered rings suggests
that the aromaticity is strongly reduced upon geminal auration at C-1, which renders this
carbon atom tetracoordinated. It is particularly noteworthy that the second auration leaves
the sulfur or oxygen donor centers untouched. This observation is one of the most remarkable
examples of the strong influence of aurophilic bonding.