Synthesis and Properties of Heterocyclic Substituted 1,2-Enedithiolates of Nickel, Palladium, and Platinum

A convenient new synthetic route to
metallo-1,2-enedithiolates was applied to the synthesis of
(dppe)M{S2C2(heterocycle)(H)}; dppe =
1,2-bis(diphenyldiphosphino)ethane, M = Ni, Pd, and Pt, and
heterocycle =
2-quinoxaline, 2-, 3-, and 4-pyridine, and 2-pyrazine. These
complexes were prepared from the corresponding
bis(hydrosulfido) complexes (dppe)M(SH)2 and the
α-bromo ketones, heterocycle−C(O)CH2Br.
In the solid state,
(dppe)Ni{S2C2(2-pyrazine)(H)}
is a slightly distorted square plane with a planar five-membered
metallo-1,2-enedithiolate ring. The metallo-1,2-dithiolate is ≈6° from
being coplanar with the pyrazine ring. These
complexes
all have a UV−visible band assignable to an intraligand transition
(ILCT) that is best described as a 1,2-enedithiolate
π → heterocycle π* charge transfer transition. The energy of
the ILCT transition tracks with the reduction potential
of the appended aromatic heterocycle. The
pKa of the protonated complexes is 1−3 units
higher than that of the
parent heterocycle, independent of the metal, and consistent with
resonance stabilization of the protonated
heterocycle by the 1,2-enedithiolate ligand.