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, indepen...