A nitrogen rich Ni(II)–dithiolate system exhibiting acid–base behavior: Synthesis, supramolecular structure and spectroscopy of [Bu4N]2[NiII(ppdt)2] (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate)

The compound [Bu4N]2[Ni(ppdt)2] (1) (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) has been synthesized, starting from pyrido[2,3-b]pyrazine-2,3-dithiol, nickel chloride and tetrabutylammonium bromide in methanol. Compound 1 crystallizes in P21/c space group (monoclinic system). Its crystal structure is characterized by interesting C–H⋯S and C–H⋯N supramolecular weak interactions. Its solution state has been described with acid–base (protonation–deprotonation) behavior, that has rarely been investigated for a metal–dithiolene system. Compound 1 is first instance of a metal–dithiolene compound that has three ring nitrogen on each dithiolate ligand. The pH dependent changes in the charge-transfer absorption band are attributed to the protonation on an imine nitrogen of the ppdt ligand. The complex is electrochemically quasi-reversible with an oxidation potential of E1/2 = +0.46 V vs. Ag/AgCl in methanol.