Acid–base behavior of a simple metal bis(dithiolate) system: Synthesis, crystal structure and spectroscopy of [Bu4N]2[MII(ppdt)2] (M = Ni, Pt; ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate)

The syntheses, crystal structures and properties of compounds [Bu4N]2[Ni(ppdt)2] (1) and [Bu4N]2[Pt(ppdt)2] (2) (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) have been described. Compound 1 crystallizes in P21/c space group (monoclinic system), whereas compound 2 crystallizes in C2/c space group (monoclinic system). The crystal structures of both compounds 1 and 2 have been characterized by C–H⋯S and C–H⋯N hydrogen bonding interactions between cation and anions resulting in three-dimensional supramolecular networks in the crystals of 1 and 2, respectively. The acid–base behavior of the ground states of both [Bu4N]2[Ni(ppdt)2] (1) and [Bu4N]2[Pt(ppdt)2] (2) and also the excited state of compound [Bu4N]2[Pt(ppdt)2] (2) in solutions has been studied. The pH dependent changes in the charge transfer absorption and emission spectra are attributed to the protonation on an imine nitrogen of the ppdt ligand. The ground-state basicity constants of the two complexes 1 and 2 have been determined from spectrophotometric analysis by titrating with an weak acid, yielding pKb1 = 8.0 for complex [Bu4N]2[Ni(ppdt)2] (1) and pKb1 = 7.8 for complex [Bu4N]2[Pt(ppdt)2] (2). The excited-state basicity constant pKb1* for complex [Bu4N]2[Pt(ppdt)2] (2) has been determined by a thermodynamic equation using a Förster analysis yielding the value of 1.8. The complex 2 is electrochemically irreversible with an oxidation potential of E1/2 = +0.41 V versus Ag/AgCl in methanol.