Palladium(II) and Platinum(II) Mononuclear Complexes and Tendency to Undergo Dehydrogenation of the Multiple N-Donor Ligand Di-(2-pyridyl)dihydropyrazine

The already known di(2-pyridyl)dihydropyrazine (dhdpp) was prepared and isolated also in the form of a bis-hydrated species, i.e., dhdpp·2H2O. As established by X-ray work, a small amount of single crystals of di(2-pyridyl)-pyrazine (dpp) was also obtained from the mother liquors, this testifying the possibility of a dehydrogenation process dhdpp -> dpp in the absence of a catalyst. Using dhdpp as a ligand, mononuclear metal derivatives of formula [(dhdpp)MCl2]·xH2O (M = PdII, PtII) were obtained as stable-to-air solids, studied by X-ray powder, IR, UV-visible, and 1H NMR spectra, and proved to exhibit a N2MCl2 coordination site involving one pyridine and one pyrazine N atom ("py-pyz" coordination). An interesting relationship has been established in terms of the observed types of coordination with the analogs of di(2-pyridyl)-pyrazine (dpp) formulated as [(dpp)MCl2]·3H2O, proved also by 1H NMR spectra to exhibit the "py-pyz" mode of coordination. Attempts to isolate from the reaction of dhdpp with Pd(OAc)2 the corresponding mononuclear derivatives were shown to lead, as definitely supported by 1H NMR spectral data and crystallographic work, to the exclusive formation of the corresponding dpp complex [(dpp)Pd(OAc)2]·5H2O ("py-pyz" coordination site), this proving the tendency of dhdpp to generate dpp under different reaction conditions. The promoted conversion of dhdpp into dpp in the complex was examined by sequential NMR analysis and established to be determined by Pd(OAc)2 which plays the role of catalyst. The new salt-like species [(CH3)(dhdpp)PdI2](I)·7H2O, prepared starting from [(dhdpp)PdCl2] in its reaction with CH3I, allowed the separation from the mother liquors of small brown crystals identified on the basis of X-ray analysis as the already known complex of formula [(dpp)PdI2] ("py-py" coordination), this result once again outlining the tendency of dhdpp to be dehydrogenated to dpp.