Crystalline products isolated from solutions with commercially available 2,3-bis(2-pyridyl)pyrazine (dpp) as reactant: Detection of a dimerized form of dpp

The compounds containing dimerized dpp were minor products (of the order of 1% yield). To determine whether ddpp had been formed in the reaction mixtures, or had been present as a contaminant in the commercially available dpp, an NMR solution study was performed. Samples of dpp, 1, 3, 4 and ddpp (obtained by treating 3 with base) were investigated by 1H (all five compounds) and 13C NMR (the three former compounds) spectroscopy. The results are consistent with the presence of ddpp as a contaminant in commercially available dpp. The study also allowed us to identify the spectral changes involved in going from neutral dpp to the partial (4) and fully (1) protonated forms.