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

From reaction mixtures of commercially available 2,3-bis(2-pyridyl)pyrazine (dpp) and perchloric acid four different solid products have been isolated and structurally characterized by X-ray crystallography; (dppH2)(ClO4)2·3H2O (1), (dppH2)(ClO4)2 (2), (ddppH2)(ClO4)2 (3) and (dppH)(ClO4) (4) (dppH is monoprotonated dpp, dppH2 is the diprotonated dication of dpp, ddppH2 is the dication 5,5′-bis(2,3-bis(2-pyridyl)pyrazinium), i.e. dimerized dppH). In 1 and 2 the nitrogen atom in both of the pyridyl rings in dpp is protonated. Hydrogen bonding and packing arrangements differ in the two compounds. In 3 a protonated and dimerized form of dpp is found; two 2,3-bis(2-pyridyl)pyrazinium units are connected by a C(sp2)C(sp2) bond in the 5-positions of the pyrazine rings, one pyridyl ring in each dpp moiety being protonated at the nitrogen. Strong intra-cation NH⋯N bonds between pyridyl rings are present. In 4 the monomeric dppH moieties display the same type of intramolecular hydrogen bonds as found in 3. Upon reacting dpp with copper(II) salts and oxalate, a dinuclear copper (II) complex, [Cu2(ddpp)(ox)2(H2O)2] (ddpp = 5,5′-bis(2,3-bis(2-pyridyl)pyrazine)), as well as mononuclear [Cu(dpp)(ox)(H2O)] have been obtained in the same reaction mixtures. In one of the isolated crystalline products a dimer of dpp is found, as described in the case of compound 3, but no protonation has occurred. The crystal structure determination reveals a dinuclear complex with the neutral ddpp (dimerized dpp) as bridging ligand. Oxalate occurs as a bidentate, terminal ligand; water completes the copper coordination sphere. The dinuclear complex cocrystallizes with a mononuclear Cu-dpp-ox complex; in the mixed crystal two mononuclear units replace one dinuclear unit in 25% of the unit cells, thus the average formula is [Cu2(ddpp)(ox)2(H2O)2]0.75[{Cu(dpp)(ox)(H2O)}2]0.25·8H2O (5). The monomeric species also crystallizes in the form of compound 6, [Cu(dpp)(ox)(H2O)]·2H2O. In 6 the dpp ligand and oxalate are both chelating to copper in equatorial positions, and [Cu(dpp)(ox)(H2O)] units are pair-wise connected through weak Cu⋯O (oxalate) interactions (semi-coordination). ry optimization through ab initio calculations gave results in good agreement with the experimental molecular geometries of dpp, dppH+ and dppH 2 2 + , respectively. An energy minimum was found for the syn conformation of pyridyl rings in the monoprotonated species, conforming to the existence of the intramolecular NH⋯N hydrogen bond. mpounds 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.