The structure and reactivity of dipyrido[1,2-a : 1′,2′-d]pyrazinium cations

The crystal structure of 6,12-dihydrodipyrido[l,2-a: 1′,2′-d]pyrazinium dibromide (1) has been determined by X-ray crystallography. In aqueous or methanolic solution (1) behaves as a strong acid and is in equilibrium with the monocation (2); the proton-transfer kinetics have been studied using stoppedflow methods and the N-protonated form of (2) has been isolated as the di-iodide. Reduction of (1) with borohydride yields a mixture of isomeric octahydro derivatives which do not appear to re-aromatise to compound (3). The relative enthalpies of formation of (1) and (3) have been calculated using MNDO and are compared to related heteroaromatic compounds.