Classical and Weak Hydrogen Bonding Interactions between 4,4′-Bipyridine and Organic Acids: From Co-Crystal to Organic Complex

Reaction of 4,4′-bipyridine (4,4′-bipy) with barbituric (BAH), orotic (ORH), and uracil-5-carboxylic acids (U5CAH) yielded three new products, which have been characterized by single-crystal X-ray diffraction. The products include a co-crystal of neutral species with Z′ = 2 (4,4′-bipy/BAH), an organic complex ion pair (4,4′-bipy/ORH) and a compound containing both ionic and hydrogen-bonded neutral species (4,4′-bipy/U5CAH). Partial proton transfer is observed in both of the ionic cases, and in all three structures the crystal packing arrangement is such that weak hydrogen bonding, π···π and molecular dipole interactions are optimized.