Partial charge transfer in the salt co-crystal of L-ascorbic acid and 4,4′-bi­pyridine

In the title 1:2 co-crystal, C10H9N2+·(C6H7.75O6·C6H7.25O6)−, L-ascorbic acid (LAA) and 4,4′-bi­pyridine (BPy) co-crystallize in the chiral space group P21 with two mol­ecules of LAA, and one mol­ecule of bpy in the asymmetric unit. The structure was modeled in two parts due to possible proton transfer from LAA to the corresponding side of the bpy mol­ecule having an occupancy of approximately 0.25 and part 2 with an occupancy of approximately 0.75. In this structure, LAA forms hydrogen bonds with neighboring LAA mol­ecules, forming extended sheets of LAA mol­ecules which are bridged by bpy mol­ecules. A comparison to a related and previously published co-crystal of LAA and 3-bromo-4-pyridone is presented.