Unexpected isomerization of maleic acid to fumaric acid on co-crystallization with 4,4′-bipyridine

Co-crystallization of maleic acid, 1, with 4,4′-bipyridine, 2, in acetone, chloroform, ethylacetate and methanol gives a hydrogen bonded 2:1 adduct of 1 with 2 (monoclinic space group, C2/c, a=24.056(8), b=6.891(2), c=11.474(6)Å, β=116.10(4)°). Co-crystallization in dimethylformamide and dimethylsulfoxide, however, yields a 1:1 adduct of fumaric acid with 2 (triclinic space group P̄1, a=3.875(1), b=8.906(1), c=10.016(1)Å, α=109.60(1)°, β=98.64(1)°, γ=96.64(1)°).