Variable-temperature neutron diffraction studies of the short, strong N...O hydrogen bonds in the 1:2 co-crystal of benzene-1,2,4,5-tetracarboxylic acid and 4,4'-bipyridyl

The 1:2 adduct of benzene-1,2,4,5-tetracarboxylic acid and 4,4'-bipyridyl at 100 K has been studied by single-crystal neutron diffraction at 20, 200 and 296 K. The structure contains two short, strong N...O hydrogen bonds: one O-H...N hydrogen bond [O...N 2.6104 (17) A at 20 K] and one short N-H...O hydrogen bond [N...O 2.5220 (17) A at 20 K]. The N-H distance in the N-H...O hydrogen bond changes from 1.207 (3) A at 20 K to 1.302 (4) A at 296 K and the N...O distance increases to 2.5315 (16) A at 296 K. At 200 K the H atom lies in an intermediate position 1.251 (6) A from the N atom with an N...O separation of 2.520 (4) A. The O-H...N hydrogen bond, on the other hand, does not change with temperature.