Phase transition and proton ordering at 50 K in 3‐(pyridin‐4‐yl)pentane‐2,4‐dione

Single‐crystal neutron diffraction experiments at 100 and 2.5 K have been performed to determine the structure of 3‐(pyridin‐4‐yl)pentane‐2,4‐dione (HacacPy) with respect to its protonation pattern and to monitor a low‐temperature phase transition. Solid HacacPy exists as the enol tautomer with a short intramolecular hydrogen bond. At 100 K, its donor···acceptor distance is 2.450 (8) Å and the compound adopts space group C 2/ c , with the N and para ‐C atoms of the pyridyl ring and the central C of the acetylacetone substituent on the twofold crystallographic axis. As a consequence of the axial symmetry, the bridging hydrogen is disordered over two symmetrically equivalent positions, and the carbon–oxygen bond distances adopt intermediate values between single and double bonds. Upon cooling, a structural phase transition to the t 2 subgroup P occurs; the resulting twins show an ordered acetylacetone moiety. The phase transition is fully reversible but associated with an appreciable hysteresis in the large single crystal under study: transition to the low‐temperature phase requires several hours at 2.5 K and heating to 80 K is required to revert the transformation. No significant hysteresis is observed in a powder sample, in agreement with the second‐order nature of the phase transition.