Crystal engineering of a 1:1 5-fluoro-cytosine-4-hy-droxy-benzaldehyde cocrystal: insights from X-ray crystallography and Hirshfeld analysis.

The 1:1 cocrystal of 5-fluoro-cytosine (5FC) and 4-hy-droxy-benzaldehyde (4HB), CHFNO·CHO has been synthesized and its structure characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The compound crystallizes in the monoclinic 2/ space group. A robust supra-molecular architecture is stabilized by N-H⋯O, N-H⋯N, C-H⋯O and C-H⋯F hydrogen bonds, forming (8), (22), (32), and (34) ring motifs. The N-H⋯O and N-H⋯N hydrogen bonds form strong directional inter-actions, contributing to the (8) and (34) motifs through dimeric and extended ring structures. O-H⋯O inter-actions link 5FC and 4HB mol-ecules, generating an (32) ring that enhances the packing. Weaker C-H⋯F bonds help form the (22) tetra-meric motif, supporting the overall three-dimensional supra-molecular framework. Additionally, C-F⋯π inter-actions between the fluorine atom and the aromatic ring add further to the crystal cohesion. Hirshfeld surface analysis and two-dimensional fingerprint plots confirm that O⋯H/H⋯O contacts are the most significant, highlighting the central role of hydrogen bonding in the stability and organization of the crystal structure.