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), C4H4FN3O·C7H6O2 has been synthesized and its structure characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The compound crystallizes in the monoclinic P21/c 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 R 2 2(8), R 4 4(22), R 6 6(32), and R 8 8(34) ring motifs. The N-H⋯O and N-H⋯N hydrogen bonds form strong directional inter-actions, contributing to the R 2 2(8) and R 8 8(34) motifs through dimeric and extended ring structures. O-H⋯O inter-actions link 5FC and 4HB mol-ecules, generating an R 6 6(32) ring that enhances the packing. Weaker C-H⋯F bonds help form the R 4 4(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.