Crystal engineering of a 1:1 5-fluorocytosine–4-hydroxybenzaldehyde 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 <i>P</i>2₁/<i>c</i> 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 <i>R</i> ₂ ²(8), <i>R</i> ₄ ⁴(22), <i>R</i> ₆ ⁶(32), and <i>R</i> ₈ ⁸(34) ring motifs. The N-H⋯O and N-H⋯N hydrogen bonds form strong directional inter-actions, contributing to the <i>R</i> ₂ ²(8) and <i>R</i> ₈ ⁸(34) motifs through dimeric and extended ring structures. O-H⋯O inter-actions link 5FC and 4HB mol-ecules, generating an <i>R</i> ₆ ⁶(32) ring that enhances the packing. Weaker C-H⋯F bonds help form the <i>R</i> 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.