Crystal engineering of a 1:1 5-fluoro­cytosine–4-hy­droxy­benzaldehyde cocrystal: insights from X-ray crystallography and Hirshfeld analysis

The cocrystal of 5-fluoro­cytosine and 4-hy­droxy­benzaldehyde (1/1), C₄H₄FN₃O·C₇H₆O₂, crystallizes in the monoclinic P2₁/c space group. The crystal structure features a robust supra­molecular network stabilized by N—H⋯O, N—H⋯N, O—H⋯O, C—H⋯O and C–H⋯F hydrogen bonds, forming diverse ring motifs including R₂²(8), R₄⁴(22), R₆⁶(32), and R₈⁸(34). Additional C—F⋯π inter­actions contribute to the crystal cohesion. Hirshfeld surface analysis reveals that O⋯H/H⋯O contacts dominate the inter­molecular inter­actions, emphasizing the key role of hydrogen bonding in the crystal packing.

The 1:1 cocrystal of 5-fluoro­cytosine (5FC) and 4-hy­droxy­benzaldehyde (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 supra­molecular architecture is stabilized by N—H⋯O, N—H⋯N, C—H⋯O and C—H⋯F hydrogen bonds, forming R₂²(8), R₄⁴(22), R₆⁶(32), and R₈⁸(34) ring motifs. The N—H⋯O and N—H⋯N hydrogen bonds form strong directional inter­actions, contributing to the R₂²(8) and R₈⁸(34) motifs through dimeric and extended ring structures. O—H⋯O inter­actions link 5FC and 4HB mol­ecules, generating an R₆⁶(32) ring that enhances the packing. Weaker C—H⋯F bonds help form the R₄⁴(22) tetra­meric motif, supporting the overall three-dimensional supra­molecular framework. Additionally, C—F⋯π inter­actions 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.