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.

Abstract

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.

1. Chemical context

Cocrystals have gained considerable attention in supra­molecular chemistry for their ability to improve the physical and chemical properties of active pharmaceutical ingredients (APIs) and functional materials without altering the mol­ecular structure of the drug. They are defined as crystalline, single-phase solids composed of two or more distinct mol­ecular and/or ionic compounds, typically in a stoichiometric ratio, which are neither simple salts nor solvates (Aitipamula et al., 2012 ▸; Almarsson & Zaworotko, 2004 ▸). Cocrystals are stabilized through non-covalent inter­actions such as hydrogen bonding, π–π stacking, halogen bonding, and van der Waals forces. Their design is guided by the principles of crystal engineering, involving the careful selection of suitable coformers and the application of supra­molecular synthons, such as the R₂²(8) hydrogen-bonded motif (Etter, 1990 ▸; Etter et al., 1990 ▸; Desiraju, 1995 ▸). In the pharmaceutical industry, cocrystallization offers a promising strategy for enhancing the solubility, stability, and bioavailability of poorly soluble drugs. (Alvani & Shayanfar, 2022 ▸; Shi et al., 2024 ▸). Compared to conventional techniques such as salt formation, micronization, solid dispersion, amorphous forms, and encapsulation, cocrystals offer the advantage of maintaining a stable crystalline structure, which facilitates detailed characterization by X-ray diffraction (Bolla & Nangia, 2016 ▸; Bolla et al., 2022 ▸).

2. Structural commentary

Single-crystal X-ray diffraction analysis reveals that the title compound crystallizes in the monoclinic P2₁/c space group with one mol­ecule each of 5-fluoro­cytosine (5FC) and 4-hy­droxy­benzaldehyde (4HB) present in the asymmetric unit. An ellipsoid plot of the compound is shown in Fig. 1 ▸. Proton transfer does not occur between the hydroxyl group of benzaldehyde and the pyrimidine ring nitro­gen atom of 5FC. The C—O bond length in the hydroxyl group of the 4HB mol­ecule is 1.3520 (13) Å, with the corresponding inter­nal bond angle [C2A—N1A—C3A = 120.00 (8)°] in agreement with reported literature values (Louis et al., 1982 ▸; Mohana et al., 2016 ▸, 2023 ▸; Sangavi et al., 2024 ▸).

Figure 1.

The mol­ecular structure of the title cocrystal with displacement ellipsoids drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines.

3. Supra­molecular features and Hirshfeld surface analysis

The primary inter­action motif is formed via N—H⋯O and C—H⋯F hydrogen bonds (Table 1 ▸). The N4A amino group and F1A atom of the 5FC mol­ecule inter­act with the O2B and C7B atoms of the 4HB mol­ecule, resulting in an (8) heterodimeric synthon. Heterodimers are further linked through a weak C—H⋯Oⁱⁱⁱ [symmetry code: (iii) −x + 2, −y + 1, −z + 1] hydrogen bond involving the C4A atom of 5FC and the O1B atom of 4HB. The inter­action leads to the formation of an (22) tetra­meric synthon. The tetra­meric motif is further extended through a homodimeric (8) synthon, formed by N—H⋯Nⁱ [symmetry code: (i) −x, y + , −z + ] and N—H⋯Oⁱⁱ [symmetry code: (ii) −x, y − , −z + ] hydrogen bonds. These inter­actions involve atoms N1A, N2A, N3A and O1A of the 5-fluoro­cytosine (5FC) mol­ecule. The formation of this homodimeric synthon bridges adjacent tetra­meric units, resulting in a large (34) ring motif. The alternating arrangement of (22) and (34) rings leads to the development of a three-dimensional supra­molecular cage-like architecture. This network is further consolidated by O—H⋯O hydrogen-bonding inter­actions between the O1A atom of the 5FC mol­ecule and the hydroxyl (–OH) group of the 4-hy­droxy­benzaldehyde (4HB) mol­ecule. The hydrogen bonding occurs via an O—H⋯O^iv [symmetry code: (iv) x + 1, −y + , z − ] inter­action, forming an (32) ring motif (Fig. 2 ▸). This inter­action strengthens the packing and adds complexity to the supra­molecular network. In addition to hydrogen bonding, the crystal structure is further consolidated by weak C—H⋯F and C—F⋯π inter­actions. The C—F⋯π inter­action (Fig. 3 ▸) is observed between 5FC mol­ecules [C1A⋯Cg^v = 3.2676 (9) Å, C1A—F1A⋯Cg = 89.41 (6)°, where Cg is the centroid of the 5FC ring; symmetry code: (v) 1 + x, y, z]. The observed angle is consistent with values reported in the literature (Sikorski et al., 2005 ▸; Vangala et al., 2002 ▸).

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2A—H1⋯N1Ai	0.88 (1)	2.06 (1)	2.9354 (12)	175 (1)
N3A—H1CC⋯O2B	0.89 (1)	2.10 (1)	2.9848 (13)	170 (1)
N3A—H1A⋯O1Aii	0.90 (1)	2.04 (1)	2.9328 (12)	176 (1)
C4A—H4A⋯O1Biii	0.93	2.48	3.2905 (14)	145
O1B—H1B⋯O1Aiv	0.86 (2)	1.85 (2)	2.6934 (13)	166 (2)
C6B—H6B⋯F1Aiii	0.93	2.56	3.3446 (14)	143
C7B—H7B⋯F1A	0.93	2.51	2.9886 (14)	112

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 2.

Three-dimensional supra­molecular cage-like architecture formed via N—H⋯O, N—H⋯N, O—H⋯O, C—H⋯F and C—H⋯O hydrogen bonds. [Symmetry codes: (i) −x, y + , −z + ; (ii) −x, y − , −z + ; (iii) −x + 2, −y + 1, −z + 1; (iv) x + 1, −y + , z − .]

Figure 3.

A view of the C—F⋯π inter­action (symmetry operation 1 + x, y, z).

Hirshfeld surface (HS) analysis was performed for the title compound to visualize and qu­antify its inter­molecular inter­actions. Fig. 4 ▸ presents the van der Waals inter­actions using a Hirshfeld surface mapped over d_norm (Spackman & Jayatilaka, 2009 ▸), generated with Crystal Explorer 21 (Spackman et al., 2021 ▸). This analysis reveals significant inter­molecular hydrogen bonds of the types N—H⋯O, N—H⋯N and O—H⋯O inter­actions. In the surface representation, red areas indicate strong hydrogen bonding, blue regions correspond to contacts close to the sum of the van der Waals radii, and white regions represent weaker inter­actions.

Figure 4.

The Hirshfeld surface mapped over d_norm showing the N—H⋯O, N—H⋯N and O—H⋯O inter­actions as dashed gray lines.

To analyze the relative contributions of different inter­molecular inter­actions, two-dimensional fingerprint plots were generated (McKinnon et al., 2007 ▸) and these are shown in Fig. 5 ▸. These plots indicate that the most prominent contacts are O⋯H/H⋯O (26.6%), followed by H⋯H (25.5%), C⋯H/H⋯C (16.7%), N⋯H/H⋯N (10.0%) and F⋯H/H⋯F (6.2%). The crystallographic analysis reveals a robust supra­molecular network in the title compound, stabilized by hydrogen bonds (N—H⋯O, N—H⋯N, O—H⋯O and C—H⋯F) and C—F⋯π inter­actions, forming a three-dimensional cage-like supra­molecular architecture. Hirshfeld surface analysis highlights prominent O⋯H/H⋯O inter­actions, alongside other significant contacts, contributing to crystal stability. The study demonstrates how non-covalent inter­actions, including hydrogen-bonding and π inter­actions, govern the mol­ecular packing and cohesion, supporting the principles of supra­molecular chemistry in crystal engineering.

Figure 5.

Fingerprint plots showing the total contribution of individual inter­actions and those delineated into O⋯H/H⋯O, H⋯H, C⋯H/H⋯C, N⋯H/H⋯N and F⋯H/H⋯F inter­actions.

4. Database survey

5-Fluoro­cytosine (5FC) is a synthetic anti­mycotic compound, first synthesized in 1957 and widely used as an anti­tumor agent. It is also active against fungal infection (Portalone & Colapietro, 2007 ▸; Vermes et al., 2000 ▸). It becomes active by deamination of 5FC into 5-fluoro­uracil by the enzyme cytosine deaminase (CD) and inhibits RNA and DNA synthesis (Morschhauser, 2003 ▸). The Cambridge Structural Database (CSD, v5.45, June 2024; Groom et al., 2016 ▸) reference codes for the monohydrate are BIRMEU, BIRMEU01, BIRMEU02, BIRMEU03, MEBQUG, MEBQIU, MEBQOA and GATMUL (Louis et al., 1982 ▸; Portalone & Colapietro, 2006 ▸; Hulme & Tocher, 2006 ▸; Portalone, 2011 ▸), and for the polymorphs: DUKWIQ, DUKWAI and DUKWEM (Tutughamiarso et al., 2009 ▸). A wide range of cocrystals has also been documented, such as XOQQUS, MECTUL, MECVEX, MECVIB, MECVOH, MECVUN, MECWAU, MECWEY, MECWOI, MECWUO, MECXEZ, MECXID, MECXOJ, GIFWIF, UJUJAM, and POCWUD (Souza et al., 2019 ▸;Tutughamiarso et al., 2012 ▸; Tutughamiarso & Egert, 2012 ▸; Mohana et al., 2016 ▸, 2023 ▸; Sangavi et al., 2024 ▸). Salts include WEWZAA01, SIJXAM, SIJXIU, SIJXUG, EDATOS, GIFWEB, POCXAK, ZAPFEE and ROLTUJ WEWZAA01, SIJXAM, SIJXIU, SIJXUG, EDATOS, GIFWEB, POCXAK, ZAPFEE and ROLTUJ (Perumalla et al., 2013a ▸,b ▸; Prabakaran et al., 2001 ▸; Mohana et al., 2017 ▸; Karthikeyan et al., 2014 ▸) have been reported in the literature. 4-Hy­droxy­benzaldehydes are potential therapeutic agents for the treatment of human angiostrongyliasis. The crystal structure of 4-hy­droxy­benzaldehyde (Jasinski et al., 2008 ▸), as well as its cocrystal (Nowak & Sikorski, 2023 ▸) and polymorphic forms (Simões et al., 2013 ▸) have also been reported. 5FC contains multiple hydrogen-bond donors and acceptors, including amino and carbonyl groups, and 4-HBA offers hydroxyl and aldehyde functionalities capable of forming hydrogen bonds, along with an aromatic ring that can engage in π–π inter­actions. The present work focuses on the supra­molecular hydrogen bonding inter­actions in the crystal structure of 1:1 cocrystals of 5-fluoro­cytosine-4-hy­droxy­benzaldehyde.

5. Synthesis and crystallization

The title compound was synthesized by mixing a hot ethano­lic solution of 5-fluoro­cytosine with 4-hy­droxy­benzaldehyde in a 1:1 molar ratio. The solution was heated in a water bath at 333 K for 30 minutes and then allowed to cool slowly to room temperature. After a few days, colorless crystals had separated out of the mother liquor.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The H atoms of the N—H, –NH₂ and OH groups were located in difference-Fourier maps and refined freely. Other H atoms were placed geometrically (C—H = 0.93 Å) and refined using a riding model with U_iso(H) = 1.2U_eq(C).

Table 2. Experimental details.

Crystal data
Chemical formula	C4H4FN3O·C7H6O2
M r	251.22
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	297
a, b, c (Å)	4.2126 (1), 9.6687 (1), 26.8628 (5)
β (°)	94.186 (1)
V (Å3)	1091.21 (3)
Z	4
Radiation type	Cu Kα
μ (mm−1)	1.07
Crystal size (mm)	0.27 × 0.21 × 0.17
Data collection
Diffractometer	XtaLAB Synergy, Dualflex, HyPix
Absorption correction	Analytical (CrysAlis PRO; Rigaku OD, 2023 ▸)
Tmin, Tmax	0.782, 0.840
No. of measured, independent and observed [I > 2σ(I)] reflections	19824, 2243, 2127
R int	0.019
(sin θ/λ)max (Å−1)	0.630
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.034, 0.101, 1.04
No. of reflections	2243
No. of parameters	180
No. of restraints	4
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.20, −0.19

Computer programs: CrysAlis PRO (Rigaku OD, 2023 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2019/2 (Sheldrick, 2015b ▸), PLATON (Spek, 2020 ▸), Mercury (Macrae et al., 2020 ▸), POVRay (Cason, 2004 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 2452037

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Crystal data

C4H4FN3O·C7H6O2	F(000) = 520
Mr = 251.22	Dx = 1.529 Mg m−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54184 Å
a = 4.2126 (1) Å	Cell parameters from 18043 reflections
b = 9.6687 (1) Å	θ = 3.3–76.2°
c = 26.8628 (5) Å	µ = 1.07 mm−1
β = 94.186 (1)°	T = 297 K
V = 1091.21 (3) Å3	Block, colorless
Z = 4	0.27 × 0.21 × 0.17 mm

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Data collection

XtaLAB Synergy, Dualflex, HyPix diffractometer	2243 independent reflections
Radiation source: micro-focus sealed X-ray tube	2127 reflections with I > 2σ(I)
Detector resolution: 10.0000 pixels mm-1	Rint = 0.019
ω scans	θmax = 76.2°, θmin = 3.3°
Absorption correction: analytical (CrysAlisPro; Rigaku OD, 2023)	h = −3→5
Tmin = 0.782, Tmax = 0.840	k = −12→12
19824 measured reflections	l = −33→33

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Refinement

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.034	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.101	w = 1/[σ2(Fo2) + (0.0563P)2 + 0.2241P] where P = (Fo2 + 2Fc2)/3
S = 1.04	(Δ/σ)max = 0.001
2243 reflections	Δρmax = 0.20 e Å−3
180 parameters	Δρmin = −0.19 e Å−3
4 restraints	Extinction correction: SHELXL2019/2 (Sheldrick, 2015b), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: dual	Extinction coefficient: 0.0088 (13)

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. The data collection, cell refinement, and data reduction were performed using CrysAlisPro (Rigaku OD, 2023). Structure solution was carried out with SHELXT 2014/5 (Sheldrick, 2015a) and refinement was done using SHELXL-2016/6 (Sheldrick, 2015b). Molecular graphics were prepared using PLATON (Spek, 2020), Mercury (Macrae et al., 2020) and POVRay (Cason 2004).

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
F1A	0.6004 (2)	0.39753 (7)	0.63425 (3)	0.0557 (2)
O1A	−0.1067 (2)	0.40145 (8)	0.79019 (3)	0.0439 (2)
N1A	0.1299 (2)	0.27556 (9)	0.73165 (3)	0.0331 (2)
N2A	0.1501 (2)	0.51932 (9)	0.73260 (3)	0.0344 (2)
H1	0.076 (3)	0.5966 (14)	0.7444 (6)	0.050 (4)*
N3A	0.3795 (3)	0.15570 (10)	0.67141 (4)	0.0401 (3)
H1CC	0.492 (3)	0.1523 (16)	0.6446 (5)	0.049 (4)*
H1A	0.294 (3)	0.0798 (14)	0.6845 (5)	0.045 (4)*
C1A	0.4154 (3)	0.40215 (11)	0.67318 (4)	0.0351 (3)
C2A	0.3072 (2)	0.27452 (11)	0.69223 (4)	0.0313 (2)
C3A	0.0531 (2)	0.39737 (10)	0.75270 (4)	0.0323 (2)
C4A	0.3331 (3)	0.52236 (11)	0.69305 (4)	0.0356 (3)
H4A	0.399417	0.605918	0.680151	0.043*
O1B	1.1638 (2)	0.27251 (10)	0.35919 (3)	0.0520 (3)
H1B	1.074 (4)	0.2080 (18)	0.3414 (7)	0.076 (5)*
O2B	0.6793 (3)	0.13585 (11)	0.57435 (3)	0.0605 (3)
C1B	1.0816 (3)	0.25581 (12)	0.40653 (4)	0.0383 (3)
C2B	0.8831 (3)	0.14938 (12)	0.42006 (4)	0.0421 (3)
H2B	0.798860	0.087622	0.396053	0.050*
C3B	0.8117 (3)	0.13552 (13)	0.46892 (4)	0.0450 (3)
H3B	0.677170	0.064757	0.477749	0.054*
C4B	0.9386 (3)	0.22637 (12)	0.50533 (4)	0.0401 (3)
C5B	1.1361 (3)	0.33238 (13)	0.49140 (4)	0.0440 (3)
H5B	1.222274	0.393527	0.515464	0.053*
C6B	1.2064 (3)	0.34827 (13)	0.44238 (5)	0.0471 (3)
H6B	1.336473	0.420430	0.433386	0.056*
C7B	0.8656 (3)	0.21566 (14)	0.55746 (4)	0.0474 (3)
H7B	0.972343	0.275878	0.579901	0.057*

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
F1A	0.0776 (5)	0.0414 (4)	0.0534 (5)	−0.0025 (3)	0.0416 (4)	0.0020 (3)
O1A	0.0648 (5)	0.0344 (4)	0.0352 (4)	0.0058 (4)	0.0229 (4)	0.0028 (3)
N1A	0.0453 (5)	0.0265 (4)	0.0287 (4)	−0.0013 (3)	0.0106 (4)	0.0013 (3)
N2A	0.0456 (5)	0.0250 (4)	0.0338 (5)	0.0007 (3)	0.0103 (4)	−0.0009 (3)
N3A	0.0588 (6)	0.0295 (5)	0.0341 (5)	−0.0013 (4)	0.0177 (4)	−0.0018 (4)
C1A	0.0421 (6)	0.0337 (6)	0.0310 (5)	−0.0024 (4)	0.0131 (4)	0.0025 (4)
C2A	0.0383 (5)	0.0296 (5)	0.0265 (5)	−0.0006 (4)	0.0053 (4)	0.0009 (4)
C3A	0.0416 (5)	0.0285 (5)	0.0274 (5)	0.0006 (4)	0.0065 (4)	0.0018 (4)
C4A	0.0423 (6)	0.0290 (5)	0.0364 (5)	−0.0035 (4)	0.0096 (4)	0.0047 (4)
O1B	0.0734 (6)	0.0513 (5)	0.0327 (4)	−0.0134 (4)	0.0137 (4)	0.0001 (4)
O2B	0.0838 (7)	0.0605 (6)	0.0404 (5)	0.0017 (5)	0.0255 (5)	0.0006 (4)
C1B	0.0477 (6)	0.0370 (5)	0.0311 (5)	0.0040 (4)	0.0082 (4)	0.0018 (4)
C2B	0.0536 (7)	0.0389 (6)	0.0343 (6)	−0.0031 (5)	0.0075 (5)	−0.0055 (4)
C3B	0.0549 (7)	0.0419 (6)	0.0397 (6)	−0.0053 (5)	0.0140 (5)	−0.0004 (5)
C4B	0.0464 (6)	0.0422 (6)	0.0324 (5)	0.0101 (5)	0.0080 (4)	−0.0005 (4)
C5B	0.0528 (7)	0.0425 (6)	0.0367 (6)	0.0017 (5)	0.0026 (5)	−0.0072 (5)
C6B	0.0595 (7)	0.0406 (6)	0.0418 (6)	−0.0090 (5)	0.0083 (5)	−0.0015 (5)
C7B	0.0569 (7)	0.0521 (7)	0.0342 (6)	0.0110 (6)	0.0102 (5)	−0.0031 (5)

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Geometric parameters (Å, º)

F1A—C1A	1.3498 (12)	O1B—H1B	0.857 (15)
O1A—C3A	1.2521 (13)	O2B—C7B	1.2122 (17)
N1A—C2A	1.3397 (13)	C1B—C6B	1.3889 (17)
N1A—C3A	1.3558 (13)	C1B—C2B	1.3907 (16)
N2A—C4A	1.3578 (14)	C2B—C3B	1.3746 (16)
N2A—C3A	1.3715 (13)	C2B—H2B	0.9300
N2A—H1	0.877 (13)	C3B—C4B	1.3920 (17)
N3A—C2A	1.3231 (14)	C3B—H3B	0.9300
N3A—H1CC	0.891 (13)	C4B—C5B	1.3887 (18)
N3A—H1A	0.900 (12)	C4B—C7B	1.4594 (15)
C1A—C4A	1.3353 (15)	C5B—C6B	1.3791 (17)
C1A—C2A	1.4235 (14)	C5B—H5B	0.9300
C4A—H4A	0.9300	C6B—H6B	0.9300
O1B—C1B	1.3520 (13)	C7B—H7B	0.9300
C2A—N1A—C3A	120.00 (8)	O1B—C1B—C2B	122.30 (10)
C4A—N2A—C3A	121.95 (9)	C6B—C1B—C2B	119.97 (10)
C4A—N2A—H1	120.1 (10)	C3B—C2B—C1B	119.96 (11)
C3A—N2A—H1	117.8 (10)	C3B—C2B—H2B	120.0
C2A—N3A—H1CC	121.8 (10)	C1B—C2B—H2B	120.0
C2A—N3A—H1A	115.6 (9)	C2B—C3B—C4B	120.64 (11)
H1CC—N3A—H1A	122.4 (14)	C2B—C3B—H3B	119.7
C4A—C1A—F1A	121.33 (9)	C4B—C3B—H3B	119.7
C4A—C1A—C2A	120.77 (10)	C5B—C4B—C3B	118.92 (10)
F1A—C1A—C2A	117.90 (9)	C5B—C4B—C7B	118.91 (11)
N3A—C2A—N1A	119.97 (9)	C3B—C4B—C7B	122.16 (11)
N3A—C2A—C1A	120.74 (9)	C6B—C5B—C4B	120.93 (11)
N1A—C2A—C1A	119.29 (9)	C6B—C5B—H5B	119.5
O1A—C3A—N1A	121.43 (9)	C4B—C5B—H5B	119.5
O1A—C3A—N2A	118.86 (9)	C5B—C6B—C1B	119.58 (11)
N1A—C3A—N2A	119.71 (9)	C5B—C6B—H6B	120.2
C1A—C4A—N2A	118.21 (9)	C1B—C6B—H6B	120.2
C1A—C4A—H4A	120.9	O2B—C7B—C4B	126.30 (12)
N2A—C4A—H4A	120.9	O2B—C7B—H7B	116.9
C1B—O1B—H1B	107.7 (13)	C4B—C7B—H7B	116.9
O1B—C1B—C6B	117.73 (11)
C3A—N1A—C2A—N3A	−179.36 (10)	O1B—C1B—C2B—C3B	178.79 (11)
C3A—N1A—C2A—C1A	0.55 (16)	C6B—C1B—C2B—C3B	−0.23 (19)
C4A—C1A—C2A—N3A	177.61 (11)	C1B—C2B—C3B—C4B	−0.64 (19)
F1A—C1A—C2A—N3A	−1.78 (16)	C2B—C3B—C4B—C5B	0.72 (18)
C4A—C1A—C2A—N1A	−2.30 (17)	C2B—C3B—C4B—C7B	179.39 (11)
F1A—C1A—C2A—N1A	178.31 (9)	C3B—C4B—C5B—C6B	0.07 (18)
C2A—N1A—C3A—O1A	−178.50 (10)	C7B—C4B—C5B—C6B	−178.65 (11)
C2A—N1A—C3A—N2A	1.84 (16)	C4B—C5B—C6B—C1B	−0.9 (2)
C4A—N2A—C3A—O1A	177.67 (10)	O1B—C1B—C6B—C5B	−178.06 (11)
C4A—N2A—C3A—N1A	−2.66 (16)	C2B—C1B—C6B—C5B	1.01 (19)
F1A—C1A—C4A—N2A	−179.10 (9)	C5B—C4B—C7B—O2B	173.98 (12)
C2A—C1A—C4A—N2A	1.53 (17)	C3B—C4B—C7B—O2B	−4.7 (2)
C3A—N2A—C4A—C1A	0.92 (17)

4-Amino-5-fluoro-1H-pyrimidin-2-one–4-hydroxybenzaldehyde (1/1) . Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2A—H1···N1Ai	0.88 (1)	2.06 (1)	2.9354 (12)	175 (1)
N3A—H1CC···O2B	0.89 (1)	2.10 (1)	2.9848 (13)	170 (1)
N3A—H1A···O1Aii	0.90 (1)	2.04 (1)	2.9328 (12)	176 (1)
C4A—H4A···O1Biii	0.93	2.48	3.2905 (14)	145
O1B—H1B···O1Aiv	0.86 (2)	1.85 (2)	2.6934 (13)	166 (2)
C6B—H6B···F1Aiii	0.93	2.56	3.3446 (14)	143
C7B—H7B···F1A	0.93	2.51	2.9886 (14)	112

Symmetry codes: (i) −x, y+1/2, −z+3/2; (ii) −x, y−1/2, −z+3/2; (iii) −x+2, −y+1, −z+1; (iv) x+1, −y+1/2, z−1/2.

Funding Statement

We thank Howard University and the National Science Foundation Major Research Instrumentation program (NSF DMR-2117502) for financially supporting the acquisition of the Rigaku Synergy single-crystal X-ray diffractometer used in this study.