The 1:1 cocrystals of the proton-transfer compound dilituric acid-phenyl­biguanide monohydrate

A proton-transfer compound, 1-phenyl­biguanidium 5-nitro-2,6-dioxo-1,2,3,6-tetra­hydro­pyrimidin-4-olate monohydrate, C₈H₁₂N₅⁺·C₄H₂N₃O₅^-·H₂O, has been synthesized by a reaction between dilituric acid (5-nitro-2,4,6-trihydroxy­pyrimi­dine, Dilit) and phenyl­biguanide (N-phenyl­imido­carbonimidic diamide, Big). This compound cocrystallized as a 1:1 adduct, and the asymmetric unit consists of two dilituric amino-oxo planar tautomeric anions (Dilit^-), two monoprotonated phenyl­biguanidium cations (BigH⁺) and two water mol­ecules of crystallization (Z' = 2). Protonation occurs at the N atom attached to the phenyl ring of Big as a result of the proton-transfer process from the acidic hydr­oxy group of Dilit. In the crystal structure, the hydrated 1:1 adduct is stabilized by 25 two- and three-center hydrogen bonds.

The 1:1 complex of cytosine and 5-fluoro­uracil monohydrate revisited

The monohydrated mol­ecular adduct cytosine-5-fluoro­uracil-water (1/1/1) (denoted CytFur) [systematic name: 4-amino­pyrimidin-2(1H)-one-5-fluoro­pyrimidine-2,4(1H,3H)-dione-water (1/1/1)], C₄H₅N₃O·C₄H₃FN₂O₂·H₂O, was determined some 40 years ago [Voet & Rich (1969). J. Am. Chem. Soc. 91, 3069-3075] and is widely cited as the first example of an inter­molecular complex between two pyrimidinic nucleobases. In view of the importance of these base associations, CytFur has been reinvestigated with modern laboratory equipment to higher precision and with the location and free refinement of the H atoms. The new experiment reaffirms the results of the original and clarifies the tautomeric form exhibited by the compounds. The asymmetric unit comprises a hydrogen-bonded adduct of the canonical amino-oxo tautomers in an exact 1:1 ratio and a water mol­ecule of crystallization. This cyclic dimer forms a layered structure approximately parallel to the bc plane by joining through hydrogen bonds other such cyclic dimers. Disordered water mol­ecules run through tunnels formed by surrounding mol­ecular adducts along the a axis.

An unusual syn conformation of 5-formyl­uracil stabilized by supra­molecular inter­actions

The asymmetric unit of the amino–oxo tautomer of 5-formyl­uracil (systematic name: 2,4-dioxo-1,2,3,4-tetra­hydropyrimi­dine-5-carbaldehyde), C₅H₄N₂O₃, comprises one planar amino–oxo tautomer, as every atom in the structure lies on a crystallographic mirror plane. At variance with all the previously reported small-mol­ecule crystal structures containing the 5-formyl­uracil residue, the formyl substituent in the title compound exhibits an unusual syn conformation. The mol­ecules are linked into planar sheets parallel to the bc plane by a combination of six N—H...O and C—H...O hydrogen bonds. Four of the hydrogen bonds are utilized to stabilize the formyl group in the syn conformation.

Hydrogen-bonded supra­molecular motifs in the 1:1 monohydrated mol­ecular adduct of acetoguanaminium chloride with acetoguanamine and in 2,4,6-triamino­pyrimidine­diium dichloride dihydrate

In the 1:1 monohydrated mol­ecular adduct 2,4-diamino-6-methyl-1,3,5-triazin-1-ium chloride 2,4-diamino-6-methyl-1,3,5-triazine monohydrate, C₄H₈N₅⁺·Cl^-·C₄H₇N₅·H₂O, formed between 2,4-diamino-6-methyl-1,3,5-triazin-1-ium chloride (acetoguanaminium chloride) and 2,4-diamino-6-methyl-1,3,5-triazine (acetoguanamine), and in triamino­pyrimidine­diium dichloride dihydrate, C₄H₉N₅²⁺·2Cl^-·2H₂O, whose cationic component lies across a twofold rotation axis, the protonation occurs at the ring N atoms and the bond distances in the protonated mol­ecules indicate significant bond alterations, consistent with charge-separated polar forms. The supra­molecular structures of both compounds are stabilized by systems of hydrogen bonds forming complex sheets, inter­linked by sets of hydrogen bonds involving the solvent water mol­ecules and the chloride anions.