The Continuum in 5‑Fluorocytosine. Toward Salt Formation

5-Fluorocytosine (5-FC) was crystallized with complementary dicarboxylic acids, aiming to achieve a controlled synthesis of structures based on the Δp<i>K</i>_a rule proposed in the salt–cocrystal continuum study and to provide structural information helpful in the comprehension of its supramolecularity. Although 5-FC tends to be basic, p<i>K</i>_a = 3.26, only three salts are reported. In this way, new 5-FC salts were obtained, the fumaric, maleic and oxalic ones, all crystallizing in the monoclinic space group <i>P</i>2₁/<i>c</i>. In the 5-FC oxalate and fumarate cases, the acid molecules are placed on an inversion center in a fashion that each half molecule exhibits one terminal donor–acceptor site, leading to the constitution of a 5-FC–acid–5-FC heterodimer. Such a heterodimer is observed in only one donor–acceptor site of the maleate of 5-FC, whose acid molecule exhibits a closed chain architecture. Infrared and Raman spectra recorded for the three compounds complement the salt characterization on the basis of the extent of proton transfer. Thermal analysis evidence that the salt formation decreases the melting point of the new compounds, ranking this molecule as a coformer candidate to improve the physical properties of other drugs