Molecular Beside Ionic: Crystal Structures of a 1/1 and a 1/4 Adduct of Pyridine and Formic Acid

Abstract

Of two low‐melting binary adducts obtained with the same universal organic acid and base components, one contains these as the neutral molecules, the other, however, in a deprotonated and a protonated form as ions. Reports on analogous results for further Brønsted acid/base systems are rare. The complex anion, new in a crystal structure, is assembled by strong hydrogen bonds O−H⋅⋅⋅O (see formula).

Observation of neutral, ionic and intermediate states in lamotrigine-acid complexes- inference from crystallographic bond geometries

“…The exact ionization state is difficult to predict and it was commented that the molecular environment in each crystal structure is unique and can affect the amount of proton transfer. For example, pyridine and formic acid form a co-crystal in 1:1 ratio, while it forms a salt in 1:4 ratio [11]. Recently, A.J.…”

Nonequimolar Mixture of Organic Acids and Bases: An Exception to the Rule of Thumb for Salt or Cocrystal

“…In Figure , the optimized unit cells of 1:1 cocrystal and 4:1 salt are shown along with their asymmetric units. The initial structures are retrieved from the Crystallographic Information Files (CIFs) of the experimental structures . In both cases, the lattice parameters get contracted by only ∼3% upon optimization with respect to the experimental crystal structure, indicating robustness of our computations.…”

“…Interestingly, however, on further increase in the ratio of formic acid to pyridine to 4:1, there is a sharp O–H···N → O – ···H–N + proton transition which leads to formation of the salt phase. It consists of one pyridinium cation, one formate anion along with three additional neutral formic acid molecules. , Previously, in the context of inorganic acid–base reactions between NH 3 and HX (X = F, Cl, Br, NO 3 , HCO 3 , and HSO 4 ), we have also shown that the proton transfer from hydrogen bonded complex (NH 3 ···HX) to ionic salt (NH 4 + X – ) is facilitated via cooperativity by increasing their stoichiometric ratio . Hence, while the 2:1 mixture of NH 3 and H 2 SO 4 is a H-bonded complex (NH 3 ···H 2 SO 4 ···NH 3 , cocrystal-like), its higher analogue (4:2 mixture) is a salt, (NH 4 ) 2 SO 4 .…”

“…The binary mixture of formic acid and pyridine has been well-studied in the context of salt–cocrystal formation. − As expected from the Δp K a rule, Mootz et al have reported that a 1:1 mixture of formic acid and pyridine interacts via a sufficiently strong O–H···N bond, resulting in cocrystal . On increasing the ratio of formic acid to pyridine to 3:1, this complex still remains as a neutral cocrystal assembly with a relatively stronger O–H···N hydrogen bond.…”