Synthon trends according to acid strength and geometry in salts of N-heterocyclic bases

MEDANHA VALDO, A. K.; SAROTTI, A.M.; MARTINS, F. T.*

CRYSTENGCOMM

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2017 vol. 19 p. 5960 - 5965

1466-8033

The hierarchy and robustness of homosynthons and heterosynthons formed by N-heterocyclic bases were assessed experimentally in salts of aminopyrazine (ampyz) and trans-1,2-bis(4-pyridyl)ethane (BPE) with common strong, moderate and weak acids, and theoretically at the M06-2X/6-31+G** level of theory. A trend to assemble the base-pairing primary homosynthon in ampyz salts as the acid strength increases can be drawn. Chloride and bromide saltsof ampyz are present with this homosynthon, which is compatible with protonation at N4 and formation of an accessory four-point heterosynthon engaging two (ampyz)+ and two halides. This robust synthon is also invoked in chloride, bromide and dihydrogen phosphate salts of BPE. Among all our BPE multicomponent crystal forms, it is not found only in the uncommon phosphoric acid cocrystal of the dihydrogen phosphate salt. When crystallizing ampyz with weaker acids such as trifluoroacetic, trichloroacetic and phosphoric acids, the primary homosynthon disappears gradually as the strength acid decreases. In the last two cases, this homosynthon is not found, but, the trifluoroacetate salt of ampyz is present with both basepairing primary homosynthon and the two-point heterosynthon with carboxylate. Both monoprotonated forms at N1 and N4 are found together in this structure. Another trend of N1 protonation in the presence of counterions from inorganic tetrahedral oxoacids such as isopropyl sulfuric and phosphoric acids is also outlined, regardless of their acid strength