Co-crystal and crystal: Supramolecular arrangement obtained from 4-aminosalicylic acid, bpa ligand and cobalt ion

In this study, the synthesis, spectroscopic properties (infrared and Raman) and crystal structures of two new compounds co-crystal and crystal named HASbpa (1) and [Co(bpa)(H2O)4]AS2⋅4H2O (2) have been reported, where bpa is trans-1,2-bis(4-pyridyl)ethane, HAS is 4-aminosalicylic acid and AS− is aminosalicylate anion. The crystalline arrangement of the compound 1 exhibits a triclinic system with space group P1¯. The formation of a structure known as co-crystal, composed by building blocks in their neutral form; being the first work of this type involving the HAS and nitrogen ligand as bpa. For compound 2, a monoclinic system was observed with P21/c space group. The crystalline arrangement of the structure consisted of a covalent one-dimensional cationic [Co(bpa)(H2O)4]2+ chain, which interacts by hydrogen bonding, π-stacking and electrostatic interactions with aminosalicylate anions and water molecules that were trapped in the crystal. These interactions form supramolecular cavities denominated as pseudo honeycombs. For compound 1, the infrared spectrum revealed the presence of bands at 1643 and 1601 cm−1 assigned to the stretching mode of CO [ν(CO)] and CC/CN groups [ν(CC/CN)]. For the Raman spectrum, these same modes appear around 1644 and 1602 cm−1 related to HAS and bpa blocks, respectively. For compound 2, the largest displacement of the bands compared to free ligand suggested the formation of covalent bonds between bpa ligand and metallic site and loss of the proton in HAS molecule. In the infrared spectrum we can observe the presence of bands around 1635 and 1618 cm−1 attributed to the stretching ν(COO−) and ν(CC/CN), for the Raman spectrum these same modes appear around 1631 and 1619 cm−1 related to AS− and bpa ligand respectively.

► Aminosalicylate anion and 1,2-bis(4-pirydil)ethane form a co-crystal (1).
► Co-crystal (1) reacts with Co(II) ion to generate the coordination polymer [Co(bpa)(H2O)4]2+ chain.
► The [Co(bpa)(H2O)4]2+ chain presents aminosalicylate anion as counterion (compound 2).
► Compound 2 presents hydrogen bonds and Pi stacking interactions as responsible for supramolecular forces.